The antimicrobial peptide LL-37 activates innate immunity at the airway epithelial surface by transactivation of the epidermal growth factor receptor

Antimicrobial peptides produced by epithelial cells and neutrophils represent essential elements of innate immunity, and include the defensin and cathelicidin family of antimicrobial polypeptides. The human cathelicidin cationic antimicrobial protein-18 is an antimicrobial peptide precursor predominantly expressed in neutrophils, and its active peptide LL-37 is released from the precursor through the action of neutrophil serine proteinases. LL-37 has been shown to display antimicrobial activity against a broad spectrum of microorganisms, to neutralize LPS bioactivity, and to chemoattract neutrophils, monocytes, mast cells, and T cells. In this study we show that LL-37 activates airway epithelial cells as demonstrated by activation of the mitogen-activated protein kinase (MAPK)/extracellular signal-regulated kinase (ERK) and increased release of IL-8. Epithelial cell activation was inhibited by the MAPK/ERK kinase (MEK) inhibitors PD98059 and U0126, by the epidermal growth factor receptor (EGFR) tyrosine kinase inhibitor AG1478, by blocking anti-EGFR and anti-EGFR-ligand Abs, and by the metalloproteinase inhibitor GM6001. These data suggest that LL-37 transactivates the EGFR via metalloproteinase-mediated cleavage of membrane-anchored EGFR-ligands. LL-37 may thus constitute one of the mediators by which neutrophils regulate epithelial cell activity in the lung.     

The human beta-defensins (-1, -2, -3, -4) and cathelicidin LL-37 induce IL-18 secretion through p38 and ERK MAPK activation in primary human keratinocytes

In addition to its physical barrier against invading microorganisms, the skin produces antimicrobial peptides, human beta-defensins (hBDs) and cathelicidin LL-37, that participate in the innate host defense. Because IL-18 is produced by keratinocytes and involved in skin diseases in which hBDs and LL-37 are highly expressed, we hypothesized that these peptides would activate keratinocytes to secrete IL-18. We found that hBD-2, -3, and -4 and LL-37, but not hBD-1, activated normal human keratinocytes to secrete IL-18; this secretion reached peak strength at 3 h. In addition, the combination of peptides resulted in a synergistic effect on IL-18 secretion. We also revealed that hBD-2, -3, and -4 and LL-37 increased IL-18 mRNA expression, and that IL-18 secretion was more enhanced in keratinocytes differentiated in vitro with high Ca2+-containing medium. Furthermore, because IL-18 secretion induced by hBDs and LL-37 could not be suppressed by caspase-1 or caspase family inhibitors, and because these peptides failed to increase caspase-1 activity, we suggest that hBD- and LL-37-induced IL-18 secretion is probably via a caspase-1-independent pathway. To determine the molecular mechanism involved, we demonstrated that IL-18 secretion was through p38 and ERK1/2 MAPK pathways, because the inhibitors of p38 and ERK1/2, but not JNK, almost completely nullified IL-18 secretion. Moreover, hBD-2, -3, and -4 and LL-37 could induce the phosphorylation of p38 and ERK1/2, but not JNK. Thus, the ability of hBDs and LL-37 to induce IL-18 secretion by keratinocytes provides a new mechanism for these peptides in innate immunity and an understanding of their role in the pathogenesis of skin disorders.     

An antimicrobial cathelicidin peptide, human CAP18/LL-37, suppresses neutrophil apoptosis via the activation of formyl-peptide receptor-like 1 and P2X7

Peptide antibiotics possess the potent antimicrobial activities against invading microorganisms and contribute to the innate host defense. An antibacterial cathelicidin, human cationic antibacterial protein of 18 kDa/LL-37, not only exhibits potent bactericidal activities against Gram-negative and Gram-positive bacteria, but also functions as a chemoattractant for immune cells, including neutrophils. During bacterial infections, the life span of neutrophils is regulated by various pathogen- and host-derived substances. In this study, to further evaluate the role of LL-37 in innate immunity, we investigated the action of LL-37 on neutrophil apoptosis. Neutrophil apoptosis was assessed using human blood neutrophils based on the morphological changes. Of note, LL-37 dose dependently (0.01-5 microg/ml) suppressed neutrophil apoptosis, accompanied with the phosphorylation of ERK-1/2, expression of Bcl-x(L) (an antiapoptotic protein), and inhibition of caspase 3 activity. Interestingly, LL-37-induced suppression of neutrophil apoptosis was attenuated by the antagonists for formyl-peptide receptor-like 1 (FPRL1) and P2X7 nucleotide receptor. Of importance, the agonists for FPRL1 and P2X7 apparently suppressed neutrophil apoptosis. Collectively, these observations indicate that LL-37 cannot only kill bacteria, but also modulate (suppress) neutrophil apoptosis via the activation of FPRL1 and P2X7 in bacterial infections. Suppression of neutrophil apoptosis results in the prolongation of their life span, and may be advantageous for host defense against bacterial invasion.     

