H2-M3 restricted presentation of a Listeria-derived leader peptide

Innate and Adaptive Immune Responses during Listeria monocytogenes Infection

It could be argued that we understand the immune response to infection with Listeria monocytogenes better than the immunity elicited by any other bacteria. L. monocytogenes are Gram-positive bacteria that are genetically tractable and easy to cultivate in vitro, and the mouse model of intravenous (i.v.) inoculation is highly reproducible. For these reasons, immunologists frequently use the mouse model of systemic listeriosis to dissect the mechanisms used by mammalian hosts to recognize and respond to infection. This article provides an overview of what we have learned over the past few decades and is divided into three sections: "Innate Immunity" describes how the host initially detects the presence of L. monocytogenes and characterizes the soluble and cellular responses that occur during the first few days postinfection; "Adaptive Immunity" discusses the exquisitely specific T cell response that mediates complete clearance of infection and immunological memory; "Use of Attenuated Listeria as a Vaccine Vector" highlights the ways that investigators have exploited our extensive knowledge of anti-Listeria immunity to develop cancer therapeutics.

Listeria Monocytogenes: A Model Pathogen Continues to Refine Our Knowledge of the CD8 T Cell Response

Listeria monocytogenes (Lm) infection induces robust CD8 T cell responses, which play a critical role in resolving Lm during primary infection and provide protective immunity to re-infections. Comprehensive studies have been conducted to delineate the CD8 T cell response after Lm infection. In this review, the generation of the CD8 T cell response to Lm infection will be discussed. The role of dendritic cell subsets in acquiring and presenting Lm antigens to CD8 T cells and the events that occur during T cell priming and activation will be addressed. CD8 T cell expansion, differentiation and contraction as well as the signals that regulate these processes during Lm infection will be explored. Finally, the formation of memory CD8 T cell subsets in the circulation and in the intestine will be analyzed. Recently, the study of CD8 T cell responses to Lm infection has begun to shift focus from the intravenous infection model to a natural oral infection model as the humanized mouse and murinized Lm have become readily available. Recent findings in the generation of CD8 T cell responses to oral infection using murinized Lm will be explored throughout the review. Finally, CD8 T cell-mediated protective immunity against Lm infection and the use of Lm as a vaccine vector for cancer immunotherapy will be highlighted. Overall, this review will provide detailed knowledge on the biology of CD8 T cell responses after Lm infection that may shed light on improving rational vaccine design.

The expanding role of murine class Ib MHC in the development and activation of Natural Killer cells

Major Histocompatibility Complex-I (MHC-I) molecules can be divided into class Ia and class Ib, with three distinct class Ib families found in the mouse. These families are designated as Q, T and M and are largely unexplored in terms of their immunological function. Among the class Ib MHC, H2-T23 (Qa-1b) has been a significant target for Natural Killer (NK) cell research, owing to its homology with the human class Ib human leukocyte antigen (HLA)-E. However, recent data has indicated that members of the Q and M family of class Ib MHC also play a critical role in the development and regulation NK cells. Here we discuss the recent advances in the control of NK cells by murine class Ib MHC as a means to stimulate further exploration of these molecules.

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.

Role of MHC class Ib molecule, H2-M3 in host immunity against tuberculosis

The MHC class I family comprises both classical (class Ia) and non-classical (class Ib) members. While the prime function of classical MHC class I molecules (MHC class Ia) is to present peptide antigens to pathogen-specific cytotoxic T cells, non-classical MHC-I (MHC class Ib) antigens perform diverse array of functions in both innate and adaptive immunity. Vaccines against intracellular pathogens such as Mycobacterium tuberculosis need to induce strong cellular immune responses. Recent studies have shown that MHC class I molecules play an important role in the protective immune response to M. tuberculosis infection. Both MHC Ia-restricted and MHC class Ib-restricted M. tuberculosis -reactive CD8(+) T cells have been identified in humans and mice, but their relative contributions to immunity is still uncertain. Unlike MHC class Ia-restricted CD8(+) T cells, MHC class Ib-restricted CD8(+) T cells are constitutively activated in naive animals and respond rapidly to infection challenge, hence filling the temporal gap between innate and adaptive immunity. The present review article summarizes the general host immunity against M. tuberculosis infection highlighting the possible role of MHC class Ib molecule, H2-M3 and their ligands (N-formylated peptides) in protection against tuberculosis.

Positive selecting cell type determines the phenotype of MHC class Ib-restricted CD8+ T cells

Several studies have demonstrated an apparent link between positive selection on hematopoietic cells (HCs) and an "innate" T-cell phenotype. Whereas conventional CD8(+) T cells are primarily selected on thymic epithelial cells (TECs) and certain innate T cells are exclusively selected on HCs, MHC class Ib-restricted CD8(+) T cells appear to be selected on both TECs and HCs. However, whether TEC- and HC-selected T cells represent distinct lineages or whether the same T-cell precursors have the capacity to be selected on either cell type is unknown. Using an M3-restricted T-cell receptor transgenic mouse model, we demonstrate that not only are MHC class Ib-restricted CD8(+) T cells capable of being selected on either cell type but that selecting cell type directly affects the phenotype of the resulting CD8(+) T cells. M3-restricted CD8(+) T cells selected on HCs acquire a more activated phenotype and possess more potent effector functions than those selected on TECs. Additionally, these two developmental pathways are active in the generation of the natural pool of M3-restricted CD8(+) T cells. Our results suggest that these two distinct populations may allow MHC class Ib-restricted CD8(+) T cells to occupy different immunological niches playing unique roles in immune responses to infection.

Genome-wide analyses reveal lineage specific contributions of positive selection and recombination to the evolution of Listeria monocytogenes

Background

The genus Listeria includes two closely related pathogenic and non-pathogenic species, L. monocytogenes and L. innocua. L. monocytogenes is an opportunistic human foodborne and animal pathogen that includes two common lineages. While lineage I is more commonly found among human listeriosis cases, lineage II appears to be overrepresented among isolates from foods and environmental sources. This study used the genome sequences for one L. innocua strain and four L. monocytogenes strains representing lineages I and II, to characterize the contributions of positive selection and recombination to the evolution of the L. innocua/L. monocytogenes core genome.

Results

Among the 2267 genes in the L. monocytogenes/L. innocua core genome, 1097 genes showed evidence for recombination and 36 genes showed evidence for positive selection. Positive selection was strongly associated with recombination. Specifically, 29 of the 36 genes under positive selection also showed evidence for recombination. Recombination was more common among isolates in lineage II than lineage I; this trend was confirmed by sequencing five genes in a larger isolate set. Positive selection was more abundant in the ancestral branch of lineage II (20 genes) as compared to the ancestral branch of lineage I (9 genes). Additional genes under positive selection were identified in the branch separating the two species; for this branch, genes in the role category "Cell wall and membrane biogenesis" were significantly more likely to have evidence for positive selection. Positive selection of three genes was confirmed in a larger isolate set, which also revealed occurrence of multiple premature stop codons in one positively selected gene involved in flagellar motility (flaR).

