The protective role of T-cell receptor Vgamma1+ T cells in primary infection with Listeria monocytogenes

From Host Defense to Metabolic Signatures: Unveiling the Role of γδ T Cells in Bacterial Infections

The growth of antibiotic-resistant bacterial infections necessitates focusing on host-derived immunotherapies. γδ T cells are an unconventional T cell subset, making up a relatively small portion of healthy circulating lymphocytes but a substantially increased proportion in mucosal and epithelial tissues. γδ T cells are activated and expanded in response to bacterial infection, having the capability to produce proinflammatory cytokines to recruit neutrophils and clear infection. They also play a significant role in dampening immune response to control inflammation and protecting the host against secondary challenge, making them promising targets when developing immunotherapy. Importantly, γδ T cells have differential metabolic states influencing their cytokine profile and subsequent inflammatory capacity. Though these differential metabolic states have not been well studied or reviewed in the context of bacterial infection, they are critical in understanding the mechanistic underpinnings of the host's innate immune response. Therefore, this review will focus on the context-specific host defense conferred by γδ T cells during infection with Staphylococcus aureus, Streptococcus pneumoniae, Listeria monocytogenes, and Mycobacterium tuberculosis.

Tissue Adaptations of Memory and Tissue-Resident Gamma Delta T Cells

Epithelial and mucosal barriers are critical interfaces physically separating the body from the outside environment and are the tissues most exposed to microorganisms and potential inflammatory agents. The integrity of these tissues requires fine tuning of the local immune system to enable the efficient elimination of invasive pathogens while simultaneously preserving a beneficial relationship with commensal organisms and preventing autoimmunity. Although they only represent a small fraction of circulating and lymphoid T cells, γδ T cells form a substantial population at barrier sites and even outnumber conventional αβ T cells in some tissues. After their egress from the thymus, several γδ T cell subsets naturally establish residency in predetermined mucosal and epithelial locations, as exemplified by the restricted location of murine Vγ5⁺ and Vγ3Vδ1⁺ T cell subsets to the intestinal epithelium and epidermis, respectively. Because of their preferential location in barrier sites, γδ T cells are often directly or indirectly influenced by the microbiota or the pathogens that invade these sites. More recently, a growing body of studies have shown that γδ T cells form long-lived memory populations upon local inflammation or bacterial infection, some of which permanently populate the affected tissues after pathogen clearance or resolution of inflammation. Natural and induced resident γδ T cells have been implicated in many beneficial processes such as tissue homeostasis and pathogen control, but their presence may also exacerbate local inflammation under certain circumstances. Further understanding of the biology and role of these unconventional resident T cells in homeostasis and disease may shed light on potentially novel vaccines and therapies.

Id3 Restricts γδ NKT Cell Expansion by Controlling Egr2 and c-Myc Activity

γδ NKT cells are neonatal-derived γδ T lymphocytes that are grouped together with invariant NKT cells based on their shared innate-like developmental program characterized by the transcription factor PLZF (promyelocytic leukemia zinc finger). Previous studies have demonstrated that the population size of γδ NKT cells is tightly controlled by Id3-mediated inhibition of E-protein activity in mice. However, how E proteins promote γδ NKT cell development and expansion remains to be determined. In this study, we report that the transcription factor Egr2, which also activates PLZF expression in invariant NKT cells, is essential for regulating γδ NKT cell expansion. We observed a higher expression of Egr family genes in γδ NKT cells compared with the conventional γδ T cell population. Loss of function of Id3 caused an expansion of γδ NKT cells, which is accompanied by further upregulation of Egr family genes as well as PLZF. Deletion of Egr2 in Id3-deficient γδ NKT cells prevented cell expansion and blocked PLZF upregulation. We further show that this Egr2-mediated γδ NKT cell expansion is dependent on c-Myc. c-Myc knockdown attenuated the proliferation of Id3-deficient γδ NKT cells, whereas c-Myc overexpression enhanced the proliferation of Id3/Egr2-double-deficient γδ NKT cells. Therefore, our data reveal a regulatory circuit involving Egr2-Id3-E2A, which normally restricts the population size of γδ NKT cells by adjusting Egr2 dosage and c-Myc expression.

