Theta-defensins (θ-defensins) are macrocyclic antimicrobial peptides expressed in leukocytes of Old World monkeys. The peptides are broad spectrum microbicides in vitro and numerous θ-defensin isoforms have been identified in granulocytes of rhesus macaques and Olive baboons. Several mammalian α- and β-defensins, genetically related to θ-defensins, have proinflammatory and immune-activating properties that bridge innate and acquired immunity. In the current study we analyzed the immunoregulatory properties of rhesus θ-defensins 1–5 (RTDs 1–5). RTD-1, the most abundant θ-defensin in macaques, reduced the levels of TNF, IL-1α, IL-1β, IL-6, and IL-8 secreted by blood leukocytes stimulated by several TLR agonists. RTDs 1–5 suppressed levels of soluble TNF released by bacteria- or LPS-stimulated blood leukocytes and THP-1 monocytes. Despite their highly conserved conformation and amino acid sequences, the anti-TNF activities of RTDs 1–5 varied by as much as 10-fold. Systemically administered RTD-1 was non-toxic for BALB/c mice, and escalating intravenous doses were well tolerated and non-immunogenic in adult chimpanzees. The peptide was highly stable in serum and plasma. Single dose administration of RTD-1 at 5 mg/kg significantly improved survival of BALB/c mice with E. coli peritonitis and cecal ligation-and-puncture induced polymicrobial sepsis. Peptide treatment reduced serum levels of several inflammatory cytokines/chemokines in bacteremic animals. Collectively, these results indicate that the anti-inflammatory properties of θ-defensins in vitro and in vivo are mediated by the suppression of numerous proinflammatory cytokines and blockade of TNF release may be a primary effect.

The alpha-defensins are not produced by CD8 cells but unexpectedly were found to be expressed in monocytes. alpha-Defensins can have anti-HIV activity but are not CD8 cell antiviral factors.

Peptidoglycan (PGN) recognition proteins (PGRPs) are pattern recognition molecules of innate immunity that are conserved from insects to humans. Various PGRPs are reported to have diverse functions: they bind bacterial molecules, digest PGN, and are essential to the Toll pathway in Drosophila. One family member, bovine PGN recognition protein-S (bPGRP-S), has been found to bind and kill microorganisms in a PGN-independent manner, raising questions about the identity of the bPGRP-S ligand. Addressing this, we have determined the binding and microbicidal properties of bPGRP-S in a range of solutions approximating physiologic conditions. In this study we show that bPGRP-S interacts with other bacterial components, including LPS and lipoteichoic acid, with higher affinities than for PCP, as determined by their abilities to inhibit bPGRP-S-mediated killing of bacteria. Where and how PGRPs act in vivo is not yet clear. Using Immunogold electron microscopy, PGRP-S was localized to the dense/large granules of naive neutrophils, which contain the oxygen-independent bactericidal proteins of these cells, and to the neutrophil phagolysosome. In addition, Immunogold staining and secretion studies demonstrate that neutrophils secrete PGRP-S when exposed to bacteria. Bovine PGRP-S can mediate direct lysis of heat-killed bacteria; however, PGRP-S-mediated killing of bacteria is independent of this activity. Evidence that bPGRP-S has multiple activities and affinity to several bacterial molecules challenges the assumption that the PGRP family of proteins recapitulates the evolution of TLRs. Mammalian PGRPs do not have a single antimicrobial activity against a narrow range of target organisms; rather, they are generalists in their affinity and activity.

