A patient with a B-cell chronic lymphocytic leukemia, whose lymphocytes also formed rosettes with sheep red cells, is described. The B-cell nature of the malignant lymphocytes was determined by surface marker analysis, and cytochemical and ultrastructural studies. The lymphocyte membrane immunoglobulin (IgG1K) did not have anti-sheep red cell activity and was not responsible for the binding of sheep erythrocytes to the leukemic cells as shown by (i) the failure to inhibit rosette formation with anti-immunoglobulin reagents and (ii) the different sensitivity to proteolysis of the membrane immunoglobulin and the sheep erythrocyte receptor. The malignant lymphocytes expressed a receptor for sheep erythrocytes similar to that of normal T cells since they stained with monoclonal antibodies directed against the sheep red cell receptors. Furthermore these antibodies blocked rosette formation. Endogenous labeling experiments demonstrated that the patient's cells produced IgG both of the membrane and of the secretory type. The latter molecular form was also actively secreted. Ultrastructural studies demonstrated that the malignant clone comprised cells at different maturational stages and with different secretory properties. These findings were confirmed by the analysis of intracytoplasmic acid hydrolases, which are normally expressed at late maturational stages. These data are consistent with the hypothesis that a process of maturation was occurring within the malignant clone.
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