Qa-2 is a glycolipid anchored, MHC encoded class I molecule expressed at high levels on all murine peripheral T lymphocytes. Anti-Qa-2 antibodies have previously been found to stimulate T cells to proliferate in the presence of crosslinking antibody and PMA. We have examined the effect of anti-Qa-2 antibodies on T cells stimulated with a suboptimal concentration of immobilized anti-CD3. When anti-Qa-2 antibodies were co-immobilized with limiting anti-CD3, in the absence of PMA, a clear augmentation of T cell proliferation was seen. Interestingly, the co-stimulatory anti-Qa-2 antibodies could be directed against epitopes mapped to either the α3 or the α1/α2 Qa-2 domains. As was the case with activation induced by soluble/crosslinked anti-Qa-2 antibodies plus PMA, CD8+ T cells were less able to be costimulated with anti-Qa-2 antibodies than CD4+ cells. Surprisingly, Ca2+ mobilization was only seen when two anti-Qa-2 antibodies reactive to separate structural domains were co-crosslinked on the surface of Indo-1 loaded T cells with a suboptimal concentration of anti-CD3. Collectively these results raise questions regarding the mechanism of Qa-2 mediated signaling and its potential role in T cell activation.
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