Rat Aortic Allograft Model Research Articles

Monocyte Chemoattractant Protein 1–Mediated Migration of Mesenchymal Stem Cells Is a Source of Intimal Hyperplasia

Intimal hyperplasia is considered to be a healing response and is a major cause of vessel narrowing after injury, where migration of vascular progenitor cells contributes to pathological events, including transplant arteriosclerosis. In this study, we used a rat aortic-allograft model to identify the predominant cell types associated with transplant arteriosclerosis and to identify factors important in their recruitment into the graft. Transplantation of labeled adventitial tissues allowed us to identify the adventitia as a major source of cells migrating to the intima. RNA microarrays revealed a potential role for monocyte chemoattractant protein 1 (MCP-1), stromal cell-derived factor 1, regulated on activation, normal T cell expressed and secreted, and interferon-inducible protein 10 in the induced vasculopathy. MCP-1 induced migration of adventitial fibroblast cells. CCR2, the receptor for MCP-1, was coexpressed with CD90, CD44, NG2, or sca-1 on mesenchymal stem cells. In vivo experiments using MCP-1-deficient and CCR2-deficient mice confirmed an important role of MCP-1 in the formation of intimal hyperplasia in a mouse model of vascular injury. The adventitia is a potentially important cellular source that contributes to intimal hyperplasia, and MCP-1 is a potent chemokine for the recruitment of adventitial vascular progenitor cells to intimal lesions.

T-cadherin expression in cardiac allograft vasculopathy: Bench to bedside translational investigation

Coronary allograft vasculopathy (CAV) is the major limiting factor for long term survival after heart transplantation. The aim of this study was to identify gene candidates implicated in human CAV using a rat aortic allograft model in tandem with microarrays and quantitative real time PCR (Q-PCR). Rat abdominal aortas were isografted (5) or allografted (5) from Brown-Norway to Lewis rats and grafts were harvested after day 8, 25 and 60. Agilent microarrays were then used to highlight differentially expressed genes between isografted and allografted rat aortas. Further investigation of a selected candidate gene was performed on human coronary arteries. 1829, 2582 and 1925 genes (fold changes >2 or <2 and p values <0.05) were differentially expressed at day 8, 25 and 60 respectively between isografs and allografts. Seventeen candidate genes were selected according to significant differential expression at day 60. These rat candidate genes were then validated by quantitative real time polymerase chain reaction (Q-PCR). One of these candidate genes, T-Cadherin (T-Cad) was further investigated, using immunohistochemistry (IHC), in human coronary arteries showing CAV compared to classical atherosclerosis present in ischemic cardiomyopathy (ICM) and normal coronary arteries present in dilated cardiomyopathy (DCM). Results showed an over expression of T-Cad in CAV and classical atherosclerosis compared to normal coronary arteries. T-Cad was found to be over expressed in CAV. T-Cad could potentially act as a trigger for smooth muscle cells (SMCs) proliferation and vascular remodelling observed in CAV leading to a diffuse narrowing of the arterial lumen.

Low Molecular Weight Fucoidan Prevents Neointimal Hyperplasia After Aortic Allografting

Fucoidan, a new low molecular weight sulfated polysaccharide (LMWF), has previously been shown to mobilize bone marrow-derived progenitors cells via stimulation of stromal derived factor (SDF)-1 release. Mobilized progenitor cells have been suggested to repair intimal lesions after immune-mediated endothelial injury and thus prevent intimal proliferation. The aim of this study was to evaluate the effect of LMWF treatment in a rat aortic allograft model of transplant arteriosclerosis (TA). Aortic grafts were performed in Brown Norway (BN, donor) and Lewis (Lew, recipient) rats. The recipient rats were treated with LMWF (5 mg/kg/day) and sacrificed at 30 days. To determine the role of SDF-1 in mediating the effects of LMWF, a specific inhibitor of the SDF-1 receptor CXCR4, AMD 3100 (20 microg/kg/day), was used. The grafted segments were evaluated by morphometric (histochemical) analyses. Untreated aortic allografts exhibited severe intimal proliferation, indicative of TA. In contrast, LMWF treatment significantly prevented allograft intimal proliferation as compared with controls (5.7+/-3 vs. 66.2+/-6 microm, P<0.01) and permitted a normalization of the intima/media ratio (0.1+/-0.1 vs. 1.7+/-0.3, P<0.01). Further, LMWF treatment stimulated allograft reendothelialization, as evidenced by strong intimal endothelial nitric oxide synthase antibody and CD31 signals. Unexpectedly, AMD treatment failed to prevent the protective effect of LMWF on intimal thickening and AMD treatment alone was found to reduced intimal proliferation in allografts. We found that LMWF treatment reduced intimal thickness and induced the presence of an endothelial cell lining in the vascular graft at 30 days. Our findings may suggest a novel therapeutic strategy in the prevention of TA.

