Antimicrobial peptides versus invasive infections

Platelet factor 4 improves survival in a murine model of antibiotic-susceptible and methicillin-resistant Staphylococcus aureus peritonitis

The complement receptor CR3, also known as integrin Mac-1 (CD11b/CD18), is one of the major phagocytic receptors on the surface of neutrophils and macrophages. We previously demonstrated that in its protein ligands, Mac-1 binds sequences enriched in basic and hydrophobic residues and strongly disfavors negatively charged sequences. The avoidance by Mac-1 of negatively charged surfaces suggests that the bacterial wall and bacterial capsule possessing net negative electrostatic charge may repel Mac-1 and that the cationic Mac-1 ligands can overcome this evasion by acting as opsonins. Indeed, we previously showed that opsonization of Gram-negative Escherichia coli with several cationic peptides, including PF4 (Platelet Factor 4), strongly augmented phagocytosis by macrophages. Here, we investigated the effect of recombinant PF4 (rPF4) on phagocytosis of Gram-positive Staphylococcus aureus in vitro and examined its impact in a mouse model of S. aureus peritonitis. Characterization of the interaction of rPF4 with nonencapsulated and encapsulated S. aureus showed that rPF4 localizes on the bacterial surface, thus making it available for Mac-1. Furthermore, rPF4 did not have direct bactericidal and bacteriostatic activity and was not toxic to host cells. rPF4 enhanced phagocytosis of S. aureus bioparticles by various primary and cultured Mac-1-expressing leukocytes by several folds. It also increased phagocytosis of live nonencapsulated and encapsulated bacteria. Notably, the augmentation of phagocytosis by rPF4 did not compromise the intracellular killing of S. aureus by macrophages. Using a murine S. aureus peritonitis model, we showed that treatment of infected mice with rPF4 caused a significant increase in the clearance of antibiotic-susceptible S. aureus and its methicillin-resistant (MRSA) variant and markedly improved survival. These findings indicate that rPF4 binding to the bacterial surface circumvents its antiphagocytic properties, improving host defense against antibiotic-susceptible and antibiotic-resistant bacteria.

PLATELET FACTOR 4 (PF4) IMPROVES SURVIVAL IN A MURINE MODEL OF ANTIBIOTIC-SUSCEPTIBLE AND METHICILLIN-RESISTANT STAPHYLOCOCCUS AUREUS PERITONITIS

The complement receptor CR3, also known as integrin Mac-1 (CD11b/CD18), is one of the major phagocytic receptors on the surface of neutrophils and macrophages. We previously demonstrated that in its protein ligands, Mac-1 binds sequences enriched in basic and hydrophobic residues and strongly disfavors negatively charged sequences. The avoidance by Mac-1 of negatively charged surfaces suggests that the bacterial wall and bacterial capsule possessing net negative electrostatic charge may repel Mac-1 and that the cationic Mac-1 ligands can overcome this evasion by acting as opsonins. Indeed, we previously showed that opsonization of Gram-negative Escherichia coli with several cationic peptides, including PF4 (Platelet Factor 4), strongly augmented phagocytosis by macrophages. Here, we investigated the effect of recombinant PF4 (rPF4) on phagocytosis of Gram-positive Staphylococcus aureus in vitro and examined its impact in a mouse model of S. aureus peritonitis. Characterization of the interaction of rPF4 with nonencapsulated and encapsulated S. aureus showed that rPF4 localizes on the bacterial surface, thus making it available for Mac-1. Furthermore, rPF4 did not have direct bactericidal and bacteriostatic activity and was not toxic to host cells. rPF4 enhanced phagocytosis of S. aureus bioparticles by various primary and cultured Mac-1-expressing leukocytes by several folds. It also increased phagocytosis of live nonencapsulated and encapsulated bacteria. Notably, the augmentation of phagocytosis by rPF4 did not compromise the intracellular killing of S. aureus by macrophages. Using a murine S. aureus peritonitis model, we showed that treatment of infected mice with rPF4 caused a significant increase in the clearance of antibiotic-susceptible S. aureus and its methicillin-resistant (MRSA) variant and markedly improved survival. These findings indicate that rPF4 binding to the bacterial surface circumvents its antiphagocytic properties, improving host defense against antibiotic-susceptible and antibiotic-resistant bacteria.

