CELL CONTACT MEDIATED SUPPRESSION OF T HELPER REACTIVITY INDUCED BY REGULATORY T SUPPRESSOR CELLS.
R Ciubotariu, AI Colovai, Z Liu, J Li, S Jiang, and N Suciu-Foca, Department of Pathology, Columbia University, New York, NY.
The mechanism of immune suppression mediated by regulatory lymphocytes is poorly understood. To study the generation and function of T suppressor cells, we have developed allospecific and xenospecific T suppressor cell lines (Ts), by performing repeated in vitro stimulations of human T cells with allogeneic or xenogeneic APC in the presence of specific cytokines (IL-2, IL-7). Ts generated in this system carry the CD3+CD8+CD28-CD45RO phenotype, and inhibit in a dose-dependent manner the proliferative response of alloreactive or xenoreactive CD4+ Th cells. Ts specifically recognize MHC class I molecules on the surface of stimulating APC and suppress their costimulatory activity by inhibiting the expression of CD86 molecule. Transwell experiments, in which Ts were separated from Th by a semipermeable membrane, indicated that cell-to-cell interaction, and not the secretion of inhibitory cytokines, is required for the suppressive effect. This finding was further supported by the cytokine profile of Ts, which showed that Ts produce IFN-gamma, but not IL-4 or IL-10. Although activation of Ts occurs upon TCR ligation, neither cell division nor protein synthesis appears to be required for Ts activity. The possibility that CTLA-4 molecule or killer inhibitory receptors (CD94, p58.2 and NKG-2) are involved in suppression was ruled out by experiments showing that mAb specific for these molecules did not block the inhibitory activity of Ts. These findings suggest that Ts activation triggers the expression or redistribution at the cell surface of a pre-existing, possibly novel molecule, which is required for suppression. These data provide an insight into the mechanism of suppression mediated by regulatory T cells, and may offer a promising alternative for the development of new therapies in transplantation.