Chlamydia Pneumoniae Infection Is Not Associated With Chronic Transplant Dysfunction in a Rat Aortic Allograft Model

Long-term survival of solid-organ transplants is limited as a result of chronic transplant dysfunction (CTD), which is characterized by occlusion of intragraft vascular tissue due to myointimal hyperplasia. Recent studies have shown a role for infections in vascular pathologies. For example, Chlamydia pneumoniae ( Cpn) has been shown to aggravate atherosclerosis, and Cpn immunoglobulin (Ig)G titers correlate with severity of allograft atherosclerosis after cardiac transplantation. In this study, we evaluated the effect of Cpn infection on CTD using a rat aortic allograft model. Orthotopic abdominal aorta transplantations (Tx) were performed with Brown Norway rats as donors and Lewis rats as recipients. Rats were humanely killed at 1 or 8 weeks after surgery. The graft was processed for DNA isolation and histological examination. Influx of macrophages and T cells was assessed using immunohistochemistry. At 1 week after Tx, the perivascular influx of inflammatory cells in the graft was not affected by Cpn infection. Furthermore, only limited numbers of Cpn DNA copies were found in the graft at 1 week after Tx. In addition, Cpn did not alter the severity of myointimal hyperplasia in the rat aortic allograft model at 8 weeks after surgery. Our data suggested that, in the rat aortic allograft model, Cpn infection did not influence the influx of inflammatory cells or the severity of CTD.

Conversion from cyclosporine A to mycophenolate mofetil protects recipient kidney and prevents intimal hyperplasia in rat aortic allografts

Background Recent studies have demonstrated that complete conversion from cyclosporine A (CsA) to mycophenolate mofetil (MMF) prolongs graft survival in patients undergoing clinical organ transplantation. We investigated the effects of conversion from CsA to MMF on recipient kidneys and transplant arteriosclerosis in a rat aortic allograft model as a high responder combination. Methods DA (MHC haplotype, RT1 a) rat abdominal aortic grafts were orthotopically transplanted into Lewis (RT1 l) rats. The recipients were divided into four oral treatment groups: (1) vehicle group, (2) CsA group (15 mg/kg/day), (3) CsA/MMF40 group (conversion from CsA 15 mg/kg/day to MMF 40 mg/kg/day on day 14), and (4) CsA/MMF20 group (conversion from CsA 15 mg/kg/day to MMF 20 mg/kg/day on day 14). On day 28 after transplantation, the rats were sacrificed and the hematoserological parameters were analyzed. The grafted aortas and recipient kidneys also were evaluated histologically and immunohistochemically. Results The CsA group developed serological renal dysfunction, arteriolar hyalinosis, and apoptosis in the recipient kidneys, whereas the CsA/MMF40 and CsA/MMF20 groups did not. In the vehicle group, we observed remarkable intimal hyperplasia and marked inflammatory cell infiltration including macrophages and T cells. In the CsA group, intimal hyperplasia was evident without infiltration of macrophages or T cells. In the CsA/MMF40 and CsA/MMF20 groups, intimal hyperplasia was abrogated, while adventitial infiltration of and adhesion to the endothelium by macrophages and T cells occurred. Conclusions Conversion from CsA to MMF protected recipient kidneys and prevented transplant arteriosclerosis. However, insufficient immunosuppression by MMF might reactivate immune cells. This conversion therapy has preventive potential in transplant patients with CsA-associated nephrotoxicity and transplant arteriosclerosis.

Overexpression of soluble fas attenuates transplant arteriosclerosis in rat aortic allografts.

