Positive regulation of Itk PH domain function by soluble IP4 is required for thymocyte positive selection but dispensable for negative selection

Abstract

Membrane recruitment of Itk and PLCγ1 require interactions between their Pleckstrin homology (PH) domains and PIP3, the phospholipid product of PI‐3 kinase. Dual anchoring via SH2 and PH domains are required for optimal Itk‐mediated activation of PLCγ1 and the subsequent generation of IP3 and DAG. We have identified a novel mechanism in which conversion of IP3 into IP4 by ItpkB creates a physiologically important soluble ligand for the Itk PH domain. IP4 specifically promotes PH domain‐dependent recruitment of Itk but not PLCγ1 to PIP3 and is required for optimal Itk activation. PH‐domain mediated oligomerization of Itk leads us to propose a model in which IP4 binding to one Itk PH domain can allosterically increase the affinity of associated Itk PH domains for membrane PIP3. IP4‐mediated positive feedback of Itk activation is required for positive selection during T cell development. Dose dependent reduction of single positive thymocytes suggests that ItpkB limits the efficiency of T cell maturation. Interestingly, our data indicates that thymocyte negative selection is normal in the absence of ItpkB and suggests that ItpkB participates in distinguishing TCR‐dependent death or survival signals.