Cationic Host Defense Peptides Research Articles

A Novel Antimicrobial Peptide on the Ocular Surface Shows Decreased Expression in Inflammation and Infection

Antimicrobial peptides (AMPs) are cationic host defense peptides with microbicidal and cell-signaling properties. They show promise as potential therapeutic agents. In the present study, a beta-defensin AMP gene was isolated from the ocular surface for the first time, and its expression was characterized in the presence of ocular inflammation and/or infection. Total RNA was obtained from impression cytology samples of the conjunctiva and cornea of normal patients and of those with bacterial, viral, acanthamoeba, or dry eye disease. The expression of the beta-defensin AMP DEFB-109 was determined by using reverse transcription-polymerase chain reaction (RT-PCR). Relative quantification of the gene in the various groups was performed by means of real-time PCR. DEFB-109 was constitutively expressed in all samples. The gene showed significantly decreased expression in the presence of all types of inflammation/infection. Reduced expression featured most prominently in acanthamoeba infection; the least change from normal was in dry eye. The discovery of DEFB-109 on the ocular surface enhances our knowledge of the profile of AMPs at this important mucosal surface. The fact that its expression is significantly reduced in both inflammatory and infective ocular surface disease reflects not only an intimate balance between this host defense gene and microbes but indicates a role other than purely microbicidal. This discovery will enable the mechanisms behind the intriguing phenomenon of reduced gene expression of an AMP in disease states to be uncovered.

Host Defense Peptide LL-37, in Synergy with Inflammatory Mediator IL-1β, Augments Immune Responses by Multiple Pathways

The human cathelicidin LL-37 is a cationic host defense peptide and serves as an important component of innate immunity. It has been demonstrated to be a multifunctional modulator of innate immune responses, although the mechanism(s) underlying this have not been well characterized. In this study, it was demonstrated that LL-37 synergistically enhanced the IL-1beta-induced production of cytokines (IL-6, IL-10) and chemokines such as macrophage chemoattractant proteins (MCP-1, MCP-3) in human PBMC, indicating a role in enhancing certain innate immune responses. Similarly, LL-37 synergistically enhanced chemokine production in the presence of GM-CSF, but IFN-gamma, IL-4, or IL-12 addition led to antagonism, indicating that the role of LL-37 in reinforcing specific immune responses is selective and restricted to particular endogenous immune mediators. The inhibition of G protein-coupled receptors and PI3K substantially suppressed the ability of IL-1beta and LL-37 to synergistically enhance the production of chemokine MCP-3. Consistent with this, the combination of IL-1beta and LL-37 enhanced the activation/phosphorylation of kinase Akt and the transcription factor CREB. The role of transcription factor NF-kappaB was revealed through the demonstration of enhanced phosphorylation of IkappaBalpha and the consequent nuclear translocation of NF-kappaB subunits p50 and p65, as well as the antagonistic effects of an inhibitor of IkappaBalpha phosphorylation. These results together indicate that the human host defense peptide LL-37 can work in synergy with the endogenous inflammatory mediator IL-1beta to enhance the induction of specific inflammatory effectors by a complex mechanism involving multiple pathways, thus reinforcing certain innate immune responses.

Failures in Clinical Treatment of Staphylococcus aureus Infection with Daptomycin Are Associated with Alterations in Surface Charge, Membrane Phospholipid Asymmetry, and Drug Binding

Increasingly frequent reports have described the in vivo loss of daptomycin susceptibility in association with clinical treatment failures. The mechanism(s) of daptomycin resistance is not well understood. We studied an isogenic set of Staphylococcus aureus isolates from the bloodstream of a daptomycin-treated patient with recalcitrant endocarditis in which serial strains exhibited decreasing susceptibility to daptomycin. Since daptomycin is a membrane-targeting lipopeptide, we compared a number of membrane parameters in the initial blood isolate (parental) with those in subsequent daptomycin-resistant strains obtained during treatment. In comparison to the parental strain, resistant isolates demonstrated (i) enhanced membrane fluidity, (ii) increased translocation of the positively charged phospholipid lysyl-phosphotidylglycerol to the outer membrane leaflet, (iii) increased net positive surface charge (P < 0.05 versus the parental strain), (iv) reduced susceptibility to daptomycin-induced depolarization, permeabilization, and autolysis (P < 0.05 versus the parental strain), (v) significantly lower surface binding of daptomycin (P < 0.05 versus the parental strain), and (vi) increased cross-resistance to the cationic antimicrobial host defense peptides human neutrophil peptide 1 (hNP-1) and thrombin-induced platelet microbicidal protein 1 (tPMP-1). These data link distinct changes in membrane structure and function with in vivo development of daptomycin resistance in S. aureus. Moreover, the cross-resistance to hNP-1 and tPMP-1 may also impact the capacity of these daptomycin-resistant organisms to be cleared from sites of infection, particularly endovascular foci.