The killing of vascular cells by activated macrophages is an important step in the process of destabilization of the arterial wall. The death receptor Fas is implicated in vascular cell death. Hence, we extended our studies in a rat aortic allograft model, using adenovirus-mediated overexpression of soluble Fas (sFas) to block Fas binding to Fas ligand (Fas-L). The contribution of Fas to vascular cell injury and consequent transplant arteriosclerosis was investigated. Activated monocytes in the presence of macrophage colony-stimulating factor induce endothelial cell apoptosis in vitro, which was significantly inhibited by adenovirus-mediated sFas overexpression. Next, donor rat abdominal aortas were either untreated or transduced with adenoviruses encoding (1) rat soluble Fas (Ad3rsFas), (2) no insert (Ad3Null), and (3) beta-galactosidase (Ad3nBg). A total of 175 aortic grafts were harvested 2 to 90 days after transplantation. Vascular cell apoptosis and CD45+ cell infiltration were significantly reduced in Ad3rsFas-transduced aortas, as compared with control allografts. Moreover, the control allografts developed marked intimal thickening, whereas Ad3rsFas-transduced allografts had significantly less neointima until the 90-day time point. sFas overexpression protects the integrity of the vessel wall from immune injury and attenuates transplant arteriosclerosis.

The combined use of prostaglandin I2 analogue (OP-2507) and thromboxane A2 synthetase inhibitor (OKY-046) strongly inhibits atherosclerosis of aortic allografts in rats

Background. Atherosclerosis is the main lesion in allografts undergoing chronic rejection. We investigated the effect of OP-2507 (prostaglandin I2 analogue) and OKY-046 (thromboxane A2 synthetase inhibitor) on graft atherosclerosis morphologically and the production of eicosanoids in grafts in a rat aortic allograft model. Methods. Abdominal aortic allografts of Lewis (RT-1l) rats were transplanted orthotopically into fully major histocompatibility complex mismatched Wistar King A/Qdj (RT-1u) rats that were subcutaneously administered OP-2507 (0.1 mg/kg/d) or OKY-046 (125 mg/kg/d), or both, with an osmotic pump. Four, 8, or 12 weeks later, the grafts were harvested and examined histologically, and the concentration of eicosanoids in the grafts were analyzed. Results. Lewis aortic allografts in Wistar King A recipients with no treatment displayed atherosclerosis, which involved gradual intimal thickening and medial thinning with continuous inflammation in adventitia. Neither OP-2507 nor OKY-046 treatment affected the intensity of adventitial inflammation. Although inhibition of medial thinning or a decrease in medial nuclear density was not observed, OKY-046 administration alone significantly inhibited an increase in intimal thickness. OP-2507 administration alone significantly inhibited a decrease in medial nuclear density and intimal thickening. Combined treatment with OP-2507 and OKY-046 further decreased the alteration of media and intima. The ratio of thromboxane B2 and 6-keto-prostaglandin F1α in the grafts was significantly reduced by OKY-046 but not by OP-2507 alone. Conclusions. We have demonstrated that atherosclerosis in aortic allografts is inhibited by the continuous administration of either OP-2507 or OKY-046, and a combination of both agents strongly increases this inhibitory effect. Amelioration of balance in eicosanoid production in the grafts by the use of thromboxane A2 synthetase inhibitor and the simultaneous usage of stable prostaglandin I2 analogue may be a strategy for preventing atherosclerosis that results from chronic rejection. (Surgery 2001;129:595-605.)

Recipient cells form the proliferative lesion in the rat heterotopic tracheal allograft model of obliterative airway disease (OAD)

Purpose: The primary pathology associated with OAD after lung transplantation is a proliferative lesion in the airways. The rat heterotopic tracheal allograft model is widely employed to study the pathobiology of OAD since it has histologic characteristics similar to those seen in lung allograft patients. We have recently demonstrated that the proliferative lesion found in the rat aortic allograft model of chronic rejection is of recipient origin. The purpose of the current work was to determine the origin of the proliferative lesion in rat OAD.

Endothelial dysfunction and denudation in rat aortic allografts.

Clinical evidence suggests that early endothelial cell (EC) dysfunction may predict the development of graft vascular disease. We wished to assess the early functional and morphological changes in the graft endothelium in a commonly used animal model of graft vascular disease, the rat aortic interposition allograft model. To assess graft EC function, regulation of vascular tone by ECs was monitored in aortic rings from grafts harvested at various times after transplantation (Tx). EC morphology was assessed by silver staining, which was followed by en face inspection of the luminal side of the grafts. Acetylcholine-induced EC-dependent vasorelaxation was reduced in allografts at post-Tx days 7 and 14, whereas in syngeneic grafts EC-dependent relaxation was unaffected at any time after Tx. In separate grafts collected at the same time points, massive leukocyte adhesion at post-Tx day 7 and EC denudation at days 14 and 28 were evident in allografts but not in syngeneic grafts. At post-Tx day 56 (a time at which vessel wall remodeling is pronounced in this model), an intact EC layer covered the grafts. EC dysfunction and morphological changes were prevented by immunosuppression of recipient rats with cyclosporine. Our study shows that Tx-induced EC dysfunction in rat aortic allografts can be observed within 1 week of Tx in rat aortic allografts and that this is occurring concomitantly with enhanced leukocyte adhesion to the graft ECs. These changes occur before any other morphological or functional changes described thus far in this model and appear to be immune-driven. Taken together, these results show that Tx-induced early EC dysfunction, as described in patients, may be studied in the model of rat aortic Tx.