Cathelicidin LL-37 bloodstream surveillance is down regulated during septic shock

Host defense peptides are ancient weapons of the innate immunity. The human cathelicidin LL-37 protects the epithelial barrier against infection and is constitutively secreted in the bloodstream by immune cells. Current knowledge claims that LL-37 is up regulated upon infection. LL-37 can protect against bacterial infections and possesses many immunomodulatory properties. Here, we show that the human host defense peptide LL-37 is down regulated during septic shock. Furthermore, we show that these effects are not related to vitamin D serum levels, a potent inducer of LL-37 gene expression, pointing out the complex regulation of cathelicidins during septic shock.     

The Human Antimicrobial Peptide LL-37, but Not the Mouse Ortholog,mCRAMP, Can Stimulate Signaling by Poly(I:C) through a FPRL1-dependent Pathway

LL-37 is an antimicrobial peptide produced by human cells that can down-regulate the lipopolysaccharide-induced innate immune responses and up-regulate double-stranded (ds) RNA-induced innate responses through Toll-like receptor 3 (TLR3). The murine LL-37 ortholog, mCRAMP, also inhibited lipopolysaccharide-induced responses, but unlike LL-37, it inhibited viral-induced responses in mouse cells. A fluorescence polarization assay showed that LL-37 was able to bind dsRNA better than mCRAMP. In the human lung epithelial cell line BEAS-2B, LL-37, but not mCRAMP, colocalized with TLR3, and the colocalization was increased in the presence of dsRNA. The presence of poly(I:C) increased the accumulation of LL-37 in Rab5 endosomes. Signaling by cells induced with both LL-37 and poly(I:C) was sensitive to inhibitors that affect clathrin-independent trafficking, whereas signaling by poly(I:C) alone was not, suggesting that the LL-37-poly(I:C) complex trafficked to signaling endosomes by a different mechanism than poly(I:C) alone. siRNA knockdown of known LL-37 receptors identified that FPRL1 was responsible for TLR3 signaling induced by LL-37-poly(I:C). These results show that LL-37 and mCRAMP have different activities in TLR3 signaling and that LL-37 can redirect trafficking of poly(I:C) to effect signaling by TLR3 in early endosomes in a mechanism that involves FPRL1.     

Selective killing of vaccinia virus by LL-37: implications for eczema vaccinatum

Possible bioterrorism with smallpox has led to the resumption of smallpox (vaccinia virus) immunization. One complication, eczema vaccinatum, occurs primarily in patients with atopic dermatitis (AD). Skin lesions of patients with AD, but not psoriasis, is deficient in the cathelicidin antimicrobial peptide (LL-37) and human beta-defensin-2 (HBD-2). We hypothesized that this defect may explain the susceptibility of patients with AD to eczema vaccinatum. The Wyeth vaccine strain of vaccinia virus was incubated with varying concentrations of human (LL-37) and murine (CRAMP) cathelicidins, human alpha-defensin (HBD-1, HBD-2), and a control peptide. Outcomes included quantification of viral PFU, vaccinia viral gene expression by quantitative real-time RT-PCR, and changes in virion structure by transmission electron microscopy. CRAMP knockout mice and control animals were inoculated by skin pricks with 2 x 10(5) PFU of vaccinia and examined daily for pox development. Physiologic amounts of human and murine cathelicidins (10-50 micro M), but not human defensins, which had antibacterial activity, resulted in the in vitro reduction of vaccinia viral plaque formation (p < 0.0001), vaccinia mRNA expression (p < 0.001), and alteration of vaccinia virion structure. In vivo vaccinia pox formation occurred in four of six CRAMP knockout animals and in only one of 15 control mice (p < 0.01). These data support a role for cathelicidins in the inhibition of orthopox virus (vaccinia) replication both in vitro and in vivo. Susceptibility of patients with AD to eczema vaccinatum may be due to a deficiency of cathelicidin.     

