The current study shows that WNV infection of human monocyte-derived macrophages (MDM) results in increased WNV mRNA, protein, and infectious virions at 24 h p.i. with a decline in titer after 48 h.
Concomitant with viral control was the robust induction of indoleamine 2,3-dioxygenase (IDO) and resultant metabolism of L-tryptophan (L-Trp) to kynurenine.
In WNV-exposed cultures, IDO protein was induced primarily in noninfected versus viral-infected MDM. Whereas WNV infection increased the production of IFN-α, IFN-β, and TNF, only antibody neutralization of TNF attenuated IDO expression and activity. WNV infection also activated NF-κB, and inhibition of this pathway with BMS-345541 abrogated IDO induction.
Similar results were also obtained with MDM infected with the related flavivirus, Japanese encephalitis virus.
Whereas IDO-mediated L-Trp metabolism can exhibit antiviral properties, inhibition of IDO activity in MDM with L-1-MT or the addition of excess L-Trp did not affect viral control. However, culturing MDM in L-Trp-deficient medium or overexpression of IDO in cells prior to infection significantly attenuated WNV replication, which was reversed by adding excess L-Trp. Together, these data support that although IDO is not required by MDM for the clearance of established viral infection, the spread of flavivirus infection is limited by IDO expressed in uninfected, neighboring cells.