Low prevalence of selective human leukocyte antigen (HLA)-A and HLA-B epitope losses in early-passage tumor cell lines

The down-regulation of human leukocyte antigen (HLA) class I molecules, esp ecially the selective down-regulation of certain allelic products, is belie ved to represent a major mechanism of tumor escape from immune surveillance . In the present report, an original approach is described to precisely eva luate and classify HLA class I epitope losses in 30 cancer patients with ma lignant melanoma and lung, breast, endometrium, ovary, and colon carcinoma tumors. Early-passage tumor cell lines were established in culture from the corresponding metastatic tumor lesions obtained in each patient. Both the cell lines and the tumor Lesions were compared, in their HLA-A and -B expre ssion, to the peripheral blood mononuclear cells (PBMCs) obtained from the same patient (autologous PBMCs). On the basis of HLA-genotyping data, the a ppropriate monoclonal antibodies identifying mono- and poly-morphic HLA-A a nd HLA-B epitopes were selected from a panel of 34 antibodies for a total o f 24 testable alleles. The selected antibodies were used not only in immuno histochemical assays on cryostatic tumor sections and cytospins of PBMCs bu t also in quantitative, sensitive flow cytometry assays on early-passage tu mor cells and PBMC suspensions. With this latter method, a low overall HLA expression was detected in 26 tumor cell explants and a complete, generaliz ed HLA-A, HLA-B, HLA-C loss in the remaining 4 cases. However, no complete, selective loss of any of the 45 tested HLA-A and HLA-B allomorphs was obse rved. Sequences from all of the HLA class I alleles could be detected at th e genomic DNA level in tumor cells and tissues. At variance from the litera ture and the results of immunohistochemical experiments performed in parall el on the corresponding tumor lesions, the relative proportions of the vari ous HLA epitopes were relatively preserved in each early-passage cell line/ PBMC pair, and selective increases, rather than decreases, in the expressio n of polymorphic HLA epitopes had the highest prevalence and greatest magni tude. Our data suggest an alternative tumor stealth strategy in which up- a nd down-regulation are equally important. This alternative model of tumor-h ost interaction better fits the available models of tumor cell recognition by CTLs and natural killer cells bearing activatory and inhibitory receptor s for HLA-A, HLA-B, HLA-C molecules.