Post-Transcriptional Control of Type I Interferon Induction by Porcine Reproductive and Respiratory Syndrome Virus in Its Natural Host Cells

Wang, Xiuqing; Christopher-Hennings, Jane

In 2004, Lee et al. first described the discrepancy between interferon- mRNA level and interferon- protein production in PRRSV-infected porcine alveolar macrophages [20] . They suggested that a post-transcriptional regulatory mechanism contributed to the inhibition of interferon- production. We have observed the same phenomenon in PRRSV-infected porcine monocyte-derived dendritic cells [32] . Despite the transient and abundant interferon- and  mRNA molecules, very little or no interferon- protein was detected in either cell lysates or supernatants of PRRSV-infected cells at different time points after virus infection, indicating a post-transcriptional control of type I interferon induction. Currently, very little is known about the translational control of type I interferon production by PRRSV. PRRSV inhibits the PI3K-dependent Akt (PI3K/Akt) pathway during late infection [33] , which makes the translation repressor, 4E-BP1, hyperactive and reduces global protein synthesis. Interestingly, a recent study has demonstrated that PI3K/Akt inhibition can also lead to the phosphorylation of eIF-2α to inhibit cellular translation [34] . We have indeed observed increased phosphorylation of eIF-2α in PRRSV-infected porcine Mo-DC during late infection (unpublished observation). Thus, it is possible that the translation of IFN-α is reduced partly by PI3K/Akt inhibition. However, it is more likely that PRRSV has employed multiple strategies to inhibit cellular protein synthesis as a simple way to evade the host defenses, which may also contribute to the inhibited type I interferon induction. In response to some virus infections, host dsRNA-activated protein kinase (PKR) phosphorylates eIF-2α to inhibit both cellular and viral translation [35, 36, 37] . PRRSV replication is known to trigger the stress-activated proteins kinases to modulate cytokine production in porcine alveolar macrophages [38] . It has also been demonstrated that cleavage of the eukaryotic translation initiation factor 4G (eIF4G) by either viral proteases or cellular proteases such as caspase 3 activated during apoptosis can result in rapid block of cellular protein synthesis [39, 40, 41, 42] . Furthermore, poly(A) tail elongation mediated by viruses may repress the efficient translation of type I interferon [43] .