Anti-Qa-2-induced T cell activation. The parameters of activation, the definition of mitogenic and nonmitogenic antibodies, and the differential effects on CD4⁺ vs CD8⁺ T cells

The MHC Ag Qa-2 is a glycolipid anchored class I molecule expressed at high levels on all peripheral T lymphocytes. In this study we found that anti-Qa-2 antibodies could stimulate the proliferation of murine T cells in vitro. Anti-Qa-2-induced proliferation required secondary cross-linking with anti-Ig antibody and the presence of PMA. Only Qa-2⁺ strains could be induced to proliferate by anti-Qa-2 antibody, but under the conditions employed, anti-CD3 could induce proliferation in Qa-2⁺ and Qa-2^- strains. Interestingly, only anti-Qa-2 reagents directed against the α3 domain of the Qa-2 class I molecule were effective in inducing proliferation. Furthermore, unlike purified CD4⁺ cells, purified CD8⁺ cells were unable to be stimulated by the anti-Qa-2 antibodies. These results lead to the inclusion of Qa-2 in a group of physiologically relevant, glycolipid-anchored, cell-surface molecules, mobilization of which can generate signals that initiate the proliferation of T cells. Such molecules may play a secondary role in cellular activation after the primary engagement of the TCR.