Background: Mice deficient in the inhibitory G protein subunit Gαi2 spontaneously develop a T helper 1 dominated colitis. We examined whether a defect in CD4(+)FoxP3(+) regulatory T cells (Treg) underpins the pathogenesis of colitis in the Gαi2(-/-) (Gαi2-deficient) colitis model.
Methodology/principal findings: Using flow cytometry, we found that thymus and colonic lamina propria, but not spleen and mesenteric lymph nodes, of colitic Gαi2(-/-) mice contained increased frequencies of Treg, whereas FoxP3 expression intensity was similar in Gαi2(-/-) compared to Gαi2(+/-) or Gαi2(+/+) wild type (WT) mice. The frequency of CD4(+)FoxP3(+) T cells expressing CD103 was significantly increased in Gαi2(-/-) compared to WT mice. Treg in colons from WT mice clustered in the T cell areas of colonic lymphoid patches (CLP), with relatively few Treg in the lamina propria, as demonstrated by immunohistochemistry. In Gαi2(-/-) mice, CLP were not observed but lamina propria Treg were increased in number and frequency within the CD4(+) infiltrate, compared to WT mice. Using an in vitro co-culture system and flow cytometric analysis of cell division we could demonstrate that the in vitro suppressive function of WT and Gαi2(-/-) CD4(+)FoxP3(+) regulatory T cells (WT-Treg and KO-Treg) was indistinguishable, but that T effector cells (CD4(+)25(-) T cells) from Gαi2(-/-) mice were less readily suppressed than WT effectors (WT-Teff) by Treg from either source. However, neither WT nor Gαi2(-/-) Treg was able to suppress colitis induced by adoptive transfer of Gαi2(-/-) effector T cells (KO-Teff) to RAG2(-/-) recipients. The enhanced inflammatory activity of Gαi2(-/-) effectors was accompanied by increased expression of an effector/memory T cell phenotype and increased cytokine secretion, especially IL-4, IL-6 and IFN-γ.
Conclusions: There is an increased frequency of Gαi2(-/-) Treg in the colon, and they demonstrate no endogenous functional defect. However, Gαi2(-/-) T effector cells are dramatically less susceptible to suppression in vitro, and in vivo, despite increased effective numbers of Treg, they cannot prevent disease.