Secreted Immunodominant Mycobacterium tuberculosis Antigens Are Processed by the Cytosolic Pathway

Abstract

Exposure to Mycobacterium tuberculosis can result in lifelong but asymptomatic infection in most individuals. Although CD8+ T cells are elicited at high frequencies over the course of infection in both humans and mice, how phagosomal M. tuberculosis Ags are processed and presented by MHC class I molecules is poorly understood. Broadly, both cytosolic and noncytosolic pathways have been described. We have previously characterized the presentation of three HLA-I epitopes from M. tuberculosis and shown that these Ags are processed in the cytosol, whereas others have demonstrated noncytosolic presentation of the 19-kDa lipoprotein as well as apoptotic bodies from M. tuberculosis-infected cells. In this paper, we now characterize the processing pathway in an additional six M. tuberculosis epitopes from four proteins in human dendritic cells. Addition of the endoplasmic reticulum-Golgi trafficking inhibitor, brefeldin A, resulted in complete abrogation of Ag processing consistent with cytosolic presentation. However, although addition of the proteasome inhibitor epoxomicin blocked the presentation of two epitopes, presentation of four epitopes was enhanced. To further examine the requirement for proteasomal processing of an epoxomicin-enhanced epitope, an in vitro proteasome digestion assay was established. We find that the proteasome does indeed generate the epitope and that epitope generation is enhanced in the presence of epoxomicin. To further confirm that both the epoxomicin-inhibited and epoxomicin-enhanced epitopes are processed cytosolically, we demonstrate that TAP transport and new protein synthesis are required for presentation. Taken together, these data demonstrate that immunodominant M. tuberculosis CD8+ Ags are processed and presented using a cytosolic pathway.

Rv3615c is a highly immunodominant RD1 (Region of Difference 1)-dependent secreted antigen specific for Mycobacterium tuberculosis infection

“…We determined that EspC-derived peptides are recognized by both CD4 + and CD8 + IFN-γ-secreting T cells, induction of the latter being consistent with EspC secretion and access to the cytosolic MHC class I antigen-processing pathway during intracellular infection in vivo (27)(28)(29). The high density of multiple CD4 + and CD8 + epitopes in EspC, with a dominance of CD4 + responses, is reminiscent of ESAT-6 (21, 22, 30, 31) and CFP-10 (21, 22, 30, 31).…”

The Goldilocks model of immune symbiosis with Mycobacteria and Candida colonizers

“…However, CD8+ T cells contribute the control of chronic M. tuberculosis infection 129 , and mycobacterial antigens can escape into the cytosol and access the MHC Class I pathway 182 . CD8+ T cells are protective in vivo, and direct cytolytic activity toward M. tuberculosis infected cells in a manner that is perforin-dependent and promoted by IL12 183–185 .…”

Annexin1 regulates DC efferocytosis and cross-presentation during Mycobacterium tuberculosis infection

“…Despite the fact that Mtb is a phagosomal pathogen and initiates antigen presentation via the MHC class II (MHC-II) endocytic pathway, Mtb antigens also access the MHC-I-restricted pathway by a mechanism termed cross-presentation (4,5). This mechanism allows antigen-presenting cells (APCs), mainly DCs, to efficiently present antigens of exogenous origins to MHC-Irestricted CD8 + T cells (6).…”