CD8+ type 1 CD44hi CD45 RBlo T lymphocytes control intracellular Brucella abortus infection as demonstrated in major histocompatibility complex class I- and class II-deficient mice

T cell immunity to brucellosis

Brucellosis is an ancient disease of animals and man that still threatens the health and prosperity of many, primarily in the third world, who depend on animal agriculture for their livelihood. Further, its pathogenicity and the facts that it is zoonotic is effectively eradicated from many Western nations make it a dangerous bioterrorism threat. Targeted human vaccination may reduce the various threats brucellosis poses. Significant effort has been expended toward this goal and many candidate vaccines exist. However, the ideal vaccine would be a subunit vaccine that specifically targets only the critical aspects of the immune response necessary to induce immunity. Much about the immune response, in particular the T cell response, remains to be discovered in order to accomplish that goal. In this review we focus on T cell responses to brucellosis with particular attention to the specific roles of T cell subtypes. We also point out areas of research on T cell responses that may allow exploitation of cutting edge vaccine technologies for the next generation vaccine for brucellosis.

Role of Toll-like receptor 4 in induction of cell-mediated immunity and resistance to Brucella abortus infection in mice

Initial host defense to bacterial infection is executed by innate immunity, and therefore the main goal of this study was to examine the contribution of Toll-like receptors (TLRs) during Brucella abortus infection. CHO reporter cell lines transfected with CD14 and TLRs showed that B. abortus triggers both TLR2 and TLR4. In contrast, lipopolysaccharide (LPS) and lipid A derived from Brucella rough (R) and smooth (S) strains activate CHO cells only through TLR4. Consistently, macrophages from C3H/HePas mice exposed to R and S strains and their LPS produced higher levels of tumor necrosis factor alpha (TNF-alpha) and interleukin-12 compared to C3H/HeJ, a TLR4 mutant mouse. The essential role of TLR4 for induction of proinflammatory cytokines was confirmed with diphosphoryl lipid A from Rhodobacter sphaeroides. Furthermore, to determine the contribution of TLR2 and TLR4 in bacterial clearance, numbers of Brucella were monitored in the spleen of C3H/HeJ, C3H/HePas, TLR2 knockout, and wild-type mice at 1, 3, and 6 weeks following B. abortus infection. Interestingly, murine brucellosis was markedly exacerbated at weeks 3 and 6 after infection in animals that lacked functional TLR4 (C3H/HeJ) compared to C3H/HePas that paralleled the reduced gamma interferon production by this mouse strain. Finally, by mass spectrometry analysis we found dramatic differences on the lipid A profiles of R and S strains. In fact, S lipid A was shown to be more active to trigger TLR4 than R lipid A in CHO cells and more effective in inducing dendritic cell maturation. In conclusion, these results indicate that TLR4 plays a role in resistance to B. abortus infection and that S lipid A has potent adjuvant activity.

Optimization of functional efficacy of phosphorothioate-modified oligonucleotides in a human CD8+ T-cell ex vivo expansion model

Antisense oligodeoxyribonucleotides (ODNs) can specifically inhibit gene expression, but their application to fresh human CD8+ T cells is limited by poor spontaneous uptake (<2%). We have examined and optimized the uptake of phosphorothioate-modified oligodeoxyribonucleotides (PS-ODNs) into these cells in an ex vivo expansion model. Optimal antisense treatments were found to be, for fresh CD8+ T cells, 1 micro m PS-ODNs complexed with lipofectin (LF), which resulted in 35% uptake and 10 micro m PS-ODNs in the absence of LF, for cultured cells, which resulted in 95% uptake. The delivered antisenses were functional, as determined by the inhibition of protein expression. In this respect, partially phosphorothioate-modified ODNs (PS-ODNs-P) were twice as effective as completely modified (PS-ODNs-C), and the antisense specific for the cap site showed the highest protein suppression of those tested (68%). Uptake mechanisms were also investigated. To our knowledge, this is the first optimization of the delivery of antisense oligonucleotides into human CD8+ T cells. This protocol could be used to study the function of a particular gene in cytotoxic T lymphocytes and also by those looking for a method to deliver short interfering RNA into cell lines to specifically suppress a gene of interest.

A DNA vaccine encoding Cu,Zn superoxide dismutase of Brucella abortus induces protective immunity in BALB/c mice

This study was conducted to evaluate the immunogenicity and protective efficacy of a DNA vaccine encoding Brucella abortus Cu,Zn superoxide dismutase (SOD). Intramuscular injection of plasmid DNA carrying the SOD gene (pcDNA-SOD) into BALB/c mice elicited both humoral and cellular immune responses. Animals injected with pcDNA-SOD developed SOD-specific antibodies which exhibited a dominance of immunoglobulin G2a (IgG2a) over IgG1. In addition, the DNA vaccine elicited a T-cell-proliferative response and also induced the production of gamma interferon, but not interleukin-10 (IL-10) or IL-4, upon restimulation with either recombinant SOD or crude Brucella protein, suggesting the induction of a typical T-helper-1-dominated immune response in mice. The pcDNA-SOD (but not the control vector) induced a strong, significant level of protection in BALB/c mice against challenge with B. abortus virulent strain 2308; the level of protection was similar to the one induced by B. abortus vaccine strain RB51. Altogether, these data suggest that pcDNA-SOD is a good candidate for use in future studies of vaccination against brucellosis.

