An Unusual Case of Membranoproliferative Glomerulonephritis: Is the Role of Vaccination in Immune Reactivation a Casual or Causal Effect?

<b>Background and Clinical Significance:</b> Membranoproliferative glomerulonephritis (MPGN) is a rare and heterogeneous pattern of immune-mediated glomerular injury, often associated with infections, autoimmune disorders, or monoclonal gammopathies. Idiopathic cases remain a diagnostic challenge and frequently require empirical immunosuppressive treatment. There is increasing interest in environmental triggers that may activate the immune system in genetically or immunologically predisposed individuals. We report an unusual case of idiopathic immune complex-mediated MPGN with a relapsing course potentially associated with vaccine-induced immune reactivation. <b>Case Presentation:</b> A 35-year-old male with no significant medical history aside from untreated dyslipidemia and active smoking presented with a hypertensive emergency and acute kidney injury (AKI). Laboratory investigations revealed nephrotic-range proteinuria, microscopic hematuria, and reduced estimated glomerular filtration rate (eGFR). Kidney biopsy demonstrated type I immune complex-mediated MPGN with a diffuse endocapillary proliferative pattern and granular subendothelial deposits (IgG+++, C3+++, C1q++). An extensive work-up ruled out secondary causes, supporting a diagnosis of idiopathic MPGN. Immunosuppressive therapy with corticosteroids and mycophenolate mofetil led to a partial clinical response. However, after receiving multiple vaccinations, the patient experienced clinical deterioration. A second biopsy revealed persistent proliferative changes and new deposits of IgM++, C4d++, and both kappa and lambda light chains. This prompted a reintroduction of immunosuppressive therapy, which resulted in subsequent clinical improvement. <b>Conclusions:</b> This case supports the hypothesis that vaccine-induced immune reactivation may serve as a potential trigger for disease relapse in idiopathic MPGN. Clinicians should remain alert to environmental stimuli that may influence disease activity in immune-mediated glomerulopathies. Further research is needed to elucidate the underlying immunopathogenic mechanisms.