Le fragment LG3 du perlécan : un nouveau régulateur de remodelage vasculaire en transplantation

In allogeneic transplanted organs, endothelial apoptosis is associated with vascular remodeling and neointima formation which in turn leads to allograft vasculopathy, a maladaptive form of vascular repair. In allograft vasculopathy, neointima results from the accumulation of leukocytes, extracellular matrix and alpha-smooth muscle actin positive (αSMA+) cells in the intima of allogeneic arteries, arterioles and capillaries. Neointimal αSMA+ cells comprise vascular smooth muscle cells (VSMC) derived from the donor and stem cells derived from the recipient, including mesenchymal stem cells (MSC). Acquisition of an anti-apoptotic phenotype of neointimal cells is central to the development of vascular obliterative changes. Dr Hébert’s team demonstrated that apoptotic endothelial cells release mediators which in turn induce a state of resistance to apoptosis of VMSC and fibroblasts. Apoptotic endothelial cells release cathepsin-L which cleaves perlecan therefore releasing a C-terminal fragment harbouring a laminin G motif and referred to as LG3. LG3 is anti-apoptotic for fibroblasts. We hypothesized that LG3 is a key mediator produced by endothelial apoptosis of importance in favoring neointima formation via the induction of an anti-apoptotic phenotype in αSMA+ neointimal cells We demonstrated that mediators released by endothelial apoptosis induce an ERK1/2-dependent anti-apoptotic phenotype in MSC. We identified LG3 as one of the mediators implicated in the induction of this anti-apoptotic response. Interactions between LG3 and beta 1 integrins expressed on MSC trigger ERK1/2 activation albeit to a lesser degree than medium conditioned by apoptotic endothelial cells. We showed that apoptotic endothelial cells also release EGF which cooperates with LG3 to induce an anti-apoptotic phenotype on MSC through cross-talk between EGF receptor and integrin-dependent pathways. Next, we characterized the impact of LG3 on allogeneic vascular remodeling in vivo. We developed a murine model of vascular rejection based on orthotopic transplantation of an aortic segment between two fully MHC-incompatible mice in absence of immunosuppression. Recombinant LG3 was injected intravenously post-transplantation in recipients resulting in higher circulating levels of LG3. In LG3-injected mice,  accumulation of αSMA+ neointimal cells was enhanced resulting in significantly increased intima/media ratios in the allogeneic aortic graft. Aortic grafts of LG3-injected allografts also showed decreased CD31+ cells. We also demonstrated, using cell-based approaches, that LG3 exerts a pro-migratory activity on VSMC through beta 1-integrin and ERK1/2 -dependent pathways. In line with these observations we also reported augmented serum LG3 in human renal transplant patients in association with acute vascular rejection episodes. Collectively these results suggest that the pro-migratory, pro-survival and angiostatic activities of LG3 contribute to neointima formation. Our results suggest that LG3 is a novel mediator of importance in the development of obliterative vascular remodeling associated with rejection of allogeneic organs.

Keyphrases: transplantation, neointima, apoptosis, endothelial cell, mesenchymal stem cell, extracellular matrix