[Adult T-cell leukemia (ATL)]

Adult T-cell leukemia (ATL) was first discovered and reported in Japan, where it has a high incidence in the southwest region. The retrovirus HTLV-I (human T-cell lymphotropic virus type I) is considered to be related to its etiology. In ATL endemic areas, HTLV-I carriers are found at a fairly high percentage even among healthy individuals. ATL shows diverse clinical features. It can be divided into 5 types (acute type, chronic type, smoldering type, crisis type, and lymphoma type). ATL cells originate from the CD4-positive subset of peripheral T cells; they show a characteristic notch in the nucleus and a lobulation tendency. ATL resists chemotherapy, and patients with acute and lymphoma types have quite a poor prognosis. A definite diagnosis of ATL is made by documenting the presence of HTLV-I proviral DNA in the DNA of tumor cells. HTLV-I infection is caused by transmission of live lymphocytes via three routes (from mother to children, from males to females, and by transfusion). Familial occurrence of ATL is frequently seen. HTLV-I infection is also seen in other countries, but its incidence is highest in Japan. It is thus an urgent task for Japanese physicians to eliminate HTLV-I infection.