CD4+ T cells recognize antigenic peptides bound to MHC class II molecules expressed on APC and thymic epithelial cells (TEC). MHC class II protein expression can also be induced on non-professional APC and tumour cells following exposure to cytokines, TLR ligands and other activators of innate signalling pathways. Traditionally, MHC class II molecules were viewed as promoting CD4+ T-cell responses to extracellular pathogens and protein Ags, as well as inactivated viral vaccines. Yet, studies of human T-cell responses also suggested a role for MHC class II molecules in promoting immunity to infectious viruses and tumour Ags.45–47 Sequencing of MHC class II ligands from a variety of APC revealed predominantly self-Ags found either on the cell surface or within the endosomal network.48,49 Yet, 10–30% of the ligands identified after release from MHC class II molecules are derived from cytoplasmic or nuclear Ags, suggesting that alternate pathways, such as autophagy, may deliver ligands to MHC class II molecules.48,50,51
MHC class II α and β subunits assemble in the ER with a conserved chaperone, the invariant chain (Ii), and transit to endosomal compartments. Within acidic endosomes, Ii is proteolytically cleaved, releasing peptide-receptive MHC class II αβ complexes. The binding of antigenic peptides to these MHC class II proteins is modulated by the editing molecules DM and peptide editor. MHC class II complexes travel through early endosomes, multivesicular bodies or mature endosomes (termed MIIC), lysosomes and phagosomes to acquire peptides before transit to the cell surface for display to CD4+ T cells.37,52 Ligand binding to MHC class II complexes at the cell surface is also observed, probably as a result of peptide exchange. Ags, including self- and foreign proteins, continuously traffick into the endosomal network for processing by cellular cathepsins and reductases. Remarkably, exposure of APC to cytokines such as IFN-γ and TLR ligands regulates autophagy pathways as well as cellular endocytosis.11,15,52,53–56 The physical intersection of autophagy pathways with endosomes and lysosomes is critical in promoting cytoplasmic and nuclear Ag processing and presentation by MHC class II molecules (Figs 1 and 2).
A wide variety of short- and long-lived cytoplasmic and nuclear Ags, including viral, tumour, self- and ectopically expressed bacterial proteins, are presented by MHC class II molecules on APC and tumours, probably via several distinct autophagy pathways.57,58 Presentation of these Ags is typically sensitive to choloroquine or lysomotrophic amines, as these agents block the acidification of endosomal compartments through which MHC class II molecules transit. Remarkably, MHC class II presentation of some cytoplasmic Ags is dependent upon proteolysis by the proteasome or calpains,59–61 while the processing of other nuclear and cytoplasmic Ags is mediated by cathepsin proteases in acidic compartments.38,39,59–62 In contrast to MHC class I proteins, TAP is not required for MHC class II presentation of cytoplasmic or nuclear Ags.60,63,64 While cytoplasmic and nuclear Ags display some heterogeneity in their processing, these endogenous Ags also appear to access distinct pathways for autophagy. Exposure of APC to 3-methyladenine failed to perturb MHC class II presentation of a number of some cytoplasmic Ags, suggesting a role for pathways other than macroautophagy.60,61 Yet, for several nuclear and cytoplasmic Ags, 3-methyladenine was a potent inhibitor of MHC class II presentation (Fig. 1).5,38,39,61 Disrupting Atg12 or Atg7 expression via RNA interference (RNAi) also confirmed a requirement for macroautophagy in the presentation of two distinct nuclear Ags.5,38,40,65 MHC class II presentation of influenza matrix protein is enhanced by conjugation to LC3 before expression in APC.5 Whether nuclear localization favours Ag processing via macroautophagy, remains controversial. MHC class II presentation of a bacterial Ag via macroautophagy was found to be equally efficient regardless of whether this protein was ectopically expressed in the cytoplasm or in the nucleus of renal tumour cells.65 By contrast, MHC class II presentation of the viral Epstein Barr virus nuclear Ag 1 (EBNA1) by macroautophagy was enhanced upon deletion of its nuclear targeting sequence.40 Notably, macroautophagy is not involved in the presentation of epitopes from two other viral nuclear Ags – EBNA2 and EBNA3C – which rely on cross-presentation and processing via the conventional MHC class II pathway.66 Several cytoplasmic Ags access the selective autophagy pathway of CMA for MHC class II presentation (Fig. 2).63 Peptides from the cytoplasmic Ags glyceraldehyde 3-phosphate dehydrogenase (GAPDH) and aspartate aminotransferase are frequently detected bound to MHC class II molecules, with the transit of both these Ags from the cytoplasm to lysosomes occurring via CMA.30,50,58 We showed previously that MHC class II presentation of cytoplasmic GAD65, a key diabetes autoantigen, is dependent upon CMA by manipulating the expression of LAMP2A and hsc70 in human B lymphoblasts.63 A recent study also demonstrated that both α and β isoforms of hsp90 are required for MHC class II presentation of cytoplasmic GAD Ag.67 Misfolded proteins in the ER are frequently translocated to the cytoplasm for proteolysis, and MHC class II presentation of one of these Ags was also dependent on CMA.63 Together these studies suggest that several autophagy pathways, which intersect the endosomal network, facilitate MHC class II presentation of cytoplasmic and nuclear Ags.