T-cell activation is the net product of competing positive and negative signals transduced by regulatory molecules on antigen-presenting cells (APCs) binding to corresponding ligands on T cells. DC-HIL as a highly glycosylated type I transmembrane protein of 125 and 95 kDa containing an extracellular Ig-like domain. Having previously identified DC-HIL as a receptor expressed by APCs that contains an extracellular immunoglobulin (Ig)-like domain. DC-HIL is a negative regulator of T lymphocyte activation, and that this native inhibitory function can be blocked by exogenous DC-HIL, leading to enhanced immune responses.