MiR-146a Regulates the TRAF6/TNF-Axis in Donor T Cells during Graft-Versus-Host Disease

Introduction: Acute graft-versus-host disease (GvHD) arises from the attack of recipient tissues by donor allogeneic T cells and represents one of the major limitations of allogeneic hematopoietic cell transplantation (allo-HCT). In spite of many clinical trials, the standard immunosuppressive regimens for prevention of acute GvHD have improved little in the last two decades. Hence, a better understanding of the biology of acute GvHD may improve therapeutic options. MicroRNA-146a (miR-146a) was found to be increased in the sera of patients with GvHD. Therefore, we aimed to decipher the role of miR-146a in allogeneic donor T cells during GvHD by functional studies and in patients undergoing allo-HCT by single nucleotide polymorphism (SNP) analysis.

Methods: We used two different murine major MHC mismatch models for acute GvHD. Recipient mice were conditioned with irradiation before transplantation of bone marrow and either wildtype or miR-146a deficient T cells from allogeneic donor mice. Furthermore, genomic DNA from 289 patients that underwent allo-HCT and their respective hematopoietic stem cell donors was isolated in order to determine their miR-146a rs2910164genotype.

Results: We observed miR-146a upregulation in T cells of mice developing acute GvHD compared to untreated mice in a major MHC and a minor histocompatibility antigen mismatch model. Transfer of miR-146a deficient T cells caused increased GvHD severity, elevated TNF serum levels and reduced survival. Conversely, the phytochemical induction of miR-146a or its overexpression in donor T cells using a specific miR-146a mimic reduced GvHD severity. TNF receptor-associated factor 6 (TRAF6), a verified target of miR-146a, was upregulated in miR-146a^-/- T cells following alloantigen stimulation. Higher TRAF6 levels translated into increased NF-κB activity and TNF production in miR-146a^-/- T cells, while other pro-inflammatory cytokine levels were unaffected. The detrimental effect of miR-146a deficiency in T cells could be antagonized by TNF blockade in vivo. Moreover, in contrast to WT T cells, over expression of miR-146a in Tnf deficient T cells had no effect on their alloreactivity. In the human system, the minor genotype of the SNP rs2910164, which causes reduced miR-146a expression, was more frequent in patients developing acute GvHD grade III/IV compared to all other allo-HCT recipients (n=289).

Conclusions: Taken together we show that miR-146a functions as a negative regulator of the TRAF6/TNF-axis in allogeneic donor T cells during GvHD, leading to reduced TNF transcription. Given our observation on the predictive role of the SNP leading to decreased miR-146a expression in acute GvHD in patients and the possibility to exogenously enhance miR-146a expression, we provide a novel and targeted molecular approach to mitigate GvHD.