The Meningococcal Vaccine Candidate Neisserial Surface Protein A (NspA) Binds to Factor H and Enhances Meningococcal Resistance to Complement

Abstract

Complement forms an important arm of innate immunity against invasive meningococcal infections. Binding of the alternative complement pathway inhibitor factor H (fH) to fH-binding protein (fHbp) is one mechanism meningococci employ to limit complement activation on the bacterial surface. fHbp is a leading vaccine candidate against group B Neisseria meningitidis. Novel mechanisms that meningococci employ to bind fH could undermine the efficacy of fHbp-based vaccines. We observed that fHbp deletion mutants of some meningococcal strains showed residual fH binding suggesting the presence of a second receptor for fH. Ligand overlay immunoblotting using membrane fractions from one such strain showed that fH bound to a ∼17 kD protein, identified by MALDI-TOF analysis as Neisserial surface protein A (NspA), a meningococcal vaccine candidate whose function has not been defined. Deleting nspA, in the background of fHbp deletion mutants, abrogated fH binding and mAbs against NspA blocked fH binding, confirming NspA as a fH binding molecule on intact bacteria. NspA expression levels vary among strains and expression correlated with the level of fH binding; over-expressing NspA enhanced fH binding to bacteria. Progressive truncation of the heptose (Hep) I chain of lipooligosaccharide (LOS), or sialylation of lacto-N-neotetraose LOS both increased fH binding to NspA-expressing meningococci, while expression of capsule reduced fH binding to the strains tested. Similar to fHbp, binding of NspA to fH was human-specific and occurred through fH domains 6–7. Consistent with its ability to bind fH, deleting NspA increased C3 deposition and resulted in increased complement-dependent killing. Collectively, these data identify a key complement evasion mechanism with important implications for ongoing efforts to develop meningococcal vaccines that employ fHbp as one of its components.

Neisseria meningitidis NalP cleaves human complement C3, facilitating degradation of C3b and survival in human serum

“…S5). The species specificity of NalP is consistent with the other Nm proteins that interact specifically with human factors, which include fHbp, NspA, opacity proteins, and transferring binding protein (13,24,31,32). The specificity of interactions with human factors may explain why Nm is strictly a human pathogen and why antibody-mediated killing of Nm in vitro is more efficient when complement is derived from rabbits, rats, or mice, with fH and C3 that are not recognized by fHbp, NspA, or NalP, respectively.…”

“…The infrequency of the nalP deletion suggests that it is disadvantageous for the bacteria and that, in the nalP-negative strain, the function could be compensated by other factors or other mechanisms. This hypothesis recalls the redundancy in fH binding by Nm, where strains lacking fHbp can use NspA to bind fH (13,14) to avoid complement activation on bacterial surface.…”

“…S4B). The interaction of Nm with several human proteins has been shown to be species-specific (13,23,24). To investigate whether the cleavage of C3 mediated by NalP was restricted to the human protein, a cleavage assay was performed using C3 purified from rabbit and mouse sera.…”

“…Bacteria can escape recognition by the complement system through the actions of cell surface structures or secreted proteins (7)(8)(9). Nm has evolved several redundant mechanisms to evade the host innate responses at sites of colonization and during systemic growth, including expression of fHbp (10,11) and NspA (12,13), that facilitate binding of fH, lipooligosaccharide (LOS) sialic acid, which inhibits complement deposition (12), and Opc protein, which binds to vitronectin (2). In a recent functional genomic study, we identified factors involved in Nm survival in human blood (14).…”

Dihydrolipoamide Dehydrogenase of Pseudomonas aeruginosa Is a Surface-Exposed Immune Evasion Protein That Binds Three Members of the Factor H Family and Plasminogen

“…Microbial proteins that bind Factor H and FHL-1 via these domains include Rck from Salmonella enterica and Gpm1 and Pra1 from C. albicans (23,35,44,45). A second group of microbial surface proteins binds the two human regulators either via SCRs 6-7 (e.g., M proteins and Fba from Streptococcus pyogenes, NspA from Neisseria meningitidis, CRASP-1 from B. burgdorferi) or via SCRs 18-20 (e.g., Scl1 from S. pyogenes and Por1B from Neisseria gonorrheae) of Factor H (30,(46)(47)(48). In addition, pathogenic microbes also bind the human terminal complement pathway regulator CFHR1.…”