Aerosol infection of BALB/c mice with Brucella melitensis and Brucella abortus and protective efficacy against aerosol challenge

Brucellosis is a zoonotic disease with a worldwide distribution that can be transmitted via intentional or accidental aerosol exposure. In order to engineer superior vaccine strains against Brucella species for use in animals as well as in humans, the possibility of challenge infection via aerosol needs to be considered to properly evaluate vaccine efficacy. In this study, we assessed the use of an aerosol chamber to infect deep lung tissue of mice to elicit systemic infections with either Brucella abortus or B. melitensis at various doses. The results reveal that B. abortus causes a chronic infection of lung tissue in BALB/c mice and peripheral organs at low doses. In contrast, B. melitensis infection diminishes more rapidly, and higher infectious doses are required to obtain infection rates in animals similar to those of B. abortus. Whether this difference translates to severity of human infection remains to be elucidated. Despite these differences, unmarked deletion mutants BAΔasp24 and BMΔasp24 consistently confer superior protection to mice against homologous and heterologous aerosol challenge infection and should be considered viable candidates as vaccine strains against brucellosis.     

Brucella abortus RB51 induces protection in mice orally infected with the virulent strain B. abortus 2308

Brucellae are gram-negative, facultative intracellular bacteria which are one of the most common causes of abortion in animals. In addition, they are the source of a severe zoonosis. In this trial, we evaluated the effect of oral inoculation of Brucella abortus RB51 in mice against a challenge infection with B. abortus 2308. First, we showed that a gastric acid neutralization prior to the oral inoculation contributed to a more homogeneous and consistent infection with both vaccine strain B. abortus RB51 and virulent strain B. abortus 2308. Successively, we assessed the clearance and the immune response following an oral infection with B. abortus RB51. Oral inoculation gave a mild infection which was cleared 42 days after infection, and it induced a delayed humoral and cell-mediated immune response. Finally, we immunized mice by oral inoculation with B. abortus RB51, and we challenged them with the virulent strain B. abortus 2308 by an oral or intraperitoneal route 42 days after vaccination. Oral inoculation of B. abortus RB51 was able to give protection to mice infected with the virulent strain B. abortus 2308 by the oral route but not to mice infected intraperitoneally. Our results indicate that oral inoculation of mice with B. abortus RB51 is able to give a protective immunity against an oral infection with virulent strains, and this protection seems to rely on an immune response at the mucosal level.     

Oral vaccination with Brucella melitensis WR201 protects mice against intranasal challenge with virulent Brucella melitensis 16M

Human brucellosis can be acquired from infected animal tissues by ingestion, inhalation, or contamination of conjunctiva or traumatized skin by infected animal products. In addition, Brucella is recognized as a biowarfare threat agent. Although a vaccine to protect humans from natural or deliberate infection could be useful, vaccines presently used in animals are unsuitable for human use. We tested orally administered live, attenuated, purine auxotrophic B. melitensis WR201 bacteria for their ability to elicit cellular and humoral immune responses and to protect mice against intranasal challenge with B. melitensis 16M bacteria. Immunized mice made serum antibody to lipopolysaccharide and non-O-polysaccharide antigens. Splenocytes from immunized animals released interleukin-2 and gamma interferon when grown in cultures with Brucella antigens. Immunization led to protection from disseminated infection and enhanced clearance of the challenge inoculum from the lungs. Optimal protection required administration of live bacteria, was related to immunizing dose, and was enhanced by booster immunization. These results establish the usefulness of oral vaccination against respiratory challenge with virulent Brucella and suggest that WR201 should be further investigated as a vaccine to prevent human brucellosis.     

Brucella abortus deficient in copper/zinc superoxide dismutase is virulent in BALB/c mice.

The gene encoding the Cu/Zn superoxide dismutase (SOD) of Brucella abortus strain 2308 was identified in a Brucella genomic library utilizing a combination of Western blotting and native gel electrophoresis. The Cu/Zn SOD gene was inactivated in vitro by ligation of a kanamycin resistance gene into the open reading frame encoding SOD. The plasmid born construct was introduced back into B. abortus by electroporation. Replacement of the wild-type Cu/Zn SOD by recombination was demonstrated by showing that both the KnR gene and the Cu/Zn SOD gene hybridized to the same band in a Southern analysis of genomic DNA. In addition, KnR strains were deficient in Cu/Zn SOD activity as assessed by lack of Cu/Zn SOD activity on a native gel and by lack of reactivity with specific serum in a Western analysis. Either strain 2308 or the Cu/Zn SOD deficient mutant injected intraperitoneally into BALB/c mice, exhibited no differences in their ability to colonize the spleen at 7 and 28 days post-inoculation. Thus, the inability to produce Cu/Zn SOD by B. abortus does not significantly impair its virulence in mice.     

