IL17A and rheumatoid arthritis: Another case-control study showed that RA patients had a lower diversity index of gut microbiota, a higher abundance of phylum Verrucomicrobiae and genus Akkermansia, and a lower abundance of Firmicutes. Moreover, the alterations in microbiota levels directly correlated with clinical and pathological characteristics like ACPA antibodies, levels of TNF-α and IL-17A, along with immune cell numbers (Chiang et al., 2019; Li Y. et al., 2021).