*Purpose: Donor specific antibody (DSA) plays a central role in chronic kidney allograft injury. As a trigger of humoral alloimmune response, follicular T helper cells(Tfh) promote DSA generation, while T-Follicular regulatory cells(Tfr) inhibit antibody production by suppressing Tfh and B cells. The key cytokine IL-21 exerts distinct effect on Tfh and Tfr. Here we studied whether blocking IL-21 by monoclonal IL-21R antibody changes the Tfh/Tfr balance and inhibits dnDSA generation.

*Methods: First, we investigated the impact of αIL-21R on Tfh and Tfr in in-vitro conditioned culture. Naïve CD4+ T cells were isolated from 3 months old C57BL6 mouse and cultured in Tfh condition with αIL-21R or isotype IgG. Proliferation and differentiation were evaluated by CFSE dilution and BCL-6, ICOS, PD-1 expression with flow cytometry. For further in-vivo investigation, a fully mismatch skin transplantation model was utilized to trigger the humoral alloimmune response. For further in-vivo investigation, a fully mismatch skin transplantation model was utilized to trigger the humoral alloimmune response.

*Results: . αIL-21R inhibits Tfh proliferation and differentiation significantly. By contrast, in modified Treg condition (conventional Treg condition plus IL-6), cell proliferation and Bcl-6 expression were not inhibited by αIL-21R. Notably, Inhibition of IL-21 on Tfr proliferation can be reversed by αIL-21R. Mechanistically, Akt signal pathway was significantly up-regulated and inhibited by αIL-21R in Tfh, while remained in constant quiescent level in Tfr regardless of administration of αIL-21R.In consistent with in-vitro data, flow cytometry and immunofluorescence revealed reduced Tfh/Tfr ratio in recipients treated with αIL-21R. Germinal center response was evaluated with lectin histochemistry. αIL-21R inhibited the formation of germinal center significantly. Most importantly, dnDSA level in different time points after transplantation revealed a significant inhibition of αIL-21R on dnDSA generation.

*Conclusions: In summary, our results demonstrate that monoclonal αIL-21R shifts Tfh/Tfr balance towards dnDSA reduction and ameliorates chronic injury by suppressing Tfh and blocking the impact of IL-21 on Tfr. The αIL-21R might, therefore, be a useful treatment in organ transplantation to prevent chronic antibody mediated rejection.

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