CD8+ T cell apoptosis induced by Escherichia coli heat-labile enterotoxin B subunit occurs via a novel pathway involving NF-κB-dependent caspase activation

Abstract

The B subunit of Escherichia coli heat‐labile enterotoxin (EtxB) is a potent immunomodulatory molecule capable of treating and preventing autoimmune disease. These properties result from its ability to bind to glycolipid receptors, principally GM1 ganglioside, and modulate immune cell function. EtxB receptor binding causes B cell activation, modulates monocyte cytokine secretion and triggers apoptosis of CD8+ T cells. These wide‐ranging effects suggest that B subunit receptor interaction triggers signaling events affecting cellular differentiation. We have investigated the processes by which EtxB induces CD8+ T cell apoptosis. We show that receptor interaction by EtxB activates caspase‐3 in CD8+ but not in CD4+ T cells. Inhibition of caspase‐3 blocks the apoptotic process. EtxB induces the activation of NF‐κB in both CD8+ and CD4+ T cells. The findings that (i) SN50, a peptide inhibitor of NF‐κB nuclear translocation, prevents caspase‐3 activation and subsequent apoptosis, and (ii) CD8+CD4– thymocytes from transgenic mice expressing a dominant‐negative form of the IκBα protein were markedly less susceptible to EtxB‐induced apoptosis than cells from wild‐type mice, indicate that NF‐κB is important in the induction of the apoptotic pathway. Further investigations revealed that while caspase‐8 activity is detected concomitant to caspase‐3, caspase‐9 activation, following mitochondrial cytochrome c release, is detectable later on. These observations are consistent with death receptor‐mediated signaling, however, experiments using lpr/lpr and p55 TNFR –/– mice rule out the involvement of Fas and the p55 TNF receptor, respectively. The data therefore indicate that EtxB‐mediated apoptosis occurs via a novel pathway involving NF‐κB.

Involvement of the intrinsic and extrinsic cell‐death pathways in the induction of apoptosis of mature lymphocytes by the Escherichia coli heat‐labile enterotoxin

“…In addition to the induction of apoptosis of developing T and B cells, LT is able to promote cell death in mature lymphocytes that in vitro is more marked in the CD8 + subpopulation [7][8][9].…”

“…This site overlaps with a NFkBbinding site, establishing a competition between ligand-bound glucocorticoid receptor and NFkB for the regulation of FasL expression [21]. Interestingly, the in vitro treatment of CD4 + and CD8 + T cells with the B subunit of LT induces a rapid activation and nuclear translocation of NFkB [9], which may interfere with the negative effect of glucocorticoids in the regulation of FasL expression and Fas-induced cell death of T cells. However, it is unknown whether such glucocorticoid and LT effects also occur in vivo.…”

“…In addition, distinct mechanisms account for the in vivo elimination of immature (intrinsic caspase-dependent cell-death pathways) versus mature lymphocytes (intrinsic and extrinsic caspase-dependent cell-death pathways) [7 and our present study]. In contrast, mature CD8 + T cells are more susceptible to LT-induced apoptosis than immature CD4 + CD8 + or mature CD4 + T cells in vitro [8,9]. The induction of CD8 + T cell apoptosis by LT in vitro correlates with the early activation of caspase 8 that is independent of Fas and TNFR engagement [8,9].…”

“…However, this early activation of caspase 8 seems independent of Fas and TNFR engagement [9]. Altogether, these results suggest that different cell-death pathways may account for the elimination of immature and mature lymphocytes after LT administration.…”

“…The LT-induced apoptosis of CD8 + T cells in vitro is preceded by a rapid nuclear translocation of NFkB, resulting in the increase of c-myc expression and the early activation of caspase 8 [8,9]. However, this early activation of caspase 8 seems independent of Fas and TNFR engagement [9].…”