LF 08-0299 In the prophylaxis and treatment of chronic rejection in a rat aortic allograft model

Abstract Chronic rejection is the major cause of late kidney allograft failure. We evaluated the efficacy of LF 08-299 (LF), an analogue of 15-deoxyspergualin, in a rat aortic allograft model of chronic rejection. BN aortic allografts were transplanted to Lew recipients. LF was administered at a dose of 6 mg/kg and 2.5 mg/kg on days 0-20 and 6 mg/kg on days 60-90. CyA was used at a dose of 5 mg/kg on days 0-20. Untreated isografts and allografts were used as controls. Histological changes and immunohistochemistry were monitored sequentially at 8, 12, 16 and 20 weeks. There were no differences in intimal proliferation between LF-treated allografts and untreated or CyA-treated controls. Only a tendency in adventitial infiltration reduction was seen in LF-treated animals. We found a significantly less pronounced reduction in media diameter in LF-treated animals. We concluded that LF 08-0299 is only able to reverse reduction in media thickness in aortic allografts, but not intimal proliferation in this model of chronic rejection.

LF 08-0299 in the prophylaxis and treatment of chronic rejection in a rat aortic allograft model.

Chronic rejection is the major cause of late kidney allograft failure. We evaluated the efficacy of LF 08-299 (LF), an analogue of 15-deoxyspergualin, in a rat aortic allograft model of chronic rejection. BN aortic allografts were transplanted to Lew recipients. LF was administered at a dose of 6 mg/kg and 2.5 mg/kg on days 0-20 and 6 mg/kg on days 60-90. CyA was used at a dose of 5 mg/kg on days 0-20. Untreated isografts and allografts were used as controls. Histological changes and immunohistochemistry were monitored sequentially at 8, 12, 16 and 20 weeks. There were no differences in intimal proliferation between LF-treated allografts and untreated or CyA-treated controls. Only a tendency in adventitial infiltration reduction was seen in LF-treated animals. We found a significantly less pronounced reduction in media diameter in LF-treated animals. We concluded that LF 08-0299 is only able to reverse reduction in media thickness in aortic allografts, but not intimal proliferation in this model of chronic rejection.

Inhibition of transplant vasculopathy in a rat aortic allograft model after infusion of anti-inflammatory viral serpin.

Transplant vasculopathy remains a difficult therapeutic problem, resulting in the majority of late cardiac graft losses. This chronic vascular disease is thought to be triggered by alloantigen-dependent and alloantigen-independent inflammatory factors. Despite improved 1-year survival, the incidence of transplant vasculopathy has not improved with current immunosuppressive protocols. Highly effective strategies have evolved in the large DNA viruses that shield infecting viruses from host inflammatory responses. Serp-1 is a secreted myxoma virus anti-inflammatory serine proteinase inhibitor. Serp-1 inhibits plasminogen activators in a manner similar to plasminogen activator inhibitor (PAI-1), a vascular protein that plays a pivotal regulatory role in vascular wound healing. In this study, we tested the ability of purified Serp-1 protein to ameliorate posttransplant vasculopathy after rat aortic allograft surgery. Serp-1 protein or controls were infused into 98 rats immediately after segmental aortic allograft transplantation. After either late (28 days, 64 rats) or early (12 to 48 hours, 24 rats) follow-up, transplanted aortic segments were harvested for morphological and immunohistochemical analysis. Significant reductions in intimal plaque growth (P<0.002) and mononuclear cell invasion (P<0.033) were detected after Serp-1 infusion at nanogram doses. Serp-1 reduced early macrophage (P<0.0016) and nonspecific lymphocyte (P<0.0179) invasion into medial and adventitial layers and inhibited associated depletion of medial smooth muscle cells (P<0.0006). Infusion of a viral anti-inflammatory serpin, Serp-1, significantly reduces early inflammatory responses and later luminal occlusion in a rat aortic allograft model.

