Phenotypic and functional characterization of human Leu1 (CD5) B cells.

Leu1 B cells have been observed in high numbers in CLL patients, fetal spleen, and in bone marrow transplant recipients. We studied these cells in normal peripheral blood, spleen, and tonsil. They were phenotypically and functionally similar to conventional B cells. Leu1 B cells in the spleen and blood displayed a mature phenotype similar to the Leu1 B cells in the fetus, some expressing high levels of both sIgM and sIgD, and some expressing high levels of sIgD but little sIgM. Most Leu1 B cells in the tonsil also expressed high levels of both sIgM and sIgD, although some tonsil Leu1 B cells had high levels of sIgM but little sIgD. The significance of the Leu1 B cell is unknown, although it may be involved in the humoral immunoincompetence of the fetus, of CLL patients, and of transplant recipients. In our data there is no evidence to suggest that these cells are activated, immature, or capable of an immunoregulatory function.