H2-M3 major histocompatibility complex class Ib-restricted CD8 T cells induced by Salmonella enterica serovar Typhimurium infection recognize proteins released by Salmonella serovar Typhimurium

The role of MHC class Ib-restricted T cells during infection

Even though major histocompatibility complex (MHC) class Ia and many Ib molecules have similarities in structure, MHC class Ib molecules tend to have more specialized functions, which include the presentation of non-peptidic antigens to non-classical T cells. Likewise, non-classical T cells also have unique characteristics, including an innate-like phenotype in naïve animals and rapid effector functions. In this review, we discuss the role of MAIT and NKT cells during infection but also the contribution of less studied MHC class Ib-restricted T cells such as Qa-1-, Qa-2-, and M3-restricted T cells. We focus on describing the types of antigens presented to non-classical T cells, their response and cytokine profile following infection, as well as the overall impact of these T cells to the immune system.

Adaptive Immune Responses during Salmonella Infection

The interaction betweenSalmonella and its host is complex and dynamic: the host mounts an immune defense against the pathogen, which in turn acts to reduce, evade, or exploit these responses to successfully colonize the host. Although the exact mechanisms mediating protective immunity are poorly understood, it is known that T cells are a critical component of immunity to Salmonella infection, and a robust T-cell response is required for both clearance of primary infection and resistance to subsequent challenge. B-cell functions, including but not limited to antibody production, are also required for generation of protective immunity. Additionally, interactions among host cells are essential. For example, antigen-presenting cells (including B cells) express cytokines that participate in CD4+ T cell activation and differentiation. Differentiated CD4+ T cells secrete cytokines that have both autocrine and paracrine functions, including recruitment and activation of phagocytes, and stimulation of B cell isotype class switching and affinity maturation. Multiple bacterium-directed mechanisms, including altered antigen expression and bioavailability and interference with antigen-presenting cell activation and function, combine to modify Salmonella's "pathogenic signature" in order to minimize its susceptibility to host immune surveillance. Therefore, a more complete understanding of adaptive immune responses may provide insights into pathogenic bacterial functions. Continued identification of adaptive immune targets will guide rational vaccine development, provide insights into host functions required to resist Salmonella infection, and correspondingly provide valuable reagents for defining the critical pathogenic capabilities of Salmonella that contribute to their success in causing acute and chronic infections.

Salmonella induces PD-L1 expression in B cells

Salmonella persists for a long time in B cells; however, the mechanism(s) through which infected B cells avoid effector CD8 T cell responses has not been characterized. In this study, we show that Salmonella infects and survives within all B1 and B2 cell subpopulations. B cells are infected with a Salmonella typhimurium strain expressing an ovalbumin (OVA) peptide (SIINFEKL) to evaluate whether B cells process and present Salmonella antigens in the context of MHC-I molecules. Our data showed that OVA peptides are presented by MHC class I K(b)-restricted molecules and the presented antigen is generated through proteasomal degradation and vacuolar processing. In addition, Salmonella-infected B cells express co-stimulatory molecules such as CD40, CD80, and CD86 as well as inhibitory molecules such as PD-L1. Thus, the cross-presentation of Salmonella antigens and the expression of activation molecules suggest that infected B cells are able to prime and activate specific CD8(+) T cells. However, the Salmonella infection-stimulated expression of PD-L1 suggests that the PD-1/PD-L1 pathway may be involved in turning off the cytotoxic effector response during Salmonella persistent infection, thereby allowing B cells to become a reservoir for the bacteria.

Salmonella modulates B cell biology to evade CD8(+) T cell-mediated immune responses

Although B cells and antibodies are the central effectors of humoral immunity, B cells can also produce and secrete cytokines and present antigen to helper T cells. The uptake of antigen is mainly mediated by endocytosis; thus, antigens are often presented by MHC-II molecules. However, it is unclear if B cells can present these same antigens via MHC-I molecules. Recently, Salmonella bacteria were found to infect B cells, allowing possible antigen cross-processing that could generate bacterial peptides for antigen presentation via MHC-I molecules. Here, we will discuss available knowledge regarding Salmonella antigen presentation by infected B cell MHC-I molecules and subsequent inhibitory effects on CD8(+) T cells for bacterial evasion of cell-mediated immunity.