Antimicrobial and antibiofilm activity of cathelicidins and short, synthetic peptides against Francisella

Francisella infects the lungs causing pneumonic tularemia. Focusing on the lung’s host defense, we have examined antimicrobial peptides as part of the innate immune response to Francisella infection. Interest in antimicrobial peptides, such as the cathelicidins, has grown due their potential therapeutic applications and the increasing problem of bacterial resistance to commonly used antibiotics. Only one human cathelicidin, LL-37, has been characterized. Helical cathelicidins have also been discovered in snakes including the Chinese King Cobra, Naja atra (NA-CATH). Four synthetic 11-residue peptides (ATRA-1, -2, -1A and -1P) containing variations of a repeated motif within NA-CATH were designed. We hypothesized that these smaller synthetic peptides could have excellent antimicrobial effectiveness with shorter length (and less cost), making them strong potential candidates for development into broad-spectrum antimicrobial compounds. We tested the susceptibility of F. novicida to four ATRA peptides, LL-37, and NA-CATH. Two of the ATRA peptides had high antimicrobial activity (μM), while the two proline-containing ATRA peptides had low activity. The ATRA peptides did not show significant hemolytic activity even at high peptide concentration, indicating low cytotoxicity against host cells. NA-CATH killed Francisella bacteria more quickly than LL-37. However, LL-37 was the most effective peptide against F. novicida (EC50 = 50 nM). LL-37 mRNA was induced in A549 cells by Francisella infection. We recently demonstrated that F. novicida forms in vitro biofilms. LL-37 inhibited F. novicida biofilm formation at sub-antimicrobial concentrations. Understanding the properties of these peptides, and their endogenous expression in the lung could lead to potential future therapeutic interventions for this lung infection.     

Antimicrobial peptides in the first line defence of human colon mucosa

Antimicrobial peptides and proteins are effector molecules in the protection of epithelial surfaces. We have evaluated the presence of antimicrobial peptides/proteins that can participate in human colonic defence against microbes. A peptide/protein extract of normal human colon mucosa was found to be active against Gram-positive bacteria, Gram-negative bacteria, and fungi. Four polypeptides with antimicrobial activity were isolated from this material and they were identified by N-terminal amino acid sequence analysis as ubiquicidin, histone H2B, eosinophil cationic protein, and phospholipase A(2) (PLA(2)). Using immunodetection and mass spectrometry, LL-37, HNP1-3, and HBD-1 were also identified. Combined, these results indicate that the colon mucosa is protected by a complex mixture of polypeptides, able to kill invading microbes and working in synergy as a barrier against bacterial invasion.     

Cationic antimicrobial peptides activate a two-component regulatory system, PmrA-PmrB, that regulates resistance to polymyxin B and cationic antimicrobial peptides in Pseudomonas aeruginosa

The two-component regulatory system PhoP-PhoQ of Pseudomonas aeruginosa regulates resistance to cationic antimicrobial peptides, polymyxin B and aminoglycosides in response to low Mg2+ conditions. We have identified a second two-component regulatory system, PmrA-PmrB, that regulates resistance to polymyxin B and cationic antimicrobial peptides. This system responds to limiting Mg2+, and is affected by a phoQ, but not a phoP mutation. Inactivation of the pmrB sensor kinase and pmrA response regulator greatly decreased the expression of the operon encoding pmrA-pmrB while expression of the response regulator pmrA in trans resulted in increased activation suggesting that the pmrA-pmrB operon is autoregulated. Interposon mutants in pmrB, pmrA, or in an intergenic region upstream of pmrA-pmrB exhibited two to 16-fold increased susceptibility to polymyxin B and cationic antimicrobial peptides. The pmrA-pmrB operon was also found to be activated by a number of cationic peptides including polymyxins B and E, cattle indolicidin and synthetic variants as well as LL-37, a component of human innate immunity, whereas peptides with the lowest minimum inhibitory concentrations tended to be the weakest inducers. Additionally, we showed that the putative LPS modification operon, PA3552-PA3559, was also induced by cationic peptides, but its expression was only partially dependent on the PmrA-PmrB system. The discovery that the PmrA-PmrB two-component system regulates resistance to cationic peptides and that both it and the putative LPS modification system are induced by cationic antimicrobial peptides has major implications for the development of these antibiotics as a therapy for P. aeruginosa infections.     

LL-37, HNP-1, and HBD2/3 modulate the secretion of cytokines TNF-α, IL-6, IFN-γ, IL-10 and MMP1 in human primary cell cultures

