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

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.

Predict prokaryotic proteins through detecting N-formylmethionine residues in protein sequences using support vector machine

Identifying prokaryotes in silico is commonly based on DNA sequences. In experiments where DNA sequences may not be immediately available, we need to have a different approach to detect prokaryotes based on RNA or protein sequences. N-formylmethionine (fMet) is known as a typical characteristic of prokaryotes. A web tool has been implemented here for predicting prokaryotes through detecting the N-formylmethionine residues in protein sequences. The predictor is constructed using support vector machine. An online predictor has been implemented using Python. The implemented predictor is able to achieve the total prediction accuracy 80% with the specificity 80% and the sensitivity 81%.

Polyclonal MHC Ib-restricted CD8+ T cells undergo homeostatic expansion in the absence of conventional MHC-restricted T cells

Naive T cells have the capacity to expand in a lymphopenic environment in a process called homeostatic expansion, where they gain a memory-like phenotype. Homeostatic expansion is dependent on competition for a number of factors, including growth factors and interactions with their selecting self-MHC molecules. In contrast to conventional T cells, it is unclear whether class Ib-restricted CD8+ T cells have a capacity to undergo homeostatic expansion. In this study, we demonstrate that polyclonal MHC Ib-restricted CD8+ T cells can undergo homeostatic expansion and that their peripheral expansion is suppressed by conventional MHC-restricted T cells. The acute depletion of CD4+ T cells in MHC class Ia-deficient Kb-/-Db-/- mice led to the substantial expansion of class Ib-restricted CD8+ T cells. Adoptive transfer of class Ib-restricted CD8+ T cells to congenic lymphopenic recipients revealed their ability to undergo homeostatic expansion in a MHC Ib-dependent manner. To further study the homeostatic expansion of MHC Ib-restricted T cells in the absence of all conventional MHC-restricted T cells, we generated mice that express only MHC Ib molecules by crossing H-2Kb-/-Db-/- with CIITA-/- mice. CD8+ T cells in these mice exhibit all of the hallmarks of naive T cells actively undergoing homeostatic expansion with constitutive memory-like surface and functional phenotype. These findings provide direct evidence that MHC Ib-restricted CD8+ T cells have the capacity to undergo homeostatic expansion. Their peripheral expansion is suppressed under normal conditions by a numerical excess of conventional MHC class Ia- and class II-restricted T cells.

Patterns of nonclassical MHC antigen presentation

The identification of pattern-recognition receptors that selectively respond to evolutionarily conserved chemical (often pathogen-derived) moieties has provided key insight into how innate immune cells facilitate rapid and relatively specific antimicrobial immune activity. In contrast, relatively slower adaptive immune responses rely on T cell clonal expansion that develops in response to variable peptides bound to the groove of classical major histocompatibility complex (MHC) proteins. For certain nonclassical 'MHC-like' class Ib proteins, such as H2-M3 and CD1d, their respective binding grooves seem to have been adapted to present to T cells unique molecular patterns analogous to those involved in innate signaling. Here we propose that another MHC class Ib protein, MR1, which is required for the gut flora-dependent development of mucosa-associated invariant T cells, presents either a microbe-produced or a microbe-induced pattern.

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.