Neutrality of the canonical NF-kappaB-dependent pathway for human and murine cytomegalovirus transcription and replication in vitro

Cytomegalovirus (CMV) is known to rapidly induce activation of nuclear factor kappaB (NF-kappaB) after infection of fibroblast and macrophage cells. NF-kappaB response elements are present in the enhancer region of the CMV major immediate-early promoter (MIEP), and activity of the MIEP is strongly upregulated by NF-kappaB in transient-transfection assays. Here we investigate whether the NF-kappaB-dependent pathway is required for initiating or potentiating human and murine CMV replication in vitro. We show that expression of a dominant negative mutant of the inhibitor of NF-kappaB-alpha (IkappaBalphaM) does not alter the replication kinetics of human or mouse CMV in cultured cells. In addition, mouse embryo fibroblasts genetically deficient for p65/RelA actually showed elevated levels of MCMV replication. Mutation of all NF-kappaB response elements within the enhancer of the MIEP in a recombinant mouse CMV containing the human MIEP (hMCMV-ES), which we have previously shown to replicate in murine fibroblasts with kinetics equivalent to that of wild-type mouse CMV, did not negatively affect replication in fibroblasts. Taken together, these data show that, for CMV replication in cultured fibroblasts activation of the canonical NF-kappaB pathway and binding of NF-kappaB to the MIEP are dispensable, and in the case of p65 may even interfere, thus uncovering a previously unrecognized level of complexity in the host regulatory network governing MIE gene expression in the context of a viral infection.