Endogenous MHC Class II Processing of a Viral Nuclear Antigen After Autophagy

Abstract

CD4+ T cells classically recognize antigens that are endocytosed and processed in lysosomes for presentation on major histocompatibility complex (MHC) class II molecules. Here, endogenous Epstein-Barr virus nuclear antigen 1 (EBNA1) was found to gain access to this pathway by autophagy. On inhibition of lysosomal acidification, EBNA1, the dominant CD4+ T cell antigen of latent Epstein-Barr virus infection, slowly accumulated in cytosolic autophagosomes. In addition, inhibition of autophagy decreased recognition by EBNA1-specific CD4+ T cell clones. Thus, lysosomal processing after autophagy may contribute to MHC class II-restricted surveillance of long-lived endogenous antigens including nuclear proteins relevant to disease.

Immune surveillance of intracellular pathogens via autophagy

“…31 Lysosomal proteases were shown to be responsible for antigen processing onto MHC class II in all three cases. For neomycin phosphotransferase II and EBNA1, autophagy was implicated in the delivery of antigens into lysosomes, while this has not been demonstrated for influenza matrix protein M1.…”

“…Subsequently, presentation of endogenous proteins on MHC class II has been described for a number of other viral antigens [28][29][30][31] as well as self-antigens, 21,32-34 model antigens, [35][36][37][38][39][40] and tumor antigens 41,42 (Table 2). On the basis of these findings, four endogenous MHC class II processing pathways can be postulated [43][44][45] (Figure 1).…”

“…47 In addition to these proteasome-dependent pathways, cytosolic and nuclear proteins can also be processed by a proteasome-and TAPindependent pathway: This fourth pathway involves the direct import of cytosolic/nuclear proteins (e.g. the EBV nuclear antigen 1 (EBNA1) 30,31 ) into endosomes/lysosomes and is in part mediated by autophagy. The latter three pathways (processing of cytosolic or nuclear proteins by proteasomedependent or -independent mechanisms) contribute more than 20% of endogenous MHC class II ligands.…”

“…47 (4) Cytosolic and nuclear proteins (e.g. the EBV nuclear antigen 1 (EBNA1) 31 ) can be processed by lysosomal proteases after direct import into endosomal/lysosomal compartments failed to be detected by M1-specific CD4 þ T cells, but stimulated M1-specific CD8 þ T cells. 48 Autophagic protein degradation in lysosomes has been suggested to mainly discard long-lived proteins (t 1/2 40.5 h), 49 whereas many short-lived substrates are degraded by the proteasome.…”

“…To date EBNA1 is the only pathogen-derived antigen for which processing onto MHC class II after autophagy has been demonstrated. 31 We could visualize EBNA1-containing autophagosomes after inhibition of lysosomal degradation by fluorescence and electron microcopy. Furthermore, MHC class II-restricted EBNA1 recognition by CD4 þ T cells was inhibited after RNA silencing of the essential autophagy gene atg12 58 as well as after 3-methyladenine treatment.…”