Pseudorabies virus infection of epithelial cells leads to persistent but aberrant activation of the NF-κB pathway, inhibiting hallmark NF-κB-induced pro-inflammatory gene expression

The nuclear factor kappa B (NF-kappaB) is a potent transcription factor, activation of which typically results in robust pro-inflammatory signalling and triggering of fast negative feedback modulators to avoid excessive inflammatory responses. Here, we report that infection of epithelial cells, including primary porcine respiratory epithelial cells, with the porcine alphaherpesvirus pseudorabies virus (PRV) results in gradual and persistent activation of NF-kappaB, illustrated by proteasome-dependent degradation of the inhibitory NF-kappaB regulator IkappaB and nuclear translocation and phosphorylation of the NF-kappaB subunit p65. PRV-induced persistent activation of NF-kappaB does not result in expression of negative feedback loop genes like IkappaBalpha or A20 and does not trigger expression of prototypical pro-inflammatory genes like TNFalpha or IL-6. In addition, PRV infection inhibits TNFalpha-induced canonical NF-kappaB activation. Hence, PRV infection triggers persistent NF-kappaB activation in an unorthodox way and dramatically modulates the NF-kappaB signalling axis, preventing typical pro-inflammatory gene expression and responsiveness of cells to canonical NF-kappaB signalling, which may aid the virus in modulating early pro-inflammatory responses in the infected host.Importance:The NF-kappaB transcription factor is activated via different key inflammatory pathways, and typically results in the fast expression of several pro-inflammatory genes as well as negative feedback loop genes to prevent excessive inflammation. In the current report, we describe that infection of cells with the porcine alphaherpesvirus pseudorabies virus (PRV) triggers a gradual and persistent aberrant activation of NF-kappaB, which does not result in expression of hallmark pro-inflammatory or negative feedback loop genes. In addition, although PRV-induced NF-kappaB activation shares some mechanistic features with canonical NF-kappaB activation, it also shows remarkable differences, e.g. being largely independent of the canonical IkappaB kinase IKK, and even renders infected cells resistant to canonical NF-kappaB activation by the inflammatory cytokine TNFalpha. Aberrant PRV-induced NF-kappaB activation may therefore paradoxically serve as a viral immune evasion strategy and may represent an important tool to unravel currently unknown mechanisms and consequences of NF-kappaB activation.