Regulatory Role of IL-10 in Experimental Obliterative Bronchiolitis in Rats Babu Naidu,* Baiya Krishnadasan,* Richard I. Whyte,* Roscoe L. Warner,† Peter A. Ward,† and Michael S. Mulligan* *Department of Cardiothoracic Surgery, University of Washington Medical School, Seattle, Washington 98115; and †Department of Pathology, University of Michigan Medical School, Ann Arbor, Michigan 48109 Received June 28, 2002 Obliterative bronchiolitis (OB) affects over half of all chronic human survivors following lung or heart–lung transplantation. Respiratory epithe- lial cell injury, peribronchial inflammation, and proliferation of fibrovas- cular connective tissue causing airway occlusion characterize this lesion. Using a rat model of experimental OB, tracheas and mainstem bronchi from Brown–Norway or Lewis (LEW) rats were transplanted subcutane- ously into LEW recipients. At 7 days, airway lumens of allografts showed minimal luminal obstruction but significant respiratory epithelial loss. By 14 days, allografts demonstrated marked peribronchial inflammation, nearly complete loss of respiratory epithelium, and extensive intraluminal proliferation of fibrovascular connective tissue, with a mean 58% reduction in airway cross-sectional diameter. However, isografts showed only limited peribronchial inflammation and no loss of airway lumen. When recipients of allotransplants were treated with anti-IL-10, OB developed more rap- idly. As early as 7 days, there was marked histologic evidence of OB and a 43% reduction in mean cross-sectional area. Allograft animals that received 5 g/day of recombinant IL-10 as a constant infusion on day 14 showed almost complete preservation of respiratory epithelium and only mild peribronchial inflammation with only a 15% reduction in airway cross-sectional area. These findings suggest that endogenous IL-10 plays a regulatory role in the development of experimental OB. © 2002 Elsevier Science (USA) INTRODUCTION Obliterative bronchiolitis (OB) affects more than 60% of long-term recipients of lung and combined heart–lung trans- plants (Sundaresan et al., 1995). This problem is character- ized clinically by a decrease in the mid portion of the expiratory flow curve and progressive dyspnea accompa- nied by a nonproductive cough and a clear chest X ray. Commonly, OB is steadily progressive and fatal (Reichen- spurner et al., 1995). While treatment usually consists of intensifying immunosuppressive therapy, there is no spe- cific and effective therapy for prevention or reversal of OB. Little is known about the pathogenesis of OB, in part because it has been difficult to model experimentally. Prob- lems related to microsurgical techniques and issues of long- term survival and maintenance protocols have limited the utility of orthotopic whole-lung transplantation in small animals. A technically simpler model of allograft airway transplantation (Hertz et al., 1993) is one effective strategy used to investigate the pathogenesis of experimental OB. Originally described in the mouse, this model has been adapted in the rat (Tazelaar et al., 1987). The histopatho- logical features of this experimental OB model largely reproduce the changes in human recipients who develop OB after lung allotransplantation. Typically, complete airway obstruction develops 3– 4 weeks following transplantation of rat airways that are heterotopically transplanted into allogeneic recipients. In the present studies, we attempted to identify a potential role for the regulatory cytokines (IL-10) in the development of experimental OB. IL-10 has been shown to have a number of anti-inflammatory effects. It reduces natural killer cell and TH 1 cell expression of IFN (Fiorentino et al., 1989) and blocks macrophage production of IL-1, TNF, IL-6, IL-8, GM-CSF, and G-CSF (Fiorentino et al., 1991). IL-10 can also blunt expression of MHC Class II antigens on monocytes and inhibit antigen-specific T-cell prolifera- tion (de Waal et al., 1991). It has been shown to diminish nitric oxide production by specifically suppressing inducible nitric oxide synthase (iNOS) expression (Cunha et al., 1992). IL-10 likewise has been shown to play important regulatory roles in acute injury in vivo. Antagonism of endogenous IL-10 with a polyclonal blocking antibody re- sults in augmentation of acute immune-complex-induced alveolitis in rat lung (Shanley et al., 1995). Conversely, intratracheal instillation of recombinant IL-10 protects, in a dose-dependent manner, against development of injury in Experimental and Molecular Pathology 73, 164 –170 (2002) doi:10.1006/exmp.2002.2466 164 0014-4800/02 $35.00 © 2002 Elsevier Science (USA) All rights reserved.