IL17A and major depressive disorder: Whilst the pathogenesis of AS, PsA and depression is yet to be fully understood, recent evidence highlights the IL-17/23 pathway as one of the most important cytokine pathways to contribute to AS and PsA.6,106, –108 Elevated IL-23 impairs T helper cells, which contributes to the overexpression of IL-17, IL-22, IL-1, IL-6 and TNF.