Cathelicidins and functional analogues as antisepsis molecules

The emergence of antibiotic-resistant bacteria together with the limited success of sepsis therapeutics has lead to an urgent need for the development of alternative strategies for the treatment of systemic inflammatory response syndrome and related disorders. Immunomodulatory compounds that do not target the pathogen directly (therefore limiting the development of pathogen resistance), and target multiple inflammatory mediators, are attractive candidates as novel therapeutics. Cationic host defence peptides such as cathelicidins have been demonstrated to be selectively immunomodulatory in that they can confer anti-infective immunity and modulate the inflammatory cascade through multiple points of intervention. The human cathelicidin LL-37, for example, has modest direct antimicrobial activity under physiological conditions, but has been demonstrated to have potent antiendotoxin activity in animal models, as well as the ability to resolve certain bacterial infections. A novel synthetic immunomodulatory peptide, IDR-1, built on this same theme has no direct antimicrobial activity, but is effective in restricting many types of infection, while limiting pro-inflammatory responses. The ability of these peptides to selectively suppress harmful pro-inflammatory responses, while maintaining beneficial infection-fighting components of host innate defences makes them a good model for antisepsis therapies that merit further investigation.

Structural and functional characterization of fowlicidin-1, a cationic host defense peptide with potent antibacterial and LPS-neutralizing activities (52.13)

Abstract Cathelicidins are a group of cationic host defense peptides with pluripotent immunomodulatory activities. We recently identified fowlicidin-1, a chicken cathelicidin possessing broad-spectrum antibacterial and LPS-neutralizing activities but with considerable toxicity toward mammalian cells. NMR spectroscopy revealed that fowlicidin-1 mainly adopts an α-helical conformation with glycine-induced kink in the center and a short flexible unstructured region at the N-terminal end. To reveal functional significance of different structural regions and identify peptide analog(s) with enhanced therapeutic potential, a series of deletion and substitution analogs of fowlicidin -1 were synthesized. Eight amino acids (residues 16–23) at the C-terminal end were found to be critically involved in bacterial killing, LPS binding and cytotoxicity. We also revealed that the N-terminal unstructured segment (residues 1–5) plays a minimum role in antibacterial and LPS-neutralizing activities. Removal of valine at position 5 reduced the cytotoxicity by 4-fold while retaining a similar antibacterial potency to the parent peptide. However, omission of tryptophan at position 6 resulted in at least 4-fold reduction in bacteria-killing efficiency, indicating the critical involvement of tryptophan. Taken together, fowlicidin-1(6–26) represents a short peptide analog with excellent potential to be explored as a novel antimicrobial. This project was supported by Oklahoma Center for the Advancement of Science and Technology, Oklahoma Agricultural Experiment Station and HHMI-Undergraduate Research Scholarship.

Cationic host defence peptides: innate immune regulatory peptides as a novel approach for treating infections.

An increase in antibiotic resistance and the emergence of new pathogens has led to an urgent need for alternative approaches to infection management. Immunomodulatory molecules that do not target the pathogen directly, but rather selectively enhance and/or alter host defence mechanisms, are attractive candidates for therapeutic development. Natural cationic host defence peptides represent lead molecules that boost innate immune responses and selectively modulate pathogen-induced inflammatory responses. This review discusses recent evidence exploring the mechanisms of cationic host defence peptides as innate immune regulators, their role in the interface of innate and adaptive immunity, and their potential application as beneficial therapeutics in overcoming infectious diseases.

