Antibody responses to Brucella abortus 2308 in cattle vaccinated with B. abortus RB51

Combined immunization with inactivated vaccine reduces the dose of live B. abortus A19 vaccine

Background

Brucella spp. is an important zoonotic pathogen responsible for brucellosis in humans and animals. Brucella abortus A19 strain is a widespread vaccine in China. However, it has a drawback of residual virulence in animals and humans.

Methods

In this study, the BALB/c mice were inoculated with either 100 μL PBS(control group, C group), 10⁹ CFU/mL inactivated B. abortus A19 strain (I group), 10⁵ CFU/mL (low-dose group, L group) 10⁶ CFU/mL live B. abortus A19 strain (high-dose group, H group), or 10⁵ CFU/mL live B. abortus A19 strain combined with 10⁹ CFU/mL inactivated B. abortus A19 strain (LI group). Mice were challenged with B. abortus strain 2308 at 7 week post vaccination. Subsequently, the immune and protective efficacy of the vaccines were evaluated by measuring splenic bacterial burden, spleen weight, serum IgG, interferon-gamma (IFN-γ), interleukin-4 (IL-4) percentage of CD4 + and CD8 + T cells of mice via bacterial isolation, weighing, ELISA and flow cytometry, respectively.

Results

The splenic bacterial burden and spleen weight of the mice in group LI were mostly equivalent to the mice of group H. Moreover, Brucella-specific serum IgG, IFN-γ, IL-4, and the percentage of CD4⁺ and CD8⁺ T cells of the LI group mice were similar to those of the H group. In the subsequent challenge test, both vaccines conferred protective immunity to wild-type (WT) 2308 strain. In addition, the levels of IL-4 and IFN-γ, CD4⁺ and CD8⁺ T cells in these mice were similar to those of the mice in the H group.

Conclusions

Combined immunization with low dose live vaccine and inactivated vaccine allowed to reduce the live B. abortus A19 vaccine, dose with an equivalent protection of the high-dose live vaccine.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12917-022-03229-0.

A safe non-toxic Brucella abortus ghosts induce immune responses and confer protection in BALB/c mice

Brucellosis, which is caused by Brucella spp., is an important zoonotic infectious disease that can cause great hazard to public health and safety. However, the current vaccines have several drawbacks, including residual virulence for animals and humans. Bacterial ghost is the empty envelopes of bacteria, which emerge as a proper vaccine candidate. With the purpose of generating B. abortus ghosts and investigating the immunogenicity of bacterial ghosts as vaccine candidate, we used homologous recombination and bacterial ghost technologies to construct 2308ΔgntR ghost strain. Mice were injected with 2308ΔgntR ghost and the safety and immunogenicity of ghost were further evaluated. The mice inoculated with ghost showed no splenomegaly. The 2308ΔgntR ghost induced high protective immunity in BALB/c mice against challenge with S2308, and elicited an anti-Brucella-specific immunoglobulin G (IgG) response and induced the secretion of interferon gamma (IFN-γ) and interleukin-4 (IL-4). Additionally, 2308ΔgntR ghost demonstrated strong spleen CD4⁺ and CD8⁺ T cell responses. These results suggest that 2308ΔgntR ghost is a potential vaccine candidate and may represent a promising new approach for vaccination against Brucella infection.

Proteomic Analysis of Membrane Blebs of Brucella abortus 2308 and RB51 and Their Evaluation as an Acellular Vaccine

Membrane blebs are released from Gram-negative bacteria, however, little is known about Brucella blebs. This work pursued two objectives, the first was to determine and identify the proteins in the membrane blebs by proteomics and in silico analysis. The second aim was to evaluate the use of membrane blebs of Brucella abortus 2308 and B. abortus RB51 as an acellular vaccine in vivo and in vitro. To achieve these aims, membrane blebs from B. abortus 2308 and RB51 were obtained and then analyzed by liquid chromatography coupled to mass spectrometry. Brucella membrane blebs were used as a "vaccine" to induce an immune response in BALB/c mice, using the strain B. abortus RB51 as a positive vaccine control. After subsequent challenge with B. abortus 2308, CFUs in spleens were determined; and immunoglobulins IgG1 and IgG2a were measured in murine serum by ELISA. Also, activation and costimulatory molecules induced by membrane blebs were analyzed in splenocytes by flow cytometry. Two hundred and twenty eight proteins were identified in 2308 membrane blebs and 171 in RB51 blebs, some of them are well-known Brucella immunogens such as SodC, Omp2b, Omp2a, Omp10, Omp16, and Omp19. Mice immunized with membrane blebs from rough or smooth B. abortus induced similar protective immune responses as well as the vaccine B. abortus RB51 after the challenge with virulent strain B. abortus 2308 (P < 0.05). The levels of IgG2a in mice vaccinated with 2308 membrane blebs were higher than those vaccinated with RB51 membrane blebs or B. abortus RB51 post-boosting. Moreover, mice immunized with 2308 blebs increased the percentage of activated B cells (CD19+CD69+) in vitro. Therefore, membrane blebs are potential candidates for the development of an acellular vaccine against brucellosis, especially those derived from the rough strains so that serological diagnostic is not affected.

