Inducible nitric oxide synthase (iNOS) mediated inhibition of T cell receptor (TCR) signaling is a mechanism for eosinophil dependent amelioration of alloimmunity in lung grafts

Abstract

Abstract Eosinophils play a surprising role in downregulating lung graft-specific alloimmunity, even without immunosuppression. We demonstrate here, a mechanism used by eosinophils to downregulate alloimmune responses. To study the mechanism of eosinophil dependent downregulation of alloimmune responses, we used a model of diphtheria toxin (DT) dependent ablation of eosinophils in iPHIL, a mouse model expressing DT receptor on the eosinophil-peroxidase gene locus. Fully MHC-mismatched Balb/c (H2Kd) lungs were engrafted to iPHIL (H2Kb) recipients without immunosuppression. We modeled alloreactivity in vitro, with mixed leukocyte cultures, comprising Th1-polarized eosinophils as modulators of T cells from C57BL/6J or Nur77-GFP mice stimulated by either allogeneic dendritic cells or anti-CD3/CD28 agonistic antibodies. Ablation of eosinophils in non-immunosuppressed lung graft recipients leads to enhanced T cell activation and graft rejection. Similarly, in vitro, we observed a dose and contact dependent downregulation of T cell activation by eosinophils. While contact between eosinophils and T cells is mediated through the upregulation of PD-L1 on both wild-type and iNOS−/− eosinophils after TH-1 polarization, the inhibition of T cell activation is iNOS dependent and is directly linked to the iNOS dependent inhibition of the TCR signaling early gene, Nr4a1, monitored in Nur77-GFP mice. Thus, following contact with T cells, iNOS-expressing eosinophils downregulate lung graft alloimmunity through a direct effect on TCR engagement early genes, leading to the inhibition of T cell activation. This provides an explanation for the surprising observation of lung-graft specific alloimmune downregulation by eosinophils.