RAG2 : GFP knockin mice reveal novel aspects of RAG2 expression in primaryand peripheral lymphoid tissues

We generated mice in which a functional RAG2:GFP fusion gene is knocked in to the endogenous RAG2 locus. In bone marrow and thymus, RAG2:GFP expressio n occurs in appropriate stages of developing B and T cells as well as in im mature bone marrow IgM(+) B cells. RAGS:GFP also is expressed in IgD(+) B c ells following cross-linking of IgM on immature IgM(+)IgD(+) B cells genera ted in vitro. RAG2:GFP expression is undetectable in most immature splenic B cells; however, in young RAG2:GFP mice, there are substantial numbers of splenic RAG2:GFP(+) cells that mostly resemble pre-B cells. The tatter popu lation decreases in size with age but reappears following immunization of o lder RAG2:GFP mice. We discuss the implications of these findings for curre nt models of receptor assembly and diversification.