MHC class-I-restricted CD8 T cells play a protective role during primary Salmonella infection

Protective immunity against Salmonella infection is known to require CD4 Th1 cells and B cells, but the role of MHC class-I-restricted CD8 T cells is less clear. Previous studies have suggested that CD8 T cells participate in secondary, but not primary, bacterial clearance. However, these studies have used experimental models that are difficult to interpret and do not clearly isolate the role of MHC class-I-restricted CD8 T cells from other cell populations. Here, we examined the role of class-I-restricted T cells in protection against Salmonella infection using mice lacking all classical MHC class-Ia molecules, perforin, or granzyme B. Immunized K(b)D(b)-, perforin-, granzyme B-, or perforin/granzyme B-deficient mice were able to resolve secondary infection with virulent Salmonella, demonstrating that class-I-restricted CTLs are not required for acquired immunity. However, during primary infection with attenuated bacteria, bacterial clearance was delayed in each of these mouse strains when compared to wild-type mice. Taken together, these data demonstrate that CD8 T cells are not required for acquired immunity to Salmonella, but can play a protective role in resolving primary infection with attenuated bacteria.

An MHC class Ib-restricted CD8+ T cell response to lymphocytic choriomeningitis virus

Conventional MHC class Ia-restricted CD8(+) T cells play a dominant role in the host response to virus infections, but recent studies indicate that T cells with specificity for nonclassical MHC class Ib molecules may also participate in host defense. To investigate the potential role of class Ib molecules in anti-viral immune responses, K(b-/-)D(b-/-)CIITA(-/-) mice lacking expression of MHC class Ia and class II molecules were infected with lymphocytic choriomeningitis virus (LCMV). These animals have a large class Ib-selected CD8(+) T cell population and they were observed to mediate partial (but incomplete) virus clearance during acute LCMV infection as compared with K(b-/-)D(b-/-)β(2)-microglobulin(-/-) mice that lack expression of both MHC class Ia and class Ib molecules. Infection was associated with expansion of splenic CD8(+) T cells and induction of granzyme B and IFN-γ effector molecules in CD8(+) T cells. Partial virus clearance was dependent on CD8(+) cells. In vitro T cell restimulation assays demonstrated induction of a population of β(2)-microglobulin-dependent, MHC class Ib-restricted CD8(+) T cells with specificity for viral Ags and yet to be defined nonclassical MHC molecules. MHC class Ib-restricted CD8(+) T cell responses were also observed after infection of K(b-/-)D(b-/-)mice despite the low number of CD8(+) T cells in these animals. Long-term infection studies demonstrated chronic infection and gradual depletion of CD8(+) T cells in K(b-/-)D(b-/-)CIITA(-/-) mice, demonstrating that class Ia molecules are required for viral clearance. These findings demonstrate that class Ib-restricted CD8(+) T cells have the potential to participate in the host immune response to LCMV.

Nonconventional CD8+ T cell responses to Listeria infection in mice lacking MHC class Ia and H2-M3

CD8(+) T cells restricted to MHC class Ib molecules other than H2-M3 have been shown to recognize bacterial Ags. However, the contribution of these T cells to immune responses against bacterial infection is not well defined. To investigate the immune potential of MHC class Ib-restricted CD8(+) T cells, we have generated mice that lack both MHC class Ia and H2-M3 molecules (K(b-/-)D (b-/-)M3(-/-)). The CD8(+) T cells present in K(b-/-)D (b-/-)M3(-/-) mice display an activated surface phenotype and are able to secrete IFN-γ rapidly upon anti-CD3 and anti-CD28 stimulation. Although the CD8(+) T cell population is reduced in K(b-/-)D (b-/-)M3(-/-) mice compared with that in K(b-/-)D (b-/-) mice, this population retains the capacity to expand significantly in response to primary infection with the bacteria Listeria monocytogenes. However, K(b-/-)D (b-/-)M3(-/-) CD8(+) T cells do not expand upon secondary infection, similar to what has been observed for H2-M3-restricted T cells. CD8(+) T cells isolated from Listeria-infected K(b-/-)D (b-/-)M3(-/-) mice exhibit cytotoxicity and secrete proinflammatory cytokines in response to Listeria-infected APCs. These T cells are protective against primary Listeria infection, as Listeria-infected K(b-/-)D (b-/-)M3(-/-) mice exhibit reduced bacterial burden compared with that of infected β(2)-microglobulin-deficient mice that lack MHC class Ib-restricted CD8(+) T cells altogether. In addition, adoptive transfer of Listeria-experienced K(b-/-)D (b-/-)M3(-/-) splenocytes protects recipient mice against subsequent Listeria infection in a CD8(+) T cell-dependent manner. These data demonstrate that other MHC class Ib-restricted CD8(+) T cells, in addition to H2-M3-restricted T cells, contribute to antilisterial immunity and may contribute to immune responses against other intracellular bacteria.

