*Purpose: The appropriate treatment for post-kidney transplant immune complex glomerulonephritis (ICGN) of unknown cause is unclear.

*Methods: From 01/2004 to 12/2018, 71 patients were diagnosed with post-transplant ICGN on transplant kidney biopsies. Forty-one of these were found to have post-transplant idiopathic ICGN, and were included in this study. The patients with active infection (12), acute rejection (11), IgA nephropathy (3), lupus (3) and monoclonal gammopathy (1) were excluded.

*Results: The mean age of the cohort at the time of transplant was 50 ± 13 years. The mean time from transplant to the diagnosis of idiopathic ICGN was 6 ± 5 years. The most common cause of kidney failure was diabetes (49%). Only 11 (27%) patients had glomerulonephritis (9 with focal segmental glomerular sclerosis and 2 with membranous nephropathy) as the cause of their native kidney failure. The mean follow-up from the time of transplant was 9 ± 5 years. The majority of patients had proteinuria (UPC > 0.3) (93%), and only 39% had hematuria (> 3RBC/hpf) at the time of biopsy. Twenty-five patients (61%) had no change in their baseline immunosuppression. Eight patients (19.5%) received steroids alone, and eight patients (19.5%) received rituximab with (7) or without (1) steroids. The patient who received rituximab had better graft survival than the patients who received no treatment (p=0.02), but the benefit of steroids compared to no treatment did not reach statistical significance (p=0.05). The multivariate analyses retained eGFR < 30 ml/min/1.73m² at time of diagnosis (HR=3.30, p=0.02; 95% Cl 1.15 to 9.46) as a significant predictor of graft loss. The multivariate analyses also showed that the treatment of ICGN was associated with lower graft loss (HR=0.22, p=0.02; 95%Cl 0.06 to 0.78).

*Conclusions: Although not well described in the literature, post-transplant idiopathic ICGN is an important cause of kidney allograft dysfunction. Treatment of idiopathic ICGN seems associated with better graft outcomes. Future studies are needed to determine risk factors and treatment strategies to improve grafts outcomes with idiopathic ICGN.

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