Biology of Human Tumors Genetic Evolution of T-cell Resistance in the Course of Melanoma Progression

Purpose: CD8þ T lymphocytes can kill autologous melanoma cells, but their activity is impaired when poorly immunogenic tumorphenotypes evolve in the course of disease progression.Here,we analyzed three consecutivemelanoma lesions obtained within one year of developing stage IV disease for their recognition by autologous T cells. Experimental Design: One skin (Ma-Mel-48a) and two lymph node (Ma-Mel-48b, Ma-Mel-48c) metastases were analyzed for T-cell infiltration. Melanoma cell lines established from the respective lesions were characterized, determining the T-cell–stimulatory capacity, expressionof surfacemolecules involved in T-cell activation, and specific genetic alterations affecting the tumor–T-cell interaction. Results:MetastasesMa-Mel-48a andMa-Mel-48b, in contrastwithMa-Mel-48c,were infiltrated byT cells. The T-cell–stimulatory capacity was found to be strong for Ma-Mel-48a, lower for Ma-Mel-48b, and completely abrogated for Ma-Mel-48c cells. The latter proved to be HLA class I–negative due to an inactivating mutation in one allele of the beta-2-microglobulin (B2M) gene and concomitant loss of the other allele by a deletion on chromosome 15q. The same deletion was already present in Ma-Mel-48a and Ma-Mel-48b cells, pointing to an early acquired genetic event predisposing to development of b2m deficiency. Notably, the same chronology of genetic alterationswas also observed in a second b2m-deficient