Figure 1: Costimulation and coinhibition. The binding of CD28 or CTLA-4 receptors on T cells by CD80 and CD86 ligands on antigen-presenting cells (APCs) can lead to either costimulation or coinhibition depending upon the precise expression patterns of the receptors and ligands and on the state of activation of the two cells. (a) CD28 is expressed on resting T cells and can be engaged by either CD80 or CD86 on APCs, leading to activation of resting T cells. This costimulation leads in the T cells to increase production of growth factor, such as IL-2 and increase cell-survival signals, such as Bcl-xL. (b) CD28 and CTLA-4 are both expressed on activated T cells. Engagement of CTLA-4 on CD80 or CD86 on APCs decreases T cell proliferation, IL-2 production, and cell cycle progression.