LL-37 attenuates inflammatory impairment via mTOR signaling-dependent mitochondrial protection

The human cationic antimicrobial protein LL-37 is a multifunctional host defense peptide with a wide range of immunomodulatory activities. Previous work has shown that LL-37 exerts both pro- and anti-inflammatory effects. The role of mitochondria in the skin inflammatory effects of LL-37 has not been well studied. Therefore, our aim was to investigate the immunomodulatory effect of LL-37 in HaCaT cells and to delineate the underlying mechanisms related to mitochondrial function. Immunohistochemistry results from tissue microarrays showed strong cytoplasmic LL-37 staining in inflammatory cells in chronic dermatic inflammation. Using exogenous LL-37 stimulation and LL-37 knockdown and overexpression, LL-37 was demonstrated to dramatically reduce the mRNA levels and protein secretion of inflammatory cytokines including IL-6, IL-8, IL-1α and tumor necrosis factor-α (TNF-α), which are induced by lipopolysaccharides (LPS). The anti-inflammatory effects of LL-37 are dependent upon its ability to increase mitochondrial biogenesis and to maintain mitochondrial homeostasis. Furthermore, we observed that LL-37 enhances the LPS-induced phosphorylation of extracellular signal-regulated kinase (ERK1/2) and mammalian target of rapamycin (mTOR). The mTOR inhibitor rapamycin can neutralize the protective effects of LL-37 on mitochondria. In conclusion, these results suggest that high LL-37 expression levels correlate with chronic skin inflammation; mitochondrial dysfunction occurs in HaCaT cells during inflammation; and LL-37 attenuates inflammatory impairment by stimulating mitochondrial biogenesis and protecting mitochondrial function, which are dependent upon mTOR signaling. These findings provide new insights into targeting mitochondria with LL-37 to prevent skin inflammatory reactions.

Recombinant probiotics with antimicrobial peptides: a dual strategy to improve immune response in immunocompromised patients

Bacterial infectious diseases are currently a serious health problem, especially in patients compromised by illness or those receiving immune-suppressant drugs. In this context, it is not only essential to improve the understanding of infectious mechanisms and host response but also to discover novel therapies with extreme urgency. Probiotics and antimicrobial peptides are also favorably viewed as novel strategies in the control of resistant bacteria. The present review will shed some light on the use of probiotic microorganisms expressing antimicrobial peptides as a dual therapy to control bacterial infectious diseases.

Citrullination alters immunomodulatory function of LL-37 essential for prevention of endotoxin-induced sepsis

Cathelicidin LL-37 plays an essential role in innate immunity by killing invading microorganisms and regulating the inflammatory response. These activities depend on the cationic character of the peptide, which is conferred by arginine and lysine residues. At inflammatory foci in vivo, LL-37 is exposed to peptidyl arginine deiminase (PAD), an enzyme released by inflammatory cells. Therefore, we hypothesized that PAD-mediated citrullination of the arginine residues within LL-37 will abrogate its immunomodulatory functions. We found that, when citrullinated, LL-37 was at least 40 times less efficient at neutralizing the proinflammatory activity of LPS due to a marked decrease in its affinity for endotoxin. Also, the ability of citrullinated LL-37 to quench macrophage responses to lipoteichoic acid and poly(I:C) signaling via TLR2 and TLR3, respectively, was significantly reduced. Furthermore, in stark contrast to native LL-37, the modified peptide completely lost the ability to prevent morbidity and mortality in a mouse model of d-galactosamine-sensitized endotoxin shock. In fact, administration of citrullinated LL-37 plus endotoxin actually exacerbated sepsis due to the inability of LL-37 to neutralize LPS and the subsequent enhancement of systemic inflammation due to increased serum levels of IL-6. Importantly, serum from septic mice showed increased PAD activity, which strongly correlated with the level of citrullination, indicating that PAD-driven protein modification occurs in vivo. Because LL-37 is a potential treatment for sepsis, its administration should be preceded by a careful analysis to ensure that the citrullinated peptide is not generated in treated patients.

High levels of circulating triiodothyronine induce plasma cell differentiation

The effects of hyperthyroidism on B-cell physiology are still poorly known. In this study, we evaluated the influence of high-circulating levels of 3,5,3'-triiodothyronine (T3) on bone marrow, blood, and spleen B-cell subsets, more specifically on B-cell differentiation into plasma cells, in C57BL/6 mice receiving daily injections of T3 for 14 days. As analyzed by flow cytometry, T3-treated mice exhibited increased frequencies of pre-B and immature B-cells and decreased percentages of mature B-cells in the bone marrow, accompanied by an increased frequency of blood B-cells, splenic newly formed B-cells, and total CD19(+)B-cells. T3 administration also promoted an increase in the size and cellularity of the spleen as well as in the white pulp areas of the organ, as evidenced by histological analyses. In addition, a decreased frequency of splenic B220(+) cells correlating with an increased percentage of CD138(+) plasma cells was observed in the spleen and bone marrow of T3-treated mice. Using enzyme-linked immunospot assay, an increased number of splenic immunoglobulin-secreting B-cells from T3-treated mice was detected ex vivo. Similar results were observed in mice immunized with hen egg lysozyme and aluminum adjuvant alone or together with treatment with T3. In conclusion, we provide evidence that high-circulating levels of T3 stimulate plasma cytogenesis favoring an increase in plasma cells in the bone marrow, a long-lived plasma cell survival niche. These findings indicate that a stimulatory effect on plasma cell differentiation could occur in untreated patients with Graves' disease.

