Binding of Complement Factor H to Loop 5 of Porin Protein 1A: A Molecular Mechanism of Serum Resistance of Nonsialylated Neisseria gonorrhoeae

Abstract

Neisseria gonorrhoeae isolated from patients with disseminated infection are often of the porin (Por1A) serotype and resist killing by nonimmune normal human serum. The molecular basis of this resistance (termed stable serum resistance) in these strains has not been fully defined but is not related to sialylation of lipooligosaccharide. Here we demonstrate that Por1A bearing gonococcal strains bind more factor H, a critical downregulator of the alternative complement pathway, than their Por1B counterparts. This results in a sevenfold reduction in C3b, which is >75% converted to iC3b. Factor H binding to isogenic gonococcal strains that differed only in their porin serotype, confirmed that Por1A was the acceptor molecule for factor H. We identified a surface exposed region on the Por1A molecule that served as the binding site for factor H. We used gonococcal strains with hybrid Por1A/B molecules that differed in their surface exposed domains to localize the factor H binding site to loop 5 of Por1A. This was confirmed by inhibition of factor H binding using synthetic peptides corresponding to the putative exposed regions of the porin loops. The addition of Por1A loop 5 peptide in a serum bactericidal assay, which inhibited binding of factor H to the bacterial surface, permitted 50% killing of an otherwise completely serum resistant gonococcal strain. Collectively, these data provide a molecular basis to explain serum resistance of Por1A strains of N. gonorrhoeae.

Complement Resistance of Borrelia burgdorferi Correlates with the Expression of BbCRASP-1, a Novel Linear Plasmid-encoded Surface Protein That Interacts with Human Factor H and FHL-1 and Is Unrelated to Erp Proteins

“…A factor H binding domain was identified previously for the borrelial BbCRASP-3 and other Erp proteins (35,37,39) as well as for the neisserial Por1A protein (32). Furthermore, a binding domain for both, factor H and FHL-1, was localized within the hypervariable region of the streptococcal M protein (17,50).…”

“…Several pathogenic microorganisms express surface proteins capable of interacting with factor H and FHL-1, such as the Mand the Fba protein of S. pyogenes (16,27,28), PspC, Hic proteins of pneumococci (20, 29 -31), Por1A protein of nonsialylated N. gonorrhoeae (32), and envelope proteins gp41 and gp120 of the human immunodeficiency virus (33). Our previous studies indicated that moderate or fully complement-resistant B. burgdorferi and Borrelia afzelii strains, but not complementsensitive Borrelia garinii strains, express up to five factor H-and FHL-1-binding surface molecules termed complement regulator-acquiring surface proteins (CRASPs) (13,34,35).…”

The Complement Regulator Factor H Binds to the Surface Protein OspE of Borrelia burgdorferi

“…Factor H binds to sialylated lipo-oligosaccharides of N. gonorrhoeae by an interaction via SCRs 16 -20 (48). In addition, factor H also binds to nonsialylated Neisseria strains by an interaction with porin proteins, the main outer membrane proteins of the pathogen (49). However, the interacting region on factor H responsible for the interaction could not be localized in the latter example.…”

“…burgdorferi-The binding of human complement regulatory proteins as one mechanism to evade complement attack and phagocytosis has been reported recently for some pathogenic organisms, like Streptococcus pyogenes (44 -46), Streptococcus pneumoniae (47), and Neisseria gonorrhoeae (48,49). Our first aim was to investigate whether the 150-kDa factor H protein and/or its truncated 42-kDa form FHL-1, the main soluble inhibitors of the alternative pathway of complement, bind to the surfaces of the spirochete bacteria of the B. burgdorferi sensu lato complex.…”

Haemophilus parasuis α-2,3-sialyltransferase-mediated lipooligosaccharide sialylation contributes to bacterial pathogenicity

“…This sialylation of N. gonorrhoeae LOS converts serum-sensitive strains to serum resistant strains and requires the exogenous addition of CMP-Neu5Ac and has been termed “unstable serum resistance” [47]. Several hypotheses concerning the mechanism of this “unstable serum resistance” have been raised, such as decreased binding of IgG, or increased binding of host complement alternative pathway inhibitor, fH [48]. Here, we observed that the binding of fH by ΔlsgB mutant was significantly less than that by both wild-type SH0165 and C- lsgB strains, indicating that the H. parasuis lsgB gene can facilitate bacterial binding of fH in response to serum challenge.…”