IL17A and Arthritis: Importantly, given that the gut microbiota shapes the development and activation of the mammalian immune system10, we showed that the transplantation of microbiota from CIA-susceptible mice into germ-free mice increased arthritis severity, the serum concentration of interleukin (IL)-17, and the proportions of CD8+ and IL-17-producing (Th17) T cells in the spleen.