Phagosomes are competent organelles for antigen cross-presentation

The ability to process microbial antigens and present them at the surface of cells is an important aspect of our innate ability to clear infections. It is generally accepted that antigens in the cytoplasm are loaded in the endoplasmic reticulum and presented at the cell surface on major histocompatibility complex (MHC) class I molecules, whereas peptides present in endo/phagocytic compartments are presented on MHC class II molecules. Despite the apparent segregation of the class I and class II pathways, antigens from intracellular pathogens including mycobacteria, Escherichia coli, Salmonella typhimurium, Brucella abortus and Leishmania, have been shown to elicit an MHC class-I-dependent CD8+ T-cell response, a process referred to as cross-presentation. The cellular mechanisms allowing the cross-presentation pathway are poorly understood. Here we show that phagosomes display the elements and properties needed to be self-sufficient for the cross-presentation of exogenous antigens, a newly ascribed function linked to phagocytosis mediated by the endoplasmic reticulum.

Neurobrucellosis presenting as leukoencephalopathy: the role of cytotoxic T lymphocytes

A 65-year-old man developed a leukoencephalopathy associated with neurobrucellosis. The disease followed a 15-month progressive course with neurologic symptoms, and magnetic resonance imaging revealed bilateral symmetrical T2 signal hyperintensities in the white matter. Biopsy of the cerebral cortex and white matter was significant for nongranulomatous meningoencephalitis with reactive microgliosis and astrogliosis. The inflammatory infiltrate was predominantly composed of T lymphocytes, including numerous cytotoxic T cells. There was no evidence of significant myelin destruction. No organisms were detected microscopically, but elevated immunoglobulin G titers to Brucella were found in the cerebrospinal fluid. An abscess formed at the biopsy site, and Brucella melitensis was cultured from abscess contents. Neurobrucellosis is difficult to diagnose outside endemic regions and is associated with leukoencephalopathy-like pathology. Cytotoxic T lymphocytes and microglia activation play an immunopathogenic role in this rare disease.

Activation and Proliferation of T Lymphocyte Subpopulations in Patients with Brucellosis

BACKGROUND

T-cell proliferation is a standard method to evaluate cellular immune responses against intracellular infectious agents. The present study was undertaken to look for expression of an early activation marker (CD69) and proliferation using a nonradioactive method to evaluate cellular immune response against a salt-extractable antigen from Brucella melitensis 16M (RCM-BM) in patients suffering from brucellosis.

METHODS

Expression of CD69 on membrane of CD4+ and CD8+ T-cells was determined by flow cytometry. Lymphoproliferation was determined by tritiated thymidine and 5-bromo-2'-deoxyuridine (BrdU) incorporation using liquid scintillation counter or flow cytometry, respectively, to evaluate DNA synthesis.

RESULTS

Thirty healthy donors and 24 patients suffering from brucellosis were included in this study. In all cases, incubation with mitogen induced expression of CD69 and proliferation of both CD4+ and CD8+ T-cells. In contrast, only brucellosis patients responded with expression of CD69 and proliferation against RCM-BM antigen from Brucella melitensis (p < 0.001).

CONCLUSIONS

Methods used in this study were useful to evaluate immune response against specific antigen or polyclonal stimulation. CD4+ and CD8+ T cells from patients became equally activated and proliferated in response to RCM-BM antigen. Our data suggest that both T-cell subpopulations play an important role in immune response against Brucella.

Delayed type hypersensitivity-associated disruption of splenic periarteriolar lymphatic sheaths coincides with temporary loss of IFN-gamma production and impaired eradication of bacteria in Brucella abortus-infected mice

A major problem of infections with facultative intracellular bacteria is their chronic course. We comprehensively evaluated the host response in murine brucellosis to study mechanisms contributing to bacterial persistence in the presence of an established immune response. Evidence is presented that the decrease in eradication kinetics, reproducibly occurring 18 d after infection of mice with Brucella abortus S19, is related to a state of downregulation of defense mechanisms. This is not due to a Th1 to Th2 switch or prostaglandin-mediated suppression by macrophages but is most probably caused by a severe disruption of spleen morphology at the height of Brucella-induced delayed type hypersensitivity. This results in a profound depletion of both CD4(+) and CD8(+) T cells in periarteriolar lymphatic sheaths, a consecutive deleterious shift in the relation of permissive macrophages and protective lymphocytes and an impaired capacity of splenocytes to produce IFN-gamma in response to soluble Brucella antigen.

Molecular host-pathogen interaction in brucellosis: current understanding and future approaches to vaccine development for mice and humans

Brucellosis caused by Brucella spp. is a major zoonotic disease. Control of brucellosis in agricultural animals is a prerequisite for the prevention of this disease in human beings. Recently, Brucella melitensis was declared by the Centers for Disease Control and Prevention to be one of three major bioterrorist agents due to the expense required for the treatment of human brucellosis patients. Also, the economic agricultural loss due to bovine brucellosis emphasizes the financial impact of brucellosis in society. Thus, vaccination might efficiently solve this disease. Currently, B. abortus RB51 and B. melitensis REV.1 are used to immunize cattle and to immunize goats and sheep, respectively, in many countries. However, these genetically undefined strains still induce abortion and persistent infection, raising questions of safety and efficiency. In fact, the REV.1 vaccine is quite virulent and apparently unstable, creating the need for improved vaccines for B. melitensis. In addition, Brucella spp. may or may not provide cross-protection against infection by heterologous Brucella species, hampering the acceleration of vaccine development. This review provides our current understanding of Brucella pathogenesis and host immunity for the development of genetically defined efficient vaccine strains. Additionally, conditions required for an effective Brucella vaccine strain as well as the future research direction needed to investigate Brucella pathogenesis and host immunity are postulated.

