A novel model for dissecting roles of IL-17 in lung transplantation

Background: The long-term success of lung transplantation is limited by the development of chronic lung allograft dysfunction (CLAD) in which IL-17 plays an important role. Direct evidence of IL-17-mediated allograft rejection has been observed when T-bet is absent. However, lack of T-bet also leads to failure in production of IFN-γ which is required for tolerance induction and allograft acceptance, as T-bet deficiency results in IL-17-expressing CD8+ T cells mediated costimulation blockade-resistant allograft rejection. Our previous research demonstrated that additional STAT6 deficiency to T-bet deficiency resulted in Th17-dominant immune responses, and importantly, restored IFN-γ production. Here we investigated whether T-bet/STAT6 double knout-out (DKO) mice as allograft recipients could provide a useful model to study IL-17 and Th17 in lung transplantation. Methods: Murine orthotopic allogeneic lung transplants were performed in C57BL/6 wild type (WT) or T-bet/STAT6 DKO (C57BL/6 background) mice using MHC fully mismatched BALB/c donors. Syngeneic transplants were also performed in WT C57BL/6 mice using C57BL/6 donors. At day 10, histopathologic characteristics and rejection status of transplanted grafts were assessed; graft-infiltrating cells were isolated and real-time RT-PCR was performed for IL-17, IFN-γ and IL-4 expressions. Results: Isografts showed no apparent rejection as anticipated. Allografts of both WT and DKO recipients displayed vigorous acute rejection and expressed comparable levels of IFN-γ; while T-bet/STAT6 double deficiency resulted in much more IL-17 and less IL-4 production. Histopathologic examination demonstrated that allografts of both WT and DKO recipients have marked inflammatory cell infiltration and pulmonary parenchyma lesion. In contrast to lymphocyte-predominant inflammation observed in WT recipients, allografts of DKO recipients displayed obvious polymorphonuclear cell infiltration and severer obliterative airway inflammation. Compared to WT recipients, the ratio of graft-infiltrating CD8+ versus CD4+ T cells increased significantly with much higher numbers of neutrophils in allografts of DKO recipients. Conclusions: T-bet/STAT6 DKO recipients of lung allografts result in IL-17-dominant transplant immunity, retain IFN-γ responses, and develop neutrophilia, obliterative airway inflammation and acute transplant rejection. Our results indicate that T-bet/STAT6 DKO mice serving as allograft recipient could be utilized as a new viable model to study the roles of IL-17 in lung transplantation.