Blockade of the IL-21 Pathway: A New Perspective for the Treatment of T and B Cell Mediated Allogeneic Responses after Transplantation

Abstract

Purpose IL-21 is a T cell growth factor for and secreted by Th17 and T follicular helper (Tfh)-cells. This cytokine regulates CD8+ T cell expansion and their effector functions, and is crucial for T cell-dependent B cell differentiation of antigen-activated B cells into antibody-producing plasma cells. Little is known about IL-21 mediated T and B cell responses in alloreactivity after transplantation. Methods First, we explored the actions of IL-21 in alloreactive T cell proliferation and cytotoxicity in mixed lymphocyte reactions. Second, to test the role of IL-21 producing Tfh cells in the regulation of B cells in an alloantigen-driven setting, we performed co-culture experiments of transplant recipient Tfh cells and B cells stimulated ith alloantigen. Third, the effect of an αIL-21R blocking antibody on allograft rejection was studied in a humanized skin transplantation model in mice reconstituted with human T and B cells. Results Alloactivated T and B cells highly express IL-21R. In the presence of IL-21, a marked increased proliferative response of alloactivated T cells was found. Also, the expanded CD8+ T cells had significant cytolytic functions, while blockade of the IL-21 route by an αIL-21R antagonist inhibited the proliferation of alloactivated T-cells. Next, we determined the role of IL-21 in T cell dependent B cell responses. Donor antigen stimulation of the co-cultured Tfh - B cells initiated expression of the activation markers ICOS and PD-1 on Tfh cells with a shift toward a mixed Tfh2 and Tfh17 phenotype. The alloantigen activated memory B cells underwent class switch recombination and differentiated toward IgM- and IgG-producing plasmablasts. Anti-IL-21R antagonists significantly inhibited B cell differentiation. Finally, in the humanized mouse skin transplant model, in mice treated with the αIL-21R antagonist reduced signs of alloreactivity were measured including significantly less CD4+ and CD8+ T and B cell infiltration and less expression of inflammatory markers Keratin 17 and Ki67. Conclusion These data show that comparing the effects of new IL-21-targeting therapeutics against the conventional immunosuppressants used in transplantation would be of considerable interest.