*Purpose: Chronic active T-cell mediated rejection (CATCMR) was newly added to the Banff classification in 2017. Currently, its response to treatment, natural course, clinical significance are largely unknown, and the best treatment regimens are also yet to be determined.

*Methods: We performed a retrospective review of renal allograft biopsy performed after 2018 (which were diagnosed on behalf of Banff 2017 classification) for cases of CATCMR. We sought to determine in which context the diagnosis of CATCMR was made and also assessed the treatment response and subsequent graft outcome.

*Results: We identified 38 biopsies from 36 patients with a diagnosis of CATCMR. Thirty-six initial biopsies of CATCMR were in 33.3% from protocol biopsies (1-year protocol biopsy, n=11; 5-year protocol biopsy n=1), and the remainder (n=24, 66.7%) were from for-cause biopsies. While all CATCMR from protocol biopsies were isolated, 29.2% of the CATCMR from for-cause biopsies had a concurrent diagnosis of antibody-mediated rejection (AMR) pathology (n=7; 2 acute AMR, 5 chronic AMR). Time from transplant to index CATCMR biopsy was longer in the for-cause biopsy group (median 4.8 yr, IQR 2.0-9.7 vs. median 1.1yr, IQR 1.0-1.2, p<0.01). Unsurprisingly, the CATCMR detected during for-cause biopsy was associated with a worse renal function (mean MDRD eGFR 33.8±17.5 vs. 55.6±14.2, p<0.01) and higher grade of proteinuria (proteinuria>2+, 50% vs. 0%, p<0.01) compared to those detected during protocol biopsy. The distribution of Banff lesion scores was not significantly different between the two groups except for a higher ah score in the for-cause group (ah>=1, 54.2% vs. 0%. P<0.01). Patients with isolated or mixed CATCMR diagnosed during for-cause biopsy received no treatment (n=2, 8.3%), steroid pulse therapy only (n=15, 62.5%), ATG (n=1, 4.2%), or AMR treatment (i.e. plasmapheresis, IVIG, rituximab) with or without steroid pulse therapy (n=6, 25%), and 50% showed complete or partial response. The estimated 1-year graft survival rate of the for-cause biopsy group was 59.9%. Graft failure was associated with renal function at the time of biopsy (HR 0.83, 95%CI 0.73-0.95, p<0.01) and higher ct score (HR 19.1, 95%CI 1.51-215.7, p=0.02). All 12 patients diagnosed with CATCMR during protocol biopsy were treated with steroid pulse therapy. Two patients experienced deterioration of graft function during the median f/u of 8.5 months, but there was no graft failure or subsequent rejection event.

*Conclusions: Patients with CATCMR diagnosed during renal dysfunction are associated with high rates of graft failure. While further long-term studies are warranted, isolated CATCMR detected during protocol biopsy should be regarded as a separate entity considering its favorable short-term outcome.

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