Structural and functional properties of the „hairy”︁ cells of leukemic reticuloendotheliosis

Structural and functional studies were performed on „hairy”︁ cells from 7 patients with leukemic reticuloendotheliosis („hairy cell leukemia”︁) (HCL). In all cases tartrate‐resistant acid phosphatase‐containing cells were demonstrated. The abnormal cells displayed complement receptors in 6 cases although there was variation in the number of abnormal cells expressing the receptor. Receptors for IgG were present in all 7 cases on a high number of abnormal cells. In 6 cases the „hairy”︁ cells showed surface immunoglobulins (SIg) when examined immediately after isolation. Procedures to eliminate in vivo bound protein substantially decreased the number of SIg‐bearing cells, indicating that most SIg represented cytophilic protein. In 2 cases, however, SIg restricted to a single light chain type remained on the abnormal cells, suggesting that in these 2 cases the SIg may have been an intrinsic cellular product. Attempts to demonstrate immunoglobulin synthesis were unsuccessful and there was no evidence that the „hairy”︁ cells contained cytoplasmic immunoglobulin. In vitro phagocytosis of latex particles by the abnormal cells was observed in all cases by transmission electron microscopy although the number of phagocytic „hairy”︁ cells varied widely from case to case. In 4 of 5 spleens with HCL, normal macrophages detected by the presence of nonspecific esterase were abundant and markedly enlarged. The electronic size distribution of HCL suspensions demonstrated a characteristic double‐peaked curve and modal volumes seldom seen in other chronic leukemias or lymphomas. Quantitative scanning electron microscopic analysis of HCL populations corroborated that the peculiar „hairy”︁ appearance of the abnormal cells was due to extensive surface ruffles which are not observed in normal or neoplastic lymphocytes. Our findings suggest that the „hairy”︁ cells are structurally and functionally unique elements, different than any other normal or abnormal cell of the lymphoreticular system known at present. Studies of cellular DNA quantitation and thymidine incorporation indicated that the growth rate of the „hairy”︁ cells is exceedingly low.