Abstract
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-γ 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.
Highlights
Mycobacterium tuberculosis (Mtb) remains a leading cause of morbidity and mortality worldwide, and is the leading cause of death in AIDS patients [1]
Major Histocompatibility Complex Class I (MHC-I) generally serves to present peptides derived from cytosolic proteins to CD8+ T lymphocytes, thereby alerting the immune system that the cell is infected
It is unclear how bacterial antigens are presented by MHC-I, as they are segregated from this machinery
Summary
Mycobacterium tuberculosis (Mtb) remains a leading cause of morbidity and mortality worldwide, and is the leading cause of death in AIDS patients [1]. The cellular immune response is vital for controlling Mtb infection and preventing development of active TB. CD8+ T cells are uniquely able to recognize intracellular infection, and may play a role in immune surveillance through direction of the granule exocytosis pathway to the Mtb-containing cell [4]. In humans, these cellular mechanisms maintain latent TB infection indefinitely in 90–95% of immunocompetent individuals. Because the phagosome is a component of the HLA-II processing pathway, the mechanisms by which Mtb-derived antigens are processed and presented on HLA-I molecules remain incompletely understood
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