Bovine T lymphocyte responses to Brucella abortus

The long-held paradigm of T lymphocyte-mediated activation of mononuclear phagocytes (Mø) as the major mechanism of protection against facultative intracellular pathogens such as Brucella has been modified to include killing of infected Mø by various subsets of T lymphocytes. Remnants of killed infected cells are phagocytosed by immunologically-activated Mø, which are much more efficient at killing such pathogens. Most of the detailed information regarding immunity in general and that of brucellosis specifically has been obtained using murine infection models rather than in cattle. However, there has been considerable definition of cellular phenotypes, cytokines and functional characteristics of T lymphocytes in cattle over the last decade. This was mainly due to development of monoclonal antibodies against cell surface markers and application of molecular cloning and polymerase chain reaction (PCR) for isolation, characterization and detection of genes encoding bovine cytokines. This review discusses cellular and molecular immunity in bovine brucellosis as pertains to T lymphocyte interactions with the Mø. Although current knowledge directly obtained from brucellosis immunity studies in the bovine host is limited and incomplete, the many parallels between the bovine and murine immune systems allow for some extrapolation in the description of bovine host defense mechanisms. Direct information from studies with immunized cattle supports the concepts of coordinate activation of uninfected Mø and killing of Brucella-infected Mø by antigen-specific T lymphocytes as major mechanisms of host defense in bovine brucellosis. There also appears to be a bias in the T lymphocyte compartment towards recognition of particular bacterial stress proteins following immunization with live Brucella vaccines.

Fundamentals of host immune response against Brucella abortus: what the mouse model has revealed about control of infection

The studies reviewed here evaluated the role cellular immune system components play in control of brucellosis by conducting comparative studies with brucella-resistant C57BL/10 or C57BL/6 mice and susceptible BALB/c mice. We have shown by both in vitro and in vivo studies that activation of macrophages with interferon-gamma (IFN-gamma) is an important factor for control of infection with B. abortus in the mouse model and that the mechanism of anti-brucella activity largely involved reactive oxygen intermediates. Differences in control of the organism by resistant and susceptible mice was not related to inherent differences in the ability of their macrophages to control infection either with or without IFN-gamma activation nor was it attributable to NK cells since we found no role for them in control of brucellosis in either mouse strain. However, relative resistance to brucellosis did correlate with increased production of IFN-gamma by CD4 T cells during the first weeks after infection while IL-10 contributed to susceptibility in BALB/c mice. Moreover, by 3 weeks post-infection splenocytes from the susceptible BALB/c mice failed to produce IFN-gamma and relied on TNF-alpha as well as CD8 T cells to control infection until the end of the plateau phase around 6 weeks post-infection when IFN-gamma production resumed and clearance began. In contrast, IFN-gamma was crucial for control throughout the infection in the more resistant C57BL/6 mice and the mice died in its absence by 6 weeks post-infection compared to 12 weeks for the more susceptible mice that relied on additional mechanisms of control. In contrast to the IFN-gamma knock-out mice, both beta2 microglobulin knock-out C57BL/6 mice, which do not express conventional MHC class I molecules and thus cannot present antigen to CD8 T cells, or perforin knock-out C57BL/6 mice, which have no T cell cytotoxic activity, controlled and cleared the infection as well as normal C57BL/6 mice. The hiatus of IFN-gamma production in BALB/c mice correlated with very high levels of total IL-12 and it was postulated that the lack of IFN-gamma was a consequence of p40 homodimer blocking activity. However, reduction of p40 IL-12 in vivo through administration of indomethacin reduced the infection without a concomitant measurable increase in IFN-gamma. Current studies are aimed at elucidating the mechanism of the IFN-gamma hiatus.

Molecular and cellular interactions between Brucella abortus antigens and host immune responses

Host protection against Brucella abortus, is thought to be mediated primarily by a Th1 type immune response. Unfortunately, only few specific bacterial antigens involved in stimulating protective cellular immunity against Brucella are known. Therefore, identifying bacterial proteins that induce a T-lymphocyte mediated response is critical to determine Brucella immunity. Several library screening methods are discussed that have been used to identify Brucella proteins that stimulate T lymphocytes including cellular immunoblotting, Escherichia coli expressed Brucella proteins, green fluorescence reporter systems, and signature tagged mutagenesis. Future studies would likely examine how bacterial proteins expressed within host cells aid pathogen survival and/or induce host responses. Some of these newly identified bacterial gene products may serve as antigens to activate a protective host immune response. Also, identifying Brucella proteins expressed at particular times during infection will also yield insights into Brucella pathogenesis.

Major histocompatibility complex class I and II expression on macrophages containing a virulent strain of Brucella abortus measured using green fluorescent protein-expressing brucellae and flow cytometry

Immune responses appropriate for control of an intracellular pathogen are generated in mice infected with Brucella abortus, shown by the ability of T cells to adoptively transfer resistance to naive mice. The infection nevertheless persists for months. It was hypothesized that one factor in maintaining the infection despite the presence of immune T cells was suboptimal expression of major histocompatibility complex (MHC) molecules on macrophages containing brucellae. This would allow B. abortus to elude detection by the host's immune system. To test this, B. abortus organisms expressing green fluorescent protein (GFP-Brucella) were constructed and three-color flow cytometry used to evaluate MHC expression on macrophages following in vitro or in vivo infection. When infected in vitro, the levels of MHC class I and class II expression on J774 macrophages containing GFP-Brucella were the same or higher than on macrophages without GFP-Brucella in the same cultures. Similarly, the MHC expression was higher on GFP(+) peritoneal exudate cells following infection or phagocytosis of heat-killed GFP-Brucella than it was on uninfected peritoneal exudate cells. Following in vivo infection of mice the level of MHC class I and II expression on GFP(+) cells in their spleens (the main site of infection) also tended to be as high as or higher than that on the GFP-negative cells. The only in vivo GFP(+) cells that showed a decreased MHC expression was a population of splenic Mac1(+) cells recovered from interferon-gamma gene-disrupted mice at the time of their death due to an overwhelming number of bacteria per spleen. Overall, it was concluded that decreased MHC expression is not a general principle associated with brucella infection of macrophages and thus not likely to contribute to maintenance of the chronic infection.

