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 <.02). In BD patients, levels of Th17 and T regulatory cells were 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.

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Keywords Bipolar disorder, Leukocyte subset, Major depressive disorder, T cell defect, T helper cell differentiation, Th17, Th2
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Journal Journal of Neuroimmunology
Becking, K. (Karlijn), Haarman, B.C.M, Grosse, L, Nolen, W.A, Claes, S, Arolt, V, … Drexhage, H.A. (2018). The circulating levels of CD4+ t helper cells are higher in bipolar disorder as compared to major depressive disorder. Journal of Neuroimmunology, 319, 28–36. doi:10.1016/j.jneuroim.2018.03.004