HLA Class II Antibody Activation of Endothelial Cells Promotes Th17 and Disrupts Regulatory T Lymphocyte Expansion

Abstract

Kidney transplantation is the most successful treatment option for patients with end-stage renal disease, and chronic antibody-mediated rejection is the principal cause of allograft loss. Predictive factors for chronic rejection include high levels of HLA alloantibodies (particularly HLA class II) and activation of graft endothelial cells (ECs). The mechanistic basis for this association is unresolved. We used an experimental model of HLA-DR antibody stimulation of microvascular ECs to examine the mechanisms underlying the association between HLA class II antibodies, EC activation and allograft damage. Activation of ECs with the F(Ab′)2 fragment of HLA-DR antibody led to phosphorylation of Akt, ERK and MEK and increased IL-6 production by ECs cocultured with allogeneic peripheral blood mononuclear cells (PBMCs) in an Akt-dependent manner. We previously showed that HLA-DR–expressing ECs induce polarization of Th17 and FoxP3bright regulatory T cell (Treg) subsets. Preactivation of ECs with anti-HLA-DR antibody redirected EC allogenicity toward a proinflammatory response by decreasing amplification of functional Treg and by further increasing IL-6–dependent Th17 expansion. Alloimmunized patient serum containing relevant HLA-DR alloantibodies selectively bound and increased EC secretion of IL-6 in cocultures with PBMCs. These data contribute to understanding of potential mechanisms of antibody-mediated endothelial damage independent of complement activation and FcR-expressing effector cells. Kidney transplantation is the most successful treatment option for patients with end-stage renal disease, and chronic antibody-mediated rejection is the principal cause of allograft loss. Predictive factors for chronic rejection include high levels of HLA alloantibodies (particularly HLA class II) and activation of graft endothelial cells (ECs). The mechanistic basis for this association is unresolved. We used an experimental model of HLA-DR antibody stimulation of microvascular ECs to examine the mechanisms underlying the association between HLA class II antibodies, EC activation and allograft damage. Activation of ECs with the F(Ab′)2 fragment of HLA-DR antibody led to phosphorylation of Akt, ERK and MEK and increased IL-6 production by ECs cocultured with allogeneic peripheral blood mononuclear cells (PBMCs) in an Akt-dependent manner. We previously showed that HLA-DR–expressing ECs induce polarization of Th17 and FoxP3bright regulatory T cell (Treg) subsets. Preactivation of ECs with anti-HLA-DR antibody redirected EC allogenicity toward a proinflammatory response by decreasing amplification of functional Treg and by further increasing IL-6–dependent Th17 expansion. Alloimmunized patient serum containing relevant HLA-DR alloantibodies selectively bound and increased EC secretion of IL-6 in cocultures with PBMCs. These data contribute to understanding of potential mechanisms of antibody-mediated endothelial damage independent of complement activation and FcR-expressing effector cells.