Immunization with recombinant GntR plasmid confers protection against Brucella challenge in BALB/c mice

It is essential to improve animal vaccine for brucellosis since conventional vaccines are residual virulent and poor protective effect, limit their applications. To solve these problems, the recombinant DNA vaccines were appeared, which could improve protective immunity and were attenuated to animals. In current research, the recombinant DNA vaccine (pVGntR) based on transcriptional regulator GntR of Brucella abortus (B. abortus) was constructed. The results show that pVGntR is significantly more protective than the conventional RB51 vaccine. Immunization with pVGntR increases the production of immunoglobulin G (IgG) and elicits elevated numbers of gamma interferon (IFN-γ) and interleukin-4 (IL-4). These results suggest that pVGntR is a highly efficacious vaccine candidate that confers protection against wild-type B. abortus challenge.

Progress in Brucella vaccine development

Brucella spp. are zoonotic, facultative intracellular pathogens, which cause animal and human disease. Animal disease results in abortion of fetuses; in humans, it manifests flu-like symptoms with an undulant fever, with osteoarthritis as a common complication of infection. Antibiotic regimens for human brucellosis patients may last several months and are not always completely effective. While there are no vaccines for humans, several licensed live Brucella vaccines are available for use in livestock. The performance of these animal vaccines is dependent upon the host species, dose, and route of immunization. Newly engineered live vaccines, lacking well-defined virulence factors, retain low residual virulence, are highly protective, and may someday replace currently used animal vaccines. These also have possible human applications. Moreover, due to their enhanced safety and efficacy in animal models, subunit vaccines for brucellosis show great promise for their application in livestock and humans. This review summarizes the progress of brucellosis vaccine development and presents an overview of candidate vaccines.

Expression and Validation of D-Erythrulose 1-Phosphate Dehydrogenase from Brucella abortus: A Diagnostic Reagent for Bovine Brucellosis

Brucella abortus is a bacterium of brucellosis causing abortion in cattle. The diagnosis of bovine brucellosis mainly relies on serologic tests using smooth lipopolysaccharide (S-LPS) from B. abortus. However, the usefulness of this method is limited by false-positive reactions due to cross-reaction with other Gram-negative bacteria. In the present study, the eryC gene encoding B. abortus D-erythrulose 1-phosphate dehydrogenase, which is involved in the erythritol metabolism in virulent B. abortus strain but is absent from a B. abortus vaccine strain (S19), was cloned. Recombinant EryC was expressed and purified for the evaluation as a diagnostic reagent for bovine brucellosis. Other B. abortus proteins, Omp16, PP26, and CP39 were also purified and their seroreactivities were compared. Recombinant EryC, Omp16, PP26, and PP39 were all reactive to B. abortus-positive serum. The specificity of recombinant Omp 16, PP26, CP39, and EryC, were shown to be approximately 98%, whereas that of B. abortus whole cell lysates was shown to be 95%. The sensitivity of Omp16, PP26, CP39, and EryC were 10%, 51%, 64%, and 43%, respectively, whereas that of B. abortus whole cell lysates was 53%. These results suggested that B. abortus EryC would be a potential reagent for diagnosis for bovine brucellosis as a single protein antigen.

