Activation-dependent Posttranscriptional Regulation of CD40L is Required for an Optimal Germinal Center (GC) response

Abstract

Abstract We have previously shown that regulation of CD40L occurs in part through a posttranscriptional mechanism of activation-induced mRNA stability. This pathway is engaged at extended times of activation and is mediated by the binding of a polypyrimidine tract binding protein (PTBP1) complex to the 3′UTR of the CD40L mRNA. This process leads to sustained expression of CD40L on CD4 T cells at a time when overall transcript levels are low. To assess the role of this regulatory pathway on an ongoing immune response we engineered a novel knock-in mouse (CD40LΔ5) that lacked the PTBP1 stability element and challenged mice with NP-KLH and SRBCs. Examination of splenic subsets prior to immunization revealed no difference in number or ratio between mutant and WT mice. However, after immunization there were significant decreases in T-dependent antibodies (Abs) and these differences were exacerbated with secondary challenge. In contrast, there were no differences in Ab response to a T-independent antigen. In total, CD40LΔ5 mice displayed significant changes in multiple aspects of the GC response including reduced levels of plasma cells, memory cells and GL7+ B cells. However, there were no observed differences in the number of Tfh cells between mutant and WT mice. Evaluation of GC structure in splenic sections revealed highly disordered GCs in CD40LΔ5 compared to WT mice at 8 days following immunization with SRBCs. Therefore, we hypothesize that the T cell activation-induced stabilization of the CD40L message and the subsequent surface expression of the protein is critical for development of a robust GC response and this occurs on one level by directly impacting the structural architecture of the GC.