Introduction: After organ transplantation, T follicular helper cells (Tfh) expressing the transcription factor BCL6 provide help to alloantigen-activated B cells and drive antibody-mediated rejection. BCL6 expression by germinal center B cells and Tfh cells plays a crucial role in germinal center development and persistence. Therefore, by targeting BCL6-expressing Tfh cells, there is a potential to suppress alloreactive responses. In this study, the inhibition of BCL6 by two small molecule inhibitors was tested in vitro to assess its effect on Tfh formation and activation in healthy controls and kidney transplant recipients.
Method: Two experimental setups were used for this study. First, isolated naïve CD4+ T helper cells from healthy volunteers (n=6) were polyclonally-activated in the presence or absence of the small molecule BCL6 inhibitor, 79-6 (25-250 µg/mL) and assessed for proliferation, BCL6 expression, and differentiation into Tfh cells based on CXCR5, ICOS and PD1 expression. Second, the inhibitory effect of BCL6 blockade on T cell activation and differentiation was investigated using PBMCs of kidney transplant recipients, after transplantation (n=16), in the presence or absence of another BCL6 inhibitor FX-1 (3-12.5 µg/ml), after alloantigen stimulation with donor cells.
Results: After polyclonal stimulation of naïve CD4+ T helper cells, proliferation and Tfh formation and activation was induced. In the presence of the BCL6 inhibitor 79-6, this proliferation was inhibited in a dose dependent manner, with >95%  inhibition  at a concentration of 250 µg/ml of 79-6. The inhibitor also prevented their differentiation into activated Tfh cells. In addition, cell imaging flow cytometry confirmed that in these stimulated T cells, 79-6 suppressed the expression of BCL6 protein. The results with patient material are in line with these findings. After allo antigen stimulation, in the presence of 12.5 µg/ml of the BCL6 inhibitor FX-1, proliferation was decreased by 60% and Tfh differentiation by more than 95%.
Conclusion: Our results show that the small molecule BCL6 inhibitors are able to suppress T cell proliferation and their differentiation into Tfh cells. These findings suggest a potential application for BCL6 inhibitors as a means to prevent allogeneic Tfh cell responses.