TNF and depressive disorder: But AS and depressive symptoms may also share a common pathophysiological pathway as both may be the result of an auto-inflammatory biological process; pro-inflammatory biomarkers, such as C-reactive protein (CRP) and tumor necrosis factor (TNF)-α, have been found to be higher in depressed individuals compared to non-depressed individuals, leading to the “inflammatory/cytokine hypothesis of depression” [4].