Protective role of gammadelta T cells in spontaneous ocular inflammation

PURPOSE

A role for gammadelta T cells in immunoregulation has been shown in a number of studies, but in the absence of infection or induced disease, mice lacking gammadelta T cells generally appear to be healthy. That certain mice lacking gammadelta T cells often spontaneously develop keratitis, characterized by a progressive and destructive inflammation of the cornea is reported here.

METHODS

The keratitis developing in these mice was characterized in terms of prevalence in males versus females, age of onset, and histologic features. Attempts were made to understand the underlying causes of the disease by removing alphabeta T cells, altering sex hormones, and reconstituting gammadelta T cells.

RESULTS

The development of keratitis in these mice depended on the C57BL/10 genetic background, and was much more common among females than males. The incidence of the disease increased with age, exceeding 80% in females greater than 18 weeks old. Evidence that the keratitis in these mice is at least partly autoimmune in nature, and that despite its prevalence in females, male hormones do not protect against the disease is presented.

CONCLUSIONS

These findings indicate an important role for gammadelta T cells in maintaining immune balance in the eye. The mice described in this study represent a potential new small animal model of keratitis.

Importance of murine Vdelta1gammadelta T cells expressing interferon-gamma and interleukin-17A in innate protection against Listeria monocytogenes infection

Murine gammadelta T cells participate in the innate immune response against infection by an intracellular pathogen Listeria monocytogenes. Vdelta1+gammadelta T cells coexpressing Vgamma6 are a major gammadelta T-cell subpopulation induced at an early stage of L. monocytogenes infection in the livers of infected mice. To investigate the protective role of the Vgamma6/Vdelta1+gammadelta T cells against L. monocytogenes infection, Vdelta1 gene-deficient (Vdelta1-/-) mice were analysed because these mice selectively lacked a Vgamma6/Vdelta1+gammadelta T-cell subpopulation in the L. monocytogenes-infected liver. The Vdelta1-/- mice showed increased bacterial burden in the liver and spleen, and decreased survival rate at an early stage of L. monocytogenes infection when compared to wild-type mice. Histological examination showed abscess-like lesions and unorganized distribution of macrophages in the liver of the Vdelta1-/- mice but not in the wild-type mice after L. monocytogenes infection. The Vgamma6/Vdelta1+gammadelta T cells produced interferon-gamma and interleukin-17A. All the results suggest that murine Vgamma6/Vdelta1+gammadelta T cells control the innate protective response against L. monocytogenes infection through production of the proinflammatory cytokines interferon-gamma and interleukin-17A in the infected liver.

IL-17 producing gammadelta T cells are required for a controlled inflammatory response after bleomycin-induced lung injury

BACKGROUND

gammadelta T cells play a key role in the regulation of inflammatory responses in epithelial tissue, and in adaptive immunity, as gammadelta T cell deficient mice have a severely impaired capacity to clear lung pathogens. gammadelta T cells regulate the initial inflammatory response to microbial invasion and thereby protect against tissue injury. Here we examined the response of gammadelta T cells to lung injury induced by bleomycin, in an effort to study the inflammatory response in the absence of any adaptive immune response to a pathogen.

RESULTS

After lung injury by bleomycin, we localized the gammadelta T cells to the lung lesions. gammadelta T cells were the predominant source of IL-17 (as detected by flow cytometry and real-time PCR). Moreover, gammadelta T cell knockout mice showed a significant reduction in cellular infiltration into the airways, reduced expression of IL-6 in the lung, and a significant delay in epithelial repair.

CONCLUSION

Mouse gammadelta T cells produce IL-17 in response to lung injury and are required for an organized inflammatory response and epithelial repair. The lack of gammadelta T cells correlates with increased inflammation and fibrosis.

Accumulation of gamma/delta T cells in the lungs and their roles in neutrophil-mediated host defense against pneumococcal infection

The present study was designed to elucidate the role of Vgamma4(+) gammadelta T cells, a major subset of pulmonary gammadelta T cells, in host defense against infection with Streptococcus pneumoniae. The proportion and number of whole gammadelta T cells, identified as CD3(+) and TCR-delta(+) cells, and Vgamma4(+) gammadelta T cells, identified as CD3(+) and TCR-Vgamma4(+) cells, increased in the lungs at 3, 6 and 12h post-infection. Survival of infected mice and lung bacterial clearance were severely impaired in TCR-Vgamma4(-/-) mice compared with control wild-type (WT) mice. The impaired host protection in TCR-Vgamma4(-/-) mice correlated well with attenuated recruitment of neutrophils in lungs. MIP-2 and TNF-alpha synthesis in the infected tissues was significantly reduced in TCR-Vgamma4(-/-) mice compared with WT mice. Similar results were noted in the synthesis of TNF-alpha, but not clearly of MIP-2, by lung leukocytes stimulated with live bacteria. Our results demonstrate that Vgamma4(+) gammadelta T cells play an important role in the neutrophil-mediated host defense against S. pneumoniae infection by promoting the synthesis of TNF-alpha and possibly of MIP-2 in the lungs.

