IL6 and depressive disorder: Previous research has provided observational evidence for an association between IL-6, depression and psychosis using population-based longitudinal (Gimeno et al., 2009, Khandaker et al., 2014) and cross-sectional studies (Dowlati et al., 2010, Goldsmith et al., 2016, Haapakoski et al., 2015, Howren et al., 2009, Miller et al., 2011, Potvin et al., 2008, Upthegrove et al., 2014).