Recipient Dendritic Cells Are Regulated By MiR-146a during Acute GvHD

Acute Graft-versus-host disease (GvHD) is a major immunological complication following allogeneic hematopoietic cell transplantation (allo-HCT) occurring in 30-40% of patients. Although immunosuppression with corticosteroids serves as standard first-line therapy in acute GvHD, sustained responses are achieved in less than 50% of patients. Therefore, the identification of risk factors and a better understanding of the molecular mechanisms underlying the disease are indispensable to develop novel therapeutic strategies. MicroRNAs control various key biological processes, including innate and adaptive immune responses. In particular, microRNA-146a (miR-146a) is emerging as a central regulator of innate immune cell signaling.

Here we found that a G/C single nucleotide polymorphism (SNP) within the pre-miR-146a gene, which reduces the amount of miR-146a produced from the C allele, was strongly associated with the risk to develop severe acute GvHD. Patients with the rs2910164 CC genotype had a significantly higher risk to develop acute GvHD grade III-IV than all other allo-HCT recipients (p=0.002) as well as an overall increased histopathological GvHD severity. In order to functionally dissect the role of miR-146a in recipient cells during GvHD, we used gene targeted (miR-146a^-/-) mice and a major histocompatibility complex (MHC)-mismatched experimental model for acute GvHD. MiR-146a deficiency of the recipient mice resulted in increased GvHD severity, elevated serum levels of pro-inflammatory cytokines and accelerated mortality compared to wild type (WT) recipients after allo-HCT. Chimeric mice with a lack of miR-146a specifically in the hematopoietic system showed an even more distinct disease exacerbation, whereas miR-146a deficiency only in non-hematopoietic recipient cells did not affect GvHD severity. Moreover, co-transplantation of recipient type miR-146a^-/-dendritic cells (DCs) elicited a more aggressive course of GvHD, while overexpression of miR-146a in DCs using a specific miR-146a mimic ameliorated disease. Phenotypically, miR-146a^-/- DCs displayed a more activated phenotype with increased MHC class II, CD80 and CD86 surface expression levels after endotoxin stimulation when compared to WT DCs. Global gene expression analysis revealed upregulation of the JAK-STAT signaling pathway in miR-146a^-/- DCs, which we confirmed at the protein and phosphoprotein level using Western blot and flow cytometry. Importantly, inhibition of JAK-STAT signaling in DCs using the JAK1/2 inhibitor ruxolitinib prevented miR-146a^-/- DC-induced GvHD exacerbation.

In conclusion, our findings indicate that miR-146a acts as a central regulator of recipient type DCs during GvHD, by dampening cytokine receptor signaling via the JAK-STAT pathway. We detected the SNP rs2910164 as a marker to identify patients at high risk for GvHD before allo-HCT, facilitating the opportunity to apply pre-emptive interventions.