Efficacy of ARV-1502, a Proline-Rich Antimicrobial Peptide, in a Murine Model of Bacteremia Caused by Multi-Drug Resistant (MDR) Acinetobacter baumannii

Acinetobacter baumannii bacteremia represents a serious and increasing clinical problem due to the high mortality and treatment failures because of high rates of antibiotic resistance. Any additional new therapies for A. baumannii bacteremia would address a growing unmet medical need. ARV-1502 (designated as Chex1-Arg20 or A3-APO monomer in prior publications) is a designer proline-rich antimicrobial peptide chaperone protein inhibitor derived from insects and has demonstrated potent activity against multi-drug resistant (MDR) Gram-negative bacteria. In the current studies, we investigated the therapeutic efficacy of ARV-1502 administered intravenously (iv) alone and in combination with imipenem/cilastatin (IPM/CIL) in a mouse bacteremia model due to a MDR clinical A. baumannii strain, HUMC1. All ARV-1502 regimens (1.25, 2.5 and 5.0 mg/kg) significantly reduced bacterial density in the target tissues in a dose-dependent manner, as compared to the untreated control and IPM/CIL monotherapy (40 mg/kg) groups in the model. In addition, ARV-1502 treatment, even at the lowest dose, significantly improved survival vs. the control and IPM alone groups. As expected, IMP/CIL monotherapy had no therapeutic efficacy in the model, since the HUMC1 strain was resistant to IMP in vitro. However, the combination of ARV-1502 and IPM/CIL significantly enhanced the efficacy of ARV-1502, except the lowest dose of ARV-1502. The superior efficacy of ARV-1502 in the bacteremia model caused by MDR A. baumannii provides further support for studying this compound in severe infections caused by other MDR Gram-positive and -negative pathogens.

Mean platelet volume to platelet count ratio as a laboratory indicator of mortality in pneumonia following ischemic stroke

Background

Platelets have a vital role in antimicrobial host defenses. The objective of this study was to evaluate if increased mean platelet volume to platelet count (MPV/PC) ratio in acute ischemic stroke patients complicated with pneumonia was associated with increased mortality risk.

Methods

The current study was conducted at Zagazig University Hospitals. It included 500 acute ischemic stroke patients classified as group 1 that included 51 patients complicated with pneumonia after admission and group 2 that included the remaining 449 patients. Clinical assessment was carried out to exclude comorbid medical illnesses likely to interfere with platelet function or morphology. Laboratory investigations including MPV/PC ratio and brain imaging were carried out for all patients.

Results

There was a significant difference between both groups regarding age, National Institutes of Health Stroke Scale (NIHSS) score, and mortality within 30 days (p = 0.02, 0.03, 0.01). There was a significant difference between survivors and non-survivors of group 1 regarding to pneumonia severity index (PSI) classes IV and V (p = 0.01 and 0.02, respectively). Also, there was a significant difference regarding confusion, urea ≥ 7 mmol/L, respiratory rater ≥ 30 breaths/min, systolic blood pressure ≤ 90 mmHg or diastolic blood pressure ≤ 60 mmHg, and age ≥ 65 years at pneumonia occurrence (CURB-65) scores 3, 4, and 5 (p = 0.03, 0.02, and 0.01, respectively). Moreover, there was a significant difference regarding decreased GCS score at pneumonia occurrence, higher NIHSS scores, PSI, and higher MPV/PC ratio (p = 0.01, 0.01, 0.028, and 0.01, respectively). Age > 65 years, need for mechanical ventilation, GCS score of > 9, PSI class ≥ IV, CURB-65 scores ≥ 3, and increased MPV/PC ratio were all significantly associated with 30-day mortality in group 1 (p = 0.03, 0.01, 0.001, 0.04, 0.01, and 0.03, respectively). The predictors of 30-day mortality risk factors were GCS less than 9, increased MPV/PC ratio, and CURB-65 scores ≥ 3 (p = 0.001, 0.05, and 0.01, respectively).

Conclusions

Once pneumonia develops, MPV/PC ratio could be considered a significant laboratory indicator of 30-day mortality.

Impact of High-Level Daptomycin Resistance in the Streptococcus mitis Group on Virulence and Survivability during Daptomycin Treatment in Experimental Infective Endocarditis

