Human cytomegalovirus directly modulates expression of chemokine CCL2 (MCP-1) during viral replication

Human cytomegalovirus (CMV) infects monocytes and other haematopoietic progenitor cells which then act as reservoirs for latency and virus dissemination. The chemokine CCL2 (monocyte chemotactic protein-1 or MCP-1) exhibits potent chemotactic activity for monocytes and is a likely target for CMV-induced immunomodulation. In this study, we demonstrate CMV modulates CCL2 expression in MRC-5 fibroblasts with multiplicity-dependent kinetics, where CCL2 is upregulated during early stage infection, followed by CCL2 inhibition at late stage infection. This CMV-induced CCL2 modulation was dependent upon virus replication, as UV-inactivated virus did not elicit any changes in CCL2 levels. Dual immunofluorescence staining showed CMV strains AD169, purified AD169, Merlin, FIX WT (FLAG-US28/WT) and pUS28-deficient FIX (FIX-ΔUS28) all induced upregulation of CCL2 primarily within infected cells. Focal upregulation of CCL2 within FIX-ΔUS28-infected cells demonstrated intracellular CCL2 accumulation was independent of CCL2 sequestration by the CMV-encoded chemokine receptor US28. Infection with purified virus confirmed CMV-induced CCL2 upregulation was not due to any CCL2-inducing factors contained within non-purified virus stocks. The CMV-induced CCL2 expression kinetics occurred concurrently with modulation of the CCL2 transcriptional activators NF-κB, interferon regulatory factor 3 and cytokine IFN-β, independent of virus strain, and with the establishment of viral replication compartments within infected cell nuclei. This is the first report to our knowledge to demonstrate CMV modulation of CCL2 expression within infected cells during viral replication. This immune modulation may facilitate virus dissemination, establishment of latency and pathogenesis of CMV-induced host disease.