Conclusion

While recombination and positive selection both contribute to evolution of L. monocytogenes, the relative contributions of these evolutionary forces seem to differ by L. monocytogenes lineages and appear to be more important in the evolution of lineage II, which seems to be found in a broader range of environments, as compared to the apparently more host adapted lineage I. Diversification of cell wall and membrane biogenesis and motility-related genes may play a particularly important role in the evolution of L. monocytogenes.

Identification of formyl peptides from Listeria monocytogenes and Staphylococcus aureus as potent chemoattractants for mouse neutrophils

The prototypic formyl peptide N-formyl-Met-Leu-Phe (fMLF) is a major chemoattractant found in Escherichia coli culture supernatants and a potent agonist at human formyl peptide receptor (FPR) 1. Consistent with this, fMLF induces bactericidal functions in human neutrophils at nanomolar concentrations. However, it is a much less potent agonist for mouse FPR (mFPR) 1 and mouse neutrophils, requiring micromolar concentrations for cell activation. To determine whether other bacteria produce more potent agonists for mFPR1, we examined formyl peptides from Listeria monocytogenes and Staphylococcus aureus for their abilities to activate mouse neutrophils. A pentapeptide (N-formyl-Met-Ile-Val-Ile-Leu (fMIVIL)) from L. monocytogenes and a tetrapeptide (N-formyl-Met-Ile-Phe-Leu (fMIFL)) from S. aureus were found to induce mouse neutrophil chemotaxis at 1-10 nM and superoxide production at 10-100 nM, similar to the potency of fMLF on human neutrophils. Using transfected cell lines expressing mFPR1 and mFPR2, which are major forms of FPRs in mouse neutrophils, we found that mFPR1 is responsible for the high potency of fMIVIL and fMIFL. In comparison, activation of mFPR2 requires micromolar concentrations of the two peptides. Genetic deletion of mfpr1 resulted in abrogation of neutrophil superoxide production and degranulation in response to fMIVIL and fMIFL, further demonstrating that mFPR1 is the primary receptor for detection of these formyl peptides. In conclusion, the formyl peptides from L. monocytogenes and S. aureus are approximately 100-fold more potent than fMLF in activating mouse neutrophils. The ability of mFPR1 to detect bacterially derived formyl peptides indicates that this important host defense mechanism is conserved in mice.

Animal model for infection with Listeria monocytogenes

This unit describes methods for infecting mice with L. monocytogenes. Optimal media for growth and methods to maintain bacterial virulence by passage through mice are included. Methods for determining the severity of splenic and hepatic infection are detailed, with strategies for distinguishing innate from specific immune responses following L. monocytogenes infection. This infection induces MHC class I-restricted CD8(+) cytolytic T lymphocytes that clear infection and provide long-term immunity. This unit describes methods that can be used for in vitro expansion of L. monocytogenes-specific T cells.

Axonal transport of Listeria monocytogenes and nerve-cell-induced bacterial killing

Listeria monocytogenes (L. monocytogenes) can cause fatal brainstem encephalitis in both sheep and humans. Here we review evidence that the bacteria can be incorporated into axons following a primary cycle of replication in macrophages/dendritic cells after subcutaneous injection in projection areas of peripheral neurons. The molecular mechanisms for the rocketing of L. monocytogenes in the cytosol by asymmetric cometic tails and the utility of this phenomenon for bacterial migration intraaxonally both in retro- and in anterograde directions to reach the central nervous system are described. The role of the immune response in the control of L. monocytogenes spread through peripheral neurons is highlighted, and a mechanism by which bacteria may be killed inside infected neurons through a nitric oxide-dependent pathway is pointed out.

Use of the CD107 mobilization assay reveals that cytotoxic T lymphocytes with novel MHC-Ib restriction are activated during Listeria monocytogenes infection

Detection of cytotoxic activity by pathogen-specific T cells of unknown antigenic specificity is difficult due to the limitations of using infected cells, instead of peptide-pulsed cells, as targets. We report here that the recently described CD107 mobilization assay readily allowed for the ex vivo detection of cytotoxic T lymphocytes (CTL) with a novel MHC-Ib restriction that specifically recognized Listeria monocytogenes-infected macrophages. The CD107 mobilization assay is likely to be a useful tool for detection of CD8(+) T cells that recognize a wide variety of intracellular pathogens.

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.

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.

Impaired response to Listeria in H2-M3-deficient mice reveals a nonredundant role of MHC class Ib-specific T cells in host defense

The major histocompatibility complex (MHC) class Ib molecule H2-M3 primes the rapid expansion of CD8+ T cells by presenting N-formylated bacterial peptides. However, the significance of H2-M3-restricted T cells in host defense against bacteria is unclear. We generated H2-M3-deficient mice to investigate the role of H2-M3 in immunity against Listeria monocytogenes (LM), a model intracellular bacterial pathogen. H2-M3-deficient mice are impaired in early bacterial clearance during primary infection, with diminished LM-specific CD8+ T cell responses and compromised innate immune functions. Although H2-M3-restricted CD8+ T cells constitute a significant proportion of the anti-listerial CD8+ T cell repertoire, the kinetics and magnitude of MHC class Ia-restricted T cell responses are not altered in H2-M3-deficient mice. The fact that MHC class Ia-restricted responses cannot compensate for the H2-M3-mediated immunity suggests a nonredundant role of H2-M3 in the protective immunity against LM. Thus, the early H2-M3-restricted response temporally bridges the gap between innate and adaptive immune responses, subsequently affecting the function of both branches of the immune system.

H2-M3-restricted CD8+ T cells are not required for MHC class Ib-restricted immunity against Listeria monocytogenes

Studies using major histocompatibility complex (MHC)-Ia-deficient mice have shown that MHC-Ib-restricted CD8+ T cells can clear infections caused by intracellular pathogens such as Listeria monocytogenes. M3-restricted CD8+ T cells, which recognize short hydrophobic N-formylated peptides, appear to comprise a substantial portion of the MHC-Ib-restricted T cell response in the mouse model of L. monocytogenes infection. In this study, we isolated formyltransferase (fmt) mutant strains of L. monocytogenes that lacked the ability to add formyl groups to nascent polypeptides. These fmt mutant Listeria strains did not produce antigens that could be recognized by M3-restricted T cells. We showed that immunization of MHC-Ia-deficient mice with fmt mutant Listeria resulted in stimulation of a protective memory response that cleared subsequent challenge with wild-type L. monocytogenes, despite the fact that M3-restricted CD8+ T cells did not proliferate in these mice. These data suggest that M3-restricted T cells are not required for protection against L. monocytogenes and underscore the importance of searching for new antigen-presenting molecules among the large MHC-Ib family of proteins.

