New Molecular Classification of Rejection in Heart Transplant Biopsies Reveals Relatively Little Three Year Graft Loss in Antibody-Mediated Rejection

Purpose The INTERHEART study previously used microarray assessment of 889 heart transplant biopsies to develop the Molecular Microscope Diagnostic System (MMDx) based on expression of rejection-associated transcripts (RATs). The present study reclassified the rejection-related states in an expanded set of 1320 prospectively collected biopsies from 645 patients from 13 centers. Methods Biopsies were classified by ensembles of classifiers and analyzed for left ventricular ejection fraction (LVEF) and survival. Results New algorithms identified 853 No rejection (NR), 179 ABMR, 76 TCMR, 13 Mixed, 161 possible ABMR (pABMR), and 38 possible TCMR (pTCMR). No rejection was subclassified as NR-Normal 462, NR-Minor 359, and NR-Early-injury 32 (Figure 1A). Compared to NR-Normal, NR-Minor biopsies had mild elevation of many inflammation transcripts (e.g. IFNG-inducible genes) and to a lesser extent parenchymal injury transcripts. NR-minor biopsies were often designated as TCMR1R by histology. In all NR biopsies, NR-Minor scores and histologic TCMR1R increased through the first year, peaking about one year, suggesting that Minor inflammation is a late response to injury, unrelated to rejection. LVEF was similar in NR-Normal and NR-Minor but depressed in TCMR and Early injury. In a 3-year post-biopsy survival analysis, NR-Minor and NR-Normal had similar survival. TCMR and Early-injury were associated with increased graft loss, but many losses were not related to rejection. Surprisingly, 76 hearts with ABMR (149 biopsies) and follow-up data had only 3 losses within 3 years post-biopsy (Figure 1B). Conclusion Many biopsies with molecular no rejection develop minor increases in rejection-related transcripts in the first year, often called TCMR1R by histology, with no apparent effects on function or survival. An unexpected finding was that molecular ABMR was associated with very few graft losses over three years. (ClinicalTrials.gov #NCT02670408).