Inhibition of T cell proliferation by cholera toxin involves the modulation of costimulatory molecules CTLA-4 and CD28

Cholera toxin (CT) is known to inhibit the proliferation of murine and human T lymphocytes. In this study we have analysed the mechanisms underlying the inhibitory effect of CT on subpopulations of human CD4+ and CD8+ T lymphocytes. We show that CT dramatically prevents the activation of resting T lymphocytes, whereas it has a minor effect on cells that have been previously activated. Analysis of DNA content of the CT-treated T cells showed an arrest in the G0/G1 phase and this correlated with high expression of the cyclin-dependent kinase inhibitor p27kip. Moreover, we show that CT up-regulates the expression of the inhibitory molecule CTLA-4 in naïve, effector and memory resting CD4+ T cells and in resting CD8+ T lymphocytes. The regulation of CTLA-4 expression by CT is at the transcriptional level. Indeed, in cells treated with CT we observed an increase of two mRNA variants coding for the membrane and the soluble CTLA-4 molecules. In parallel with the up-regulation of the inhibitory CTLA-4, CT down-modulates the costimulatory molecule CD28 on CD4+ and CD8+ resting T cells. The increased expression of CTLA-4 played a role in controlling T cell activation and function as blocking anti-CTLA-4 F(ab′)2 mAbs partially inhibited anti-CD3 mAbs induced proliferation. These findings show that the inhibition of T cell proliferation by CT affects early stages of the T cell activation and involves the modulation of costimulatory molecules CTLA-4 and CD28 on resting T cells.