| Abstract: | Natural killer (NK) cells serve as a crucial first-line defense against tumors and virus-infected cells. We previously showed that lysis of influenza virus (IV)-infected cells is mediated by the interaction between the NK receptor, NKp46, and the IV hemagglutinin (HA) type 1 expressed by the infected cells. This interaction requires the presence of sialyl groups on the NKp46-T225 O-glycoforms. In the current study, we analyzed the O-glycan sequences that are imperative for the interaction between recombinant NKp46 (rNKp46) and IV H1N1 strains. We first showed that rNKp46 binding to IV H1N1 is not mediated by a glycoform unique to the Thr225 site. We then characterized the O-glycan sequences that mediate the interaction of rNKp46 and IV H1N1; we employed rNKp46s with dissimilar glycosylation patterns and IV H1N1 strains with different sialic acid α2,3 and α2,6 linkage preferences. The branched α2,3-sialylated O-glycoform Neu5NAcα2,3-Galβ1,4-GlcNAcβ1,6[Neu5NAcα2,3-Galβ1,3]GalNAc competently mediated the interaction of rNKp46 with IV H1N1, manifesting a preference for α2,3 linkage. In contrast, the linear α2,3-sialylated O-glycoform Neu5NAcα2,3-Galβ1,3-GalNAc was not correlated with enhanced interaction between rNKp46 and IV H1N1 or a preference for α2,3 linkage. The branched α2,3- and α2,6-sialylated O-glycoform Neu5NAcα2,3-Galβ1,3[Neu5NAcα2,6]GalNAc competently mediated the interaction of rNKp46 with IV H1N1, manifesting a preference for α2,6 linkage. Previous viral HA-binding-specificity studies were performed with glycopolymer conjugates, free synthetic sialyl oligosaccharides, and sialidase-treated cells. This study shed light on the O-glycan sequences involved in the interaction of glycoprotein and viral hemagglutinins and may help in the design of agents inhibitory to hemagglutinin for influenza treatment.Hemagglutinin (HA) is the receptor-binding and membrane fusion protein of influenza virus (IV), as well as the target for infectivity-neutralizing antibodies (27). Terminal sialic acids of glycoproteins and glycolipids are the cellular receptors for the IV HA (27). Two major linkages between sialic acid and the penultimate galactose residues of carbohydrate side chains are found in nature, Neu5NAcα(2,3)-Gal and Neu5NAcα(2,6)-Gal (27); different HAs have different recognition specificities for these linkages and the sugar backbone beneath (23, 26, 30). However, all of the HA-binding specificity studies were performed with glycopolymer conjugates, free synthetic sialyl oligosaccharides, and sialidase-treated cells (8, 10, 20, 25). This could be sufficient for the design of IV-inhibitory agents, and yet, it contributes only partially to the understanding of the interaction of IV HAs with glycoproteins and glycolipids. We aimed to further explore the exact glycoform sequences conjugated to a specific glycoprotein''s glycosylation site that is recognized by different IV strains.For this purpose, we took advantage of our findings on the interaction of natural cytotoxicity receptors (NCRs) and IV HAs (2, 3, 13, 18, 19, 22, 34). We showed that the NKp44 and NKp46 NCRs but not the NKp30 NCR interact with IV HAs. This interaction requires the sialylation of NKp44 and NKp46 oligosaccharides, and the binding of these NCRs to viral HA is required for the lysis of virus-infected cells by NK cells (3, 13, 18). NKp46 displays two putative O-linked glycosylation sites at Thr125 and Thr225 and one N-linked glycosylation site at Asn216. In order to determine the specific sugar-carrying residue that is important for the HA1 recognition, site-directed mutagenesis of the three residues was performed to carry the glycan modifications. Only when Thr225 was replaced was a sharp decrease in the enhanced binding to IV HA1 and IV H1N1-infected cells observed (2). Therefore, for the NKp46 receptor, the interaction with IV HA1 is restricted to Thr225, one of its three glycosylation sites (2).We already showed that producing recombinant NKp46 (rNKp46) in different cell lines resulted in dissimilar glycosylation patterns and had a strong effect on the binding to its ligands (11). Therefore, we analyzed the O-glycan patterns of rNKp46 produced from various cell lines and utilized the dissimilar glycosylation patterns to elucidate the NKp46 O-glycan sequences that mediate the interaction with IV H1N1 strains. To associate the results with the IV preference for sialic acid α2,3 and/or α2,6 linkages, we employed A/PR/8/34 (H1N1), A/NC/20/99 (H1N1), and A/Brisbane/59/2007 (H1N1) grown in either hen egg amnion or Madin-Darby canine kidney (MDCK) cells. Our results pointed to two branched O-glycan sequences that mediated the interaction of the NKp46 glycoprotein with IV H1N1 in correlation with the sialic acid linkage preference of the IV strain. |