Delineation of the function of a major gamma delta T cell subset during infection

Gammadelta T cells play important but poorly defined roles in pathogen-induced immune responses and in preventing chronic inflammation and pathology. A major obstacle to defining their function is establishing the degree of functional redundancy and heterogeneity among gammadelta T cells. Using mice deficient in Vgamma1+ T cells which are a major component of the gammadelta T cell response to microbial infection, a specific immunoregulatory role for Vgamma1+ T cells in macrophage and gammadelta T cell homeostasis during infection has been established. By contrast, Vgamma1+ T cells play no significant role in pathogen containment or eradication and cannot protect mice from immune-mediated pathology. Pathogen-elicited Vgamma1+ T cells also display different functional characteristics at different stages of the host response to infection that involves unique and different populations of Vgamma1+ T cells. These findings, therefore, identify distinct and nonoverlapping roles for gammadelta T cell subsets in infection and establish the complexity and adaptability of a single population of gammadelta T cells in the host response to infection that is not predetermined, but is, instead, shaped by environmental factors.

A defective Th1 response of the spleen in the initial phase may explain why splenectomy helps prevent a Listeria infection

Listeria monocytogenes (Listeria) are known to grow and proliferate in the liver while a splenectomy induces host resistance against a Listeria infection despite the fact that a splenectomy inhibits the Th1 response. Therefore, the mechanism by which a splenectomy helps to prevent the growth of Listeria still remains to be elucidated. After an i.v. challenge of Listeria (1 x 10(6) CFU) in C57BL/6 mice, Listeria rapidly increased in the spleen but not in the liver until 48 h. However, after this initial phase, Listeria remarkably grew in the liver. In contrast, when the mice received a splenectomy beforehand, no remarkable growth of Listeria in the liver was observed after Listeria challenge despite the fact that serum IFN-gamma and IL-12 levels at 24 h after Listeria challenge were significantly lower than those in the sham mice. However, the liver leucocytes from mice by 6 h after infection produced a substantial amount of IFN-gamma while spleen MNC did not, whereas spleen leucocytes at 24 h after Listeria challenge did. Consistently, the IFN-gamma and IL-12 levels in the tissue homogenates of the spleen were significantly lower than in those of the liver until 6 h after infection. This defective spleen Th1 response in the early phase of Listeria infection was corrected by an IL-18 i.p. injection just after the Listeria challenge. Our findings suggest that Listeria exploit the defective Th1 environment of the spleen in the initial phase and afterwards overcome the host defense mechanism of the liver.

The Interaction of γδ T Cells with Activated Macrophages Is a Property of the Vγ1 Subset

Immunoregulation is an emerging paradigm of gammadelta T cell function. The mechanisms by which gammadelta T cells mediate this function, however, are not clear. Studies have identified a direct role for gammadelta T cells in resolving the host immune response to infection, by eliminating populations of activated macrophages. The aim of this study was to identify macrophage-reactive gammadelta T cells and establish the requirements/outcomes of macrophage-gammadelta T cell interactions during the immune response to the intracellular bacterium, Listeria monocytogenes (Lm). Using a macrophage-T cell coculture system in which peritoneal macrophages from naive or Lm-infected TCRdelta(-/-) mice were incubated with splenocytes from naive and Lm-infected alphabeta/gammadelta T cell-deficient and wild-type mice, the ability to bind macrophages was shown to be restricted to gammadelta T cells and the GV5S1 (Vgamma1) subset of gammadelta T cells. Macrophage adherence resulted in a 4- to 10-fold enrichment of Vgamma1(+) T cells. Enrichment of Vgamma1 T cells was dependent upon the activation status of macrophages, but independent of the activation status of gammadelta T cells. Vgamma1 T cells were cytotoxic for activated macrophages with both the binding to and killing of macrophages being TCR dependent because anti-TCRgammadelta Abs inhibited both Vgamma1 binding and killing activities. These studies establish the identity of macrophage cytotoxic gammadelta T cells, the conditions under which this interaction occurs, and the outcome of this interaction. These findings are concordant with the involvement of Vgamma1 T cells in macrophage homeostasis during the resolution of pathogen-mediated immune responses.

