The Battle Between Influenza and the Innate Immune Response in the Human Respiratory Tract

Nicholls, John M

Three classes of lectin have been demonstrated to show antiinfluenza activity -the C-type lectins, S-type lectins and pentraxins (reviewed in Ng and colleagues [34] . The first type. C-lectins, are calcium dependent (C-type) and recognize mannose or galactose. The C-type lectins have a subgroup called collectins (because of a collagenous domain). The collectins have a trimer composed of polypeptides that have a carbohydrate recognition domain. In the lung the 2 main lectins involved in anti-influenza activity are surfactant proteins A and D. These 2 collectins have different mechanisms. SP-A is sialylated and so acts as a decoy receptor (so called γ-inhibition) -thus its ability to prevent influenza infection is not dependent on the degree of HA glycosylation [38] . In contrast SP-D binds mannose type oligosaccharides and has been demonstrated to interact with influenza virus HA involved in haemagglutination inhibition, and is also able to mediate aggregation of viral particles and inhibit HA activity through interaction of the viral HA or NA (Fig. 2) . Since SP-D interacts with the mannose, its antiviral effect will be dependent on the degree of glycosylation present, thus the changes in HA associated with drift due to increasing glycosylation will lead to greater neutralization by SP-D and thus reduced virulence.