Activation of the TLR2/NF-κB signaling axis by capsid proteinVP1 of feline calicivirus promotes IL-1β expression in vitro

Abstract

Feline calicivirus (FCV) induces systemic inflammation in felines, posing a serious threat to feline health worldwide. Severe cases may lead to chronic stomatitis and other inflammatory conditions. However, the precise mechanisms underlying FCV-induced inflammation remain unclear. To investigate the involvement of toll-like receptors (TLRs), in vitro experiments assessed changes in TLR gene expression following FCV infection. FCV infection significantly upregulated TLR2 and TLR7 transcription, as well as expression of the NLRP3 gene. Functional assays revealed that inhibition or silencing of TLR2 markedly reduced FCV-induced transcription and secretion of proinflammatory cytokines IL-1β, IL-6, and TNF-α. Conversely, TLR2 overexpression enhanced IL-1β transcription and secretion, further implicating TLR2 in FCV-mediated inflammatory signaling. Mechanistically, FCV infection increased the expression of TLR2 and MyD88, promoted IκBα phosphorylation and degradation, and facilitated NF-κB (p65) phosphorylation and nuclear translocation. Disruption of TLR2 or MyD88 abrogated these events, thereby blocking NF-κB activation and downstream IL-1β expression. Inhibition of any component within the TLR2/MyD88/NF-κB axis—including NF-κB or IκBα—similarly suppressed IL-1β transcription, expression, and secretion, establishing the central role of this pathway in FCV-induced inflammation. Further experiments demonstrated that FCV virus-like particles (VLPs) can induce IL-1β gene transcription through the TLR2/MyD88/NF-κB axis but are insufficient to trigger IL-1β secretion. Dual-luciferase assays confirmed that FCV VP1 alone is capable of activating IL-1β gene transcription via this pathway and directly interacts with TLR2. Collectively, these findings demonstrate that FCV VP1 binds to TLR2, initiates IκBα phosphorylation through MyD88, promotes nuclear translocation of NF-κB (p65), and activates the NF-κB signaling cascade. This cascade primes the inflammasome by inducing transcription of NLRP3, Caspase-1, and IL-1β, as well as expression of the pro-IL-1β precursor, thereby initiating the first signal required for NLRP3 inflammasome activation in FCV-infected cells.