The aim of this study was to evaluate the effects of the LL-37, HNP-1 and HBD2/3 peptides on cytokine andMMPproduction in human polymorphonuclear cells, mononuclear cells and chondrocytes. The levels of cytokines in supernatants from mononuclear and polymorphonuclear cell cultures were measured with a cytometric bead array by flow cytometry. Likewise, the levels of metalloproteinase/MMP-1, 3, and 13 were measured in supernatants from chondrocyte cultures using an ELISA. The expression of RANKL on lymphocytes was analyzed by flow cytometry.We observed increased levels of TNF-α, IL-6 and IL-10 in mononuclear and polymorphonuclear cell cultures stimulated with HBD-2/3.We also observed increased levels of IFN-γ, IL-10, and IL-6 in mononuclear cell cultures stimulated with HNP-1, and increased IL-6 levels were observed in polymorphonuclear cell cultures exposed to HNP-1. We also found that the MMP-1 level increased in the chondrocyte cultures stimulated with HBD-3, whereas the MMP-1 level was decreased in cultures exposed to LL-37. The present report is the first study to determine that HNP-1and HBD2/3 promote the secretion of pro-inflammatory cytokines by polymorphonuclear and mononuclear cells and the secretion of MMP by chondrocytes, whereas LL-37 diminishes MMP1 secretion. Our results suggest that HBD-2/3 and HNP1 might play a pathological role in rheumatoid arthritis, while LL-37 might have a protective role.     

Inactivation of human beta-defensins 2 and 3 by elastolytic cathepsins

beta-Defensins are antimicrobial peptides that contribute to the innate immune responses of eukaryotes. At least three defensins, human beta-defensins 1, 2, and 3 (HBD-1, -2, and -3), are produced by epithelial cells lining the respiratory tract and are active toward Gram-positive (HBD-3) and Gram-negative (HBD-1, -2, and -3) bacteria. It has been postulated that the antimicrobial activity of defensins is compromised by changes in airway surface liquid composition in lungs of patients with cystic fibrosis (CF), therefore contributing to the bacterial colonization of the lung by Pseudomonas and other bacteria in CF. In this report we demonstrate that HBD-2 and HBD-3 are susceptible to degradation and inactivation by the cysteine proteases cathepsins B, L, and S. In addition, we show that all three cathepsins are present and active in CF bronchoalveolar lavage. Incubation of HBD-2 and -3 with CF bronchoalveolar lavage leads to their degradation, which can be completely (HBD-2) or partially (HBD-3) inhibited by a cathepsin inhibitor. These results suggest that beta-defensins are susceptible to degradation and inactivation by host proteases, which may be important in the regulation of beta-defensin activity. In chronic lung diseases associated with infection, overexpression of cathepsins may lead to increased degradation of HBD-2 and -3, thereby favoring bacterial infection and colonization.     

Bacterial products increase expression of the human cathelicidin hCAP-18/LL-37 in cultured human sinus epithelial cells

The respiratory epithelium plays a major role in the primary defense of the airways against infection. It has been demonstrated that bacterial products are involved in the induction of inflammatory reactions of the upper airways. Little is known about the effects of bacterial products on expression of the antimicrobial peptide hCAP-18/LL-37, the only human cathelicidin identified so far. The aim of this study was to investigate the effects of bacterial products from both gram-positive and gram-negative bacteria on the expression of hCAP-18/LL-37 by sinus epithelial cells using an air-exposed tissue culture model. Lipopolysaccharide and lipoteichoic acid both increased hCAP-18/LL-37 expression in cultured sinus epithelium as assessed by immunohistochemistry, where maximal stimulation occurred at 100 ng ml(-1) lipopolysaccharide or 10 microg ml(-1) lipoteichoic acid. The stimulatory effect of lipopolysaccharide and lipoteichoic acid was not restricted to expression of hCAP-18/LL-37, since also mucin expression and IL-8 release from cultured sinus epithelium cells were increased by lipopolysaccharide and lipoteichoic acid. This suggests that bacterial products may stimulate innate immunity in the upper airways.     

Antimicrobial peptides are present in immune and host defense cells of the human respiratory and gastroinstestinal tracts

Previous studies have implicated antimicrobial peptides in the host defense of the mammalian intestinal and respiratory tract. The aim of the present study has been to characterize further the expression of these molecules in non-epithelial cells of the human pulmonary and digestive systems by detailed immunohistochemical analysis of the small and large bowel and of the large airways and lung parenchyma. Additionally, cells obtained from bronchoalveolar lavage were analyzed by fluorescent activated cell sorting and immunostaining of cytospin preparations. hBD-1, hBD-2, and LL-37 were detected in lymphocytes and macrophages in the large airways, lung parenchyma, duodenum, and colon. Lymphocytes positive for the peptides revealed a staining pattern and distribution that largely matched that of CD3-positive and CD8-positive T-cells. Macrophages with positive staining for the antimicrobial peptides also stained positively for CD68 and CD74. In view of the morphology of the LL-37-positive and hBD-2-positive mucosal lymphocytes, they are probably also B-cells. Thus, antimicrobial peptides of the defensin and cathelicidin families are present in a variety of non-epithelial cells of mucosal organs. These findings confirm that antimicrobial peptides have multiple functions in the biology of the mucosa of these organs. This work was supported by grants from the Deutsche Forschungsgemeinschaft (Ba 1641/5–1 and Ba 1641/6–1)     