Antigen-specific activation and proliferation of CD4+ and CD8+ T lymphocytes from brucellosis patients

Salt-extractable antigen from Brucella melitensis 16M (RCM-BM) was used to evaluate the immune response from acute and chronic patients suffering from Brucella infections (in Mexico); their responses were compared with those of healthy controls. As a readout we used upregulation of CD69 (a well-established early activation marker for lymphocytes), lymphocyte proliferation by 3[H]thymidine or 5-bromo-2-deoxyuridine (BrdU) incorporation measured by liquid scintillation or flow cytometry, respectively, and production of gamma interferon (IFN gamma). We compared the antigen-specific response with the response induced by phytohaemagglutinin (PHA) as a positive control. There was no difference between acute patients and the healthy controls in the percentages of CD3+, CD4+ or CD8+ lymphocytes. However, we found that chronic patients had a significant (P < 0.05) increase in the CD8+ T cells, in line with previous studies. Antigen-specific responses to RCM-BM showed a significant (P < 0.05) upregulation of CD69 in both CD4+ and CD8+ T lymphocytes in acute brucellosis patients and in CD8+ T lymphocytes in chronic patients, indicating that both populations became activated by this antigen preparation. Moreover, lymphocyte proliferation from both acute and chronic patients in response to RCM-BM was highly significant (P < 0.001) when compared with healthy controls. However, there were no apparent differences between acute and chronic patients. Although the incorporation of BrdU showed similar results it provided additional information, since we demonstrated that both CD4+ and CD8+ T lymphocytes from acute and chronic patients proliferated equally well in response to RCM-BM. Similar results were observed with intracellular IFN gamma determination. As a whole, our data suggest an important role for both CD4+ and CD8+ T lymphocytes in Brucella infection in humans. As has been reported in mice, it is feasible that activated CD8+ T cells participate in protection against Brucella in humans through cytotoxicity or/and by the production of factors such as interferon and granulysin. The role of these cells should be carefully analysed to understand better their participation in human infection by Brucella.

Yersinia enterocolitica as a vehicle for a naked DNA vaccine encoding Brucella abortus bacterioferritin or P39 antigen

Brucella is a facultative intracellular parasite that causes brucellosis in animals and humans. The protective immune response against Brucella involves both humoral and cell-mediated immunity. In previous studies, we demonstrated that the T-dominant Brucella antigens bacterioferritin (BFR) and P39 administered either as CpG adjuvant recombinant proteins or as naked-DNA plasmids induced a specific Th1-biased immune response in mice. In order to improve the protection conferred by the BFR and P39 vaccines and to evaluate the additive role of antilipopolysaccharide (anti-LPS) antibodies, we used live attenuated Yersinia enterocolitica serotypes O:3 and O:9 as delivery vectors for naked-DNA plasmids encoding these BFR and P39 antigens. Following two intragastric immunizations in BALB/c mice, the Yersinia vectors harboring a DNA vaccine encoding BFR or P39 induced antigen-specific serum immunoglobulin and Th1-type responses (both lymphocyte proliferation and gamma interferon production) among splenocytes. Moreover, as expected, antibodies recognizing Brucella abortus 544 lipopolysaccharide were detected in O:9-immunized mice but not in O:3-treated animals. Animals immunized with O:9 organisms carrying pCI or with O:9 organisms alone were found to be significantly resistant to infection by B. abortus 544. Our data demonstrated that pCI plasmids encoding BFR or P39 and delivered with live attenuated strains of Yersinia O:3 or O:9 can trigger Th1-type responses. The fact than only O:9 vectors induced a highly significant protective immunity against B. abortus 544 infection pointed out the crucial role of anti-LPS antibodies in protection. The best protection was conferred by a serotype O:9 strain carrying pCIP39, confirming the importance of the P39 T-cell antigen in this mechanism.

Susceptibility of IFN regulatory factor-1 and IFN consensus sequence binding protein-deficient mice to brucellosis

IFN-gamma is a key cytokine controlling Brucella infection, and the diverse functions of this cytokine are mediated by IFN regulatory factors (IRFs) such as IRF-1, IRF-2, and IFN consensus sequence binding protein (ICSBP). However, the roles of these three IRFs in Brucella infection have not been investigated. The infection of each IRF-deficient mouse strain provides an opportunity to determine not only the significance of each IRF molecule but also the crucial immune components necessary for host defense during in vivo infection, because respective IRF-deficient mouse strains contain unique immunodeficient phenotypes. Brucella abortus S2308-infected IRF-1-/- mice were dead within 2 wk postinfection, while IRF-2-/- mice contained less splenic Brucella CFU than wild-type mice at the early stage of infection. Infected ICSBP-/- mice maintained a plateau of splenic Brucella CFU throughout the infection. Additional infection of IL-12p40-, NO synthase 2-, and gp91(phox)-deficient mice indicates that these immune components are crucial for Brucella immunity and may contribute to the susceptibility of IRF-1-/- and ICSBP-/- mice. Immunologic and histopathological analyses of infected IRF-1-/- mice indicate that the absence of IL-12p40 induction and serious hepatic damage are involved in the death of IRF-1-/- mice. These results indicate that 1) IRF-1 and ICSBP are essential transcriptional factors for IFN-gamma-mediated protection against Brucella; 2) IL-12, reactive nitrogen intermediates, and reactive oxygen intermediates are crucial immune components against Brucella, and their absence may contribute to the susceptibility of IRF-1-/- and ICSBP-/- mice; and 3) hepatic damage caused by Brucella virulence contributes to the death of IRF-1-/- mice.

