IL17A and psoriatic arthritis: On the other hand, although the “signature cytokines” of AxSpA, i.e. IL-17 and TNFα, do not depend on JAK-STAT activation, they are either downstream of JAK-dependent cytokines, such as IL-23, or their effects on target cells are modulated by other cytokines signalling via JAK.12–15 Indeed, data from animal models suggest a link connecting JAKis and the IL-23/IL-17 axis and therefore could at least partially explain the efficacy of this drug class in psoriatic arthritis (PsA) and SpA.16,17