Immune escape of cancer cells with beta2‐microglobulin loss over the course of metastatic melanoma

Cancer cells escape T‐cell‐mediated destruction by losing human leukocyte antigen (HLA) class I expression via various mechanisms, including loss of beta2‐microglobulin (β2m). Our study illustrates the immune escape of HLA class I‐negative tumor cells and chronological sequence of appearance of tumor β2m gene mutation in successive lesions obtained from a patient with metastatic melanoma. We observed a gradual decrease in HLA expression in consecutive lesions with few HLA‐negative nodules in the primary tumor and the emergence of a totally negative lesion at later stages of the disease. We detected loss of β2m in β2m‐negative nests of the primary tumor caused by a combination of two alterations: ( i ) a mutation (G to T substitution) in codon 67 in exon 2 of β2m gene, producing a stop codon and ( ii ) loss of the second gene copy by loss of heterozygosity (LOH) in chromosome 15. The same β2m mutation was found in a homogeneously β2m‐negative metastasis 10 months later and in a cell line established from a biopsy of a postvaccination lymph node. Microsatellite analysis revealed the presence of LOH in chromosomes 6 and 15 in tumor samples, showing an accumulation of chromosomal loss at specific short tandem repeats in successive metastases during disease progression. HLA loss correlated with decreased tumor CD8+ T‐cell infiltration. Early incidence of β2m defects can cause an immune selection and expansion of highly aggressive melanoma clones with irreversible genetic defects causing total loss of HLA class I expression and should be taken into consideration as a therapeutic target in the development of cancer immunotherapy protocols.