Background: Understanding the role of alternative complement pathway dysregulation in membranoproliferative\nglomerulonephritis (MPGN) has led to a dramatic shift in its classification into two subgroups: immune complexmediated\nMPGN and complement-mediated MPGN, consisting of dense deposit disease and C3 glomerulonephritis\n(C3GN). A limited number of C3GN cases have been published to date with not yet conclusive results since the\nnovel therapeutic approach with eculizumab was introduced.\nCase presentation: We report the clinical follow-up of a 16-year-old patient in whom a diagnosis of C3GN was\nconfirmed by immunofluorescence and electron microscopy in second and third kidney biopsies, while the first\nbiopsy revealed idiopathic immune complex-mediated MPGN type III, Anders and Strife variant, which failed to\nimprove after several attempts at conventional immunosuppression therapy. Although applied late in an already\nfairly advanced stage of the severe active form of MPGN, the efficacy of eculizumab on C3GN was evidenced\nclinically and pathohistologically. Its beneficial influence on pathomorphogenesis was demonstrated by a unique\nfollow-up in the last three biopsies, despite the recent observation, confirmed in this study, of eculizumab binding\nwithin the kidney tissue.\nConclusions: Clinicians and pathologists should be aware that, in some patients, an underlying genetic or acquired\ncomplement alternative pathway abnormality can be masked by an initial immune complex-mediated mechanism,\nwhich subsequently triggers an unbalanced excessive continual driving of complement terminal pathway activation\nand the development of C3GN. In such a patient, supplementary steroids in addition to eculizumab appear\nnecessary to achieve an adequate response.
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