Does transforming growth factor beta 1 play a role in the pathogenesis of chronic allograft rejection?

To investigate the potential role of Transforming Growth Factor beta 1 (TGF beta 1) in the pathogenesis of chronic allograft rejection, we studied TGF beta 1 expression in a rat aortic allograft model. mRNA and protein expression of total and endogenously active TGF beta 1 were analysed in infra-renal orthotopic aortic syngeneic and allogeneic grafts and matched with the histological appearances of the grafts, 2, 4 and 12 weeks post-transplantation. Serum levels of TGF beta 1 were also measured. The level of TGF beta 1 m RNA and protein expression appeared highest 2 and 4 weeks following transplantation in both syngeneic and allogeneic grafts, with significantly elevated levels of mRNA expression in the 2 week allograft specimens. These time-points correlate histologically with maximal inflammatory cell infiltration of the grafts. By 12 weeks post-transplantation, TGF beta 1 mRNA expression is reduced in allogeneic grafts compared to syngeneic grafts. However, detectable levels of total and endogenously active TGF beta 1 protein levels in the allografts exceed those measured in the syngeneic grafts at this time point. These results demonstrate the complex expression pattern of this growth factor during the progression of chronic rejection and suggest an aetiological link between TGF beta 1 and the process of accelerated graft atherosclerosis.

Mixed hematopoietic chimerism prevents allograft vasculopathy

Background Mixed hematopoietic chimerism has been shown to induce long-term acceptance of transplant organs. We determined whether mixed chimerism prevented allograft vasculopathy, using the rat aortic allograft model. Methods Mixed chimeras were prepared by reconstituting lethally irradiated (1100 cGy) WF rats with a mixture of T-cell depleted (TCD) syngeneic (WF) plus TCD allogeneic (ACI) bone marrow. Donor-specific (ACI) or third-party (F344) aortic grafts were transplanted into mixed chimeric animals 1 to 2 months after bone marrow reconstitution. No immunosuppressive drugs were administered. At 30 days postoperatively, aortic allografts were harvested for histology and measurement of cytokine mRNA by semiquantitative RT-PCR. Some aortic grafts were harvested at 90 and 180 days after transplantation for histological analysis. The degree of intimal hyperplasia and cytokine gene expression were compared among 4 groups: I (syngeneic; ACI donors to ACI recipients), II (allografts; ACI to WF), III (donor specific; ACI donor to chimeras) and IV (third-party; F344 to chimeras). Results There was no difference in the degree of intimal hyperplasia (IH) between groups I and III. Groups II and IV had significantly more IH than group I. Compared to group I, levels of mRNA for IFN-γ, IL-2, IL-10 and iNOS in groups II and IV were higher, while there was no difference in mRNA levels between group I and III. Conclusions These data suggest that mixed chimerism prevents allograft vasculopathy. Mixed chimerism holds great promise in clinical transplantation as a means to prevent allograft vasculopathy.

Medial cell apoptosis precedes the development of intimal hyperplasia after transient allogeneic exposure in a rat aortic allograft model of chronic rejection

Medial cell apoptosis precedes the development of intimal hyperplasia after transient allogeneic exposure in a rat aortic allograft model of chronic rejection

Gene therapy with soluble FAS decreases AGA in a rat aortic allograft model

Gene therapy with soluble FAS decreases AGA in a rat aortic allograft model

Detection of transplant vasculopathy in a rat aortic allograft model by fluorescence spectroscopic optical analysis.

Transplant vasculopathy is a leading cause of late cardiac graft loss. We have examined laser-induced fluorescence (LIF) spectroscopy as an optical diagnostic tool for detection of intimal plaque development and inflammatory cellular invasion in a rat model of aortic allograft transplant. Infrarenal aortic segments were transplanted from Lewis to Sprague Dawley rats. A range of vasculopathy development was produced by treatment with a viral anti-inflammatory protein. LIF spectra were recorded from the intima of aortic implants at 28 days. Fluorescence intensity was analyzed for correlation with vasculopathy development. Significant differences in LIF intensity at 400-450 nm (P < or = 0.05 by ANOVA) were detected. LIF emission was correlated with plaque growth (R2 = 0.980), vessel narrowing (R2 = 0.964), and cellular invasion (R2 = 0.971) by regression analysis. LIF optical analysis provides a nontraumatic diagnostic approach for detection of atherosclerosis prior to cardiac transplant or during development of vasculopathy after transplant.

