The Co-Stimulatory Signal During T-cell Activation
Full description or abstract
For a T cell to be activated by a specific antigen, the T cell receptor must recognize complexes of MHCI with the antigen on the surface of an antigen-presenting cell. T cells and the T cell receptor complex do not respond to antigen in solution, but even for the specific antigen they only respond to antigen-MHC-1 complexes on the cell surface. This interaction is necessary for T cell activation, but it is not sufficient. T cell activation also requires a co-stimulatory signal involving interaction of CD28 on the T cell with CD80 or CD86 (B7 family genes) on the antigen-presenting cell. CD28 activates a signal transduction pathway acting through PI-3K, Lck and Grb-2/ITK to provide its co-stimulatory signal for T cell activation. Another means to control T cell activation is by expressing factors that down-regulate T cell activation. Signaling by activated T cell receptors induces expression of CTLA-4, a receptor that opposes T cell activation. CTLA-4 has a higher affinity than CD28 for B7 proteins, terminating T cell activation. ICOS is a protein related to CD28 that is only expressed on activated T cells, and that provides another important co-stimulatory signal. The requirement for co-stimulatory signals provides additional control mechanisms that prevent inappropriate and hazardous T cell activation.
