Major Histocompatibility Complex Class II-dependent Unfolding, Transport, and Degradation of Endogenous Proteins

Abstract

We have analyzed the ability of major histocompatibility (MHC) class II molecules to capture proteins in the biosynthetic pathway and whether this may be associated with MHC class II-dependent antigen processing. When coexpressed with HLA-DR 4 molecules in HeLa cells, influenza hemagglutinin was inhibited from folding and trimerization in the biosynthetic pathway, targeted to endosomal compartments, and rapidly degraded. Due to the interaction with MHC class II molecules, therefore, unfolded forms of hemagglutinin were bypassing the quality control mechanism of the secretory pathway. More important, however, the transport, endocytosis, and rapid degradation of unfolded hemagglutinin in the presence of MHC class II molecules suggest that proteins captured in the endoplasmic reticulum by class II molecules may become substrates for antigen processing and presentation to CD4-positive T cells. In insect cells we show that this phenomenon is not restricted to a few proteins such as hemagglutinin. A highly heterogeneous mixture of proteins from the endoplasmic reticulum including coexpressed hemagglutinin can form stable complexes with soluble HLA-DR ␣ and ␤ chains that were transported into the supernatant. This mechanism may gain biological significance in abnormal situations associated with accumulation of unfolded or malfolded proteins in the endoplasmic reticulum, for example during viral infections.

Amino Acid Substitutions in the Putative MHC Class II “Dimer of Dimers” Interface Inhibit CD4+ T Cell Activation

“…However, the presence or absence of high-molecular-mass bands showed no correlation with the ability of these HLA-DR molecules to induce T cell activation. Therefore, it seems more likely that the strong complexes detected in SDS gels are either aggregates of incompletely folded MHC class II molecules or complexes of a class II molecule and other proteins of undefined nature that have been shown to form SDS-stable aggregates (32,33). However, the failure to detect preformed MHC class II complexes here could be explained if the MHC class II intermolecular avidity is too low to isolate them in the presence of the detergent Nonidet P-40.…”

Immunodominant T‐cell epitopes of hnRNP‐A2 associated with disease activity in patients with rheumatoid arthritis

“…Immunization studies in DR4-Tg mice with the T-cell epitopes 117-133/120-133 and various B-cell epitopes (includ-ing peptide [19][20][21][22][23][24][25][26][27][28][29][30][31] are currently in progress in our laboratory to further elucidate the role of hnRNP-A2 in RA.…”

“…Recombinant HLA class II DRA1 Ã 0101/DRB1 Ã 0101, DRA1 Ã 0101/DRB1 Ã 0401, DRA1 Ã 0101/DRB1 Ã 0404, molecules were expressed in insect cells and purified as described [30].…”

Immune surveillance of intracellular pathogens via autophagy

“…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).…”

“…Firstly, secreted/ transmembrane proteins (e.g. influenza hemagglutinin (HA) 28 ) can associate with newly synthesized MHC class II molecules in the ER, and then follow MHC class II-I i complexes to endosomal compartments, where processing and peptide loading occurs. This pathway contributes the majority of endogenous MHC class II ligands, which were found to be derived from secreted/transmembrane proteins that intersect with the endocytic pathway.…”