Elucidating the impact of a Pik3cd GOF mutation on T cell development and function

Activated PI3K Delta Syndrome 1 (APDS1) is a rare condition caused by heterozygous gain-of-function (GOF) mutation in PIK3CD, which encodes the leukocyte-restricted p110δ catalytic subunit of phosphoinositol 3-kinase (PI3K). PI3K is activated downstream of many T cell receptors and has been implicated in the control of T cell differentiation, activation and proliferation. APDS1 patients have increased PI3K activity leading to several immune manifestations including hepatosplenomegaly, chronic herpes-virus infections, recurrent respiratory tract infections and impaired Ab responses. Here we phenotyped the T cells in a CRISPR/Cas9 mouse model harbouring the ortholog of the common mutation seen in ~80% of patients (Pik3cdE1020K). This revealed the peripheral blood T cells in these mice phenocopy what is observed in the blood of APDS1 patients. Analysis of the spleen revealed increased CD4+ T cell numbers which contributed to the splenomegaly seen in Pik3cd GOF mice. Moreover, T cell activation was deeply disturbed across the lymphoid organs, with a decreased frequency of naïve T cells and a concurrent increase in memory T cells. Mixed bone marrow chimeras revealed the T cell activation was extrinsically driven. Further, chimeras revealed that Pik3cd GOF in myeloid cells was not sufficient to drive this T cells activation. Similarly, we saw few changes in CD4+ T cells activation in vitro when they interacted with Pik3cd GOF APCs during antigen presentation. Thus, it remains unclear what drives this T cell activation. In addition, this thesis examined the effect of Pik3cd GOF on T cell function. In vitro polarising cell cultures of Pik3cd GOF naïve CD4+ and CD8+ T cells revealed altered cytokine expression, indicating Pik3cd GOF alters T cell differentiation. As impaired humoral response is a marked characteristic of these patients, we also analysed Pik3cd GOF Tfh cell function by using an in vivo antigen specific model. We observed similar frequencies of Tfh cells generated, however the Pik3cd GOF Tfh cells showed impaired ability to sustain GCs. Together, the results presented in this thesis reveal that PI3K plays a key role in modulating T cell activation, differentiation and function and provide new insights into the pathophysiology of APDS1 patients