Cathelicidin antimicrobial peptides inhibit hyaluronan-induced cytokine release and modulate chronic allergic dermatitis

The Contribution of Antimicrobial Peptides to Immune Cell Function: A Review of Recent Advances

The development of novel antimicrobial agents to replace antibiotics has become urgent due to the emergence of multidrug-resistant microorganisms. Antimicrobial peptides (AMPs), widely distributed in all kingdoms of life, present strong antimicrobial activity against a variety of bacteria, fungi, parasites, and viruses. The potential of AMPs as new alternatives to antibiotics has gradually attracted considerable interest. In addition, AMPs exhibit strong anticancer potential as well as anti-inflammatory and immunomodulatory activity. Many studies have provided evidence that AMPs can recruit and activate immune cells, controlling inflammation. This review highlights the scientific literature focusing on evidence for the anti-inflammatory mechanisms of different AMPs in immune cells, including macrophages, monocytes, lymphocytes, mast cells, dendritic cells, neutrophils, and eosinophils. A variety of immunomodulatory characteristics, including the abilities to activate and differentiate immune cells, change the content and expression of inflammatory mediators, and regulate specific cellular functions and inflammation-related signaling pathways, are summarized and discussed in detail. This comprehensive review contributes to a better understanding of the role of AMPs in the regulation of the immune system and provides a reference for the use of AMPs as novel anti-inflammatory drugs for the treatment of various inflammatory diseases.

Deletion of TNFAIP6 Gene in Human Keratinocytes Demonstrates a Role for TSG-6 to Retain Hyaluronan Inside Epidermis

TSG-6 is a soluble protein secreted in the extracellular matrix by various cell types in response to inflammatory stimuli. TSG-6 interacts with extracellular matrix molecules, particularly hyaluronan (HA), and promotes cutaneous wound closure in mice. Between epidermal cells, the discrete extracellular matrix contains HA and a tiny amount of TSG-6. However, challenges imposed to keratinocytes in reconstructed human epidermis revealed strong induction of TSG-6 expression, after exposure to T helper type 2 cytokines to recapitulate the atopic dermatitis phenotype or after fungal infection that causes secretion of cytokines and antimicrobial peptides. After both types of challenge, enhanced release of TSG-6 happens simultaneously with increased HA production. TSG-6 deficiency in N/TERT keratinocytes was created by inactivating TNFAIP6 using CRISPR/Cas9. Some TSG-6 -/- keratinocytes analyzed through scratch assays tend to migrate more slowly but produce reconstructed human epidermis that exhibits normal morphology and differentiation. Few significant alterations were noticed by transcriptomic analysis. Nevertheless, reduced HA content in TSG-6 -/- reconstructed human epidermis was observed, along with enhanced HA release into the culture medium, and this phenotype was even more pronounced after the challenging conditions. Reintroduction of cells producing TSG-6 in reconstructed human epidermis reduced HA leakage. Our results show a role for TSG-6 in sequestering HA between epidermal cells in response to inflammation.

Skin inflammation activates intestinal stromal fibroblasts and promotes colitis

Inflammatory disorders of the skin are frequently associated with inflammatory bowel diseases (IBDs). To explore mechanisms by which these organs communicate, we performed single-cell RNA-Seq analysis on fibroblasts from humans and mice with IBD. This analysis revealed that intestinal inflammation promoted differentiation of a subset of intestinal stromal fibroblasts into preadipocytes with innate antimicrobial host defense activity. Furthermore, this process of reactive adipogenesis was exacerbated if mouse skin was inflamed as a result of skin wounding or infection. Since hyaluronan (HA) catabolism is activated during skin injury and fibroblast-to-adipocyte differentiation is dependent on HA, we tested the hypothesis that HA fragments could alter colon fibroblast function by targeted expression of human hyaluronidase-1 in basal keratinocytes from mouse skin. Hyaluronidase expression in the skin activated intestinal stromal fibroblasts, altered the fecal microbiome, and promoted excessive reactive adipogenesis and increased inflammation in the colon after challenge with dextran sodium sulfate. The response to digested HA was dependent on expression of TLR4 by preadipocytes. Collectively, these results suggest that the association between skin inflammation and IBD may be due to recognition by mesenchymal fibroblasts in the colon of HA released during inflammation of the skin.