Immune control of Brucella abortus 2308 infections in BALB/c mice

BALB/c mice infected with Brucella abortus strain 2308 have 10-fold higher levels of bacteria during the plateau phase of infection (the time period when the number of colony-forming units in vivo remains consistent) than the more resistant C57BL/10 mice. This is due to a cessation of interferon-gamma (IFN-gamma) production that begins after the first week of infection and continues until the end of the plateau phase at least 6 weeks post infection. Despite the lack of IFN-gamma production during this time BALB/c mice are able to prevent an increase in bacterial colony-forming units. Here it was shown that both tumor necrosis factor (TNF)-alpha and CD8 T cells were involved in controlling bacterial numbers in BALB/c mice during this time. That is, neutralization of TNF-alpha or depletion of CD8 T cells with monoclonal antibodies resulted in a significant increase in the number of splenic colony-forming units recovered at 3 weeks post infection. In the absence of CD8 T cells there was also a significant increase in splenic macrophages. The role of TNF-alpha may depend upon the presence of interferon-gamma early in the infection since when TNF-alpha was neutralized in interferon-gamma gene knockout mice there was a marked increase in splenic macrophages, NK cells and neutrophils but not a significant increase in colony-forming units.

Induction of immune response in BALB/c mice with a DNA vaccine encoding bacterioferritin or P39 of Brucella spp

In this study, we evaluated the ability of DNA vaccines encoding the bacterioferritin (BFR) or P39 proteins of Brucella spp. to induce cellular and humoral immune responses and to protect BALB/c mice against a challenge with B. abortus 544. We constructed eukaryotic expression vectors called pCIBFR and pCIP39, encoding BFR or P39 antigens, respectively, and we verified that these proteins were produced after transfection of COS-7 cells. PCIBFR or pCIP39 was injected intramuscularly three times, at 3-week intervals. pCIP39 induced higher antibody responses than did the DNA vector encoding BFR. Both vectors elicited a T-cell-proliferative response and also induced a strong gamma interferon production upon restimulation with either the specific antigens or Brucella extract. In this report, we also demonstrate that animals immunized with these plasmids elicited a strong and long-lived memory immune response which persisted at least 3 months after the third vaccination. Furthermore, pCIBFR and pCIP39 induced a typical T-helper 1-dominated immune response in mice, as determined by cytokine or immunoglobulin G isotype analysis. The pCIP39 delivered by intramuscular injection (but not the pCIBFR or control vectors) induced a moderate protection in BALB/c mice challenged with B. abortus 544 compared to that observed in positive control mice vaccinated with S19.

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.

Interferon-gamma is crucial for surviving a Brucella abortus infection in both resistant C57BL/6 and susceptible BALB/c mice

Brucella abortus is an intracellular bacterial pathogen that causes chronic infections in humans and a number of agriculturally important species of animals. It has been shown that BALB/c mice are more susceptible to infections with virulent strains of Brucella abortus than C57BL/6 or C57BL/10 strains. In experiments described here, gene knock-out mice were utilized to elucidate some of the salient components of resistance. Resistant C57BL/6 mice with gene deletions or disruptions in the interferon-gamma (IFN-gamma), perforin or beta(2)-microglobulin genes had decreased abilities to control intracellular infections with B. abortus strain 2308 during the first week after infection. However, only the IFN-gamma knock-out mice had a sustained inability to control infections and this resulted in death of the mice at approximately 6 weeks post-infection. These mice had a continual increase in the number of bacterial colony-forming units (CFU) in their spleens until death. When BALB/c mice with the disrupted IFN-gamma gene were infected they had more splenic CFU at one week post-infection than control mice but the increase was not statistically significant and by 3 weeks they did not have more CFU than control mice. Moreover, the number of splenic bacteria did not increase in the BALB/c IFN-gamma knock-out mice between 6 and 10.5 weeks, although they died at 10.5 weeks, the time by which normal BALB/c mice were clearing the infection. Death in both strains of IFN-gamma gene disrupted mice coincided with symptoms of cachexia and macrophages comprised > or= 75% of the splenic leucocytes.

Protection of BALB/c mice against Brucella abortus 544 challenge by vaccination with bacterioferritin or P39 recombinant proteins with CpG oligodeoxynucleotides as adjuvant

The P39 and the bacterioferrin (BFR) antigens of Brucella melitensis 16M were previously identified as T dominant antigens able to induce both delayed-type hypersensivity in sensitized guinea pigs and in vitro gamma interferon (IFN-gamma) production by peripheral blood mononuclear cells from infected cattle. Here, we analyzed the potential for these antigens to function as a subunitary vaccine against Brucella abortus infection in BALB/c mice, and we characterized the humoral and cellular immune responses induced. Mice were injected with each of the recombinant proteins alone or adjuvanted with either CpG oligodeoxynucleotides (CpG ODN) or non-CpG ODN. Mice immunized with the recombinant antigens with CpG ODN were the only group demonstrating both significant IFN-gamma production and T-cell proliferation in response to either Brucella extract or to the respective antigen. The same conclusion holds true for the antibody response, which was only demonstrated in mice immunized with recombinant antigens mixed with CpG ODN. The antibody titers (both immunoglobulin G1 [IgG1] and IgG2a) induced by P39 immunization were higher than the titers induced by BFR (only IgG2a). Using a B. abortus 544 challenge, the level of protection was analyzed and compared to the protection conferred by one immunization with the vaccine strain B19. Immunization with P39 and CpG ODN gave a level of protection comparable to the one conferred by B19 at 4 weeks postchallenge, and the mice were still significantly protected at 8 weeks postchallenge, although to a lesser extent than the B19-vaccinated group. Intriguingly, no protection was detected after BFR vaccination. All other groups did not demonstrate any protection.

