We describe a regulated antigen delivery system (RADS) that has (a) a vector that includes (1) a gene encoding a desired gene product operably linked to a control sequence, (2) an origin of replication conferring vector replication using DNA polymerase III, and (3) an origin of replication conferring vector replication using DNA polymerase I, where the second origin of replication is operably linked to a control sequence that is repressible by a repressor. The RADS microorganism also has a gene encoding a repressor, operably linked to an activatible control sequence. The RADS described provide high levels of the desired gene product after repression of the high copy number origin of replication is lifted. The RADS are particularly useful as live bacterial vaccines. Also described is a delayed RADS system, in which there is a delay before the high copy number origin is expressed after the repression is lifted. The delayed RADS is also particularly useful for live bacterial vaccines. Also described are several control elements useful for these systems, as well as methods for providing immunity to a pathogen in a vertebrate immunized with the RADS microorganisms.

Description of the Invention

Unless otherwise indicated, the practice of the present invention employs conventional techniques of cell culture, molecular biology, microbiology, recombinant DNA manipulation, immunology and animal science, which are within the skill of the art. Such techniques are explained fully in the literature. See, e.g., DNA CLONING, Volumes I and II (D. N. Glover, ed., 1985); OLIGONUCLEOTIDE SYNTHESIS (M. J. Gait ed., 1984); NUCLEIC ACID HYBRIDIZATION (B. D. Hames and S. J. Higgins, eds., 1984); B. Perbal, A PRACTICAL GUIDE TO MOLECULAR CLONING (1984); the series, METHODS IN ENZYMOLOGY (Academic Press, Inc.); VECTORS: A SURVEY OF MOLECULAR CLONING VECTORS AND THEIR USES (R. L. Rodriguez and D. T. Denhardt, eds., 1987, Butterworths); Sambrook et al. (1989), MOLECULAR CLONING, A LABORATORY MANUAL, second ed., Cold Spring Harbor Laboratory Press; Sambrook et al. (1989), MOLECULAR CLONING, A LABORATORY MANUAL, second ed., Cold Spring Harbor Laboratory Press; and Ausubel et al. (1995), SHORT PROTOCOLS IN MOLECULAR BIOLOGY, John Wiley and Sons.

The present invention is based on the discovery that the production of a vector-borne recombinant desired gene product in a microorganism can be conveniently increased by utilizing vector control elements that can be induced to increase the copy number of the vector.

The invention is directed toward a Regulated Antigen Delivery System (RADS), which utilizes microorganisms that comprise extrachromosomal vectors, called runaway vectors (RAVs), as well as genes encoding at least one repressor whose synthesis is under the control of an activatible control sequence. Essential elements of a RAV are (a) a gene encoding a desired gene product operably linked to a control sequence, (b) a first origin of replication ("ori") conferring vector replication using DNA polymerase I, (c) a second ori conferring vector replication using DNA polymerase III. The second ori is operably linked to a first control sequence that is repressible by the repressor.

In a RADS, the microorganism is maintained under conditions in which the first control sequence is repressed. Since the first control sequence controls utilization of the second ori, replication under these maintenance conditions is controlled by the first ori. However, under conditions where the repressor affecting the second ori is not made, derepression of the second ori takes place and control of vector replication switches to the second ori. The second ori then confers uncontrolled vector replication, thus producing very large amounts of the vector and the desired gene product encoded on the vector.

As further discussed infra, preferred use of RADS microorganisms is as a live bacterial vaccine. In those embodiments, the desired gene product is an antigen.

In a RADS, levels of expression of the desired gene product is controlled, at least in part, by controlling vector copy number. Copy number is controlled through the use of more than one origin of replication (ori) on the RAV.

As is well known, an ori is the region on a chromosome of extrachromosomal vector where DNA replication is initiated. For a review of plasmid replication in bacteria, see Helinski et al., pp. 2295-2324 in Escherichia coli and Salmonella, Cellular and Molecular Biology, Second Ed. Neidhardt et al., Eds. 1996. In the ColE1-type plasmids, replication is initiated by the synthesis of a 700-base preprimer RNA, designated RNA II. Transcription of RNA II is initiated 555 bases upstream from the on. Upon transcription, the 3' end of RNA II forms a hybrid with the plasmid at the ori. Cleavage of this RNA-DNA hybrid occurs at the replication origin, exposing a 3'-hydroxyl group that serves as the primer for DNA synthesis catalyzed by host DNA polymerase I. ColE1-type replicons do not specify an essential replication protein, however, they do require DNA polymerase I. Replication control in the ColE1-type plasmids is normally regulated by the binding of an unstable RNAI transcript, which is complementary to the RNAII pre-primer transcript. RNAI is transcribed divergent to, but within the RNAII coding region. Additional levels of regulation are provided by a protein encoded by rop elsewhere on the plasmid, which interacts with the RNAI and RNAII transcripts to prevent DNA replication. The RAV as described has been obtained by replacing the native promoter for RNAII with a-strong regulated promoter, capable of producing excess RNAII transcripts and thus uncontrolled replication of the ColE1-type replicon when the repressor for the new promoter is not present.

The choice of ori's for use in the present invention is not narrowly limited, provided the first ori confers replication by DNA polymerase III (e.g., the product of the dnaE gene in E. coli) and the second ori confers replication by DNA polymerase I (e.g., the product of the polA gene in E. coli). Preferably, the first ori confers a low copy number to the vector, to minimize any disadvantage imparted by replication of the vector. A preferred first ori is a pSC ori, which is known to confer about six to eight vector copies per chromosomal DNA equivalent in E. coli or Salmonella spp. A preferred second ori is a pUC ori.

The second ori is operably linked to a promoter that is repressible by a repressor. Preferred promoters for this purpose are P.sub.lac or P.sub.trc, which are repressed by the LacI repressor, and P22 P.sub.R, which is repressed by the C22 repressor.

The LacI repressor and C22 repressor are preferred repressors. Preferably, these repressors are encoded on the chromosome of the microorganism. The repressor gene is operably linked to an activatible control sequence that allows synthesis of the repressor only when the inducer of the control sequence is present. As more fully discussed infra, a preferred activatible control sequence is araCP.sub.BAD, which is activated by arabinose. Therefore, in a RADS microorganism using araCP.sub.BAD to control repressor synthesis, the presence of arabinose activates expression of the repressor, which prevents the utilization of the second ori. Vector replication is then is under the control of the first ori. However, when arabinose is withdrawn, the repressor is no longer made and the first control sequence is derepressed, allowing runaway replication of the vector. Regulation with arabinose is also useful since free arabinose is not generally available in nature. For example, arabinose is absent from avian and vertebrate tissues.