Mouse Cytokine Profiles Associated with Brucella abortus RB51 Vaccination or B. abortus 2308 Infection

This study indicated that mice immunized with Brucella abortus RB51 bacteria and subsequently challenged with B. abortus 2308 were protected from reinfection. After vaccination, both Th1 and Th2 cytokine patterns were observed. Of those, the early production of gamma interferon seems to have the prominent role in inducing an immunologically based protection.     

Brucella abortus RB51 and Hot Saline Extract from Brucella ovis as Antigens in a Complement Fixation Test Used To Detect Sheep Vaccinated with Brucella abortus RB51

The efficacy of Brucella abortus RB51 and hot saline extract (HSE) from Brucella ovis as antigens in complement fixation (CF) tests was comparatively evaluated in detecting immune responses of sheep vaccinated with B. abortus strain RB51. For this study, four 5-month-old sheep were vaccinated subcutaneously with 5 × 10⁹ CFU of RB51, and two sheep received saline. Serum samples collected at different times after vaccination were tested for the presence of antibodies to RB51 by a CF test with RB51 as antigen, previously deprived of anticomplementary activity, and with HSE antigen, which already used as the official antigen to detect B. ovis-infected sheep. The results showed that vaccinated sheep developed antibodies which reacted weakly against HSE antigen and these antibodies were detectable for 30 days after vaccination. However, antibodies to RB51 could be detected for a longer period after vaccination by using homologous RB51 antigen in CF tests. In fact, high titers were still present at 110 days postvaccination with RB51 antigen. Sera from sheep naturally infected with B. ovis also reacted to RB51 but gave lower titers than those detected by HSE antigen. As expected, all sera from RB51-vaccinated sheep remained negative when tested with standard S-type Brucella standard antigens.     

Brucella abortus RB51 and hot saline extract from Brucella ovis as antigens in a complement fixation test used To detect sheep vaccinated with Brucella abortus RB51

The efficacy of Brucella abortus RB51 and hot saline extract (HSE) from Brucella ovis as antigens in complement fixation (CF) tests was comparatively evaluated in detecting immune responses of sheep vaccinated with B. abortus strain RB51. For this study, four 5-month-old sheep were vaccinated subcutaneously with 5 x 10(9) CFU of RB51, and two sheep received saline. Serum samples collected at different times after vaccination were tested for the presence of antibodies to RB51 by a CF test with RB51 as antigen, previously deprived of anticomplementary activity, and with HSE antigen, which already used as the official antigen to detect B. ovis-infected sheep. The results showed that vaccinated sheep developed antibodies which reacted weakly against HSE antigen and these antibodies were detectable for 30 days after vaccination. However, antibodies to RB51 could be detected for a longer period after vaccination by using homologous RB51 antigen in CF tests. In fact, high titers were still present at 110 days postvaccination with RB51 antigen. Sera from sheep naturally infected with B. ovis also reacted to RB51 but gave lower titers than those detected by HSE antigen. As expected, all sera from RB51-vaccinated sheep remained negative when tested with standard S-type Brucella standard antigens.     

Immune Responses of Bison and Efficacy after Booster Vaccination with Brucella abortus Strain RB51

