Monocytes-macrophages are a potential target in human infection with West Nile virus through blood transfusion

West Nile virus (WNV) transmission by transfusion was documented in 2002. Approximately 80 percent of WNV infections are asymptomatic and 1 percent develop severe neurological illness. In animals, Langerhans-dendritic cells support initial viral replication, followed by replication in lymphoid tissues and dissemination to organs and possibly to the CNS. The cellular tropism of WNV infection after transfusion and the particular human blood cells that sustain viral replication remain largely unknown. Whether primary monocyte-derived macrophages (MDMs) support WNV infection-replication and produce infectious virions, with an in vitro system, was investigated.

Elutriated monocytes (CD33+/CD14+) from suitable blood donors were cultured in the presence of macrophage-colony-stimulating factor, infected with WNV-NY99 at different time points, washed, and cultivated for up to 47 days. Supernatants were tested for WNV replication by TaqMan reverse transcription-polymerase chain reaction (RT-PCR), with primers for the envelope and/or 3'NC regions, and by cDNA-PCR to detect WNV minus-strand RNA and for the presence of functional virions by infectivity assays in Vero cells.

RT-PCR TaqMan of supernatants demonstrated productive infection of MDMs. Viral load reached 2 to 5 log above baseline in 3 to 6 days and then declined, with detectable viral replication persisting for up to 47 days. WNV minus-strand RNA was detected in Day 4 cultures, indicating active viral replication. Infected MDM cultures showed no cytopathic changes. Supernatants that were TaqMan-positive for the presence of WNV-infected Vero cells and produced cytopathic effects within 3 to 5 days of culture.

The susceptibility of monocytes-macrophages to productive infection in vitro is compatible with a potential role in initial WNV replication and propagation after transmission by transfusion.