The proto-oncogene c-fgr is expressed in normal mantle zone B lymphocytes and is developmentally regulated during myelomonocytic differentiation in vivo.

The proto-oncogene c-fgr is a member of the c-src gene family of cytoplasmic tyrosine kinases. Previous studies have suggested that it is normally expressed in neutrophils, monocytes, macrophages, and natural killer cells. c-fgr is also expressed in the B cells of certain lymphoproliferative disorders, namely, Epstein-Barr virus-associated lymphoproliferative disease, and in chronic lymphocytic leukemia, but it has not previously been detected in normal or reactive human lymphoid tissue. In this study we have determined the pattern of p55c-fgr protein expression in normal human hematopoietic and lymphoid tissues at the single-cell level using immunohistochemical and immunofluorescent techniques. We show that p55c-fgr expression is developmentally regulated with high-level expression first evident at the myelocyte stage of myeloid differentiation. In addition, we show that p55c-fgr is expressed in circulating B lymphocytes isolated from chronic lymphocytic leukemia patients but is not expressed in normal circulating B lymphocytes. Surprisingly, p55c-fgr is also expressed in a subpopulation of normal B lymphocytes, the mantle zone B lymphocytes. This demonstration that p55c-fgr is expressed in a normal B-lymphocyte subpopulation suggests that its expression in certain B-cell lymphoproliferative disorders may be an indirect consequence of, rather than a primary cause of, the neoplastic transformation process.