TLR3 regulation and cytokine response during BoAHV-1 and BoAHV-5 infection of neuronal-like cells.

Varicellovirus bovinealpha (BoAHV) 1 and 5 are alphaherpesviruses that differ in their neuropathogenic potential. While BoAHV-5 causes necrotizing meningoencephalitis in calves, neurological cases associated with BoAHV-1 are less frequent. In this study we used differentiated the human neuroblastoma cell line (SH-SY5Y) to evaluate the mRNA expression of Toll-like receptor 3 (TLR3), its adaptor molecule TRIF and the production of interferons and pro-inflammatory cytokines during infection with both alphaherpesviruses. TLR3 was activated with polyinosinic acid: polycytidylic acid (Poly I:C) and it was genetically silenced using TLR3-targeted siRNA. BoAHV infection induced an initial upregulation of TLR3, followed by a notable decrease, particularly in BoAHV-5-infected cells. TLR3 knockdown was effective for 72 h in uninfected cells although it was reversed shortly after BoAHV infection. In the presence of activated TLR3, virus titers remained high, indicating limited antiviral activity of TLR3 signaling. TRIF was downregulated early after infection, implying viral interference with the innate immune response. In contrast, Poly I:C upregulated TLR3 and TRIF. IFN-β was upregulated following infection and IFN-α/β, TNF-α and IL-6 were induced even in TLR3-silenced cells, implicating the involvement of alternative signaling pathways. These findings demonstrate how bovine alphaherpesviruses modulate the innate immune mechanisms, highlighting differential viral strategies to evade the immune response which may contribute to neuropathogenesis.