Human mitochondria-derived N-formylated peptides are novel agonists equally active on FPR and FPRL1, while Listeria monocytogenes-derived peptides preferentially activate FPR

N-formyl peptides are cleavage products of bacterial and mitochondrial proteins, and can attract leukocytes to sites of infection or tissue damage. In this study, HL-60 cell lines expressing the human N-formyl peptide receptor FPR or its two homologues (FPRL1, FPRL2) were used to determine the receptor selectivity of N-formylated peptides derived from Listeria monocytogenes or from human mitochondrial proteins. Bacterial peptides were 100-fold more potent on FPR than on FPRL1, whereas none of them could trigger intracellular signaling through FPRL2. In contrast, N-formylated hexapeptides corresponding to the N terminus of mitochondrial NADH dehydrogenase subunits 4 (fMLKLIV) and 6 (fMMYALF), and cytochrome c oxidase subunit I (fMFADRW) were equally potent on FPR and FPRL1. They triggered cellular responses with the following order of potency: fMMYALF > fMLKLIV > fMFADRW, with an EC50, in a Fura-2 calcium mobilization assay, of 10 nM, 44 nM, and 160 nM on FPR-expressing cells, and 15 nM, 55 nM and 120 nM on FPRL1-expressing cells. fMMYALF was also a low-affinity agonist of FPRL2 (EC50 of 1 microM) and was chemotactic for both FPRL1- and FPRL2-expressing cells. We identified novel mitochondrial host-derived agonists for human N-formyl-peptide receptors that might play a role in inflammatory or degenerative processes linked to their stimulation.

Cross-recognition of N-formylmethionine peptides is a general characteristic of H2-M3-restricted CD8+ T cells

H2-M3-restricted CD8+ T cells can exhibit cross-reactivity to different bacterially derived N-formylmethionine peptides. The extent of this promiscuity is unclear. We deleted the nonredundant fMIVTLF epitope and found that Listeria monocytogenes still primed fMIVTLF-specific T cells. Thus, cross-reactivity appears to be a more general characteristic of H2-M3-restricted T cells.

Maturational stage-dependent thymocyte responses to TCR engagement

Thymocyte positive and negative selection are dependent on avidity-driven TCR-mediated recognition events in the thymus. High-avidity recognition events result in negative selection, while low-avidity recognition events result in positive selection. However, it has not been established how thymocytes maturation stages affect their responses to TCR signals of different avidities. We gained insight into this question when we reduced thymocyte selection to an in vitro system, in which full maturation of developmentally synchronized immature double-positive thymocytes was induced on a cloned line of thymic epithelial cells. Our analysis of the kinetics of thymocyte development supports a multi-phasic model of thymic selection. In it, thymocyte maturation stages as well as interaction avidity control the outcome TCR stimulation. Positive selection is initiated during a primary recognition event that proceeds independently of the TCR avidity. During a secondary recognition event the final fate of thymocyte, full maturation versus negative selection, is determined by TCR avidity.

Immune responses to Listeria monocytogenes

Listeria monocytogenes is a Gram-positive bacterium that is often used to study the mammalian immune response to infection because it is easy to culture, is relatively safe to work with and causes a highly predictable infection in laboratory mice. The broad application of this mouse model has resulted in a torrent of studies characterizing the contributions of different cytokines, receptors, adaptors and effector molecules to resistance against infection with Listeria monocytogenes. These studies, which are yielding one of the most comprehensive pictures of the 'battle' between host and microorganism, are reviewed here.

Promiscuity of MHC class Ib-restricted T cell responses

Murine infection with the Gram-positive intracellular bacterium Listeria monocytogenes activates CD8(+) T cells that recognize bacterially derived N-formyl methionine peptides in the context of H2-M3 MHC class Ib molecules. Three peptides, fMIGWII, fMIVIL, and fMIVTLF, are targets of L. monocytogenes-specific CD8(+) T cells. To investigate epitope cross-recognition by H2-M3-restricted CD8(+) T cells, we deleted the sequence encoding fMIGWII from a virulent strain of L. monocytogenes. Infection with fMIGWII-deficient L. monocytogenes unexpectedly primed CD8(+) T cells that stain with fMIGWII/H2-M3 tetramers and lyse fMIGWII-coated target cells in vivo. Because the fMIGWII sequence is nonredundant, we speculated that other bacterially derived Ags are priming these responses. HPLC peptide fractionation of bacterial culture supernatants revealed several distinct L. monocytogenes-derived peptides that are recognized by fMIGWII-specific T cells. Our results demonstrate that the dominant H2-M3-restricted CD8(+) T cell population, although reactive with fMIGWII, is primed by other, non-fMIGWII peptides derived from L. monocytogenes. Although this degree of Ag receptor promiscuity is unusual for the adaptive immune system, it may be a more common feature of T cell responses restricted by nonpolymorphic MHC class Ib molecules.

The Listeria monocytogenes lemA gene product is not required for intracellular infection or to activate fMIGWII-specific T cells

Clearance of the intracellular bacterial pathogen Listeria monocytogenes requires antigen-specific CD8(+) T cells. Recently it was shown that activation of class Ib major histocompatibility complex (MHC)-restricted CD8(+) T cells alone is sufficient for immune protection against listeriae. A major component of the class Ib MHC-restricted T-cell response is T cells that recognize formylated peptide antigens presented by M3 molecules. Although three N-formylated peptides derived from L. monocytogenes are known to bind to M3 molecules, fMIGWII is the immunodominant epitope presented by M3 during infection of mice. The source of fMIGWII peptide is the L. monocytogenes lemA gene, which encodes a 30-kDa protein of unknown function. In this report, we describe the generation of two L. monocytogenes lemA deletion mutants. We show that lemA is not required for growth of listeriae in tissue culture cells or for virulence during infection of mice. Surprisingly, we found that fMIGWII-specific T cells were still primed following infection with lemA mutant listeriae, suggesting that L. monocytogenes contains at least one additional antigen that is cross-reactive with the fMIGWII epitope. This cross-reactive antigen appears to be a small protease-resistant molecule that is secreted by L. monocytogenes.