Regulation with arabinose is especially useful for delayed RADS. As discussed more fully infra, in delayed RADS, the induction of the high copy number ori is delayed after being triggered. Such a system is useful for live bacterial vaccines, which are generally grown in culture under conditions in which the runaway plasmid replication is not initiated (e.g., with arabinose). The bacteria are then inoculated into the vertebrate. In such a system, the inoculated bacteria colonize the lymphoid tissue before runaway plasmid replication is initiated, which allows production of large amounts of antigen. The delay in initiating runaway vector replication by the high copy number ori avoids the interference of the bacteria's ability to colonize caused by the production of high antigen levels and high vector levels. A delay system where araC-P.sub.BAD controls synthesis of the repressor, to allow the second ori to initiate runaway replication when arabinose is not-present; is effective because, once arabinose is no longer supplied, it takes time for the arabinose concentration to decline sufficiently to allow the AraC protein to begin acting as a repressor. However, this delay is not very long because the araCBAD operon efficiently metabolizes arabinose, thus rapidly (within about 15 min) reducing arabinose levels to the point where repressor synthesis does not occur. This short delay system can nonetheless allow sufficient delay of the initiation of runaway replication to provide an advantage, e.g., in live bacterial vaccines where the vaccine can be delivered to lymphoid tissue quickly, as is the case with intranasal inoculation. However, with oral inoculation, runaway replication would commence well before the vaccine reaches the GALT. In that situation, much of the antigen produced by the runaway vector is not exposed to lymphoid tissue and is thus wasted. Therefore, for situations where a longer delay is desired, as with oral administration of a live bacterial vaccine, an enhanced delay RADS can be utilized. In that system, the time of temporary viability is extended by utilizing a microorganism with an inactivation mutation in the operon that controls production of enzymes that degrade the inducer. Where arabinose is the inducer, its metabolism can be eliminated by an inactivating deletion mutation in the araCBAD operon. Such a deletion prevents metabolism of arabinose, leading to a higher intracellular level of arabinose after arabinose exposure has been withdrawn. This results in a longer delay before arabinose levels decline sufficiently to allow the AraC protein to begin acting as a repressor. This enhanced delay is sufficient to allow orally administered vaccines comprising the enhanced delayed RADS system to colonize the GALT before runaway vector replication is initiated.

An example of a RAV is the plasmid vector pMEG-771 (FIG. 1 (see Original Patent)). pMEG-771 must be maintained in a bacterial strain which possesses genes for two repressor proteins, the P22 C2 repressor and the LacI repressor of the lac operon, whose syntheses are regulated by the araCP.sub.BAD control sequence so that transcription of the c2 and lacI genes are dependent upon the presence of arabinose in the growth medium. The araC gene product in the presence of arabinose is a transcription activator causing transcription from P.sub.BAD of any structural genes linked to it. In the absence of arabinose the araC gene product serves as a repressor at P.sub.BAD to preclude transcription of structural genes fused to the P.sub.BAD promoter. Within the chromosome of the strain harboring pMEG-771 are the deletion/insertion mutations .DELTA.asdA19::TTaraCP.sub.BADc2TT and .DELTA.ilvG3::TTaraCP.sub.BADlacITT. When this strain is grown in the presence of arabinose, the c2 repressor gene is expressed and the C2 repressor binds to the P22 P.sub.R to preclude transcription of the pUC RNAII gene, which is fused to P.sub.R. The RNAII gene specifies an initiator RNA that, in the presence of DNA polymerase I (encoded by the chromosomal polA gene), initiates replication at the pUC ori. The plasmid continues to replicate making use of the pSC101 replicon control so that there are about six plasmid copies per chromosome DNA equivalent. This strain also produces copious quantities of the LacI repressor protein when the strain is grown in the presence of arabinose. In that situation, the lacI gene product binds to P.sub.TRC and represses transcription from this promoter so that any foreign gene sequence inserted into the multi cloning site from NcoI to HindIII is not synthesized. In this case, the strain grows exceedingly well under cultural conditions such as in a fermenter since energy demands to maintain the pMEG-771 plasmid vector are minimal due both to low plasmid copy number and the inability to express large amounts of the foreign protein. However, when the vaccine strain is administered to an immunized animal host, there is no exogenous arabinose and the arabinose within the bacterial vaccine cells disappears due to its catabolism or also by possible leakage out of the cell. Under those conditions, C2 and LacI repressor proteins cease to be synthesized and the density of these repressors within the cell decreases permitting not only transcription of the pUC RNA II sequence needed to initiate plasmid replication from the pUC ori in a completely unregulated manner, but also initiation of transcription of a gene for a foreign antigen. The overall level of production of the desired gene product is thus greatly increased due to increasing plasmid copy number along with increased transcription of the foreign gene from P.sub.TRC.

Some elements of RADS were previously described in WO96/40947. See in particular FIG. 8 of that publication. However, in that system the RAV was used primarily for the purpose of preventing the bacterial vaccine from retaining viability in nonpermissive environments such as below 30.degree., since in moving from body temperature to ambient temperature the cI857 vector-borne gene product becomes functional so that c2 repressor gene expression ceases, which leads to the derepression of the lysis genes.

The RAV exemplified in pMEG-771 utilizes various regulatory elements that work alone or in conjunction with other regulatory elements to achieve the desired result. The regulatory elements used in a RADS are not narrowly limited to those used in pMEG-771 and its associated host (araCP.sub.BAD, P.sub.R, P.sub.TRC, lacI repressor, C2 repressor); these elements may be substituted for others with similar functions, described as follows and as known in the art.

In general, the genes in a RADS of the present invention can be regulated 1) by linking the coding sequences to control sequences that promote or prevent transcription under permissive and non-permissive conditions, 2) by regulating the expression of trans regulatory elements that in turn promote or prevent transcription of the genes of the RAV and/or the host chromosome, 3) by adapting or altering trans regulatory elements, which act on the genes of the RAV and/or host chromosome, to be active or inactive under either high copy number or low copy number conditions, or by using combinations of these schemes. The RAVs of the present invention require various promoters to coordinate expression of different elements of the system. Some elements, such as temperature-sensitive repressors or environment-specific regulatory elements, use inducible, derepressible or activatible promoters. Preferred promoters for use as regulatory elements in an RAV are the cspA gene promoter, the phoA gene promoter, P.sub.BAD (in an araC-P.sub.BAD system), the trp promoter, the tac promoter, the trc promoter, .lamda.P.sub.L, P22 P.sub.R, mal promoters, and the lac promoter. These promoters mediate transcription at low temperature, at low phosphate levels, in the presence of arabinose, in the presence of low tryptophan levels, and in the presence of lactose (or other lac inducers), respectively. Each of these promoters and their regulatory systems are well known.

Trans Regulatory Elements. As used herein, "trans regulatory element" refers to a molecule or complex that modulates the expression of a gene. Examples include repressors that bind to operators in a control sequence, activators that cause transcription initiation, and antisense RNA that binds to and prevents translation of a mRNA. For use in RADS of the present invention, expression from regulated promoters is modulated by promoter regulatory proteins. These promoter regulatory proteins can function to activate or repress transcription from the promoter. Preferred trans regulatory elements are proteins mediating regulation of the cspA gene promoter, the phoA gene promoter, P.sub.BAD (in an araC-P.sub.BAD system), the trp promoter, the tac promoter, the trc promoter, the mal promoters, and the lac promoter.

Another type of trans regulatory element is RNA polymerase. Genes of the RADS can be regulated by linking them to promoters recognized only by specific RNA polymerases. By regulating the expression of the specific RNA polymerase, expression of the gene is also regulated. For example, T7 RNA polymerase requires a specific promoter sequence that is not recognized by bacterial RNA polymerases. A T7 RNA polymerase gene can be placed in the host cell and regulated to be expressed only in the permissive or non-permissive environment. Expression of the T7 RNA polymerase will in turn express any gene linked to a T7 RNA polymerase promoter. A description of how to use T7 RNA polymerase to regulate expression of a gene of interest, including descriptions of nucleic acid sequences useful for this regulation appears in Studier et al., Methods Enzymol. 185:60-89 (1990).

Another type of trans regulatory element is antisense RNA. Antisense RNA is complementary to a nucleic acid sequence, referred to as a target sequence, of a gene to be regulated. Hybridization between the antisense RNA and the target sequence prevents expression of the gene. Typically, antisense RNA complementary to the mRNA of a gene is used and the primary effect is to prevent translation of the mRNA. Expression of the genes of a RADS can be regulated by controlling the expression of the antisense RNA. Expression of the antisense RNA in turn prevents expression of the gene of interest. A complete description of how to use antisense RNA to regulate expression of a gene of interest appears in U.S. Pat. No. 5,190,931.

