Background: Antigen-specific antibody responses rely on IL-21+ T follicular helper (Tfh) cells that regulate B cell differentiation. In transplantation, a large proportion of renal allograft recipients develop a donor-specific antibody response which is associated with an increased risk for acute and chronic rejection. Current immunosuppressive agents are mainly aimed at T-cell-mediated alloimmunity, whereas agents that effectively target humoral effectors are still insufficient. Therefore, to prevent rejections, there is a need to develop such agents.

Aim: Here, we tested in an allogeneic setting whether Tfh cell help signals control B cell differentiation with its dependency on IL-21.

Methods: Patient PBMCs obtained pre kidney transplantation (n=17) were FACS sorted into CD4+CXCR5+ Tfh cells and CD19+CD27+ memory B cells and in vitro stimulated with alloantigen of the corresponding donor in the presence or absence of an IL-21 receptor antagonist (αIL-21R). Phospho-flow cytometry was used to determine the STAT3 phosphorylation levels of IL-21 stimulated T and B cells.

Results: Stimulation of Tfh and memory B cells with alloantigen initiated expression of the activation markers ICOS and PD-1 on Tfh cells, and a shift towards a mixed Tfh2 and Tfh17 phenotype. The co-culture also initiated memory B cell class switch recombination and differentiation towards IgM and IgG producing plasmablasts. In the presence of αIL-21R, a dose dependent inhibition of STAT3 phosphorylation, a downstream activation molecule of the IL21R, was measured in both T and B cells. Blockade of the IL-21R did not have an effect on PD-1 and ICOS expression on Tfh cells but significantly inhibited B cell differentiation. The proportion of plasmablasts decreased by 78% in the presence of αIL-21R (p=0.004). Moreover, secreted IgM and IgG2 levels were significantly lower in the presence of αIL-21R (p=0.004, p=0.004, respectively).

Conclusion: Our results demonstrate that IL-21 produced by alloantigen activated Tfh cells controls B cell differentiation towards antibody producing plasmablasts. The IL-21R might therefore be a useful target in organ transplantation to prevent alloantibody mediated immune responses leading to graft failure.

CITATION INFORMATION: de Leur K, Dor F, Dieterich M, van der Laan L, Hendriks R, Baan C. IL-21R Antagonist Inhibits Differentiation of B Cells Towards Plasmablasts upon Alloantigen Stimulation. Am J Transplant. 2017;17 (suppl 3).

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