Immune–inflammatory markers in patients with seasonal affective disorder: effects of light therapy

Background: There is increasing evidence that an activation of the immune–inflammatory system is involved in the pathophysiology of depressive disorders. The purposes of this study were to (1) compare immune–inflammatory markers in patients with seasonal affective disorder (SAD) with those in matched normal controls; and (2) examine the effects of light therapy on the immune–inflammatory markers in patients with SAD. Methods: Plasma concentrations of interleukin-6 (IL-6), soluble IL-6 receptor (sIL-6R) and soluble IL-2 receptor (sIL-2R) were measured in 15 patients with SAD and 15 age- and sex-matched normal controls. Of the 15 patients, 14 had repeated blood sampling for these variables following 2 weeks of light therapy. Results: We found that patients with SAD had significantly increased IL-6 levels compared to normal controls (P<0.0005). There was a trend toward increased sIL-2R in patients with SAD (P=0.09). There was no significant difference in sIL-6R level between the two diagnostic groups (P=0.18), but the product term (IL-6×sIL-6R) was significantly higher in patients with SAD than that in normal control controls (P<0.0003). Furthermore, all 14 patients who completed the study improved with 2 weeks of light therapy and nine of them (64%) had 50% reduction in score of the Hamilton Depression Rating Scale–SAD version post-treatment compared to baseline. However, the initially increased immune markers in SAD patients were not significantly altered by the therapeutic light therapy. Limitations: This study was limited to a small sample size and other immune inflammatory markers should be measured for further evidence of immune activation in seasonal depression. Conclusions: Our results of increased IL-6, IL-6×sIL-6R, and sIL-2R in patients with SAD suggest an activation of the immune–inflammatory system in winter depression, which is not altered by 2 weeks of successful light therapy.