Other types of trans regulatory elements are elements of the quorum sensing apparatus. Quorum sensing is used by some cells to induce expression of genes when the cell population reaches a high density. The quorum sensing system is activated by a diffusible compound that interacts with a regulatory protein to induce expression of specific genes (Fuqua et al., J. Bacteriol. 176:269-275 (1994)). There is evidence that the diffusible compound, referred to as an autoinducer, interacts directly with a transcriptional activator. This interaction allows the activator to bind to DNA and activate transcription. Each quorum sensing transcriptional activator is typically activated only by a specific autoinducer, although the activator can induce more than one gene. It has also been shown that quorum sensing regulation requires only the transcriptional activator and a gene that contains a functional binding site for the activator (Gray et al., J. Bacteriol. 176:3076-3080 (1994)). This indicates that quorum sensing regulation can be adapted for the regulation of genes in a RADS of the present invention. For example, a gene encoding a quorum sensing transcriptional activator can be expressed in a RADS host, and another gene of the RADS can be under the control of a promoter that is controlled by the quorum sensing transcriptional activator. This will cause the RADS gene to be expressed when the cognate autoinducer is present and not expressed in the absence of the autoinducer. A gene under such control is referred to herein as being under quorum control. Where the RADS host produces the autoinducer, the gene under quorum control will be expressed when cell density is high, and will not be expressed when cell density is low. Any of the genes in a RADS can be placed under quorum control, including essential genes, lethal genes, replication genes and regulatory genes, which are described in WO96/40947. For operation of the RADS, the autoinducer can be supplied, for example, by the RADS host through the action of endogenous genes (that is, genes responsible for the synthesis of the autoinducer), in culture medium, or both. In the later case, the autoinducer supplied in the medium mimics the permissive conditions of high cell density. Alternatively, a gene for the production or synthesis of the autoinducer can be incorporated as an element of the RADS. Such an autoinducer gene would be considered a regulatory gene as used herein.

Examples of quorum sensing transcriptional activator genes and genes for the production of their cognate autoinducer are luxR and luxI (Gray et al., J. Bacteriol. 176:3076-3080 (1994)), lasR and lasI (Gambello and Iglewski, J. Bacteriol. 173:3000-3009 (1991)), traR and traI (Piper et al., Nature 362:448-450 (1993)), rhlI and rhlR (Latifi et al., Mol Microbiol 17(2):333-343 (1995)), and expR and expI (Pirhonen et al., EMBO J 12:2467-2476 (1993)). Autoinducers for these pairs include N-(3-oxohexanoyl)homoserine lactone (VAI; for LuxR), N-(3-oxododecanoyl)homoserine lactone (PAI; for LasR), and N-(3-oxo-octanoyl) homoserine lactone (AAI; for TraR). Some promoters that are induced by the quorum sensing transcriptional activators are luxI promoters, the lasB promoter, the traA promoter, and the traI promoter.

Quorum control can be used to effect regulated antigen delivery in a number of ways, for example, by obtaining production of desired gene products such as antigens, and non-expression of high copy number ori primer RNA II sequences under permissive conditions of high cell density in, for example, a fermenter, with the opposite expression pattern appearing as cell density decreases when, for example, the cells are introduced into an animal or released into the environment. As another example, a regulatory gene such c2 can be placed under quorum control. Then other elements of the RADS can be placed under control of the product of the regulatory gene, using, for example, P22P.sub.R. The regulatory gene will be expressed in the presence of the autoinducer, and not expressed in the absence of the autoinducer. Where the regulatory gene is c2 and a RADS gene is linked to P22P.sub.R, the RADS gene will be expressed (that is, derepressed) when the autoinducer is not present (since no C2 protein will be made), and repressed when the autoinducer is present. Where an essential gene or a replication gene is under quorum control (the autoinducer induces expression), it is preferred that the autoinducer be present under permissive conditions and absent under non-permissive conditions. Where a regulatory gene is under quorum control, the presence or absence of the autoinducer under permissive or non-permissive conditions will depend on whether the product of the regulatory gene is a positive or negative regulator.

Trans regulatory elements, such as repressors or antisense RNA, can be expressed from either the chromosome or a plasmid. To limit the size and complexity of the plasmid portion of the system, however, it is preferred that these regulatory elements be expressed from the bacterial chromosome.

Temperature-Sensitive Regulation. A preferred type of regulation for microorganisms intended for growth in humans or other warm-blooded animals is temperature regulation. This is based on the contrast between the high and constant body temperature present in mammals and birds and the low and variable temperature present in the ambient environment into which microorganisms are shed. To accomplish this, a preferred RADS expresses genes ensuring survival at about 37.degree. C. It is preferred that, where an RADS is intended to be administered to an animal, any temperature-based regulation should take into account the normal body temperature of the target animal. For example, chickens have a body temperature of 41.5.degree. C., and pigs have a body temperature of around 40.degree. C.

Temperature-regulated gene expression suitable for use in the RADS is described by Neidhardt et al., Annu. Rev. Genet. 18:295-329 (1984). There are well-defined heat shock genes that are strongly expressed at high temperature. Although the expression of these genes is temperature-regulated, there is frequently some low basal level of expression at the restrictive temperatures (Jones et al., J. Bacteriol. 169:2092-2095 (1987)). Temperature-regulated promoters exhibiting tighter control are described by Tobe et al., Mol. Micro. 5:887-893 (1991), Hromockyi et al., Mol. Micro. 6:2113-2124 (1991), and Qoronfleh et al., J. Bacteriol. 174:7902-7909 (1992).

For desired genes such as antigens, the S. flexneri virB promoter can be used, with S. flexneri virF gene and promoter elsewhere on the same plasmid, on a separate plasmid, or on the chromosome (Hromockyi et al. (1992); Tobe et al. (1991). A Yersina two component system for temperature regulation can also be used involving the structural gene for the temperature-regulated positive activator virF (Lambert de Rouvroit et al., Molec. Microbiol. 6:395-409 (1992) in combination with promoters of the yopH or yadR genes, with or without modification of the histone-like YmoA protein encoded by ymoA (Cornelis, in Molecular Biology of Bacterial Infections (Cambridge University Press, Cambridge, 1992)). The Shigela virF gene is equivalent to IcrF in Y. pestis (Hoe et al., J. Bacteriol. 174:4275-4286 (1992). Many other repressor-promoter combinations can be adapted to express genes in a temperature-specific manner by using temperature-sensitive forms of the repressor. Methods for obtaining temperature-sensitive mutant repressors are well established.

Cold-specific expression can also be accomplished by coupling a gene to a cold-shock promoter or a cold-sensitive promoter. Cold shock promoters may be obtained from known cold-shock genes. Cold shock genes with promoters have been described (Jones et al. (1987)). An example of a useful cold-shock promoter is the promoter from cspA (Vasina and Baneyx, Appl. Environ. Micro. 62:1444-1447 (1996)). Promoters with temperature-specific expression can be identified by a promoter probe vector. Such vectors have flanking DNA from a gene that is dispensable and which can readily be selected for or identified using, for example, a chromogenic substrate. Other cold-specific promoters useful for expression of the essential gene can be identified by screening for cold-sensitive lack of expression of .beta.-galactosidase in an S. typhimurium lacZ fusion library (Tanabe et al., J. Bacteriol. 174:3867-3873 (1992)).

