Abstract
T cells are mediators of the adaptive immune response. To properly mount a response, T cells use extracellular
receptors to sense their environment and transduce signals to intracellular signaling networks. While many
signaling pathways relevant to T cell function are established, less is known about how these pathways are
modulated to discriminate between different types of signals and thus represents a significant gap in our
knowledge base. Such knowledge would aid in controlling T cell activation and differentiation in multiple
therapeutic settings. One dominant signaling input is T cell receptor (TCR) signaling strength, which regulates T
cell differentiation, thymic development and cytokine signaling. In previous work, we identified that the strength
of the T cell receptor signal differentially regulated the AKT/mTOR signaling axis. TCR signal strength regulated
the phosphorylation of AKT which in turn controls AKT substrate specificity so that different TCR signal strengths
engage qualitatively different AKT signaling networks. While these results are intriguing, the basic biochemical
mechanisms that couple TCR signal strength to downstream signaling networks including differential AKT
activation remains ill defined. One pathway that could couple TCR signal strength to intracellular signaling
networks is phosphatidylinositol (PIP) metabolism. Many PIP species are bioactive and regulate signaling,
transcription, metabolism and RNA splicing. Following pMHC binding to TCR, PI3K phosphorylates PI(4,5)P2 to
generate PIP3 at the cell membrane. PIP3 has garnered interest because it activates kinases important for
immune function, including AKT and PDK1. However, other bioactive PIP lipid species are generated and their
functions in T cells are ill established. Based on a computational model we built to study the AKT activation in a
T cell, our simulation unexpectedly predicted that different TCR signal strengths would generate different PIPs.
Experimentally, we found that other bioactive PIPs in addition to PIP3 are generated at appreciable levels during
T cell activation and that different TCR signal strengths generate different PIP species. Our proteomic screen
identified proteins in a T cell that bind to specific PIPs, which positions us to identify novel pathways that are
engaged during T cell activation. The novel result that T cells transduce TCR signal strength by generating
different PIPs has the potential to illuminate a basic biochemical mechanism for how T cell interprets extracellular
signals. These preliminary data serve as the basis of our central hypothesis that T cells encode TCR signal
strength by generating different phosphatidylinositols to control T cell fate decisions, which will be tested by: 1)
identifying mechanisms that control differential generation of phosphatidylinositols in response to TCR signal
strength and 2) identifying how differential generation of phosphatidylinositols functions in the Treg versus T
helper cell fate choice and the Th1 versus Th2 cell fate choice. Taken together, results from this work will provide
novel mechanisms of receptor signal integration at the molecular level and identify functions of differential
phosphatidylinositol generation in the context of CD4+ T cell fate choices.