Rhesus -defensin 1 (RTD-1) is a unique tridisulfide, cyclic antimicrobial peptide formed by the ligation of two 9-residue sequences derived from heterodimeric splicing of similar 76-amino acid, ␣-defensin-related precursors, termed RTD1a and RTD1b (Tang, Y. Q., Yuan, J., Osapay, G., Osapay, K., Tran, D., Miller, C. J., Ouellette, A. J., and Selsted, M. E. (1999) Science 286, 498 -502). The structures of RTD-2 and RTD-3 were predicted to exist if homodimeric splicing of the RTD1a and RTD1b occurs in vivo. Western blotting disclosed the presence of putative -defensins, distinct from RTD-1, in leukocyte extracts. Two new -defensins, RTD-2 and RTD-3, were purified by reverse-phase high performance liquid chromatography and characterized by amino acid analysis, matrix-assisted laser desorption/ionization time-offlight mass spectroscopy, and comparison to the synthetic standards. RTD-2 and RTD-3 are the predicted homodimeric splicing products of RTD1b and RTD1a, respectively. The cellular abundances of RTD-1, -2, and -3 were 29:1:2, indicating that there is a preference for the heterodimeric ligation that generates RTD-1. RTD-1, -2, and -3 had similar antimicrobial activities against Staphylococcus aureus, Candida albicans, and Cryptococcus neoformans, whereas the activity of RTD-2 against Escherichia coli was 2-3-fold less than those of RTD-1 and RTD-3. Equal amounts of each -defensin bound to E. coli cells, indicating that the differences in antibacterial activities are the result of post-binding processes.Antimicrobial peptides are essential components of the innate immune system (1-4). They play a significant role at the epithelial defense barrier (5-9) and as the antibacterial arsenals in neutrophils and macrophages (10 -13). In mammals, defensins and cathelicidins are the two major antimicrobial peptide families (9,11,14). Cathelicidins are heterogeneous peptides that share homology in the proregion with cathelin (14). ␣-and ␤-defensins are highly conserved tridisulfide peptides from two genetically distinct families (15, 16). In humans, there are four neutrophilic ␣-defensins, HNP-1-4 (13); two enteric ␣-defensins, HD-5 and HD-6 (17); and four epithelial ␤-defensins, h␤D-1-4 (18 -22). Although h␤D-1 is constitutively expressed in epithelia, the expression of h␤D-2, -3, and -4 is inducible by inflammatory cytokines (23) or bacterial infection (18,21).The characterization of the host defense components of Rhesus macaque granulocytes disclosed two distinct subfamilies of ␣-defensins (24) and a new tridisulfide peptide termed rhesus -defensin 1 (RTD-1) 1 (25). RTD-1 is a macrocyclic 18-amino acid antimicrobial peptide formed by the ligation of two 9-residue sequences derived from similar 76-amino acid, ␣-defensinrelated precursors, termed RTD1a and RTD1b (25). RTD-1 shares some structural similarities with the pig neutrophil protegrins and the horseshoe crab tachyplesins (25-27). The cyclic structure of RTD-1 is an important determinant for microbicidal potency and resistance to the inhibitory effect of physiologic...

Rhesus macaque -defensins (RTDs) are unique macrocyclic antimicrobial peptides. The three RTDs (RTD 1-3), isolated from macaque leukocytes, have broad-spectrum antimicrobial activities in vitro and share certain structural features with acyclic porcine protegrins, which are microbicidal peptides of the cathelicidin family. To understand the structural features that confer the respective cytocidal properties to -defensins and protegrins, we determined and compared the biological properties of RTD 1-3 and protegrin 1 (PG-1) in assays for antimicrobial activity, bacterial membrane permeabilization, and toxicity to human cells. RTD 1-3 and PG-1 had similar microbicidal potencies against Escherichia coli, Staphylococcus aureus, and Candida albicans in low-ionic-strength (10 mM) buffers at pH 7.4. The inclusion of physiologic sodium chloride partially inhibited the microbicidal activities of the RTDs, and the degree of inhibition depended on the buffer used in the assay. Similarly, the inclusion of 10% normal human serum partially antagonized the bactericidal activities of all four peptides. In contrast, the microbicidal activities of PG-1 and RTD 1-3 against E. coli were unaffected by physiologic concentrations of calcium chloride and magnesium chloride. Treatment of E. coli ML35 cells with RTD 1-3 or PG-1 rapidly rendered the bacteria permeable to -nitrophenyl-␤-D-galactopyranoside, and this was accompanied by the rapid entry of the RTDs. Finally, although PG-1 was toxic to human fibroblasts and caused a marked lysis of erythrocytes, the RTDs were not cytotoxic or hemolytic. Thus, compared to PG-1, RTD 1-3 possess substantially greater cytocidal selectivity against microbes. Surprisingly, the low cytotoxicity of the RTDs did not depend on the peptides' cyclic conformation.

