Phenotypic changes of phytohaemagglutinin-stimulated hairy cells

Hairy cell leukaemia (leukaemic reticuloendotheliosis) appears to be a homogeneous and well defined disease on the basis of clinical presentation, light and electron microscopic features and cytochemical characteristics, but the study of immunological markers of hairy cells (HC) from many patients reveals apparent heterogeneity. The most common phenotype associates B-cell and monocytic properties: HC usually express monoclonal surface (and in certain cases cytoplasmic) immunoglobulins, receptors for IgM and IgG Fc, and mouse erythrocytes, and la-like antigens. Additionally, they are capable of phagocytosis, glass adherence, lysozyme and peroxidase synthesis. However, most of these features are not constant and cases have been reported in which HC fail to express one or more of these properties. In certain cases HC even display a T-cell phenotype, while, in others, features of both T and B cells are expressed. Moreover, in two recently studied patients, the phenotype of HC in the blood differed from that in the spleen (B + T in the blood and B in the spleen). These surprising discrepancies led us to hypothesize that HC from the same individual might be able to express different phenotypes following an appropriate stimulus. We therefore studied immunological parameters of HC stimulated by mitogens and the results indeed showed that after stimulation by phytohaemagglutinin (PHA) the cells switched from B to T or B + T phenotypes.