Antigen-derived peptides directly engage the unfolded-protein sensor IRE1α to curb cross-presentation by dendritic cells
Dendritic cells (DCs) promote adaptive immunity by cross-presenting antigen-based epitopes to CD8+ T cells. DCs process internalized protein antigens into peptides that enter the endoplasmic reticulum (ER) and upload onto major histocompatibility type I (MHC-I) protein complexes for cell-surface transport and cross-presentation. Perplexingly, DCs often exhibit activation of the ER-stress sensor IRE1 in the absence of classical ER stress--leaving the underlying mechanism unexplained. Here we show that antigen-derived hydrophobic peptides directly engage ER-resident IRE1 by masquerading as unfolded proteins. Furthermore, IRE1 activation depletes MHC-I heavy-chain mRNAs through regulated IRE1-dependent decay (RIDD), thereby curtailing antigen cross-presentation. In tumor-bearing mice, IRE1 disruption increased MHC-I expression on tumor-infiltrating DCs, and enhanced recruitment and activation of CD8+ T cells. Moreover, IRE1 inhibition synergized with anti-PD-L1 antibody treatment to cause tumor regression. Our findings elucidate the mechanism and consequence of antigen-driven IRE1 activation in DCs, yielding a promising combination strategy for cancer immunotherapy.