Activation of human CD4+ cells with CD3 and CD46 induces a T-regulatory cell 1 phenotype

The immune system must distinguish not only between self and non-self, but also between innocuous and pathological foreign antigens to prevent unnecessary or self-destructive immune responses. Unresponsiveness to harmless antigens is established through central and peripheral processes(1). Whereas clonal deletion and anergy are mechanisms of peripheral tolerance(2,3), active suppression by T-regulatory 1 (Tr1) cells has emerged as an essential factor in the control of autoreactive cells(4). Tr1 cells are CD4(+) T lymphocytes that are defined by their production of interleukin 10 (IL-10)(5) and suppression of T-helper cells(6); however, the physiological conditions underlying Tr1 differentiation are unknown. Here we show that co-engagement of CD3 and the complement regulator CD46 in the presence of IL-2 induces a Tr1-specific cytokine phenotype in human CD4(+) T cells. These CD3/CD46-stimulated IL-10-producing CD4(+) cells proliferate strongly, suppress activation of bystander T cells and acquire a memory phenotype. Our findings identify an endogenous receptor-mediated event that drives Tr1 differentiation and suggest that the complement system has a previously unappreciated role in T-cell-mediated immunity and tolerance.