Th17 immunity emerged as a distinct lineage of T cell effector function in 2005. Since then, the type 17 immune response has been the focus of much research culminating in a number of therapeutic approaches that target this pathway. The role of this pathway in human disease was initially highlighted by genetic polymorphisms implicating it in a number of immune-mediated inflammatory diseases. In addition to the many successes in clinical trials, there have also been failures that have provided the opportunity to understand the role of this pathway in a number of human diseases. Targeting IL-17A and F in inflammatory arthritis has shown efficacy in the MHC-I associated diseases such as ankylosing spondylitis and psoriatic arthritis but not in MHC-II associated rheumatoid arthritis. Additionally, targeting the effector cytokines IL-17A and F has proven more successful for arthritis than the upstream cytokine IL-23 suggesting a role for IL-23-indpendent type 17 immunity in inflammatory joint disease.
