At 23 wk of gestation, the fetal spleen contains follicles of lymphocytes that coexpress B cell differentiation antigens, surface Ig, and the 67-kD pan-T lymphocyte antigen, CD5 (Leu-1). Such cells are thought to represent the normal equivalent cells of B chronic lymphocytic leukemia (CLL). This B cell leukemia is distinctive in that high proportions of patients have leukemic cells that express sIg bearing one or more crossreactive idiotypes (CRIs) that commonly are found on IgM autoantibodies. We performed immunohistochemical studies on fetal spleen at 23 wk of gestation using a panel of mAbs specific for autoantibody-associated CRIs. We find that high proportions (5-17%) of the lymphocytes within each follicle react with any one of the anti-CRI mAbs. Furthermore, there is little variation between primary follicles in the proportions of cells that express a particular CRI. Using a cocktail of four anti-CRI mAbs, we detect autoantibody-associated CRIs on approximately one-third of the lymphocytes within each of the primary B cell follicles. These data indicate that the many of the Igs produced during early B cell development may be structurally related to IgM autoantibodies and to Ig expressed in CLL and related CD5 B cell malignancies. Furthermore, these studies suggest that the repertoire of Ig V genes expressed in each primary B cell follicle may be representative of the total restricted Ig V gene repertoire expressed during early B cell ontogeny.

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