IL6 and major depressive disorder: IL-6 quantification via ELISA. The multiple linear regression model, which included four predictor terms (IL-6, social support, social strain, and W1 MDD symptoms) and two interaction terms (IL-6 × social support and IL-6 × social strain) accounted for 13.6% of the variance in future MDD symptoms, F(6, 1047) = 28.65, p < .001 (see Table 2).