Thirty-one bison heifers were randomly assigned to receive saline or a single vaccination with 10¹⁰ CFU of Brucella abortus strain RB51. Some vaccinated bison were randomly selected for booster vaccination with RB51 at 11 months after the initial vaccination. Mean antibody responses to RB51 were greater (P < 0.05) in vaccinated bison after initial and booster vaccination than in nonvaccinated bison. The proliferative responses by peripheral blood mononuclear cells (PBMC) from the vaccinated bison were greater (P < 0.05) than those in the nonvaccinated bison at 16 and 24 weeks after the initial vaccination but not after the booster vaccination. The relative gene expression of gamma interferon (IFN-γ) was increased (P < 0.05) in the RB51-vaccinated bison at 8, 16, and 24 weeks after the initial vaccination and at 8 weeks after the booster vaccination. The vaccinated bison had greater (P < 0.05) in vitro production of IFN-γ at all sampling times, greater interleukin-1β (IL-1β) production in various samplings after the initial and booster vaccinations, and greater IL-6 production at one sampling time after the booster vaccination. Between 170 and 180 days of gestation, the bison were intraconjunctivally challenged with approximately 1 × 10⁷ CFU of B. abortus strain 2308. The incidences of abortion and infection were greater (P < 0.05) in the nonvaccinated bison after experimental challenge than in the bison receiving either vaccination treatment. Booster-vaccinated, but not single-vaccinated bison, had a reduced (P < 0.05) incidence of infection in fetal tissues and maternal tissues compared to that in the controls. Compared to the nonvaccinated bison, both vaccination treatments lowered the colonization (measured as the CFU/g of tissue) of Brucella organisms in all tissues, except in retropharyngeal and supramammary lymph nodes. Our study suggests that RB51 booster vaccination is an effective vaccination strategy for enhancing herd immunity against brucellosis in bison.     

Efficacy of Dart or Booster Vaccination with Strain RB51 in Protecting Bison against Experimental Brucella abortus Challenge

This study characterized the efficacy of the Brucella abortus strain RB51 vaccine in bison when delivered by single intramuscular vaccination (hand RB51), by single pneumatic dart delivery (dart RB51), or as two vaccinations approximately 13 months apart (booster RB51) in comparison to control bison. All bison were challenged intraconjunctivally in midgestation with 10⁷ CFU of B. abortus strain 2308 (S2308). Bison were necropsied and sampled within 72 h of abortion or delivery of a live calf. Compared to nonvaccinated bison, bison in the booster RB51 treatment had a reduced (P < 0.05) incidence of abortion, uterine infection, or infection in maternal tissues other than the mammary gland at necropsy. Bison in single-vaccination treatment groups (hand RB51 and dart RB51) did not differ (P > 0.05) from the control group in the incidence of abortion or recovery of S2308 from uterine, mammary, fetal, or maternal tissues at necropsy. Compared to nonvaccinated animals, all RB51 vaccination groups had reduced (P < 0.05) mean colonization or incidence of infection in at least 2 of 4 target tissues, with the booster RB51 group having reduced (P < 0.05) colonization and incidence of infection in all target tissues. Our data suggest that booster vaccination of bison with RB51 enhances protective immunity against Brucella challenge compared to single vaccination with RB51 by hand or by pneumatic dart. Our study also suggests that an initial vaccination of calves followed by booster vaccination as yearlings should be an effective strategy for brucellosis control in bison.     

Induction of specific cytotoxic lymphocytes in mice vaccinated with Brucella abortus RB51

A safe, more sensitive, nonradioactive, neutral red uptake assay was adopted to replace the traditional 51Cr release assay for detection of Brucella-specific cytotoxic T lymphocyte (CTL) activity. Our studies indicated that Brucella abortus strain RB51 vaccination of mice induced specific CTLs against both strain RB51- and strain 2308-infected J774.A1 macrophages but not against Listeria monocytogenes-infected J774.A1 cells. The antigen-specific cytotoxic activity was exerted by T lymphocytes but not by NK cells. CD3+ CD4+ T cells secreted the highest level of gamma interferon (IFN-gamma) and were able to exert a low but significant level of specific lysis of Brucella-infected macrophages. They also exerted a low level of nonspecific lysis of noninfected macrophages. In contrast, CD3+ CD8+ T cells secreted low levels of IFN-gamma but demonstrated high levels of specific lysis of Brucella-infected macrophages with no nonspecific lysis. These findings indicate that B. abortus strain RB51 vaccination of mice induces specific CTLs and suggest that CD3+ CD4+ and CD3+ CD8+ T cells play a synergistic role in the anti-Brucella activity.     