-Defensins are macrocyclic antimicrobial peptides that were previously isolated from leukocytes of a single species, the rhesus macaque. We now report the characterization of baboon -defensins (BTDs) expressed in bone marrow and peripheral blood leukocytes. Four cDNAs encoding -defensin precursors were characterized, allowing for the prediction of 10 theoretical -defensins (BTD-1 to BTD-10) produced by binary, headto-tail splicing of nonapeptides excised from paired precursors. Five of the predicted -defensins were purified from baboon leukocytes, and synthetic versions of each were prepared. Anti--defensin antibody localized the peptides in circulating neutrophils and monocytes and in immature and mature myeloid elements in bone marrow. Each of the BTDs possessed antimicrobial activity against bacterial and fungal test organisms in vitro. Peptide activities varied markedly despite a high degree of sequence conservation among the -defensins tested. Thus, baboons express numerous -defensins which appear to differentially contribute to host defense against diverse pathogens.Antimicrobial peptides (AMPs) are effectors of the innate immune system. AMPs are expressed in cells (epithelia, neutrophils, and macrophages) that come into contact with potentially invasive microorganisms (17). In mammals, the two major classes of AMPs are defensins and cathelicidins. Cathelicidins are characterized by a conserved cathelin prodomain which lies N terminal to highly variable mature peptides that are released by activating proteases (27). Defensins are small, cationic peptides that are composed of three structural subclasses, ␣-, ␤-, and -defensins, differentiated by the spacing and pairing of their six disulfide-bonded cysteines (7, 9, 18). -Defensins are further distinguished by their macrocyclic backbone and as such represent the only known cyclic protein motif expressed in animals (16).The biosynthesis of -defensins requires head-to-tail splicing of two 9-amino-acid sequences derived from -defensin precursors (16). -Defensins were first identified in neutrophils and monocytes of the rhesus monkey (21). Subsequently, Nguyen et al. (15) conducted a phylogenetic survey that revealed the existence of -defensin genes in other Old World monkeys and two apes (the siamang and orangutan), but none in New World monkeys or prosimians. Humans, chimpanzees, bonobos, and gorillas express -defensin pseudogenes in which the precursor mRNA contains a mutation producing a stop codon in the signal sequence, thus preventing translation of the -defensin precursor (15).Rhesus -defensin-1 (RTD-1) is produced from the heterodimeric splicing of two -defensin precursors, proRTD1a and proRTD1b. Homodimeric excision/ligation reactions involving proRTD1a and proRTD1b were revealed by the isolation of RTD-2 and RTD-3 (12, 23). RTD-1, -2, and -3 have potent microbicidal activities against bacteria and fungi (23) and have antiviral activities against human immunodeficiency virus type 1 (3, 24) and herpes simplex virus (26). A synthetic -defensin designed b...

Mammalian defensins are cationic, antimicrobial peptides that play a central role in innate immunity. The peptides are composed of three structural subfamilies: α-, β-, and θ-defensins. θ-defensins are macrocyclic octadecapeptides expressed only in Old World monkeys and orangutans and are produced by the pair-wise, head-to-tail splicing of nonapeptides derived from their respective precursors. The existence of three active θ-defensin genes predicts that six different RTDs (1-6) are produced in this species. In this study, we isolated and quantified RTDs 1-6 from the neutrophils of 10 rhesus monkeys. RTD-1 was the most abundant θ-defensin, constituting ~50% of the RTD content; total RTD content varied by as much as threefold between animals. All peptides tested were microbicidal at ∼1 μM concentrations. The contribution of θ-defensins to macaque neutrophil antimicrobial activity was assessed by analyzing the microbicidal properties of neutrophil granule extracts after neutralizing θ-defensin content with a specific antibody. θ-defensin neutralization markedly reduced microbicidal activities of the corresponding extracts. Macaque neutrophil granule extracts had significantly greater microbicidal activity than those of human neutrophils, which lack θ-defensins. Supplementation of human granule extracts with RTD-1 markedly increased the microbicidal activity of these preparations, further demonstrating a prominent microbicidal role for θ-defensins.

Invasive candidiasis is an increasingly frequent cause of serious and often fatal infections in hospitalized and immunosuppressed patients. Mortality rates associated with these infections have risen sharply due to the emergence of multidrug resistant (MDR) strains of C. albicans and other Candida spp., highlighting the urgent need of new antifungal therapies. Rhesus theta (θ) defensin-1 (RTD-1), a natural macrocyclic antimicrobial peptide, was recently shown to be rapidly fungicidal against clinical isolates of MDR C. albicans in vitro. Here we found that RTD-1 was rapidly fungicidal against blastospores of fluconazole/caspofungin resistant C. albicans strains, and was active against established C. albicans biofilms in vitro. In vivo, systemic administration of RTD-1, initiated at the time of infection or 24 h post-infection, promoted long term survival in candidemic mice whether infected with drug-sensitive or MDR strains of C. albicans. RTD-1 induced an early (4 h post treatment) increase in neutrophils in naive and infected mice. In vivo efficacy was associated with fungal clearance, restoration of dysregulated inflammatory cytokines including TNF-α, IL-1β, IL-6, IL-10, and IL-17, and homeostatic reduction in numbers of circulating neutrophils and monocytes. Because these effects occurred using peptide doses that produced maximal plasma concentrations (Cmax) of less than 1% of RTD-1 levels required for in vitro antifungal activity in 50% mouse serum, while inducing a transient neutrophilia, we suggest that RTD-1 mediates its antifungal effects in vivo by host directed mechanisms rather than direct fungicidal activity. Results of this study suggest that θ-defensins represent a new class of host-directed compounds for treatment of disseminated candidiasis.