Type I Interferons promote maintenance and function of follicular T helper cells in vitro

The hepatitis B and C viruses (HBV, HCV) are known to induce highly different interferon (IFN) signatures in infected hepatocytes. As a stealth virus, HBV avoids the induction of an IFN response, whereas HCV induces a strong type I IFN response early after and throughout chronic infection. During chronic HCV (cHCV) infection follicular T helper (Tfh) cells are more frequent in the periphery and accumulate in the liver. A robust Tfh cell response is one major requirement for viral clearance since these specialized CD4 T cells provide help to B cells to facilitate a strong antibody response. Type I IFNs are known to suppress Tfh cell differentiation in vitro, which is at odds with the observation that cHCV features a strong type I IFN signature as well as a Tfh cell accumulation in the liver.

Aim of the study was to investigate the influence of IFNs on maintenance and function of Tfh cells to elucidate in how far IFN-signaling influences Tfh cell responses. IFN concentrations in plasma of patients (HCV: n = 35, HBV: n = 71) and healthy donors (HD: n = 20) were analyzed by a custom-plex assay (Eve Technologies). To analyze IFN signal transduction, STAT1 phosphorylation in Tfh cell clones (n = 2) was determined after IFN exposure (50 – 500 pmol/ml) and analyzed by flow cytometry. PBMCs of healthy donors (n = 10) were phenotypically characterized and regulation of relevant cytokines and transcription factors in Tfh cell clones (n = 6;7, respectively) was determined by flow cytometry after 3 d IFN exposure at 500 pmol/ml. In addition, the B cell helper capacity of Tfh clones (n = 11) under the IFN influence was analyzed by co-culture with autologous naïve B cells. Immunoglobulins in supernatants were measured by ELISA.

IFN levels during HBV and HCV infection appeared not to be substantially elevated. Interestingly STAT1 phosphorylation was detected after type I, but not type II or III IFN exposure, suggesting that especially type I IFNs directly signal to Tfh cells. A decrease of the follicular homing molecule CXCR5 within CD4 T cells was observed under the influence of type I IFNs on PBMCs. However, CD4 T cells that maintained CXCR5 expression showed a Tfh-like phenotype (PD1+, CXCR3-) and increased activation (CD38+ and ICOS+). The transcriptional activity (TCF-1 and FoxP3) was significantly elevated in Tfh clones after type I IFN influence as well as the expression of IFN γ and the Tfh signature cytokine IL-21. Correspondingly, B cell helper capacity appeared elevated, indicated by increased IgG and IgM concentrations in coculture supernatants. Interferons alone showed no effect on naïve B cells.

In conclusion our results suggest an activation of Tfh cells and stabilization of their phenotype by type I IFNs. This supports the strategy of antiviral therapy with pegylated IFN α and would explain Tfh accumulation in the infected liver in the HCV-infected liver.