Mohlin, F. | bioRxiv Science

bioRxiv2026

C4BP occludes the non-opsonic interaction of Neisseria gonorrhoeae with human neutrophil CEACAMs

Neisseria gonorrhoeae (Gc) causes the sexually transmitted infection gonorrhea, an urgent public health concern. Gc infection elicits a robust neutrophil response and serum leakage, but Gc has developed specialized defenses to evade both complement and neutrophils. We recently reported that the classical complement pathway inhibitor C4b-binding protein (C4BP) binds to Gc and reduces phagocytic killing by neutrophils in a complement-independent manner. Here, we used a Chinese hamster ovary (CHO) expression system and engineered C4BP constructs to define the underlying molecular mechanisms. C4BP inhibited interactions between opacity protein (Opa)-expressing Gc and carcinoembryonic antigen-related cell adhesion molecules (CEACAMs), receptors that drive non-opsonic phagocytosis of Gc by neutrophils. The degree of C4BP-mediated inhibition varied among CEACAMs. By using wild-type and chimeric CEACAMs, we found C4BP was more inhibitory towards the granulocyte-restricted CEACAM3 than the ubiquitously expressed CEACAM1, which we ascribed to CEACAM3s shorter extracellular domain. C4BP also inhibited the association between Opa-expressing Gc and the GPI-anchored CEACAM6. Molecules containing C4BP domains 1 and 2 fused to IgM (C4BP-IgM) or to a hexameric IgG Fc construct (C4BP-Hexa-IgG), proteins similar in diameter and degree of multimerization to native C4BP, inhibited the association of Opa-expressing Gc with CEACAM3-CHO cells to the same degree as C4BP, while C4BP domains 1 and 2 fused to dimeric Fc (C4BP-IgG) did not. C4BP-IgM, but not C4BP-IgG bearing mutations to abrogate Fc gamma receptor interactions, blocked Opa-mediated phagocytosis by primary human neutrophils. These results support a model in which C4BP occludes Opa-CEACAM interactions, which protects Gc from phagocytic killing by neutrophils.

microbiology↗

bioRxiv2022

Neisseria gonorrhoeae co-opts C4b-binding protein to enhance complement-independent survival from neutrophils

Neisseria gonorrhoeae (Gc) is a human-specific pathogen that causes the sexually transmitted infection gonorrhea. Gc survives in neutrophil-rich gonorrheal secretions, and recovered bacteria predominantly express phase-variable, surface-expressed opacity-associated (Opa) proteins (Opa+). However, expression of Opa proteins like OpaD decreases Gc survival when exposed to human neutrophils ex vivo. Here, we made the unexpected observation that incubation with normal human serum, which is found in inflamed mucosal secretions, enhances survival of Opa+ Gc from primary human neutrophils. We directly linked this phenomenon to a novel complement-independent function for C4b-binding protein (C4BP). When bound to the bacteria, C4BP was necessary and sufficient to suppress Gc-induced neutrophil reactive oxygen species production and prevent neutrophil phagocytosis of Opa+ Gc. This research identifies for the first time a complement-independent role for C4BP in enhancing the survival of a pathogenic bacterium from phagocytes, thereby revealing how Gc exploits inflammatory conditions to persist at human mucosal surfaces. Author SummaryGonorrhea is considered an urgent threat to public health with an estimated 98 million cases occurring annually worldwide, growing antimicrobial resistance, and the absence of a gonococcal vaccine. Currently, we do not understand how N. gonorrhoeae expressing opacity (Opa) proteins survive neutrophil defenses and are recovered viable from infected patients. Here, we investigated how soluble elements of gonorrhea infection, present in human serum, contribute to N. gonorrhoeae survival from neutrophils. We found that the serum component C4b-binding protein (C4BP) protects N. gonorrhoeae from neutrophil killing and suppresses neutrophil activation. C4BP limited neutrophil phagocytosis of N. gonorrhoeae that expressed Opa proteins that bound to neutrophil receptors of the CEACAM family. This work provides novel insight into the interplay between the noncellular and cellular aspects of the innate immune response to N. gonorrhoeae.

microbiology↗