Delineating Early Events in B1 B‐cell Development

Abstract

The recent description of CD19+CD45R/B220lo/neg B1‐specified progenitors (B1P) by Montecino‐Rodriguez and Dorshkind adds support for the notion that these specialized lymphocytes arise via a distinct lineage. However, the origin of B1P was not described and developmental relationships to conventional B2 cells were unclear. Therefore, RAG‐1+ early lymphoid progenitors (ELP) and Lin‐ Sca‐1+ c‐KitLo IL‐7Rα+ common lymphoid progenitors (CLP) were isolated from bone marrow of adult Rag1/GFP knock‐in reporter mice. Both categories of conventional lymphoid progenitors generated CD19+CD45R/B220lo/neg B1P cells in culture and when transplanted to sublethally irradiated recipients. Moreover, significant numbers of peritoneal CD11b+ CD5+ B1a and CD11b+ CD5‐ B1b cells were produced. B1 lineage development was also found to be IL‐7Rα‐dependent, and developmentally blocked in Btk−/−mice. Further study is needed to determine the principal differentiation route(s) utilized by B1 cells. However, our findings indicate at least some B1P can be produced in adult bone marrow from primitive B2 progenitors, suggesting a developmental relationship between these lineages.