Transplant Immunology 11 (2003) 179–185 0966-3274/03/$ - see front matter  2003 Elsevier Science B.V. All rights reserved. PII: S0966-3274 Ž 03 . 00004-2 Chronic cardiac allograft rejection in a rat model disparate for one single class I MHC molecule is associated with indirect recognition by CD4 T q cells Haval Shirwan *, Anna Mhoyan , Esma S. Yolcu , Xingyi Que , Sherif Ibrahim a, a a a b Institute for Cellular Therapeutics and Department of Microbiology and Immunology, University of Louisville, Louisville, KY 40202, USA a Department of Pathology, New York University, New York, NY 10016, USA b Received 30 September 2002; accepted 24 January 2003 Abstract T-cell mediated immune responses play a critical role in chronic allograft dysfunction. The complex nature of allograft rejection, particularly with respect to the vast repertoire of alloantigens and their mode of recognition by T cells, presents a major challenge for the design of well-controlled studies into the immunobiology of chronic rejection. The purpose of this study was to develop a rat model with restricted antigenic specificity that develops chronic rejection without any immunologic manipulation to study the T-cell response. PVG.1U allogeneic hearts disparate for one single class I antigen, RT.1A , were transplanted into PVG.R8 u rat recipients. Grafts from PVG.R8 were used as syngeneic controls. Chronic rejection was studied by histological analysis of the grafted hearts at various time points posttransplantation (20–100 days). Donor specific alloreactive response was studied in a mixed lymphocyte reaction assay. All allografts survived more than 90 days and showed extensive evidence of chronic rejection, which was characterized by interstitial fibrosis, vasculitis, and occlusive myointimal thickening. Chronic rejection was evident by day 20 and most extensive by day 100 posttransplantation. In marked contrast, syngeneic grafts remained free of chronic lesions. Lymphocytes harvested from graft recipients showed a more vigorous proliferative response to allogeneic splenocytes as compared with that of lymphocytes from naıve animals. The proliferative response was primarily mediated by CD4 T cells recognizing q ¨ the RT1.A molecule via the indirect pathway. A single class I disparity in this model generates chronic rejection associated with a potent CD4 T-cell responses induced by the indirect recognition pathway. The use of this antigenically restricted model may q facilitate the design of well-controlled studies for the characterization of immune mechanisms responsible for chronic rejection.  2003 Elsevier Science B.V. All rights reserved. Keywords: Heart transplantation; Chronic rejection; Indirect recognition; CD4 T cells q 1. Introduction Although the use of potent immunosuppressive phar- maceuticals curtailed acute allograft rejection in the clinic, most of these grafts succumb to slowly progress- ing immunopathological reactions that culminate in chronic rejection. The most significant pathologic find- ing in chronically rejected organ grafts is diffuse con- centric intimal proliferation in the arterial system. Although the etiology of this pathogenic process is Abbreviations: H&E, hematoxylin and eosin staining; MHC, major histocompatibility complex; TCR, T-cell receptor. *Corresponding author. Institute for Cellular Therapeutics, 570 S, Preston Street, Suite 404, University of Louisville, Louisville, KY 40202, USA. Tel.: q1-502-852-2066; fax: q1-502-852-2085. E-mail address: haval.shirwan@louisville.edu (H. Shirwan). multifactorial, alloreactive immune responses play the major role w1–3x. Evidence substantiating this contention was provided by a study demonstrating that grafts performed between genetically different individuals develop chronic rejection; whereas, those between genet- ically identical individuals remain free of chronic rejec- tion w4x. Additionally, regimens that are targeted at specific blockade of T-cell responses are able to prevent chronic rejection w5,6x. Allograft rejection is primarily initiated by alloreac- tive T cells that recognize the foreign histocompatibility antigens of the graft through specific T-cell receptors (TCRs). This recognition process initiates a set of intra- and intercellular reactions that result in the destruction of the graft. MHC molecules are the most potent of histocompatibility antigens that serve as targets for