Requirement of the Pseudomonas aeruginosa CbrA sensor kinase for full virulence in a murine acute lung infection model

Pseudomonas aeruginosa is an opportunistic pathogen that is a major cause of respiratory tract and other nosocomial infections. The sensor kinase CbrA is a central regulator of carbon and nitrogen metabolism and in vitro also regulates virulence-related processes in P. aeruginosa. Here, we investigated the role of CbrA in two murine models of infection. In both peritoneal infections in leukopenic mice and lung infection models, the cbrA mutant was less virulent since substantially larger numbers of cbrA mutant bacteria were required to cause the same level of infection as wild-type or complemented bacteria. In contrast, in the chronic rat lung model the cbrA mutant grew and persisted as well as the wild type, indicating that the decrease of in vivo virulence of the cbrA mutant did not result from growth deficiencies on particular carbon substrates observed in vitro. In addition, a mutant in the cognate response regulator CbrB showed no defect in virulence in the peritoneal infection model, ruling out the involvement of certain alterations of virulence properties in the cbrA mutant including defective swarming motility, increased biofilm formation, and cytotoxicity, since these alterations are controlled through CbrB. Further investigations indicated that the mutant was more susceptible to uptake by phagocytes in vitro, resulting in greater overall bacterial killing. Consistent with the virulence defect, it took a smaller number of Dictyostelium discoideum amoebae to kill the cbrA mutant than to kill the wild type. Transcriptional analysis of the cbrA mutant during D. discoideum infection led to the conclusion that CbrA played an important role in the iron metabolism, protection of P. aeruginosa against oxidative stress, and the regulation of certain virulence factors.

Immunotherapy for pulmonary TB: antimicrobial peptides and their inducers

TB is an infectious disease that still has an enormous impact on public health worldwide. With the continuous increasing epidemic of multidrug-resistant TB, new drugs and vaccines are urgently needed. In the last decade there has been a broad advance in the knowledge of innate immunity in TB. Together with the growing research regarding immunomodulators, new promising insights have been developed that can contribute in the control of TB. This is the case of antimicrobial peptides, which can be potential therapeutic or adjuvant agents. The current high cost of antimicrobial peptide synthesis may be a current deterrent for treatment; antimicrobial peptide-inducers can be an alternative for low-cost treatment and/or adjuvants.

Defensin-based anti-infective strategies

Cationic and amphiphilic peptides are widely distributed in eukaryotic organisms and constitute a first line of host defense against invading pathogens. Some of these host defense peptides (HDPs) combine specific antibiotic activities with modulation of immune responses. Moreover, they are active against bacteria resistant to conventional antibiotics and show only modest resistance development under in vitro selection pressure. Based on these features, HDPs and particularly defensins are considered a promising source of novel anti-infective agents. This review summarizes the current knowledge about defensins from different kingdoms and discusses their potential for therapeutic application.

Cathelicidin-like helminth defence molecules (HDMs): absence of cytotoxic, anti-microbial and anti-protozoan activities imply a specific adaptation to immune modulation

Host defence peptides (HDPs) are expressed throughout the animal and plant kingdoms. They have multifunctional roles in the defence against infectious agents of mammals, possessing both bactericidal and immune-modulatory activities. We have identified a novel family of molecules secreted by helminth parasites (helminth defence molecules; HDMs) that exhibit similar structural and biochemical characteristics to the HDPs. Here, we have analyzed the functional activities of four HDMs derived from Schistosoma mansoni and Fasciola hepatica and compared them to human, mouse, bovine and sheep HDPs. Unlike the mammalian HDPs the helminth-derived HDMs show no antimicrobial activity and are non-cytotoxic to mammalian cells (macrophages and red blood cells). However, both the mammalian- and helminth-derived peptides suppress the activation of macrophages by microbial stimuli and alter the response of B cells to cytokine stimulation. Therefore, we hypothesise that HDMs represent a novel family of HDPs that evolved to regulate the immune responses of their mammalian hosts by retaining potent immune modulatory properties without causing deleterious cytotoxic effects.

In mammals, secreted host defence peptides (HDPs) protect against a wide range of infectious pathogens. They also perform a range of immune modulatory functions which regulate the immune response to pathogens, ensuring that the protective inflammatory response is not exacerbated and that post-infection repair mechanisms are initiated. We identified a novel family of molecules secreted by medically-important helminth pathogens (termed helminth defence molecules; HDMs) that exhibit striking structural and biochemical similarities to the HDPs. To further investigate the extent of this similarity, we have performed a comparative functional study between several well characterized, anti-microbial, mammalian HDPs and a series of parasite-derived peptides. The parasite HDMs displayed immune modulatory properties that were similar to their HDP homologs in mammals, but possessed no antimicrobial or cytotoxic activity. We propose that HDMs of these helminth pathogens underwent specific adaptation, losing their anti-microbial activity but retaining their ability to regulate the immune responses of their mammalian hosts. This absence of cytotoxicity and retention of immune-modulatory activity offers an opportunity to design novel immunotherapeutics derived from the HDMs which could be used to combat destructive inflammatory responses associated with microbial infection and immune-related disorders.

