IN-VIVO AND IN-VITRO GENERATION OF A NEW ALTERED HLA PHENOTYPE IN MELANOMA-TUMOR-CELL VARIANTS EXPRESSING A SINGLE HLA-CLASS-I ALLELE

A new HLA-class-I altered phenotype is described in melanoma, This phe notype is the result of a combination of HLA-B-locus down-regulation a nd HLA-haplotype loss. The alteration was found in 2 melanoma cell lin es generated from 2 patients; one was derived from an in vivo lesion ( FM37) and the other was obtained after in vitro immunoselection (R22.2 ). The R22.2 cell line was isolated from FM55P, a cell line derived fr om a primary melanoma, after in vitro treatment with a heterologous HL A-A2-restricted cytotoxic-T-lymphocyte (CTL) clone. Two additional cel l lines from patient 55 were obtained from 2 s.c. metastases (FM55M1 a nd FM55M2). Iso-electric focusing and flow-cytometric studies showed a significant reduction in the expression of both HLA-B alleles in all cell lines studied. The expression of HLA-B-locus products recovered c ompletely after IFN-gamma treatment of FM55P, M1 and M2. In contrast, FM37 and R22.2 tumour cells showed an additional HLA defect: the absen ce of one HLA haplotype. Simple tandem-repeat polymorphism markers spa nning chromosome 6 showed that DNA from the 2 samples (FM37 and R22.2) showed loss of heterozygosity (LOH). In both cases, homozygosity was observed on 6p, which maps the HLA region, the final consequence being a tumour cell that expressed a single HLA-class-I allele (HLA-A3 and HLA-A1 respectively). FM37 cells may thus reflect the in vivo counterp art of resistance to lysis by HLA-A2-restricted tumour-infiltrating ly mphocytes. (C) 1998 Wiley-Liss, Inc.