CD4 ⁺ T Cells and Complement Independently Mediate Graft Ischemia in the Rejection of Mouse Orthotopic Tracheal Transplants

Abstract

Rationale While microvascular injury is associated with chronic rejection, the cause of tissue ischemia during alloimmune injury is not yet elucidated. Objective We investigated the contribution of T lymphocytes and complement to microvascular injury-associated ischemia during acute rejection of mouse tracheal transplants. Methods and Results Using novel techniques to assess microvascular integrity and function, we evaluated how lymphocyte subsets and complement specifically affect microvascular perfusion and tissue oxygenation in MHC-mismatched transplants. To characterize T cell effects on microvessel loss and recovery, we transplanted functional airway grafts in the presence and absence of CD4+ and CD8+ T cells. To establish the contribution of complement-mediated injury to the allograft microcirculation, we transplanted C3-deficient and C3-inhibited recipients. We demonstrated that CD4+ T cells and complement are independently sufficient to cause graft ischemia. CD8+ T cells were required for airway neovascularization to occur following CD4-mediated rejection. Activation of antibody-dependent complement pathways mediated tissue ischemia even in the absence of cellular rejection. Complement inhibition by CR2-Crry attenuated graft hypoxia, complement/antibody deposition on vascular endothelium and promoted vascular perfusion by enhanced angiogenesis. Finally, there was a clear relationship between the burden of tissue hypoxia (ischemia × time duration) and the development of subsequent airway remodeling. Conclusions These studies demonstrated that CD4+ T cells and complement operate independently to cause transplant ischemia during acute rejection and that sustained ischemia is a precursor to chronic rejection.

Targeting complement component 5a promotes vascular integrity and limits airway remodeling

“…This model system faithfully replicates lymphocytic bronchitis, a large airway precursor of chronic rejection (bronchiolitis obliterans syndrome) (7). We demonstrated that either CD4 T-cell or antibody-triggered complement activity is independently sufficient to induce airway ischemia during allograft rejection, whereas CD8 T-cell activity is not (8). Although decreasing complement component 3 (C3) activity greatly reduces the duration of tissue ischemia associated with allograft rejection, it also paradoxically leads to vasodilatation, and increases microvessel permeability early in rejection.…”

“…C3 −/− recipients of tracheal allografts exhibit improved graft perfusion over time because of enhanced neovascularization following acute rejection (8). However, during the first week of acute rejection, there is evidence of vasodilatation and increased permeability in allograft microvessels in C3 −/− recipients (8).…”

“…In this study, we addressed the paradoxical finding of microvascular vasodilatation and leakiness previously observed in C3 −/− and C3-inhibited recipients of allograft transplants (8). Vasodilatation and increased capillary permeability can be triggered by C5a, an anaphylatoxin.…”

“…Further, the simultaneous loss of complement C3 and C5a activity is sufficient to completely prevent airway ischemia, even in the absence of other immunosuppression. Preservation of blood flow in the allograft is highly correlated with maintenance of normal airway architecture (5,6,8), and C5a-inhibited C3 −/− recipients of tracheal allografts maintain normal airway architecture as defined by relatively little subepithelial fibrosis and normal pseudostratified epithelium.…”

“…The Veterans Affairs Palo Alto Animal Care and Use Committee approved experiments used in this study. Orthotopic tracheal transplants were prepared according to our established procedure as described previously with slight modifications (6,8). All mice (4-6 wk) were purchased from the Jackson Laboratory.…”