De Novo IgAN with C1q Deposition After ABO-Incompatible Kidney Transplantation: A Case Report

Introduction: De novo IgA nephropathy (IgAN) in kidney transplant recipients is increasingly recognized. However, its clinical features remain unclear, and distinguishing it from non-specific IgA deposition can be challenging. Here, we experienced de novo IgAN with C1q deposition after ABO-incompatible transplantation with diagnosis supported by Gd-IgA1 staining. Case Description: A 29-year-old woman with a history of ANCA-associated vasculitis underwent an ABO-incompatible living-donor kidney transplantation. The post-transplant immunosuppressive regimen comprised prednisolone, tacrolimus, azathioprine, and everolimus, maintaining stable graft function (sCr 1.0–1.2 mg/dL). Nine months before presentation, she contracted COVID-19. Three months later, she exhibited microscopic hematuria (5–9/HPF) and nephrotic-range proteinuria (3.0 g/gCr) with no evidence of ANCA recurrence. A kidney biopsy revealed mesangial expansion and fibrocellular crescents. Immunofluorescence revealed granular mesangial deposition of IgG, IgA, C3, and C1q, with a predominance of IgA staining. Electron microscopy confirmed mesangial and sub-endothelial electron-dense deposits. Gd-IgA1 co-localized with IgA, supporting the diagnosis of de novo IgAN. According to the Pozzi protocol, the patient underwent steroid pulse therapy and a tonsillectomy. Nevertheless, follow-up biopsies conducted at 6 and 12 months revealed a progression to global glomerulosclerosis and the persistence of crescents. The decline in kidney function (sCr 1.5 mg/dL; proteinuria 1.1 g/gCr) indicates resistance to the treatment regimen. Discussion: A unique feature of this de novo IgAN case was C1q positivity following transplantation, coupled with resistance to treatments. Infections and vaccinations have been reported as potential triggers of de novo IgAN, and COVID-19 may have played a role in this case. Notably, we demonstrated that Gd-IgA1 staining can be useful in distinguishing de novo IgAN from non-specific IgA deposition. C1q positivity in native IgAN is linked to poor prognosis, possibly via classical complement activation. This case raises the possibility that C1q positivity may also be relevant in de novo IgAN, although further cases are needed to clarify its significance.