Negative regulation of T cell activation.

T cell activation is negatively regulated by cytotoxic T lymphocyte antigen 4 (CTLA-4) and the killer cell inhibitory receptors. Endocytosis and signaling of CTLA-4 are regulated by tyrosine phosphorylation. While T cell activation is mediated by phosphorylation of immunoreceptor tyrosine-based activation motifs and tyrosine kinases, inhibitory signals are delivered by tyrosine phosphatases. Unresponsiveness is also induced by modulation of signaling components of the T cell receptor complex.