Promotion of autoimmune diabetes by cereal diet in the presence or absence of microbes associated with gut immune activation, regulatory imbalance, and altered cathelicidin antimicrobial Peptide

We are exposed to millions of microbial and dietary antigens via the gastrointestinal tract, which likely play a key role in type 1 diabetes (T1D). We differentiated the effects of these two major environmental factors on gut immunity and T1D. Diabetes-prone BioBreeding (BBdp) rats were housed in specific pathogen-free (SPF) or germ-free (GF) conditions and weaned onto diabetes-promoting cereal diets or a protective low-antigen hydrolyzed casein (HC) diet, and T1D incidence was monitored. Fecal microbiota 16S rRNA genes, immune cell distribution, and gene expression in the jejunum were analyzed. T1D was highest in cereal-SPF (65%) and cereal-GF rats (53%) but inhibited and delayed in HC-fed counterparts. Nearly all HC-GF rats remained diabetes-free, whereas HC-fed SPF rats were less protected (7 vs. 29%). Bacterial communities differed in SPF rats fed cereal compared with HC. Cereal-SPF rats displayed increased gut CD3(+) and CD8α(+) lymphocytes, ratio of Ifng to Il4 mRNA, and Lck expression, indicating T-cell activation. The ratio of CD3(+) T cells expressing the Treg marker Foxp3(+) was highest in HC-GF and lowest in cereal-SPF rats. Resident CD163(+) M2 macrophages were increased in HC-protected rats. The cathelicidin antimicrobial peptide (Camp) gene was upregulated in the jejunum of HC diet-protected rats, and CAMP(+) cells colocalized with CD163. A cereal diet was a stronger promoter of T1D than gut microbes in association with impaired gut immune homeostasis.

Looped host defense peptide CLP-19 binds to microtubules and inhibits surface expression of TLR4 on mouse macrophages

The looped host defense peptide CLP-19 is derived from a highly functional core region of the Limulus anti-LPS factor and exerts robust anti-LPS activity by directly interacting with LPS in the extracellular space. We previously showed that prophylactic administration of CLP-19 even 20 h prior to LPS challenge might significantly increase the survival rate in a lethal endotoxin shock mouse model. Such an effect may be associated with immune regulation of CLP-19. To investigate the underlying mechanisms, peptide affinity chromatography, immunofluorescence, and Western blotting procedures were used to identify α- and β-tubulin as direct and specific binding partners of CLP-19 in the mouse macrophage cell line RAW 264.7. Bioinformatic analysis using the AutoDock Vina molecular docking and PyMOL molecular graphics system predicted that CLP-19 would bind to the functional residues of both α- and β-tubulin and would be located within the groove of microtubules. Tubulin polymerization assay revealed that CLP-19 might induce polymerization of microtubules and prevent depolymerization. The immunoregulatory effect of CLP-19 involving microtubules was investigated by flow cytometry, immunofluorescence, and Western blotting, which showed that CLP-19 prophylactic treatment of RAW 264.7 cells significantly inhibited LPS-induced surface expression of TLR4. Taken together, these results suggest that CLP-19 binding to microtubules disrupts the dynamic equilibrium of microtubules, reducing the efficacy of microtubule-dependent vesicular transport that would otherwise translocate TLR4 from the endoplasmic reticulum to the cell surface.

Safety evaluation of the antimicrobial peptide bovicin HC5 orally administered to a murine model

BACKGROUND

Bovicin HC5 is an antimicrobial peptide that shows a broad spectrum of activity and potential for biotechnological and therapeutic applications. To gain insight about the safety of bovicin HC5 application, the histological and immunostimulatory effects of orally administrated bovicin HC5 to BALB/c mice were evaluated. BALB/c mice were divided into three groups: negative control (NC group); mice given purified bovicin HC5 (Bov group); mice given ovalbumin (positive control, PC group; a murine model of enteropathy). The mice were initially pre-sensitized, and PBS, bovicin HC5 or ovalbumin were administered for 30 days by daily gavages. Histological and morphometric analysis were performed and the relative expression of cytokines was analyzed by real-time RT-PCR.

RESULTS

The oral administration of bovicin HC5 to BALB/c mice reduced weight gain and caused alterations in the small intestine, although absorptive changes have not been detected. The number of total goblet cells and the mucopolysaccharides production were not affected by bovicin HC5 administration. A hypertrophy of Paneth cells and an increase in the number of mitotic cells were observed in Bov group, while the number of mast cells remained unaltered. Increased expression of TNF-α, INF-γ and IL-12 was observed in the small intestine upon bovicin HC5 administration.

CONCLUSION

Bovicin HC5 has only minor effects on intestinal permeability and did not elicit an allergenic response upon oral administration to animal models. Considering the low in vivo toxicity of bovicin HC5, it might be a good candidate for enteral applications.

