PU.1 and interferon consensus sequence-binding protein regulate the myeloid expression of the human toll-like receptor 4 gene

The protein product of the Toll-like receptor (TLR) 4 gene has been implica ted in the signal transduction events induced by lipopolysaccharide (LPS). In mice, destructive mutations of Tlr4 impede the normal response to LPS an d cause a high susceptibility to Gramnegative infection. Expression of TLR4 mRNA in humans is restricted to a small number of cell types, including LP S-responsive myeloid cells, B-cells, and endothelial cells. To investigate the molecular basis for TLR4 expression in cells of myeloid origin, we clon ed the human TLR4 gene and analyzed its putative 5'-proximal promoter. In t ransient transfections a region of only 75 base pairs upstream of the major transcription initiation site was sufficient to induce maximal luciferase activity in THP-I cells, The sequence of this region is similar in human an d mouse TLR4 genes and lacks a TATA box, typical Spl-sites or CCAAT box seq uences. Instead, it contains consensus-binding sites for Ets family transcr iption factors, octamer-binding factors, and a composite interferon respons e factor/Ets motif. The activity of the promoter in macrophages was strictl y dependent on the integrity of both half sites of the composite interferon response factor/Ets motif, which was constitutively bound by the myeloid a nd B-cell-specific transcription factor PU.1 and interferon consensus seque nce-binding protein. These results indicate that the two tissue-restricted transcription factors PU.1 and interferon consensus sequence-binding protei n participate in the basal regulation of human TLR4 in myeloid cells. Cloni ng of the human TLR4 gene provides a basis for further investigation of the possible impact of genetic variations on the susceptibility to infection a nd sepsis.