Identification of Novel Synthetic Toll-like Receptor 2 Agonists by High Throughput Screening

Abstract

Toll-like receptors (TLRs) play a central role in host defense by inducing inflammatory and adaptive immune responses following infection. Drugs that target TLRs are of considerable interest as potential inflammatory regulators, vaccine adjuvants, and novel immunotherapeutics. TLR2, in cooperation with either TLR1 or TLR6, mediates responses to a wide variety of microbial products as well as products of host tissue damage. In an effort to understand the structural basis of TLR2 recognition and uncover novel TLR2 agonists, a synthetic chemical library of 24,000 compounds was screened using an IL-8-driven luciferase reporter in cells expressing these human receptors. The screening yielded several novel TLR2-dependent activators that utilize TLR1, TLR6, or both as co-receptors. These novel small molecule compounds are aromatic in nature and structurally unrelated to any known TLR2 agonists. The three most potent compounds do not exhibit synergistic activity, nor do they act as pseudoantagonists toward natural TLR2 activators. Interestingly, two of the compounds exhibit species specificity and are inactive toward murine peritoneal macrophages. Mutational analysis reveals that although the central extracellular region of TLR1 is required for stimulation, there are subtle differences in the mechanism of stimulation mediated by the synthetic compounds in comparison with natural lipoprotein agonists. The three most potent compounds activate cells in the nanomolar range and stimulate cytokine production from human peripheral blood monocytes. Our results confirm the utility of high throughput screens to uncover novel synthetic TLR2 agonists that may be of therapeutic benefit. Toll-like receptors (TLRs) play a central role in host defense by inducing inflammatory and adaptive immune responses following infection. Drugs that target TLRs are of considerable interest as potential inflammatory regulators, vaccine adjuvants, and novel immunotherapeutics. TLR2, in cooperation with either TLR1 or TLR6, mediates responses to a wide variety of microbial products as well as products of host tissue damage. In an effort to understand the structural basis of TLR2 recognition and uncover novel TLR2 agonists, a synthetic chemical library of 24,000 compounds was screened using an IL-8-driven luciferase reporter in cells expressing these human receptors. The screening yielded several novel TLR2-dependent activators that utilize TLR1, TLR6, or both as co-receptors. These novel small molecule compounds are aromatic in nature and structurally unrelated to any known TLR2 agonists. The three most potent compounds do not exhibit synergistic activity, nor do they act as pseudoantagonists toward natural TLR2 activators. Interestingly, two of the compounds exhibit species specificity and are inactive toward murine peritoneal macrophages. Mutational analysis reveals that although the central extracellular region of TLR1 is required for stimulation, there are subtle differences in the mechanism of stimulation mediated by the synthetic compounds in comparison with natural lipoprotein agonists. The three most potent compounds activate cells in the nanomolar range and stimulate cytokine production from human peripheral blood monocytes. Our results confirm the utility of high throughput screens to uncover novel synthetic TLR2 agonists that may be of therapeutic benefit.