Human mucosal associated invariant T cells detect bacterially infected cells.

Control of infection with Mycobacterium tuberculosis (Mtb) requires Th1-type immunity, of which CD8+ T cells play a unique role. High frequency Mtb-reactive CD8+ T cells are present in both Mtb-infected and uninfected humans. We show by limiting dilution analysis that nonclassically restricted CD8+ T cells are universally present, but predominate in Mtb-uninfected individuals. Interestingly, these Mtb-reactive cells expressed the Valpha7.2 T-cell receptor (TCR), were restricted by the nonclassical MHC (HLA-Ib) molecule MR1, and were activated in a transporter associated with antigen processing and presentation (TAP) independent manner. These properties are all characteristics of mucosal associated invariant T cells (MAIT), an "innate" T-cell population of previously unknown function. These MAIT cells also detect cells infected with other bacteria. Direct ex vivo analysis demonstrates that Mtb-reactive MAIT cells are decreased in peripheral blood mononuclear cells (PBMCs) from individuals with active tuberculosis, are enriched in human lung, and respond to Mtb-infected MR1-expressing lung epithelial cells. Overall, these findings suggest a generalized role for MAIT cells in the detection of bacterially infected cells, and potentially in the control of bacterial infection.

Nitric oxide-sensitive and -insensitive interaction of Bacillus subtilis NsrR with a ResDE-controlled promoter.

NsrR is a nitric oxide (NO)-sensitive transcription repressor that controls NO metabolism in a wide range of bacteria. In Bacillus subtilis, NsrR represses transcription of the nitrite reductase (nasDEF) genes that are under positive control of the ResD-ResE two-component signal transduction system. Derepression is achieved by reaction of NO with NsrR. Unlike some NsrR orthologues that were shown to contain a NO-sensitive [2Fe-2S] cluster, B. subtilis NsrR, when purified anaerobically either from aerobic or from anaerobic Escherichia coli and B. subtilis cultures, contains a [4Fe-4S] cluster. [4Fe-4S]-NsrR binds around the -35 element of the nasD promoter with much higher affinity than apo-NsrR and binding of [4Fe-4S]-NsrR, but not apo-protein, is sensitive to NO. RNA polymerase and phosphorylated ResD make a ternary complex at the nasD promoter and NsrR dissociates the preformed ternary complex. In addition to the -35 region, NsrR binds to two distinct sites of the upstream regulatory region where ResD also binds. These interactions, unlike the high-affinity site binding, do not depend on the NsrR [4Fe-4S] cluster and binding is not sensitive to NO, suggesting a role for apo-NsrR in transcriptional regulation.

The Mycobacterium tuberculosis phagosome is a HLA-I processing competent organelle.

Mycobacterium tuberculosis (Mtb) resides in a long-lived phagosomal compartment that resists maturation. The manner by which Mtb antigens are processed and presented on MHC Class I molecules is poorly understood. Using human dendritic cells and IFN-gamma release by CD8(+) T cell clones, we examined the processing and presentation pathway for two Mtb-derived antigens, each presented by a distinct HLA-I allele (HLA-Ia versus HLA-Ib). Presentation of both antigens is blocked by the retrotranslocation inhibitor exotoxin A. Inhibitor studies demonstrate that, after reaching the cytosol, both antigens require proteasomal degradation and TAP transport, but differ in the requirement for ER-golgi egress and new protein synthesis. Specifically, presentation by HLA-B8 but not HLA-E requires newly synthesized HLA-I and transport through the ER-golgi. Phenotypic analysis of the Mtb phagosome by flow organellometry revealed the presence of Class I and loading accessory molecules, including TAP and PDI. Furthermore, loaded HLA-I:peptide complexes are present within the Mtb phagosome, with a pronounced bias towards HLA-E:peptide complexes. In addition, protein analysis also reveals that HLA-E is enriched within the Mtb phagosome compared to HLA-A2. Together, these data suggest that the phagosome, through acquisition of ER-localized machinery and as a site of HLA-I loading, plays a vital role in the presentation of Mtb-derived antigens, similar to that described for presentation of latex bead-associated antigens. This is, to our knowledge, the first description of this presentation pathway for an intracellular pathogen. Moreover, these data suggest that HLA-E may play a unique role in the presentation of phagosomal antigens.