MIF rediscovered: cytokine, pituitary hormone, and glucocorticoid-induced regulator of the immune response.

The protein that has been historically called macrophage migration inhibitory factor (MIF) was one of the first cytokine activities to be discovered and was originally described to be a T lymphocyte product that inhibited the random migration of macrophages. Over the years, additional molecules with MIF "activity" have been described and the precise role of the original MIF "protein" remained enigmatic. Recent studies have led to the discovery of a pituitary mediator that appears to act as the counterregulatory hormone for glucocorticoid action within the immune system. Isolated as a product of murine anterior pituitary cells, this peptide was sequenced and found to be the mouse homolog of MIF. MIF has the unique property of being released from macrophages and T cells in response to physiological concentrations of glucocorticoids. The secretion of MIF is tightly regulated and decreases at high, anti-inflammatory steroid concentrations. Once released, MIF "overrides" or counterregulates the immunosuppressive effects of steroids on immune cell activation and cytokine production. These observations suggest that MIF fills an important gap in our understanding of how the host initiates and controls immunity. Because glucocorticoids are an integral part of the host's global response to infection or tissue invasion, the physiological role of MIF is to act at an inflammatory site or lymph node to counterbalance the profound inhibitory effects of steroids on the immune response.