A preferred system that is less complex involves the interaction of the bacteriophage lambda promoters, .lamda.P.sub.L and .lamda.P.sub.R, with the CI857 temperature-sensitive repressor. This system has been described, for example, by Lieb, J. Mol. Biol. 16:149-163 (1966). The lambda phage promoters .lamda.P.sub.L and .lamda.P.sub.R, with their mutant temperature-sensitive repressor CI857, provide a tightly regulated system used in expression vectors to provide controlled expression of toxic genes (O'Connor and Timmis, J. Bacteriol. 169:4457-4462 (1987)) and could also be used to regulate the synthesis of the initiator RNA II to initiate RNA replication at the DNA polymerase I dependent ori of high copy number plasmid vectors. The cI857 gene product is synthesized but inactive at 37.degree. C., and especially at even higher temperatures found in birds and some mammals, and is synthesized but actively represses expression of genes at 30.degree. C. and below whose transcription is controlled by either .lamda.P.sub.L or .lamda.P.sub.R.

Leaky expression from the control sequences of a RADS, if encountered, can be eliminated in several ways. For example, the level of CI repressor produced can be increased by placing the cI857 gene under the control of a strong promoter, such as Ptrc, thus providing an excess of the thermosensitive repressor. In addition, more binding sites for the CI repressor can be introduced within the operator region of .lamda.P.sub.R to reduce transcriptional starts at non-permissive temperatures, or engineered into regions downstream of the promoter element to hinder transcription at lower temperatures. Additionally, an antisense RNA for the regulated gene could be transcribed from a differently regulated promoter oriented in the opposite direction to .lamda.P.sub.R.

Arabinose Regulation. As previously discussed, a preferred regulatory system for triggering the expression switch when a microorganism is moved from a permissive to a non-permissive environment is the araC-P.sub.BAD system. The araC-P.sub.BAD system is a tightly regulated expression system which has been shown to work as a strong promoter induced by the addition of low levels of arabinose (see Guzman et al., J. Bacteriol. 177(14):4121-4130 (1995)). The araC-araBAD promoter is a bidirectional promoter controlling expression of the araBAD genes in one direction, and the araC gene in the other direction. For convenience, the portion of the araC-araBAD promoter that mediates expression of the araBAD genes, and which is controlled by the araC gene product, is referred to herein as P.sub.BAD. For use in the vectors and systems described herein, a cassette with the araC gene and the araC-araBAD promoter should be used. This cassette is referred to herein as araC-P.sub.BAD. The AraC protein is both a positive and negative regulator of P.sub.BAD. In the presence of arabinose, the AraC protein is a positive regulatory element that allows expression of P.sub.BAD. In the absence of arabinose, the AraC protein represses expression of P.sub.BAD. This can lead to a 1,200-fold difference in the level of expression from P.sub.BAD.

Enteric bacteria contain arabinose regulatory systems homologous to the araC araBAD system from E. coli. For example, there is homology at the amino acid sequence level between the E. coli and the S. typhimurium AraC proteins, and less homology at the DNA level. However, there is high specificity in the activity of the AraC proteins. For example, the E. coli AraC protein activates only E. coli P.sub.BAD (in the presence of arabinose) and not S. typhimurium P.sub.BAD. Thus, a RADS can employ multiple arabinose regulatory sequences from multiple enterics to differentially regulate different components in the same system.

Maltose Regulation. Another preferred regulatory system for triggering the expression switch when high copy number is desired is the malT system. malT encodes MalT, a positive regulator of four maltose-responsive promoters (P.sub.PQ, P.sub.EFG, P.sub.KBM, and P.sub.S). The combination of malT and a mal promoter creates a tightly regulated expression system which has been shown to work as a strong promoter induced by the addition of maltose (see Schleif, "Two Positively Regulated Systems, ara and mal" pp. 1300-1309 in Escherichia coli and Salmonella Cellular and Molecular Biology, Second Edition, Neidhardt et al., eds., ASM Press, Washington, D.C., 1996. Unlike the araC-P.sub.BAD system, malT is expressed from a promoter (P.sub.T) functionally unconnected to the other mal promoters. P.sub.T is not regulated by MalT. The malEFG-malKBM promoter is a bidirectional promoter controlling expression of the malKBM genes in one direction, and the malEFG genes in the other direction. For convenience, the portion of the malEFG-malKBM promoter that mediates expression of the malKBM gene, and which is controlled by the malT gene product, is referred to herein as P.sub.KBM, and the portion of the malEFG-malKBM promoter that mediates expression of the malEFG gene, and which is controlled by the malT gene product, is referred to herein as P.sub.EFG. Full induction of P.sub.KBM requires the presence of the MalT binding sites of P.sub.EFG. For use in the vectors and systems described herein, a cassette with the malT gene and one of the mal promoters should be used. This cassette is referred to herein as malT-P.sub.mal. In the presence of maltose, the MalT protein is a positive regulatory element which allows expression of P.sub.mal.

As with arabinose and araC-P.sub.BAD, regulation with maltose is useful for delayed RADS. This is because, once maltose is no longer supplied, it takes time for the maltose concentration to decline sufficiently to abolish induction by the MalT protein. To extend the time of temporary viability, it is preferred that strains for use with a maltose regulated RADS contain a deletion of the one or more elements of the mal operon. Such a deletion prevents metabolism of maltose, leading to a higher intracellular level of maltose. This results in a longer delay before maltose levels decline sufficiently to abolish induction by the MalT protein. Regulation with maltose is also useful since free maltose is not generally available in nature.

Delayed Death

As an alternative to rapid induction of the high copy number ori, the RADS can be designed to allow the host microorganism to remain viable for a limited time after the factor repressing the high copy number ori is withdrawn. This is referred to herein as a delayed RADS and results in RAVs which temporarily continue to be under the control of the lower copy number ori after host exposure to the environmental signal to switch to the high copy number ori. A preferred mechanism for delaying RAV high copy number is to base regulation on a trans regulatory element which must be degraded or diluted before the RAV can switch to the runaway condition. In such a system, upon moving the host microorganism from a high copy number repressed to a high copy number induced environment, a trans regulatory element which maintains the low copy number regime ceases to be produced. However, as long as the trans regulatory elements already on hand remain in sufficient quantity, the low copy number regime can remain in effect. Depending on the turnover of the trans regulatory element and the relationship between the amount of trans regulatory element on hand and the amount of trans regulatory element needed to maintain the low copy number regime, the low copy number regime can be maintained for several generations after transfer to the high copy number environment. Such temporary low copy number condition can be useful, for example, for allowing the host microorganism to colonize the host in a high copy number environment (e.g., without arabinose), such as an animal, but not remain indefinitely. As such, the RADS is a containment system even without the phage lysis genes described in WO96/40947. A delayed RADS is also useful when the desired gene product is harmful to the host cell, as in Example 3. Additionally, the delayed RADS can be used to depend an essential gene of a balanced lethal host system, such as asd, on an activatible control sequence such as araCP.sub.BAD, to provide for a weakening of the cell wall upon immunization (and withdrawal of, in this case, arabinose). See Example 5.

A preferred trans regulatory element for use in a delayed RADS consists of the AraC protein and arabinose, its inducer. The AraC protein will continue to repress the high copy number orn operatively linked to P.sub.BAD until the concentration of arabinose falls below a critical level.

Vectors

As used herein, "vector" refers to an autonomously replicating nucleic acid unit. The present invention can be practiced with any known type of vector, including viral, cosmid, phasmid, and plasmid vectors. The most preferred type of vector is a plasmid vector.

