Simultaneous ligation of CD5 and CD28 on resting T lymphocytes induces T cell activation in the absence of T cell receptor/CD3 occupancy.

The T cell surface molecules CD5 and CD28 have been shown to be receptors for accessory signals in T cell activation. We here demonstrate that in the absence of any other activating stimulus, simultaneous ligation of CD5 and CD28 by mAb induces polyclonal T cell activation. Immobilization of the anti-CD28 and anti-CD5 mAb was an essential requirement for T cell stimulation. This was done either through coating of the culture plates with goat anti-mouse Ig, or by coculture with mitomycin C-treated Fc gamma R-bearing P815 mouse mastocytoma cells. Most importantly, T cells could also be stimulated with B7, the natural ligand of CD28, and anti-CD5 presented on irradiated 3T6 mouse fibroblasts co-transfected with human Fc gamma RII and with B7. Neither immobilized mAb 9.3 (anti-CD28) nor any of four different anti-CD5 mAb were mitogenic as a sole stimulus. Immobilized mAb identifying CD4, CD7, or LFA-1 were not co-mitogenic with either mAb 9.3 or one of the anti-CD5 mAb. The T cell proliferation induced by cross-linking of CD5 and CD28 is IL-2-dependent, as was demonstrated by the cell-surface expression of the p55 chain of the IL-2R, the production of IL-2, and inhibition of the proliferative response by the anti-IL-2R mAb anti-Tac. CD5/CD28 ligation induced production of TNF-alpha, but not of IL-4, and did not induce modulation of the TCR/CD3 complex. Expression of IL-2R (p55) and of CD69 preferentially occurred on CD29-low naive cells, and indicated that about 50% of the cultured cells were activated. Cell proliferation was not increased by adding monocytes to the cultures and it was inhibited by PKC inhibitors (H7 and staurosporine) and by cyclosporine A. In conclusion, our data provide evidence for a pathway of Ag-independent T cell activation via CD5 and CD28, which preferentially stimulates native T cells.