Efficacy of strain RB51 vaccine in protecting infection and vertical transmission against Brucella abortus in Sprague-Dawley rats

Immunizing animals in the wild against Brucella (B.) abortus is essential to control bovine brucellosis because cattle can get the disease through close contact with infected wildlife. The aim of this experiment was to evaluate the effectiveness of the B. abortus strain RB51 vaccine in protecting infection as well as vertical transmission in Sprague-Dawley (SD) rats against B. abortus biotype 1. Virgin female SD rats (n = 48) two months of age were divided into two groups: one group (n = 24) received RB51 vaccine intraperitoneally with 3 × 10¹⁰ colony forming units (CFU) and the other group (n = 24) was used as non-vaccinated control. Non-vaccinated and RB51-vaccinated rats were challenged with 1.5 × 10⁹ CFU of virulent B. abortus biotype 1 six weeks after vaccination. Three weeks after challenge, all rats were bred. Verification of RB51-vaccine induced protection in SD rats was determined by bacteriological, serological and molecular screening of maternal and fetal tissues at necropsy. The RB51 vaccine elicited 81.25% protection in SD rats against infection with B. abortus biotype 1. Offspring from rats vaccinated with RB51 had a decreased (p < 0.05) prevalence of vertical transmission of B. abortus biotype 1 compared to the offspring from non-vaccinated rats (20.23% and 87.50%, respectively). This is the first report of RB51 vaccination efficacy against the vertical transmission of B. abortus in the SD rat model.

Protective properties of rifampin-resistant rough mutants of Brucella melitensis

Vaccination against Brucella infections in animals is usually performed by administration of live attenuated smooth B. abortus strain S19 and B. melitensis strain Rev1. They are proven effective vaccines against B. abortus in cattle and against B. melitensis and B. ovis in sheep and goats, respectively. However, both vaccines have the main drawback of inducing O-polysaccharide-specific antibodies that interfere with serologic diagnosis of disease. In addition, they retain residual virulence, being a cause of abortion in pregnant animals and infection in humans. To overcome these problems, one approach is to develop defined rough mutant Brucella strains lacking O antigen of lipopolysaccharide. B. abortus rough strain RB51, a rifampin-resistant mutant of virulent strain B. abortus 2308, is used as a vaccine against B. abortus infection in cattle in some countries. However, RB51 is not effective in sheep, and there is only preliminary evidence that it is effective in goats. In this study, we tested the efficacies of six rifampin-resistant rough strains of B. melitensis in protecting BALB/c mice exposed to B. melitensis infection. The protective properties, as well as both humoral and cellular immune responses, were assessed in comparison with those provided by B. melitensis Rev1 and B. abortus RB51 vaccines. The results indicated that these rough mutants were able to induce a very good level of protection against B. melitensis infection, similar to that provided by Rev1 and superior to that of RB51, without inducing antibodies to O antigen. In addition, all B. melitensis mutants were able to stimulate good production of gamma interferon. The characteristics of these strains encourage further evaluation of them as alternative vaccines to Rev1 in primary host species.

Genetic bases of the rifampin resistance phenotype in Brucella spp

Rifampin is one of the most potent and broad-spectrum antibiotics against bacterial pathogens. Its bactericidal activity is due to its ability to bind to the beta subunit of the DNA-dependent RNA polymerase encoded by the rpoB gene. Mutations of the rpoB gene have been characterized in rifampin-resistant (Rif(r)) strains of Escherichia coli and Mycobacterium tuberculosis. The genetic bases of Rif(r) in Brucella spp. are still unknown. In the present study, the nucleotide sequences of the rpoB gene of the Rif(r) vaccine strain Brucella abortus RB51 and of 20 Rif(r) clones derived in our laboratory from two Brucella melitensis isolates were determined. These sequences were then compared to those of the respective rifampin-susceptible (Rif(s)) parental strains and to the published B. melitensis strain 16M. All Rif(r) strains carried one or more missense mutations mapping in two regions of the rpoB gene. These two "hot" regions were investigated in eight additional Rif(r) Brucella laboratory mutants and in 20 reference Rif(s) Brucella strains. rpoB mutations were found in all Rif(r) mutants. In contrast, no missense mutations were found in any analyzed Rif(s) strains. Our results represent the first from a study of the molecular characterization of rpoB mutations in resistant Brucella strains and provide an additional proof of the association of specific rpoB mutations with the development of the Rif(r) phenotype in prokaryotes. In addition, because of the relationship between Rif(r) and the attenuation of virulence in Brucella spp., studies of virulence in these mutants may provide useful information about the genetic basis of pathogenesis in Brucella.