As is well known in the art, plasmids and other vectors possess a wide array of promoters, multiple cloning sequences, etc., and these replicons can be used so that the amount of a synthesized foreign antigen can be controlled by the relative number of gene copies. For example, vectors with p15A, pBR and pUC replicons can be constructed, all of which are dependent on the polA gene encoding DNA polymerase I for their replication. Determination of whether replication of a vector is dependent on DNA polymerase I can be accomplished by growing the vector in a host with a temperature-sensitive polA mutation, such as .chi.1891 (see Table 1 (see Original Patent)), and checking for vector maintenance as a function of temperature. Preferably, vectors used in RAD systems do not use antibiotic resistance to select for maintenance of the vector.

Preferred vectors have all of the essential elements of a RAV, that is (a) a gene encoding a desired gene product operably linked to a control sequence, (b) an origin of replication ("ori") conferring a low or intermediate copy number of the vector in the microorganism, and (c) an ori conferring a high copy number. Other elements which are part of any particular RADS may be on the vector or on another compatible vector or on the chromosome of the host microorganism.

Transfer Vectors. Rather than expressing an expression product directly, a microorganism can harbor a RAV for transfer to, and expression in, another cell in the environment into which the microorganism is placed. As used herein, a transfer vector is an expression vector which can be transferred from a RADS microorganism into a cell, and which directs the expression of an expression gene encoded by the transfer vector. It is intended that the transfer vector can contain any expression gene, including genes encoding antigens, immunomodulators, enzymes, and expression products that regulate gene expression or cellular activity in the recipient cell.

Preferred recipients for transfer vectors are cells of animals treated with the vector. For this purpose, RADS microorganisms containing a RAV transfer vector can be administered to an animal. It is preferred that the microorganisms invade cells of the animal in order to deliver the transfer vector. For this purpose, it is preferred that the microorganism lyses once it enters a cell of the animal. A preferred method for causing this lysis is an ELVS system, as described in WO96/40947. In that system, vector-borne lethal genes such as the phage lysis genes lys 13 and lys 19 are operably linked to P22 P.sub.R and the chromosome-encoded C2 repressor is operably linked to araCP.sub.BAD. Introduction of the strain into an environment without arabinose, such as in an inoculated animal, results in a dilution of the C2 repressor present until the lethyl gene products kill the cell. In addition, the RADS with a RAV comprising a transfer vector can be designed as an ELVS that lysis due to regulated lysis genes inserted into the chromosome. Such expression of lysis genes would exhibit delayed expression such that lysis would only occur after the vertebrate cells with the transfer vector had entered a eukaryotic cell and conferred runaway vector replication. See also Example 6, which describes novel transfer vector adaptations to the RADS. When properly designed, the ELVS system is fully compatible with the RADS system and may share control elements. In this case, lysis of the cell, for example caused by an ELVS, will release the transfer vector inside the recipient cell. For expression of genes on the transfer vector in recipient cells, it is preferred that the expression genes be operatively linked to expression control sequences operable in the recipient cell. For example, where the recipient cell is an animal cell, it is preferred that the expression genes be operatively linked to a promoter functional in the animal and possess sequences ensuring polyadenylation of the mRNA. Methods for engineering such sequences are well known in the art.

Transfer vectors may also contain replication sequences operable in the recipient cell. This would allow replication of the transfer vector, resulting in increased or longer expression of expression genes present on the transfer vector. Transfer vectors are especially useful for expression of antigens and other proteins that need to be glycosylated or post-translationally modified in a eukaryotic cell. In this way a bacterial cell with a RAV/ELVS vector can be used for delivery of a protein requiring eukaryotic processing by expressing the protein from a transfer vector.

A preferred use for transfer vectors is in a RADS for stimulation of an immune response in an animal. For this purpose it is preferred that the bacteria is avirulent Salmonella, Shigella, Yersinia, or invasive Escherichia that would invade and then lyse to liberate a transfer vector designed for expression in cells of the animal. This can be useful in stimulating an immune response for viruses, parasites or against gamete antigens in which the antigens are normally glycosylated or post translationally modified in some way that can only be accomplished when the antigen product is synthesized within the eukaryotic cell.

The efficiency of transfer of transfer vector can be improved by including an endA mutation, mutations in recBC (with or without sbc suppressor mutations), and/or mutations in other nuclease genes. Such mutations can reduce degradation of the transfer vector upon lysis of the bacterial cell. It is also possible to influence the cell type and the mucosal surface to which the microorganism containing the transfer vector would adhere to and invade. This can be achieved by blocking or turning on the expression of specific adhesins and/or invasins.

Many vectors are known for DNA immunization or introduction into cells in an animal. Such vectors can be used as transfer vectors in microorganisms containing a RAV with an ELVS. In this case, the RADS provides a useful means for introducing such vectors into cells. Preferred promoters for expression of expression genes on transfer vectors are adenovirus, herpes virus and cytomegalovirus promoters. Expression of the expression gene can also be increased by placing a bacterial promoter upstream of the-eukaryotic promoter, so that the bacterial strain would already express some of the expression product. This expression product would be liberated upon lysis of the bacterium.

Preferred bacterial hosts/strains and vectors useful in, or useful for constructing, Environmentally Limited Viability Systems are listed in Tables 1 and 2 (see Original Patent).

As previously discussed, the host cells of the present invention also have a desired recombinant gene encoding the polynucleotide of a desired gene product such as a polypeptide or a mRNA. The choice of desired gene is not narrowly limited and may include genes encoding, for example, viral, bacterial, fungal or parasite antigens, etc.

In order for the desired gene to be useful in the present invention, the gene must be expressed. Gene expression means that the information encoded in the sequence of DNA bases is transformed into a physical product in the form of a RNA molecule, polypeptide or other biological molecule by the biochemical mechanisms of the cell in which the gene is located. The biological molecule so produced is called the gene product. The term gene product as used here refers to any biological product or products produced as a result of expression of the gene. The gene product may be, for example, an RNA molecule, a peptide, or a product produced under the control of an enzyme or other molecule that is the initial product of the gene, i.e., a metabolic product. For example, a gene may first control the synthesis of an RNA molecule that is translated by the action of ribosomes into an enzyme that controls the formation of glycans in the environment external to the original cell in which the gene was found. The RNA molecule, the enzyme, and the glycan are all gene products as the term is used here. Any of these as well as many other types of gene products, such as glycoproteins and polysaccharides, will act as antigens if introduced into the immune system of an animal. Protein gene products, including glycoproteins and lipoproteins, are preferred gene products for use as antigens in vaccines.

Preferred embodiments of the present invention relate to the use of the above-described RADS microorganisms as constituents of live vaccines. In these cases, the desired recombinant gene would encode an antigen of a fungal, bacterial, parasitic, or viral disease agent. Live vaccines are particularly useful where localized immunity to the disease agent is important and might be a first line of defense. However, in this case it is essential that the host cells be attenuated to the individual being vaccinated.

Preferably, the host cells used in live vaccines are attenuated derivatives of pathogens. Most preferably, the attenuated derivatives are able to attach to, invade and persist in the gut-associated lymphoid tissue (GALT) or bronchial-associated lymphoid tissue (BALT). Such attenuated host cells are preferred because they are known to be able to persist in the inoculated animal, causing exposure to the antigen for an extended time period. Such a long exposure period is known to be highly effective in inducing an immunogenic response to the antigen.

