Recently, we showed that porcine
sialoadhesin (pSn) mediates internalization of the arterivirus porcine reproductive and respiratory syndrome virus (PRRSV) in alveolar macrophages (Vanderheijden et al., J. Virol. 77:8207-8215, 2003). In rodents and humans,
sialoadhesin, or
Siglec-1, has been described as a macrophage-restricted molecule and to specifically bind
sialic acid moieties. In the current study, we investigated whether pSn is a
sialic acid binding protein and, whether so, whether this property is important for its function as a PRRSV receptor. Using untreated and
neuraminidase-treated sheep erythrocytes, we showed that pSn binds
sialic acid. Furthermore, pSn-specific
monoclonal antibody 41D3, which blocks PRRSV
infection, inhibited this interaction. PRRSV attachment to and
infection of porcine alveolar macrophages (PAM) were both shown to be dependent on the presence of
sialic acid on the virus:
neuraminidase treatment of virus but not of PAM blocked
infection and reduced attachment. Enzymatic removal of all N-linked
glycans on the virus with
N-glycosidase F reduced PRRSV
infection, while exclusive removal of nonsialylated N-linked
glycans of the high-
mannose type with
endoglycosidase H had no significant effect. Free
sialyllactose and
sialic acid containing (neo)
glycoproteins reduced
infection, while
lactose and (neo)
glycoproteins devoid of
sialic acids had no significant effect. Studies with linkage-specific neuraminidases and
lectins indicated that alpha2-3- and alpha2-6-linked
sialic acids on the virion are important for PRRSV
infection of PAM. From these results, we conclude that pSn is a
sialic acid binding
lectin and that interactions between
sialic acid on the
PRRS virion and pSn are essential for PRRSV
infection of PAM.