Mechanism of murine Vgamma1+ gamma delta T cell-mediated innate immune response against Listeria monocytogenes infection

Murine gamma delta T cells participate in innate immune response against infection of the intracellular bacterium Listeria monocytogenes. In the present report, we analyzed the mechanism of the gamma delta T cell-mediated response against L. monocytogenes infection. gamma delta T cell-enriched spleen cells of L. monocytogenes-infected mice produced IFN-gamma in vitro in response to L. monocytogenes-infected spleen cells. The IFN-gamma production was abrogated by depletion of Vgamma1+ gamma delta T cells. IFN-gamma production of the Vgamma1+ gamma delta T cells in response to L. monocytogenes-infected spleen cells required IL-12. However, addition of Fab fragment of anti-TCR gamma delta monoclonal antibodies (mAb) failed to block the response, suggesting that the response requires no TCR-mediated antigen recognition. Interestingly, Vgamma1+ gamma delta T cells of naive mice also produced IFN-gamma in response to L. monocytogenes-infected spleen cells in an IL-12-dependent manner. Furthermore, the IL-12 receptor (IL-12R) gene was expressed on the Vgamma1+ gamma delta T cells of naive mice as well as those of L. monocytogenes-infected mice although naive alpha beta T cells lack IL-12R expression. All the results suggest that the Vgamma1+ gamma delta T cells participate in immune surveillance against intracellular bacterial infection through quick production of IFN-gamma in response to infection-induced IL-12 without antigen-driven clonal expansion and maturation.

Modulation of immune responses to Mycobacterium bovis in cattle depleted of WC1(+) gamma delta T cells

It is accepted that cell-mediated immune responses predominate in mycobacterial infections. Many studies have shown that CD4(+) T cells produce Th1 cytokines, such as gamma interferon (IFN-gamma), in response to mycobacterial antigens and that the cytolytic activity of CD8(+) cells toward infected macrophages is important. However, the extent and manner in which gamma delta T cells participate in this response remain unclear. In ruminants, gamma delta T cells comprise a major proportion of the peripheral blood mononuclear cell population. We have previously shown that WC1(+) gamma delta T cells are involved early in Mycobacterium bovis infection of cattle, but their specific functions are not well understood. Here we describe an in vivo model of bovine tuberculosis in which the WC1(+) gamma delta T cells were depleted from the peripheral circulation and respiratory tract, by infusion of WC1(+)-specific monoclonal antibody, prior to infection. While no effects on disease pathology were observed in this experiment, results indicate that WC1(+) gamma delta T cells, which become significantly activated (CD25(+)) in the circulation of control calves from 21 days postinfection, may play a role in modulating the developing immune response to M. bovis. WC1(+)-depleted animals exhibited decreased antigen-specific lymphocyte proliferative response, an increased antigen-specific production of interleukin-4, and a lack of specific immunoglobulin G2 antibody. This suggests that WC1(+) gamma delta TCR(+) cells contribute, either directly or indirectly, toward the Th1 bias of the immune response in bovine tuberculosis--a hypothesis supported by the decreased innate production of IFN-gamma, which was observed in WC1(+)-depleted calves.

Cytokine production by Vgamma(+)-T-cell subsets is an important factor determining CD4(+)-Th-cell phenotype and susceptibility of BALB/c mice to coxsackievirus B3-induced myocarditis