Fowlicidin‐3 is an α‐helical cationic host defense peptide with potent antibacterial and lipopolysaccharide‐neutralizing activities

Cathelicidins are an important family of cationic host defense peptides in vertebrates with both antimicrobial and immunomodulatory activities. Fowlicidin-1 and fowlicidin-2 are two newly identified chicken cathelicidins with potent antibacterial activities. Here we report structural and functional characterization of the putatively mature form of the third chicken cathelicidin, fowlicidin-3, for exploration of its therapeutic potential. NMR spectroscopy revealed that fowlicidin-3 comprises 27 amino-acid residues and adopts a predominantly alpha-helical structure extending from residue 9 to 25 with a slight kink induced by a glycine at position 17. It is highly potent against a broad range of Gram-negative and Gram-positive bacteria in vitro, including antibiotic-resistant strains, with minimum inhibitory concentrations in the range 1-2 microM. It kills bacteria quickly, permeabilizing cytoplasmic membranes immediately on coming into contact with them. Unlike many other host defense peptides with antimicrobial activities that are diminished by serum or salt, fowlicidin-3 retains bacteria-killing activities in the presence of 50% serum or physiological concentrations of salt. Furthermore, it is capable of suppressing lipopolysaccharide-induced expression of proinflammatory genes in mouse macrophage RAW264.7 cells, with nearly complete blockage at 10 microM. Fowlicidin-3 appears to be an excellent candidate for future development as a novel antimicrobial and antisepsis agent, particularly against antibiotic-resistant pathogens.

Antimicrobial and host-defense peptides as new anti-infective therapeutic strategies

Short cationic amphiphilic peptides with antimicrobial and/or immunomodulatory activities are present in virtually every life form, as an important component of (innate) immune defenses. These host-defense peptides provide a template for two separate classes of antimicrobial drugs. Direct-acting antimicrobial host-defense peptides can be rapid-acting and potent, and possess an unusually broad spectrum of activity; consequently, they have prospects as new antibiotics, although clinical trials to date have shown efficacy only as topical agents. But for these compounds to fulfill their therapeutic promise and overcome clinical setbacks, further work is needed to understand their mechanisms of action and reduce the potential for unwanted toxicity, to make them more resistant to protease degradation and improve serum half-life, as well as to devise means of manufacturing them on a large scale in a consistent and cost-effective manner. In contrast, the role of cationic host-defense peptides in modulating the innate immune response and boosting infection-resolving immunity while dampening potentially harmful pro-inflammatory (septic) responses gives these peptides the potential to become an entirely new therapeutic approach against bacterial infections.

Bovine and human cathelicidin cationic host defense peptides similarly suppress transcriptional responses to bacterial lipopolysaccharide

Genomic approaches can be exploited to expose the complexities and conservation of biological systems such as the immune network across various mammalian species. In this study, temporal transcriptional expression profiles were analyzed in human and bovine monocytic cells in response to the TLR-4 agonist, LPS, in the presence or absence of their respective host defense peptides. The cathelicidin peptides, human LL-37 and bovine myeloid antimicrobial peptide-27 (BMAP-27), are homologs, yet they have diverged notably in terms of sequence similarity. In spite of their low sequence similarities, both of these cathelicidin peptides demonstrated potent, antiendotoxin activity in monocytic cells at low, physiologically relevant concentrations. Microarray studies indicated that 10 ng/ml LPS led to the up-regulation of 125 genes in human monocytes, 106 of which were suppressed in the presence of 5 mug/ml of the human peptide LL-37. To confirm and extend these data, temporal transcriptional responses to LPS were assessed in the presence or absence of the species-specific host defense peptides by quantitative real-time PCR. The transcriptional trends of 20 LPS-induced genes were analyzed in bovine and human monocytic cells. These studies demonstrated conserved trends of gene responses in that both peptides were able to profoundly suppress many LPS-induced genes. Consistent with this, the human and bovine peptides suppressed LPS-induced translocation of NF-kappaB subunits p50 and p65 into the nucleus of monocytic cells. However, there were also distinct differences in responses to LPS and the peptides; for example, treatment with 5 mug/ml BMAP-27 alone tended to influence gene expression (RELA, TNF-alpha-induced protein 2, MAPK phosphatase 1/dual specificity phosphatase 1, IkappaBkappaB, NFkappaBIL1, TNF receptor-associated factor 2) to a greater extent than did the same amount of human LL-37. We hypothesize that the immunomodulatory effects of the species-specific host defense peptides play a critical role in regulating inflammation and represent an evolutionarily conserved mechanism for maintaining homeostasis, although the sequence divergence of these peptides is substantial.

