Alternate costimulatory molecules in T cell activation: differential mechanisms for directing the immune response.

T cells are required for an effective immune response against a wide range of pathogens and for the generation of immunological memory. T cell activation can be divided into two phases: an antigen-specific signal delivered through the T cell antigen receptor, and a costimulatory signal delivered through accessory molecules on the T cell surface. Following activation, T cells differentiate to acquire distinct effector functions depending on the costimulatory signal, cytokine environment, and the pathogen itself. Although CD28 has been identified as the dominant costimulatory molecule, several other molecules have been described as having a costimulatory function. This review will focus on recent evidence for the existence of alternate costimulatory molecules, and the differential roles they might play in the activation, development, and survival of T cells.