PI3K p110δ Is Expressed by gp38- CD31+ and gp38+CD31+ Spleen Stromal Cells and Regulates Their CCL19, CCL21, and LTβR mRNA Levels

The role of p110 delta PI3K in lymphoid cells has been studied extensively, showing its importance in immune cell differentiation, activation and development. Altered T cell localization in p110 delta-deficient mouse spleen suggested a role for p110d in non-hematopoietic stromal cells, which maintain hematopoietic cell segregation. We tested this hypothesis using p110 delta(WT/WT) mouse bone marrow to reconstitute lethally irradiated p110dWT/WT or p110 delta(D910A/D910A) (which express catalytically inactive p110d) recipients, and studied localization, number and percentage of hematopoietic cell subsets in spleen and lymph nodes, in homeostatic conditions and after antigen stimulation. These analyses showed diffuse T cell areas in p110d (D910A/D910A) and in reconstituted p110 delta(D910A/D910A) mice in homeostatic conditions. In these mice, spleen CD4(+) and CD8(+) T cell numbers did not increase in response to antigen, suggesting that a p110 delta(D910A/D910A) stroma defect impedes correct T cell response. FACS analysis of spleen stromal cell populations showed a decrease in the percentage of gp38(-)CD31(+) cells in p110 delta(D910A/D910A) mice. qRT-PCR studies detected p110 delta mRNA expression in p110 delta(WT/WT) spleen gp38(-)CD31(+) and gp38(+)CD31(+) subsets, which was reduced in p110 delta(D910A/D910A) spleen. Lack of p110 delta activity in these cell populations correlated with lower LT beta R, CCL19 and CCL21 mRNA levels; these molecules participate in T cell localization to specific spleen areas. Our results could explain the lower T cell numbers and more diffuse T cell areas found in p110 delta(D910A/D910A) mouse spleen, as well as the lower T cell expansion after antigen stimulation in p110 delta(D910A/D910A) compared with p110 delta(WT/WT) mice.