Receptor determinants for the avian coronavirus infectious bronchitis virus: Roles of host cell lectins and glycans

The Coronavirus family consists of a variety of single‐stranded RNA, enveloped viruses that infect a wide range of animals, with many new coronaviruses from unanticipated hosts discovered in recent years. The prototype avian coronavirus, infectious bronchitis virus (IBV) is a continuing economic concern in the poultry industry around the globe and can be used as model for the study of pathogenesis for the Coronavirus family. Even though the putative receptor for IBV has not been identified, sialic acid, a terminal sugar on glycoproteins or glycolipids, has been proposed to be an attachment factor for IBV. Intriguingly, IBV is able to infect many different tissues/organs in the host chicken; yet IBV is unable to infect any cells from other animal species than chicken. The sialic acid binding pattern in multiple chicken organs has also been implicated in the pathogenesis in the host, but the correlation of binding could not explain the limited susceptibility of IBV to non‐chicken cells since sialic acid is ubiquitous in animal tissues. The lack of knowledge in IBV receptor leaves a big gap in our understanding in the relatively wide chicken tissue tropism and strict species tropism of IBV. When we expressed DC‐SIGN, a human C‐type lectin that may resemble a chicken analog lectin, into mammalian cell lines, we were surprised to find that not only IBV infection is rescued in these cell lines but also the infection is not dependent on the level of sialic acid on the cell surface. In addition, we examined whether sialic acid also plays a role in the IBV infection in chicken peripheral blood derived monocytes (chPBMCs), cells that potentially harbor the authentic proteinaceous receptor for IBV, unexpectedly we also found that the established attachment factor sialic acid does not play a critical role in the IBV infection in PBMC. Our data suggest that, sialic acid may be dispensable or may play a lesser role in virus‐receptor complex formation in the presence of a potential high‐affinity receptor molecule.