Synthetic cationic peptide IDR-1018 modulates human macrophage differentiation

Macrophages play a critical role in the innate immune response. To respond in a rapid and efficient manner to challenges in the micro-environment, macrophages are able to differentiate towards classically (M1) or alternatively (M2) activated phenotypes. Synthetic, innate defense regulators (IDR) peptides, designed based on natural host defence peptides, have enhanced immunomodulatory activities and reduced toxicity leading to protection in infection and inflammation models that is dependent on innate immune cells like monocytes/macrophages. Here we tested the effect of IDR-1018 on macrophage differentiation, a process essential to macrophage function and the immune response. Using transcriptional, protein and systems biology analysis, we observed that differentiation in the presence of IDR-1018 induced a unique signature of immune responses including the production of specific pro and anti-inflammatory mediators, expression of wound healing associated genes, and increased phagocytosis of apoptotic cells. Transcription factor IRF4 appeared to play an important role in promoting this IDR-1018-induced phenotype. The data suggests that IDR-1018 drives macrophage differentiation towards an intermediate M1-M2 state, enhancing anti-inflammatory functions while maintaining certain pro-inflammatory activities important to the resolution of infection. Synthetic peptides like IDR-1018, which act by modulating the immune system, could represent a powerful new class of therapeutics capable of treating the rising number of multidrug resistant infections as well as disorders associated with dysregulated immune responses.

Host defense peptides: an alternative as antiinfective and immunomodulatory therapeutics

Host defense peptides are conserved components of innate immune response present among all classes of life. These peptides are potent, broad spectrum antimicrobial agents with potential as novel therapeutic compounds. Also, the ability of host defense peptides to modulate immunity is an emerging therapeutic concept since its selective modulation is a novel antiinfective strategy. Their mechanisms of action and the fundamental differences between pathogens and host cells surfaces mostly lead to a not widely extended microbial resistance and to a lower toxicity toward host cells. Biological libraries and rational design are novel tools for developing such molecules with promising applications as therapeutic drugs.

Alveolar macrophage cathelicidin deficiency in severe sarcoidosis

BACKGROUND

Dysfunctional immune responses characterize sarcoidosis, but the status of cathelicidin, a potent immunoregulatory and antimicrobial molecule, has not been established in clinical disease activity.

METHODS

Alveolar macrophage cathelicidin expression was determined in biopsy-proven sarcoidosis patients classified clinically as 'severe' (requiring systemic treatment) or 'non-severe' (never requiring treatment). Bronchoalveolar lavage (BAL) cells from sarcoidosis patients and healthy controls were analyzed for mRNA expression of cathelicidin, vitamin D receptor (VDR) and the VDR coactivator steroid receptor coactivator-3 (SRC3) by quantitative PCR. Cathelicidin-derived peptide LL-37 was determined by immunocytochemistry. Serum calcidiol (25-hydroxyvitamin D2; vitD2) and calcitriol (1,25-dihydroxyvitamin D3; vitD3) were quantified.

RESULTS

The results indicated reduced BAL cell expression of cathelicidin and SRC3 in severe but not non-severe sarcoidosis compared to controls. Serum levels of biologically active vitD3 in both severe and non-severe patients were within the control range even though vitD2 levels in both groups were below the recommended level (30 ng/ml). Sarcoidosis and control alveolar macrophages were studied in vitro to determine cathelicidin responses to vitD3 and tumor necrosis factor-α (TNFα), a vitD3 antagonist elevated in active sarcoidosis. Alveolar macrophage cathelicidin was stimulated by vitD3 but repressed by TNFα, which also repressed SRC3.

CONCLUSIONS

These findings suggest that TNFα-mediated repression of SRC3 contributes to alveolar macrophage cathelicidin deficiency in severe sarcoidosis despite healthy vitD3 levels. Deficiency of cathelicidin, a multifunctional regulator of immune cells and proinflammatory cytokines, may impede resolution of inflammation in the lungs of patients with severe sarcoidosis.

Signaling pathways mediating chemokine induction in keratinocytes by cathelicidin LL-37 and flagellin

Cathelicidin LL-37 is a multifunctional immunomodulatory and antimicrobial host defense peptide that has an important role in the immune defenses of the skin and other epithelial barriers. We have previously demonstrated that at physiological concentrations LL-37 synergistically augments the production of immune mediators in response to microbial compounds in human primary keratinocytes. Here we define the signaling mechanisms responsible for this activity. We demonstrate that inhibition of Src family kinases (SFKs) strongly inhibited the synergistic chemokine production in response to LL-37 and flagellin in keratinocytes. SFK activation was induced by LL-37 stimulation and was required for the downstream activation of Akt (protein kinase B) and the transcription factors CREB and ATF1. In cells stimulated with LL-37 and flagellin together, Akt activation was primarily induced by LL-37, while both flagellin and LL-37 contributed to the activation of CREB and ATF1 and consequently chemokine induction. The purinergic receptor P2X₇ was identified as the receptor upstream of SFK activation in LL-37-stimulated keratinocytes. Overall, these findings established the P2X₇-SFK-Akt-CREB/ATF1 signaling pathway activated by LL-37 in primary keratinocytes. These signaling mechanisms mediated the synergistic effects of LL-37 on chemokine production in flagellin-stimulated keratinocytes, and thus might have a role in the immune defenses of the skin and possibly other epithelial barriers.

Specificity of antimicrobial peptide LL-37 to neutralize periodontopathogenic lipopolysaccharide activity in human oral fibroblasts

BACKGROUND

The antimicrobial peptide LL-37 is known to have a potent lipopolysaccharide (LPS)-neutralizing activity in various cell types. Because of observed heterogeneity within periodontopathogenic LPS, the authors hypothesized that LL-37 had specificity to neutralize such LPS activity. The present study, therefore, aims to investigate the LPS-neutralizing activity of LL-37 to various periodontopathogenic LPS in interleukin-8 (IL-8) production after challenging them in human oral fibroblasts.

