The Essential of PI3Kγ and PI3Kδ in Thymocyte Survival.

Class I phosphoinositide 3-kinases (PI3Ks), consisting of PI3Kα, β, γ and δ, are a family of intracellular signalling molecules that play an important role in both innate and adaptive immune responses. In thymocytes, however, their role is less clear as PI3Kγ, but not other class I members, has been postulated to be partially involved in pre-TcR-dependent differentiation without effecting overall T cell numbers. We now report that PI3Kγ in conjunction with its delta counterpart are essential for thymocyte survival and ultimately T cell production. Surprisingly, genetic deletion of the both p110γ and p110δ catalytic subunits resulted in a dramatic reduction in thymic size, cellularity, and lack of cortico-medullary differentiation. Total thymocyte counts in these animals were 27-fold less than observed for wild-type (WT) controls due to a diminished number of DP cells and was associated with T cell depletion in both peripheral blood and secondary lymphoid organs. This phenotype could also be recapitulated in vitro as incubation of thymi harvested from day 14 p110γ−/− embryos with the p110δ specific inhibitor IC87114, but not vehicle control, yielded identical results. Moreover, the observed reduction in the DP population appears to be intrinsic to thymocytes as reconstitution of animals deficient in both p110γ and p110δ catalytic subunits with WT fetal liver cells restored the proportions of DN, DP, SP to that observed in WT controls. The observed defects, however, appear to be due to an excessive amount of apoptosis of the DP thymocyte population, while TCRbeta expression, pre-TCR selection and DP cell production appear to proceed unperturbed. Thus, this study provides evidence that these two distinct class I PI3Ks isoforms work in concert to protect developing thymocytes from apoptotic signals rather than by participating directly in thymocyte differentiation.