Immune Selection of Hot-Spot β2-Microglobulin Gene Mutations, HLA-A2 Allospecificity Loss, and Antigen-Processing Machinery Component Down-Regulation in Melanoma Cells Derived from Recurrent Metastases following Immunotherapy

摘要

Scanty information is available about the mechanisms underlying HLA class I Ag abnormalities in malignant cells exposed to strong T cell-mediated selective pressure. In this study, we have characterized the molecular defects underlying HLA class I Ag loss in five melanoma cell lines derived from recurrent metastases following initial clinical responses to T cell-based immunotherapy. Point mutations in the translation initiation codon (ATG→ATA) and in codon 31 (TCA→TGA) of the β2-microglobulin (β2m) gene were identified in the melanoma cell lines 1074MEL and 1174MEL, respectively. A hot-spot CT dinucleotide deletion within codon 13–15 was found in the melanoma cell lines 1106MEL, 1180MEL, and 1259MEL. Reconstitution of β2m expression restored HLA class I Ag expression in the five melanoma cell lines; however, the HLA-A and HLA-B , -C gene products were differentially expressed by 1074MEL, 1106MEL, and 1259MEL cells. In addition, in 1259MEL cells, the Ag-processing machinery components calnexin, calreticulin, and low m.w. polypeptide 10 are down-regulated, and HLA-A2 Ags are selectively lost because of a single cytosine deletion in the HLA-A2 gene exon 4. Our results in conjunction with those in the literature suggest the emergence of a preferential β 2 m gene mutation in melanoma cells following strong T cell-mediated immune selection. Furthermore, the presence of multiple HLA class I Ag defects within a tumor cell population may reflect the accumulation of multiple escape mechanisms developed by melanoma cells to avoid distinct sequential T cell-mediated selective events.