Regulation of CD40 function by its isoforms generated through alternative splicing

CD40 is a member of the tumor necrosis factor receptor superfamily. The interaction between CD40 and CD40 ligand (CD154) activates NF-κB, Jun N-terminal kinase, and Janus kinase/signal transducers and activators of transcription pathways and promotes B cell growth, differentiation, and survival as well as IL-12 production in macrophages and dendritic cells. We demonstrate here the existence of multiple isoforms of CD40 mRNA generated by alternative splicing and show that their expression is regulated differentially in activated macrophages and dendritic cells. Pre-CD40 RNA is spliced preferentially out to signal-transducible CD40 mRNA in the early stage of activation; half of the CD40 mRNA is replaced by the signal-nontransducible CD40 mRNAs in the later stages (24 h). Using IL-12 p40 gene expression as a reporter for CD40 signaling, we show that three of the alternative isoforms can disable signaling through CD40. The major alternative isoform lacks the membrane-associated endodomain and seems to reduce the amount of the signal-transducible form available on the cell surface. It would seem, therefore, that CD40 expression is controlled by posttranscriptional and posttranslational regulation through alternative splicing. Modulation of isoform expression may provide a mechanism by which cells regulate their susceptibility to CD40L signaling.

Trichostatin A reverses skewed expression of CD154, interleukin-10, and interferon-γ gene and protein expression in lupus T cells

In systemic lupus erythematosus (SLE), T helper cells exhibit increased and prolonged expression of cell-surface CD40 ligand (CD154), spontaneously overproduce interleukin-10 (IL-10), but underproduce interferon-gamma (IFN-γ). We tested the hypothesis that the imbalance of these gene products reflects skewed expression of CD154, IL-10, and IFN-γ genes. Here, we demonstrate that the histone deacetylase inhibitor, trichostatin A, significantly down-regulated CD154 and IL-10 and up-regulated IFN-γ gene expression in SLE T cells. This reversal corrected the aberrant expression of these gene products, thereby enhancing IFN-γ production and inhibiting IL-10 and CD154 expression. That trichostatin A can simultaneously reverse the skewed expression of multiple genes implicated in the immunopathogenesis of SLE suggests that this pharmacologic agent may be a candidate for the treatment of this autoimmune disease.