Limitations of biopsy-based transcript diagnostics to detect T-cell-mediated allograft rejection.

Background: Isolated tubulitis, borderline changes and isolated arteritis suspicious for histologic T-cell-mediated rejection (hTCMR) remain findings of uncertain significance. Although the Molecular Microscope Diagnostics System (MMDx) has not been trained on those lesions, it was suggested that MMDx might reclassify a subgroup to molecular TCMR (mTCMR).

Methods: In this single-center cohort of 326 consecutive, unselected kidney allograft biopsies assessed by histology and MMDx, we analyzed 249 cases with isolated tubulitis (i0, t1-3, v0; n = 101), borderline changes (according to Banff 2022, v0; n = 9), isolated arteritis (no borderline, v1; n = 37), no inflammation (i0, t0, v0; n = 67) and a positive control cohort (hTCMR, n = 27; mixed histologic rejection, n = 8; both according to Banff 2022; total n = 35). The first three groups were summarized as TCMR-suspicion (n = 147). Subcategorization included the presence and absence of microvascular inflammation (MVI); g+ptc ptc ≥2. Molecular rejection rates and differentiation were investigated.

Results: Molecular rejection rates were 37/147 cases (25.2%; 32 with MVI) in TCMR-suspicion, 6/67 (9%; 4 with MVI) in no inflammation and 30/35 (85.7%; 19 with MVI) in the positive control cohort. Molecular antibody-mediated rejection (mAMR) was present in 39/73 (53.4%) of cases. The presence of donor-specific antibodies at the time of the biopsy was high (127/249, 51%). Only 3 mAMR/TCMR and 0 pure mTCMR cases were detected in TCMR-suspicion and no inflammation, compared with 12 mAMR/TCMR and 10 mTCMR cases in the positive control cohort (P < .001). Even though the TCMR-specific molecular (Classifier) score differentiated between TCMR-suspicion and no inflammation (P = 0.005), rejection phenotype scores (R2 and R3) did not (P = .157 and .121).

Conclusions: MMDx did not identify pure mTCMR among isolated tubulitis, borderline changes or isolated arteritis, likely due to low sensitivity for TCMR lesions. However, it identified mAMR or mAMR/TCMR, especially in cases with MVI. Subthreshold findings remain to be further studied.