RAPID COMMUNICATION In Vitro Reconstitution of Human B-Cell Ontogeny: From CD341 Multipotent Progenitors to Ig-Secreting

We describe a long-term, in vitro culture system initiated with CD341 or CD341CD382 umbilical cord blood hematopoi-etic progenitors that supports normal human B-lineage development, including the production of mature Ig-secret-ing B cells. In the first stage (human B-progenitor long-term culture [HB-LTC]), CD341 hematopoietic progenitors are cul-tured on the murine stromal cell line, S17, leading to the sustained production of large numbers of CD101, CD191 early B progenitors. Reverse transcriptase-polymerase chain reaction (RT-PCR) and three-parameter flow cytometry for VpreB (surrogate light chain), cytoplasmic m chain, and surface IgM expression were used to characterize the CD191 B progenitors present within these cultures. This analysis showed distinct B-lineage subpopulations, including pro-B cells, cycling pre-B cells, and IgM1, IgD2/1 immature B cells