Attenuation can be conferred upon the microbes by any known means, including chemical mutagenesis and the use of various recombinant genes. Preferred methods of conferring attenuation render the host cells unable to revert to the pathogenic condition. The most preferred methods of conferring attenuation on host cells are though the introduction of stable mutations or gene insertions by recombinant methods. Non-limiting examples of such methods include (1) introducing mutations that impose a requirement for aromatic amino acids and vitamins derived from precursors in this pathway (Stocker et al., 1983, Dev. Biol. Stand. 53:47-54; Hoiseth and Stocker, 1981, Nature 291:238-9); (2) mutating genes for global regulators such as cya and cyp (U.S. Pat. Nos. 5,389,368; 5,855,879; 5,855,880; 5,294,441 and 5,468,485), phoP (U.S. Pat. No. 5,424,065), ompR (Dorman et al., 1989, Infect. Immun. 57:2136-40), and poxA (U.S. patent application Ser. No. 08/829,402); (3) mutating genes for lipopolysaccharide (LPS) synthesis, such as galE (Germanier et al., 1975, J. Infect. Dis. 131:553-8), although this alone may be insufficient (Hone et al., 1988 Infect. Immun. 56:1325-33); (4) mutating genes needed for colonization of deep tissues, such as cdt (U.S. Pat. No. 5,387,744); or (5) by preventing expression of genes for proteases required at high temperature, such as htrA (Johnson et al., 1991, Mol. Microbiol. 5:401-7).

Once rendered attenuated, the microbes can serve as the immunogenic component of a vaccine to induce immunity against the microbe. Thus, the use of any microbe possessing the characteristics of the host cells described supra, including avirulence, are contemplated by this invention, including but not limited to E. coli, Salmonella spp., E. coli-S. typhimurium hybrids, Shigella spp., Yersinia spp., Pasteurella spp., Legionella spp. or Brucella spp. Preferred microbes are members of the genus Salmonella such as S. typhimurium, S. typhi, S. paratyphi, S. gallinarum, S. enteritidis, S. choleraesius, S. arizona, or S. dublin.

Live bacterial antigen delivery systems. Preferred hosts for use as antigen delivery systems are enteric bacteria. As used herein, the terms "antigen delivery system" and "antigen delivery microorganism" refer to a microorganism that produces an antigen or that harbors a transfer vector encoding an antigen. As used herein, "enteric bacteria" refers to any Enterobacteriaceae. Many of the preferred genes and regulatory elements described herein are operable in most enteric bacteria, thus allowing use of the many well developed E. coli and Salmonella regulatory systems. Most preferably, the bacterial host is an attenuated derivative of a pathogenic Salmonella.

In one embodiment of the system described herein, an attenuated derivative of a pathogenic microbe that attaches to, invades and persists in the gut-associated lymphoid tissue (GALT) or bronchial-associated lymphoid tissue (BALT) is used as a carrier of the gene product which is used for stimulating immune responses against a pathogen or allergen. Attenuated does not mean that a microbe of that genus or species can not ever cause disease, but that the particular microbe being used is attenuated with respect to the particular animal being treated. The microbe may belong to a genus or even a species that is normally pathogenic, but must belong to a strain that is attenuated. By pathogenic is meant capable of causing disease or impairing normal physiological functioning. Attenuated strains are incapable of inducing a full suite of symptoms of the disease that is normally associated with its virulent pathogenic counterpart. Microbes as used herein include bacteria, protozoa, parasites, unicellular fungi, and multicellular fungi.

Shigella or an enteroinvasive E. coli can be useful in antigen delivery systems since invasion into colonic mucosa could stimulate lymphoid tissues adjacent to the colon, so as to stimulate a strong mucosal immune response in the reproductive tract. Rectal immunization can be effective because of anatomical features such as the proximity of lymph nodes and lymphatics to the colon.

In order for a vaccine to be effective in inducing antibodies, the antigenic material must be released in such a way that the antibody-producing mechanism of the vaccinated animal can come into play. Therefore the microbe carrier of the gene product must be introduced into the animal. In order to stimulate a preferred response of the GALT or BALT cells as discussed previously, introduction of the microbe or gene product directly into the gut or bronchus is preferred, such as by oral administration, gastric intubation or in the form of intranasal, although other methods of administering the vaccine, such as intravenous, intramuscular, subcutaneous injection or intramammary or intrapenial or vaginal administration, are possible.

When the attenuated microbe is used as a vaccine, the antigen needs to become available to the animal's immune system. This may be accomplished when the carrier microbe dies so that the antigen molecules are released. Of course, the use of "leaky" attenuated mutants that release the contents of the periplasm without lysis is also possible. Alternatively, a gene may be selected that controls the production of an antigen that will be made available by the carrier cell to the outside environment prior to the death of the cell.

Antigens. Live recombinant RADS microorganisms can be used to deliver any product that can be expressed in the host microorganism. Preferred expression products for this purpose are antigens. For example, antigens can be from bacterial, viral, mycotic and parasitic pathogens, to protect against bacterial, viral, mycotic, and parasitic infections, respectively; gametes, provided they are gamete specific, to block fertilization; and tumor antigens, to halt cancers. It is specifically contemplated that antigens from organisms newly identified or newly associated with a disease or pathogenic condition, or new or emerging pathogens of animals or humans, including those now known or identified in the future, can be used in a RADS. Furthermore, antigens for use in a RADS are not limited to those from pathogenic organisms. The selection and recombinant expression of antigens has been previously described by Schodel (1992) and Curtiss (1990). Immunogenicity of the microorganisms can be augmented and/or modulated by constructing strains that also express genes for cytokines, adjuvants, and other immunomodulators.

Some examples of microorganisms useful as a source for antigen are listed below. Theses include microoganisms for the control of plague caused by Yersinia pestis and other Yersinia species such as Y. pseudotuberculosis and Y. enterocolitica, of gonorrhea caused by Neisseria gonorrhoea, of syphilis caused by Treponema pallidum, and of venereal diseases as well as eye infections caused by Chlamydia trachomatis. Species of Streptococcus from both group A and group B, such as those species that cause sore throat or heart diseases, Erisipelothrix rhusiopathiae, Neisseria meningitidis, Mycoplasmapneumoniae and other Mycoplasma species, Hemophilus influenza, Bordetella pertussis, Mycobacterium tuberculosis, Mycobacterium leprae, Bordetella species, Escherichia coli, Streptococcus equi, Streptococcus pneumoniae, Brucella abortus, Pasteurella hemolytica and P. multocida, Vibrio cholera, Shigella species, Borrellia species, Bartonella species, Heliobacter pylori, Campylobacter species, Pseudomonas species, Moraxella species, Brucella species, Francisella species, Aeromonas species, Actinobacillus species, Clostridium species, Rickettsia species, Bacillus species, Coxiella species, Ehrlichia species, Listeria species, and Legionella pneumophila are additional examples of bacteria within the scope of this invention from which antigen genes could be obtained. Viral antigens can also be used in a RADS. Viral antigens can be used in antigen delivery microorganisms directed against viruses, either DNA or RNA viruses, for example from the classes Papovavirus, Adenovirus, Herpesvirus, Poxvirus, Parvovirus, Reovirus, Picornavirus, Myxovirus, Paramyxovirus, Flavivirus or Retrovirus. Antigen delivery microorganisms using antigens of pathogenic fungi, protozoa and parasites can also be used.

Certain vaccine embodiments comprise utilization of microorganisms comprising a RADS system where the desired gene encodes an allergen. Such a vaccine may be used in an exposure regimen designed to specifically desensitize an allergic host. Allergens are substances that cause allergic reactions in an animal that is exposed to them. Allergic reactions, also known as Type I hypersensitivity or immediate hypersensitivity, are vertebrate immune responses characterized by IgE production in conjunction with certain cellular immune reactions. Many different materials may be allergens, such as animal dander and pollen, and the allergic reaction of individual animals will vary for any particular allergen. It is possible to induce tolerance to an allergen in an animal that normally shows an allergic response. The methods of inducing tolerance are well-known and generally comprise administering the allergen to the animal in increasing dosages.

