In vivo monocyte tropism of pathogenic feline immunodeficiency viruses

Virus-infected monocytes rarely are detected in the bloodstreams of animals or people infected with immunodeficiency-inducing lentiviruses, yet tissue macrophages are thought to be a major reservoir of virus-infected cells in vivo. We have identified feline immunodeficiency virus (FIV) clinical isol ates that are pathogenic in cats and readily transmitted vertically. We rep ort here that five of these FIV isolates are highly monocytotropic in vivo. However, while FIV-infected monocytes were numerous in the blood of experi mentally infected cats, viral antigen was not detectable in freshly isolate d cells. Only after a short-term (at least 12-h) in vitro monocyte culture were FIV antigens detectable (by immunocytochemical analysis or enzyme-link ed immunosorbent assay). In vitro experiments suggested that monocyte adher ence provided an important trigger for virus antigen expression. In the blo od of cats infected with a prototype monocytotropic isolate (FIV subtype B strain 2542), infected monocytes appeared within 2 weeks, correlating with high blood mononuclear-cell-associated viral titers and CD4 cell depletion. By contrast, infected monocytes could not be detected in the blood of cats infected with a less pathogenic FIV strain (FIV subtype A strain Petaluma) . We concluded that some strains of FIV are monocytotropic in vivo, Moreove r, this property may relate to virus virulence, vertical transmission, and infection of tissue macrophages.