Protection of Brucella abortus RB51 revaccinated cows, introduced in a herd with active Brucellosis, with presence of atypical humoral response

It is a dogma, that RB51 vaccination does not induce antibodies that interfere with Brucellosis diagnosis, therefore any animal positive to serological test is considered as an infected animal. To determine protection against Brucellosis virulent field strain, 35 pregnant cows from a free-Brucellosis herd, previously vaccinated as calves with 1 x 10(10) CFU of RB51, were revaccinated with RB51 reduced dose, and then introduced into a herd with an active outbreak. Seventeen cows resulted positive in card test after revaccination. All 35 pregnant revaccinated cows had normal parturition; nevertheless, RB51 vaccine strain was isolated from milk and vaginal exudates from two cows after delivery at day 120 post-revaccination. At 150 days post-revaccination, two cows were positives to card and rivanol test and the field virulent strain was isolated. Revaccination with a reduced dose of RB51 in endemic zones did not cause abortion and protected 94% of animals against field infection, but caused an atypical response to conventional serological tests.

Development and evaluation as vaccines in mice of Brucella melitensis Rev.1 single and double deletion mutants of the bp26 and omp31 genes coding for antigens of diagnostic significance in ovine brucellosis

The live attenuated Brucella melitensis Rev.1 strain is considered the best vaccine available for the prophylaxis of brucellosis in sheep caused by either B. melitensis or Brucella ovis. However, its application stimulates antibody responses in vaccinated animals indistinguishable by the current conventional serological tests from those observed in infected animals. The periplasmic protein BP26 and the outer membrane protein (OMP) Omp31 are immunodominant antigens in the serological responses of B. melitensis and B. ovis infected sheep, respectively. Accordingly, vaccine strain Rev.1 single and double deletion mutants of the bp26 and omp31 genes were developed, based on the principle that the use of such mutants as vaccines in association with diagnostic tests based on BP26 and Omp31 antigens would allow the serological differentiation between infected and vaccinated animals. The deletion mutants obtained were indistinguishable from the parental Rev.1 strain by conventional bacteriological and typing tests. The expression of their major surface antigens, as determined by reactivity with specific monoclonal antibodies (MAbs), remained unaffected, i.e. smooth-lipopolysaccharide (S-LPS) and OMPs besides in the expression of the antigens whose respective genes were deleted. The bp26 and omp31 deletions did not modify the kinetics of splenic infection nor the residual virulence of Rev.1 in the BALB/c mouse model. Vaccination of BALB/c mice with the deletion mutants conferred significant protective immunity against B. melitensis strain H38 or B. ovis strain PA challenges, to the same extent as that induced by parental Rev.1 strain. Thus, these Rev.1 bp26 or omp31 deletion mutants are promising vaccine candidates against B. melitensis and B. ovis infections and will be further evaluated in sheep.

Brucella abortus rough mutants are cytopathic for macrophages in culture

Rough mutants of Brucella spp. are attenuated for survival in animal models. However, conflicting in vitro evidence has been obtained concerning the intracellular survival of rough mutants. Transposon-derived rough mutants isolated in our laboratory were previously shown to exhibit small but significant reductions in intracellular survival in a 12-h in vitro assay. Several recent publications report that rough mutants exhibited increased macrophage uptake relative to their smooth parental strains, and a reduction in numbers at the end of the assay has been interpreted as intracellular killing. In an effort to explore the role of O antigen in the interaction between Brucella abortus and macrophages, we have monitored the uptake of rough mutants and survival in vitro by using the murine macrophage cell line J774.A1. The results confirm a 10- to 20-fold-increased uptake of rough mutants over that of smooth organisms under standard conditions. Recovery of the rough mutants persisted up to 8 h postinfection, but at the point when intracellular replication of the smooth organisms was observed, the number of rough organisms recovered declined. Fluorescence microscopy revealed the intracellular multiplication of both smooth and rough organisms, and assays performed in the absence of antibiotic confirmed the replication of the rough organisms. Examination by phase-contrast microscopy revealed the lytic death of macrophages infected with the rough mutants, which was confirmed by the release of lactate dehydrogenase (LDH) from the cell cytoplasm. Thus, the decline in the number of rough organisms was the result of the lysis of macrophages and not from intracellular killing. The cytopathic effect is characterized as necrotic rather than apoptotic cell death based on early LDH release, annexin V and propidium iodide staining, morphological changes of infected cells and nuclei, and glycine protection. The cytopathic effect was observed with macrophages at multiplicities of infection (MOIs) of as low as 20 and was not observed with epithelial cells at MOIs of as high as 2000. These findings suggest a role for O antigen during the early stages of host-agent interaction that is essential in establishing an intracellular niche that maintains and supports persistent intracellular infection resulting in disease.

