Interleukin-6 is associated with anxiety and anhedonia, but not with the severity of bipolar and unipolar depression

Objective: to compare the levels of interleukin-6 (IL-6) in patients with a current depressive episode (DE) in bipolar disorder (BD) and major depressive disorder (MDD), as well as to evaluate the associations of IL-6 with clinical characteristics, including anhedonia and anxiety. Material and methods: the study participants were examined using a semi–structured diagnostic interview and psychometric tools such as the Montgomery–Asberg Depression Rating Scale (MADRS), the Snaith-Hamilton Pleasure Scale (SHAPS), the Hospital Anxiety and Depression Scale (HADS), and the Columbia-Suicide Severity Rating Scale (C-SSRS). Serum levels of IL-6 were determined by enzyme immunoassay. Nonparametric methods, correlation and regression analysis were used for statistical processing.

Results: The study included 50 patients with current DE (BD: n=20, MDD: n=30). There were no differences between the BD and MDD groups in IL-6 levels, as well as in the severity of anhedonia (SHAPS) and anxiety (HADS-A). Patients with BD had an earlier onset of the disorder and a higher number of DE compared with MDD. In BD diagnosis stratification, IL-6 correlated with anxiety (HADS-A) and age of onset, while in MDD it correlated with anhedonia (SHAPS). Nevertheless, in the regression analysis conducted after the correlation stage, higher levels of IL-6 showed a stable, independent positive association only with the severity of anhedonia; the effect persisted when the diagnosis was taken into account (BD/ MDD) and clinical covariates. Conclusion: in patients with current DE, IL-6 may be more associated with individual domains of symptoms, namely, the severity of anhedonia, rather than the overall severity of depression. Moreover, this relationship manifests itself regardless of the diagnosis of BD and MDD, which indicates its key role in the immune-inflammatory mechanisms of mood disorders.

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