Efficacy of Antigonococcal CMP-Nonulosonate Therapeutics Require Cathelicidins

Novel therapies to counteract multidrug-resistant gonorrhea are urgently needed. A unique gonococcal immune evasion strategy involves capping of lipooligosaccharide (LOS) with sialic acid by gonococcal sialyltransferase (Lst), utilizing host-derived CMP-sialic acid (CMP-Neu5Ac in humans). LOS sialylation renders gonococci resistant to complement and cationic peptides, and down-regulates the inflammatory response by engaging siglecs. CMP-sialic acid analogs (CMP-nonulosonates [CMP-NulOs]) such as CMP-Leg5,7Ac2 and CMP-Kdn are also utilized by Lst. Incorporation of these NulO analogs into LOS maintains gonococci susceptible to complement. Intravaginal administration of CMP-Kdn or CMP-Leg5,7Ac2 attenuates gonococcal colonization of mouse vaginas. Here, we identify a key mechanism of action for the efficacy of CMP-NulOs. Surprisingly, CMP-NulOs remained effective in complement C1q-/- and C3-/- mice. LOS Neu5Ac, but not Leg5,7Ac2 or Kdn, conferred resistance to the cathelicidins LL-37 (human) and mouse cathelicidin-related antimicrobial peptide in vitro. CMP-NulOs were ineffective in Camp-/- mice, revealing that cathelicidins largely mediate the efficacy of therapeutic CMP-NulOs.

Hyaluronan Degradation by Cemip Regulates Host Defense against Staphylococcus aureus Skin Infection

Staphylococcus aureus is a major human bacterial pathogen responsible for deep tissue skin infections. Recent observations have suggested that rapid, localized digestion of hyaluronic acid in the extracellular matrix (ECM) of the dermis may influence bacterial invasion and tissue inflammation. In this study we find that cell migration-inducing protein (Cemip) is the major inducible gene responsible for hyaluronan catabolism in mice. Cemip^−/− mice failed to digest hyaluronan and had significantly less evidence of infection after intradermal bacterial challenge by S. aureus. Stabilization of large-molecular-weight hyaluronan enabled increased expression of cathelicidin antimicrobial peptide (Camp) that was due in part to enhanced differentiation of preadipocytes to adipocytes, as seen histologically and by increased expression of Pref1, PPARg, and Adipoq. Cemip^−/− mice challenged with S. aureus also had greater IL-6 expression and neutrophil infiltration. These observations describe a mechanism for hyaluronan in the dermal ECM to regulate tissue inflammation and host antimicrobial defense.

In this paper, Dokoshi et al. describe how the mammalian hyaluronidase Cemip is induced in the dermis during S. aureus infection. Cemip digests hyaluronan in the skin to regulate reactive adipogenesis and subsequent antimicrobial activity and skin inflammation.

Mast Cell Functions Linking Innate Sensing to Adaptive Immunity

Although mast cells (MCs) are known as key drivers of type I allergic reactions, there is increasing evidence for their critical role in host defense. MCs not only play an important role in initiating innate immune responses, but also influence the onset, kinetics, and amplitude of the adaptive arm of immunity or fine-tune the mode of the adaptive reaction. Intriguingly, MCs have been shown to affect T-cell activation by direct interaction or indirectly, by modifying the properties of antigen-presenting cells, and can even modulate lymph node-borne adaptive responses remotely from the periphery. In this review, we provide a summary of recent findings that explain how MCs act as a link between the innate and adaptive immunity, all the way from sensing inflammatory insult to orchestrating the final outcome of the immune response.

Hyaluronan Fragmentation During Inflammatory Pathologies: A Signal that Empowers Tissue Damage

The mechanisms that modulate the response to tissue injury are not fully understood. Abnormalities in the repair response are associated with a variety of chronic disease states characterized by inflammation, followed subsequently by excessive ECM deposition. As cell-matrix interactions are able to regulate cellular homeostasis, modification of ECM integrity appears to be an unspecific factor in promoting the onset and progression of inflammatory diseases. Evidence is emerging to show that endogenous ECM molecules supply signals to damage tissues and cells in order to promote further ECM degradation and inflammation progression. Several investigations have been confirmed that HA fragments of different molecular sizes exhibit different biological effects and responses. In fact, the increased deposition of HA into the ECM is a strong hallmark of inflammation processes. In the context of inflammatory pathologies, highly polymerized HA is broken down into small components, which are able to exacerbate the inflammatory response by inducing the release of various detrimental mediators such as reactive oxygen species, cytokines, chemokines and destructive enzymes and by facilitating the recruitment of leukocytes. However, strategies involving the modulation of the HA fragment with specific receptors on cell surface could represent different promising effects for therapeutic scope. This review will focus on the inflammation action of small HA fragments in recent years obtained by in vivo reports.

