Molecular mechanisms of isotype switch recombination.

点击翻译按钮获取中文摘要

Mouse B cells express CD38 whose ligation by anti-CD38 antibody induces their proliferation and protection from apoptosis. We previously showed that stimulation of mouse splenic B cells with interleukin 5 (IL-5) together with CS/2, an anti-mouse CD38 monoclonal antibody, induces production of IgG1 and IgM.Here we examined the role of IL-5 and CS/2 in the expression of germline γ1 transcripts and the generation of reciprocal products forming DNA circles as by products of μ-γ1 switch recombination. By itself, CS/2 induced significant expression of germline γ1 transcripts in splenic naive B cells, whereas IL-5 neither induced nor enhanced germline γ1 expression. Increased cellular content of reciprocal product, which is characteristic of μ-γ1 recombination, was not observed after culturing B cells with CS/2, but increased reciprocal product along with high levels of IgG1 secretion was found when B cells were cultured with CS/2 plus IL-5. Although IL-4 did not, by itself, induce μ-γ1 recom … More bination in B cells stimulated with CS/2, in conjunction with CS/2 plus IL-5, IL-4 dramatically enhanced sterile γ1 transcription and IgG1 production. These results demonstrate that CD38 ligation induces only germline γ1 transcription and that IL-5 promotes both μ-γ1 switch recombination and IgG1 secretion in an IL-4 independent manner.Although some post-receptor signaling events of IL-5 in activated B cells have been characterized, the involvement of the Janus kinase/signal transducer and activator (Jak/Stat) pathway in IL-5 signaling has not been thoroughly evaluated. In this study, we examined whether IL-5 activates the Jak/Stat pathway in CD38-activated mouse splenic B cells. Both Stat5a and Stat5b were activated by IL-5 stimulation. The role of Stat5a and Stat5b in IL-5-induced μ-γ1 switch recombination and IgG1 production were documented, as IL-5 did not act on CD38-stimnulated splenic B cells of Stat5a^<-/-> and Stat5b^<-/-> mouse. Expression levels of germline γ1 transcripts to CD38 and of activation-induced cytidine deaminase (AID) in Stat5a^<-/-> and Stat5b^<-/-> B cells upon IL-5 stimulation were comparable to these of wild type B cells. Thus, both Stat5a and Stat5b are essential for at IL-5-dependent μ-γ1 switch recombination, and their targets may not be g1 and AID genes. The impaired μ-γ1 switch recombination by Stat5b^<-/-> B cells, but not by Stat5a^<-/-> B cells, were rescued in part by IL-4, as the addition of IL-4 to the culture of CD38- and IL-5-stimulated B cells did induce μ-γ1 switch recombination leading to significant IgG1 production. Our data support the notion that Stat5a and Stat5b are not redundant, but rather are at least partially distinctive in their function on B cell differentiation. Analysis of cell division cycle number of B cells, examined by using 5-, 6-carboxyfluorescein diacetate, succinimidyl ester (CFSE), revealed that μ-γ1 switch recombination and surface IgG1-positive cells were observed after 5 to 6 division cycles upon CS/2 and IL-5. Stat5a^<-/-> and stat5b^<-/-> B cells showed 5 to 6 cell division cycles, but they could express neither μ-γ1 switching nor surface IgG1. Less