The complement receptor 3, CR3 (CD11b/CD18), on T lymphocytes: activation-dependent up-regulation and regulatory function

The complement receptor 3 (CR3; CD11b/CD18) is present exclusively on leukocytes, particularly on NK cells, monocytes and polymorphonuclear neutrophils. Approximately 10% of peripheral T lymphocytes and, as we found now mainly CD8(+) cells, expressed CD11b. Upon stimulation, however, expression of CD11b was up-regulated also on CD4(+) cells. Stimulation of T cells either by cross-linked anti-CD3 and IL-2 or by mononuclear cells and mitogen yielded up to 28% CD11b(+) T cells. The majority of CD11b(+) T cells also expressed CD56. T cell lines established from healthy donors were also found to express CR3. When restimulated up to 90% of cells became positive for CD11b making those cells an ideal tool for studying the functional role of CD11b. Antibodies to CD11b and bona fide ligands for the complement receptor inhibited the anti-CD3-induced T cell proliferation and as well as IL-2 release. In contrast, proliferation of a CD11b(-) T cell line was not inhibited. Taken together, our data indicate an activation-dependent expression of the complement receptor on T cells and suggest a regulatory function.