Expression and Function of Host Defense Peptides at Inflammation Sites

There is a growing interest in the complex role of host defense peptides (HDPs) in the pathophysiology of several immune-mediated inflammatory diseases. The physicochemical properties and selective interaction of HDPs with various receptors define their immunomodulatory effects. However, it is quite challenging to understand their function because some HDPs play opposing pro-inflammatory and anti-inflammatory roles, depending on their expression level within the site of inflammation. While it is known that HDPs maintain constitutive host protection against invading microorganisms, the inducible nature of HDPs in various cells and tissues is an important aspect of the molecular events of inflammation. This review outlines the biological functions and emerging roles of HDPs in different inflammatory conditions. We further discuss the current data on the clinical relevance of impaired HDPs expression in inflammation and selected diseases.

LL-37 alone and in combination with IL17A enhances proinflammatory cytokine expression in parallel with hyaluronan metabolism in human synovial sarcoma cell line SW982-A step toward understanding the development of inflammatory arthritis

LL-37 is the only human cathelicidin-family host defense peptide and has been reported to interact with invading pathogens causing inflammation at various body sites. Recent studies showed high levels of LL-37 in the synovial-lining membrane of patients with rheumatoid arthritis, a common type of inflammatory arthritis. The present study aims to investigate the role of LL-37 on mechanisms associated with pathogenesis of inflammatory arthritis. The effects of LL-37 on the expression of proinflammatory cytokines, hyaluronan (HA) metabolism-related genes, cell death-related pathways, and cell invasion were investigated in SW982, a human synovial sarcoma cell line. Time-course measurements of proinflammatory cytokines and mediators showed that LL-37 significantly induced IL6 and IL17A mRNA levels at early time points (3–6 hr). HA-metabolism-related genes (i.e., HA synthase 2 (HAS2), HAS3, hyaluronidase 1 (HYAL1), HYAL2, and CD44) were co-expressed in parallel. In combination, LL-37 and IL17A significantly enhanced PTGS2, TNF, and HAS3 gene expression concomitantly with the elevation of their respective products, PGE2, TNF, and HA. Cell invasion rates and FN1 gene expression were also significantly enhanced. However, LL-37 alone or combined with IL17A did not affect cell mortality or cell cycle. Treatment of SW982 cells with both LL-37 and IL17A significantly enhanced IKK and p65 phosphorylation. These findings suggest that the chronic production of a high level of LL-37 may synchronize with its downstream proinflammatory cytokines, especially IL17A, contributing to the co-operative enhancement of pathogenesis mechanisms of inflammatory arthritis, such as high production of proinflammatory cytokines and mediators together with the activation of HA-metabolism-associated genes and cell invasion.

Hyaluronidase inhibits reactive adipogenesis and inflammation of colon and skin

In this study we evaluated the role of hyaluronan (HA) in reactive adipogenesis, a local expansion of preadipocytes that provides host defense by release of antimicrobial peptides. We observed that HA accumulated during maturation of adipocytes in vitro and was associated with increased expression of preadipocyte factor 1, zinc finger protein 423, and early B cell factor 1. Although HA is normally abundant in the extracellular matrix, a further increase in HA staining occurred in mice at sites of reactive adipogenesis following injury of colon by dextran sodium sulfate or injury of skin from infection with Staphylococcus aureus. HA also abundantly accumulated around adipocytes seen in the colons of patients with inflammatory bowel disease. This HA was necessary for adipocyte maturation because digestion of HA by administration of soluble hyaluronidase or transgenic expression of hyaluronidase 1 inhibited adipogenesis in vitro and in vivo. Furthermore, hyaluronidase also suppressed inflammation of both skin and colon and decreased antimicrobial peptide expression by developing preadipocytes. This resulted in increased bacterial transit across the epithelial barrier despite decreased tissue injury from inflammation. These observations suggest HA plays an important role in reactive adipogenesis and host defense after injury.

Design of peptide mimetics to block pro-inflammatory functions of HA fragments

Hyaluronan is a simple extracellular matrix polysaccharide that actively regulates inflammation in tissue repair and disease processes. The native HA polymer, which is large (>500 kDa), contributes to the maintenance of homeostasis. In remodeling and diseased tissues, polymer size is strikingly polydisperse, ranging from <10 kDa to >500 kDa. In a diseased or stressed tissue context, both smaller HA fragments and high molecular weight HA polymers can acquire pro-inflammatory functions, which result in the activation of multiple receptors, triggering pro-inflammatory signaling to diverse stimuli. Peptide mimics that bind and scavenge HA fragments have been developed, which show efficacy in animal models of inflammation. These studies indicate both that HA fragments are key to driving inflammation and that scavenging these is a viable therapeutic approach to blunting inflammation in disease processes. This mini-review summarizes the peptide-based methods that have been reported to date for blocking HA signaling events as an anti-inflammatory therapeutic approach.

