Ly6C(high) monocytes facilitate transport of Murid herpesvirus 68 into inflamed joints of arthritic mice.

Viruses, particularly the Epstein-Barr virus (EBV) has long been suspected to exacerbate acute arthritic symptoms. However, the cell populations that contribute to import viruses into the inflamed tissues remain to be identified. In the present study, we have investigated the role of monocytes in the transport of Murid herpesvirus 68 (MHV-68), a mouse virus closely related to EBV, using the serum transfer-induced arthritis (STIA) model. We found compelling evidence that MHV-68 infection markedly increased disease severity in NR4A1(-/-) mice, which are deficient for Ly6C(low) monocytes. In contrast, the MHV-68-induced enhancement of joint inflammation was lessened in CCR2(-/-) mice, suggesting the involvement of inflammatory Ly6C(high) monocytes in viral transport. We also observed that following selective depletion of monocyte subsets by administration of clodronate, MHV-68 transport into the synovium occurs only in the presence of Ly6C(high) monocytes. Tracking of adoptively transferred Ly6C(high) GFP infected monocytes into arthritic CCR2(-/-) mice by two-photon intravital microscopy showed that this monocyte subset has the capacity to deliver viruses to inflamed AR joints, as confirmed by the detection of viral DNA in inflamed tissues of recipient mice. We thus conclude that Ly6C(high) monocytes import MHV-68 when they are mobilized to the inflamed arthritic joint.