Assessment of antimicrobial peptide LL-37 as a post-exposure therapy to protect against respiratory tularemia in mice

Early activation of the innate immune response is important for protection against infection with Francisella tularensis live vaccine strain (LVS) in mice. The human cathelicidin antimicrobial peptide LL-37 is known to have immunomodulatory properties, and therefore exogenously administered LL-37 may be suitable as an early post-exposure therapy to protect against LVS infection. LL-37 has been evaluated for immunostimulatory activity in uninfected mice and for activity against LVS in macrophage assays and protective efficacy when administered post-challenge in a mouse model of respiratory tularemia. Increased levels of pro-inflammatory cytokine IL-6, chemokines monocyte chemoattractant protein 1 (MCP-1) and CXCL1 with increased neutrophil influx into the lungs were observed in uninfected mice after intranasal administration of LL-37. Following LVS challenge, LL-37 administration resulted in increased IL-6, IL-12 p70, IFNγ and MCP-1 production, a slowing of LVS growth in the lung, and a significant extension of mean time to death compared to control mice. However, protection was transient, with the LL-37 treated mice eventually succumbing to infection. As this short course of nasally delivered LL-37 was moderately effective at overcoming the immunosuppressive effects of LVS infection this suggests that a more sustained treatment regimen may be an effective therapy against this pathogen.     

1,25-Dihydroxyvitamin D3 Induces LL-37 and HBD-2 Production in Keratinocytes from Diabetic Foot Ulcers Promoting Wound Healing: An In Vitro Model

Diabetic foot ulcers (DFU) are one of the most common diabetes-related cause of hospitalization and often lead to severe infections and poor healing. It has been recently reported that patients with DFU have lower levels of antimicrobial peptides (AMPs) at the lesion area, which contributes with the impairment of wound healing. The aim of this study was to determine whether 1,25-dihydroxyvitamin D₃ (1,25 (OH)₂ D₃) and L-isoleucine induced HBD-2 and LL-37 in primary cultures from DFU. We developed primary cell cultures from skin biopsies from 15 patients with DFU and 15 from healthy donors. Cultures were treated with 1,25 (OH)₂D₃ or L-isoleucine for 18 h. Keratinocytes phenotype was identified by western blot and flow cytometry. Real time qPCR for DEFB4, CAMP and VDR gene expression was performed as well as an ELISA to measure HBD-2 and LL-37 in supernatant. Antimicrobial activity, in vitro, wound healing and proliferation assays were performed with conditioned supernatant. The results show that primary culture from DFU treated with 1,25(OH)₂D₃, increased DEFB4 and CAMP gene expression and increased the production of HBD-2 and LL-37 in the culture supernatant. These supernatants had antimicrobial activity over E. coli and induced remarkable keratinocyte migration. In conclusion the 1,25(OH)₂D₃ restored the production of AMPs in primary cell from DFU which were capable to improve the in vitro wound healing assays, suggesting their potential therapeutic use on the treatment of DFU.     

Innate immune responses to Mycobacterium ulcerans via toll-like receptors and dectin-1 in human keratinocytes

Mycobacterium ulcerans (MU), an environmental pathogen, causes Buruli ulcer, a severe skin disease. We hypothesized that epidermal keratinocytes might not be a simple barrier, but play a role during MU infection through pattern-recognition receptors expressed in keratinocytes. We found that keratinocyte Toll-like receptors (TLRs) 2 and 4 and Dectin-1 actively participate in the innate immune response to MU, which includes the internalization of bacteria, the production of reactive oxygen species (ROS), and the expression of chemokines and LL-37. Human keratinocytes constitutively expressed TLRs 2 and 4 and induced Dectin-1 in response to MU. Exposing keratinocytes to MU resulted in rapid ROS production, which in turn contributed to the mRNA and protein expression of LL-37. In addition, TLR2, Dectin-1 and, to an extent, TLR4 are essential for the MU-mediated expression of CXCL8, CCL2 and LL-37 in keratinocytes. Furthermore, confocal analysis showed that the Dectin-1 is necessary for keratinocytes to internalize bacilli. Importantly, blockade of ROS and LL-37 significantly increased the intracellular MU growth in keratinocytes, suggesting an important role of these mediators for cutaneous innate immune responses. Our results demonstrate that TLR2, TLR4 and Dectin-1 actively sense, internalize and respond in an innate way to MU in human epidermal keratinocytes.