Evaluation of protection afforded by Brucella abortus and Brucella melitensis unmarked deletion mutants exhibiting different rates of clearance in BALB/c mice

Research for novel Brucella vaccines has focused upon the development of live vaccine strains, which have proven more efficacious than killed or subunit vaccines. In an effort to develop improved vaccines, signature-tagged mutant banks were screened to identify mutants attenuated for survival. Mutants selected from these screens exhibited various degrees of attenuation characterized by the rate of clearance, ranging from a failure to grow in macrophages after 24 h of infection to a failure to persist in the mouse model beyond 8 weeks. Ideal vaccine candidates should be safe to the host, while evoking protective immunity. In the present work, we constructed unmarked deletion mutants of three gene candidates, manBA, virB2, and asp24, in both Brucella abortus and Brucella melitensis. The Deltaasp24 mutants, which persist for extended periods in vivo, are superior to current vaccine strains and to other deletion strains tested in the mouse model against homologous challenge infection after 12, 16, and 20 weeks postvaccination. The Deltaasp24 mutants also display superior protection compared to DeltamanBA and DeltavirB2 mutants against heterologous challenge in mice. From this study, a direct association between protection against infection and cytokine response was not apparent between all vaccine groups and, therefore, correlates of protective immunity will need to be considered further. A distinct correlation between persistence of the vaccine strain and protection against infection was corroborated.     

Immune and pathologic responses in mice infected with Brucella abortus 19, RB51, or 2308.

Immune and pathologic responses were measured for 20 weeks after infection of mice with Brucella abortus 19, RB51, or 2308. Live bacteria and bacterial antigens of 19 and RB51 persisted in spleens for 10 and 4 weeks after infection, respectively, whereas 2308 bacteria and bacterial antigens persisted for at least 20 weeks. Small germinal centers and profound lymphoid depletion occurred in spleens of mice during the first 4 weeks of infection with strain 19 or 2308; however, mice infected with strain RB51 had much larger germinal centers but no lymphoid depletion. At 4 weeks, only spleen cells from RB51-infected mice proliferated when incubated with 2308 bacteria. Large germinal centers in the spleen and spleen cell proliferative responses to 2308 did not appear in strain 19-infected mice until 6 weeks or in strain 2308-infected mice until 10 weeks. Similar proliferative responses to 2308 occurred in mice infected with strain 19 or RB51 at 6 weeks and in mice infected with strain 19, RB51, or 2308 at 10 weeks. However, at 20 weeks, spleen cell proliferative responses to 2308 occurred in mice infected with strain 19 or 2308 but not in mice infected with strain RB51. Mice infected with strain RB51 had lower and less persistent antibody titers to 2308 than did mice infected with strain 19 or 2308. Collectively, these results indicate that RB51-infected mice have less persistent immune responses to 2308 than do mice infected with 19 or 2308. The shorter duration of the responses probably resulted because RB51 is considerably less pathogenic and is cleared more rapidly from mice than are 19 and 2308.     

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.     

Comparison of living and nonliving vaccines for Brucella abortus in BALB/c mice.

The BALB/c mouse was selected as a model for infection with Brucella abortus on the basis of protracted nonclinical infection produced by strain 2308, virulent for cattle, and relatively rapid clearance of strain 19, an attenuated strain used to vaccinate cattle. Protection in mice vaccinated with strain 19 was compared with that obtained with nonliving vaccines at early (1 week) and later (4 weeks) intervals after challenge with strain 2308 and assessed by enumeration of B. abortus organisms in the spleen. Mice challenged 4 weeks after vaccination with strain 19 exhibited significant protection at 1 and 4 weeks postinfection (p.i.), with an increased magnitude of protection at the later time. When challenged 6 weeks after vaccination with strain 19, the level of protection diminished between 1 and 4 weeks p.i. and at the later time was not always significantly different from controls. Mice immunized 4 weeks earlier with nonliving vaccines in mineral oil with t trehalose dimycolate (TDM) and muramyl dipeptide (MDP) demonstrated patterns of protection similar to those obtained following the 6 week vaccination-challenge interval with strain 19. Vaccination with cell envelopes derived from strain 2308 produced equivalent protection at 1 week p.i. whether administered in phosphate-buffered saline, incomplete Freund adjuvant, or the TDM and MDP adjuvant. Equivalent protection also followed vaccination with strain 2308 killed whole cells, cell envelopes, or outer membrane proteins in phosphate-buffered saline or in the TDM and MDP adjuvant. The TDM and MDP adjuvant alone induced nonspecific resistance, which peaked at 1 day p.i. and was still present at 1 week p.i., although by this time its magnitude was significantly less than the protection induced by antigen combined with the adjuvant. These data, together with the results of antibody assays and passive and adoptive transfer studies, suggested that protection at 1 week p.i. could be accounted for largely by an effect of O antibodies, with T cell-mediated immune responses having a subsidiary role.     