The human cationic host defense peptide LL-37 mediates contrasting effects on apoptotic pathways in different primary cells of the innate immune system

The human cathelicidin LL-37 is a cationic host defense peptide (antimicrobial peptide) expressed primarily by neutrophils and epithelial cells. This peptide, up-regulated under conditions of inflammation, has immunomodulatory and antimicrobial functions. We demonstrate that LL-37 is a potent inhibitor of human neutrophil apoptosis, signaling through P2X(7) receptors and G-protein-coupled receptors other than the formyl peptide receptor-like-1 molecule. This process involved modulation of Mcl-1 expression, inhibition of BID and procaspase-3 cleavage, and the activation of phosphatidylinositol-3 kinase but not the extracellular signal-regulated kinase 1/2 mitogen-activated protein kinase pathway. In contrast to the inhibition of neutrophil apoptosis, LL-37 induced apoptosis in primary airway epithelial cells, demonstrating alternate consequences of LL-37-mediated modulation of apoptotic pathways in different human primary cells. We propose that these novel immunomodulatory properties of LL-37 contribute to peptide-mediated enhancement of innate host defenses against acute infection and are of considerable significance in the development of such peptides and their synthetic analogs as potential therapeutics for use against multiple antibiotic-resistant infectious diseases.

HIV-1 Adapts to a Retrocyclin with Cationic Amino Acid Substitutions That Reduce Fusion Efficiency of gp41

Retrocyclin (RC)-101 is a cationic theta-defensin that inhibits HIV-1 entry. Passaging HIV-1(BAL) under selective pressure by this cyclic minidefensin resulted in only a 5- to 10-fold decrease in viral susceptibility to RC-101. Emergent viral isolates had three amino acid substitutions in their envelope glycoprotein. One was in a CD4-binding region of gp120, and the others were in the heptad repeat (HR) domains of gp41 (HR1 and HR2). Each mutation replaced an electroneutral or electronegative residue with one that was positively charged. These mutations were evaluated either alone or in combination in a single-round viral entry assay. Although the mutation in gp120 did not affect viral entry, the mutation in HR1 of gp41 conferred relative resistance to RC-101. Interestingly, the envelope with the HR2 mutation was less efficient and became codependent on the presence of RC-101 for entry. The adaptive response of HIV-1 to this cationic host defense peptide resembles the responses of bacteria that modulate their surface or membrane charge to evade analogous host defense peptides. These findings also suggest that interactions between theta-defensins and gp41 may contribute to the ability of these cyclic minidefensins to prevent HIV-1 entry into target cells.

The Cationic Host Defense Peptide rCRAMP Promotes Gastric Ulcer Healing in Rats

Cathelicidin, a cationic host defense peptide, has been shown to promote cutaneous wound repair and reaches high levels in the gastric mucosa during infection and inflammation. Therefore, we investigated whether this peptide contributes to gastric ulcer healing in rats. Ulcer induction increased the expression of rat cathelicidin rCRAMP in the gastric mucosa. Further increase in expression of rCRAMP by local injection of rCRAMP-encoding plasmid promoted ulcer healing by enhancing cell proliferation and angiogenesis. rCRAMP directly stimulated proliferation of cultured rat gastric epithelial cells (RGM-1), which was abolished by inhibitors of matrix metalloproteinase (MMP), epidermal growth factor receptors (EGFR) tyrosine kinase, or mitogen-activated protein kinase/extracellular signal-regulated kinase (ERK) kinase. rCRAMP also increased EGFR and ERK1/2 phosphorylation via an MMP-dependent mechanism. Knockdown of transforming growth factor alpha (TGFalpha), which is a ligand of EGFR, by small interfering RNA completely nullified the mitogenic signals evoked by rCRAMP in RGM-1 cells. These findings suggest that rCRAMP exhibits prohealing activity in stomachs through TGFalpha-dependent transactivation of EGFR and its related signaling pathway to induce proliferation of gastric epithelial cells.

Antimicrobial peptides: therapeutic potential

A significant component of the innate immune system of a wide variety of animals and plants is arbitrated by cationic host defence peptides. In man, these peptides, in addition to exhibiting a direct antimicrobial activity, seems to provide a range of non-antimicrobial bioactivities related to defence, inflammation and wound healing. Despite the fact that such peptides have so far failed to reach the market, there are continued initiatives to advance such potential therapeutics to, and through, the clinic. The reasons behind such initiatives include: reduced manufacturing costs for peptides; allowing entry into therapeutic areas previously inaccessible due to cost; the continued identification of previously unknown bioactivities of such peptides; and the resurgence of interest in peptide therapeutics. As a result, clinical programmes based on cationic host defence peptides exist in the areas of infection, dermatology, cancer and inflammation. The probability of clinical success for host defence peptide-based therapeutics is on the rise as options for a wider range of clinical indications emerge.

