Generation of antigen-loaded MR1 tetramers that specifically stain MAIT cells identifies heterogeneity in phenotypes and TCR repertoires in humans and mice.

NCT00231907.

Mucosal associated invariant T (MAIT) cells have a semi-invariant TCR Vα chain, and their optimal development is dependent upon commensal flora and expression of the non-polymorphic MHC class I-like molecule MR1. MAIT cells are activated in an MR1-restricted manner by diverse strains of bacteria and yeast suggesting a widely shared Ag. Recently, human and mouse MR1 were found to bind bacterial riboflavin metabolites (ribityllumazines, RL Ag) capable of activating MAIT cells. Here we use MR1/RL tetramers to study MR1-dependency, subset heterogeneity and protective effector functions important for tuberculosis (TB) immunity. Although tetramer+ cells were detected in both MR1+/+ and MR1−/− TCR Vα19i transgenic (Tg) mice, MR1 expression resulted in significantly increased tetramer+ cells co-expressing TCR Vβ6/8, NK1.1, CD44 and CD69, that displayed more robust in vitro responses to IL-12+IL-18 and RL Ag, indicating that MR1 is necessary for the optimal development of the classic murine MAIT cell memory/effector subset. In addition, tetramer+ MAIT cells expressing CD4, CD8 or neither developing in MR1+/+ Vα19i Tg mice had disparate cytokine profiles in response to RL Ag. Therefore, murine MAIT cells are considerably more heterogeneous than previously thought. Most notably, after mycobacterial pulmonary infection heterogeneous subsets of tetramer+ Vα19i Tg MAIT cells expressing CXCR3 and α4β1 were recruited into the lungs and afforded early protection. In addition, Vα19iCα−/−MR+/+ mice were significantly better protected than Vα19iCα−/−MR1−/−, wild type and MR1−/− non-transgenic mice. Overall, we demonstrate considerable functional diversity of MAIT cell responses, and also that MR1-restricted MAIT cells are important for TB protective immunity.

Mouse models of orthopoxvirus disease provide great promise for probing basic questions regarding host responses to this group of pathogens, which includes the causative agents of monkeypox and smallpox. However, some essential tools for their study that are taken for granted with other mouse models are not available for these viruses. Here we map and characterize the initial CD8 ؉ T-cell determinants for poxviruses in H-2 d -haplotype mice. CD8 ؉ T cells recognizing these three determinants make up around 40% of the total responses to vaccinia virus during and after resolution of infection. We then use these determinants to test if predicted conservation across orthopoxvirus species matches experimental observation and find an unexpectedly cross-reactive variant peptide encoded by ectromelia (mousepox) virus.

Human γ9δ2 T cells potently inhibit pathogenic microbes, including intracellular mycobacteria, but the key inhibitory mechanism(s) involved have not been identified. We report a novel mechanism involving the inhibition of intracellular mycobacteria by soluble granzyme A. γ9δ2 T cells produced soluble factors that could pass through 0.45 µm membranes and inhibit intracellular mycobacteria in human monocytes cultured below transwell inserts. Neutralization of TNF-α in co-cultures of infected monocytes and γ9δ2 T cells prevented inhibition, suggesting that TNF-α was the critical inhibitory factor produced by γ9δ2 T cells. However, only siRNA- mediated knockdown of TNF-α in infected monocytes, but not in γ9δ2 T cells, prevented mycobacterial growth inhibition. Investigations of other soluble factors produced by γ9δ2 T cells identified a highly significant correlation between the levels of granzyme A produced and intracellular mycobacterial growth inhibition. Furthermore, purified granzyme A alone induced inhibition of intracellular mycobacteria, while knockdown of granzyme A in γ9δ2 T cell clones blocked their inhibitory effects. The inhibitory mechanism was independent of autophagy, apoptosis, nitric oxide production, type I interferons, Fas/FasL and perforin. These results demonstrate a novel microbial defense mechanism involving granzyme A-mediated triggering of TNF-α production by monocytes leading to intracellular mycobacterial growth suppression. This pathway may provide a protective mechanism relevant for the development of new vaccines and/or immunotherapies for macrophage-resident chronic microbial infections.