Two coxsackievirus B3 (CVB3) variants (H3 and H310A1) differ by a single amino acid mutation in the VP2 capsid protein. H3 induces severe myocarditis in BALB/c mice, but H310A1 is amyocarditic. Infection with H3, but not H310A1, preferentially activates Vgamma4 Vdelta4 cells, which are strongly positive for gamma interferon (IFN-gamma), whereas Vgamma1 Vdelta4 cells are increased in both H3 and H310A1 virus-infected animals. Depletion of Vgamma1(+) cells using monoclonal anti-Vgamma1 antibody enhanced myocarditis and CD4(+)-, IFN-gamma(+)-cell responses in both H3- and H310A1-infected mice yet decreased the CD4(+)-, IL-4(+)-cell response. Depleting Vgamma4(+) cells suppressed myocarditis and reduced CD4(+) IFN-gamma(+) cells but increased CD4(+) IL-4(+) T cells. The role of cytokine production by Vgamma1(+) and Vgamma4(+) T cells was investigated by adoptively transferring these cells isolated from H3-infected BALB/c Stat4 knockout (Stat4ko) (defective in IFN-gamma expression) or BALB/c Stat6ko (defective in IL-4 expression) mice into H3 virus-infected wild-type BALB/c recipients. Vgamma4 and Vgamma1(+) T cells from Stat4ko mice expressed IL-4 but no or minimal IFN-gamma, whereas these cell populations derived from Stat6ko mice expressed IFN-gamma but no IL-4. Stat4ko Vgamma1(+) cells (IL-4(+)) suppress myocarditis. Stat6ko Vgamma1(+) cells (IFN-gamma(+)) were not inhibitory. Stat6ko Vgamma4(+) cells (IFN-gamma(+)) significantly enhanced myocarditis. Stat4ko Vgamma4(+) cells (IL-4(+)) neither inhibited nor enhanced disease. These results show that distinct gammadelta-T-cell subsets control myocarditis susceptibility and bias the CD4(+)-Th-cell response. The cytokines produced by the Vgamma subpopulation have a significant influence on the CD4(+)-Th-cell phenotype.

Gamma delta T cell-deficient mice have a down-regulated CD8+ T cell immune response against Encephalitozoon cuniculi infection

Gamma(delta) T cells have been reported to play an essential effector role during the early immune response against a wide variety of infectious agents. Recent studies have suggested that the gamma(delta) T cell subtype may also be important for the induction of adaptive immune response against certain microbial pathogens. In the present study, an early increase of gamma(delta) T cells during murine infection with Encephalitozoon cuniculi, an intracellular parasite, was observed. The role of gamma(delta) T cells against E. cuniculi infection was further evaluated by using gene-knockout mice. Mice lacking gamma(delta) T cells were susceptible to E. cuniculi infection at high challenge doses. The reduced resistance of delta(-/-) mice was attributed to a down-regulated CD8+ immune response. Compared with parental wild-type animals, suboptimal Ag-specific CD8+ T cell immunity against E. cuniculi infection was noted in delta(-/-) mice. The splenocytes from infected knockout mice exhibited a lower frequency of Ag-specific CD8+ T cells. Moreover, adoptive transfer of immune TCR(alpha)beta+ CD8+ cells from the delta(-/-) mice failed to protect naive CD8(-/-) mice against a lethal E. cuniculi challenge. Our studies suggest that gamma(delta) T cells, due to their ability to produce cytokines, are important for the optimal priming of CD8+ T cell immunity against E. cuniculi infection. This is the first evidence of a parasitic infection in which down-regulation of CD8+ T cell immune response in the absence of gamma(delta) T cells has been demonstrated.

Persistent infection with Listeria monocytogenes in the kidney induces anti-inflammatory invariant fetal-type gammadelta T cells

After intraperitoneal inoculation with Listeria monocytogenes, gammadelta T cells appear in the peritoneal cavity preceding the appearance of alphabeta T cells. Such gammadelta T cells predominantly express T-cell receptor (TCR)Vgamma1/Vdelta6, develop through an extrathymic pathway, and contribute to host defence against the bacteria. We have observed a gradual increase in gammadelta T cells in kidneys of mice after intrarenal inoculation with L. monocytogenes, which resulted in an unusually long-lasting local infection. In this study, we examined the characteristics and the roles of the gammadelta T cells induced in this model. It was found that these gammadelta T cells predominantly expressed TCRVgamma6/Vdelta1 with canonical junctional sequences identical to those expressed on fetal thymocytes. Although depletion of such gammadelta T cells in vivo did not affect the number of bacteria, it resulted in histologically exacerbated inflammation in the kidneys. These results indicate that a persistent infection with L. monocytogenes in kidneys induces a different kind of gammadelta T cell from that induced after intraperitoneal infection. The former expresses invariant fetal-type Vgamma6/Vdelta1+TCR and plays a regulatory role in resolution of inflammation.