Immunity and protection against Brucella abortus

Brucella abortus is an intracellular pathogen that causes disease in cattle and in humans. The response against B. abortus involves the whole gamut of the immune system, from innate to adaptive immunity resulting from stimulation of antigen-presenting cells, NK cells, CD4(+) and CD8(+) T cells, and B cells.

Impaired control of Brucella melitensis infection in Rag1-deficient mice

After intranasal inoculation, Brucella melitensis chronically infects the mononuclear phagocyte system in BALB/c mice, but it causes no apparent illness. Adaptive immunity, which can be transferred by either T cells or antibody from immune to naive animals, confers resistance to challenge infection. The role of innate, non-B-, non-T-cell-mediated immunity in control of murine brucellosis, however, is unknown. In the present study, we documented that BALB/c and C57BL/6 mice had a similar course of infection after intranasal administration of 16M, validating the usefulness of the model in the latter mouse strain. We then compared the course of infection in Rag1 knockout mice (C57BL/6 background) (referred to here as RAG-1 mice) which have no B or T cells as a consequence of deletion of Rag1 (recombination-activating gene 1), with infection in normal C57BL/6 animals after intranasal administration of B. melitensis 16M. C57BL/6 mice cleared brucellae from their lungs by 8 to 12 weeks and controlled infection in the liver and spleen at a low level. In contrast, RAG-1 mice failed to reduce the number of bacteria in any of these organs. From 1 to 4 weeks after inoculation, the number of splenic bacteria increased from 2 to 4.5 logs and remained at that level. In contrast to the consistently high numbers of brucellae observed in the spleens, the number of bacteria rose in the livers sampled for up to 20 weeks. Immunohistologic examination at 8 weeks after infection disclosed foci of persistent pneumonia and large amounts of Brucella antigen in macrophages in lung, liver, and spleen in RAG-1, but not C57BL/6, mice. These studies indicate that T- and B-cell-independent immunity can control Brucella infection at a high level in the murine spleen, but not in the liver. Immunity mediated by T and/or B cells is required for clearance of bacteria from spleen and lung and for control of bacterial replication in the liver.

Effects of opsonization and gamma interferon on growth of Brucella melitensis 16M in mouse peritoneal macrophages in vitro

Entry of opsonized pathogens into phagocytes may benefit or, paradoxically, harm the host. Opsonization may trigger antimicrobial mechanisms such as reactive oxygen or nitric oxide (NO) production but may also provide a safe haven for intracellular replication. Brucellae are natural intramacrophage pathogens of rodents, ruminants, dogs, marine mammals, and humans. We evaluated the role of opsonins in Brucella-macrophage interactions by challenging cultured murine peritoneal macrophages with Brucella melitensis 16M treated with complement- and/or antibody-rich serum. Mouse serum rich in antibody against Brucella lipopolysaccharide (LPS) (aLPS) and human complement-rich serum (HCS) each enhanced the macrophage uptake of brucellae. Combinations of suboptimal levels of aLPS (0. 01%) and HCS (2%) synergistically enhanced uptake. The intracellular fate of ingested bacteria was evaluated with an optimal concentration of gentamicin (2 microg/ml) to control extracellular growth but not kill intracellular bacteria. Bacteria opsonized with aLPS and/or HCS grew equally well inside macrophages in the absence of gamma interferon (IFN-gamma). Macrophage activation with IFN-gamma inhibited replication of both opsonized and nonopsonized brucellae but was less effective in inhibiting replication of nonopsonized bacteria. IFN-gamma treatment of macrophages with opsonized or nonopsonized bacteria enhanced NO production, which was blocked by N(G)-monomethyl L-arginine (MMLA), an NO synthesis inhibitor. MMLA also partially blocked IFN-gamma-mediated bacterial growth inhibition. These studies suggest that primary murine macrophages have limited ability to control infection with B. melitensis, even when activated by IFN-gamma in the presence of highly opsonic concentrations of antibody and complement. Additional cellular immune responses, e.g., those mediated by cytotoxic T cells, may play more important roles in the control of murine brucellosis.

Protection of mice against brucellosis by vaccination with Brucella melitensis WR201(16MDeltapurEK)

Human brucellosis can be acquired from infected animal tissues by ingestion, inhalation, or contamination of the conjunctiva or traumatized skin by infected animal products. A vaccine to protect humans from occupational exposure or from zoonotic infection in areas where the disease is endemic would reduce an important cause of morbidity worldwide. Vaccines currently used in animals are unsuitable for human use. We tested a live, attenuated, purine-auxotrophic mutant strain of Brucella melitensis, WR201, for its ability to elicit cellular and humoral immune responses and to protect mice against intranasal challenge with B. melitensis 16M. Mice inoculated intraperitoneally with WR201 made serum antibody to lipopolysaccharide and non-O-polysaccharide antigens. Splenocytes from immunized animals released interleukin-2 (IL-2), gamma interferon, and IL-10 when cultured with Brucella antigens. Immunization led to protection from disseminated infection but had only a slight effect on clearance of the challenge inoculum from the lungs. These studies suggest that WR201 should be further investigated as a vaccine to prevent human brucellosis.

