Competition between colitogenic Th1 and Th17 cells contributes to the amelioration of colitis

Th17 cells and Th1 cells coordinate to play a critical role in the formation of inflammatory bowel diseases. To examine how Th17 and Th1 cells are regulated at inflammatory sites, we used Th1-dominant CD4⁺CD45RB ^high T cell-transferred RAG-2^-/- and Th1/Th17-mixed IL-10^-/- mice. Interestingly, not only did colitic RAG-2 ^-/- mice that were parabiosed with WT mice show significant amelioration of colitis, but amelioration of disease was also observed in those parabiosed with colitic IL-10^-/- mice. To assess the interference between Th1 and Th17 colitogenic T cells, we co-transferred colitogenic CD4 ⁺ T cells from the lamina propria (LP) of CD4⁺CD45RB ^high T cell-transferred RAG-2^-/- mice and IL-10 ^-/- mice into RAG-2^-/- mice. Surprisingly, the co-transferred RAG-2^-/- mice showed a vast cellular infiltration of LP CD4⁺ T cells similar to that seen in RAG-2^-/- mice re-transferred with the cells from colitic RAG-2^-/- mice alone, but the co-transferred RAG-2^-/- mice did not have the wasting symptoms, which are also absent in RAG-2^-/- mice transferred with cells from colitic IL-10^-/- mice alone. Furthermore, the percentages of Th1 and Th17 cells originating from IL-10^-/- mice and those of Th1 cells originating from colitic RAG-2^-/- mice were all significantly decreased in the co-transferred mice as compared with the singly-transferred paired RAG-2^-/- mice, suggesting that Th1 and Th17 cells are in competition, and that their orchestration results in a merged clinical phenotype of the two types of murine colitis.