CD8 T cell responses to infectious pathogens

CD8 T cells respond to viral infections but also participate in defense against bacterial and protozoal infections. In the last few years, as new methods to accurately quantify and characterize pathogen-specific CD8 T cells have become available, our understanding of in vivo T cell responses has increased dramatically. Pathogen-specific T cells, once thought to be quite rare following infection, are now known to be present at very high frequencies, particularly in peripheral, nonlymphoid tissues. With the ability to visualize in vivo CD8 T cell responses has come the recognition that T cell expansion is programmed and, to a great extent, independent of antigen concentrations. Comparison of CD8 T cell responses to different pathogens also highlights the intricate relationship between microbially induced innate inflammatory responses and the kinetics, magnitude, and character of long-term T cell responses. This review describes recent progress in some of the major murine models of CD8 T cell-mediated immunity to viral, bacterial, and protozoal infection.

Class Ia MHC-deficient BALB/c mice generate CD8+ T cell-mediated protective immunity against Listeria monocytogenes infection

CD8(+) T cells are required for protective immunity against intracellular pathogens such as Listeria monocytogenes. In this study, we used class Ia MHC-deficient mice, which have a severe reduction in circulating CD8(+) T cells, to determine the protective capacity of class Ib MHC-restricted T cells during L. monocytogenes infection. The K(b-/-)D(b-/-) mutation was backcrossed onto a C.B10 (BALB/c congenic at H-2 locus with C57BL/10) background, because BALB/c mice are more susceptible to Listeria infection than other commonly studied mouse strains such as C57BL/6. C.B10 K(b-/-)D(b-/-) mice immunized with a sublethal dose of L. monocytogenes were fully protected against a subsequent lethal infection. Adoptive transfer of Listeria-immune splenocyte subsets into naive K(b-/-)D(b-/-) mice indicated that CD8(+) T cells were the major component of this protective immune response. A CD8(+) T cell line isolated from the spleen of a Listeria-infected class Ia MHC-deficient mouse was shown to specifically recognize Listeria-infected cells in vitro, as determined by IFN-gamma secretion and cytotoxicity assays. Adoptive transfer of this T cell line alone resulted in significant protection against L. monocytogenes challenge. These results suggest that even a limited number of class Ib MHC-restricted T cells are sufficient to generate the rapid recall response required for protection against secondary infection with L. monocytogenes.

Recombinant lemA without adjuvant induces extensive expansion of H2-M3-restricted CD8 effectors, which can suppress primary listeriosis in mice

Mice infected with Listeria monocytogenes (LM) produce large numbers of H2-M3-restricted CD8 T cells directed against the formylated peptides, f-MIGWII and f-MIVIL. To examine responsiveness to these epitopes in the absence of infection, we inoculated mice with recombinant lemA (r-lemA) containing f-MIGWII or r-vemA (a variant of r-lemA containing f-MIVIL in place of f-MIGWII) without adjuvant. To monitor responses, we measured peptide-specific cytoplasmic IFN-gamma production ex vivo by freshly harvested splenocytes at varying times post-inoculation. B6 mice inoculated with r-lemA produced substantial numbers of epitope-specific CD8 cells with peak levels on day 7 when there were 1.1 x 10(6) f-MIGWII-specific CD8 cells in the spleen (8.2% of total CD8 splenocytes). The r-vemA-treated animals accumulated 0.25 x 10(6) cells (1.8% of total CD8 cells) at this time point. Comparable responses were observed after rechallenge of immunized animals. Other elements in the lemA moiety distinct from the immunogenic peptide were required since mice did not respond to equimolar amounts of synthetic f-MIGWII or f-MIVIL alone. In comparative studies, B6 and C3H/HeJ mice responded to r-lemA much more vigorously than BALB/c animals. When r-lemA- or r-vemA-treated B6 animals were challenged i.v. with LM 7 days later, they suppressed splenic accumulation of bacteria much more effectively than controls. On the other hand, antigen-treated animals were not protected against infection 1 month later. Thus, responsive strains of mice respond vigorously to H2-M3-restricted epitopes, even in the absence of bacterial infection or adjuvant. The resulting effectors acutely enhance antimicrobial resistance but do not confer long-term memory protection.

Immunization with gp96 from Listeria monocytogenes-infected mice is due to N-formylated listerial peptides

N-Formylated (N-f-met) peptides derived from proteins of the intracellular bacterium Listeria monocytogenes generate a protective, H2-M3-restricted CD8 T cell response in C57BL/6 mice. N-f-met peptide-specific CTL were generated in vitro when mice previously immunized with gp96 isolated from donor mice infected with L. monocytogenes were stimulated with these peptides. No significant peptide-specific CTL activity was observed in mice immunized with gp96 from uninfected animals. Masses corresponding to one N-f-met peptide were found by matrix-assisted laser desorption/ionization-mass spectrometry on gp96 isolated from C57BL/6 mice infected with L. monocytogenes, but not on gp96 from noninfected mice. Therefore, bacterial N-f-met peptides from intracellular bacteria can bind to gp96 in the infected host, and gp96 loaded with these peptides can generate N-f-met-peptide-specific CTL. We assume a unique role of gp96 in Ag processing through the H2-M3 pathway.

Bacterial antigens elicit T cell responses via adaptive and transitional immune recognition

T cells are a critical component of host immune responses against bacterial pathogens. T cell activation relies on recognition of antigen(s) derived from the bacteria, and this activation triggers potent biological effector mechanisms. Therefore, the characterization of antigens that are stimulatory for T cells provides insight into host-pathogen interactions and advances rational vaccine design. The adaptive immune response is defined by its ability to detect variable or unique single-gene products, whereas a 'transitional' immune system recognizes more conserved structures or products of multigene pathways. This transitional system functionally overlaps the canonical innate and adaptive immune responses. Antigen identification has relied upon biochemistry, genetics and expression cloning strategies. The development of computational approaches, fuelled by advances in immunology and genomic information, will facilitate the discovery of antigens and expand our understanding of both beneficial and pathological immune responses.