Hyperglycaemia inhibits REG3A expression to exacerbate TLR3-mediated skin inflammation in diabetes

Dysregulated inflammatory responses are known to impair wound healing in diabetes, but the underlying mechanisms are poorly understood. Here we show that the antimicrobial protein REG3A controls TLR3-mediated inflammation after skin injury. This control is mediated by REG3A-induced SHP-1 protein, and acts selectively on TLR3-activated JNK2. In diabetic mouse skin, hyperglycaemia inhibits the expression of IL-17-induced IL-33 via glucose glycation. The decrease in cutaneous IL-33 reduces REG3A expression in epidermal keratinocytes. The reduction in REG3A is associated with lower levels of SHP-1, which normally inhibits TLR3-induced JNK2 phosphorylation, thereby increasing inflammation in skin wounds. To our knowledge, these findings show for the first time that REG3A can modulate specific cutaneous inflammatory responses and that the decrease in cutaneous REG3A exacerbates inflammation in diabetic skin wounds.

Patients with diabetes often have delayed wound healing, associated with excessive inflammation. Here the authors report that REG3A inhibits TLR3-driven inflammation in skin wounds, and show that REG3A is reduced in models of diabetes, which exacerbates inflammation in diabetic wounds.

Functions of Cationic Host Defense Peptides in Immunity

Cationic host defense peptides are a widely distributed family of immunomodulatory molecules with antimicrobial properties. The biological functions of these peptides include the ability to influence innate and adaptive immunity for efficient resolution of infections and simultaneous modulation of inflammatory responses. This unique dual bioactivity of controlling infections and inflammation has gained substantial attention in the last three decades and consequent interest in the development of these peptide mimics as immunomodulatory therapeutic candidates. In this review, we summarize the current literature on the wide range of functions of cationic host defense peptides in the context of the mammalian immune system.

Transcriptional regulation of cathelicidin genes in chicken bone marrow cells

Cathelicidins form a family of vertebrate-specific immune molecules with an evolutionarily conserved gene structure. We analyzed the expression patterns of cathelicidin genes (CAMP, CATH3, and CATHB1) in chicken bone marrow cells (BMCs) and chicken embryonic fibroblasts (CEFs). We found that CAMP and CATHB1 were significantly up-regulated in BMCs, whereas the expression of CATH3 did not differ significantly between BMCs and CEFs. To study the mechanism underlying the up-regulation of cathelicidin genes in BMCs, we predicted the transcription factors (TFs) that bind to the 5'-flanking regions of cathelicidin genes. CEBPA, EBF1, HES1, MSX1, and ZIC3 were up-regulated in BMCs compared to CEFs. Subsequently, when a siRNA-mediated knockdown assay was performed for MSX1, the expression of CAMP and CATHB1 was decreased in BMCs. We also showed that the transcriptional activity of the CAMP promoter was decreased by mutation of the MSX1-binding sites present within the 5'-flanking region of CAMP. These results increase our understanding of the regulatory mechanisms controlling cathelicidin genes in BMCs.

Antimicrobial peptide LL-37 along with peptidoglycan drive monocyte polarization toward CD14(high)CD16(+) subset and may play a crucial role in the pathogenesis of psoriasis guttata

