Fig. 1.

T cell activation. A: simplified model of early events in T cell signaling. In the absence of CD28 costimulation, canonical T cell signaling is mostly stalled at the linker for activation of T cells (LAT) signalosome stage, where phospholipase C (PLC)-γ remains unactivated. MHC, major histocompatibility complex; TCR, T cell receptor; P, phosphorylation; ZAP-70, ζ-chain-associated protein kinase 70; IP₃, inositol 1,4,5-trisphosphate; PI3K, phosphatidylinositol 3-kinase; B: sustained interaction between a T cell and an antigen-presenting cell (APC) results in the formation of an immunological synapse. Membrane reorganization creates a greater apposition of surface areas into which signaling and adhesion molecules are ordered into a “bulls-eye” arrangement. The relevant molecules are listed according to location. SMAC, supramolecular activation complex; L, ligand; LFA-1, lymphocyte function-associated antigen 1.