Platelets interact with bacterial pathogens through a wide array of cellular and molecular mechanisms. The consequences of this interaction may significantly influence the balance between infection and immunity. On the one hand, recent data indicate that certain bacteria may be capable of exploiting these interactions to gain a virulence advantage. Indeed, certain bacterial pathogens appear to have evolved specific ways in which to subvert activated platelets. Hence, it is conceivable that some bacterial pathogens exploit platelet responses. On the other hand, platelets are now known to possess unambiguous structures and functions of host defense effector cells. Recent discoveries emphasize critical features enabling such functions, including expression of toll-like receptors that detect hallmark signals of bacterial infection, an array of microbicidal peptides, as well as other host defense molecules and functions. These concepts are consistent with increased risk and severity of bacterial infection as correlates of clinical abnormalities in platelet quantity and quality. In these respects, the molecular and cellular roles of platelets in host defense against bacterial pathogens are explored with attention on advances in platelet immunobiology.

The rise of microbial pathogens refractory to conventional antibiotics represents one of the most urgent and global public health concerns for the 21st century. Emergence of Candida auris isolates and the persistence of invasive mold infections that resist existing treatment and cause severe illness has underscored the threat of drug-resistant fungal infections. To meet these growing challenges, mechanistically novel agents and strategies are needed that surpass the conventional fungistatic or fungicidal drug actions. Host defense peptides have long been misunderstood as indiscriminant membrane detergents. However, evidence gathered over the past decade clearly points to their sophisticated and selective mechanisms of action, including exploiting regulated cell death pathways of their target pathogens. Such peptides perturb transmembrane potential and mitochondrial energetics, inducing phosphatidylserine accessibility and metacaspase activation in fungi. These mechanisms are often multimodal, affording target pathogens fewer resistance options as compared to traditional small molecule drugs. Here, recent advances in the field are examined regarding regulated cell death subroutines as potential therapeutic targets for innovative anti-infective peptides against pathogenic fungi. Furthering knowledge of protective host defense peptide interactions with target pathogens is key to advancing and applying novel prophylactic and therapeutic countermeasures to fungal resistance and pathogenesis.

Background

Neuromyelitis optica spectrum disorder (herein called NMO) is an inflammatory demyelinating disease that can be initiated by binding of immunoglobulin G autoantibodies (AQP4-IgG) to aquaporin-4 on astrocytes, causing complement-dependent cytotoxicity (CDC) and downstream inflammation. The increased NMO pathology in rodents deficient in complement regulator protein CD59 following passive transfer of AQP4-IgG has suggested the potential therapeutic utility of increasing the expression of complement regulator proteins.

Methods

A cell-based ELISA was developed to screen for pharmacological upregulators of endogenous CD55 and CD59 in a human astrocyte cell line. A statin identified from the screen was characterized in cell culture models and rodents for its action on complement regulator protein expression and its efficacy in models of seropositive NMO.

Results

Screening of ~ 11,500 approved and investigational drugs and nutraceuticals identified transcriptional upregulators of CD55 but not of CD59. Several statins, including atorvastatin, simvastatin, lovastatin, and fluvastatin, increased CD55 protein expression in astrocytes, including primary cultures, by three- to four-fold at 24 h, conferring significant protection against AQP4-IgG-induced CDC. Mechanistic studies revealed that CD55 upregulation involves inhibition of the geranylgeranyl transferase pathway rather than inhibition of cholesterol biosynthesis. Oral atorvastatin at 10-20 mg/kg/day for 3 days strongly increased CD55 immunofluorescence in mouse brain and spinal cord and reduced NMO pathology following intracerebral AQP4-IgG injection.

Conclusion

Atorvastatin or other statins may thus have therapeutic benefit in AQP4-IgG seropositive NMO by increasing CD55 expression, in addition to their previously described anti-inflammatory and immunomodulatory actions.

Persistent bacteremia caused by Staphylococcus aureus (SA), especially methicillin-resistant SA (MRSA), is a significant cause of morbidity and mortality. Despite susceptibility phenotypes in vitro, persistent MRSA strains fail to clear with appropriate anti-MRSA therapy during bacteremia in vivo. Thus, identifying the factors that cause such MRSA persistence is of direct and urgent clinical relevance. To address the dynamics of MRSA persistence in the face of host immunity and typical antibiotic regimens, we developed a mathematical model based on the overarching assumption that phenotypic heterogeneity is a hallmark of MRSA persistence. First, we applied an ensemble modeling approach and obtained parameter sets that satisfied the condition of a minimum inoculum dose to establish infection. Second, by simulating with the selected parameter sets under vancomycin therapy which follows clinical practices, we distinguished the models resulting in resolving or persistent bacteremia, based on the total SA exceeding a detection limit after five days of treatment. Third, to find key determinants that discriminate resolving and persistent bacteremia, we applied a machine learning approach and found that the immune clearance rate of persister cells is a key feature. But, fourth, when relapsing bacteremia was considered, the growth rate of persister cells was also found to be a key feature. Finally, we explored pharmacological strategies for persistent and relapsing bacteremia and found that a persister killer, but not a persister formation inhibitor, could provide for an effective cure the persistent bacteremia. Thus, to develop better clinical solutions for MRSA persistence and relapse, our modeling results indicate that we need to better understand the pathogen-host interactions of persister MRSAs in vivo.

