Identification of novel B-1 transitional progenitors by B-1 lymphocyte fate-mapping transgenic mouse model Bhlhe41 (dTomato-Cre)

B-1 lymphocytes exhibit specialized roles in host defense against multiple pathogens. Despite the fact that CD19(+)CD93(+)B220(lo/-) B cells have been identified as B-1 progenitors, the definition for B-1 progenitors remains to be elucidated as CD19(+)CD93(+)B220(+) B cells are capable to give rise to B-1 cells. Given that transcription factor Bhlhe41 is highly and preferentially expressed in B-1 cells and regulates B-1a cell development, we generated a transgenic mouse model, Bhlhe41(dTomato-Cre) , for fate mapping and functional analysis of B-1 cells. Bhlhe41(dTomato-Cre) mice efficiently traced Bhlhe41 expression, which was mainly restricted to B-1 cells in B-cell lineage. We showed an efficient and specific Cre-mediated DNA recombination in adult B-1 cells and neonatal B-1 progenitors rather than B-2 cells by flow cytometric analysis of Bhlhe41 (dTomato-Cre/+) Rosa26 (EYFP) mice. Treatment of Bhlhe41 (dTomato-Cre/+) Rosa26 (iDTR) mice with diphtheria toxin revealed a robust efficacy of B-1 cell depletion. Interestingly, using Bhlhe41 (dTomato-Cre) mice, we demonstrated that neonatal B-1 progenitors (CD19(+)CD93(+)B220(lo/-)) expressed Bhlhe41 and were identical to well-defined transitional B-1a progenitors (CD19(+)CD93(+)B220(lo/-)CD5(+)), which only gave rise to peritoneal B-1a cells. Moreover, we identified a novel population of neonatal splenic CD19(hi)dTomato(+)B220(hi)CD43(lo)CD5(lo) B cells, which differentiated to peritoneal B-1a and B-1b cells. Bhlhe41 deficiency impaired the balance between CD19(hi)dTomato(+)B220(lo/-)CD5(hi) and CD19(hi)dTomato(+)B220(hi)CD5(lo) cells. Hence, we identified neonatal CD19(hi)dTomato(+)B220(hi)CD43(lo)CD5(lo) B cells as novel transitional B-1 progenitors. Bhlhe41 (dTomato-Cre/+) mouse can be used for fate mapping and functional studies of B-1 cells in host-immune responses.