Evaluation of the effects of activation dependent regulation of CD40L on germinal center response using CD40LΔ5 mouse model
Creators: name: Narayanan, Bitha
Publisher: name: No Publisher Supplied
Year: 2020

description: The interaction between cognate T and B cells decides the progression of an immune response to a pathogen or self-antigen. Of the multiple signals that synchronize to fine-tune this union, the binding of CD40 on the surface of B cells to CD40L expressed on CD4 T cells is of paramount importance. Ligation of CD40 on antigen-experienced B cells is associated with the initiation and development of germinal centers (GCs) resulting in the subsequent generation of high affinity antibodies and B cell memory. Post-transcriptional regulation of CD40L has been implicated in regulating the activation dependent expression of this protein. Our lab has previously shown a polypyrimidine tract binding protein complex binds to the 3’ UTR of the CD40L mRNA and that the deletion of a PTBP1 binding stability element in the same results in a significant decrease in the half-life of the CD40L transcript and subsequently the surface expression at later stages of activation in vitro. To understand the biological basis for activation-induced posttranscriptional regulation of CD40L, a mouse was engineered with a deletion of two segments in the PTBP1 stability element that provide stability to the transcript and termed CD40LΔ5 (data not shown, work done by K. Voskoboynik and J. La Porta). Notably, splenic CD4 cells from the CD40LΔ5 mouse showed decreased CD40L expression at later stages following ex vivo activation. It has been well established that CD40L knock-out mice are unable to surmount an adequate immune response when subject to antigen challenge. The question we asked was whether the activation-induced pathway of CD40L mRNA stability was required to achieve an optimal immune response. It has been shown that GC B cells constantly shuttle between the T follicular (Tfh) rich, light zone (LZ) and the dark zone (DZ) where they undergo somatic hypermutation. B cells rely on interactions with the Tfh cells to provide direction as to how to evolve to optimize the immune response. We hypothesized that this interaction would be disrupted in CD40LΔ5 mice and subsequently result in the Tfh cells being unable to provide durable support to the B cells to develop into functioning antibody factories.To test our hypothesis, we assessed the immune response in CD40LΔ5 and wild type mice in response to a T dependent antigen, NP-KLH. We observed significantly reduced levels of antigen-specific antibodies in the mutant mice when compared to the wild type by ELISA. On evaluating whether this decreased antibody generation was subsequent to a reduced number of plasma cells (by ELISPOT), we found that there was indeed a decrease in the number of NP-KLH specific clones of plasma cells in the CD40LΔ5 mice. The splenic sections from WT and mutant mice revealed that the mutant mice were able to produce GCs in contrast to the CD40L knock out mice, but these GCs were much less organized with scattered GL7 (marker of GC B cells) positive cells when compared to the wildtype where distinct GL7 positive cores were surrounded by IgD positive rings. This data suggested that the CD40LΔ5 is allowing for seeding and initiation of GCs, but at subsequent stages, deregulated CD40L results in less effective development of the immune response.Based on phenotypic and genotypic evaluation we conclude that the CD40LΔ5 mice have reduced number of organized GCs and in turn lower levels of antibody secreting plasma cells and consequently lower titers of antibodies from isotypes IgG1, IgG2b, IgG2c, and IgM against NP. We also found decreased affinity maturation in the CD40LΔ5 mice, but the SHM rate was similar to that of the wild type. In addition, the alteration in CD40L mRNA stability alters the development of the memory B cell and long-term plasma cell populations. When we looked at differentially expressed genes by RNAseq, several genes associated with proliferation were down regulated in the CD40LΔ5 mice. On the contrary, genes linked to apoptosis were upregulated in the CD40LΔ5. We were able to confirm this pattern by flow cytometry on GC B cells. The sum total of our data strongly supports a major role for the posttranscriptional pathway of CD40L mRNA stabilization for regulating expression and ensuring the timely availability of CD40L on Tfh cells to signal activated B cells to produce a robust humoral response.