Cutting edge: Absence of expression of RAG1 in peritoneal B-1 cells detected by knocking into RAG1 locus with green fluorescent protein gene

It has been proposed that Ig gene rearrangement in the peritoneal cavity (P c) B-1 cells might be involved in autoantibody generation. To study possibl e secondary B cell maturation, we prepared mice carrying a target integrati on of gfp gene into a rag1 locus (rag1/gfp mice). The GFP(+) cells express rag1 mRNA and are undergoing Ig gene rearrangement. RAG1 expression was stu died in Pc B-1 cells to detect cells during the stage of Ig gene rearrangem ent, In contrast to previous reports, Pc B-1cells did not show RAG1 express ion in adolescent or elderly mice. RAG1 expression was not induced in Pc B- 1cells in vivo after stimulation by oral or i.p. administration of LPS, Our results suggest that RAG1 expression in Pc B-1cells is inhibited for a lon g period under normal condition and that this suppression is an essential s tate which maintains allelic exclusion of Ig genes.