METHODS

Human periodontal ligament fibroblasts (PDLF) and gingival fibroblasts (GF) were cultured from biopsies of periodontal ligament and gingival tissues. After cell confluence in 24-well plates, LPS (10 μg/mL) from Porphyromonas gingivalis, Prevotella intermedia, Fusobacterium nucleatum, and Aggregatibacter actinomycetemcomitans were added with or without LL-37 (10 μg/mL). After 18 hours, the supernatant was collected and analyzed in IL-8 production by enzyme-linked immunosorbent assay.

RESULTS

All periodontopathogenic LPS statistically significantly induced IL-8 production in both PDLF and GF (P <0.01). After neutralization with LL-37, both PDLF and GF showed a statistically significant reduction in IL-8 production compared with LPS-treated groups without LL-37 (P <0.01), and the percentage of reduction in IL-8 production in PDLF appeared to be higher than in GF. In addition, the percentage of reduction in IL-8 production varied considerably according to each periodontopathogenic LPS.

CONCLUSIONS

The antimicrobial peptide LL-37 had an ability to suppress periodontopathogenic LPS-induced IL-8 production in both PDLF and GF. Its LPS-neutralizing activity revealed specificity to periodontopathogenic LPS and seemed to be dependent on the heterogeneity within LPS between different genera.

Modulating immunity as a therapy for bacterial infections

Despite our efforts to halt the increase and spread of antimicrobial resistance, bacteria continue to become less susceptible to antimicrobial drugs over time, and rates of discovery for new antibiotics are declining. Thus, it is essential to explore new paradigms for anti-infective therapy. One promising approach involves host-directed immunomodulatory therapies, whereby natural mechanisms in the host are exploited to enhance therapeutic benefit. The objective is to initiate or enhance protective antimicrobial immunity while limiting inflammation-induced tissue injury. A range of potential immune modulators have been proposed, including innate defence regulator peptides and agonists of innate immune components such as Toll-like receptors and NOD-like receptors.

Cationic host defence peptides: multifaceted role in immune modulation and inflammation

Host defence peptides (HDPs) are innate immune effector molecules found in diverse species. HDPs exhibit a wide range of functions ranging from direct antimicrobial properties to immunomodulatory effects. Research in the last decade has demonstrated that HDPs are critical effectors of both innate and adaptive immunity. Various studies have hypothesized that the antimicrobial property of certain HDPs may be largely due to their immunomodulatory functions. Mechanistic studies revealed that the role of HDPs in immunity is very complex and involves various receptors, signalling pathways and transcription factors. This review will focus on the multiple functions of HDPs in immunity and inflammation, with special reference to cathelicidins, e.g. LL-37, certain defensins and novel synthetic innate defence regulator peptides. We also discuss emerging concepts of specific HDPs in immune-mediated inflammatory diseases, including the potential use of cationic peptides as therapeutics for immune-mediated inflammatory disorders.

Cathelicidin peptide LL-37 modulates TREM-1 expression and inflammatory responses to microbial compounds

Inflammatory diseases remain an important cause of morbidity and mortality. Cathelicidins are immunomodulatory and antimicrobial peptides with potent anti-endotoxic properties. Although the effects of the human cathelicidin LL-37 on cellular responses to Toll-like receptor (TLR) ligands have been investigated, its effects on responses to other pro-inflammatory stimuli have not been well studied. Triggering receptor expressed on myeloid cells (TREM-1) acts to amplify inflammatory responses and plays important roles in the pathogenesis of endotoxemia. In this work, the effects of LL-37 on responses to TREM-1 stimulation, alone and in the presence of a range of microbial compounds, were analyzed. It was shown that in peripheral blood mononuclear cells LL-37 strongly suppressed synergistic responses to TREM-1 and TLR4 stimulation, partly through the inhibition of TREM-1 expression on monocytes; similar effects were observed using the TLR2 ligand lipoteichoic acid. In contrast, LL-37 stimulated TREM-1 upregulation by peptidoglycan (PGN, TLR2 ligand that is also recognized via nucleotide-binding oligomerization domain containing 2 after fragmentation and intracellular uptake), as well as the responses to combined TREM-1 and PGN stimulation, possibly via the p38 mitogen-activated protein kinase pathway. LL-37 did not affect TREM-1-induced neutrophil degranulation or the production of reactive oxygen species and interleukin-8 by neutrophils. These findings provide further insight into the roles of LL-37 during inflammation and may have implications for its in vivo immunomodulatory properties and for the design of synthetic cathelicidin derivatives as anti-inflammatory and anti-endotoxic molecules.

New insights into cathelicidin modulation of adaptive immunity

Cathelicidins are a family of host defence peptides that are known to selectively alter innate immunity in response to infection and other changes in immune status. A study in this issue of the European Journal of Immunology elucidates a new role for mouse cathelin-related antimicrobial peptide in the adaptive immune response by clearly demonstrating for the first time that a cathelicidin can alter T-cell-dependent activation of the humoral response in vivo and thus modulate the activities of both B and T lymphocytes.

