Abstract
Members of the pentraxin family, including PTX3 and serum amyloid P component (SAP), have been reported to play a role in innate host defence against a range of microbial pathogens, yet little is known regarding their antiviral activities. In this study, we demonstrate that human SAP binds to human influenza A virus (IAV) strains and mediates a range of antiviral activities, including inhibition of IAV-induced hemagglutination (HA), neutralization of virus infectivity and inhibition of the enzymatic activity of the viral neuraminidase (NA). Characterization of the anti-IAV activity of SAP after periodate or bacterial sialidase treatment demonstrated that α(2,6)-linked sialic acid residues on the glycosidic moiety of SAP are critical for recognition by the HA of susceptible IAV strains. Other proteins of the innate immune system, namely human surfactant protein A and porcine surfactant protein D, have been reported to express sialylated glycans which facilitate inhibition of particular IAV strains, yet the specific viral determinants for recognition of these inhibitors have not been defined. Herein, we have selected virus mutants in the presence of human SAP and identified specific residues in the receptor-binding pocket of the viral HA which are critical for recognition and therefore susceptibility to the antiviral activities of SAP. Given the widespread expression of α(2,6)-linked sialic acid in the human respiratory tract, we propose that SAP may act as an effective receptor mimic to limit IAV infection of airway epithelial cells.
Highlights
Mammalian serum and airway fluids contain a number of soluble proteins that are known to recognize and inactivate influenza viruses
In contrast to previous studies indicating that serum amyloid P component (SAP) mediated anti-influenza A virus (IAV) activity in a manner characteristic of b-type inhibitors [28,29], our preliminary studies demonstrated that lectin-mediated binding of SAP to IAV was not a critical determinant of its anti-IAV activity
Human SAP and PTX3 Bind to the HKx31 Strain of IAV Enzyme-linked Immunosorbent Assays (ELISA) was used to assess the ability of PTX3, SAP and Creactive protein (CRP) to bind to purified IAV strain HKx31
Summary
Mammalian serum and airway fluids contain a number of soluble proteins that are known to recognize and inactivate influenza viruses. Collectins express carbohydrate recognition domains (CRDs) that bind to mannose-rich glycans on the viral HA and, in some cases, to the neuraminidase (NA) [5,6], to mediate a range of anti-IAV activities including inhibition of IAV hemagglutination and NA enzyme function, neutralization of virus infectivity, virus aggregation, increased IAV uptake by neutrophils and opsonization of virus to enhance neutrophil respiratory burst responses to IAV [7,8,9]. Surfactant protein (SP)-D, a collectin constitutively expressed in the lung, acts as a classical b-type inhibitor against highly glycosylated IAV [10,11] and contributes to anti-IAV activity in human bronchoalveolar lavage (BAL) fluids [10,12]. The enhanced susceptibility of mice deficient in SP-D [15,16,17] or MBL [18] to glycosylated IAV suggests an important role for each collectin in innate host defence in vivo
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