The human cathelicidin LL-37 peptide is overexpressed in psoriasis and has been demonstrated to be a multifunctional modulator of innate immune response elements, including monocytes. Monocytes, categorized into three populations based on the cell surface expression of CD14 and CD16, are activated in psoriasis guttate and are commonly triggered by streptococcal infections. Peptidoglycan (PGN) is a major cell-wall component of streptococcus, and an increasing number of PGN-containing cells have been detected in psoriasis. Since there are independent reports of both PGN and LL-37 influencing monocytes, we tried to evaluate the effect of human LL-37 on PGN-induced monocyte activity and differentiation and subsequently studied their correlation with the pathogenesis of psoriasis guttate. The results revealed that monocytes from the peripheral blood of healthy individuals resulted in their polarization toward the CD14(high)CD16(+) subset, when cultured with PGN in the presence of the LL-37 peptide. This peptide further induced PGN-driven differentiated monocytes into immature dendritic cells (iDC), as evident by the increased expression of CD1a, CD86, and HLA-DR markers, resulting in the induction of T cell proliferation and Th17 polarization. Furthermore, our data suggested that psoriasis guttata patients have significantly higher percentages of CD14(high)CD16(+) monocytes as well as circulating levels of LL-37, soluble form of triggering receptor expressed on myeloid cells (sTREM-1) levels, and anti-streptolysin O (ASO) levels, as compared to healthy controls. Psoriasis guttata patients also showed a positive correlation between the percentage of CD14(high)CD16(+) monocytes and the serum levels of sTREM-1 as well as the Psoriasis Area and Severity Index (PASI) scores. Therefore, we concluded that LL-37 in synergy with PGN directs monocyte polarization and differentiation into a proinflammatory phenotype, which might play a crucial role in the pathogenesis of psoriasis.

Intra-amniotic LPS modulates expression of antimicrobial peptides in the fetal sheep lung

BACKGROUND

Damage-associated molecular patterns (DAMPs) and antimicrobial peptides (AMPs) are components of pulmonary innate immunity and tissue repair. We hypothesized that DAMPs and AMPs would increase in response to fetal pulmonary inflammation caused by chorioamnionitis in a time-dependent manner.

METHODS

Fetal sheep were exposed to intra-amniotic saline or lipopolysaccharide (LPS) (10 mg) between 5 h and 15 d prior to preterm delivery at 125 ± 2 d. Lung tissue mRNAs for proinflammatory cytokines; AMPs: myeloid AMP-29 (MAP29), dodecapeptide, sheep β-defensin-1 (SBD1), and sheep β-defensin-2 (SBD2); and DAMPs: interleukin (IL)-1α, lactoferrin, heat-shock protein-70 (HSP70), high-mobility group box protein-B1 (HMGB1), and receptor for advanced glycation endproducts (RAGE) were measured by reverse-transcriptase quantitative polymerase chain reaction. Immunohistochemistry of DAMPs and in situ hybridization of AMPs was performed.

RESULTS

IL-1α, IL-1β, IL-6, IL-8, IL-10, MCP-1, and tumor necrosis factor (TNF)-α mRNA increased after LPS exposure. MAP29, dodecapeptide, SBD1, and SBD2 mRNA were suppressed at 24 h. MAP29 and dodecapeptide mRNA then increased at 8 d. Lactoferrin increased at 24 h. There were no changes for HMGB1, HSP70, or RAGE. MAP29 and dodecapeptide localized to alveolar cells, increased 8 d after exposure to LPS.

CONCLUSION

AMPs are initially suppressed in the fetal lung by LPS-induced chorioamnionitis. The late induction of MAP29 and dodecapeptide may be related to lung repair.

Actin enables the antimicrobial action of LL-37 peptide in the presence of microbial proteases

Host defense peptides play an important host-protective role by their microcidal action, immunomodulatory functions, and tissue repair activities. Proteolysis is a common strategy of pathogens used to neutralize host defense peptides. Here, we show that actin, the most abundant structural protein in eukaryotes, binds the LL-37 host defense peptide, protects it from degradation by the proteases of Pseudomonas aeruginosa and Porphyromonas gingivalis, and enables its antimicrobial activity despite the presence of the proteases. Co-localization of LL-37 with extracellular actin was observed in necrotized regions of samples from oral lesions. Competition assays, cross-linking experiments, limited proteolysis, and mass spectrometry revealed that LL-37 binds by specific hydrophobic interactions to the His-40-Lys-50 segment of actin, located in the DNase I binding loop. The integrity of the binding site of both LL-37 and actin is a prerequisite to the binding. Our results demonstrate that actin, presumably released by dead cells and abundant in infected sites, might be utilized by the immune system to enhance spatio-temporal immunity in an attempt to arrest infection and control inflammation.

