MDD: Major Depressive Disorder. BMI: Body Mass Index. HR: Heart Rate. SBP: Systolic Blood Pressure. DBP: Diastolic Blood Pressure. Global Severity Index, depression, anxiety and hostility scores measured by Brief Symptom Inventory scale. Stress score measured by Depression, Anxiety, Stress Scale. ^ Two-Way ANOVA. Significance at p < 0.05
3.2 Plasma cytokine concentrations:
Each cytokine showed positive skewed distribution (IL1-α skewness = 5.684, IL6 skewness = 8.562, TNF-α skewness= 9.047). Shapiro-Wilk tests for all the cytokines data were significant (p < 0.001) indicating that the data did not follow normal distribution. Two outliers were removed from the IL-1α data (one MDD and one control) with concentrations of 8.31 pg/mL and 8.06 pg/mL, respectively; one outlier from the control group was removed from each of IL-6 and TNF-α data with concentrations of 165.8 pg/mL and 42.3 pg/mL, respectively. The ranges of IL-1α, IL-6 and TNF-α were 0.36 - 4.69 pg/mL, 2.67 - 42.3 pg/mL and 0.39 - 9.32 pg/mL respectively. Mean cytokine values (IL-1α, IL-6 and TNF-α) are shown in Table 2. All values were logarithmic-transformed for all analyses to reduce skewness.
As shown in Table 2, both the means of IL-1α and IL-6 concentrations were higher in MDD than controls, with no effect of sex. However, there was a significant group by sex interaction for IL-6, whereby depressed females had significantly higher plasma IL-6 than females in the control group (0.95 ± 0.25 versus 0.77 ± 0.21; p = 0.002), while males in the MDD and control groups had similar plasma IL-6 (0.84 ± 0.16 versus 0.83 ± 0.21, p = 0.862). There were no significant effects or interaction for TNF-α. When age and BMI were considered as covariates, they were not significant cofounders for any of the cytokines. Among the cytokines, IL-6 and TNF-α were strongly correlated (r = 0.651, p < 0.001), however IL-1α did not correlate with IL-6 (r = 0.058, p = 0.365) nor with TNF-α (r = 0.130, p = 0.162).
Table 2 : Plasma concentrations of IL-1α, IL-6 and TNF-α per group and sex. Mean values with standard deviation in parentheses