Expansion of Th/Tc17 T Cells and Loss of CX3CR1⁺ Macrophages Play a Critical Role in the Pathogenesis of Steroid-Refractory Acute Intestinal Gvhd

Acute gut graft-versus-host disease (GVHD) is the major contributor to mortality and morbidity of allogeneic hematopoietic stem cell transplantation recipients. Steroids are the first-line therapy for acute GVHD, but approximately 30% of patients develop steroid refractory GVHD (SR-GVHD). The pathophysiology of SR-GVHD is poorly understood.

In the current studies, we explored the mechanisms of SR-GVHD, using a murine model of MHC-mismatched HCT with C57BL/6 donors and BALB/c recipients. Acute GVHD can be mediated by either Th/Tc1 or Th/Tc17 T cells. We evaluated the impact of dexamethasone (DEX) treatment on acute GVHD induced by spleen and BM cells from WT, IFN-γ^-/- or RORγt^-/- C57BL/6 donors. WT donor T cells give rise predominantly to Th/Tc1 cells early after HCT, but Th/Tc17 cells can expand as time goes on. IFN-γ^-/- donor T cells give rise to Th/Tc17 cells but not to IFN-γ-producing Th/Tc1 cells, while RORγt^-/- donor T cells give rise to Th/Tc1 cells but not to Th/Tc17 cells. BALB/c recipients were treated with single dose dexamethasone (DEX) at 5 mg/Kg on day 3 after HCT. DEX treatment effectively prevented acute GVHD in recipients given WT or RORγt^-/- grafts for up to 20 days after HCT. GVHD then recurred in recipients grafted with WT cells but not in recipients grafted with RORγt^-/- cells. In contrast, DEX treatment had no effect on acute GVHD in recipients grafted with IFN-γ^-/- cells. Recurrence of GVHD in DEX-treated recipients grafted with WT cells and the lack of DEX effect in recipients grafted with IFN-γ^-/- cells was associated with expansion of Th/Tc17 cells that produced both IL-17 and IL-22. In recipients grafted with IFN-γ^-/- cells, we also observed marked reduction of donor-type CX3CR1⁺ intestinal macrophages that play a critical role in controlling bacterial translocation from the intestinal lumen to the liver and blood. In addition, the DEX-treated recipients showed augmentation of Th/Tc17 differentiation and exacerbation of acute GVHD, When CX3CR1^GFP/GFP mice that are deficient in generating CX3CR1⁺ intestinal macrophages were used as donors.

These results indicate that DEX-treatment can cause expansion of Th/Tc17 and loss of protective donor-type CX3CR1⁺ intestinal macrophages, leading to SR-GVHD.