Exploring the pharmacological potential of promiscuous host-defense peptides: from natural screenings to biotechnological applications

In the last few years, the number of bacteria with enhanced resistance to conventional antibiotics has dramatically increased. Most of such bacteria belong to regular microbial flora, becoming a real challenge, especially for immune-depressed patients. Since the treatment is sometimes extremely expensive, and in some circumstances completely inefficient for the most severe cases, researchers are still determined to discover novel compounds. Among them, host-defense peptides (HDPs) have been found as the first natural barrier against microorganisms in nearly all living groups. This molecular class has been gaining attention every day for multiple reasons. For decades, it was believed that these defense peptides had been involved only with the permeation of the lipid bilayer in pathogen membranes, their main target. Currently, it is known that these peptides can bind to numerous targets, as well as lipids including proteins and carbohydrates, from the surface to deep within the cell. Moreover, by using in vivo models, it was shown that HDPs could act both in pathogens and cognate hosts, improving immunological functions as well as acting through multiple pathways to control infections. This review focuses on structural and functional properties of HDP peptides and the additional strategies used to select them. Furthermore, strategies to avoid problems in large-scale manufacture by using molecular and biochemical techniques will also be explored. In summary, this review intends to construct a bridge between academic research and pharmaceutical industry, providing novel insights into the utilization of HDPs against resistant bacterial strains that cause infections in humans.

Cathelin-related antimicrobial peptide differentially regulates T- and B-cell function

Mammalian antimicrobial peptides (AMPs) play an important role in host defense via direct antimicrobial activity as well as immune regulation. The mouse cathelin-related antimicrobial peptide (mCRAMP), produced from the mouse gene Camp, is the only mouse cathelicidin identified and the ortholog of the human gene encoding the peptide LL-37. This study tested the hypothesis that mouse B and T cells produce and respond to mCRAMP. We show that all mature mouse B-cell subsets, including follicular (FO), marginal zone (MZ), B1a, and B1b cells, as well as CD4(+) and CD8(+) T cells produce Camp mRNA and mCRAMP protein. Camp(-/-) B cells produced equivalent levels of IgM, IgG3, and IgG2c but less IgG1 and IgE, while Camp(-/-) CD4(+) T cells cultured in Th2-inducing conditions produced more IL-4-expressing cells when compared with WT cells, effects that were reversed upon addition of mCRAMP. In vivo, Camp(-/-) mice immunized with TNP-OVA absorbed in alum produced an enhanced TNP-specific IgG1 response when compared with WT mice. ELISpot analysis revealed increased numbers of TNP-specific IgG1-secreting splenic B cells and FACS analysis revealed increased CD4(+) T-cell IL-4 expression. Our results suggest that mCRAMP differentially regulates B- and T-cell function and implicate mCRAMP in the regulation of adaptive immune responses.

Antimicrobial peptides in the duodenum at the acute and convalescent stages in patients with diarrhea due to Vibrio cholerae O1 or enterotoxigenic Escherichia coli infection

Patients with acute watery diarrhea caused by Vibrio cholerae O1 or enterotoxigenic Escherichia coli (ETEC) were analyzed for innate immune factors produced by the epithelium during the disease process. Duodenal biopsies were obtained from study participants at the acute (day 2) and convalescent (day 21) stages of disease. Levels of α-defensin (HD-5 and -6), β-defensin (hBD-1-4), and cathelicidin (LL-37) mRNAs were determined by real-time qRT-PCR. hBD-2, HD-5, LL-37 peptides were analyzed in duodenal epithelium by immunomorphometry. Concentration of hBD-2 in stool was determined by ELISA. Specimens from healthy controls were also analyzed. hBD-2 mRNA levels were significantly increased at acute stage of diarrhea; hBD-2 peptide was detected in fecal specimens but barely in duodenal epithelium at acute stage. Immunomorphometry analysis showed that Paneth cells contain significantly higher amounts of HD-5 pre/propeptide at convalescence (P<0.01) and in healthy controls (P<0.001) compared to acute stage, LL-37 peptide levels also decreased at acute stage while mRNA levels remained unchanged. mRNA expression levels of the other antimicrobial peptides remained unchanged with higher levels of α-defensins than β-defensins. V. cholerae induced an innate immune response at the acute stage of disease characterized by increased expression of hBD-2, and continued expression of hBD-1, HD-5-6, and LL-37.

Antibacterial effects of vitamin D

Interaction between vitamin D and the immune system has been recognized for many years, but its relevance to normal human physiology has only become evident in the past 5 years. Studies of innate immune responses to pathogens such as Mycobacterium tuberculosis have shown that pathogen-recognition receptor-mediated activation of localized vitamin D metabolism and signaling is a key event associated with infection. Vitamin D, acting in an intracrine fashion, is able to induce expression of antibacterial proteins and enhance the environment in which they function. The net effect of these actions is to support increased bacterial killing in a variety of cell types. The efficacy of such a response is highly dependent on vitamin D status; in other words, the availability of circulating 25-hydroxyvitamin D for intracrine conversion to active 1,25-dihydroxyvitamin D by the enzyme 25-hydroxyvitamin D-1α-hydroxylase. The potential importance of this mechanism as a determinant of human disease is underlined by increasing awareness of vitamin D insufficiency across the globe. This Review will explore the molecular and cellular systems associated with antibacterial responses to vitamin D in different tissues and possible consequences of such a response for the prevention and treatment of human immune disorders.

