216.4: Wide Spectrum of Molecular Injury Highlights Heterogeneity of Banff Tubulitis and Interstitial Inflammation Lesions

Introduction: The pathological definition and clinical significance of borderline T cell-mediated rejection (BL-TCMR) remains an area of active debate, leading to inconsistencies in therapeutic strategy. Previously published data suggests that donor-derived cell-free DNA (dd-cfDNA) levels at the time of BL-TCMR diagnosis may identify patients at risk of adverse long-term outcomes. We characterized dd-cfDNA levels associated with BL-TCMR among patients enrolled in the Kidney allograft Outcomes AlloSure Registry (KOAR, NCT03326076). Methods: Patients with BL-TCMR findings (Banff 2019) on either for-cause or surveillance biopsy and a dd-cfDNA result within 30 days were included in the analysis. Patients with biopsies showing isolated tubulitis or interstitial inflammation without tubulitis were also analyzed. Results: We identified 56 cases of BL-TCMR with paired dd-cfDNA results (obtained within 30 days); median dd-cfDNA among these patients was 0.34% (IQR:0.17 - 1.00) (Figure 1a). The differences in dd-cfDNA among individual BL-TCMR combinations (t1/i1, t2/i1, t3/i1, t1/i2, t1/i3) were not significant, though the number of t3/i1 (n = 3, dd-cfDNA = 0.04%, 4.85%, 9.06%) and t1/i3 (n = 1, dd-cfDNA = 3.03%) cases was small. No differences were observed between biopsies with BL-TCMR and those with isolated tubulitis (t1/i0, t2/i0) or isolated inflammation without tubulitis (t0/i1) (Figure 1b). 31 of 56 BL-TCMR cases had prior dd-cfDNA measurement, with median result of 0.24% (IQR: 0.20 - 0.37) obtained 63 (IQR: 53.5 - 100) days before the index biopsy. The median percent increase between these sequential results was 55% (IQR: -9 - 235%). Conclusions: Substantial heterogeneity is observed both with regards to dd-cfDNA levels at the time of BL-TCMR and the trajectory of dd-cfDNA preceding index biopsy. More importantly, no differences in dd-cfDNA are observed between BL-TCMR and t/i lesions not presently included in Banff criteria for BL-TCMR. These findings suggest that BL-TCMR, isolated tubulitis, and isolated inflammation without tubulitis represent a spectrum of molecular injury that may be further.