Immune Responses and Safety after Dart or Booster Vaccination of Bison with Brucella abortus Strain RB51

One alternative for management of brucellosis in Yellowstone National Park bison (Bison bison) is vaccination of calves and yearlings. Although Brucella abortus strain RB51 vaccination protects bison against experimental challenge, the effect of booster vaccinations was unknown. This study characterized immunologic responses after dart or booster vaccination of bison with Brucella abortus strain RB51. In two studies, 8- to 10-month-old female bison were inoculated with saline (n = 14), hand vaccinated with 1.1 × 10¹⁰ to 2.0 × 10¹⁰ CFU of RB51 (n = 21), or dart vaccinated with 1.8 × 10¹⁰ CFU of RB51 (n = 7). A subgroup of hand vaccinates in study 1 was randomly selected for booster vaccination 15 months later with 2.2 × 10¹⁰ CFU of RB51. Compared to single vaccinates, booster-vaccinated bison had greater serologic responses to RB51. However, there was a trend for antigen-specific proliferative responses of peripheral blood mononuclear cells (PBMC) from booster vaccinates to be reduced compared to responses of PBMC from single vaccinates. PBMC from booster vaccinates tended to have greater gamma interferon (IFN-γ) production than those from single vaccinates. In general, dart vaccination with RB51 induced immunologic responses similar to those of hand vaccination. All vaccinates (single hand, dart, or booster) demonstrated greater (P < 0.05) immunologic responses at various times after vaccination than nonvaccinated bison. Booster vaccination with RB51 in early gestation did not induce abortion or fetal infection. Our data suggest that booster vaccination does not induce strong anamnestic responses. However, phenotypic data on resistance to experimental challenge are required to fully assess the effect of booster vaccination on protective immunity.     

Lymphocyte proliferation in response to Brucella abortus 2308 or RB51 antigens in mice infected with strain 2308, RB51, or 19.

Lymphocyte proliferation to 22 protein fractions (106 to 18 kDa) of Brucella abortus 2308 or the lipopolysaccharide O-antigen-deficient mutant of 2308, strain RB51, was measured for 20 weeks after infection of mice with strain 2308, RB51, or 19. Throughout the 20-week study, the 22 protein fractions of 2308 and RB51 induced a similar pattern of proliferation when they were incubated with lymphocytes from the infected mice. In addition, during the 20 weeks, lymphocytes from all groups of infected mice exhibited the highest proliferation when the lymphocytes were incubated with 18-kDa or smaller proteins from either 2308 or RB51. Lymphocytes obtained from mice at 6 weeks after infection with strain RB51 or 19 exhibited similar proliferation to the 18-kDa proteins of S2308 or SRB51. Lymphocytes from strain 2308-infected mice did not proliferate to these proteins until 10 weeks after infection, and the responses were similar to those in strain RB51-infected mice but lower than those in strain 19-infected mice. Lymphocytes obtained from mice at 20 weeks after infection with strain 19 or 2308 proliferated to most of the 22 fractions of 2308 or RB51, which contained 106- to 18-kDa proteins. However, lymphocytes obtained from strain RB51-infected mice at 20 weeks did not proliferate to any of these fractions. These results indicate that mice infected with RB51 have less-persistent lymphocyte proliferative responses to 2308 proteins than do mice infected with 2308 or 19. In addition, all 2308 proteins that stimulate lymphocyte proliferation appear to be present in RB51.     

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

Cattle vaccinated with Brucella abortus rough strain RB51 (SRB51) produced small amounts of serum immunoglobulin G (IgG) but no IgM antibody to smooth strain 2308 (S2308) bacteria and produced no IgG or IgM antibody to S2308 lipopolysaccharide (LPS). Western immunoblot analysis revealed that antiserum from SRB51-vaccinated cattle contained IgG antibody that reacted with S2308 proteins of 84 to <20 kDa. However, antiserum from the vaccinated cattle did not contain agglutinating B. abortus antibody in the tube agglutination test for brucellosis. These results suggest that SRB51-vaccinated cattle produced no antibody to S2308 LPS, although they did produce nonagglutinating IgG antibody that reacted with S2308 bacteria and bacterial proteins of 84 to <20 kDa.     

Enhancement of the Brucella AMOS PCR assay for differentiation of Brucella abortus vaccine strains S19 and RB51.

