Targeting the TLR Co-Receptor CD14 with TLR2-Derived Peptides Modulates Immune Responses to Pathogens

Toll-like receptor–mediated responses to pathogens can be modulated by targeting the TLR co-receptor CD14. Influencing Immunity Toll-like receptors (TLRs) are a type of pattern recognition receptor; they respond to conserved motifs on pathogens or produced during cellular stress and affect the downstream immune response. However, TLRs do not stand alone: One co-receptor that contributes to TLR activation is CD14. Now, Raby et al. show that targeting CD14 can modulate TLR-mediated immune responses to pathogens. TLRs are attractive therapeutic targets because of their contributions to inflammation. The authors hypothesized that one way to target TLR activity was through CD14. They used peptide screening and mutagenesis to identify regions in TLR2 that could be involved in the TLR2-CD14 interaction. Peptides derived from these putative interacting regions enhanced the proinflammatory response of TLR2 and TLR4 to bacterial pathogens both in vitro and in vivo in a mouse model of peritonitis. Raby et al. then further identified the mechanism of this enhancement—their peptides accelerated microbial ligand transfer from CD14 to TLR2, resulting in prolonged activation. What’s more, these CD14-targeting peptides could overcome sepsis-induced immunosuppression in patient cells ex vivo. Thus, targeting CD14 may provide a therapeutic strategy for targeting TLRs. Dysregulation of Toll-like receptor (TLR) responses to pathogens can lead to pathological inflammation or to immune hyporesponsiveness and susceptibility to infections, and may affect adaptive immune responses. TLRs are therefore attractive therapeutic targets. We assessed the potential of the TLR co-receptor CD14 as a target for therapeutics by investigating the magnitude of its influence on TLR responses. We studied the interaction of CD14 with TLR2 by conducting peptide screening and site-directed mutagenesis analysis and found TLR2 leucine-rich repeats 5, 9, 15, and 20 involved in interaction with CD14. Peptides representing these regions interacted with CD14 and enhanced TLR2- and TLR4-mediated proinflammatory responses to bacterial pathogens in vitro. Notably, the peptides’ immune boosting capacity helped to rescue proinflammatory responses of immunosuppressed sepsis patients ex vivo. In vivo, peptide treatment increased phagocyte recruitment and accelerated bacterial clearance in murine models of Gram-negative and Gram-positive bacterial peritonitis. Up-modulating CD14’s co-receptor activity with TLR2-derived peptides also enhanced antigen-induced dendritic cell (DC) maturation and interleukin-2 production and, most notably, differentially affected DC cytokine profile upon antigen stimulation, promoting a T helper 1–skewed adaptive immune response. Biochemical, cell imaging, and molecular docking studies showed that peptide binding to CD14 accelerates microbial ligand transfer from CD14 to TLR2, resulting in increased and sustained ligand occupancy of TLR2 and receptor clustering for signaling. These findings reveal the influence that CD14 exerts on TLR activities and describe a potential therapeutic strategy to amplify responses to different pathogens mediated by different TLRs by targeting the common TLR co-receptor, CD14.