Secreted Proteins fromMycobacterium tuberculosisGain Access to the Cytosolic MHC Class-I Antigen-Processing Pathway

Abstract

CD8+ T cells play an important role in the host response to infection with Mycobacterium tuberculosis (Mtb). Mtb resides in an arrested phagosome that is phenotypically similar to an early endosome. The mechanisms by which Mtb-derived Ags gain access to the HLA-I-processing pathway are incompletely characterized. Studies with CD8+ T cell lines have suggested that Mtb Ags gain access to the HLA-I pathway in an alternate vacuolar pathway that is both brefeldin A (BFA) and TAP independent. To define the requirements of entry of Ag into the HLA-I pathway, we have used human CD8+ T cell clones specific for the secreted Mtb Ag CFP10. Human monocyte-derived dendritic cells were pulsed with CFP10 expressed in a recombinant adenovirus, surface adsorbed to microspheres, or in its native form by Mtb. When delivered by adenovirus, processing and presentation of CFP10 were blocked by both BFA and the proteasomal blocker lactacystin. In contrast, processing of CFP10 adsorbed to the surface of microspheres was not affected by either of these Ag-processing inhibitors. BFA, lactacystin, and TAP inhibition blocked the recognition of Mtb-infected dendritic cells, suggesting that processing was via a cytosolic pathway for this secreted protein Ag. We conclude that secreted proteins from Mtb can be processed in a BFA- and proteasome-dependent manner, consistent with egress of Ag into the cytosol and subsequent loading of proteasomally derived peptides.

Antigen Load Governs the Differential Priming of CD8 T Cells in Response to the Bacille Calmette Guérin Vaccine or Mycobacterium tuberculosis Infection

“…This is particularly relevant as the vacuolar pathway appears less efficient than the phagosome-to-cytosol pathway in the generation of immune responses in vivo (30). It should be noted that the absence of RD1-encoded proteins does not appear to affect the capacity of DCs to present secreted M. tuberculosis Ags to human CD8 T cells via the cytosolic pathway (35), however detailed comparison of the precise Ag processing and presentation mechanisms used to activate either BCG-or M. tuberculosis-reactive CD8 T cells has yet to be performed.…”

“…In the vacuolar model, peptides generated in the phagosome are loaded on to MHC class I molecules and processing does not require TAP or trafficking through the endoplasmic reticulum. Ags such as OVA can be presented to CD8 T cells by the two pathways (31,32), and both cytosolic and alternative models of Ag presentation have been shown to contribute to the recognition of mycobacterial Ags (33)(34)(35). It is possible that both these pathways contribute to CD8 T cell activation after BCG or M. tuberculosis infection, and the greater Ag load available for MHC class I loading after infection with M. tuberculosis results in improved T cell priming.…”

The Type I IFN Response to Infection withMycobacterium tuberculosisRequires ESX-1-Mediated Secretion and Contributes to Pathogenesis

“…A more interesting possibility is that the ESX-1 system functions in a manner similar to type III or type IV secretion systems of other pathogenic bacteria for delivery of bacterial effectors directly into the cytosol of infected macrophages (44). Recently, it was shown that CFP-10 presentation by MHC class I requires components specific to the cytosolic Ag-processing pathway, providing evidence that CFP-10 gains access to the cytosol during infection (13). Additional evidence that M. tuberculosis is capable of accessing cytosolic signaling pathways was provided by Ferwerda et al (38), who showed that nod proteins are involved in the response to infection with M. tuberculosis.…”

“…M. tuberculosis, unlike L. monocytogenes, is found exclusively in membrane bound compartments, suggesting that type I IFN production proceeds through a different pathway. Lewinsohn et al (13) showed that culture filtrate protein-10 (CFP-10), a protein secreted by the ESX-1 system of M. tuberculosis, gains access to the cytosol during infection and is presented by the MHC class-I Ag processing pathway, demonstrating the possibility that products from M. tuberculosis may gain access to cytosolic signaling receptors during infection.…”

Mycobacterium tuberculosis Hip1 Dampens Macrophage Proinflammatory Responses by Limiting Toll-Like Receptor 2 Activation

“…It is likely that NLR ligands are able to access the macrophage cytosol. Possible mechanisms for the cytosolic localization and recognition of M. tuberculosis NLR ligands include the rupture of phagosomes due to the particulate nature of M. tuberculosis antigens and the release of PAMPs into the cytosol, as well as entry of M. tuberculosis components into the macrophage cytoplasm (27,57,62). This may be mediated in part by ESAT-6, which is implicated in mediating membrane damage and has been reported to be required for activation of caspase-1 and the NLRP3 inflammasome (37).…”