Cell Surface MHC Class I Expression Is Limited by the Availability of Peptide‐Receptive “Empty” Molecules Rather than by the Supply of Peptide Ligands

Abstract

While antigen processing and presentation (APP) by the major histocompatibility complex class I (MHC-I) molecules have been extensively studied, a question arises as to whether the level of MHC-I expression is limited by the supply of peptide-receptive (empty) MHC molecules, or by the availability of peptide ligands for loading. To this end, the effect of interferons (IFNs) on the MHC peptidomes of human breast cancer cells (MCF-7) were evaluated. Although all four HLA allotypes of the MCF-7 cells (HLA-A*02:01, B*18, B*44, and C*5) present peptides of similar lengths and C-termini, which should be processed similarly by the proteasome and by the APP chaperones, the IFNs induced differential modulation of the HLA-A, B, and C peptidomes. In addition, overexpression of recombinant soluble HLA-A*02:01, introduced to compete with the identical endogenous membrane-bound HLA-A*02:01 for peptides of the MCF-7 cells, did not alter the expression level or the presented peptidome of the membrane-bound HLA-A*02:01. Taken together, these results indicate that a surplus supply of peptides is available inside the ER for loading onto the MHC-I peptide-receptive molecules, and that cell surface MHC-I expression is likely limited by the availability of empty MHC molecules.

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

“…Although ERAP1 has been extensively studied for its ability to generate antigenic peptides from N-terminally elongated precursors, it can also over-trim antigenic peptides to lengths not suitable for binding onto MHCI, essentially destroying the epitope (3). Recent analysis on the effect of ERAP1 on the immunopeptidome of cells have suggested that the destructive properties of ERAP1 may have been underestimated and could actually be the dominant function of the enzyme (47,52,53). The data presented herein highlight the ability of MHCI to protect peptides from ERAP1mediated degradation and are thus consistent with the hypothesis that ERAP1 acts to limit available peptides for MHCI.…”

Redundancy and Complementarity between ERAP1 and ERAP2 Revealed by their Effects on the Behcet's Disease-associated HLA-B*51 Peptidome*[S]

“…This was also supported by the pattern of P(-1) usage in the absence of both enzymes and is consistent with the observation in this and other studies (77) that many peptides are unaffected, even in their expression levels, by ERAP1/ ERAP2 silencing. This may reflect that there is an excess of preformed ligands in the ER (84), or that several peptides are either not processed by these enzymes or their generation/ destruction balance results in unaltered peptide levels (38). Yet the B*51:01 peptidome is optimized by ERAP1 and ERAP2, as revealed by its increased affinity and higher HLA-B*51 surface expression on WT cells.…”

Biogenesis of HLA Ligand Presentation in Immune Cells Upon Activation Reveals Changes in Peptide Length Preference

“…For example, if a given HLA-I allele binds preferentially 80% 9, 10% 10, and 10% 11 mers, a two-fold increase in its expression level will still display the same ratio (80:10:10), unless some perturbations would enforce a supply of peptide repertoire with different length characteristics, or if the supply of 9 mer peptides is the major limiting step. In a recent paper by Komov et al IFN induced differential modulation of the HLA-A, B, and C peptidomes was investigated ( 50 ). They elegantly showed that overexpression of recombinant soluble HLA-A * 02:01, introduced to compete for peptides with the endogenous membrane-bound HLA-A * 02:01, did not alter the expression level or features of the presented peptidome of the membrane-bound HLA-A * 02:01.…”