Mammalian antimicrobial peptide influences control of cutaneous Leishmania infection

Cathelicidin-type antimicrobial peptides (CAMP) are important mediators of innate immunity against microbial pathogens acting through direct interaction with and disruption of microbial membranes and indirectly through modulation of host cell migration and activation. Using a mouse knock-out model in CAMP we studied the role of this host peptide in control of dissemination of cutaneous infection by the parasitic protozoan Leishmania. The presence of pronounced host inflammatory infiltration in lesions and lymph nodes of infected animals was CAMP-dependent. Lack of CAMP expression was associated with higher levels of IL-10 receptor expression in bone marrow, splenic and lymph node macrophages as well as higher anti-inflammatory IL-10 production by bone marrow macrophages and spleen cells but reduced production of the pro-inflammatory cytokines IL-12 and IFN-γ by lymph nodes. Unlike wild-type mice, local lesions were exacerbated and parasites were found largely disseminated in CAMP knockouts. Infection of CAMP knockouts with parasite mutants lacking the surface metalloprotease virulence determinant resulted in more robust disseminated infection than in control animals suggesting that CAMP activity is negatively regulated by parasite surface proteolytic activity. This correlated with the ability of the protease to degrade CAMP in vitro and co-localization of CAMP with parasites within macrophages. Our results highlight the interplay of antimicrobial peptides and Leishmania that influence the host immune response and the outcome of infection.

Immunological characterization of the equine airway epithelium and of a primary equine airway epithelial cell culture model

Our understanding of innate immunity within the equine respiratory tract is limited despite growing evidence for its key role in both the immediate defense and the shaping of downstream adaptive immune responses to respiratory disease. As the first interface to undergo pathogen invasion, the respiratory epithelium is a key player in these early events and our goal was to examine the innate immune characteristics of equine respiratory epithelia and compare them to an in vitro equine respiratory epithelial cell model cultured at the air-fluid interface (AFI). Respiratory epithelial tissues, isolated epithelial cells, and four-week old cultured differentiated airway epithelial cells collected from five locations of the equine respiratory tract were examined for the expression of toll-like receptors (TLRs) and host defense peptides (HDPs) using conventional polymerase chain reaction (PCR). Cultured, differentiated, respiratory epithelial cells and freshly isolated respiratory epithelial cells were also examined for the expression of TLR3, TLR9 and major histocompatibility complex (MHC) class I and class II using fluorescence-activated cell sorting (FACS) analysis. In addition, cytokine and chemokine profiles from respiratory epithelial tissues, freshly isolated respiratory epithelial cells, and cultured, differentiated, epithelial cells from the upper respiratory tract were examined using real-time PCR. We found that respiratory epithelial tissues and isolated epithelial cells expressed TLRs 1-4 and 6-10 as well as HDPs, MxA, 2'5' OAS, β-defensin-1, and lactoferrin. In contrast, epithelial cells cultured at the AFI expressed TLRs 1-4 and 6 and 7 as well as MxA, 2'5' OAS, β-defensin-1, but had lost expression of TLRs 8-10 and lactoferrin. In addition, MHC-I and MHC-II surface expression decreased in epithelial cells cultured at the AFI compared to isolated epithelial cells whereas TLR3 and TLR9 were expressed at similar levels. Lastly, we found that equine respiratory epithelial cells express an array of pro-inflammatory, antiviral and regulatory cytokines and that after four weeks of in vitro growth conditions, equine respiratory epithelial cells cultured at the AFI retained expression of GM-CSF, IL-10, IL-8, TGF-β, TNF-α, and IL-6. In summary, we describe the development of an in vitro equine respiratory epithelial cell culture model that is morphologically similar to the equine airway epithelium and retains several key immunological properties. In the future this model will be a used to study equine respiratory viral pathogenesis and cell-to-cell interactions.

Spotlight on Human LL-37, an Immunomodulatory Peptide with Promising Cell-Penetrating Properties

Cationic antimicrobial peptides are major components of innate immunity and help control the initial steps of the infectious process. They are expressed not only by immunocytes, but also by epithelial cells. They share an amphipathic secondary structure with a polar cationic site, which explains their tropism for prokaryote membranes and their hydrophobic site contributing to the destructuration of these membranes. LL-37 is the only cationic antimicrobial peptide derived from human cathelicidin. LL-37 can also cross the plasma membrane of eukaryotic cells, probably through special domains of this membrane called lipid rafts. This transfer could be beneficial in the context of vaccination: the activation of intracellular toll-like receptors by a complex formed between CpG oligonucleotides and LL-37 could conceivably play a major role in the building of a cellular immunity involving NK cells.

Suppressive effect of the antimicrobial peptide LL-37 on expression of IL-6, IL-8 and CXCL10 induced by Porphyromonas gingivalis cells and extracts in human gingival fibroblasts

Porphyromonas gingivalis is a major periodontogenic bacterium and possesses immunostimulatory components, such as lipopolysaccharides (LPS) and fimbriae. The host antimicrobial peptide, LL-37, suppresses proinflammatory responses of immune cells but its effect on human gingival fibroblasts (HGFs) is not known. In this study, we assessed the effect of LL-37 on the proinflammatory responses of HGFs stimulated with P. gingivalis cells and their components. Live P. gingivalis cells did not induce proinflammatory responses of HGFs, and LL-37 did not alter these responses. However, LL-37 was able to suppress the killed P. gingivalis cell-induced secretion of interleukin (IL)-6 and IL-8. LL-37 also suppressed the expression of IL6, IL8, and CXCL10 genes that was induced by P. gingivalis components, including phenol-water extracts, lipid A, and fimbriae, and the induction of phosphorylation of p38 and extracellular signal-regulated kinase (ERK) by P. gingivalis lipopolysaccharide (LPS). CAMP was found to be expressed in oral epithelial cells but not in HGFs, despite stimulation with P. gingivalis components. Therefore, LL-37 can exert a suppressive effect on P. gingivalis-induced proinflammatory responses of HGFs in a paracrine manner, suggesting that excess inflammatory responses to P. gingivalis in the gingival tissue are suppressed by LL-37 in vivo.

