The circulating levels of CD4+ t helper cells are higher in bipolar disorder as compared to major depressive disorder

Abstract Introduction Clinical differentiation between bipolar disorder (BD) and major depressive disorder (MDD) is difficult. Research has therefore focused on discriminatory biological markers. Previous studies in MDD reported T cell deficits, while the limited studies in BD reported T cell activation. Studies directly comparing circulating numbers of T cells and T cell subsets between BD and MDD are lacking. The studies in the MOODINFLAME consortium make such a comparison possible. Methods The number of circulating leukocyte populations (lymphocytes, monocytes, NK cells, B cells, T cells, CD3+CD8+ T cytotoxic cells, CD3+CD4+ T helper cells, Th1, Th2, Th17 and T regulatory cells) was determined using FACS technology in a cohort of 83 euthymic BD patients, 8 BD patients with a current mood episode and 165 healthy controls (HC). Data were compared to those of 34 moderately and 56 severely depressed MDD patients. Results Compared to MDD patients, BD patients showed significantly increased levels of Th17, Th2, Th1 and T regulatory cells (all p  increased compared to HC (p = .03, p = .02, respectively), while MDD patients showed decreased levels of Th17 and Th2 compared to HC (p = .03, p = .01, respectively). Of the various medications only SSRI/SNRI usage could explain part of the Th2 decrease in MDD. Conclusion This study shows CD4+ T helper cell deficits in MDD patients, while normal or even raised levels of these cells were found in BD patients. The differences in CD4+ T helper cell differentiation was most outspoken for Th17 cells.