IL6 and major depressive disorder: In support of a causal association, IL-6 has been shown to interfere with the production of serotonin from tryptophan by increasing the breakdown of tryptophan, thus reducing serotonin levels (and increasing depression risk) and preferentially increasing the synthesis of kynurenine and its neurotoxic metabolites, 3-hydroxykynurenine and quinolinic acid.