Recombinant attenuated Salmonella are capable of stimulating strong mucosal, systemic and cellular immune responses against foreign antigens and thus against the pathogen that is the source of the foreign antigen. It is not necessary that the antigen gene be a complete gene as present in the parent organism, which was capable of producing or regulating the production of a macromolecule, for example, a functioning polypeptide. It is only necessary that the gene be capable of serving as the template used as a guide in the production of an antigenic product. The product may be one that was not found in that exact form in the parent organism. For example, a functional gene coding for a polypeptide antigen comprising 100 amino acid residues may be transferred in part into a carrier-microbe so that a peptide comprising only 75, or even 10, amino acid residues is produced by the cellular mechanism of the host cell. Alternatively, if the amino acid sequence of a particular antigen or fragment thereof is known, it is possible to chemically synthesize the DNA fragment or analog thereof by means of automated gene synthesizers, PCR, or the like and introduce said DNA sequence into the appropriate expression vector. At the other end of the spectrum is a long section of DNA coding for several gene products, one or all of which can be antigenic.

Multiple antigens can also be expressed by a recombinant avirulent Salmonella strain. In addition, antigens, or even parts of antigens, that constitute a B cell epitope or define a region of an antigen to which an immune response is desired, can be expressed as a fusion to a carrier protein that contains a strong promiscuous T cell epitope and/or serves as an adjuvant and/or facilitates presentation of the antigen to enhance, in all cases, the immune response to the antigen or its component part. This can easily be accomplished by genetically engineering DNA sequences to specify such fusions for expression by attenuated strains. Fusion to tenus toxin fragment C, CT-B, LT-B and hepatitis virus B core are particularly useful for these purposes, although other epitope presentation systems are well known in the art.

In order for the expression gene to be effective in eliciting an immune response, the expression gene must be expressed, which can be accomplished as described above. In order for an antigen delivery microorganism to be effective in immunizing an individual, the antigenic material must be released in such a way that the immune system of the vaccinated animal can come into play. Therefore the live avirulent microorganism must be introduced into the animal. In order to stimulate a preferred response of the GALT or BALT cells as discussed previously, introduction of the microbe or gene product directly into the gut or bronchus is preferred, such as by oral administration, intranasal administration, gastric intubation or in the form of aerosols, although other methods of administering the antigen delivery microorganism, such as intravenous, intramuscular, subcutaneous injection or intramammary, intrapenial, intrarectal, or vaginal administration, is possible.

Antigen Delivery Compositions. A preferred use of antigen delivery microorganisms is as vaccines for stimulating an immune response to the delivered antigens. Oral immunization in a suitable animal host with live recombinant Salmonella vaccine strains leads to colonization of the gut-associated lymphoid tissue (GALT) or Peyer's patches, which leads to the induction of a generalized mucosal immune response to both Salmonella antigens and the foreign antigens synthesized by the recombinant Salmonella (Curtiss et al., Adv. Exp. Med. Biol. 251:33-47 (1989)). Further penetration of the vaccine strain into the mesenteric lymph nodes, liver and spleen augments the induction of systemic and cellular immune responses directed against Salmonella antigens and the foreign antigens made by the recombinant Salmonella (Doggett and Curtiss (1992)). Thus the use of recombinant avirulent Salmonella vaccines for oral immunization stimulates all three branches of the immune system, particularly important when immunizing against infectious disease agents which colonize on and/or invade through mucosal surfaces.

By vaccine is meant an agent used to stimulate the immune system of a living organism so that an immune response occurs. Preferably, the vaccine is sufficient to stimulate the immune system of a living organism so that protection against future harm is provided. Immunization refers to the process of inducing a continuing high level of antibody and/or cellular immune response in which T-lymphocytes can either kill the pathogen and/or activate other cells (for example, phagocytes) to do so in an organism, which is directed against a pathogen or antigen to which the organism has been previously exposed. Although the phrase "immune system" can encompass responses of unicellular organisms to the presence of foreign bodies, that is, interferon production, as used herein the phrase is restricted to the anatomical features and mechanisms by which a multi-cellular organism responds to an antigenic material which invades the cells of the organism or the extra-cellular fluid of the organism. The antibody so produced may belong to any of the immunological classes, such as immunoglobulins A, D, E, G or M. Of particular interest are vaccines which stimulate production of immunoglobulin A (IgA) since this is the principle immunoglobulin produced by the secretory system of warm-blooded animals, although the vaccines described herein are not limited to those which stimulate IgA production. For example, vaccines of the nature described herein are likely to produce a broad range of other immune responses in addition to IgA formation, for example, cellular and humoral immunity. Immune responses to antigens are well studied and widely reported. A survey of immunology is given in Paul, Ed. (1999), Fundamental Immunology, fourth ed., Philadelphia: Lippincott-Raven, Sites et al., Basic and Clinical Immunology (Lange Medical Books, Los Altos, Calif., 1994), and Orga et al., Handbook of Mucosal Immunology (Academic Press, San Diego, Calif., 1994). Mucosal immunity is also described by Ogra et al., Eds. (1999), Mucosal Immunology, second ed., Academic Press, San Diego.

An individual treated with a vaccine of the invention is defined herein as including all vertebrates, for example, mammals, including domestic animals and humans, various species of birds, including domestic birds, particularly those of agricultural importance. Preferably, the individual is a warm-blooded animal.

The dosages of live recombinant vaccines required to elicit an immune response will vary with the antigenicity of the cloned recombinant expression product and need only be a dosage sufficient to induce an immune response typical of existing vaccines. Routine experimentation will easily establish the required dosage. Typical initial dosages of vaccine for oral administration could be 1.times.10.sup.7 to 1.times.10.sup.11 CFU depending upon the size and age of the individual to be immunized. Administering multiple dosages can also be used as needed to provide the desired level of protective immunity. The pharmaceutical carrier in which the vaccine is suspended can be any solvent or solid material for encapsulation that is non-toxic to the inoculated animal and compatible with the carrier organism or antigenic gene product. Suitable pharmaceutical carriers include liquid carriers, such as normal saline and other non-toxic salts at or near physiological concentrations, and solid carriers not used for humans, such as talc or sucrose, or animal feed. Adjuvants may be added to enhance the antigenicity if desired. When used for administering via the bronchial tubes, the vaccine is preferably presented in the form of an aerosol.

Immunization with a pathogen derived gene product can also be used in conjunction with prior immunization with the attenuated derivative of a pathogenic microorganism acting as a carrier to express the gene product specified by a recombinant gene from a pathogen. Such parenteral immunization can serve as a booster to enhance expression of the secretory immune response once the secretory immune system to that pathogen-derived gene product has been primed by immunization with the carrier microbe expressing the pathogen derived gene product to stimulate the lymphoid cells of the GALT or BALT. The enhanced response is known as a secondary, booster, or anamnestic response and results in prolonged immune protection of the host. Booster immunizations may be repeated numerous times with beneficial results.

Although it is preferred that antigen delivery microorganisms be administered by routes that stimulate a mucosal immune response, namely oral, intranasal, intravaginal, and interrectal, these microorganisms can also be delivered intramuscularly, intravenously, and in other parenteral routes. Administration of an antigen delivery microorganism can also be combined with parenteral administration of purified antigenic components. In case where an ELVS is used to control or treat cancer, it is preferred that the ELVS be administered parenterally.

