Characterization of phenotypically distinct B-cell subsets and receptor-stimulated mitogen-activated protein kinase activation in human cord blood B cells

Human cord blood (CB) is a valuable source of hematopoietic stem cells, but clinical reports have indicated slow recovery of B‐cell development and function after CB transplantation. To investigate the basis of these B‐cell defects in reconstitution, we characterized B cells purified from CB. We compared B‐cell receptor activation and B‐cell subsets in CB, bone marrow (BM), and peripheral blood (PB). We found that in CB B cells activation of extracellular signal‐regulated kinase (ERK) and p38 following ligation of CD40 but not of the B‐cell antigen receptor (BCR) was inefficient. The patterns of expression of CD5, CD34, and CD40 in the B‐cell population of CB were similar to those in PB rather than in BM. The B cells in CB contained an increased proportion of B cells expressing a high level of CD24 and a low proportion of B cells expressing CD27, pointing to the presence of circulating CD24high immature transitional and CD27− naive B cells. CD40‐mediated activation of ERK and p38 was also minimal in these B cells of CB. These findings may account for the functional defects of B cells in transplanted CB.