Induction of M3-restricted cytotoxic T lymphocyte responses by N-formylated peptides derived from Mycobacterium tuberculosis

Major histocompatibility complex (MHC) class I-restricted CD8(+) T cells play a critical role in the protective immunity against Mycobacterium tuberculosis (Mtb). However, only a few Mtb peptides recognized by MHC class Ia-restricted CD8(+) T cells have been identified. Information on epitopes recognized by class Ib-restricted T cells is even more limited. M3 is an MHC class Ib molecule that preferentially presents N-formylated peptides to CD8(+) T cells. Because bacteria initiate protein synthesis with N-formyl methionine, the unique binding specificity of M3 makes it especially suitable for presenting these particular bacterial epitopes. We have scanned the full sequence of the Mtb genome for NH2-terminal peptides that share features with other M3-binding peptides. Synthetic peptides corresponding to these sequences were tested for their ability to bind to M3 in an immunofluorescence-based peptide-binding assay. Four of the N-formylated Mtb peptides were able to elicit cytotoxic T lymphocytes (CTLs) from mice immunized with peptide-coated splenocytes. The Mtb peptide-specific, M3-restricted CTLs lysed the Mtb-infected macrophages effectively, suggesting that these N-formylated Mtb peptides are presented as the naturally processed epitopes by Mtb-infected cells. Furthermore, T cells from Mtb-infected lungs, spleen, and lymph nodes responded to N-formylated Mtb peptides in an M3-restricted manner. Taken together, our data suggest that M3-restricted T cells may participate in the immune response to Mtb.

Lack of expansion of major histocompatibility complex class Ib-restricted effector cells following recovery from secondary infection with the intracellular pathogen Listeria monocytogenes

Sublethal infection of BALB/c mice with the intracellular bacterial pathogen Listeria monocytogenes leads to the development of antilisterial immunity with concurrent stimulation of major histocompatibility complex (MHC) class Ia- and Ib-restricted CD8+ effector T cells. Secondary L. monocytogenes infection is followed by an accelerated increase in the number of Listeria-specific CD8+ cells and rapid clearance of the bacterium from the murine host. Recovery from secondary infection is associated with increased levels of effector cell function, as measured by gamma interferon secretion following coculture of immune cells with L. monocytogenes infected APCs in vitro, as well as antilisterial cytotoxicity, as measured by effector cell recognition of L. monocytogenes-infected target cells. We assessed the frequency of L. monocytogenes-specific MHC class I-restricted cells following secondary infection by ELISPOT assays utilizing coculture of immune cells with L. monocytogenes-infected antigen-presenting cells that express MHC class Ia and/or Ib molecules. We found that the antilisterial Qa-1b (MHC class Ib)-restricted effector subset is not detected as an expanded population following secondary infection compared to the frequency of this effector population as measured following recovery from primary infection. This is in contrast to the frequency of antilisterial H2-Kd (MHC class Ia)-restricted effector cells, which following recovery from secondary infection are detected as an expanded population, and appears to undergo a substantial expansion event 3 to 4 days post-secondary infection. These results are consistent with the conclusion that although Listeria-specific MHC class Ib-restricted effector cells are present following recovery from secondary infection, this subset does not appear to undergo the expansion phase that is detected for the MHC class Ia-restricted effector cell response.

Identification of new genes involved in the virulence of Listeria monocytogenes by signature-tagged transposon mutagenesis

Listeria monocytogenes is a gram-positive, facultative intracellular pathogen that can cause severe food-born infections in humans and animals. We have adapted signature-tagged transposon mutagenesis to L. monocytogenes to identify new genes involved in virulence in the murine model of infection. We used transposon Tn1545 carried on the integrative vector pAT113. Forty-eight tagged transposons were constructed and used to generate banks of L. monocytogenes mutants. Pools of 48 mutants were assembled, taking one mutant from each bank, injected into mice, and screened for those affected in their multiplication in the brains of infected animals. From 2,000 mutants tested, 18 were attenuated in vivo. The insertions harbored by these mutants led to the identification of 10 distinct loci, 7 of which corresponded to previously unknown genes. The properties of four loci involving putative cell wall components were further studied in vitro and in vivo. The data suggested that these components are involved in bacterial invasion and multiplication in the brain.

Variable immunodominance hierarchies for H2-M3-restricted N-formyl peptides following bacterial infection

H2-M3-restricted presentation of N-formyl methionine (f-Met) peptides to CD8(+) T cells provides a mechanism for selective recognition of bacterial infection. In this report we demonstrate that Listeria monocytogenes infection induces distinct CD8(+) T cell populations specific for each of the known Listeria-derived formyl methionine peptides presented by M3. The sum H2-M3-restricted, Listeria-specific T cell response constitutes a major fraction of the total CD8(+) T cell response to primary infection. H2-M3-restricted T cell populations expand synchronously in vivo and achieve peak frequencies approximately 2 days earlier than MHC class Ia-restricted T cell populations. Although cross-recognition of different f-Met peptides by M3-restricted T cells was previously described, costaining of CD8(+) T cells ex vivo with H2-M3 tetramers complexed with different f-Met peptides shows that the majority of Listeria-specific, M3-restricted CD8(+) T cells are peptide specific. In contrast to the highly predictable size and immunodominance hierarchies of MHC class Ia-restricted T cell responses, the magnitudes of T cell responses specific for H2-M3-restricted peptides are remarkably variable between genetically identical mice. Our findings demonstrate that H2-M3-restricted T cell responses are distinct from classically restricted T cell responses to bacterial infection.

MHC class Ib-restricted CTL provide protection against primary and secondary Listeria monocytogenes infection

Infection of B6 mice with the intracellular pathogen Listeria monocytogenes (LM) results in the activation of CD8(+) T cells that respond to Ag presented by both MHC class Ia and class Ib molecules. Enzyme-linked immunospot analysis reveals that these CTL populations expand and contract at different times following a primary sublethal LM infection. Between days 4 and 6 postinfection, class Ib-restricted CTL exhibit a rapid proliferative response that is primarily H2-M3 restricted. The peak response of class Ia-restricted CD8(+) T cells occurs a few days later, after the majority of bacteria have been cleared. Although class Ia-restricted CTL exhibit a vigorous recall response to secondary LM infection, we observe limited expansion of class Ib-restricted memory CTL, even in MHC class Ia-deficient mice (B6.K(b-/-)D(b-/-)). Despite this lack of enhanced expansion in vivo, class Ib-restricted memory CTL retain the ability to proliferate and expand when provided with Ag in vitro. Furthermore, we demonstrate that in vivo depletion of CD8(+) T cells in LM-immune B6.K(b-/-)D(b-/-) mice severely impairs memory protection. Together, these data demonstrate that class Ib-restricted CTL play an important role in clearing a primary LM infection and generate a memory population capable of providing significant protection against subsequent infection.