Adaptation of RADS to Useful Host Strains. Avirulent strains of S. typhimurium are known to be totally attenuated and highly immunogenic in mice, chickens, and pigs, inducing protective immunity to infection with 10,000 times a lethal dose with the virulent wild-type strain. Similarly, avirulent strains of S. choleraesuis are attenuated and-immunogenic in mice and pigs and also offer significant protective immunity. Avirulent strains: of S. dublin have been isolated and tested and found to be avirulent, immunogenic, and protective in calves. Attenuated S. typhi strains have also been constructed and found to induce significant immune responses in human volunteers. Attenuated derivatives of Vibrio cholerae and Shigella flexneri have also been constructed and used as vaccines to induce significant immune responses in human volunteers. Mycobacterium bovis strain BCG has also been used to orally immunize humans. Attenuated Listeria monocytogenes has also been used as a live vaccine for immunization of mice. In addition to serving as vaccines to immunize animals and human hosts against infection with related virulent wild-type strains, avirulent derivatives of the above cited microorganisms can also be used as antigen delivery vectors by genetically engineering them to express foreign antigens. These antigens could be from bacterial, viral, fungal and parasitic pathogens or they could be allergens or they could be gamete specific antigens in a contraceptive vaccine or tumor antigens in anti cancer vaccines. Immunization of animal and/or human hosts with these live recombinant avirulent vaccines is known to induce mucosal, systemic and cellular immune responses directed against the foreign antigen and against the pathogen from which the gene specifying the foreign antigen was isolated or against allergens or against sperm or ova or against tumor cells, respectively.

Bacterial pathogens can be attenuated by introducing deletion mutations in various genes as described above or as known to the skilled artisan. Any of these strains are suitable for introduction of a RADS of the sort disclosed herein, although modifications would be needed to make the system operable in gram-positive bacteria. Specifically these modifications would require modification of Shine-Dalgarno sequences to permit translation of mRNA, and slight changes in promoter sequences to cause transcription to be more efficient, as is known in the art.

Administration of a live vaccine of the type disclosed above to an animal may be by any known or standard technique. These include oral ingestion, gastric intubation, or broncho-nasal spraying. All of these methods allow the live vaccine to easily reach the GALT or BALT cells and induce antibody formation and are the preferred methods of administration. Other methods of administration, such as intravenous injection to allow the carrier microbe to reach the animal's blood stream may be acceptable. Intravenous, intramuscular or intramammary injection is also acceptable with other embodiments of the invention, as is described later.

Since preferred methods of administration are oral ingestion, aerosol spray and gastric intubation, preferred carrier microbes are those that belong to species that home preferentially to any of the lymphoepithelial structures of the intestines or of the bronchi of the animal being vaccinated. Preferably, these strains are attenuated derivatives of enteropathogenic strains produced by genetic manipulation of enteropathogenic strains. Strains that home to Peyer's patches and thus directly stimulate production of IgA are most preferred. In animals these include specific strains of Salmonella, and Salmonella-E. coli hybrids that home to the Peyer's patches.

The dosages required will vary with the antigenicity of the gene product and need only be an amount sufficient to induce an immune response typical of existing vaccines. Routine experimentation will easily establish the required amount. Typical initial dosages of vaccine could be 0.001-0.1 mg antigen/kg body weight, with increasing amounts or multiple dosages used as needed to provide the desired level of protection.

The pharmaceutical carrier in which the vaccine is suspended or dissolved may be any solvent or solid or encapsulated in a material that is non-toxic to the inoculated animal and compatible with the carrier organism or antigenic gene product. Suitable pharmaceutical carriers include liquid carriers, such as normal saline and other non-toxic salts at or near physiological concentrations, and solid carriers not used for humans, such as talc, sucrose, and feed for farm animals. Adjuvants may be added to enhance the antigenicity if desired. When used for administering via the bronchial tubes, the vaccine is preferably presented in the form of an aerosol.

Immunization with a pathogen-derived gene product can also be used in conjunction with prior immunization with the attenuated derivative of a pathogenic microorganism acting as a carrier to express the gene product specified by a recombinant gene from a pathogen. Such parenteral immunization can serve as a booster to enhance expression of the secretory immune response once the secretory immune system to that pathogen-derived gene product has been primed by immunization with the carrier microbe expressing the desired gene product to stimulate the lymphoid cells of the GALT or BALT. The enhanced response is known as a secondary, booster, or anamnestic response and results in prolonged immune protection of the host. Booster immunizations may be repeated numerous times with beneficial results.

In other embodiments of the invention, a recombinant attenuated derivative of a pathogenic microbe can be used to express, in the animal host, gene products that are therapeutic in the inoculated animal. Non-limiting examples of such products include lymphokines or cytokines to modulate the immune response (Saltzman et al., 1996, Cancer Bio. Ther. Radiol. Pharm. 11: 145-153; Saltzman et al., 1997, J. Pediatric. Surg. 32:301-306; Whittle et al., 1997, J. Med. Microbiol. 46:1029-1038, 1997; Dunstan et al., 1996, Infect. Immun. 64:2730-2736), sperm-specific and egg-specific autoantigens to arrest fertility (U.S. Pat. No. 5,656,488), blood products such as clotting factors, specific antibodies, e.g., which bind to tumors or pathogens such as viruses, fungi, parasites, or bacteria; growth factors, essential enzymes or structural proteins which are insufficiently produced in the host, ribozymes or antisense RNA which cleaves or inactivates a nucleic acid encoding an undesirable gene product (e.g., a gene product essential for tumor metastasis or angiogenesis of tumors; a gene product essential for a pathogen to cause disease), or enzymes that have the potential to convert prodrugs into toxic drugs within a tumor cell mass in an individual with a solid tumor (Pawelek et al., 1997, Cancer Res. 57:4537-44).

Because the avirulent microbes of this invention are able to traverse a variety of immunocompetent structures including the GALT, mesenteric lymph nodes and spleen, such microbes may also be used to modulate the immune system by producing a variety of immunoregulatory products. Accordingly, one or more genes encoding immunoregulatory proteins or peptides may be recombinantly introduced as a desired gene into the attenuated microbes such that the microbes are capable of taking up residence in the appropriate immunocompetent tissue and express the recombinant desired gene product to suppress, augment or modify the immune response in the host. Nonlimiting examples of immunoregulatory molecules include colony stimulating factors (macrophage, granulocyte, or mixed), macrophage chemotoxin, macrophage inhibition factor, leukocyte inhibitory factors, lymphotoxins, blastogenic factor, interferons, and interleukins.

Derivatives of attenuated microbes are also contemplated to be within the scope of this invention. By derivative is meant sexually or asexually derived progeny and mutants of the avirulent strains including single or multiple base substitutions, deletions, insertions or inversions which retain the basic functioning of the host cells previously described.
 

Claim 1 of 23 Claims

1. A microorganism comprising a regulated antigen delivery system (RADS), wherein the RADS comprises: (a) a vector comprising (1) a gene encoding a desired gene product operably linked to a second control sequence; (2) a first origin of replication (ori) conferring vector replication using DNA Polymerase III; and (3) a second ori conferring vector replication using DNA Polymerase I, wherein the second ori is operably linked to a first control sequence repressible by a first repressor, and wherein the runaway vector does not comprise a phage lysis gene; and (b) a gene encoding a first repressor operably linked to a first activatible control sequence, wherein said microorganism additionally comprises a balanced lethal host vector system comprising a lack of a functioning essential gene, wherein the essential gene is a gene necessary for synthesis of essential cell wall constituent diaminopimelic acid (DAP), on the chromosome and the presence of a recombinant functioning copy of the essential gene on the vector.

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