Predominance of a distinct subtype of hairy cell leukemia in Japan.

Forty Japanese patients with hairy cell leukemia (HCL) were reviewed. Nine cases were diagnosed as typical HCL, and two cases had the features of HCL variant (prolymphocytic variant). The remaining 29 cases (72.5%) differed morphologically and hematologically from the other two groups in that they usually had a moderately high leukocyte count (average 27.9 x 10(3)/microliters), and abnormal cells showing a densely stained round nucleus and an inconspicuous nucleolus. Tartrate-resistant acid phosphatase reaction was weak, and their cells exhibited generally smooth or slightly irregular, cellular outlines in smears. The cells showed weak expression of surface immunoglobulin G (IgG) with kappa-chain predominance. CD25 antigen was not detected. Some of these findings resemble those of B-cell chronic lymphocytic leukemia, but the patients also had several important features of HCL. They had splenomegaly without significant lymphadenopathy. The abnormal cells were CD20+, CD11c+ and showed typical 'hairy morphology' under phase-contrast and scanning electron microscopy. Furthermore, spleen sections revealed diffuse infiltration by the abnormal cells in the red pulp. From these findings, we speculated that this group of patients constitute a distinct subtype of HCL which is commonly seen in Japan. We propose to term the disease as HCL Japanese variant.