Kinetics and cellular sources of cathelicidin during the course of experimental latent tuberculous infection and progressive pulmonary tuberculosis

In spite of advances in immunology on mycobacterial infection, there are few studies on the role of anti-microbial peptides in tuberculosis. The cathelin-related anti-microbial peptide (CRAMP) is the only cathelicidin isolated from mice. In this work we investigated the cellular sources and the production kinetics of this molecule during experimental tuberculosis, using two well-characterized models of latent or chronic infection and progressive disease. The lung of non-infected control mice expressed CRAMP at very low levels. In both models of experimental tuberculosis the main cells immunolabelled for CRAMP were bronchial epithelial cells, macrophages and pneumocytes types II and I. After intratracheal infection with a high bacilli dose (H37Rv strain) in Balb/c mice to produce progressive disease, a high CRAMP gene expression was induced showing three peaks: very early after 1 day of infection, at day 21 when the peak of protective immunity in this model is raised, and at day 28 when the progressive phase starts and the immunoelectronmicroscopy study showed intense immunolabelling in the cell wall and cytoplasm of intracellular bacilli, as well as in cytoplasmic vacuoles. Interestingly, at day 60 post-infection, when advanced progressive disease is well established, characterized by high bacillary loads and extensive tissue damage, CRAMP gene expression decreased but strong CRAMP immunostaining was detected in vacuolated macrophages filled with bacilli. Thus, cathelicidin is highly produced during experimental pulmonary tuberculosis from diverse cellular sources and could have significant participation in its pathogenesis.

The human cathelicidin hCAP18/LL-37: a multifunctional peptide involved in mycobacterial infections

Antimicrobial peptides are predominantly small cationic polypeptides that are classified together on the basis of these molecules to directly kill or inhibit the growth of microorganisms including mycobacteria, and to activate mechanisms of cellular and adaptive immunity. Various families of antimicrobial peptides have been identified, including the cathelicidins. The cathelicidin family is characterised by a conserved N-terminal cathelin domain and a variable C-terminal antimicrobial domain that can be released from the precursor protein after cleavage by proteinases. LL-37 is the C-terminal part of the only human cathelicidin identified to date called human cationic antimicrobial protein (hCAP18), which is mainly expressed by neutrophils and epithelial cells. The cathelicidin hCAP18/LL-37 is a multifunctional molecule that may mediate various host responses, including bactericidal action, chemotaxis, epithelial cell activation, angiogenesis, epithelial wound repair and activation of chemokine secretion. The antimicrobial peptide LL-37 is produced from human cells during infection of mycobacteria and exerts a microbicidal effect. The discussion will (1) describe recent work on the antimicrobial and immunomodulatory functions of the cathelicidin hCAP18/LL-37, (2) highlight the effectiveness of the cathelicidin hCAP18/LL-37 as a potent component in antimycobacterial immune responses and (3) summarise current progress in the understanding of the therapeutic application of hCAP18/LL-37 and its derivates antimicrobial peptides in mycobacterial infection.

LL-37 directs macrophage differentiation toward macrophages with a proinflammatory signature

The human cathelicidin LL-37 has broad-spectrum antimicrobial activity. It also participates at the interface of innate and adaptive immunity by chemoattracting immune effector cells, modulating the production of a variety of inflammatory mediators by different cell types, and regulating the differentiation of monocytes into dendritic cells. In this study, we investigated the effects of LL-37 on the differentiation of human monocytes into anti-inflammatory macrophages (MPhi-2; driven by M-CSF) versus proinflammatory macrophages (MPhi-1; driven by GM-CSF) as well as on fully differentiated MPhi-1 and MPhi-2. Results revealed that monocytes cultured with M-CSF in the presence of LL-37 resulted in macrophages displaying a proinflammatory signature, namely, low expression of CD163 and little IL-10 and profound IL-12p40 production on LPS stimulation. The effects of LL-37 on M-CSF-driven macrophage differentiation were dose- and time-dependent with maximal effects observed at 10 microg/ml when the peptide was present from the start of the cultures. The peptide enhanced the GM-CSF-driven macrophage differentiation. Exposure of fully differentiated MPhi-2 to LL-37 for 6 d resulted in macrophages that produced less IL-10 and more IL-12p40 on LPS stimulation than control MPhi-2. In contrast, LL-37 had no effect on fully differentiated MPhi-1. Peptide mapping using a set of 16 overlapping 22-mer peptides covering the complete LL-37 sequence revealed that the C-terminal portion of LL-37 is responsible for directing macrophage differentiation. Our results furthermore indicate that the effects of LL-37 on macrophage differentiation required internalization of the peptide. Together, we conclude that LL-37 directs macrophage differentiation toward macrophages with a proinflammatory signature.