NxGEN Vector Solutions’ Cardiac Gene Therapy is featured in the December 2022 issue of Genetic Engineering & Biotechnology magazine (GEN): Click here to read the Article entitled “Viral Vectors for Gene Therapy Get the Industrial Design Treatment.”

Chronic allograft rejection (CR) is the leading cause of late graft failure following organ transplantation. CR is a progressive disease, characterized by proliferation of fibroblasts within the graft, which promotes interstitial fibrosis as well as thickening and narrowing of the coronary vessels, a process referred to as transplant associated vasculopathy. These pathological changes result in deteriorating graft function for which there is no cure except for re-transplantation and CR will occur despite the administration of anti-rejection medication.

Programmed death-1 (PD-1) is an immune checkpoint receptor on cytotoxic T cells, which acts as an off-switch for T cell activation, proliferation, and survival. Upregulation of the PD-1 receptor on T cells and interaction with its ligand, Programmed death-ligand 1 (PD-L1), plays a key role in the debilitating process of T cell exhaustion, as well as being an important factor during the normal immune response to prevent autoimmunity.

For our gene therapy vector, we employed a PD-L1 transgene and assessed the impact of intragraft PD-L1 expression on CR of a transplanted heart. In order to study the immune mediators in CR, we used the heterotopic cardiac allograft mouse model in which a C57BL/6 mouse is transplanted with an intact BALB/c cardiac allograft. The donor’s heart is anastomosed to the great vessels of the abdomen and perfused with recipient blood. Once vascularized, the heart resumes contraction so that through palpation we are able to monitor function and we can study events in the cardiac graft as well as relevant lymphoid tissues. Donor cardiac allografts were transduced by perfusion with self-complimentary AAV vectors that encoded a mouse PD-L1 gene (scAAV9-PD-L1) or a Null vector encoding a scrambled mouse PD-L1 gene sequence (scAAV9-Null) and subsequently heterotopically transplanted into fully-MHC disparate recipients that received transient immunosuppression (transient CD4+ T cell depletion at the time of transplantation (administration of anti-CD4+ mAb on days -1, 0, and 7 relative to transplant) employing a well-established experimental mouse model of CR).

PD-L1 gene transfer in cardiac allografts combined with transient immunosuppression improved graft function and attenuated cardiac hypertrophy, which correlated with a significant reduction in the percentage of graft-infiltrating T cells. Donor-reactive T and B cells remained in a hyporesponsive state in the PD-L1-expressing recipients. Interstitial fibrosis represents a hallmark of CR and results in pathogenic cardiac remodeling and graft dysfunction. Quantitative analysis revealed a significant reduction in the level of interstitial fibrosis in the PD-L1-expressing allografts compared to the Null vector transduced grafts (p<0.05). Strikingly, allografts transduced with scAAV9-PD-L1 combined with transient depletion of CD4+ T cells exhibited comparable collagen deposition to allografts in a mouse transplant model that remains free of CR.

Based on our results, we believe that we have achieved Sir Peter Medawar’s “acquired tolerance” or “The Holy Grail’ by combining early, transient immunosuppression and AAV-mediated PD-L1 cardiac gene transfer. We demonstrated that local PD-L1 production at the site of inflammation—the transplanted organ–permits modulation of the immune response and prevents the development of CR without the requirement for long-term immunosuppressants. Our PD-L1 gene therapy provides a new strategy to overcome a critical challenge to transplant success and can be used for in the treatment for other fibrotic diseases as well for the treatment of autoimmune diseases.

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