Lineage tracing and transplantation studies indicate endothelial origin of B-1a cells in the mouse embryo

Abstract

Abstract Innate-immune like B-1a cells are unique in its functions and developmental origin compared to conventional adaptive immune B-cells (B-2 cells). Especially, their origin and developmental pathway have been argued over four decades between two proposed models: the lineage model and the selection model. Although many transplantation studies support the lineage model in which B-1a cells are derived from fetal progenitors, contradictory results of fetal liver transplantation studies raise a question as to whether fetal liver hematopoietic stem cells (HSCs) are the primary source of the peritoneal B-1a cells. In order to address this question, we took two approaches using transplantation assays and lineage tracing mouse models that enable us to trace Tomato+ progeny of endothelial cells (ECs, Cdh5CreERT mice) and HSCs (Fgd5CreERT2 mice) at a time of Tamoxifen (Tam) injection, respectively. Results: First, we transplanted HSC-precursors (pre-HSCs) from the aorta-gonad-mesonephros (AGM) region of the mouse embryo at embryonic (E) 11.5 into irradiated NSG neonates. Interestingly, we found more B-1a repopulating capacity than multi-lineage in the pre-HSC population. The lineage tracing of hemogenic ECs revealed that B-1a progenitors and HSCs were simultaneously produced from ECs during E9.5 to 10.5, but B-1a production is diminished after E11.5 while HSC were still labeled at E11.5. The HSC-tracing model demonstrated the minimal contribution of FL HSCs to the postnatal B-1a cell pool. Conclusion: Our data addressed the long-lasting controversy and showed HSC-independent B-1a cell development in the mouse embryo.