C3 glomerulopathy associated with anti-factor H autoantibodies in a child with inflammatory bowel disease.

Complement dysregulation may occur secondary to systemic immune activation, but its relationship with inflammatory bowel disease (IBD) remains poorly defined. We report a 10-year-old boy presenting with severe anemia and newly diagnosed IBD who developed acute kidney injury with microscopic hematuria and proteinuria. Complement studies showed markedly reduced C3 and CH50 with normal C4. Kidney biopsy demonstrated diffuse proliferative glomerulonephritis with dominant C3 deposition and subendothelial immune complexes, consistent with C3 glomerulopathy. Additional complement testing revealed factor H depletion, high titers of anti-factor H autoantibodies, and elevated soluble C5b-9, while C3 nephritic factor and genetic testing of complement genes were negative. Multiple systemic autoantibodies were simultaneously present. Treatment with glucocorticoids resulted in rapid clinical improvement with normalization of kidney function, complement parameters, and disappearance of anti-factor H antibodies during follow-up. This case illustrates how transient systemic immune activation associated with intestinal inflammation may induce acquired complement dysregulation and trigger C3 glomerulopathy.