Apoptosis of airway epithelial cells: human serum sensitive induction by the cathelicidin LL-37.

LL-37 is a human cationic host defense peptide that is present in the specific granules of neutrophils, produced by epithelial cells from a variety of tissues, and is upregulated during inflammation, infection, and injury. It has been proposed to have a variety of antimicrobial functions, including both direct antimicrobial activity and immunomodulatory functions. Using the TUNEL assay it was demonstrated that LL-37 induced apoptosis in vitro in the A549 human lung and 16 HBE4o- human airway epithelial cell lines, and in vivo in the murine airway. Peptide-induced apoptosis in vitro involved the activation of caspase pathways and was substantially inhibited by an inhibitor of caspase 3. Apoptosis was also inhibited by human serum, but not fetal bovine serum. Similarly, human but not fetal bovine serum inhibited the cellular internalization of LL-37 and the production of IL-8 in response to LL-37 treatment of epithelial cells. The protective effects of human serum were also observed with high-density lipoproteins but not by the core peptide apolipoprotein A1, providing one possible mechanism of human serum inhibition of apoptosis. We propose that LL-37-induced apoptosis of epithelial cells at low serum tissue sites may have a protective role against bacterial infection.

Function and therapeutic potential of host defence peptides

Cationic host defence (antimicrobial) peptides are an important component of the innate immune systems of a wide variety of plants, animals, and bacteria. Although most of these compounds have direct antimicrobial activities under specific conditions, a greater appreciation for the diversity of functions of these molecules is beginning to develop in the field. In addition to their directly antimicrobial activities, they also have a broad spectrum of activity on the host immune system, with both pro-inflammatory and anti-inflammatory effects being invoked. Increasingly sophisticated approaches to understand the role of host defence peptides in modulating innate immunity are already serving to guide the development of novel therapeutics.

Anti-endotoxin properties of cationic host defence peptides and proteins

The innate immune system of mammals contains a series of peptides with overall positive charge and an amphipathic structure which have a variety of important properties in host defences. Although these are often termed cationic antimicrobial peptides, they have numerous roles in innate defences in all complex species of life and thus we prefer to refer to them as host defence peptides. These roles include: (i) an ability to kill micro-organisms directly, ranging from bacteria to viruses, fungi, parasites and helminths; (ii) an adjuvant activity in the adaptive response; and (iii) a multiplicity of roles in modulating innate immunity, including an apparent ability to stimulate protective innate immunity while suppressing harmful inflammatory/septic responses. This latter property may be one of the more important activities of these peptides in vivo. Innate immunity is thought to be triggered by the interaction of conserved bacterial components with particular receptors including Toll-like receptors (TLRs) on host cells. However, the initiation of the innate immune response through this route may trigger a pro-inflammatory cascade that is the principle cause of harmful conditions such as sepsis. Since we are exposed to potentially dangerous pathogens on a daily basis, the host response must contain certain checks and balances. We propose that host defence peptides have a role in feed-back modulation of inflammation under normal (low-pathogen exposure) conditions. This review surveys the available information regarding the anti-endotoxic/anti-inflammatory properties of host defence peptides, and will address whether this potential might be exploited for therapeutic benefit in sepsis.

Anti-endotoxin properties of cationic host defence peptides and proteins

The innate immune system of mammals contains a series of peptides with overall positive charge and an amphipathic structure which have a variety of important properties in host defences. Although these are often termed cationic antimicrobial peptides, they have numerous roles in innate defences in all complex species of life and thus we prefer to refer to them as host defence peptides. These roles include: (i) an ability to kill micro-organisms directly, ranging from bacteria to viruses, fungi, parasites and helminths; (ii) an adjuvant activity in the adaptive response; and (iii) a multiplicity of roles in modulating innate immunity, including an apparent ability to stimulate protective innate immunity while suppressing harmful inflammatory/septic responses. This latter property may be one of the more important activities of these peptides in vivo. Innate immunity is thought to be triggered by the interaction of conserved bacterial components with particular receptors including Toll-like receptors (TLRs) on host cells. However, the initiation of the innate immune response through this route may trigger a pro-inflammatory cascade that is the principle cause of harmful conditions such as sepsis. Since we are exposed to potentially dangerous pathogens on a daily basis, the host response must contain certain checks and balances. We propose that host defence peptides have a role in feed-back modulation of inflammation under normal (low-pathogen exposure) conditions. This review surveys the available information regarding the antiendotoxic/anti-inflammatory properties of host defence peptides, and will address whether this potential might be exploited for therapeutic benefit in sepsis.

