Binding of human factor H to outer membrane protein P5 of non‐typeable Haemophilus influenzae contributes to complement resistance

Abstract

Summary Non-typeable Haemophilus influenzae is an opportunistic pathogen of the human upper respiratory tract and is often found to cause inflammatory diseases that include sinusitis, otitis media and exacerbations of chronic obstructive pulmonary disease. To persist in the inflammatory milieu during infection, non-typeable H. influenzae must resist the antimicrobial activity of the human complement system. Here, we used Tn-seq to identify genes important for resistance to complement-mediated killing. This screen identified outer membrane protein P5 in evasion of the alternative pathway of complement activation. Outer membrane protein P5 was shown to bind human complement regulatory protein factor H directly, thereby, preventing complement factor C3 deposition on the surface of the bacterium. Furthermore, we show that amino acid variation within surface-exposed regions within outer membrane P5 affected the level of factor H binding between individual strains.

Naturally occurring bactericidal antibodies specific for Haemophilus influenzae Lipooligosaccharide are present in healthy adult individuals

“…In contrast to Rd, which originally was an encapsulated serotype d strain that lost the capsule, various NTHi strains are constantly under selective pressure for the acquisition of virulence factors that can contribute to serum resistance. Recent studies have shown that NTHi are able to evade IgM binding [31,32] and to scavenge complement inhibitors, C4BP and Factor H [33,34]. The NTHi 375 strain may potentially employ some mechanisms of immune evasion facilitating the observed serum resistance.…”

Uptake of Sialic Acid by Nontypeable Haemophilus influenzae Increases Complement Resistance through Decreasing IgM-Dependent Complement Activation

“…The magnitude of sialic acid-mediated complement resistance differed from strain to strain. Large effects were seen for strains 11P6H, 3655, and 86-028NP, whereas only a modest effect was seen for strain R2866, which might be due to its intrinsic resistance to complement-mediated killing due to other complement evasion mechanisms, such as a complex LOS structure preventing IgM antibody binding and factor H binding to outer membrane protein P5 (10,(20)(21)(22). By measuring incorporation of Neu5Az on the bacterial surface, detected Neu5Az incorporation was high for NTHi strain 11P6H, modest for 86-028NP, and lowest for 3655 and R2866, which supports the largest effect of sialic acid on complement binding and complement-mediated killing of NTHi strain 11P6H.…”

“…NTHi bacteria have evolved multiple mechanisms to prevent binding of IgM to its bacterial surface. Outer membrane protein P5, which was also seen to bind human factor H (21,22), was shown to decrease binding of IgM by unknown mechanisms (22). Modification of the LOS was shown to decrease binding of IgM, resulting in lowered complement and opsonophagocytic killing (21,28).…”

Hijacking Complement Regulatory Proteins for Bacterial Immune Evasion

“…Outer membrane loops 1 and 2 of OmpP5 contribute to the binding of FH via unidentified aa residues. Heterogeneity in these outer membrane loops have been reported and are associated with the levels of FH binding to specific strains (Langereis et al, 2014). …”