From Asymptomatic Myeloperoxidase-Antineutrophil Cytoplasmic Antibody (MPO-ANCA) Seropositivity to Crescentic Glomerulonephritis and Cold Agglutinin Disease Following Chlamydia Infection: A Second-Hit Hypothesis

Myeloperoxidase (MPO)-antineutrophil cytoplasmic antibody (ANCA)-associated vasculitis (AAV) is a recognised cause of rapidly progressive glomerulonephritis (GN), while cold agglutinin disease (CAD) is a rare autoimmune haemolytic disorder that is frequently associated with infections.The simultaneous occurrence of MPO-ANCA GN and CAD is uncommon and presents significant diagnostic and therapeutic challenges.We describe a patient with longstanding asymptomatic MPO-ANCA seropositivity who developed rapidly progressive renal dysfunction, active urinary sediment, and biopsy-proven crescentic GN in association with cold agglutinin-mediated haemolysis following a recent respiratory illness attributed to Chlamydia pneumoniae infection.This case suggests a second-hit mechanism in which C. pneumoniae may have contributed to the development of two immune-mediated processes in a susceptible host and highlights the importance of evaluating for infectious triggers when AAV presents with atypical haematological features.