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).