Combined S99/RB51 antigen for complement fixation test for serological diagnosis of brucellosis in cattle and sheep

AIMS

To assess the efficiency of a single antigen for the complement fixation (CF) test, prepared by combining Brucella abortus smooth strain 99 (S99) with Brucella abortus rough strain RB51(RB51), in detecting cattle and sheep infected or vaccinated with Brucella spp.

METHODS AND RESULTS

Serum samples from B. abortus-infected and RB51-vaccinated cattle were tested by the CF test using S99, RB51 and the combined S99/RB51 as antigens. Likewise, serum samples from Brucella melitensis-infected, RB51-vaccinated and Brucella ovis-infected sheep were tested by the CF test using S99, RB51, hot saline (HS) and combined S99/RB51 as antigens. Comparative analysis of the CF results showed that no reduction of sensitivity or specificity occurs when S99/RB51 antigen is used instead of specific antigens used separately.

CONCLUSIONS

The results of this study indicated that combined S99/RB51 antigen used in the CF test, because of its specificity and sensitivity, could be used in animal brucellosis surveillance systems to improve the efficiency of the preliminary screening of herds.

SIGNIFICANCE AND IMPACT OF THE STUDY

This study proposes an improved antigen for the CF test for the epidemiological survey of animal brucellosis. It could represent advantages over standard protocols because of its ability to detect antibody responses following infection or vaccination withBrucella strains of rough and smooth phenotype.

Comparison between immune responses and resistance induced in BALB/c mice vaccinated with RB51 and Rev. 1 vaccines and challenged with Brucella melitensis bv. 3

BALB/c mice were immunized with live rough Brucella abortus RB51 or smooth Brucella melitensis Rev. 1 vaccines and challenged with a B. melitensis field strain. Protection was assessed by a variety of serological tests and recovery of vaccinal and challenge strains by culture. Mice vaccinated with RB51 gave negative results in the conventional serological tests prior to challenge, namely; standard tube agglutination test (SAT), Rose Bengal plate test (RBPT), buffered acidified plate antigen test (BAPAT), and mercaptoethanol test (MET). Sero-conversion took place to a whole-cell bacterial buffered RB51 antigen after vaccination and persisted for 7 weeks post-vaccination. Mice challenged with B. melitensis were assessed for bacterial load and immune response for 12 weeks after challenge. Protection units were showed that Rev. 1 vaccine was superior to RB51 vaccine in protection of mice against B. melitensis. However, RB51 vaccine has the advantage that it would not elicit antibodies to standard serological tests based on the LPS O antigen. RB51 vaccine could therefore be used for control of B. melitensis infection and avoid confusion in the use of standard sero-diagnostic tests.

Brucella abortus vaccine strain RB51 produces low levels of M-like O-antigen

Brucella abortus RB51 is a rough (R) stable vaccine strain used in cattle and is believed to be devoid of O-side chain. We analyzed by use of a panel of monoclonal antibodies (MAbs) directed against seven previously defined O-polysaccharide (O-PS) epitopes the O-chain expression in strain RB51. Two MAbs specific for the C/Y (A=M) and C (M>A) epitopes showed low bindings in ELISA to strain RB51. O-chain expression was further confirmed by Western blot after SDS-PAGE of strain RB51. In particular, the MAb of C (M>A) specificity, showing preferential binding to M-dominant smooth (S) Brucella strains, revealed in strain RB51 a typical smooth-lipopolysaccharide (S-LPS) pattern which resembled that of M-dominant S-LPS. Thus, the results clearly show that strain RB51 produces low levels of M-like O-antigen.

Use of Brucella abortus vaccine strain RB51 in pregnant cows after calfhood vaccination with strain 19 in Argentina

One hundred and seven pregnant cows, which had been calfhood vaccinated with Brucella abortus strain 19 (S-19) were revaccinated with either S-19 or strain RB51 (S-RB51). All S-19-revaccinated animals seroconverted, while none of the RB51-revaccinated animals seroconverted. Two out of 25 (8%) S-19-revaccinated animals aborted, while none of the 57 RB51-revaccinated group aborted. Four of the S-19-revaccinated animals shed S-19 in the milk for at least 7 days, while only 1 cow shed S-RB51 for at least 3 days (but <7 days) post-parturition. Revaccination of strain 19 calfhood-vaccinated, pregnant cattle with S-RB51 appears to be a safe procedure with no diagnostically negative consequences.

