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

Abstract

Although nontypeable Haemophilus influenzae (NTHi) is a humanspecific nasopharyngeal commensal bacterium, it also causes upper respiratory tract infections in children and lower respiratory tract infections in the elderly, resulting in frequent antibiotic use. The transition from symbiotic colonizing bacterium to opportunistic pathogen is not completely understood. Incorporation of sialic acids into lipooligosaccharides is thought to play an important role in bacterial virulence. It has been known for more than 25 years that sialic acids increase resistance to complement-mediated killing; however, the mechanism of action has not been elucidated thus far. Here, we provide evidence that growth of NTHi in the presence of sialic acids Neu5Ac and Neu5Gc decreases complement-mediated killing through abrogating the classical pathway of complement activation by preventing mainly IgM antibody binding to the bacterial surface. Therefore, strategies that interfere with uptake or incorporation of sialic acids into the lipooligosaccharide, such as novel antibiotics and vaccines, might be worth exploring to prevent or treat NTHi infections.

Underlying Glycans Determine the Ability of Sialylated Lipooligosaccharide To Protect Nontypeable Haemophilus influenzae from Serum IgM and Complement

“…Sialic acid significantly reduced the amount of IgM bound to this strain (Fig. 3A), consistent with a previous report demonstrating that sialic acid uptake confers decreased IgM binding to NTHi (23). However, IgG binding was unaffected by the presence of sialic acid (Fig.…”

“…The majority of naturally occurring antibody directing the complement-mediated killing of NTHi is specific to the LOS, and anti-LOS IgM titers correlate with increased serum bactericidal activity (25). A recent report showed that sialic acid uptake-dependent inhibition of IgM binding correlated with the reduced sensitivity of NTHi to classical pathway-mediated killing in NHS (23). Consistent with these observations, we confirmed that growth on sialic acid conferred to NTHi protection from killing by the classical pathway of complement (Fig.…”

“…NTHi, and specific structures, including sialic acid, can inhibit the binding of complement proteins, such as C3b and C4b (19)(20)(21)(22)(23). Recently, Oerlemans et al showed that NTHi can utilize sialic acid to evade IgM binding and the bactericidal effect of complement in pooled human serum (23). Natural IgM antibodies present in healthy individuals exhibit varied specificity against self and nonself glycans (24).…”

Exploring the Impact of Ketodeoxynonulosonic Acid in Host-Pathogen Interactions Using Uptake and Surface Display by Nontypeable Haemophilus influenzae

“…Unlike Neu5Ac, Kdn does not protect against serum and whole blood killing. Incorporation of Neu5Ac on LOS allows NTHi to resist complement-mediated serum killing (88)(89)(90)(91). Since Kdn can be incorporated into LOS, we asked whether its presence affected NTHi survival in human sera.…”

Antibody Binding and Complement-Mediated Killing of Invasive Haemophilus influenzae Isolates from Spain, Portugal, and the Netherlands

“…Binding of IgM to the bacterial surface is associated with increased bacterial killing of NTHi. Previously, we used IgA/IgM-deficient serum to determine the contribution of IgM in complement-mediated killing of NTHi strains (4,9,10). In order to determine the contribution of IgM in complement-mediated killing of invasive NTHi strains, we determined binding of IgG and IgM to the bacterial surface and bacterial killing of 48 NTHi strains divided into four equal groups based on the quantiles for survival in 5% NHS (Fig.…”