Hyaluronan digestion controls DC migration from the skin

The breakdown and release of hyaluronan (HA) from the extracellular matrix has been hypothesized to act as an endogenous signal of injury. To test this hypothesis, we generated mice that conditionally overexpressed human hyaluronidase 1 (HYAL1). Mice expressing HYAL1 in skin either during early development or by inducible transient expression exhibited extensive HA degradation, yet displayed no evidence of spontaneous inflammation. Further, HYAL1 expression activated migration and promoted loss of DCs from the skin. We subsequently determined that induction of HYAL1 expression prior to topical antigen application resulted in a lack of an antigenic response due to the depletion of DCs from the skin. In contrast, induction of HYAL1 expression concurrent with antigen exposure accelerated allergic sensitization. Administration of HA tetrasaccharides, before or simultaneously with antigen application, recapitulated phenotypes observed in HYAL1-expressing animals, suggesting that the generation of small HA fragments, rather than the loss of large HA molecules, promotes DC migration and subsequent modification of allergic responses. Furthermore, mice lacking TLR4 did not exhibit HA-associated phenotypes, indicating that TLR4 mediates these responses. This study provides direct evidence that HA breakdown controls the capacity of the skin to present antigen. These events may influence DC function in injury or disease and have potential to be exploited therapeutically for modification of allergic responses.

Allergic contact dermatitis in children: review of the past decade

Allergic contact dermatitis (ACD) is a type IV delayed hypersensitivity reaction. During the last decade, there has been a heightened awareness of this disease in the pediatric population. The gold standard for diagnosis is patch testing. The prevalence of positive patch tests in referred children with suspected ACD ranges from 27 to 95.6 %. The most common allergens in children in North America are nickel, neomycin, cobalt, fragrance, Myroxylon pereirae, gold, formaldehyde, lanolin/wool alcohols, thimerosal, and potassium dichromate. The relationship between ACD and atopic dermatitis (AD) is complicated with conflicting reports of prevalence in the literature; however, in a patient with dermatitis not responding to traditional therapies, or with new areas of involvement, ACD should be considered as part of the work-up.

Potential role of microorganisms in the pathogenesis of rosacea

Rosacea is a skin condition of abnormal inflammation and vascular dysfunction. The active contribution of a microbial agent in the development or progression of rosacea continues to be debated. Research supports the presence of commensal Demodex folliculorum mites at increased density in the skin and associates Helicobacter pylori infection of the gut with rosacea. Fewer studies implicate Staphylococcus epidermidis, Chlamydophila pneumoniae, and the Demodex-associated bacteria Bacillus oleronius. No research, however, provides a mechanism by which colonization by a microorganism translates to manifestation of the condition. Prevailing and emerging principles in the biology of the microbiome and the pathophysiology of rosacea may help to reconcile these lingering questions. Here the microorganisms implicated in rosacea are reviewed and the reaction of the microbiome to inflammation and to changes in microenvironments and macroenvironments are discussed to explain potential roles for microorganisms in rosacea pathophysiology.

A comprehensive summary of LL-37, the factotum human cathelicidin peptide

Cathelicidins are a group of antimicrobial peptides. Since their discovery, it has become clear that they are an exceptional class of peptides, with some members having pleiotropic effects. Not only do they possess an antibacterial, antifungal and antiviral function, they also show a chemotactic and immunostimulatory/-modulatory effect. Moreover, they are capable of inducing wound healing, angiogenesis and modulating apoptosis. Recent insights even indicate for a role of these peptides in cancer. This review provides a comprehensive summary of the most recent and relevant insights concerning the human cathelicidin LL-37.

Antimicrobial peptides in the pathogenesis of psoriasis

One characteristic abnormality of lesional skin in psoriasis is the excessive production of antimicrobial peptides and proteins (AMPs). AMPs typically are small (12-50 amino acids), have positive charge and amphipathic structure, and are found in all living organisms including mammals, insects, plants and invertebrates. These peptides are best known for their integral role in killing pathogenic microorganisms; however, in vertebrates, they are also capable of modifying host inflammatory responses by a variety of mechanisms. In psoriatic lesions, many AMPs are highly expressed, and especially the associations between psoriasis and cathelicidin, β-defensins or S100 proteins have been well studied. Among them, a cathelicidin peptide, LL-37, has been highlighted as a modulator of psoriasis development in recent years. AMPs had been thought to worsen psoriatic lesions but recent evidence has also suggested the possibility that the induction of AMPs expression might improve aspects of the disease. Further investigations are needed to uncover a previously underappreciated role for AMPs in modulating the immune response in psoriasis, and to improve disease without the risks of systemic immunosuppressive approaches.

Early immune events in the induction of allergic contact dermatitis

The skin is a barrier site that is exposed to a wide variety of potential pathogens. As in other organs, pathogens that invade the skin are recognized by pattern-recognition receptors (PRRs). Recently, it has been recognized that PRRs are also engaged by chemical contact allergens and, in susceptible individuals, this elicits an inappropriate immune response that results in allergic contact dermatitis. In this Review, we focus on how contact allergens promote inflammation by activating the innate immune system. We also examine how innate immune cells in the skin, including mast cells and dendritic cells, cooperate with each other and with T cells and keratinocytes to initiate and drive early responses to contact allergens.

