Animal models could be used to study the processes and investigate underlying mechanisms and therapeutic interventions of lung transplant disease pathogenesis. A variety of lung transplant models was reviewed and discussed in this session
on how we can use animal models to improve our understanding of lung transplant pathophysiology.
Rat Models: A Window of Opportunity
Dong Tian, MD
, The University of Tokyo Graduate School of Medicine, Tokyo, Japan
The ideal animal should be able to provide physiological and anatomical similarities to the human disease process. It should be cost-efficient, technically stable, and easy to handle. The rat model was first developed in 1971 and has since been used to study several lung transplantation models. The advantages of the rat model include weight and size, which allows one performer to develop the model compared to larger animals. Complicated techniques such as EVLP and re-transplantation can be applied in the rat model, and serial blood sampling is more feasible in rats, compared to the mice model (researchers are able to obtain more blood from rats).
Another advantage of the rat model is low surgical complexity. There is no technical difficulty in heterotypic or orthotropic tracheal or bronchial implantation. Orthotopic left lung transplantation can be done by Dr. Tian’s group’s “Pendulum” model within 48 minutes without intraoperative failing. The cost of the rat model is cheap, and this is helpful when trying to screen novel treatments prior to proceeding to large animal models.
The short gestation period and life span of rats also allows investigators to increase sample size in a short timeframe, which increases the replicative value in research, leading to more robust and credible results. A rat model has been used to develop a model to study ischemia-reperfusion injury, allograft rejection, ex vivo lung perfusion, and decellularization (bioartificial lung).
Mouse Models: The Golden Standard?
Andrew E. Gelman, PhD
, Washington University School of Medicine, St. Louis, MO USA
Dr. Gelman discussed the murine model in lung transplantation in this session. The mice model has been used in lung transplantation research due to the similarity of general anatomic organization of human and mouse lungs, the cost-effectiveness of the model, and the opportunities to investigate genetic manipulations.
The first model using mice (heterotrophic tracheal transplantation) was developed in 1993 by a group of investigators at the University of Minnesota. This model produces a histopathological appearance of obliterative bronchiolitis in seven days, and continues to be a highly used BOS model due to technical ease. The drawback of this model is that the airway was not vascularized, and there is a lot of immunologic stress.
Orthotopic tracheal transplantation from a mice model was developed at Mount Sinai in 2002 with an end-to-end anastomosis. This model demonstrates a loss of ciliated cells and high amounts of lymphocyte infiltrate by three weeks, and subbasement fibrosis by four weeks. The drawback of this model is allografts undergo recipient-derived re-epithelialization, which does not happen in humans.
Orthotopic lung transplantation was developed in 2006 at Washington University. It is technically challenging and took about six months to learn. It has been used to study PGD, ACR, AMR, and CLAD. There are several drawbacks of the mice model. The regenerative response to lung tissue injury is prominent in this model. Mice have a different spatial distribution of club cells, and immune system dissimilarities. Finally, an altered microbiome and sterile facility housing can alter the immune response.
Xenotransplantation and Beyond
Megan Sykes, III, MD
, Columbia Center for Translational Immunology, Columbia, NY USA
Xenotransplantation is transplantation from another species. It could provide unlimited organ supply, alleviate organ shortages, and allow transplants to be performed electively as there is no need to wait for donors. Organ survival in xenotransplantation has improved since the 1980s from minutes to months due to better immunosuppression.
There are three major approaches to overcoming immune barriers to xenograft: immunosuppression, genetic engineering, and tolerance. Recent studies showed that the survival time of xenotransplantation of a heart from pigs to non-human primates lasted for about six months. In 2021, that has been prolonged to nine months by using an organ from genetic modification “10-GE pig,” including growth hormone receptor knockout. This is the same approach that lead to the first pig to human heart transplantation that was performed recently in the United States.
For lung and liver xenotransplantation, the graft survival was only two and four weeks, respectively. Tolerance is necessary to get permanent graft survival without excess immunosuppression. There are two approaches for xenograft tolerance: mixed chimerism and thymic transplantation. Mixed chimerism involves the coexistence of donor and recipient hematopoietic systems. It can tolerate most of the immune system (T cell, B cell, and partial NK cell tolerance). The second technique of xenograft tolerance is thymic transplantation. The combination of these two techniques using further genetic modification could make xenotolerance clinically achievable and safe.
The Ex-Vivo Lung as an Experimental Mechanistic Model of Transplant
Ciara Shaver, MD, PhD
, Vanderbilt University Medical Center, Nashville, TN USA
Ex vivo lung perfusion (EVLP) provides a platform for transcriptomic, proteomic, and metabolic assessment of potential donor lungs. Moreover, advanced assessment can be performed in EVLP to evaluate gas exchange, biomarkers, and imaging studies. The possible disadvantages are variability between lungs (especially if using declined donor lungs), limited duration, and logistic challenges. EVLP with declined human donor lungs is a useful tool for mechanistic research.
Injury can be induced in this model by either intravenous or intrabronchial instillation by infectious or sterile insults, and injury can be detected within two hours. This model can test the causal roles of molecules and pathways. However, the current EVLP systems are limited by 6-8 hours due to mitochondrial injury, altered glucose utilization, impaired amino acid clearance, and shift in lipid utilization. Bioenergetic and metabolic support is limited to 6-12 hours and is associated with lung health deterioration. Temperature management and xenogeneic platforms may facilitate organ recovery and rehabilitation.
Clinical Interventions with Ex-Vivo Perfused Lungs
Marcelo Cypel, MD
, Toronto General Hospital, Toronto, ON Canada
Dr. Cypel reviewed existing evidence for EVLP therapeutic interventions, and covered novel interventions in EVLP. EVLP allows the transplant team to assess and optimize organs prior to transplantation. The conditions of organs that could be optimized by EVLP include pulmonary edema, pneumonia, aspiration, chronic viral infection (HCV), or long warm ischemia in controlled and uncontrolled DCD.
Treatment strategies that have been studied in EVLP are perfusion therapy (solution with UVC light for HCV treatment, Rituximab for EBV infection), drugs (high dose antibiotics to reduce bacteria), inhaled gases (high dose inhaled nitric oxide above 200 ppm acts as an antimicrobial through nitrosylation of bacterial chromosome), cell therapy, immuno-cloaking, and gene therapy (conversion human donor blood type ex vivo).
In the future, EVLP could be a platform for major advances in organ transplantation including cell and genetic modification, organ modification in xenotransplantation, and a platform for patients’ own organ repair.
How Well Do Animal Models Mimic the Human Condition?
Fiorella Calabrese, MD
, University of Padova, Padova, Italy
Dr. Calabrese concluded the session with things we should consider when using animal models to conduct a study on lung transplantation. The ideal animal model should provide strong physiological/anatomical similarity to a human disease process.
Unfortunately, the ideal model does not exist yet. Investigators should know the lung anatomy of animal models, which animal (small, large, genetically modified) is suitable for which disease pathologies, and graft pathology lesions. In conclusion, there is no perfect animal model and continuous effort should be made to collect translatable data to select an appropriate model.
– Summary by Prangthip Charoenpong, MD, MPH