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

Nicholls, John M

The functional unit of the lung is the alveolus and this is involved in gas exchange. To achieve this function requires as narrow a thickness as possible for efficient gas exchange, thus the presence of mucin in this environment would be a disadvantage as it would create a barrier to gas exchange. The body, however, has another line of defence to replace mucin and there are compounds called lectins which are proteins that bind to specific carbohydrate structures (reviewed by Ng and colleagues [34] ). As the influenza HA and NA are glycoproteins, they undergo a process called glycosylation which means that glycans are added to the Asn residue of the sequence Asn-X-Ser/Thr, where X can be any amino acid except proline. In the normal glycosylation pathway the N-glycans are capped with Sia, however owing to the action of the viral NA, influenza itself is not sialylated but has an exposed galactose [35] . The glycosylation of HA is important in the development of antigenic variation but also plays a role in recognition by the host lectin mechanism [36] . In 2007 Reading and colleagues showed that the degree of glycosylation affected the ability of certain viruses to replicate in the respiratory tract of mice [37] . Viruses that lacked glycosylation of HA were able to cause more disease in mice than viruses that were glycosylated.