The appearance of C1q deposition in transplanted kidney allografts and its clinical and histopathologic features.

Background: C1q nephropathy is an uncommon type of glomerulonephritis characterized by extensive and dominant C1q mesangial deposition in the absence of systemic lupus erythematosus. However, there are limited studies about C1q deposition in renal allografts. This study aimed to report the prevalence of C1q deposition in transplanted kidney allograft biopsies and describe its clinical and histopathologic features. Methods: Between January 2005 and December 2018, a total of 1,742 kidney transplantations were performed at Seoul National University Hospital. All renal allograft biopsies (n=10,217) of these patients were retrospectively screened for C1q deposition. C1q deposition was detected in the renal allograft biopsies of 104 patients (6.0%). Only 28 cases (1.6%) had intense (≥2+) C1q dominance and were reviewed in this study. Results: Among the 28 cases, only four (14.3%) had accompanying proliferative glomerulonephritis. Most did not have any other glomerular changes on light microscopy. No patients had nephrotic-range proteinuria at the time of biopsy. A follow-up biopsy was undertaken in 15 of the cases (53.6%). In these follow-up biopsies, C1q deposition either completely disappeared (n=13, 86.7%) or showed diminished staining (n=2, 13.3%). Conclusions: The prevalence of dominant or codominant C1q deposition in transplanted renal allograft biopsies was 1.6%. Most cases did not have any other accompanying glomerular changes. The follow-up biopsies of these allografts showed spontaneous disappearance or diminished staining of C1q deposition. These findings suggest that C1q deposition found in renal allografts is most likely clinically benign, although this possibility should be confirmed in further large-scale studies.