ASSESSMENT OF PATHOLOGICAL-CHANGES ASSOCIATED WITH CHRONIC ALLOGRAFT-REJECTION AND TOLERANCE IN 2 EXPERIMENTAL-MODELS OF RAT LUNG TRANSPLANTATION

Lung transplantation is now routinely performed for a wide range of end-stage cardiopulmonary disorders. Despite overcoming the problems associated with early acute rejection, chronic rejection (CR) in the form of obliterative bronchiolitis has emerged as the primary cause of late graft loss. The mechanisms involved in the development of CR of lung allografts are poorly understood, and no effective therapy is currently available. To better understand the pathological events associated with CR and tolerance, we examined two models of lung allograft rejection established in our laboratory. First, we exchanged left lung allografts between moderately histoincompatible inbred rat strains (WKY-->F344: n=42 and F344-->WKY: n=40). The WKY-->F344 model was previously shown to develop spontaneous tolerance, while the converse model (F344-->WKY) showed persistent acute rejection. The purpose of this investigation was to assess histopathological changes associated with long-term grafts left in place up to 140 days after transplant. To confirm that tolerance had developed, skin-grafting experiments were performed. Five skin grafts from each strain were placed on lung allograft recipients on day 35 after transplant and skin allograft survival was assessed and compared with controls. Acute rejection (AR) was graded histologically (stage 0-IV) and the pathologic intensity of inflammation and CR were graded (0-4: 0=0%, 1=1-25%, 2=26-50%, 3=51-75%, and 4=76-100%) on percentage of involvement with the following categories being examined: (a) lymphocytic infiltration (perivascular, peribronchial, and peribronchiolar) and (b) vasculitis, edema, hemorrhage, and necrosis. Finally, chronic rejection was diagnosed by the presence of intimal hyperpiesia, interstitial fibrosis, peribronchiolar fibrosis, bronchiolitis obliterans, and bronchiectasis. The WKY-->F344 animals showed progressive AR (stage III, day 21). Thereafter, the AR subsided spontaneously and was stage 0 on day 140. There were no signs of CR in these animals. In the F344-->WKY model, the AR progressed up to stage III-IV (day 21) and maintained for several weeks at stage III. Thereafter, pictures of the lungs showed CR on days 49, 70, and 98. There were significant differences between the two models during the chronic phase, such as interstitial fibrosis (0+/-0 vs. 1.8+/-1.3, P<0.005), peribronchiolar fibrosis (0+/-0 vs. 3.6+/-0.55, P<0.01), vasculitis (0.2+/-0.45 vs. 2.0+/-0, P<0.008), and intimal hyperplasia (0.2+/-0.45 vs. 2.6+/-0.9, P<0.008). A significant prolonged survival of skin grafts was seen in the WKY-->F344 animals who received WRY lung compared with the controls (mean survival time = 63 days vs. 12 days, P<0.009). Third-party skin grafts were rejected within 10 days. Data obtained from these studies suggest that: (a) the WKY-->F344 model develops true tolerance (documented by skin-grafting experiments) and that the absence of AR prevents CR. (b) The persistent AR seen in the F344-->WKY model allows for the development of CR and presents an opportunity to study the immunopathogenic mechanisms responsible for CR in this model.