SummaryWe previously identified a cis-acting element within the 3 0 untranslated region of CD40 ligand messenger RNA (mRNA) that is composed of three complex binding sites and acts to increase mRNA stability in both in vitro and in vivo systems. We now demonstrate the functional consequences of the three binding sites with respect to increasing both luciferase activity and mRNA stability in a heterologous transcript expressed in a T-cell line. The internal region B was shown to be a bona fide stability element because its presence increased luciferase activity fourfold over the unmodified transcript and its removal from the XbaI-HaeIII region resulted in rapid degradation of the transcript. Region A contained both a binding site for a polypyrimidine-tract-binding protein (PTB)-mediated complex (Complex I) and an upstream, adjacent sequence that was a negative regulator of mRNA stability. Region C bound Complex II, which contained both PTB and heterogeneous nuclear ribonucleoproteinL (hnRNPL), and was less effective as a stability element on its own compared to region B. Our findings demonstrate differential levels of activity for the three binding sites relative to the turnover of CD40 ligand mRNA, suggesting that the lack of binding of Complex I/II during the early stages of T-cell activation contributes to the rapid degradation of the CD40 ligand mRNA transcript.

Our previous results demonstrated that B cells from a patient (pt1) with non-X-linked hyper-IgM syndrome (HIGM) possess an atypical CD23(lo) phenotype that is unaffected by CD40-mediated activation. To investigate the molecular mechanism underlying defective CD23 expression in pt1 B cells, we used lymphoblastoid cell lines that express LMP1 under the control of a tetracycline-inducible promoter (LCL(tet)). Our analysis revealed that the CD23(lo) phenotype in the pt1-LCL(tet) cells is a direct consequence of diminished CD23 transcription. We demonstrate a marked decrease in c-Rel-containing complexes that bind to the proximal CD23a/b promoters in pt1-LCL(tet) extracts, resulting from an overall lower expression of c-Rel in pt1-LCL(tet) cells. Analysis of c-Rel mRNA revealed relatively equal amounts in pt1-LCL(tet) and control LCL(tet) cells, indicating that diminished c-Rel protein expression is unrelated to decreased transcription. Finally, a critical role for c-Rel in CD23 regulation was demonstrated by effectively altering c-Rel expression that resulted in the direct modulation of CD23 surface expression. Collectively, these findings demonstrate that low levels of c-Rel are the underlying cause of aberrant CD23 expression in pt1 B cells and are likely to play a critical role in the pathophysiology of this form of HIGM.

Analysis of subclass-specific germline transcription in activated peripheral B cells revealed a highly biased expression pattern of the four Iγ transcripts to signals through CD40 and IL-4. This difference was most pronounced when comparing the profile of Iγ1 and Iγ4 transcripts and was not expected given the very high degree of sequence conservation between promoters. In this report, the influence of sequence differences on the regulation of the Iγ1 and Iγ4 promoters has been investigated given the highly muted transcriptional activity of the Iγ4 promoter. Two regions were analyzed where single nucleotide differences corresponded to major changes in transcriptional activity. These regions were the previously defined CD40 response region containing three putative NF-κB-binding sites and the downstream 36-bp region containing CREB/activating transcription factor and κB6 sites. Mutation of a single nucleotide at position 6 within the Iγ4 κB6 site increased promoter activity to ∼50% of the activity of the Iγ1 promoter. Furthermore, elevated promoter strength corresponded with increased binding of p50, p65, c-Rel, RelB, and p300 proteins to a level comparable with that of Iγ1. Minor nucleotide changes to both the Iγ4 CD40 response region and the 36-bp element resulted in a response undistinguishable from an Iγ1 response, suggesting cooperation between the two regulatory regions for optimal transcriptional activity. Collectively, these mutational analyses suggest that minor sequence differences contribute to the composition and affinity of transcriptional protein complexes regulating subclass-specific germline transcription, which in part impacts the overall level of class switch recombination to targeted CH regions.

LMP1-mediated activation of nuclear factor of kappaB (NF-κB) is critical for the ligand independent proliferation and cell survival of in vitro EBV-transformed lymphoblastoid cell lines (LCLs). Previous experiments revealed that a majority of LMP1-dependent responses are regulated by NF-κB. However, the extent that individual NF-κB family members are required for these responses, in particular, c-Rel, whose expression is restricted to mature hematopoietic cells, remains unclear. Here we report that low c-Rel expression in LCLs derived from a patient with hyper-IgM syndrome (Pt1), resulted in defects in proliferation and cell survival. In contrast to studies that associated loss of NF-κB with increased apoptosis, Pt1 LCLs failed to initiate apoptosis and alternatively underwent autophagy and necrotic cell death. Whereas the proliferation defect appeared linked to a c-Rel-associated decrease in c-myc expression, identified pro-survival and pro-apoptotic targets were expressed at or near control levels consistent with the absence of apoptosis. Ultrastructural examination of Pt1 LCLs revealed a high level of cellular and ER stress that was further supported by gene expression profiling showing the upregulation of several genes involved in stress and inflammation. Apoptosis-independent cell death was accompanied by increased expression of the inflammatory marker, caspase-4. Using gene overexpression and siRNA knockdown we demonstrated that levels of c-Rel directly modulated expression of caspase-4 as well as other ER stress genes. Overall, these findings reveal the importance of c-Rel in maintaining LCL viability and that decreased expression results in ER stress and a default response leading to necrotic cell death.