Elevated levels of serum IL-5 are associated with an increased likelihood of major depressive disorder

Background: Inflammatory mediators in both the peripheral circulation and central nervous system (CNS) are dysregulated in major depressive disorder (MDD). Nevertheless, relatively little is known about the role of the T-helper (Th)-2 effector cytokines interleukin (IL)-5 and IL-13 in MDD. Methods: We examined the serum levels of these cytokines and a Th-1 comparison cytokine, interferon (IFN)-gamma, in 116 individuals (MDD, n = 58; controls, n = 58). Results: In our basic multivariate model controlling for the effects of potential confounders on the associations between MDD and the examined cytokines, each 1-unit increase in the serum IL-5 level increased the likelihood of belonging to the MDD group by 76% (OR 1.76, 95% CI 1.03-2.99, p = 0.04; model covariates: age, gender, marital status, daily smoking and alcohol use). The likelihood further increased in models additionally controlling for the effects of the use of antidepressants and NSAIDS, and a diagnosis of asthma. No such associations were detected with regard to IL-13 (OR 1.08, 95% CI 0.96-1.22, p = 0.22) or IFN-gamma (OR 1.02, 95% CI 0.99-1.05, p = 0.23). Conclusions: Elevated levels of IL 5, which uses the neural plasticity related RAS GTPase extracellular signal regulated kinase (Ras-ERK) pathway to mediate its actions in the central nervous system (CNS), could be one of the factors underlying the depression-related changes in CNS plasticity.