Structural and functional analysis of β2 microglobulin abnormalities in human lung and breast cancer

Abstract

The escape of tumor cells from immune recognition is a central problem in tumor immunology. Here, we examined the functional role of somatic beta 2-microglobulin (β2m) gene mutations in human lung and breast cancers. Using single-strand conformational polymorphism (SSCP) analysis and DNA sequencing, we found mutations in the β2m gene in 2 of 110 tested lung, colon and breast tumors and tumor cell lines. No mutations were identified in 63 breast cancer tumors, in B-lymphoblastoid cell lines or normal tissues from these or other patients. In these cell lines, β2m protein was undetectable by Western blot analysis and there was no MHC class I on their cell surface even after treatment with interferon-γ. Transfection of these tumor cell lines with the β2m gene, but not addition of purified β2m protein restored MHC expression without addition of exogenous peptides, indicating that endogenous β2m expression is necessary for proper intracellular MHC assembly and stabilization by endogeneous peptides. Mutation in β2m caused cell line H2009 to be resistant to specific lysis by influenza virus-specific CTL from HLA matched donors, and transfection of the β2m gene restored this killing. A small cell lung cancer cell line with low class I expression and with a normal β2m genomic sequence nonetheless also demonstrated increased class I expression after transfection of the β2m expression vector alone, indicating that the availability of β2m may be rate limiting for MHC assembly in this line. Our results indicate that somatic mutations or selective loss of expression of the β2m gene in human lung cancer is rare, but can cause defective MHC class I expression and function allowing these cells to escape recognition by cytotoxic T cells.