CD4 and Arthritis: While γδ17 and Th17 cells may have complementary pathogenic roles in the development of IL1rn−/− arthritis, our data suggest that IL-17-producing cells located in lamina propria and induced by IL1rn−/− intestinal microbiota are TCRβ-expressing CD4+ Th17 cells (Fig. 4 and Additional file 1: Figure S4).