UC Davis

AbstractThe direction of the B cell response determines the quantity and quality of antibody produced, a balance of which is essential for an effective immune response. Quality of secreted antibody is regulated by the germinal center (GC) reaction, in which germinal center B cells (GCBs) undergo proliferation, somatic hypermutation of the BCR, and selection, collectively known as affinity maturation, to ultimately increase GCB affinity to antigen and thus improve antibody quality. Understanding these processes and the transcriptional networks involved in their control are essential to better understanding selection itself. The transcription factor IRF4 is an immediate early gene induced upon BCR and CD40 signaling and is required for activated B cell proliferation and differentiation into GCBs or plasma cells, situating it as a master regulator of B cell fate decisions. Despite its critical role in control of early B cell fates, the requirement for IRF4 in GCBs has been less understood. We hypothesized that, given its importance in early B cell fate decisions, IRF4 also functions during the GC response to control efficient selection of GCBs. By halving Irf4 gene copy number in an antigen-specific BCR transgenic mouse model, we were able to study the requirement for IRF4 in GC processes. Here, we show that IRF4 is expressed in GCBs at levels greater than naive B cells throughout the course of the GC response, and levels of IRF4 expression directly regulate acquisition of high affinity mutations. While GCB phenotype, GC zonation, proliferation, class-switching, BCR expression, and rates of somatic hypermutation were comparable to wild-type cells, Irf4 haploinsufficient GCBs were impaired in the generation of high affinity mutations. Further, in a competition setting, Irf4 haploinsufficient cells were disadvantaged for GC entry, indicating a role for B cell-intrinsic IRF4 in the acquisition of T cell help. We propose a mechanism whereby IRF4 acts to promote effective T cell interactions during both the GC entry and GCB selection checkpoints, consequently regulating affinity maturation.