NF-κB is involved in regulation of CD40 ligand expression on Mycobacterium bovis bacillus Calmette-Guérin-activated human T cells

Interaction between CD40L (CD154) on activated T cells and its receptor CD40 on antigen-presenting cells has been reported to be important in the resolution of infection by mycobacteria. However, the mechanism(s) by which Mycobacterium bovis bacillus Calmette-Guérin (BCG) up-regulates membrane expression of CD40L molecules is poorly understood. This study was done to investigate the role of the nuclear factor κB (NF-κB) signaling pathway in the regulation of CD40L expression in human CD4⁺ T cells stimulated with BCG. Specific pharmacologic inhibition of the NF-κB pathway revealed that this signaling cascade was required in the regulation of CD40L expression on the surface of BCG-activated CD4⁺ T cells. These results were further supported by the fact that treatment of BCG-activated CD4⁺ T cells with these pharmacological inhibitors significantly down-regulated CD40L mRNA. In this study, inhibitor κBα (IκBα) and IκBβ protein production was not affected by the chemical protease inhibitors and, more importantly, BCG led to the rapid but transient induction of NF-κB activity. Our results also indicated that CD40L expression on BCG-activated CD4⁺ T cells resulted from transcriptional up-regulation of the CD40L gene by a mechanism which is independent of de novo protein synthesis. Interestingly, BCG-induced activation of NF-κB and the increased CD40L cell surface expression were blocked by the protein kinase C (PKC) inhibitors 1-[5-isoquinolinesulfonyl]-2-methylpiperazine and salicylate, both of which block phosphorylation of IκB. Moreover, rottlerin a Ca²⁺-independent PKC isoform inhibitor, significantly down-regulated CD40L mRNA in BCG-activated CD4⁺ T cells. These data strongly suggest that CD40L expression by BCG-activated CD4⁺ T cells is regulated via the PKC pathway and by NF-κB DNA binding activity.