Outer Membrane Protein P5 Is Required for Resistance of Nontypeable Haemophilus influenzae to Both the Classical and Alternative Complement Pathways

Abstract

The complement system is an important first line of defense against the human pathogen Haemophilus influenzae. To survive and propagate in vivo, H. influenzae has evolved mechanisms for subverting this host defense, most of which have been shown to involve outer surface structures, including lipooligosaccharide glycans and outer surface proteins. Bacterial defense against complement acts at multiple steps in the pathway by mechanisms that are not fully understood. Here we identify outer membrane protein P5 as an essential factor in serum resistance of both H. influenzae strain Rd and nontypeable H. influenzae (NTHi) clinical isolate NT127. P5 was essential for resistance of Rd and NT127 to complement in pooled human serum. Further investigation determined that P5 expression decreased cell surface binding of IgM, a potent activator of the classical pathway of complement, to both Rd and NT127. Additionally, P5 expression was required for NT127 to bind factor H (fH), an important inhibitor of alternative pathway (AP) activation. Collectively, the results obtained in this work highlight the ability of H. influenzae to utilize a single protein to perform multiple protective functions for evading host immunity.

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.…”

Serum bactericidal antibody assays – The role of complement in infection and immunity

“…Decrease cell surface binding of the complement activator IgM [2] Vitronectin Inhibition of complement [12] …”

“…uses CspA for binding of host-derived proteins and/or direct interaction with the formation of the terminal complement complex, as a mechanism for evasion [5] and [6]. These and other examples, including mechanisms used by N. meningitidis, are shown in Table 1 [2][3][4][5][6][7][8][9][10][11][12][13][14][15][16].…”

“…Haemophilus influenzae has developed various mechanisms, mostly involving outer surface structures such as lipooligosaccharide glycans and outer surface proteins, for evading the host's immune system [2]. Similarly, Staphylococcus aureus has the staphylococcal superantigen-like protein 7 (SSL7) that binds both immunoglobulin A and complement C5, in order to inhibit complement mediated haemolytic and bactericidal activity [3].…”

Identification of a Haemophilus influenzae Factor H–Binding Lipoprotein Involved in Serum Resistance

“…It was recently shown that the interaction between NTHi (strain NT127), but not the interaction between the encapsulated strain RdKW20, and FH is mediated by P5, a protein previously known to be involved in bacterial attachment to mucosal surfaces (51,52). This difference between NTHi and H. influenzae type d (RdKW20) is likely due to the sequence variations in P5, and may also account for the lack of affinity for FH observed with the Hib and Hif PH mutants (53,54).…”