Differential Role of TLR- and RLR-Signaling in the Immune Responses to Influenza A Virus Infection and Vaccination

Abstract

The innate immune system recognizes influenza A virus via TLR 7 or retinoic acid-inducible gene I in a cell-type specific manner in vitro, however, physiological function(s) of the MyD88- or interferon-β promoter stimulator 1 (IPS-1)-dependent signaling pathways in antiviral responses in vivo remain unclear. In this study, we show that although either MyD88- or IPS-1-signaling pathway was sufficient to control initial antiviral responses to intranasal influenza A virus infection, mice lacking both pathways failed to show antiviral responses, resulting in increased viral load in the lung. By contrast, induction of B cells or CD4 T cells specific to the dominant hemagglutinin or nuclear protein Ags respectively, was strictly dependent on MyD88 signaling, but not IPS-1 signaling, whereas induction of nuclear protein Ag-specific CD8 T cells was not impaired in the absence of either MyD88 or IPS-1. Moreover, vaccination of TLR7- and MyD88-deficient mice with inactivated virus failed to confer protection against a lethal live virus challenge. These results strongly suggest that either the MyD88 or IPS-1 signaling pathway is sufficient for initial antiviral responses, whereas the protective adaptive immune responses to influenza A virus are governed by the TLR7-MyD88 pathway.

Role of human Toll‐like receptors in naturally occurring influenza A infections

“…The alternative explanation of a higher level of viral replication inducing a smaller increase in these viral‐sensing TLRs also appeared unlikely 3, 4, 5, 7, 8. Our results are in line with recent mice experiments which showed that TLR activation rapidly produces virus inhibitory responses (predominantly through type I IFNs and IFN‐stimulated mechanisms), conferring protection against influenza challenge 3, 4, 10, 14. Tuvim et al .…”

“…Increased TLR (7, 8, and 9) expression was found to correlate significantly with the key intracellular signaling molecules (MAPKs, NF‐κB/IκB) and higher levels of pro‐inflammatory cytokines including IL‐6 and sTNFR‐1 3, 5, 7, 14, 20, 22, 27, 28, 32, 33. Consistent with earlier in vitro studies, associations with the “adaptive” cytokines (e.g., Th1‐related IFN‐γ, CXCL10/IP‐10, CXCL9/MIG) were also observed, particularly for TLR9 and TLR8, which signal through the MyD88 pathway 3, 4, 10, 34. TLR's active role in cytokine induction was supported by our ex vivo experiments, which showed significant differences in cellular cytokine responses between patients with influenza and controls upon TLR‐specific ligand stimulation (e.g., CpG DNA‐TLR9 and imiquimod‐TLR7; the resultant response pattern governed by the ligand tested, cell type studied, and disease stage at the time of sampling/“immune exhaustion”) and a dynamic change in their responsiveness during clinical recovery 20, 32.…”

“…This reflects the less specific nature of innate immunity, which can be advantageous when considering TLR targeting as a means of prophylaxis in influenza (discussed below). There is evidence to show that the RLRs are concomitantly upregulated and possibly play a contributory role in mediating the pro‐inflammatory cytokine responses 3, 7, 10, 15, 33. Although both RIG‐1 and MDA‐5 often showed upregulation in influenza and other respiratory viral infections, innate responses against influenza viruses are likely dependent on RIG‐1 and suppressible by the viral NS‐1 protein 7, 13, 26, 33…”

“…Broadly, viral nucleic acids (dsRNA, ssRNA, CpG oligodeoxynucleotides) are detected by the endosomal TLRs (3, 7, 8, 9), whereas components of bacteria (peptidoglycans/lipoproteins, lipopolysaccharides) are detected by the cell surface TLRs (2, 4) 3, 4. Recent in vitro and in vivo studies on influenza pathogenesis have shown that TLR activation induces expression of type I interferons and pro‐inflammatory cytokines (e.g., IL‐6, TNF‐α), limiting viral replication and dissemination, mediating tissue inflammation, and link to adaptive immunity development 3, 7, 8, 9, 10, 11, 123, 7, 10, 13.…”

“…Recent in vitro and in vivo studies on influenza pathogenesis have shown that TLR activation induces expression of type I interferons and pro‐inflammatory cytokines (e.g., IL‐6, TNF‐α), limiting viral replication and dissemination, mediating tissue inflammation, and link to adaptive immunity development 3, 7, 8, 9, 10, 11, 123, 7, 10, 13. Recently, data from animal studies have further shown that targeting the TLRs (with agonists) may rapidly upregulate the innate immunity to provide broad‐range, virus strain non‐specific protection against lethal influenza challenge 4, 11, 14, 15, 16, 17.…”