Molecular mechanisms of the spatial and temporal regulation of T cell receptor signal transduction

The T lymphocyte is essential for establishing an acquired immune response. A T cell's functional status is regulated through complex networks of molecules from the cell surface to the nucleus. In an effort to understand T cell biology at the molecular level, we investigated how (1) spatial and (2) temporal control of the proximal signaling networks may direct the character and extent of T cell activation. (1) Lateral mobility and spatial organization of proteins within the plasma membrane are likely to mediate the initial events coordinating T cell activation. Lipid rafts, distinct cholesterol/sphingolipid-rich membrane microdomains, provide a mechanism for this regulation by concentrating or excluding signaling proteins. We demonstrate in peripheral blood T cell lymphoblasts that immediate early phosphotyrosine signal transduction through the T cell receptor complex is functionally dependent on a distinct population of lipid rafts. Specifically, cholesterol extraction destabilizes the membrane microdomains containing Lck, while the rafts containing the adapter protein LAT remain intact. After T cell activation Lck and LAT co-localize to 50–100 nm microdomains in the plasma membrane, indicating that sequestration of these proteins into distinct lipid rafts may function to regulate the initiation of T cell signal transduction. (2) The temporal nature of regulating signal transduction is in part dependent on the aforementioned spatial aspect, but we have recently uncovered an as yet unappreciated feature of signal transduction through the T cell receptor complex. While it has been known for some time that kinases and phosphatases both take part in T cell activation (and inhibition), we show that these two opposing enzymatic activities are occurring co-temporally in a dynamic manner, which we believe acts to adjust the gain for signal transduction through the T cell receptor. This kind of molecular amplifying or dampening of the proximal signaling pathway could play an essential role in rapid discrimination of antigen from the vastly more abundant self-peptides continuously presented to T cells, as well as differentially directing second messenger pathways required for cytokine synthesis. Understanding these complex interactions among T cell signaling molecules will create targets for directed pharmacological interventions with the potential for highly specific immune modulation.