Semisynthetic Viral Inhibitor for Light Control of the MHC I Peptide Loading Complex

The immune system detects virally or malignantly transformed cells via peptide-loaded major histocompatibility complex class I (pMHC I) molecules on the cell surface. MHC I molecules are loaded with cargo peptides in the endoplasmic reticulum (ER) by the highly dynamic multiprotein peptide loading complex (PLC). Here, we developed a semisynthetic approach to generate a photocleavable immune modulator ICP47 of Herpes simplex virus. Using this nanotool, we revealed key mechanistic events of the purified PLC, such as peptide binding and translocation coupled to ATP hydrolysis, triggered by light. We established a single-organelle flow cytometry assay to monitor light-controlled activation of the antigen processing machinery in native ER membranes. This photochemical modulation opens new opportunities for a comprehensive mechanistic analysis of the antigen processing machinery in vitro and native membrane environment.

A systematic re-examination of processing of MHCI-bound antigenic peptide precursors by endoplasmic reticulum aminopeptidase 1

Endoplasmic reticulum aminopeptidase 1 (ERAP1) trims antigenic peptide precursors to generate mature antigenic peptides for presentation by major histocompatibility complex class I (MHCI) molecules and regulates adaptive immune responses. ERAP1 has been proposed to trim peptide precursors both in solution and in preformed MHCI-peptide complexes, but which mode is more relevant to its biological function remains controversial. Here, we compared ERAP1-mediated trimming of antigenic peptide precursors in solution or when bound to three MHCI alleles, HLA-B*58, HLA-B*08, and HLA-A*02. For all MHCI-peptide combinations, peptide binding onto MHCI protected against ERAP1-mediated trimming. In only a single MHCI-peptide combination, trimming of an HLA-B*08-bound 12-mer progressed at a considerable rate, albeit still slower than in solution. Results from thermodynamic, kinetic, and computational analyses suggested that this 12-mer is highly labile and that apparent on-MHC trimming rates are always slower than that of MHCI-peptide dissociation. Both ERAP2 and leucine aminopeptidase, an enzyme unrelated to antigen processing, could trim this labile peptide from preformed MHCI complexes as efficiently as ERAP1. A pseudopeptide analogue with high affinity for both HLA-B*08 and the ERAP1 active site could not promote the formation of a ternary ERAP1/MHCI/peptide complex. Similarly, no interactions between ERAP1 and purified peptide-loading complex were detected in the absence or presence of a pseudopeptide trap. We conclude that MHCI binding protects peptides from ERAP1 degradation and that trimming in solution along with the dynamic nature of peptide binding to MHCI are sufficient to explain ERAP1 processing of antigenic peptide precursors.