Cell Surface MHC Class I Expression Is Limited by the Availability of Peptide-Receptive "Empty" Molecules Rather than by the Supply of Peptide Ligands.

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.