Fig. 2.

a Beyond TNF and IL-17A other cytokines are indirectly involved in the axis. For example, IL-1, IL-6 and TGF-beta can prime IL-17A, IL-17F and IL-22 production and genetic polymorphisms in IL-1 an IL-6 are linked to AS. Independently of the impact on IL-17 pathway biology these cytokines can also directly impact on stromal and MSC biology. b The balance between immune activation in response to mechanical stress. Overactivation of the immune system in response to mechanical damage can lead to excessive repair as seen in SpA (red). Conversely blocking the immune system when excessive mechanical stress induced injury can be detrimental to the repair process or may render repair ineffective