Bootcongres

Background: Understanding and defining the process of injury to the transplanted kidney is essential for improving short and long term allograft survival. In this process B cells are harmful through the generation of donor-specific alloantibody. Important cells that stimulate differentiation of B-lymphocytes into harmful immunoglobulin-producing plasmablasts (through IL-21) are follicular T-helper (T_FH) cells. We studied the function of peripheral T_FH-cells in patients before and after kidney-transplantation (KT) in vitro and in vivo. Methods: Numbers of plasmablasts and T_FH-cells (CD4+CXCR5+ T-cells) including their IL-21 production were measured before and after KT (n=30), as well as DSA (positive when MFI≥4000). Function was studied by co-culture experiments of T_FH-cells with endotoxin-activated B-cells (CD4+CXCR5- T-cells as negative control). After 7 days of co-culture, we measured B-cell differentiation into plasmablasts and immunoglobulin(Ig)-production. To define the role of IL-21 , the IL-21-receptor was blocked. Finally, by immunehistochemistry the presence of T_FH-cells was established in rejection biopsies. Results: While the numbers of T_FH-cells remained stable after KT, the numbers of plasmablasts were reduced after KT (p<0.01). However, after KT T_FH-cells were three times higher in patients with pre-existing DSA than in patients without pre-existent DSA (p<0.02). Moreover, the IL-21 production capacity by T_FH-cells decreased after KTx in patients without pre-existing DSA (p<0.01), but not in patients with DSA. In vitro, T_FH-cells induced B-cell differentiation into Ig-producing plasmablasts (p<0.05). T_FH-cells obtained after KT were still able to stimulate B-cell differentiation and Ig-production (p<0.05). Blocking the IL-21-R suppressed these processes (p<0.05). Follicular-like structures were observed in the biopsies of the transplants with type I cellular rejections, which showed the presence of T-cells expressing the transcription factor for T_FH-cells, BCL-6. Moreover, these type I cellular biopsies stained positive for Ig. 

Conclusions: In KT-patients the differentiation of B cells into Ig-producing plasmablasts depends on IL-21 producing T_FH-cells. Inhibition of the IL-21 pathway could be a novel approach for the treatment of humoral responses after KT and might improve allograft survival in patients with pre-existing DSA.