Immunoglobulin isotype switching during early B cell development (IRM8P.702)

Abstract

Abstract During B cell lymphopoiesis in the bone marrow (BM), V(D)J joining assembles immunoglobulin (Ig) VH and VL genes mediated by RAG recombinase. Mature splenic B cells undergo class switch recombination (CSR) and somatic hypermutation initiated by activation induced deaminase (AID). However, the strict ontological separation of these processes may breakdown under some circumstances. Several immunodeficiencies including Omenn syndrome display increased serum IgE despite undetectable numbers of B cells. We hypothesized that CSR occurs in BM pro- or pre-B cells. Earlier, we showed that Rag1 and Mb1 deficient pro-B cells can be induced to undergo robust CSR, albeit to a restricted subset of IgH isotypes, including IgG2b and IgE. Immunization of Rag1-/- mice with LPS, promoted IgG2b switching in BM pro-B cells. We demonstrated that V(D)J joining can be induced to follow CSR. We found a unique stage specific chromatin conformation of Igh locus in pro-B cells that may repress IgG3 and IgG1 switching (Genes Dev. 27:2439). Here we show that CSR is inducible in WT pro-B cells. Using an elegant mouse model to irreversibly mark AID expressing cells, a knockin carrying AIDcre/+RosaYFP (Nat Imm. 10:1292), we assess the frequency of AID expression in BM pro-B cells activated in vitro and in vivo with LPS. These findings have implications for the hyper-IgE phenotype in Omenn syndrome, autoimmune repertoire development and leukemagenesis via the co-expression of the RAG and AID recombinases.