There was a significant baseline IL-8 by condition (endotoxin vs. placebo) interaction for POMS depression change from baseline to T2 (approximately 2 h following endotoxin or placebo infusion), controlling for age, (β = -0.274, p = 0.03), such that higher baseline IL-8 was associated with less increase in depressed mood among endotoxin-exposed participants (β = − 0.304, p = 0.03; effect size (sr2) = 0.09) but not placebo-exposed participants (β = 0.056, p = 0.69; effect size (sr2) = 0.003) (see Fig. 1A). Here, CXCL8 is linked to major depressive disorder.