Among the viridans group streptococci, the Streptococcus mitis group is the most common cause of infective endocarditis. These bacteria have a propensity to be β-lactam resistant, as well as to rapidly develop high-level and durable resistance to daptomycin (DAP). We compared a parental, daptomycin-susceptible (DAPs) S. mitis/S. oralis strain and its daptomycin-resistant (DAPr) variant in a model of experimental endocarditis in terms of (i) their relative fitness in multiple target organs in this model (vegetations, kidneys, spleen) when animals were challenged individually and in a coinfection strategy and (ii) their survivability during therapy with daptomycin-gentamicin (an in vitro combination synergistic against the parental strain). The DAPr variant was initially isolated from the cardiac vegetations of animals with experimental endocarditis caused by the parental DAPs strain following treatment with daptomycin. The parental strain and the DAPr variant were comparably virulent when animals were individually challenged. In contrast, in the coinfection model without daptomycin therapy, at both the 106- and 107-CFU/ml challenge inocula, the parental strain outcompeted the DAPr variant in all target organs, especially the kidneys and spleen. When the animals in the coinfection model of endocarditis were treated with DAP-gentamicin, the DAPs strain was completely eliminated, while the DAPr variant persisted in all target tissues. These data underscore that the acquisition of DAPr in S. mitis/S. oralis does come at an intrinsic fitness cost, although this resistance phenotype is completely protective against therapy with a potentially synergistic DAP regimen.

Atomic Force Microscopy Reveals the Mechanobiology of Lytic Peptide Action on Bacteria

Increasing rates of antimicrobial-resistant medically important bacteria require the development of new, effective therapeutics, of which antimicrobial peptides (AMPs) are among the promising candidates. Many AMPs are membrane-active, but their mode of action in killing bacteria or in inhibiting their growth remains elusive. This study used atomic force microscopy (AFM) to probe the mechanobiology of a model AMP (a derivative of melittin) on living Klebsiella pneumoniae bacterial cells. We performed in situ biophysical measurements to understand how the melittin peptide modulates various biophysical behaviors of individual bacteria, including the turgor pressure, cell wall elasticity, and bacterial capsule thickness and organization. Exposure of K. pneumoniae to the peptide had a significant effect on the turgor pressure and Young's modulus of the cell wall. The turgor pressure increased upon peptide addition followed by a later decrease, suggesting that cell lysis occurred and pressure was lost through destruction of the cell envelope. The Young's modulus also increased, indicating that interaction with the peptide increased the rigidity of the cell wall. The bacterial capsule did not prevent cell lysis by the peptide, and surprisingly, the capsule appeared unaffected by exposure to the peptide, as capsule thickness and inferred organization were within the control limits, determined by mechanical measurements. These data show that AFM measurements may provide valuable insights into the physical events that precede bacterial lysis by AMPs.

Identification of Aspergillus fumigatus Surface Components That Mediate Interaction of Conidia and Hyphae With Human Platelets

BACKGROUND

Platelets were recently identified as a part of innate immunity. They are activated by contact with Aspergillus fumigatus; putative consequences include antifungal defense but also thrombosis, excessive inflammation, and thrombocytopenia. We aimed to identify those fungal surface structures that mediate interaction with platelets.

METHODS

Human platelets were incubated with Aspergillus conidia and hyphae, isolated wall components, or fungal surface mutants. Interaction was visualized microscopically; activation was quantified by flow cytometry of specific markers.

RESULTS

The capacity of A. fumigatus conidia to activate platelets is at least partly due to melanin, because this effect can be mimicked with "melanin ghosts"; a mutant lacking melanin showed reduced platelet stimulating potency. In contrast, conidial hydrophobin masks relevant structures, because an A. fumigatus mutant lacking the hydrophobin protein induced stronger platelet activation than wild-type conidia. A. fumigatus hyphae also contain surface structures that interact with platelets. Wall proteins, galactomannan, chitin, and β-glucan are not the relevant hyphal components; instead, the recently identified fungal polysaccharide galactosaminogalactan potently triggered platelet activation.

CONCLUSIONS

Conidial melanin and hydrophobin as well as hyphal galactosaminogalactan represent important pathogenicity factors that modulate platelet activity and thus might influence immune responses, inflammation, and thrombosis in infected patients.

Nafcillin enhances innate immune-mediated killing of methicillin-resistant Staphylococcus aureus

Based on in vitro synergy studies, the addition of nafcillin to daptomycin was used to treat refractory methicillin-resistant Staphylococcus aureus (MRSA) bacteremia. Daptomycin is a de facto cationic antimicrobial peptide in vivo, with antistaphylococcal mechanisms reminiscent of innate host defense peptides (HDPs). In this study, the effects of nafcillin on HDP activity against MRSA were examined in vitro and in vivo. Exposures to β-lactam antimicrobials in general, and nafcillin in particular, significantly increased killing of S. aureus by selected HDPs from keratinocytes, neutrophils, and platelets. This finding correlated with enhanced killing of MRSA by whole blood, neutrophils, and keratinocytes after growth in nafcillin. Finally, nafcillin pretreatment ex vivo reduced MRSA virulence in a murine subcutaneous infection model. Despite the lack of direct activity against MRSA, these studies show potent, consistent, and generalized nafcillin-mediated "sensitization" to increased killing of MRSA by various components of the innate host response. The use of nafcillin as adjunctive therapy in MRSA bacteremia merits further study and should be considered in cases refractory to standard therapy.