A physiological ligand of positive selection is recognized as a weak agonist

Positive selection is a process that ensures that peripheral T cells express TCR that are self-MHC restricted. This process occurs in the thymus and requires both self-MHC and self-peptides. We have recently established a TCR transgenic (TCR(trans)(+)) mouse model using the C10.4 TCR restricted to the MHC class Ib molecule, H2-M3. Having defined H2-M3 as the positively selecting MHC molecule, the severely limited number of H2-M3 binding peptides allowed us to characterize a mitochondrial NADH dehydrogenase subunit 1-derived 9-mer peptide as the physiological ligand of positive selection. Here, we demonstrate that the NADH dehydrogenase subunit 1 self-peptide is seen by mature C10.4 TCR(trans)(+) T cells as a weak agonist and induces positive selection at a defined concentration range. We also found that the full-length cognate peptide, a strong agonist for mature C10.4 TCR(trans)(+) T cells, initiated positive selection, albeit at significantly lower concentrations. At increased peptide concentrations, and thus increased epitope densities, either peptide only induced the development of partially functional T cells. We conclude that successful positive selection only proceeded at a defined, yet fairly narrow window of avidity.

Partially TAP-independent protection against Listeria monocytogenes by H2-M3-restricted CD8+ T cells

Effective protection against Listeria monocytogenes requires Ag-specific CD8(+) T cells. A substantial proportion of CD8(+) T cells activated during L. monocytogenes infection of C57BL/6 mice are restricted by the MHC class Ib molecule H2-M3. In this study, an H2-M3-restricted CD8(+) T cell clone specific for a known H2-M3 epitope (fMIGWII) was generated from L. monocytogenes-infected mice. The clone was cytotoxic, produced IFN-gamma, and could mediate strong protection against L. monocytogenes when transferred to infected mice. Macrophages pulsed with heat-killed LISTERIAE: presented Ag to the clone in a TAP-independent manner. Both TAP-independent and -dependent processing occurred in vivo, as TAP-deficient mice infected with L. monocytogenes were partially protected by adoptive transfer of the clone. This is the first example of CD8(+) T cell-mediated, TAP-independent protection against a pathogen in vivo, confirming the importance of alternative MHC class I processing pathways in the antibacterial immunity.

Host immune response to intracellular bacteria: A role for MHC-linked class-Ib antigen-presenting molecules

MHC-linked class-Ib molecules are a subfamily of class-I molecules that display limited genetic polymorphism. At one time these molecules were considered to have an enigmatic function. However, recent studies have shown that MHC-linked class-Ib molecules can function as antigen presentation structures that bind bacteria-derived epitopes for recognition by CD8+ effector T cells. This role for class-Ib molecules has been demonstrated across broad classes of intracellular bacteria including Listeria moncytogenes, Salmonella typhimurium, and Mycobacterium tuberculosis. Additionally, evidence is emerging that MHC-linked class-Ib molecules also serve an integral role as recognition elements for NK cells as well as several TCR alpha/beta and TCR gamma/delta T-cell subsets. Thus, MHC-linked class-Ib molecules contribute to the host immune response by serving as antigen presentation molecules and recognition ligands in both the innate and adaptive immune response to infection. In this review, we will attempt to summarize the work that supports a role for MHC-linked class-Ib molecules in the host response to infection with intracellular bacteria.

Immunization with f-Met peptides induces immune reactivity against Mycobacterium tuberculosis

OBJECTIVE

To determine whether synthetic peptides containing an amino terminal formyl-methionine residue and corresponding to the sequence of several proteins produced by Mycobacterium tuberculosis, would elicit an immune response in mice.

DESIGN

Peptides corresponding to the amino termini of 8 M. tuberculosis proteins and initiating with formyl methionine residues were synthesized. The ability of these peptides to bind to the mouse non-classical MHC class I molecule H-2M3a was determined by flow microfluorimetry. These peptides were used to pulse dendritic cells that were then injected into normal mice. These mice were subsequently challenged with aerosolized M. tuberculosis and, 30 days later, the number of viable bacteria in the lungs was determined.

RESULTS

Four of the 8 synthetic peptides bound to H-2M3a and stabilized its expression on the cell surface. Injection of mice with dendritic cells pulsed with H-2M3a binding peptides elicited non-MHC restricted cytotoxic T lymphocytes that killed peptide pulsed target cells and macrophages infected with M. tuberculosis. Immunization of mice with syngeneic dendritic cells pulsed in vitro with 2 of these peptides led to retardation of the growth of M. tuberculosis following aerosol challenge.

CONCLUSION

Peptides that bind to non-polymorphic class I molecules can elicit immune reactivity directed towards M. tuberculosis.

Immunity to Listeria infection

Infection with Listeria monocytogenes is a well studied model for understanding host resistance to intracellular bacteria. Recent advances in the study of Listeria have carefully quantitated the response of CD8(+) T cells to infection and analyzed the effector functions of these cells in vivo. A surprising role for antibody in mediating resistance to Listeria has also recently emerged, providing new insight into the mechanisms of host defense.

Immune responses in tuberculosis

The initial interaction of Mycobacterium tuberculosis with phagocytes through Toll-like receptors can affect induction of the adaptive response by inflammatory cytokine production, as well as begin the process of bacterial containment by induction of antimycobacterial functions. Advances in T cell research in tuberculosis include identification of antigens recognized by CD8(+) T cells in infected hosts, elucidation of antimycobacterial mechanisms of T cells and the discovery of CD8(+) T cells that recognize antigens presented by a variety of non-classical molecules.

CD8+ T cell effector mechanisms in resistance to infection

Based on T cell subset depletion studies and the analysis of gene knockout mice, it is evident that CD8(+) T cells contribute to resistance against intracellular infections with certain viral, protozoan, and bacterial pathogens. Although they are known primarily for their capacity to kill infected cells, CD8(+) T cells elaborate a variety of effector mechanisms with the potential to defend against infection. Microbes use multiple strategies to cause infection, and the nature of the pathogenhost interaction may determine which CD8(+) T cell effector mechanisms are required for immunity. In this review, we summarize our current understanding of the effector functions used by CD8(+) T cells in resistance to pathogens. Analyses of mice deficient in perforin and/or Fas demonstrate that cytolysis is critical for immunity against some, but not all, infections and also reveal the contribution of cytolysis to the pathogenesis of disease. The role of CD8(+) T cell-derived cytokines in resistance to infection has been analyzed by systemic treatment with neutralizing antibodies and cytokine gene knockout mice. These studies are complicated by the fact that few, if any, cytokines are uniquely produced by CD8(+) T cells. Thus, the requirement for CD8(+) T cell- derived cytokines in resistance against most pathogens remains to be defined. Finally, recent studies of human CD8(+) T cells reveal the potential for novel effector mechanisms in resistance to infection.

