Co-stimulation with TLR3 and TLR21 Ligands Synergistically Up-regulates Th1-cytokine IFN-γ and Regulatory Cytokine IL-10 Expression in Chicken Monocytes

Toll-like receptors (TLRs) are pattern recognition receptors of the innate immune system for various conserved pathogen-associated molecular motifs. Chicken TLR3 and TLR21 (avian equivalent to mammalian TLR9) recognize poly I:C (double-stranded RNA) and CpG-ODN (a CpG-motif containing oligodeoxydinucleotide), respectively. Interaction between TLR3 and TLR21 agonists poly I:C and CpG-ODN has been reported to synergize in expression of proinflammatory cytokines and chemokines and the production of nitric oxide in chicken monocytes. However, the interaction between poly I:C and CpG-ODN on the expression of interferons (IFNs) and Th1/Th2 cytokines remains unknown. The objective of the present study was to investigate the effect of the interaction between poly I:C and CpG-ODN on the mRNA expression levels of IFN-α and IFN-β, Th1 cytokines IFN-γ and IL-12, Th2 cytokine IL-4, and regulatory IL-10 in chicken monocytes. When stimulated with either agonist alone, CpG-ODN significantly up-regulated the expression of INF-γ, IL-10, and IL-12p40, but not IFN-α and IFN-β; whereas poly I:C induced the expression of INF-γ, IFN-α, IFN-β, and IL-10; but not IL-12p40. However, stimulation with a combinatory CpG-ODN and poly I:C further synergistically increased the expression of IFN-γ and IL-10 mRNA. Our results provide strong evidence supporting the critical role of TLR3 and TLR21 in avian innate immunity against both viral and bacterial infections; and the synergistic interaction between the TLR3 and TLR21 pathways produces a stronger Th1-biased immune response in chicken monocytes. Our result also suggest a potential use of poly I:C and CpG-ODN together as a more efficient adjuvant for poultry vaccine development.