Signaling features of T cells: Implications for the regulation of the anti-allograft response
T cell activation is contingent upon the antigenic signal and an additional (costimulatory) signal provided by the antigen presenting cell (APCs). The antigenic signal is initiated by the physical contact between the clonally variant T cell receptor and the antigenic peptide presented in the context of major histocompatibility complex proteins expressed on the surface of APCs. A number of cell-surface proteins are candidate molecules for the generation of the obligatory costimulatory signal. Our primary data that engendered the hypothesis that the T cell surface CD2 antigen functions as a receptor for APCs and generates the costimulatory signal obligatory for antigen-dependent T cell activation and some of the molecular mechanisms for the synergism between the T cell receptor and the CD2 antigen-derived signals are reviewed in this report. The clinical implications of the formulation that the CD2 antigen-derived signals complement TCR/CD3 complex-derived signals in promoting T cell activation and the therapeutic potential of a CD2 antigen-targeted therapy are also outlined in this review.