Hairy cell leukaemia: seven cases with probable B-lymphocytic origin.

Mononuclear cells from seven patients with hairy cells leukaemia were examined for features suggestive of either a lymphocytic or monocytic origin. Immunofluorescent staining of both methanol fixed and incubated cells, using monospecific antisera, revealed a predominant cell-associated immunoglobulin in each case. Three were positive for mu and kappa chains, two for gamma and kappa chains, one for delta and kappa chain determinants and one reacted only with antigamma chain serum. Formation of EAC rosettes, a feature of both B lymphocytes and monocytes, was variable. T cells, as judged by E rosettes, were not elevated in any patient. Phytohaemagglutinin reactivity was normal in six and depressed in one case. With the exception of minimal activity in assays for glass adherence and latex particle phagocytosis, none of the cells showed features typical of monocytes. Hairy cells were negative by peroxidase stain and lacked the electron microscopic characteristics of monocytes. They did not react in either rosette or phagocytic assays with anti-A or anti-D coated erythrocytes nor did they elaborate granulocyte colony stimulating factor, a monocyte-derived in vitro granulopoietin. Although unequivocal classification of these abnormal cells is not possible, the data storngly suggests that this represents a variant of a B lymphocytic neoplasm.