3.4. Influenza C Viruses Use Neu5,9Ac2
In 1947, a new influenza virus without cross-reactive antisera against IAV (PR8) and IBV (Lee) was first isolated by R.M. Taylor from throat washings of a New York man during an influenza outbreak [177]. It was later designated type C and the first strain was named C/Taylor/1233/1947. ICV usually causes mild upper respiratory infection but can cause lower respiratory infection in children less than 2 years of age [178]. Most humans acquire antibodies to ICV at a young age [178,179] and antigenicity of ICV is stable, with no antigenic change being detected for at least 30 years [71]. These facts may be related to the limited outbreaks of ICV in humans, mainly in children. Although ICV antigenicity is stable, comparison of HE gene sequences in viruses isolated from 1947 to 2014 demonstrated that there are six lineages comprised of C/Taylor/1233/1947, C/Kanagawa/1/1976, C/Mississippi/1980, C/Aichi/1/1981, C/Yamagata/26/1981 and C/Sao Paulo/378/1982 [71]. ICVs have also been isolated from pigs [69] and cattle [70] (Table 1).
Different from IAVs and IBVs, ICV possesses hemagglutinin and receptor-destroying enzyme (RDE) on the same homotrimeric glycoprotein having multifunctional hemagglutinin (receptor-binding and membrane fusion activities) and esterase (receptor-destroying activity) and so-called hemagglutinin-esterase-fusion (HEF) protein [180]. The glycoprotein HEF spikes are encoded by the fourth gene segment, and only the ICV (-)ssRNA genome is comprised of only seven gene segments [181].
Thin-layer chromatography (TLC), gas-liquid chromatography (GLC) and high-performance liquid chromatography (HPLC) analyses of rat alpha 1-macroglobulin (RMG) and bovine submaxillary mucin (BSM) incubated with ICV in comparison with those incubated with neuraminidase from A. ureafaciens revealed that RMG and BSM incubated with ICV have a reduced amount of Neu5,9Ac2 but an increased amount of Neu5Ac. After confirmation by using purified Neu5,9Ac2 instead of RMG and BSM, it was concluded that RDE of ICV is neuraminate O-acetylesterase (9-O-acetyl N-acetylneuraminate O-acetylhydrolase (EC 3.1.1.53) catalyzing removal of the 9-O-acetyl group from Neu5,9Ac2, not cleaving the terminal Neu5Ac from glycoconjugate [182]. RMG and BSM can potentially inhibit hemagglutination by ICV at 4oC, and their inhibitory effects were abolished by pre-incubation of RMG and BSM with ICV at 37oC [182]. This evidence suggested that Neu5,9Ac2 is a receptor of ICV on the cell surface.
Receptor binding analysis of C/Johannesburg/1/66 classified in C/Aichi lineage [71] on a sialoglycan microarray showed that the virus predominantly binds to Neu5,9Ac2α2,6Galβ1,4GlcNAc β1,2Manα3(Neu5,9Ac2α2,6Galβ1,4GlcNAcβ1,2Manα6)Manβ1,4GlcNAcβ1,4GlcNAcitol-AEAB [72]. Further studies by using ICVs from other lineages may help to clarify whether receptor binding specificity of all ICVs to Neu5,9Ac2 depends on the α2,6 linkage or not.