THE CD40 LIGAND - AT THE CENTER OF THE IMMUNE UNIVERSE

For several years, the primary function of CD40 ligand (CD40L) has bee n believed to be in regulation of contact-dependent, CD40-CD40L-mediat ed signals between B- and T-cells, which are essential for the regulat ion of thymus-dependent (TD) humoral immune responses. Recently, a flu rry of reports indicate that CD40 is expressed by variety of cell type s other than B-cells that include dendritic cells, follicular dendriti c cells, monocytes, macrophages, fibroblasts, and endothelial cells. T hese studies show that CD40-CD40L interactions are important in inflam matory process. For the past few years, through the availability of CD 40L-knockout mice, new data have emerged to support the belief that CD 40L has many more functions than its role in TD humoral immunity. CD40 L-deficient mice have provided significant information towards our und erstanding of the in vivo role of CD40L. The current picture that emer ges indicates that CD40-CD40L interactions mediate many cell-mediated immune responses and T-cell-mediated effector functions that are requi red for proper functioning of the host defense system. This article fo cuses on the in vivo role of the CD40L in regulation of cell-mediated effector functions.