Evaluation of Leu-M5 (CD11c) in hairy cell leukemia by the alkaline phosphatase anti-alkaline phosphatase technique.

The concomitant presence of B antigens and of the antigen recognized by the monoclonal antibody Leu-M5 (CD11c) on neoplastic lymphoid cells has been reported to be largely restricted to hairy cell leukemia (HCL). The authors studied Leu-M5 reactivity of neoplastic cells from 59 patients whose specimens were referred with a stated diagnosis of HCL by using the alkaline phosphatase anti-alkaline phosphatase technique on peripheral blood (PB) and bone marrow (BM) specimens. Tartrate-resistant acid phosphatase (AcP-T) activity was also studied. In 49 patients, HCL had been confirmed previously by BM biopsy, and specimens were evaluated for disease status during or after therapy with interferon (IFN) or 2'-deoxycoformycin. The remaining ten patients were newly referred for confirmation of the diagnosis of HCL before therapy. In all 55 patients in whom the BM biopsy demonstrated HCL, virtually every leukemic cell was Leu-M5 reactive, and the reaction proved, in some cases, to be helpful in the detection of small numbers of hairy cells in PB or BM preparations. AcP-T reactivity was demonstrated in the neoplastic cells of 52 of these 55 patients, including all but 3 of those receiving IFN, and was helpful in confirming persistent leukemia when interpretation of BM biopsy sections was difficult because the numbers of hairy cells were small. However, in four of the ten newly referred patients, BM biopsy showed features of splenic lymphoma with villous lymphocytes, rather than HCL. The neoplastic cells of these four patients were of B-cell origin and in three were Leu-M5 reactive. The authors' study indicates that Leu-M5 is present in nearly all hairy cells, but its presence in conjunction with other B-cell markers is not specific for HCL.