Characterization of the attachment and infection by Porcine reproductive and respiratory syndrome virus 1 isolates in bone marrow-derived dendritic cells

Porcine reproductive and respiratory syndrome virus (PRRSV) is known to infect porcine dendritic cells (DC). Previous studies indicated that different PRRSV1 isolates regulated differently the cytokine profiles and expression of surface molecules of DC. However, the characterisation of the infection is lacking. The current study aimed to characterise the replication and attachment of different PRRSV1 isolates in bone marrow-derived DC (BMDC). For this purpose, immature (i) and mature (m) BMDC were infected with three PRRSV1 isolates. The replication kinetics showed that titres in iBMDC were significantly (p < 0.05) higher than in mBMDC by 24 hpi, and for two isolates titres peaked earlier in iBMDC, suggesting that iBMDC were more efficient in supporting PRRSV1 replication than mBMDC. The attachment was revealed by a three-colour confocal microscopy staining. All three isolates were seen attached to iBMDC even in cells lacking CD163 -the essential receptor for PRRSV- or porcine sialoadhesin (PoSn). The attachment was not fully avoided after removal of heparan sulphate by heparinase I. Furthermore, the infection was examined with regards to CD163 expression. By flow cytometry and confocal microscopy, positive signals of PRRSV1 nucleocapsid could be observed in CD163(-) iBMDC. Additional sorting experiment demonstrated that CD163(-) iBMDC were infected only when CD163(lo/hi) cells were present. This can be interpreted in different ways: susceptible CD163(-) cells arose as result of milieu created by CD163(+) infected BMDC; CD163(-) cells were infected by receptor-independent mechanisms (i.e. exosomes) or, some cells expressed CD163 at levels beyond the technical sensitivity.