A novel beta-glucoside-specific PTS locus from Streptococcus mutans that is not inhibited by glucose

A regulon from Streptococcus mutans that plays a role in the utilization of beta-glucosides has been isolated, sequenced and subjected to sequence analysis. This regulon encodes a beta-glucoside-specific Enzyme II (EII) component (bglP) of the phosphoenolpyruvate-dependent phosphotransferase system (PTS) and a phospho-beta-glucosidase (bglA) which is responsible for the breakdown of the phospho-beta-glucosides within the cell. Both the bglP and bglA gene products have significant similarity with proteins that have similar functions from Clostridium longisporum, Listeria monocytogenes, Erwinia chrysanthemi, Escherichia coli, Klebsellia oxytoca and Bacillus subtilis. The potential functions of the BglP and BglA proteins are supported by phenotypic data from both S. mutans and E. coli. A chromosomal deletion in S. mutans spanning the bglP and bglA genes resulted in a strain that was unable to hydrolyse the beta-glucoside aesculin in the presence of glucose. When glucose was removed from the medium, the deletion strain regained the ability to break down aesculin. These data suggest that S. mutans possesses an alternative mechanism from the one described in this report for breaking down beta-glucosides. This second mechanism was repressed by glucose while the regulon described here was not. Complementation studies in E. coli CC118 also suggest a potential role for this regulon in the utilization of other beta-glucosides. When a plasmid containing the 8 kb beta-glucoside-specific regulon was transformed into E. coli CC118, the transformed strain was able to break down the beta-glucoside arbutin.

Eradication of Cryptosporidium parvum infection by mice with ovalbumin-specific T cells

CD154 is necessary for mice to clear a Cryptosporidium parvum infection, but whether this ligand has to be expressed on T cells with specificity for C. parvum has not been determined. We infected DO11.10 (ovalbumin specific) T-cell receptor transgenic mice that had been bred to a RAG(-/-) background with C. parvum and found that the infection was cleared within 6 weeks, while RAG(-/-) controls were unable to clear C. parvum infection. Recovery was accompanied by an increase in the number of splenic T cells with the CD44(high) phenotype that characterizes memory cells. To determine whether a C. parvum-infected environment sufficed to activate transgenic T cells, we reconstituted C. parvum-infected BALB/c SCID mice with DO11.10 RAG(-/-) splenocytes. Fecal excretion of C. parvum antigen ceased in the 12 weeks following the adoptive transfer, unless the mice were also injected with tolerizing doses of ovalbumin. DO11.10 T cells were found in the submucosa of C. parvum-infected, but not uninfected, BALB/c SCID hosts within 48 h of injection. The transferred DO11.10 T cells divided and acquired a CD44(high) memory phenotype in C. parvum-infected, but not uninfected, recipients. DO11.10 splenocytes from CD154 knockout donors failed to clear a C. parvum infection, confirming a requirement for CD154 in recovery. In vitro, the DO11.10 cells did not proliferate in response to C. parvum antigen, and a tBlast GenBank search revealed no matches between the ovalbumin peptide and C. parvum DNA sequences. C. parvum-infected SCID mice given RAG(-/-) CD8(+) T cells with a Listeria-specific transgene did not recover from C. parvum infection. Our data suggest that antigen-nonspecific CD4(+) T-cell effector mechanisms in combination with the innate arm of the immune system are sufficient for the eradication of C. parvum infection.

A physiological ligand of positive selection is seen with high specificity

Positive selection is a process that ensures that peripheral T cells express TCR that are restricted to self-MHC molecules. This process requires both self-MHC and self-peptides. We have recently established a TCR transgenic mouse model (C10.4 TCRtrans+) in which the transgenic TCR was selected on the nonclassical MHC class Ib molecule H2-M3 in conjunction with a physiologically occurring peptide derived from the mitochondrial NADH-dehydrogenase subunit 1 gene (9-mer peptide). Here, the specificity of positive selection of C10.4 TCRtrans+ T cells was examined using a fetal thymic organ culture system. We demonstrated that at low peptide concentrations, shortening the NADH-dehydrogenase subunit 1 gene 9-mer peptide or mutating its surface-exposed side chains severely impaired its ability to induce positive selection. We concluded that under physiological conditions positive selection of C10.4 TCRtrans+ T cells was highly specific and occurred at low epitope densities.

The selection of M3-restricted T cells is dependent on M3 expression and presentation of N-formylated peptides in the thymus

The major histocompatibility complex (MHC) class Ib molecule H2-M3 binds N-formylated peptides from mitochondria and bacteria. To explore the role of M3 expression and peptide supply in positive and negative selection, we generated transgenic mice expressing an M3-restricted TCR-alpha/beta from a CD8(+) T cell hybridoma (D7) specific for a listerial peptide (LemA). Development of M3-restricted transgenic T cells is impaired in both beta2-microglobulin-deficient and transporter associated with antigen processing (TAP)-deficient mice, but is not diminished by changes in the H-2 haplotype. Maturation of M3/LemA-specific CD8(+) single positive cells in fetal thymic organ culture was sensitive to M3 expression levels as determined by antibody blocking and use of the castaneus mutant allele of M3. Positive selection was rescued in TAP(-/-) lobes by nonagonist mitochondrial and bacterial peptides, whereas LemA and a partial agonist variant caused negative selection. Thus, M3-restricted CD8(+) T cells are positively and negatively selected by M3, with no contribution from the more abundant class Ia molecules. These results demonstrate that class Ib molecules can function in thymic education like class Ia molecules, despite limited ligand diversity and low levels of expression.

The Adjacent Flanking Region Plays a Critical Role in Facilitating the Presentation of the Listeria monocytogenes Product lemA to H2 M3wt-Restricted, Peptide-Specific Murine CD8 Cells

Mice infected with Listeria monocytogenes (LM) generate CD8 effectors specific for f-MIGWII, the amino terminus of the bacterial product lemA presented by the class Ib MHC molecule H2 M3wt. lemA has several distinctive properties: 1) it is readily presented as an exogenous Ag in the absence of bacterial infection; 2) it is processed by a TAP-independent pathway, which is sensitive to chloroquine, pepstatin, and brefeldin; and 3) the immunogenic portion of the molecule is extremely resistant to proteolytic degradation even by proteinase K. To assess the structural basis for these findings, we expressed a truncated variant (t-lemA) containing the amino-terminal hexapeptide and the subsequent 27 amino acids linked to a histidine tail in Escherichia coli, and purified the product by affinity chromatography. Purified t-lemA could be presented to f-MIGWII-specific effectors by macrophages and fibroblasts at 1–10 nM. Unlike f-MIGWII, which binds directly to H2 M3wt, t-lemA required processing by a chloroquine-, pepstatin-, and brefeldin-sensitive pathway. Brefeldin sensitivity often implies endogenous processing in the cytoplasm, but several lines of evidence suggest translocation to the cytoplasm and proteosomal degradation are not critical for t-lemA presentation. Unlike f-MIGWII, t-lemA was profoundly resistant to proteinase K, and, using 35S-labeled t-lemA, we could identify the region from position 1 to ∼30 as the protease-resistant element. Thus, the hydrophobic peptide sequence following f-MIGWII can account for the unusual properties of lemA noted above. Analogous modification could be used to alter the properties of other peptide Ags presented by class I MHC products.

