Varicellovirus bovinealpha (BoAHV) 1 and 5 activate distinct toll-like receptors signaling pathways in neural cells

Abstract

Varicellovirus bovinealpha (BoAHV) types 5 and 1 are closely-related, neurotropic alphaherpesviruses. BoAHV-5 is the etiological agent of non-suppurative meningoencephalitis in calves, whereas BoAHV-1 is responsible for several syndromes in cattle, including respiratory and reproductive diseases. The innate immune response mediated by TLR3 and TLR7 is crucial in controlling infection and modulating pro-inflammatory cytokines, such as IFNs. In this study, it was evaluated whether TLR3 and TLR7 agonists affect BoAHV replication and whether TLR stimulation has an effect on the IFN-λ3 response in neural cells. TLR3 and TLR7 expression in neural cells was induced by the TLR agonists, Poly I:C and Imiquimod, respectively. The antiviral effect of the agonists varied with the virus strain. TLR7 was suppressed early after BoAHV-5 infection and it was upregulated during BoAHV-1 infection. Imiquimod pre-treatment of neural cells induced higher levels of TLR7 mRNA and reduced the replication of the natural BoAHV-5/1 recombinant. In this study, TLR3 expression was completely inhibited during infection with BoAHV-5 and there was a marked up-regulation of TLR3 mRNA during BoAHV-1 infection. Poly I:C treatment up-regulated TLR3 expression in infected cells but a detrimental effect on BoAHV-5 replication was not observed. Infection of neural cells with the recombinant virus A665 stimulated TLR3 expression late in the infectious cycle. Steady levels of BoAHV-1 replication were maintained in the presence of IFN-λ3 and this cytokine was unable to slow the replication of BoAHV-5. For BoAHV-5/1 A663 strain there was a consistent induction of IFN-λ3 throughout the infection period and maximum A663 titers at advanced stages of the replication cycle were in agreement with a decrease in expression levels. The study emphasizes the importance of strain-specific factors, the infection phase and the cell type involved in virus- and agonist-induced TLR and IFN-λ3 expression. Furthermore, these results evidenced that a deeper analysis on the role and activity of TLR agonists on BoAHV infection should be conducted to evaluate their potential as preventive or therapeutic molecules.