Vascular smooth muscle cells and neointimal hyperplasia in chronic transplant rejection.

Intimal hyperplasia is the characteristic pathological hallmark in the arterial tree of chronically rejecting solid organ grafts. Mechanisms underlying the development of this lesion are poorly understood. One strongly held hypothesis is that vascular smooth muscle cells (vs.mcs) migrate from the medial layer of arteries contained within the graft into the intima forming the "neointima" characteristic of intimal hyperplasia. This study investigated this theory in a rat aortic allograft model of intimal hyperplasia. It also examined the possibility, using a combination of immunocytochemistry and electron microscopy, that aortic vs.mcs may undergo a phenotypic change during this process. Intimal area in syngeneic grafts was 3509 +/- 4325 pixels (n = 5) compared with 240,896 +/- 87,042 in allogenic grafts (n = 9, P < 0.001) 12 weeks after transplantation. At that time medial nuclear density was markedly reduced in the same allografts compared with corresponding syngeneic grafts (0.83 +/- 0.14 versus 2.64 +/- 0.60, P < 0.001, respectively). The most striking finding was that there was strongly positive staining for alpha-actin cells in the neointima in association with an almost acellular medial layer. Immunocytochemical staining also demonstrated the presence of beta-actin cells in the neointima of allografts while electron microscopy showed these cells to be secretory in phenotype. These results support the hypothesis that vs.mcs form an important component of the lesion of intimal hyperplasia and also propose that a phenotypic change may occur in these cells once they are present in the neointima.

Mycophenolate mofetil (MMF, RS-61443) inhibits inflammation and smooth muscle cell proliferation in rat aortic allografts

To investigate the impact of mycophenolate mofetil (MMF) on allograft arteriosclerosis (chronic rejection) in rat aortic allograft model, we administered MMF 20 mg/kg/day from the day of transplantation and sarcrificed the rats at 1–12 months afterwards. MMF significantly suppressed all major histological manifestations of allograft arteriosclerosis, i.e. adventitial inflammation, media necrosis and intimal thickening and cellularity. There was a significant decrease in the replication rate ( 3H-thymidine incorporation) of inflammatory cells in the adventitia and of smooth muscle cells (SMC) in the media. MMF did not have any major effect on mRNA expression of several growth factors, (determined by polymerase chain reaction with inbuilt glyceraldehyde-3-phosphate dehydrogenase control), which have previously been demonstrated to be elevated in nonimmunosuppressed allografts. Immunoperoxidase staining showed a 40% reduction in the number of adventitial interleukin-2 receptors expressing lymphoid cells in MMF-treated allografts. The intensity of SMC α-actin staining was also significantly reduced. As the results suggested that MMF may have a direct antiproliferative effect on SMC, this possibility was investigated in primary SMC cultures in vitro and using the carotid denudation model in vivo. Both approaches showed inhibition of SMC proliferation by MMF. Our results indicate that MMF inhibits histopathological changes of chronic rejection by reducing the immune response and possible replication of SMC.

Sequential immunological targeting of chronic experimental arterial allograft.

Arterial wall is the main site involved in the chronic rejection process. The rat aortic allograft model was used here to characterize and describe the sequential evolution of the different targets and effectors of arterial wall immunological injury and response during arterial allograft rejection. Rat abdominal aortae were isografted or allografted from Brown-Norway to Lewis rats. Endothelial and smooth muscle cell injury and humoral and cellular immunological effectors were characterized from 0 to 60 days after transplantation using a battery of specific antibodies. The intimal proliferative response was also characterized over this time. Isografted Brown-Norway aorta adventitia had very few cellular components, which suggests that donor adventitia would be poorly antigenic in allografts. In contrast, allograft adventitia was the site of a major inflammatory cell invasion in which the expression of an adhesion molecule by colonizing capillary endothelial cells could play a main role. This adventitial infiltration continued as long as medial smooth muscle persisted. The luminal endothelial cells disappeared early, probably associated with macrophage margination. In contrast, medial smooth muscle cell disappearance occurred later and was specifically targeted by immunoglobulins. Intimal proliferation was the most delayed phenomenon, involving both inflammatory cell infiltration at an early stage and later myofibroblastic proliferation, and could be related to the specific expression of growth factors in this layer. The rat aortic allograft model appeared useful for characterizing specific targets and effectors of chronic arterial graft rejection, demonstrating an early stage of endothelial injury and the presence of immunoglobulins involved in chronic medial smooth muscle cell injury.