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.

Mechanisms of chemical-induced innate immunity in allergic contact dermatitis

Allergic contact dermatitis (ACD) is one of the most prevalent occupational skin diseases and causes severe and long-lasting health problems in the case of chronification. It is initiated by an innate inflammatory immune response to skin contact with low molecular weight chemicals that results in the priming of chemical-specific, skin-homing CD8(+) Tc1/Tc17 and CD4(+) Th1/Th17 cells. Following this sensitization step, T lymphocytes infiltrate the inflamed skin upon challenge with the same chemical. The T cells then exert cytotoxic function and secrete inflammatory mediators to produce an eczematous skin reaction. The recent characterization of the mechanisms underlying the innate inflammatory response has revealed that contact allergens activate innate effector mechanisms and signalling pathways that are also involved in anti-infectious immunity. This emerging analogy implies infection as a potential trigger or amplifier of the sensitization to contact allergens. Moreover, new mechanistic insights into the induction of ACD identify potential targets for preventive and therapeutic intervention. We summarize here the latest findings in this area of research.

Extra domain A-positive fibronectin-positive feedback loops and their association with cutaneous inflammatory disease

Cutaneous inflammation can show Th1 or Th2 predominance, but the precise mechanisms by which such selectivity is determined are unknown. A recent study has demonstrated that Th1 cells, but not Th2 cells, produce an endogenous ligand for Toll-like receptor (TLR) 4, namely extradomain A+ fibronectin containing extra type III domain A (FnEDA+). As TLR4 stimulation leads to production of proinflammatory cytokines that recruit (via altered endothelial adhesion molecule expression and chemokine production) more Th1/Th17 cells, a positive feedback mechanism for Th1/Th17 inflammation exists. We propose that FnEDA+ positive feedback loops are a potential driver of Th1/Th17 inflammation. Conversely, the inflammatory EDA+ fibronectin loop is negatively regulated in atopic dermatitis, Th2 cytokines actively suppress TLR4 expression of Th1 cytokines, and recruited Th2 cells do not produce FnEDA+. In psoriasis, there are multiple FnEDA+ loops, comprising inflammatory, keratinocyte, and autoimmune loops. In allergic contact dermatitis, a single inflammatory loop operates. In atopic dermatitis, the FnEDA+ loop is actively suppressed by Th2 cytokines, and recruited Th2 cells do not "feedback" FnEDA+. We review endogenous ligands for TLR in relation to inflammatory disease, FnEDA+ function, and the potential role for FnEDA+ in psoriasis, allergic contact dermatitis, and atopic dermatitis.

Protecting the boundary: the sentinel role of host defense peptides in the skin

The skin is our primary shield against microbial pathogens and has evolved innate and adaptive strategies to enhance immunity in response to injury or microbial insult. The study of antimicrobial peptide (AMP) production in mammalian skin has revealed several of the elegant strategies that AMPs use to prevent infection. AMPs are inducible by both infection and injury and protect the host by directly killing pathogens and/or acting as multifunctional effector molecules that trigger cellular responses to aid in the anti-infective and repair response. Depending on the specific AMP, these molecules can influence cytokine production, cell migration, cell proliferation, differentiation, angiogenesis and wound healing. Abnormal production of AMPs has been associated with the pathogenesis of several cutaneous diseases and plays a role in determining a patient's susceptibility to pathogens. This review will discuss current research on the regulation and function of AMPs in the skin and in skin disorders.

Chlamydial infection increases gonococcal colonization in a novel murine coinfection model

Genital tract infections caused by Neisseria gonorrhoeae and Chlamydia trachomatis serovars D to K occur at high incidence in many areas of the world. Despite high rates of coinfection with these pathogens, investigations of host-parasite interactions have focused on each pathogen individually. We describe here a coinfection model in which female BALB/c mice were first infected with the mouse Chlamydia species C. muridarum and then inoculated with N. gonorrhoeae following treatment with water-soluble 17β-estradiol to promote long-term gonococcal infection. Viable gonococci and chlamydiae were recovered for an average of 8 to 10 days, and diplococci and chlamydial inclusions were observed in lower genital tract tissue by immunohistochemical staining. Estradiol treatment reduced proinflammatory cytokine and chemokine levels in chlamydia-infected mice; however, coinfected mice had a higher percentage of vaginal neutrophils compared to mice infected with either pathogen alone. We detected no difference in pathogen-specific antibody levels due to coinfection. Interestingly, significantly more gonococci were recovered from coinfected mice compared to mice infected with N. gonorrhoeae alone. We found no evidence that C. muridarum increases gonococcal adherence to, or invasion of, immortalized murine epithelial cells. However, increased vaginal concentrations of inflammatory mediators macrophage inflammatory protein 2 and tumor necrosis factor alpha were detected in C. muridarum-infected mice prior to inoculation with N. gonorrhoeae concurrently with the downregulation of cathelicidin-related antimicrobial peptide and secretory leukocyte peptidase inhibitor genes. We conclude that female mice can be successfully infected with both C. muridarum and N. gonorrhoeae and that chlamydia-induced alterations in host innate responses may enhance gonococcal infection.

