Gene Therapy: Will the Promise of Optimizing Lung Allografts Become Reality?

Abstract

Lung transplantation is the definitive therapy for patients living with end-stage lung disease. Despite significant progress made in the field, graft survival remains the lowest of all solid organ transplants. Additionally, the lung has among the lowest of organ utilization rates-among eligible donors, only 22% of lungs from multi-organ donors were transplanted in 2019. Novel strategies are needed to rehabilitate marginal organs and improve graft survival. Gene therapy is one promising strategy in optimizing donor allografts. Over-expression or inhibition of specific genes can be achieved to target various pathways of graft injury, including ischemic-reperfusion injuries, humoral or cellular rejection, and chronic lung allograft dysfunction. Experiments in animal models have historically utilized adenovirus-based vectors and the majority of literature in lung transplantation has focused on overexpression of IL-10. Although several strategies were shown to prevent rejection and prolong graft survival in preclinical models, none have led to clinical translation. The past decade has seen a renaissance in the field of gene therapy and two AAV-based in vivo gene therapies are now FDA-approved for clinical use. Concurrently, normothermic ex vivo machine perfusion technology has emerged as an alternative to traditional static cold storage. This preservation method keeps organs physiologically active during storage and thus potentially offers a platform for gene therapy. This review will explore the advantages and disadvantages of various gene therapy modalities, review various candidate genes implicated in various stages of allograft injury and summarize the recent efforts in optimizing donor lungs using gene therapy.

Keywords: Adeno-associated viral vector (AAV vector); Adenoviral (Ad) vector; gene therapy; lung transplant; viral vector.

Figure 1

Approaches to gene therapy in lung transplantation. The basic principle of gene therapy is that gene sequences are delivered to target tissues using a vector, which is then taken up by these tissues allowing for gene expression. Gene therapy can be delivered using multiple routes, temperatures, and vectors, with numerous gene targets and uses. Intratracheal delivery is most commonly used, as it allows for localized treatment without systemic effects, although intravascular delivery has been used to target the vascular endothelium. Gene therapy is often delivered at physiologic temperature (37°C), although studies have also been performed at subnormothermic temperature (25°C) and cold storage temperature (4°C). Adenovirus is the most widely employed vector. Other viral vectors include AAV, lentivirus, and sendai virus. Nucleotide-based vectors include naked plasmid, shRNA, and siRNA, and nanoparticles have also been used to deliver gene products. The transfected genes include anti-inflammatory cytokines (IL-10, TGF-β), pro-inflammatory cytokines (IL-1, TNF-α), cytoprotective genes (NOS), anti-apoptotic molecules (caspase-3, FasL), and immune modulatory molecules (CTLA4-Ig, IDO).

Figure 2

Gene therapy targets in lung transplantation. Graft injury at many timepoints in transplantation, from procurement to years following transplant, has been targeted using gene therapy. Proteins that modulate the immune system, prevent apoptosis, or protect the endothelium have been transfected using vectors, namely viral vectors such as adenovirus. Anti-inflammatory cytokines such as IL-10 and TGF-beta decrease innate immune responses, T cell effector and helper functions, and B cell costimulation, which makes them a target in the treatment of IRI, TCMR, BCMR, and CLAD. Tregs are activated by TGF-beta, produce IL-10, and have immune-modulating functions. Other immune-modulating proteins such as CTLA4Ig, which blocks T cell activation, and soluble IL1 and TNF-alpha receptors (sIL1R, sTNF-alphaR), which block the downstream effects of proinflammatory cytokines, have been used in gene therapy models. NOS is a target for ameliorating IRI, as the production of NO decreases both platelet and leukocyte adhesion. Anti-apoptotic molecules, such as IkB and Bcl2 have also been used in IRI models. (IRI, ischemia-reperfusion injury; TCMR, T cell-mediated rejection; CLAD, chronic lung allograft dysfunction; BCMR, B cell-mediated rejection).