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^hidTomato⁺B220^hiCD43^loCD5^lo B cells, which differentiated to peritoneal B-1a and B-1b cells. Bhlhe41 deficiency impaired the balance between CD19^hidTomato⁺B220^lo/-CD5^hi and CD19^hidTomato⁺B220^hiCD5^lo cells. Hence, we identified neonatal CD19^hidTomato⁺B220^hiCD43^loCD5^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.