Cathelicidins Modulate TLR-Activation and Inflammation

Abstract

Cathelicidins are short cationic peptides that are part of the innate immune system. At first, these peptides were studied mostly for their direct antimicrobial killing capacity, but nowadays they are more and more appreciated for their immunomodulatory functions. In this review, we will provide a comprehensive overview of the various effects cathelicidins have on the detection of damage- and microbe-associated molecular patterns, with a special focus on their effects on Toll-like receptor (TLR) activation. We review the available literature based on TLR ligand types, which can roughly be divided into lipidic ligands, such as LPS and lipoproteins, and nucleic-acid ligands, such as RNA and DNA. For both ligand types, we describe how direct cathelicidin-ligand interactions influence TLR activation, by for instance altering ligand stability, cellular uptake and receptor interaction. In addition, we will review the more indirect mechanisms by which cathelicidins affect downstream TLR-signaling. To place all this information in a broader context, we discuss how these cathelicidin-mediated effects can have an impact on how the host responds to infectious organisms as well as how these effects play a role in the exacerbation of inflammation in auto-immune diseases. Finally, we discuss how these immunomodulatory activities can be exploited in vaccine development and cancer therapies.

Keywords: DAMPs; LL-37; MAMPs; Toll-like receptors; antimicrobial peptides; cathelicidins; dendritic cells; macrophages.

Figure 1

Cathelicidins inhibit the activation of lipid-sensing TLRs and modulate the response of other extracellular receptors. Activation of TLR4 by LPS, TLR1/2, or-2/6 heterodimers by LTA, and TLR5 by bacterial flagellin leads to activation of the intracellular domain. Through the adaptor MyD88 and subsequent downstream signaling through MAPKs, NFkB, and AP-1 are released and translocated to the nucleus, leading to transcription of inflammatory cytokines. LPS can also activate TLR4 located in endosomal compartments, which leads to activation through adaptors TRIF and TRAM. Additionally, P₂X₇ receptor responds to extracellular ATP, which leads to inflammasome formation. In general, cathelicidins inhibit lipid-induced TLR-activation and thereby the downstream inflammatory response by neutralizing lipid MAMPs. LPS-cathelicidin complexes can be taken up, after which the cathelicidins are degraded to allow LPS-induced TLR4 stimulation. Finally, cathelicidins can signal through the P₂X₇ receptor by which the peptides can stimulate inflammasome formation and autophagy. Black lines: normal signaling; red lines: effects exerted by cathelicidins.

Figure 2

Cathelicidins influence the response to nucleic acids depending on the cell type. Extracellular nucleotides are an inflammatory signal through various pathways. Phagocytosis or endocytosis of DNA and/or RNA is required for their delivery to endosomal compartments leads to activation of TLR3 by dsRNA, TLR7/8 by ssRNA, and TLR9 by bacterial or self-DNA. Activation of the TLR response leads to activation of the NFkB and/or IRF transcription factors, an subsequently to inflammatory cytokine production. Extracellular DNAses and RNAses degrade the nucleotides, which prevents uptake by phagocytic cells and reduces TLR activation in the endosomal compartements. Cathelicidins can bind to extracellular DNA/RNA to stabilize it and prevent it from degradation by DNAses or RNAses. The uptake of DNA/RNA-cathelicidin complexes is enhanced and this complex formation enhances TLR9 and possibly TLR7 activation in pDCs (A). The uptake of DNA-cathelicidin complexes by macrophages (B) or RNA-cathelicidin-complexes by bronchial epithelial cells (C) is enhanced as well, but the cathelicidin is degraded in order to DNA or RNA to activate their respective TLRs. In keratinocytes, the dsRNA-cathelicidin complex is transported to the endosome and enhances the TLR3 stimulation (D). Black lines: normal signaling or route; red lines: effects exerted by cathelicidins.

Figure 3

Cathelicidins balance the immune response to bacteria. Live bacteria activate various TLRs through their MAMPs, leading to NFkB and AP-1 activation and inflammatory signaling. Bacteria killed by for example UV, heat or gentamycin still activate these TLRs. Addition of cathelicidins to viable bacteria does not inhibit TLR activation; however, cathelicidin-killed bacteria or addition of cathelicidins to non-viable bacteria strongly inhibits TLR activation. This silent killing reduces the inflammatory response and thereby the subsequent inflammatory tissue damage when the bacteria are already killed and are no longer a threat. Black lines: normal signaling; red lines: effects exerted by cathelicidins.