Depletion of a gamma delta T cell subset can increase host resistance to a bacterial infection

Gammadelta T lymphocytes have been shown to regulate immune responses in diverse experimental systems. Because distinct gammadelta T cell subsets, as defined by the usage of certain TCR V genes, preferentially respond in various diseases and disease models, we have hypothesized that the various gammadelta T cell subsets carry out different functions. To test this, we compared one particular gammadelta T cell subset, the Vgamma1(+) subset, which represents a major gammadelta T cell type in the lymphoid organs and blood of mice, to other subsets and to gammadelta T cells as a whole. Using Listeria monocytogenes infection as an infectious disease model, we found that bacterial containment improves in mice depleted of Vgamma1(+) gammadelta T cells, albeit mice lacking all gammadelta T cells are instead impaired in their ability to control Listeria expansion. Our findings indicate that Vgamma1(+) gammadelta T cells reduce the ability of the innate immune system to destroy Listeria, even though other gammadelta T cells as a whole promote clearance of this pathogen.

Vgamma1+ gammadelta T cells play protective roles at an early phase of murine cytomegalovirus infection through production of interferon-gamma

Cytomegalovirus (CMV) causes severe opportunistic infection in immunocompromised hosts. The importance of conventional alphabeta T cells in protection against CMV infection has been well documented. However, the role of the second T-cell population (which express the gammadelta T-cell receptor) in CMV infection is not known. In the present study, we analysed the function and protective role of gammadelta T cells in a murine cytomegalovirus (MCMV) infection model. After intraperitoneal infection with MCMV, the number of gammadelta T cells increased in the liver and peritoneal cavity from day 3, and reached a peak on day 5. The gammadelta T cells showed an activated T-cell phenotype and predominantly expressed Vgamma1, which is known to be expressed by heat-shock protein 65 (hsp 65)-specific gammadelta T cells. Analysis of cytokine expression demonstrated that the MCMV-induced gammadelta T cells expressed interferon-gamma (IFN-gamma) and tumour necrosis factor-alpha (TNF-alpha) but not interleukin-4 (IL-4), implying their participation in the cell-mediated immune response against MCMV. Depletion of gammadelta T cells by anti-T-cell receptor (TCR) gammadelta monoclonal antibody (mAb) treatment resulted in significant increase of virus titre and decrease of IFN-gamma in the liver on day 3 after MCMV infection, which further supports the importance of gammadelta T cells in early protection against infection. Finally, the MCMV-induced gammadelta T cells produced IFN-gamma in vitro in response to hsp 65. Our results suggest that gammadelta T cells participate in early protection against MCMV infection through recognition of hsp 65 and production of IFN-gamma.

Escherichia coli infection induces only fetal thymus-derived gamma delta T cells at the infected site

Intraperitoneal infection of mice with Escherichia coli induced activated TCR gamma delta T cells in the peritoneal cavity. We provide evidence that the E. coli-induced gamma delta T cells are derived only from the fetal thymus on the following grounds. The gamma delta T cells were not induced in athymic nude mice and irradiated bone marrow-transferred mice which lack fetal thymus-derived T cells. However, E. coli infection of fetal thymus-grafted nude mice did induce fetal thymus-derived gamma delta T cells. These results suggest that the fetal thymus-derived gamma delta T cells colonize the periphery during early ontogeny, and are maintained until adult age. The E. coli-induced gamma delta T cells express only the Vdelta1 gene. Vgamma6 was predominantly expressed whereas anti-Vgamma1 and anti-Vgamma4 monoclonal antibodies stained less than 3 % of the cells. Direct sequencing of PCR products revealed that Vgamma6 and Vdelta1 genes expressed by the E. coli-induced gamma delta T cells were invariant sequences identical to those expressed in the fetal thymus. The antigen (Ag) specificity of a T cell hybridoma expressing the fetal type Vgamma6 / Vdelta1(+) TCR could not be identified as the cells failed to respond to lipopolysaccharide, E. coli Ag, mycobacterial heat shock protein 65, or isopentenyl pyrophosphate. These results suggest that the Vgamma6 / Vdelta1(+) gamma delta T cells derived from fetal thymus can participate in immune responses against bacterial infection through recognition of a novel class of Ag which is not yet identified.