Relatively normal human lymphopoiesis but rapid turnover of newly formed B cells in transplanted nonobese diabetic/SCID mice

Human B lineage lymphocyte precursors in chimeric nonobese diabetic/SCID mice transplanted with umbilical cord blood cells were directly compared with those present in normal bone marrow. All precursor subsets were represented and in nearly normal proportions. Cell cycle activity and population dynamics were investigated by staining for the Ki-67 nuclear Ag as well as by incorporation experiments using 5-bromo-2′-deoxyuridine. Again, this revealed that human B lymphopoiesis in chimeras parallels that in normal marrow with respect to replication and progression through the lineage. Moreover, sequencing of Ig gene rearrangement products showed that a diverse repertoire of V_H genes was utilized by the newly formed lymphocytes but there was no evidence for somatic hypermutation. The newly formed B cells frequently acquired the CD5 Ag and had a short life span in the periphery. Thus, all molecular requirements for normal B lymphocyte formation are present in nonobese diabetic/SCID mice, but additional factors are needed for recruitment of B cells into a fully mature, long-lived pool. The model can now be exploited to learn about species restricted and conserved environmental cues for human B lymphocyte production.