IntroductionB cells are generated in the fetal liver or the adult bone marrow (BM) after immunoglobulin gene rearrangements that create a primary repertoire of random antibody (Ab) specificity. 1-3 B cells that react to various self-antigens (Ags) at immature stages are thought to experience anergy after encountering Ags or to be eliminated by apoptotic mechanisms in the BM microenvironment. [4][5][6] It has been suggested that a substantial proportion of B cells reactive to self-Ags undergoes a process known as receptor editing in which secondary immunoglobulin gene rearrangements replace variable (V) gene segments, generating a new Ab specificity as a part of the primary Ab repertoire. 7-11 B cells recruited to the circulation are stimulated by Ags and helper-T (Th) cells in the T cell area of secondary lymphoid organs. 12-14 Such Ag-driven B cells subsequently undergo rapid proliferation to become centroblasts located in the dark zone of germinal centers (GCs) and then arrest cell cycling and become small centrocytes in the light zone. [15][16][17][18][19][20] Recent studies proposed an interesting model in which mature B cells also undergo a similar process, termed receptor editing, in GCs of secondary lymphoid follicles after stimulation by Tdependent (TD) Ags. [21][22][23][24][25] This notion has been supported by the detection of rag1/rag2 transcripts and by the evidence of immunoglobulin gene rearrangements in situ, suggesting that receptor editing occurs in the GC light zone. 21 In addition, stimulation with anti-CD40 monoclonal antibody (mAb) plus interleukin-4 (IL-4), or lipopolysaccharide (LPS) plus IL-4, induced the up-regulation of rag1/rag2 transcripts and immunoglobulin gene rearrangements in mature surface immunoglobulin M (sIgM) ϩ B cells in vitro. 22,[24][25][26] These results suggested that mature B cells frequently undergo receptor editing in GCs after stimulation with Ags. The receptor editing mechanism might provide a more flexible and diverse Ab specificity response than the one produced solely by the somatic mutation mechanism. The receptor editing in GCs could be a requisite molecular event for an immediate and effective response producing high-affinity Abs against various antigenic stimuli. Alternatively, receptor editing could salvage B cells carrying anti-self specificities.The reactivation of rag gene expression has been suggested as one mechanism for the immunoglobulin gene rearrangements that occur in receptor revision. 27,28 To demonstrate direct in vivo evidence of the induction of RAG molecules in GCs and to measure the frequency of cells undergoing receptor editing (or receptor revision), we examined lymphoid organs for RAG1/green fluorescent protein (GFP) signal using immunized or unimmunized rag1/gfp knockin mice. Unimmunized heterozygous rag1/gfp knockin mice express GFP that can be detected at high sensitivity by microscopic observation of splenic sections. Surprisingly, when we examined GCs of immunized mice, we observed that the RAG1/GFP signal was not significantly indu...

It has been proposed that Ig gene rearrangement in the peritoneal cavity (Pc) B-1 cells might be involved in autoantibody generation. To study possible secondary B cell maturation, we prepared mice carrying a target integration 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 studied in Pc B-1 cells to detect cells during the stage of Ig gene rearrangement. In contrast to previous reports, Pc B-1 cells did not show RAG1 expression in adolescent or elderly mice. RAG1 expression was not induced in Pc B-1 cells in vivo after stimulation by oral or i.p. administration of LPS. Our results suggest that RAG1 expression in Pc B-1 cells is inhibited for a long period under normal condition and that this suppression is an essential state which maintains allelic exclusion of Ig genes.