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Background: There is evidence that major depression (MDD) and bipolar disorder (BD) are accompanied by activated immune & oxidative (I&O) pathways.

Methods: To compare I&O biomarkers between MDD and BD we assessed serum levels of thiobarbituric acid reactive substances (TBARS; a lipid peroxidation marker), soluble interleukin-2 receptor (sIL-2R), sIL-6R, IL-[alpha], sIL-1R antagonist (sIL-1RA), tumor necrosis factor receptor 60 kDa/80 kDa (sTNFR60/R80) in 114 MDD and 133 BD patients, and 50 healthy controls. We computed z-unit weighted indices reflecting the 5 cytokine receptor levels (zCytR), cell-mediated immunity (zCMI) and I&O pathways (zCMI TBARS).

Results: There are no significant differences in biomarkers between MDD and BD. BD/MDD with atypical features is characterized by increased sIL-6R and TBARS, whereas melancholia is associated with higher TBARS and lower sTNFR60 levels. Severity of illness, as measured with the Hamilton Depression Rating Scale, is correlated with increased sIL-6R, sTNFR80, TBARS, zCytR and zCMI TBARS. The number of episodes the year prior to blood sampling is positively associated with sTNFR80, TBARS, zCMI, zCMI TBARS, while number of hospitalizations is positively associated with sIL-1RA. Prior suicidal attempts are associated with increased sIL-1RA, IL-1[alpha], zCMI, TBARS and zCMI TBARS, while TBARS is associated with current suicidal ideation.

Conclusions: There are no I&O biomarker differences between MDD and BD. Atypical depression is associated with increased IL-6 trans-signaling and lipid peroxidation. Severity of depression, number of episodes and suicidal attempts are associated with activated I&O pathways. Increased TBARS is the single best predictor of BD/MDD, atypical depression, melancholia and current suicidal ideation.

Highlights:

* There are no immune & oxidative (I&O) biomarker differences between major depression (MDD) and bipolar disorder (BD).

* Atypical depression is associated with increased IL-6 trans-signaling and lipid peroxidation.

* Severity of depression, number of episodes and suicidal attempts are associated with activated I&O pathways.

* Increased TBARS is the single best predictor of BD/MDD, atypical depression, melancholia and current suicidal ideation.

(C) 2018Elsevier, Inc.