Fc receptors, immunoglobulin-binding factors and B chronic lymphocytic leukemia.

FcRs for all isotypes of Ig are found on lymphocytes and on lymphocyte derived tumour cells. Due to the availability of monoclonal antibodies, FcR for IgE (Fc epsilon R) and for IgG (Fc gamma R) have been characterized and their genes have been cloned. These receptors are markers of leucocyte sub-populations. Fc epsilon RII(CD23) in almost exclusively expressed on B lymphocytes and B tumour cells. Fc gamma RII(CDw32) is found on B lymphocytes, but also on monocytes, neutrophils, eosinophils and platelets. Fc gamma RIII(CD16) is mostly borne by granulocytes, but is also a marker of NK cells ans some T lymphocytes. FcR are molecules which are readily released in the sera where they can be detected by radioimmunoassays and dot-blot assays. The amount of circulating Fc epsilon RII is strongly increased in sera from patients with B-CLL and this increase seems to be related to the Rai stage of the disease. Circulating Fc gamma RIII is not grossly modified in B-CLL, although some patients have very low amounts of these molecules. The significance of these data has to be envisioned in view of the possible role of immunoglobulin-Binding Factors (IBF), FcR-related immunoregulatory lymphokines, in the control of B cell proliferation. For instance, IgE-BF, a cleavage product of Fc epsilon RII has been proposed to act as an autocrine growth factor on malignant B cells. IgG-BF, antigenically related to Fc gamma RII in the mouse and to Fc gamma RIII in the human, inhibits the IgG production and the growth of malignant B cells. Thus, the dosage of FcRs and IBFs could bring new insights in the monitoring of B cell proliferations, including B-CLL.