Immunomodulatory Activities of Small Host Defense Peptides

Recent studies have demonstrated that in addition to their antimicrobial activity, cationic host defense peptides, like the human cathelicidin LL-37, perform many activities relating to innate immunity, including the induction or modulation of chemokine and cytokine production, alteration of gene expression in host cells, and inhibition of proinflammatory responses of host cells to bacterial components such as lipopolysaccharide (LPS) in vitro and in vivo. To investigate if these properties are shared by smaller peptides, two cathelicidin peptides derived from bovine neutrophils, the 13-mer indolicidin and Bac2A, a linear 12-amino-acid derivative of bactenecin, were compared to the 37-amino-acid peptide LL-37. Indolicidin, like LL-37, inhibited LPS-induced tumor necrosis factor alpha (TNF-alpha) secretion, even when added up to an hour after the addition of Escherichia coli O111:B4 LPS to the human macrophage/monocyte-like THP-1 cell line. In contrast, Bac2A demonstrated no significant antiendotoxin activity. At low concentrations, indolicidin and LL-37 acted synergistically to suppress LPS-induced production of TNF-alpha. Indolicidin was analogous to LL-37 in its ability to induce the production of the chemokine interleukin-8 (IL-8) in a human bronchial cell line, 16HBE14o(-), but it was unable to induce production of IL-8 in THP-1 cells. In contrast, Bac2A was unable to induce IL-8 in either cell type. Conversely, Bac2A was chemotactic for THP-1 cells at concentrations between 10 and 100 mug/ml, while indolicidin and LL-37 were not chemotactic at these concentrations for THP-1 cells. This indicates that in addition to the potential for direct microbicidal activity, cationic host defense peptides may have diverse and complementary abilities to modulate the innate immune response.

Interaction and Cellular Localization of the Human Host Defense Peptide LL-37 with Lung Epithelial Cells

LL-37 is a human cationic host defense peptide that is an essential component of innate immunity. In addition to its modest antimicrobial activity, LL-37 affects the gene expression and behavior of effector cells involved in the innate immune response, although its mode of interaction with eukaryotic cells remains unclear. The interaction of LL-37 with epithelial cells was characterized in tissue culture by using biotinylated LL-37 and confocal microscopy. It was demonstrated that LL-37 was actively taken up into A549 epithelial cells and eventually localized to the perinuclear region. Specific inhibitors were used to demonstrate that the uptake process was not mediated by actin but required elements normally involved in endocytosis and that trafficking to the perinuclear region was dependent on microtubules. By using nonlinear regression analysis, it was revealed that A549 epithelial cells have two receptors for LL-37B, with high and low affinity for LL-37, respectively. These results indicate the mode of interaction of LL-37 with epithelial cells and further our understanding of its role in modulating the innate immune response.

Impact of LL-37 on anti-infective immunity

Host defense peptides (often called cationic antimicrobial peptides) have pleiotropic immunomodulatory functions. The human host defense peptide LL-37 is up-regulated at sites of infection and has little or no antimicrobial activity in tissue-culture media but under the same conditions, demonstrates immunomodulatory effects on epithelial cells, monocytes, and dendritic cells (DC). These effects include the induction of chemokine production in a mitogen-activated protein kinase-dependent manner in epithelial cell lines and monocytes and profound alterations of DC differentiation, resulting in the capacity to enhance a T helper cell type 1 response. Although the exact mechanisms of interaction between LL-37 and these cell types have not been elucidated, there is evidence for specific (i.e., receptor-mediated) and nonspecific interactions. The relative significance of the direct antimicrobial activities and immunomodulatory properties of LL-37 and other cationic host defense peptides in host defense remains unresolved. To demonstrate that antimicrobial activity was not necessarily required for protection in vivo, model peptides were synthesized and tested for antimicrobial and immunomodulatory activities. A peptide with no antimicrobial activity was found to be protective in animal models of Staphylococcus aureus and Salmonella infection, implying that a host defense peptide can protect by exerting immunomodulatory properties.