Objective: This study aims to ascertain the effect of baseline IL-1Ra and IL-8 in the treatment response of patients with major depressive disorder (MDD) and to clarify the relationship between inflammation markers and depression.

Methods: We recruited 242 patients with a Beck Depression Inventory (BDI) score ≥ 17 referred to secondary care in Finland. The patients’ serum IL-1Ra and IL-8 concentrations were measured at baseline. Montgomery-Åsberg Depression Rating Scale (MADRS) tests and Alcohol Use Disorders Identification Tests (AUDIT) were administered at baseline and six weeks. The Antidepressant treatments varied: somewere started, others changed or continued their previous medication, and others had their doses adjusted. Patients started behavioral activation therapy. Linear regression was used with a relative MADRS score change during six weeks as the dependent variable and patient age, AUDIT score, BMI, daily number of cigarettes smoked, sex, and serum IL-1Ra and IL-8 concentrations as independent variables.

Results: Higher baseline serum IL-1Ra and IL-8 levels were associated with a smaller relative change in the MADRS-score within the first six weeks of treatment in linear regression analysis (p<0.001 and p=0.007, respectively). In further analysis comparing groups with ≤ 24 and > 24 MADRS score only the ≤ 24 MADRS score group showed a similar association.

Conclusion: Higher baseline IL-1Ra and IL-8 concentrations were associated with a lesser relative response to depression treatment, particularly in patients with mild depression. Results on IL-8 concur with earlier findings whereas the association between higher IL-1Ra serum concentrations reduced treatment response is a novel finding.