The p110δ subunit of class I A PI3K modulates signaling in innate immune cells. We previously demonstrated that mice harboring a kinase-dead p110δ subunit (p110δ KD) develop spontaneous colitis. Macrophages contributed to the Th1/Th17 cytokine bias in p110δ KD mice through increased IL-12 and IL-23 expression. In this study, we show that the enteric microbiota is required for colitis development in germfree p110δ KD mice. Colonic tissue and macrophages from p110δ KD mice produce significantly less IL-10 compared with wild-type mice. p110δ KD APCs cocultured with naive CD4+ Ag-specific T cells also produce significantly less IL-10 and induce more IFN-γ–and IL-17A–producing CD4+ T cells compared with wild-type APCs. Illustrating the importance of APC–T cell interactions in colitis pathogenesis in vivo, Rag1−/−/p110δ KD mice develop mild colonic inflammation and produced more colonic IL-12p40 compared with Rag1−/− mice. However, CD4+ CD45RB high/low T cell Rag1−/−/p110δ KD recipient mice develop severe colitis with increased percentages of IFN-γ–and IL-17A–producing lamina propria CD3+ CD4+ T cells compared with Rag1−/− recipient mice. Intestinal tissue samples from patients with Crohn’s disease reveal significantly lower expression of PIK3CD compared with intestinal samples from non–inflammatory bowel disease control subjects (p< 0.05). PIK3CD expression inversely correlates with the ratio of IL12B: IL10 expression. In conclusion, the PI3K subunit p110δ controls homeostatic APC–T cell interactions by altering the balance between IL-10 and IL-12/23. Defects in p110δ expression and/or function may underlie the pathogenesis of human inflammatory bowel disease and lead to new therapeutic strategies.