Optimism moderates the relationship between inflammatory polygenic risk and major depressive disorder in U.S. Military veterans

Major depressive disorder (MDD) is a leading cause of disability, and chronic inflammation is a contributing factor to its onset and progression. This study examined the relationship between genetic predisposition to inflammation and MDD risk in a nationally representative sample of U.S. military veterans, as well as psychosocial moderators of this association. A composite polygenic risk score (PRS) for inflammatory biomarkers was derived from the UK Biobank and examined in relation to a positive MDD screen in 1,660 European-American veterans. The analysis adjusted for known correlates of inflammation and MDD, including medical conditions and cumulative trauma burden. Each standard deviation increase in the inflammatory PRS was associated with more than two-fold increased odds of screening positive for MDD (OR = 2.51, 95% CI = 1.39–4.54). Interaction analyses revealed that optimism moderated this association; among those in the highest PRS tertile, individuals with high optimism were more than 30 times less likely to screen positive for MDD compared to those with low optimism (0.7% vs. 22.6%). Pathway-based analyses identified enrichment of immune- and brain-related gene sets, highlighting potential biological mechanisms linking inflammation and MDD. Findings suggest genetic risk for inflammation contributes to MDD vulnerability and that optimism may buffer this risk.