Simultaneous cross-linking of CD6 and CD28 induces cell proliferation in resting T cells

In the present study, we showed that simultaneous ligation of the monoclonal antibodies (mAb) against CD6 and CD28 induces T-cell proliferation in purified resting T lymphocytes in the absence of T-cell receptor (TCR) occupancy. No cell proliferation was observed when the mAb were cross-linked alone or used simultaneously in the soluble form. T-cell proliferation mediated through CD6/CD28 is accompanied by the up-regulation of interleukin-2 (IL-2) mRNA and expression of IL-2 receptors on the cell surface. In the presence of IL-2-neutralizing mAb the proliferative response of the T cell induced through CD6/CD28 was inhibited dose dependently. Cross-linking mAb to CD6 and CD28 alone or together did not down-regulate the CD3/TCR complex. T-cell proliferation mediated through CD6/CD28 was only partially blocked by the immunosuppressive drug, cyclosporin A (CsA), whereas anti-CD28-induced T-cell proliferation in the presence of the phorbol ester, 12-O-tetradecanoylphorbol-13-acetate (TPA), was unaffected. In sharp contrast T-cell proliferation mediated by anti-CD6 in the presence of TPA was efficiently blocked by CsA. In addition, two protein kinase C (PKC) inhibitors, GF 109203X and H-7 dose-dependently inhibited T-cell proliferation mediated through CD6/CD28, suggesting that PKC activation may be involved. Furthermore, there was a marked differential dose-dependent inhibitory effect of the PKC inhibitors on T-cell proliferation mediated by the co-ligation of anti-CD6 or anti-CD28 in the presence of anti-CD3, with the former being more sensitive to PKC inhibition. Taken collectively, our results suggest that T-cell activation can occur through an antigen-independent pathway by cross-linking the accessory molecules, CD6 and CD28, and that these two cell surface antigens may have distinct signalling pathways.