Novel phenotype associated with in vivo activated CTL precursors

A previously undefined phenotype of CD8(+) cells that appears to represent in vivo activated CTL precursors (CTLP*) has been identified in the spleens of C57Bl/6 mice responding to a P815 tumor allograft. This population was first evident by the transient expression of very high levels of CD28 and CD44 on day 5 of the allograft response and reached maximal levels on days 7 and 8 before declining on day 9. A transient increase in CD69 expression was also observed on these cells on day 5. In contrast, CTL effectors (CTLE), identified by their CD8(+)CD44(hi)CD62LloCD45RBlo phenotype, were not appreciably detected in the spleen until day 8 and reached maximal levels on day 10. Further characterization of CTLP* on day 7 revealed that they represented blasting cells by increased light scatter and also expressed very high levels of CD54 but not CD122, CD152, or CD154. In addition, the cells had already up-regulated CD49d, asialo GM1, CD11a, and CD95L, and down-regulated their expression of CD62L. A small percentage of these cells also expressed CD25. Day 7 CTLP* sorted on the basis of their CD44(xhi) and CD54(xhi) phenotype did not exhibit cytolytic activity in a standard chromium release assay but became cytotoxic when they were cultured in the presence of exogenous murine IL-2 for 5 days. Granzyme B activity, however, was detected in CTLP* on day 7 at levels equivalent to CTLE on day 10. In order to establish a potential precursor relationship between CTLP* and CTLE, mice were treated with various doses of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), a chemical that has been shown to dose-dependently suppress the in vivo generation of CTLE to P815 tumor cells by altering an early stage of CTLP activation. Results indicated that CTLP* were suppressed by TCDD on day 7 to the same degree that CTLE were suppressed on day 10. Importantly, for controls and for all doses of TCDD, there were approximately 12.5 CTLE on day 10 for every CTLP* detected on day 7. These results suggested that TCDD acted identically across all doses to inhibit the early stages of activation of CTLP but did not affect the final stages of differentiation and expansion to CTLE. This interpretation supports the previous observation that TCDD exposure had to occur within the first 3 days of the allograft response in order to induce suppression of CTLE activity. Taken together, these results support the conclusion that in vivo activated CTLP can be identified by their unique expression of very high levels of CD44, CD28, and/or CD54 prior to their full maturation and clonal expansion to functional CTLE.

Quantitative assessment by flow cytometry of T-lymphocytes producing antigen-specific gamma-interferon in Brucella immune cattle

Peripheral blood mononuclear cells (PBMC) isolated from cattle infected with Brucella secreted gamma-interferon (IFN-gamma) after antigen-specific stimulation with Brucellergene, which is a mixture of cytoplasmic proteins of rough Brucella melitensis B115. Following the depletion of the monocyte-macrophages from the PBMC, the enriched lymphocyte populations stimulated with Brucellergene did not produce IFN-gamma. Two-colour immunofluorescence staining of intracellular IFN-gamma and bovine cell surface molecules identified the cells producing IFN-gamma among the PBMC stimulated with Brucellergene. Moreover, this method could be used to estimate the number of T-cells specifically producing IFN-gamma. For a given animal, there is a significant correlation (p < 0.05) between the production of IFN measured by an ELISA of the supernatant of whole blood stimulated with Brucellergene and the number of T-cells producing IFN-gamma after in vitro stimulation with Brucellergene. The development of the immunofluorescence staining technique provides a new tool for analysing and for measuring the T-cell immune response in cattle.

Cloning and sequencing of yajC and secD homologs of Brucella abortus and demonstration of immune responses to YajC in mice vaccinated with B. abortus RB51

To identify Brucella antigens that are potentially involved in stimulating a protective cell-mediated immune response, a gene library of Brucella abortus 2308 was screened for the expression of antigens reacting with immunoglobulin G2a antibodies from BALB/c mice vaccinated with B. abortus RB51. One selected positive clone (clone MCB68) contained an insert of 2.6 kb; nucleotide sequence analysis of this insert revealed two open reading frames (ORFs). The deduced amino acid sequences of the first and second ORFs had significant similarities with the YajC and SecD proteins, respectively, of several bacterial species. Both the YajC and SecD proteins were expressed in Escherichia coli as fusion proteins with maltose binding protein (MBP). In Western blots, sera from mice vaccinated with B. abortus RB51 recognized YajC but not SecD. Further Western blot analysis with purified recombinant YajC protein indicated that mice inoculated with B. abortus 19 or 2308 or B. melitensis RM1 also produced antibodies to YajC. In response to in vitro stimulation with recombinant MBP-YajC fusion protein, splenocytes from mice vaccinated with B. abortus RB51 were able to proliferate and produce gamma interferon but not interleukin-4. This study demonstrates, for the first time, the involvement of YajC protein in an immune response to an infectious agent.

The CD4+ T-cell response to protein immunization is independent of accompanying IFN-gamma-producing CD8+ T cells

By virtue of their strong bias towards production of interferon-gamma (IFN-gamma), CD8+ T cells have the potential to promote the development of type 1 immune responses. We have previously shown that the CD4+ T-cell response to immunization with the protein antigen keyhole limpet haemocyanin (KLH) has a mixed interleukin-4 (IL-4)/IFN-gamma production profile. Here we show that this immunization regimen also stimulates accumulation in the draining lymph nodes of CD8+ T cells, which preferentially contain IFN-gamma mRNA ex vivo and secrete IFN-gamma protein in vitro. This provides a model to test whether CD8+ cell-derived IFN-gamma participates in the normal control of the immune response to a non-viable exogenous antigen. To investigate regulation of the anti-KLH response by the CD8+ population or IFN-gamma produced by this or other cell types, mice were administered depleting antibodies. Depletion of CD8+ cells had no effect on the frequency of clonogenic KLH-specific CD4+ T cells, the IL-4/IFN-gamma profiles of their progeny, or the isotype profiles of the serum antibody response to KLH. In contrast, IFN-gamma neutralization diminished cell accumulation in the lymph nodes and reduced both the frequency of KLH-specific CD4+ T cells that gave rise to IFN-gamma-producing clones and serum titres of KLH-specific IgG2a and IgG3. Therefore, despite the potential for cross-regulation, the CD4+ T-cell response to this immunogen is independent of the IFN-gamma-skewed CD8+ response.