Because the brucellosis eradication program uses slaughter and quarantine as control measures, it would benefit from faster methods of bacterial identification. Distinguishing vaccine strains from strains that cause infections among vaccinated herds in the field is essential. To accomplish this, our PCR-based, species-specific assay (B. J. Bricker and S. M. Halling, J. Clin. Microbiol. 32:2660-2666, 1994) was updated to identify Brucella abortus vaccine strains S19 and RB51. Three new oligonucleotide primers were added to the five-primer multiplex Brucella AMOS PCR assay. Identification is based on the number and sizes of six products amplified by PCR.     

Variation of Brucella abortus 2308 infection in BALB/c mice induced by prior vaccination with salt-extractable periplasmic proteins from Brucella abortus 19.

The study compared the immune and protective responses induced in BALB/c mice vaccinated with six salt-extractable periplasmic protein fractions (Brucella cell surface proteins [BCSP]) of Brucella abortus 19 and later challenge exposed with B. abortus 2308. BCSP70 was precipitated with ammonium sulfate at 70% saturation, and BCSP100 was precipitated with ammonium sulfate at 100% saturation by use of supernatant fluid of BCSP70 that had been precipitated with 70% ammonium sulfate. Four subfractions were separated from BCSP100 by anion-exchange high-performance liquid chromatography (HPLC). Monophosphoryl lipid A (MPL) from Salmonella typhimurium Re mutant strain was used as a potential immune response modifier in some vaccines. Reduced or increased numbers of CFU and increased spleen size in the principal groups of mice relative to that of the nonvaccinated control group were considered protectiveness or virulence (survival) criteria. Results indicated that vaccines prepared from BCSP70 and BCSP100 were moderately protective and immunogenic. The subfractions designated BCSP100-A through BCSP100-D purified by anion-exchange HPLC were not protective when MPL was not used as an immune response modifier. However, two subfractions were associated with significant (P < 0.05) increases in CFU per spleen and splenomegaly in vaccinated mice compared with those in nonvaccinated challenge-exposed mice. MPL enhanced protection or was neutral when used with BCSP70, BCSP100, BCSP100-C, and BCSP100-D. Serologic results of an enzyme-linked immunosorbent assay indicated that MPL modulated the immunoglobulin G responses induced by BCSP70, BCSP100, and subfraction BCSP100-B vaccines only. The overall results suggest that certain proteinaceous periplasmic fractions might serve as virulence or survival factors in B. abortus infections.     

Antibody-mediated protection against Brucella abortus in BALB/c mice at successive periods after infection: variation between virulent strain 2308 and attenuated vaccine strain 19.

In BALB/c mice antibodies specific for the O polysaccharide (OPS) as well as T lymphocytes mediate protective immunity to Brucella abortus. We performed quantitative analyses of isotypes of OPS antibodies generated during primary infections, and tested the protective qualities of antisera at successive stages of infection against B. abortus strain 2308, representative of the wild type, and attenuated vaccine strain 19. IgM antibodies predominated during the first 3-4 weeks of infection. IgG3 antibodies increased slowly for the first 3 weeks but then rose rapidly and persisted at high levels (> 300 micrograms/ml). IgG1, IgG2a and IgG2b antibodies had increased slightly by week 4 and then remained at low to moderate levels (< 70 micrograms/ml). Week 2 serum pools (IgM high, IgG3 low or undetectable) transferred substantial protection against 2308 (> or = 1 log unit) which increased relatively little (to 1.2-1.5 log units) with later sera that were high in IgG antibodies. In contrast, week 2 sera conferred low levels of protection against 19 (< 0.6 log units), but protection was dramatically increased (to > or = 2.3 log units) with sera obtained 1 week later that had slightly increased IgG antibodies. Monoclonal IgM antibodies also provided better protection against 2308 than 19, while monoclonal IgG3 antibodies protected much better against 19. Strain 19 opsonized with antibodies taken at any stage of infection was killed within normal macrophages, whereas comparably opsonized 2308 underwent intracellular replication. Phagocytosis of 2308 was better than of 19 when brucellae were opsonized with either polyclonal IgM or IgG3 antibodies, and the difference between strains was more extreme following IgM opsonization. The data suggest an explanation for differences in the growth curves of 2308 and 19 in spleens of BALB/c mice. Higher numbers achieved by 19 at week 2 could result from extracellular replication owing to ineffectual opsonization by IgM antibodies, while the precipitous decline of 19 beginning at week 3 could be caused by the increase in more effective IgG3 opsonins that facilitate its rapid intracellular destruction.