Allograft solid-organ transplantation has become a standard of care in patients with end-stage organ disease. In some patients, these treatments, along with other advances in care, have transformed fatal disease into treatable and preventable disease1-3 After transplantation, patients are placed on immunosuppressant drug therapy and routinely monitored to prolong the survival of the donor allograft. The cost for managing a failed allograft may be 500% more than a patient with a functioning transplant.4 Early detection of AR has led to significant improvement in allograft survival in the first 12 months posttransplantation.5
The importance of graft rejection and immunosuppression was discovered early on following the development of transplantation, a challenge that started to be overcome with the availability of immunosuppressants, though with the current standard-of-care for managing solid organ transplant patients, rejection remains a common problem with a high frequency of graft failure at 5 and 10 years.4,6-8 Acute rejection occurs as cellular rejection (ACR) or antibody-mediated rejection (AMR).9
Graft assessment is used clinically to assist in the management of immunosuppression; the clinical value it brings is that it allows modification of immunosuppressive therapy so as to maximize graft longevity, which is a focus of post-transplant care. Histology has traditionally been used, potentially in conjunction with serologic markers as a common graft assessment tool.8,10-15 While histology is considered the gold standard of diagnosis at this point in time, this requires a biopsy, which is invasive and may be associated with significant risks and access to care barriers.
Molecular diagnostic methods have emerged in an attempt to address limitations in current diagnostics including the measurement of donor-derived cell-free DNA (hereon cfDNA) and gene expression profile (GEP) assays, which have been developed in a number of organ allografts including kidney, heart, liver, and lung.16-24 The principle underlying cfDNA assays to assess rejection is that the transplantation of a new organ involves transplantation of new genetic material, and genetic material is shed into the bloodstream as part of rejection.25 The fraction of donor-derived cell-free DNA in the blood-stream may serve as a marker of rejection.18-21 While this is a straightforward principle, DNA concentration in the bloodstream is quite small, and therefore tests relying on cell-free DNA require sophisticated methods to accurately capture and quantify the presence of cfDNA specific to the allograft.18,21,25 GEP tests tend to quantify expression of numerous genes in the allograft recipient and use these data in algorithms developed with sophisticated modeling or machine learning to determine whether rejection is occurring.22,24,26 These tests can not only provide information about graft status in a minimally-invasive manner, but they can be sensitive enough to be able to detect AR before it is histologically evident.27
However, these molecular tests have different strengths and weaknesses and can be leveraged for different populations. For example, some GEP tests have high negative predictive value for the likelihood of AR, but may be limited in their ability as a positive predictor for ACR or even detecting AMR, which may still be useful in a stable patient at low risk for rejection.28 Other tests may have higher sensitivity or positive predictive value, suitable for higher-risk patients.18,21
While these technologies are new, large and multicenter studies have supported their use in renal and heart transplantation as minimally and non-invasive methods to assess allograft status, modify immunosuppression regimens, and avoid unnecessary biopsies.18,19,29-31 Evidence continues to develop for other transplant allograft organs and other analytes.16,17,32-35 Additionally, there is evidence that while some cfDNA and GEP tests may have different intended uses, combining both may further improve graft rejection determination.36