KEY MESSAGES

Nafcillin has been used as adjunctive therapy to clear persistent MRSA bacteremia. Nafcillin enhances killing of MRSA by a cadre of innate host defense peptides. Nafcillin increases binding of human cathelicidin LL-37 to the MRSA membrane. Nafcillin enhances killing of MRSA by neutrophils. Nafcillin reduces virulence of MRSA in a murine subcutaneous infection model.

Facile expression and purification of the antimicrobial peptide histatin 1 with a cleavable self-aggregating tag (cSAT) in Escherichia coli

Human histatin 1 (Hst1), a member of the histatin family, possesses antimicrobial properties. In this study, we applied a previously developed cleavable self-aggregating tag (cSAT) for the expression and purification of histatin 1 to demonstrate its utility for peptide expression and purification. The tag consists of a self-cleavable intein and a self-assembling peptide ELK16 (I-ELK16). First, an active insoluble aggregate of the recombinant histatin 1-Mxe GyrA intein-ELK16 (Hst1-I-ELK16) fusion protein was produced with a yield of 28.9 μg/mg wet cell pellet. The thiol reagent dithiothreitol (DTT) was then used to induce the intein-mediated cleavage and peptide release into the soluble fraction with a yield of 2.06 μg/mg wet cell pellet and a purity of 70%. The peptide was further purified by high performance liquid chromatography. These results were comparable to the yield and purity achieved when the more conventional glutathione transferase (GST) tag was used. The antimicrobial activities of this recombinant histatin 1 were confirmed against three Candida strains. This cSAT technique offers considerable advantages in terms of its simplicity and speed, eliminating the need for an exogenous protease, and reducing the number of chromatography purification steps. This technique should also be useful for the expression and purification of other AMPs.

Host defense peptides of thrombin modulate inflammation and coagulation in endotoxin-mediated shock and Pseudomonas aeruginosa sepsis

Gram-negative sepsis is accompanied by a disproportionate innate immune response and excessive coagulation mainly induced by endotoxins released from bacteria. Due to rising antibiotic resistance and current lack of other effective treatments there is an urgent need for new therapies. We here present a new treatment concept for sepsis and endotoxin-mediated shock, based on host defense peptides from the C-terminal part of human thrombin, found to have a broad and inhibitory effect on multiple sepsis pathologies. Thus, the peptides abrogate pro-inflammatory cytokine responses to endotoxin in vitro and in vivo. Furthermore, they interfere with coagulation by modulating contact activation and tissue factor-mediated clotting in vitro, leading to normalization of coagulation responses in vivo, a previously unknown function of host defense peptides. In a mouse model of Pseudomonas aeruginosa sepsis, the peptide GKY25, while mediating a modest antimicrobial effect, significantly inhibited the pro-inflammatory response, decreased fibrin deposition and leakage in the lungs, as well as reduced mortality. Taken together, the capacity of such thrombin-derived peptides to simultaneously modulate bacterial levels, pro-inflammatory responses, and coagulation, renders them attractive therapeutic candidates for the treatment of invasive infections and sepsis.

Pathogen sensing, subsequent signalling, and signalosome in human platelets

Beyond haemostasis, platelets exert a potent role in innate immunity and particularly in its inflammatory arm. The extent of this action remains however debatable, despite clear - and old - evidence of a link between platelets and infection. Platelets can sense infectious pathogens by pathogen recognition receptors and they can even discriminate between various types of infectious signatures. In reply, they can shape their capacity to respond by activating a signalosome and by producing different profiles of pro-inflammatory cytokines and related products. The links between pathogen sensing, signalosome activation and protein production, and their finely tuned regulation are still under investigation since platelets lack a nucleus and thus, canonical molecular biology and genomics apparati.

Unravelling the mechanism and significance of thrombin binding to platelet glycoprotein Ib

The main question concerning the mechanism of a-thrombin binding to platelet membrane glycoprotein (GP)Ib is whether it involves both thrombin exosite I and exosite II. The solution of two independent crystal structures suggests alternative explanations that may actually reflect different modes of binding with distinct pathophysiological significance. With respect to function, it is still unclear whether thrombin binding to GPIb promotes procoagulant and prothrombotic pathways of response to vascular injury or limits such responses by sequestering, at least temporarily, the active enzyme. We review here published information on these topics and touch upon ongoing studies aimed at finding definitive answers to outstanding questions relevant for a better understanding of thrombosis and haemostasis.