CD28, Costimulator or Agonist Receptor?

Changes in the state of a cell as it responds to signals delivered by soluble mediators as well as by engagement of ligands displayed on cell surfaces or on extracellular matrices are the basis of differentiation, development, and the activation of effector functions. The molecular mechanisms by which ligands of cell surface receptors elicit such changes occupy not only a large proportion of the time and interest of cell and molecular biologists, but also account for an extensive fraction of the prokaryotic and eukaryotic genomes and proteomes. Complex systems that lend themselves to a deep analysis of such cell-signaling events include the network by which the immune system communicates within and among its major components: T cells, B cells, NK cells, and APCs. The study of T cells and the means of their activation have been augmented by our understanding of the primary sensor of...