Immunologic control of human T-cell leukemia virus type I and adult T-cell leukemia.

Host T-cell responses to human T-cell leukemia virus type I (HTLV-I) control the expansion of HTLV-I-infected cells and are determinants of the equilibrium proviral load in vivo. Insufficient T-cell responses are regarded as an immunologic risk factor for adult T-cell leukemia (ATL) because they allow increased proviral loads, which represent an epidemiologic risk factor for ATL. ATL cells from approximately half of ATL cases retain the ability to express HTLV-I Tax, a major target antigen of HTLV-I-specific cytotoxic T-lymphocytes (CTL), whereas Tax-specific CTL in ATL patients are inactive. Tax-specific CTL responses are strongly activated after hematopoietic stem cell transplantation in some ATL patients in long-term remission, indicating that HTLV-I Tax is expressed in vivo rather than being silent, and that the donor-derived T-cell system can recognize it. These findings strongly suggest that reactivation of Tax-specific CTL by vaccines may be promising for prophylaxis of ATL in the high-risk group of HTLV-I carriers and for therapy of ATL in patients whose tumor cells are capable of expressing Tax.