Background.The purpose of this study was to investigate whether obliterative bronchiolitis might occur after xenogenic pulmonary transplantation. A model for obliterative airway disease (OAD) after tracheal allograft transplantation in the rat undergoes tracheal obliteration with histologic features characteristic of obliterative bronchiolitis in human lung transplant recipients. Using this model, the pathogenesis of OAD and its prevention with immunosuppressive drugs was studied in rat recipients of hamster tracheal grafts.Methods.Tracheae from 30 hamsters (xenografts) or 23 Brown-Norway rats (allografts) were implanted and wrapped in the greater omentum of untreated Lewis rats. The grafts were removed on day 1, 3, 7, 14, 21, or 28 after transplantation and stained with hematoxylin and eosin and Masson's trichrome and by immunohistochemistry and immunofluorescence (IFL) techniques. In addition, 25 recipients were treated with cyclosporine (CsA, 10 mg/kg p.o.), leflunomide (LFM, 20 mg/kg p.o.), or rapamycin (RPM, 6 mg/kg i.p.) for 14 or 21 days (5 animals per treatment group). Visual and morphometric analyses were used to evaluate the extent of airway obliteration, luminal coverage by respiratory or flattened cuboidal epithelium, and extent and density of peritracheal cellular inflammation.Results.In all xenografts, a neutrophilic infiltration of the mucosa and submucosa was observed from day 1 until day 14 and was associated with complete loss of tracheal epithelium by day 14. A marked peritracheal mononuclear cellular infiltrate mixed with plasma cells and eosinophils was seen on days 7 and 14. Both the extent of peritracheal inflammation and the density of the mononuclear cell infiltrate were significantly increased in xenograft tracheae when compared with the allografts. Tracheal obliteration began on day 14 and reached a maximum of 43% on day 21 with evidence of intraluminal fibrosis. In contrast to IFL of allografts, IFL of xenografts demonstrated marked deposition of rat immunoglobulin in the peritracheal tissue on days 7 and 14. The effects of treatment with immunosuppressive drugs on tracheal graft narrowing and protection of respiratory epithelium were as follows: After 14 days of treatment, the percentage of tracheal graft narrowing was 12%, 23%, and 19% in the no treatment, CsA, and LFM groups, respectively; the percentage of respiratory epithelium at 14 days was 0%, 21%, and 95%. After 21 days of treatment, the percentage of tracheal graft narrowing was 43%, 49%, 12%, and 5% for the no treatment, CsA, LFM, and RPM groups, respectively; the percentage of respiratory epithelium at 21 days was 0%, 39%, 86%, and 0%. Using computerized morphometry, the extent and densities of the peritracheal cellular infiltrates were significantly reduced in LFM- and CsA-treated groups when compared with untreated xenograft controls. LFM and RPM, but not CsA, significantly reduced the degree of luminal obliteration compared with no treatment (P<0.05). LFM and, to a lesser extent, CsA were able to prevent the loss of normal respiratory epithelium. Analysis by IFL revealed a marked decrease in rat immunoglobulin deposition in xenografts from LFM- and RPM-treated groups compared with xenografts from CsA-treated or untreated rats.Conclusions.(1) OAD occurs not only after tracheal allotransplantation but also after xenotransplantation. (2) Subepithelial infiltration of neutrophils and the appearance of plasma cells and eosinophils in the peritracheal infiltrates distinguished the histology of rejected xenografts from allografts. (3) Antibody deposition was detected by IFL only in xenografts. (4) Treatment with LFM or RPM significantly decreased the severity of luminal obliteration. Importantly, LFM also prevented the loss of respiratory epithelium.