Development of a validated peripheral blood biomarker for subAR
Identifying the need for a non-invasive replacement for biopsies in stable patients, we set out to discover and validate a peripheral blood biomarker to detect subAR in these patients as a “rule in” test, similar to biopsies. While our clinical trials and sample collection regimens were well designed, the evidentiary data and biomarker performance that resulted caused us to rethink the context of use (COU) of the biomarker.
Subclinical acute rejection (subAR), also referred to as “silent” rejection, is histologically defined acute rejection characterized by tubulointerstitial mononuclear cell infiltration identified from a biopsy specimen in a patient with normal or stable renal function (4-8). In the NIH-sponsored CTOT-08 study of 307 kidney transplant recipients (7), the natural prevalence of subAR, based on surveillance biopsies, was 20% at 3-6 months, and 25% at 12 and 24 months surveillance biopsies, with an overall prevalence of 35% (7). Of note, 80% of the subAR was of the borderline variety when classified by central pathology using the Banff criteria (14), and importantly, the biopsy was normal in 75% of cases. At the two year time point, patients with subAR on surveillance biopsies had worse outcomes than patients who did not. This was based on a composite clinical endpoint (CCE) consisting of biopsy-proven acute rejection (BPAR) on any “for-cause biopsy” by central read, or a 24-month biopsy (central read) showing evidence of chronic injury measured by interstitial fibrosis and tubular atrophy (IFTA) of Banff grade ≥II IFTA (ci ≥2 or ct ≥2), or a decrease in estimated glomerular filtration rate (GFR) by >10 mL/ min/1.73 m2 between 4 and 24 months post-transplant (7). SubAR was also associated with a higher frequency of both class I and class II de novo donor specific antibody (dn DSA) development (7, 15).
In addition to the CTOT-08 data shown above, a number of clinical studies have also recently associated subAR with poor outcomes (4-8, 15-19). A study in recipients with a rapid steroid withdrawal protocol compared outcomes in patients with no inflammation and those with subclinical inflammation on a 3-month surveillance biopsy. In the patients with subclinical inflammation, the serum creatinine levels were significantly higher at 24 months, and the allograft chronicity index on biopsy, the rate of subsequent BPAR and development of dn DSA were all significantly increased at 12 months (16). A large Australian study compared outcomes in patients with normal biopsies, those with borderline rejection, and those with T cell mediated acute rejection. Compared to patients with normal biopsies, patients with borderline rejection had worse renal function, more IFTA, subsequent acute rejection, allograft failure and patient mortality (17). A recent study in 103 pediatric renal transplant recipients that examined subclinical inflammation phenotypes and long-term outcomes after pediatric kidney transplantation, highlights the importance and treatment of subAR (18). In this study, surveillance biopsies were performed in first 6 months and a composite endpoint (CEP) of acute rejection and graft failure was measured at 5 years. The CEP was reached by 41% for treated borderline rejection vs. 67% for untreated (p<0.001) (18). Additionally, another recent publication has shown that borderline early acute rejection is associated with the development of late acute rejection and graft loss (19).