We hypothesise that inhibition of IL-6 signalling with a single intravenous infusion of anti-IL6R monoclonal antibody tocilizumab will attenuate somatic symptoms of depression, improve cognitive function, reduce serum proinflammatory cytokine levels and indoleamine 2,3-dioxygenase (IDO) activation in individuals with depression who show evidence of low-grade inflammation and poor response to antidepressant. This evidence concerns the gene IDO2 and major depressive disorder.