Effect of vaccination with Brucella abortus strain RB51 on heifers and pregnant cattle

Thirteen cows, which had been vaccinated as calves with strain 19, were revaccinated twice or three times as adults with I x 109 cfu of B. abortus strain RB51. Their serological responses following adult vaccination remained negative to conventional brucellosis surveillance tests. Vaccination with strain RB51 during the eighth month of pregnancy did not induce abortion, although strain RB51 was recovered from milk for up to 69 days after vaccination. In a parallel study, thirteen 8- to 10-month-old heifers were vaccinated as calves with 109 cfu of strain RB51. The heifers remained seronegative to conventional brucellosis surveillance tests but antibody responses to RB51 could be demonstrated using an indirect ELISA. This study showed that multiple vaccination with strain RB51 did not induce seroconversion to brucellosis surveillance tests. In addition, this study suggests that 109 cfu of strain RB51 is safe for use in pregnant cattle.

Complement fixation test to assess humoral immunity in cattle and sheep vaccinated with Brucella abortus RB51

The live attenuated Brucella abortus strain RB51 is a rifampin-resistant, lipopolysaccharide (LPS) O-chain-deficient mutant of virulent B. abortus 2308. The reduced O-chain content in RB51 prevents this bacterium from inducing antibodies detectable by the conventional serologic tests for bovine brucellosis diagnosis that mainly identify antibodies to LPS. The absence of available serologic tests for RB51 also complicates the diagnosis of possible RB51 infections in humans exposed to this strain. The purpose of this study was to evaluate the suitability of a complement fixation (CF) test performed with the rough strain B. abortus RB51, previously deprived of anticomplementary activity, in detecting anti-B. abortus RB51 antibodies in cattle and sheep experimentally vaccinated with this strain. The results of this study showed that a CF test with RB51 as the antigen is able to specifically detect antibodies following RB51 vaccination in cattle and sheep. In addition, this method could be a useful tool for detecting B. abortus RB51 infection in humans.

Characterization of smooth lipopolysaccharides and O polysaccharides of Brucella species by competition binding assays with monoclonal antibodies

Previously, four epitope specificities on the O chain of Brucella species were reported: M, A, C, and C/Y. In this work, according to monoclonal antibody binding to smooth lipopolysaccharides of Yersinia enterocolitica 0:9, Brucella abortus W99 (A-dominant strain), and B. melitensis Rev1 (M-dominant strain), seven O-chain epitope specificities were defined: M, A, C (M > A), C (M = A), C/Y (M > A), C/Y (M = A) and C/Y (A > M). Competitive binding assays between these monoclonal antibodies suggested that these different epitopes are probably overlapping structures.

Immune responses and resistance to brucellosis in mice vaccinated orally with Brucella abortus RB51

Immune responses and resistance to infection with Brucella abortus 2308 (S2308) were measured in mice following oral or intraperitoneal (i.p.) vaccination with strain RB51 (SRB51). Bacteria persisted in the parotid lymph node for 4 weeks following oral vaccination of mice with 5 x 10(8) or 5 x 10(6) CFU of SRB51. Bacteria did not appear in the spleen during 12 weeks after oral vaccination, whereas they did appear in the spleen for 8 weeks following i.p. vaccination of mice with SRB51 (5 x 10(8) or 5 x 10(6) CFU). Increased resistance to S2308 infection occurred at 12 to 20 weeks in mice vaccinated i.p. with SRB51 (5 x 10(8) or 5 x 10(6) CFU) but occurred at 12 weeks only in mice vaccinated orally with SRB51 (5 x 10(8) CFU). Oral SRB51 vaccination induced lower levels of antibodies to the surface antigens of intact SRB51 bacteria than did i.p. vaccination. However, neither route of vaccination induced anamnestic antibody responses to the surface antigens of intact S2308 bacteria after challenge infection of the vaccinated mice with S2308. Mice vaccinated orally with SRB51 and challenged with S2308 at 12 to 20 weeks had lower and less persistent spleen cell proliferation and production of gamma interferon in response to S2308 and certain immunodominant S2308 proteins (32 to < or = 18 kDa) than did mice vaccinated i.p. with SRB51. However, mice vaccinated orally or i.p. with SRB51 and challenged with S2308 had similar spleen cell tumor necrosis factor alpha production. These results indicate that oral vaccination of mice with SRB51 was effective in inducing protective immunity to S2308 infection, although the immunity was lower and less persistent than that induced by i.p. vaccination. The lower protective immunity induced by oral vaccination may have resulted from lower and less persistent cell-mediated immunity and gamma interferon production in response to S2308 and S2308 proteins.