CD25-NEGATIVE HAIRY-CELL LEUKEMIA - INTRACYTOPLASMIC DETECTION OF TACANTIGEN AND INTERFERON-INDUCED SURFACE EXPRESSION

Hairy cell leukaemia (HCL) is a chronic lymphoproliferative disease of B-cell lineage. One of the peculiar immunophenotypic markers is the s trong expression of the p55 chain of the interleukin-2 receptor (IL2R) , recognized by anti-CD25 (or anti-Tac) monoclonal antibody. However, it is known that in rare cases CD25 may not be detectable, even when v ariant forms of HCL are excluded. The possibility has not been investi gated that in these situations CD25 is present in the cytoplasm of the neoplastic cells. This paper describes a case in which the clinical, histological, and electron microscopic features were consistent with a typical HCL. Immunophenotype analysis showed the whole spectrum of ma rkers of HCL, except for the expression of IL2R. The soluble form of t he molecule was, however, increased in the patient's serum. Cytospin s taining of the neoplastic B cells with anti-CD25 clearly demonstrated the presence of IL2R in the cytoplasm of hairy cells. When the cells w ere cultivated in vitro in the presence of interferon-a2b, CD25 was de tectable at the membrane level. These findings suggest that at least s ome cases of CD25-negative HCL may express cytoplasmic IL2R.