The role of T cell subsets and cytokines in the regulation of intracellular bacterial infection

Cellular immune responses are a critical part of the host's defense against intracellular bacterial infections. Immunity to Brucella abortus crucially depends on antigen-specific T cell-mediated activation of macrophages, which are the major effectors of cell-mediated killing of this organism. T lymphocytes that proliferate in response to B. abortus were characterized for phenotype and cytokine activity. Human, murine, and bovine T lymphocytes exhibited a type 1 cytokine profile, suggesting an analogous immune response in these different hosts. In vivo protection afforded by a particular cell type is dependent on the antigen presented and the mechanism of antigen presentation. Studies using MHC class I and class II knockout mice infected with B. abortus have demonstrated that protective immunity to brucellosis is especially dependent on CD8+ T cells. To target MHC class I presentation we transfected ex vivo a murine macrophage cell line with B. abortus genes and adoptively transferred them to BALB/c mice. These transgenic macrophage clones induced partial protection in mice against experimental brucellosis. Knowing the cells required for protection, vaccines can be designed to activate the protective T cell subset. Lastly, as a new strategy for priming a specific class I-restricted T cell response in vivo, we used genetic immunization by particle bombardment-mediated gene transfer.

Characterization, occurrence, and molecular cloning of a 39-kilodalton Brucella abortus cytoplasmic protein immunodominant in cattle

Monoclonal antibodies and polyclonal antisera recognizing a 39-kDa protein (P39) of brucellin, a cytoplasmic extract from Brucella melitensis rough strain B115, were produced. The P39 was purified by anion-exchange chromatography. Eleven of fourteen Brucella-infected cows whose infections had been detected by the delayed-type hypersensitivity (DTH) test with brucellergen also developed a DTH reaction when purified P39 was used as the trigger. The T-cell proliferative responses to P39 of peripheral blood lymphocytes from Brucella-infected cows were also positive. None of the animals infected with other bacterial species that are presumed to induce immunological cross-reactions with Brucella spp. reacted to P39, either in DTH tests or in lymphocyte proliferation assays. A lambda gt11 genomic library of Brucella abortus was screened with a monoclonal antibody specific for P39, and the gene coding for this protein was subsequently isolated. The nucleotide sequence of the P39 gene was determined, and the deduced amino acid sequence is in accordance with the sequence of an internal peptide isolated from P39.

Outer membrane protein of Neisseria meningitidis as a mucosal adjuvant for lipopolysaccharide of Brucella melitensis in mouse and guinea pig intranasal immunization models

A mucosal vaccine against brucellosis consisting of the lipopolysaccharide (LPS) of Brucella melitensis complexed with the outer membrane protein (GBOMP) of group B Neisseria meningitidis was tested in small-animal models of intranasal immunization. Mice given two doses of the vaccine developed high levels of immunoglobulin G (IgG) and IgA antibodies specific for B. melitensis LPS in lung lavages and specific IgG and IgA antibody-secreting cells in the lungs and spleen. Similarly, in guinea pigs immunized twice intranasally, IgG and IgA LPS-specific antibodies were detected in lung lavages, and specific antibody-secreting cells were isolated from the spleen and cervical nodes. In mice immunized with LPS only, pulmonary responses consisted mostly of IgM antibodies, while guinea pigs given LPS alone developed local antibody of all three isotypes, but at lower levels compared to animals given the complex vaccine. Both mice and guinea pigs also developed high levels of serum IgG and moderate levels of IgA as a result of intranasal immunization with the complex vaccine. The serum antibodies in both cases were found to cross-react with the LPS of B. abortus, which shares an immunogenic epitope with B. melitensis LPS. In mice given the complex vaccine, there was a prominent serum IgG1 response that was absent in the mice given LPS alone. In conclusion, the N. meningitidis GBOMP was an effective mucosal adjuvant for secretory IgA and IgG responses in the lungs of both mice and guinea pigs. The IgG1 subclass response in mice suggests that GBOMP may have favored a Th2 type of response to the LPS. A vaccine capable of stimulating high levels of antibody at local sites has the potential to protect against brucellae, since these pathogens gain entry to the host via mucosal routes.

T lymphocyte mediated protection against facultative intracellular bacteria

Acquired immunity against intracellular bacteria is T cell dependent. T cells play a major role in protection against intracellular bacteria, but bacterial antigens recognized by T cells have been studied less extensively than bacterial antigens recognized by B cells. Using T lymphocytes from animals immunized against Brucella abortus, we have screened a bacterial genomic library for genes encoding antigens recognized by T cells. Lymphocytes that proliferated to B. abortus proteins were characterized for phenotype and cytokine activity. Bovine and murine lymphocytes recognized common bacterial antigens and possessed similar cytokine profiles, suggesting an analogous immune response in these two animal species. In vivo protection afforded by a particular cell type is dependent on the bacterial antigens presented and mechanisms of antigen presentation. MHC class I and class II gene knockout animals infected with B. abortus have demonstrated that protection to B. abortus is especially dependent on CD8+ T cells. Knowing the cells required for protection, vaccines can be designed to elicit the protective subset of lymphocytes. Currently, we are testing several recombinant B. abortus proteins using different immunization strategies. Finally, bacterial genes activated following intracellular phagocytosis are being examined using a novel, reporter system adapted to B. abortus.