Preliminary characterization of protein binding factor for porcine reproductive and respiratory syndrome virus on the surface of permissive and non-permissive cells

In its natural host, porcine reproductive and respiratory syndrome virus (PRRSV) has been reported to have a restricted tropism for cells of the monocyte/macrophage lineage. To date, cloned monkey kidney cell lines, such as MARC-145 and CL2621 cells which have been established from MA-104 cells, are the only non-porcine cells known to support PRRSV replication. In the present study, a binding assay was set up to follow by flow cytometry the attachment of PRRSV on the surface of porcine and non-porcine cells. PRRSV was found to be able to bind permissive cells like porcine alveolar macrophages and MARC-145. Further binding assays with porcine peripheral blood leukocytes showed that only monocytes can attach the virus. By their lack of binding factor, lymphocytes appeared to be refractory to PRRSV infection. Pre-incubation of MARC-145 cells with chymotrypsin and pronase E, but not neuraminidase, blocked their binding activity for PRRSV. The binding activity of the protease-treated cells was regenerated 8 hours after treatment, but cells remained unable to bind PRRSV if maintained in the presence of cycloheximide, thus confirming the proteinic nature of the specific binding factor(s). Experiments conduced with cells that have been previously characterized as non-permissive to PRRSV infection showed that many of them were able to bind the virus. Data obtained suggest that interaction of PRRSV with a specific binding factor on the surface of some cells is not sufficient to lead to a productive infection, and that a second putative receptor or other phenomena are probably required to pursue later events.