Patients with adult T-cell leukemia showed altered expression of class I HLA antigen in their peripheral blood lymphocytes. Acute type adult T-cell leukemia showed increased levels of the antigen expression compared to those of control group and smoldering type (P < 0.001 and 0.01, respectively). Natural killer sensitivity of infected cell lines with different levels of class I HLA expression showed an inverse relationship with the antigen expression. Further, various cell lines including human T-cell leukemia virus type I-infected cell lines treated with acid buffer, which selectively eliminated the surface class I HLA molecules from cell membrane, became more sensitive to natural killer-mediated lysis. These data suggested that the enhanced expression of class I HLA on peripheral blood lymphocytes of patients with acute type adult T-cell leukemia may contribute to escaping from the immunosurveillance system of natural killer cells in vivo.

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