Processing of Listeria monocytogenes antigens and the in vivo T-cell response to bacterial infection

Presentation of antigens to T lymphocytes is a critical step in the clearance of pathogens from their hosts and in the establishment of protective immunity. Several animal models have been developed to study this process, but few have been as informative as the murine immune response to Listeria monocytogenes infection. Herein we review the presentation of L. monocytogenes proteins by the MHC class I antigen-processing pathway and the in vivo T-cell response to these bacterial antigens. These studies demonstrate the following: 1) The size of a peptide-specific T-cell response does not correlate with the amount of epitope presented by infected cells; 2) T cells specific for dominant epitopes do not, in the case of L. monocytogenes infection, inhibit responses to subdominant epitopes; 3) T cells responding to different epitopes presented by MHC class Ia molecules expand, contract and enter the memory pool synchronously; 4) Repeated in vivo expansion of antigen-specific T-cell populations results in a narrowing of their T-cell receptor repertoire and in an increase in their affinity for antigen; and 5) T cells restricted by H2-M3 MHC class Ib molecules constitute a major part of the primary response to bacterial infection, but appear to play a relatively smaller role in memory responses. These studies have provided a novel glimpse of the relationship between antigen processing and in vivo T-cell responses to infection, and provide a foundation for more detailed analyses of T-cell mediated adaptive immunity.

MHC class Jb-restricted cell responses to Listeria monocytogenes infection

Murine infection with Listeria monocytogenes induces CD8+ T cell responses specific for bacterial peptides that are presented on the infected cell surface by MHC class Ia and MHC class Ib molecules. We have used MHC tetramers to demonstrate that CD8+ T cells restricted by the H2-M3 MHC class Ib molecules constitute a substantial portion of the T cell response to L. monocytogenes infection. The in vivo size and kinetics of MHC class Ib-restricted T cell populations suggests that they play a prominent role in bacterial clearance following primary L. monocytogenes infection.

MHC class I restricted T cell responses to Listeria monocytogenes, an intracellular bacterial pathogen

Studies of the murine immune response to infection with the intracellular bacterial pathogen Listeria monocytogenes have provided a wealth of information about innate and acquired immune defenses in the setting of an infectious disease. Our studies have focused on the MHC class I restricted, CD8+ T cell responses of Balb/c mice to L. monocytogenes infection. Four peptides that derive from proteins that L. monocytogenes secretes into the cytosol of infected cells are presented to cytotoxic T lymphocyte (CTL) by the H2-Kd major histocompatibility complex (MHC) class I molecule. We have found that bacterially secreted proteins are rapidly degraded in the host cell cytosol by proteasomes that utilize, at least in part, the N-end rule to determine the rate of degradation. The MHC class I antigen processing pathway is remarkably efficient at generating peptides that bind to MHC class I molecules. The magnitude of in vivo T cell responses, however, is influenced to only a small degree by the amount of antigen or the efficiency of antigen presentation. Measurements of in vivo T cell expansion following L. monocytogenes infection indicate that differences in the sizes of peptide-specific T cell responses are more likely owing to differences in the repertoire of naive T cells than to differences in peptide presentation. This notion is supported by our additional finding that dominant T cell populations express a more diverse T cell receptor (TCR) repertoire than do subdominant T cell populations.

Peptide Affinity and Concentration Affect the Sensitivity of M3-Restricted CTLs Induced In Vitro

In vitro stimulation of mouse splenocytes with hemagglutinin (HA) 173–190, a peptide derived from influenza virus hemagglutinin (A/JAP/305/57, H2N2), induces CTLs that are directed to the MHC class Ib molecule, H2-M3. M3 preferably binds peptides bearing an N-terminal formylmethionine. In this study, we show that several related nonformylated peptides can induce anti-HA CTLs in vitro: MLIIW (the minimal epitope), derived from HA186–190 at the C-terminal end of HA173–190; MLIIWG; MLIIWGV; and MLIIWGI, as well as formylated MLIIW. The heptamer peptides correspond to a polymorphism of HA192 in H2 strains of influenza; they have the highest relative affinities for M3 of the nonformylated peptides and higher affinities than some formylated mitochondrial peptides. Depending on the affinity of the peptide, a range of concentrations can be used to induce CTLs. One nanomolar of the high affinity f-MLIIW peptide can induce anti-HA CTLs, whereas 100-fold more of the lower affinity MLIIW peptide is needed. Lines induced with high concentrations (1 μM or greater) of f-MLIIW recognize Ag poorly, and the most efficient CTLs are induced with the lowest concentrations of peptide. Analysis with a panel of anti-TCRVβ Abs shows that different T cells respond to high vs low peptide; the repertoire of cells responding to higher concentrations is more diverse, consistent with the expansion of more, but less efficient, clones. Thus, peptide affinity and concentration should be considered together for generating efficient antipeptide CTLs in vitro.

The majority of H2-M3 is retained intracellularly in a peptide-receptive state and traffics to the cell surface in the presence of N-formylated peptides

We used a new monoclonal antibody (mAb 130) to analyze the intracellular trafficking and surface expression of H2-M3, the major histocompatibility complex class Ib molecule that presents N-formylated peptides to cytotoxic T cells. M3 surface expression is undetectable in most cell types due to the paucity of endogenous antigen. M3 is induced on the cell surface by addition of high-affinity N-formylated peptides from mitochondria and listeria. Peptide-induced M3 expression is most efficient on antigen presenting cells. Basal and inducible expression of M3 is transporter associated with antigen processing (TAP)-dependent, distinguishing M3 from the class Ib molecules TL and CD1. Unlike the expression of class Ia molecules and a previously described M3/L(d) chimera, surface expression of M3 cannot be rescued by lowered temperature, suggesting that the alpha3 domain and transmembrane region of M3 may control trafficking. Pulse-chase analysis and use of trafficking inhibitors revealed a pool of empty M3 in the endoplasmic reticulum or early Golgi apparatus. Addition of exogenous peptide allows maturation with kinetics matching those of D(d). The lack of endogenous N-formylated peptide allows discovery of novel pathogen-derived peptides in normal antigen presenting cells. The nonpolymorphic nature of M3 and its ability to present bacterial antigens rapidly and dominantly make it an attractive target for peptide vaccination strategies.