Biological roles of host defense peptides: lessons from transgenic animals and bioengineered tissues

Host defense peptides (HDPs) have long been recognized as microbicidal agents, but their roles as modulators of innate and adaptive immunity have only more recently been appreciated. The study of transgenic animal and tissue models has provided platforms to improve our understanding of the immune modulatory functions of HDPs. Here, the characterization of transgenic animals or tissue models that over-express and/or are deficient for specific HDPs is reviewed. We also attempt to reconcile this data with evidence from human studies monitoring HDP expression at constitutive levels and/or in conjunction with inflammation, infection models, or disease states. We have excluded activities ascribed to HDPs derived exclusively from in vitro experiments. An appreciation of the way that HDPs promote innate immunity or influence the adaptive immune response is necessary in order to exploit their therapeutic or adjuvant potential and to open new perspectives in understanding the basis of immunity. The potential applications for HDPs are discussed.

Kallikrein expression and cathelicidin processing are independently controlled in keratinocytes by calcium, vitamin D(3), and retinoic acid

Cathelicidin has dual functions in the skin, acting as an innate antibiotic and as an immunomodulator in diseases such as rosacea and psoriasis. The serine proteases kallikrein 5 (KLK5) and kallikrein 7 (KLK7) control enzymatic processing of cathelicidin precursor in the skin and regulate the eventual function of the final forms of these peptides. We analyzed factors that control expression of KLK5 and KLK7 in normal human epidermal keratinocytes to better understand how these may influence cathelicidin processing and function. Increased extracellular calcium-induced KLK5 and KLK7 mRNA expression and protein release in a time-dependent manner that is similar to induction of differentiation markers such as keratin 10 and involucrin. However, 1,25(OH)(2) vitamin D(3), 9-cis retinoic acid (RA), and 13-cis RA also induced the KLKs, but the timing and pattern of KLK induction for each were different and distinct from changes in differentiation markers. Increased protease activity and differential processing of cathelicidin accompanied increased KLK expression. These findings show that the expression and activity of KLK are under fine control and can be distinctly influenced by variables such as differentiation, calcium, vitamin D, and RA. Thus, these variables may further control the functions of antimicrobial peptides in the skin.

Alarmins link neutrophils and dendritic cells

Neutrophils are the first major population of leukocyte to infiltrate infected or injured tissues and are crucial for initiating host innate defense and adaptive immunity. Although the contribution of neutrophils to innate immune defense is mediated predominantly by phagocytosis and killing of microorganisms, neutrophils also participate in the induction of adaptive immune responses. At sites of infection and/or injury, neutrophils release numerous mediators upon degranulation or death, among these are alarmins which have a characteristic dual capacity to mobilize and activate antigen-presenting cells. We describe here how alarmins released by neutrophil degranulation and/or death can link neutrophils to dendritic cells by promoting their recruitment and activation, resulting in the augmentation of innate and adaptive immune responses.

AMPed up immunity: how antimicrobial peptides have multiple roles in immune defense

Antimicrobial peptides (AMPs) are widely expressed and rapidly induced at epithelial surfaces to repel assault from diverse infectious agents including bacteria, viruses, fungi and parasites. Much information suggests that AMPs act by mechanisms that extend beyond their capacity to serve as gene-encoded antibiotics. For example, some AMPs alter the properties of the mammalian membrane or interact with its receptors to influence diverse cellular processes including cytokine release, chemotaxis, antigen presentation, angiogenesis and wound healing. These functions complement their antimicrobial action and favor resolution of infection and repair of damaged epithelia. Opposing this, some microbes have evolved mechanisms to inactivate or avoid AMPs and subsequently become pathogens. Thus, AMPs are multifunctional molecules that have a central role in infection and inflammation.