Differential induction of gene promoter constructs by constitutively active human TLRs

Antigen presenting cells can sense microorganisms through activation of members of the Toll like receptor family (TLRs), which initiate signals leading to transcription of many inflammation-associated genes. TLRs and IL-1R, through their TIR domains, activate NFkappaB and mitogen-activated protein kinase pathways and upregulate a set of specific target genes. Recent evidence points to several differences in signaling pathways activated by individual TLRs. To evaluate the basic signaling potential of individual TIR signaling domains, we generated constitutively active versions of all known human TLRs by fusing mouse CD4 extracellular portion with the TLR transmembrane and TIR domains. A panel of promoters from genes known to be activated by TLRs as well as artificial promoter constructs with transcription factor binding sites were selected to measure their response in the presence of constitutively active CD4TLR fusion molecules. These studies show for the first time that a unique panel of promoters appears to be highly induced by CD4TLR1, 6 (TLRs that usually function through heterodimerisation with TLR2), and CD4TLR10. We also observed that CD4TLR4 is the most potent gene activator compared to all other ten human TLRs. Preliminary analyses of several promoter deletions showed that TLRs use different sequence elements to activate these reporters. In addition, since different ligands for a single TLR (e.g., TLR9) can induce different pathways, the CD4TLR fusions seem to activate all the pathways and therefore can be used to assess the overall signaling capacity of a given TLR. Finally, analysis of promoter constructs induced by the only orphan TLR, TLR10, allowed the identification of the ENA78 promoter as a tool for screening its ligands.