Complement and neutrophils are actively involved at the cervicovaginal interface in cases of adverse microbial composition, cervical shortening and spontaneous preterm birth

weeks, women with Community State Type (CST) IV had significantly higher concentrations of C1q, C4, C4b, Factor B, D, C3b/iC3b, and Factor H, I than those with CST I. Women who later developed a short cervix and delivered preterm showed lower L. crispatus abundance and elevated complement activation. Neutrophils were the dominant local immune cell and exhibited enhanced activation relative to peripheral neutrophils, with altered expression of CD11b, CD62L, CD63 and CD66b. Cervical neutrophil CD63, CD66b, and CD88 differed between CST IV and CST I, though not by pregnancy outcome. Neutrophil abundance correlated with cytokines, complement proteins, and MMPs, suggesting roles in inflammation and tissue remodelling. These findings highlight a microbiota-driven complement-neutrophil axis present before cervical remodelling and sPTB, identifying potential complement-based predictors and therapeutic targets.