Several strategies are opted by the intracellular protozoan pathogens to hide from the host-immune surveillance. Meddling with the process of antigen processing/presentation is one of the primary ways by which they can evade host immunity. For intracellular parasites, like Leishmania, which resides within the parasitophorous vacuole (PV) of the host, parasite-specific proteins from PV are transported into the host cytosol and processed by proteasomes. These fragmented proteins are transported to the endoplasmic reticulum by TAP (Transporter associated with antigen processing) and presented through the MHC-I-dependent cross-presentation pathway. 1,2 In general, intracellular pathogens have the intrinsic ability to downregulate the surface expression of MHC-I or modulate several parameters related to MHC-I mediated presentation or cross-presentation of its antigen as it replicates within the infected host. 3 Hence these infected cells remain largely invisible to cytotoxic CD8 T cells. Similarly, exogenous proteins which might be injected into the host cytoplasm by the invading pathogens, or micro-organism endocytosed by the host, enter into a vesicular pathway, resulting in progressive acidification of the vesicles, lysosomal fusion and eventual degradation of parasite proteins which are then presented by MHC-II to the CD4 T cells.

No abstract