The etiology of CAN is multifactorial and involves both immune-dependent and immune-independent factors. Immune-dependent factors include cell-mediated immune responses and antibody-mediated responses to donor antigens. Immune-independent factors include donor characteristics, cold ischemia time, cytomegalovirus infection, hypertension, dyslipidemia, and the use of calcineurin inhibitors (CNI), notably CsA, and more recently Tac. In our present study, we didn’t found T-cell infiltration and C4d sediment, so we focus on the immune-independent injury of CAN. There has Ahmedabad Tolerance Induction Protocol reported for treatment of immune injuries of CAN , but in death of reports for non-immune injury of CAN, since the mechanism of which still needed to be fully revealed.
In the present study, we investigated the expression of MMP-9 and its role, with F334 to LEW transplant rat model, in the early stage (12 weeks post-transplantation) of chronic allograft nephropathy which represents the most common cause of late graft failure. We also investigated the correlation between MMP-9 expression and the infiltration of mononuclear cells and SMCs replication in the intima of vascular wall which are the cardinal pathological changes of early stage of CAN [9, 19, 20].
It was shown in our data that MMP-9 expression was upregulated in the early stage of CAN, at the same time there was significant mononuclear cells infiltration in the interstitium of allografts, and the MMP-9 expression was correlated positively to the mononuclear cell infiltration and SMC replication. Studies in other disorders such as tumor, atherosclerosis and many inflammatory diseases indicate that extracellular matrix degradation should be prerequisite for the migration of mononuclear cells and other cells in the tissue, and the ability of MMP-9 to degrade components of the extracellular matrix and to regulate the activity of a number of soluble proteins confers an important role in various physiological and pathological processes . Therefore, the upregulated MMP-9 in the early stage of CAN, showed in our data, might degrade extracellular matrix, which further aggravate the mononuclear cells infiltration, and lead to tubulointerstitial fibrosis and chronic renal failure eventually. We could conclude that MMP-9 might play an important role in the early stage of CAN, the degradation of extracellular matrix caused by increased expression of MMP-9 might be essential for the migration and proliferation of mononuclear cells in the early stage of CAN.
Our data suggest that upregulated MMP-9 expression in the early stage of CAN has significant correlation with SMC replication in the vascular intima of the allografts. Bendeck MP et al. found that administration of a metalloproteinase inhibitor after injury resulted in a 97% reduction in the number of SMCs migrating into the intima in rat carotid artery . Recent research also provides evidence that MMP-9-deficient aortic SMCs had not only decreased the migratory activity, but also decreased the capacity to make collagen contraction compared with wild-type cells . Cho A et al. found a denuding injury to the arteries of wild-type mice promoted the medial SMC replication and the migration of medial SMCs into the neointima, but SMC replication was significantly lower, and SMC migration and arterial lesion growth were significantly impaired in MMP-9 knockout arteries. SMCs, isolated from MMP-9 knockout mouse arteries, showed an impairment of migration and replication in vitro . Thus, based on above evidence, we can reach a conclusion that upregulated MMP-9 expression in the early stage of CAN, shown with our data, may breakdown vascular basement membrane and, therefore, is critical for the development of arterial lesions of CAN by regulating both SMC migration and proliferation.
Our data showed MMP-9 expression was positively correlated to the increased TGF-beta1 expression in the tubulointerstitium and arteriolar wall of the allografts. It is well known that TGF-beta1 is a key fibrogenetic cytokine involved in the pathogenesis of chronic renal allograft dysfunction. It was demonstrated in mammary cancer cells derived from Tenascin-C (TN-C) - deficient mice, that TGF-beta1 induce MMP-9 expression in a dose-dependent manner, and this inducement was significantly enhanced by addition of TN-C. Neutralization with specific anti-TGF-beta1 antibody showed decreased expression of MMP-9, indicating that TGF-beta controls the baseline MMP-9 expression by a direct autocrine mechanism . Kobayashi T et al. found MMP-9 expression by fibroblasts was induced by the addition of TGF-beta1 or tumour necrosis factor (TNF)-alpha to the culture medium . Palosaari H et al. found TGF-beta1 significantly upregulated MMP-9 mRNA in odontoblasts . All the evidences above suggest that TGF-beta1 may be one of the important causes leading to the upregulation of MMP-9 expression in the early stage of CAN.
The pathological mechanism of CAN is complicated; there are many contributors, such as MMPs and TIMPs regulated extracellular matrix (ECM) synthesis and degradation, introducing inflammation cells infiltration. In our previous study , we found that matrix metalloproteinase-2 (MMP-2) and tissue inhibitor of metalloproteinase-1 (TIMP-1) were important cytokines for ECM synthesis and degradation, and the excess accumulation of ECM is the main pathological mechanism of fibrosis in antibody-mediated renal graft rejection. Our present study reveals that TGF-beta1 induced expression of mmp-9 may induce mononuclear cells infiltration in the interstitium of allografts and SMC replication. However, the signaling pathway of MMPs and TIMPs regulate the synthesis and degradation of ECM remains to reveal. We are going to study the role of GSK-3β-β-actin signaling pathway, because our pilot study had found that expression of GSK-3β has positive correlation with either inflammatory cell infiltration or interstitial fibrosis/tubular atrophy in human renal allograft tissue . Further studies are needed.