Body traffic: ecology, genetics, and immunity in inflammatory bowel disease

The abundant bacteria and other microbial residents of the human intestine play important roles in nutrient absorption, energy metabolism, and defense against microbial pathogens. The mutually beneficial relationship of host and commensal microbiota represents an ancient and major coevolution in composition and mutual regulation of the human mucosa and the resident microbial community. Inflammatory bowel disease (IBD) is a set of chronic, relapsing inflammatory intestinal diseases in which rules of normal host-microbial interaction have been violated. This review considers the components of this host-microbial mutualism and the ways in which it is undermined by pathogenic microbial traits and by host immune and epithelial functions that confer to them susceptibility in patients with IBD. Recent advances in understanding the genetics of IBD and the immunology of host-microbial interaction are opening new strategies for treatments that target host susceptibility, candidate microbial pathogens, and intestinal ecology.

CD8+ T cell protective immunity against Chlamydia pneumoniae includes an H2-M3-restricted response that is largely CD4+ T cell-independent

CD8+ T cells are important for immunity to the intracellular bacterial pathogen Chlamydia pneumoniae (Cpn). Recently, we reported that type 1 CD8+ (Tc1) from Cpn-infected B6 mice recognize peptides from multiple Cpn Ags in a classical MHC class Ia-restricted fashion. In this study, we show that Cpn infection also induces nonclassical MHC class Ib-(H2-M3)-restricted CD8+ T cell responses. H2-M3-binding peptides representing the N-terminal formylated sequences from five Cpn Ags sensitized target cells for lysis by cytolytic effectors from the spleens of infected B6 mice. Of these, only peptides fMFFAPL (P1) and fMLYWFL (P4) stimulated IFN-gamma production by infection-primed splenic and pulmonary CD8+ T cells. Studies with Cpn-infected Kb-/-/Db-/- mice confirmed the Tc1 cytokine profile of P1- and P4-specific CD8+ T cells and revealed the capacity of these effectors to exert in vitro H2-M3-restricted lysis of Cpn-infected macrophages and in vivo pulmonary killing of P1- and P4-coated splenocytes. Furthermore, adoptive transfer of P1- and P4-specific CD8+ T cells into naive Kb-/-/Db-/- mice reduced lung Cpn loads following challenge. Finally, we show that in the absence of MHC class Ia-restricted CD8+ T cell responses, CD4+ T cells are largely expendable for the control of Cpn growth, and for the generation, memory maintenance, and secondary expansion of P1- and P4-specific CD8+ T cells. These results suggest that H2-M3-restricted CD8+ T cells contribute to protective immunity against Cpn, and that chlamydial Ags presented by MHC class Ib molecules may represent novel targets for inclusion in anti-Cpn vaccines.

A structural perspective on MHC class Ib molecules in adaptive immunity

The highly polymorphic MHC class Ia molecules have a central role in adaptive immunity. By contrast, the closely related MHC class Ib molecules, which show limited polymorphism, are best known for regulating innate immune responses. Nevertheless, a recent area of interest is the emerging role of class Ib molecules in adaptive immunity, particularly in response to tumours and pathogens such as Mycobacteria, Listeria and Salmonella. Here, we review recent findings in this area, highlighting the structure of a T-cell receptor complexed with a cytomegalovirus peptide bound to the class Ib molecule, HLA-E. Collectively, these findings have implications for immunity, transplantation and autoimmunity, and our understanding of the evolution and plasticity of the molecular interactions mediating adaptive immunity.

H2–M3-restricted T cell response to infection

H2-M3 is a major histocompatibility complex class Ib molecule that presents N-formylated peptides to specific CD8+ T cells. Prokaryotic, but not eukaryotic, translation begins with the addition of N-formyl methionine, suggesting a role for these H2-M3-restricted T cells in response to bacterial infection. Indeed, these cells constitute a non-redundant "early" component of anti-microbial response.