Introduction: In kidney transplantation, antibody-mediated rejection is a major cause of graft loss. The currently prescribed immunosuppressive treatments insufficiently control this humoral B cell-mediated immune response. This humoral response is not only B cell-mediated but is also dependent on follicular T helper cells. To prevent or inhibit this T cell-dependent humoral allo-response, we investigated whether targeting Bcl-6, the transcription factor mediating germinal center formation and the T – B reaction, inhibits this humoral response. For this reason, the effects of the small molecule Bcl-6 inhibitor 79-6 on T and B cell proliferation and differentiation were studied.
Material and Methods: First, the effect on T cell proliferation by 79-6 was tested using a mixed lymphocyte reaction in the presence of different concentrations of 79-6.
Next, we determined whether 79-6 affected the generation of Tfh cells. Magnetic-sorted naïve peripheral CD4 helper T cells were stimulated with anti-CD3/28 along with the polarizing cytokines IL-12 and IL-21.
To examine the direct effects of this agent on B cell differentiation, naïve B cells were stimulated with anti-IgM/anti-CD40 and IL-21 in the presence and absence of 79-6.
Results and Discussion: In the mixed lymphocyte reaction, we found a dose dependent effect of 79-6 on T cell proliferation. This includes the proliferation response of the specialized Bcl6+ T helper subset named follicular T cells (Tfh) that are fundamental in the B cell differentiation.
Differentiation of naïve T helper cells under cocktail-stimulated conditions resulted in a large population of “Tfh-like” cells with significantly reduced numbers in the presence of 76-9. These  “Tfh-like” cells formed in the presence of 76-9, exhibited higher levels of CXCR3, thereby exhibiting a Th1-Tfh phenotype which is associated with poor antibody responses.
In the presence of the Bcl-6 inhibitor less expression of the B cell memory markers CD27 and CD38 was measured, as well as a decreased formation of plasma cells. In addition, 79-6 affected B cell class switching, as in the presence of the BCL-6 inhibitor only low numbers of class switched IgG B cells were measured.
Conclusion: In summary, our studies show promising first results that targeting Bcl-6 transcription affects the functionality of activated T and B cells, thereby preventing the differentiation into B cell plasma blasts, the cell population secreting immunoglobulins.