Abstract
BACKGROUND: Our previous research revealed a novel function of berberine (BBR), a clinically relevant plant-derived alkaloid, as a suppressor of follicular T helper (T(fh)) cell proliferation in secondary lymphoid organs of BBR-treated mice that underwent immunization for collagen-induced arthritis (CIA) in DBA1/J mice. Due to the importance of T(fh) cell and B cell interactions in the generation of T cell-dependent humoral responses, the suppression of T(fh) cell activity may have implications for the general safety of BBR as a prophylactic dietary supplement, and its potential use in antibody-driven autoimmune and hypersensitivity disorders. PURPOSE: This research aims to characterize BBR's impact on the activation, differentiation, and proliferation of T(fh) cells by examining the expression of key extracellular signaling molecules, as well as the activity of intracellular signaling molecules involved in the Ca(2+)-calcineurin-NFAT pathway and STAT3 phosphorylation, following activation. STUDY DESIGN: In vitro experimental study using primary tissues. METHODS: To explore the direct effects of BBR on the proliferation and differentiation of Tfh cells, isolated naïve CD4(+) T cells (>95% pure) were activated and differentiated into pre- T(fh) cells in the presence or absence of BBR. The resulting T(fh) cell populations and the expression of the key extracellular molecules CXCR5, ICOS, and PD-1 were measured. In addition, we examined the impact of BBR treatment on the activity of key intracellular signaling molecules involved in T(fh) cell activation and differentiation following TCR ligation and/or CD28 signaling (p-ZAP-70, p-Lck, p-PLCγ1, NFATc1 and intracellular calcium, Ca(2+), concentrations), as well as IL-6 signaling (p-STAT3). RESULTS: Treatment with BBR significantly reduced the expression of both CXCR5 (p < 0.01) and ICOS (p < 0.005), but not PD-1, and reduced the percentage of T(fh) cells within the total CD4(+) T cell population. BBR treatment also led to a reduction in intracellular Ca(2+) flux, activation of p-STAT3, and IL-21 production. CONCLUSION: Our observations provide insight into the mechanism of BBR-mediated T(fh) cell suppression and suggest that BBR treatment can directly inhibit T(fh) cell activity, perhaps through interfering with cytokine receptor or downstream signaling.