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.