Evidence for dissociation of TLR mRNA expression and TLR agonist-mediated functions in bovine macrophages

Toll-like receptors are of key importance in the recognition of and response to infectious agents by cells of the innate immune system. TLR mRNA expression and TLR-mediated functions were determined in bovine macrophages (MΦ) infected with bovine viral diarrhea virus (BVDV) or stimulated with interferon-γ (IFN-γ) in order to see whether they are correlated under these conditions. As parameters quantitative real time RT-PCR (QRT-PCR) for TLR2, TLR3 and TLR4, NO and TNF production were measured. Triggering of bovine MΦ with bona fide TLR2 and TLR4 agonists (lipopolysaccharide, lipoteichoic acid, peptidoglycan, lipopetide) led to NO and TNF production but neither TLR3 nor TLR9 agonists (double-stranded RNA, CpG DNA) showed this effect. The mRNA expression of TLR2, TLR3 and TLR4 was neither influenced by MΦ costimulation with IFN-γ nor by MΦ preinfection with BVDV nor by the ligands themselves. However, NO production induced by TLR2 or TLR4 agonists was strongly modulated either by IFN-γ costimulation or BVDV preinfection. Thus costimulation of MΦ with IFN-γ resulted in an increase of both NO synthesis and TNF expression by cells stimulated simultaneously by TLR2 or TLR4 agonists. Preinfection of bovine MΦ by BVDV resulted in upregulation of TLR2- and TLR4-mediated NO synthesis. Collectively, these data show that TLR-mediated functions may be modulated by viral infection or activation via IFN-γ of MΦ whereas the mRNA concentrations of relevant TLR members were not significantly influenced. Thus, the amount of TLR2, TLR3 and TLR4 mRNA transcripts is stable at least under the conditions tested. More importantly, modulation of TLR-mediated responses was dissociated from mRNA expression of TLR members.