Profiling immune responses across several dimensions, including time, patients, molecular features, and tissue sites, can deepen our understanding of immunity as an integrated system. These studies require new analytical approaches to realize their full potential. We highlight recent applications of tensor methods and discuss several future opportunities.

Persistent methicillin-resistant Staphylococcus aureus (MRSA) bacteremia (PB) represents an important subset of S. aureus infection and correlates with poor clinical outcomes. MRSA isolates from patients with PB differ significantly from those of resolving bacteremia (RB) with regard to several in vitro phenotypic and genotypic profiles. For instance, PB strains exhibit less susceptibility to cationic host defense peptides and vancomycin (VAN) killing under in vivo-like conditions, greater damage to endothelial cells, thicker biofilm formation, altered growth rates, early activation of many global virulence regulons (e.g., sigB, sarA, sae and agr) and higher expression of purine biosynthesis genes (e.g., purF) than RB strains. Importantly, PB strains are significantly more resistant to VAN treatment in experimental infective endocarditis as compared to RB strains, despite similar VAN minimum inhibitory concentrations (MICs) in vitro. Here, we review relevant phenotypic and genotypic characteristics related to the PB outcome. These and future insights may improve our understanding of the specific mechanism(s) contributing to the PB outcome, and aid in the development of novel therapeutic and preventative measures against this life-threatening infection.

Recent perspectives forecast a new paradigm for future "third generation" vaccines based on commonalities found in diverse pathogens or convergent immune defenses to such pathogens. For Staphylococcus aureus, recurring infections and a limited success of vaccines containing S. aureus antigens imply that native antigens induce immune responses insufficient for optimal efficacy. These perspectives exemplify the need to apply novel vaccine strategies to high-priority pathogens. One such approach can be termed convergent immunity, where antigens from non-target organisms that contain epitope homologs found in the target organism are applied in vaccines. This approach aims to evoke atypical immune defenses via synergistic processes that (1) afford protective efficacy; (2) target an epitope from one organism that contributes to protective immunity against another; (3) cross-protect against multiple pathogens occupying a common anatomic or immunological niche; and/or (4) overcome immune subversion or avoidance strategies of target pathogens. Thus, convergent immunity has a potential to promote protective efficacy not usually elicited by native antigens from a target pathogen. Variations of this concept have been mainstays in the history of viral and bacterial vaccine development. A more far-reaching example is the pre-clinical evidence that specific fungal antigens can induce cross-kingdom protection against bacterial pathogens. This trans-kingdom protection has been demonstrated in pre-clinical studies of the recombinant Candida albicans agglutinin-like sequence 3 protein (rAls3) where it was shown that a vaccine containing rAls3 provides homologous protection against C. albicans, heterologous protection against several other Candida species, and convergent protection against several strains of S. aureus. Convergent immunity reflects an intriguing new approach to designing and developing vaccine antigens and is considered here in the context of vaccines to target S. aureus.

The mechanisms by which Candida glabrata resists host defense peptides and caspofungin are incompletely understood. To identify transcriptional regulators that enable C. glabrata to withstand these classes of stressors, a library of 215 C. glabrata transcriptional regulatory deletion mutants was screened for susceptibility to both protamine and caspofungin. We identified 8 mutants that had increased susceptibility to both host defense peptides and caspofungin. Of these mutants, 6 were deleted for genes that were predicted to specify proteins involved in histone modification. These genes were ADA2, GCN5, SPT8, HOS2, RPD3 , and SPP1 . The ada2 Δ and gcn5 Δ mutants also had increased susceptibility to other stressors such as H 2 O 2 and SDS. In the Galleria mellonella model of disseminated infection, the ada2 Δ and gcn5 Δ mutants had attenuated virulence, whereas in neutropenic mice, the virulence of the ada2 Δ and rpd3 Δ mutants was decreased. Thus, histone modification plays a central role in enabling C. glabrata to withstand host defense peptides and caspofungin, and Ada2 is essential for the maximal virulence of this organism during disseminated infection.

We previously described a transposon mutant in Staphylococcus aureus strain SH1000 that exhibited reduced susceptibility to cationic thrombin-induced platelet microbicidal proteins (tPMPs). The transposon insertion site was mapped to the gene snoD, the staphylococcal nuo orthologue. Hence, further studies have been performed to understand how this mutation impacts susceptibility to tPMP, by comparing proteomics profiling and membrane lipid analyses of the parent vs. mutant strains. Surprisingly, the mutant showed differential regulation of only a single protein when cultivated aerobically (FadB), and only a small number of proteins under anaerobic growth conditions (AdhE, DapE, Ddh, Ald1, IlvA1, AgrA, Rot, SA2366, and SA2367). Corresponding to FadB impact on lipid remodeling, membrane fatty acid analyses showed that the snoD mutant contained more short chain anteiso-, but fewer short chain iso-branched chain fatty acids under both aerobic and anaerobic conditions vs. the parental strain. Based upon these proteomic and membrane compositional data, a hypothetical "network" model was developed to explain the impact of the snoD mutation upon tPMP susceptibility.

Objectives

Persistent MRSA infections are especially relevant to endovascular infections and correlate with suboptimal outcomes. However, the virulence signatures of Staphylococcus aureus that drive such persistence outcomes are not well defined. In the current study, we investigated correlations between accessory gene regulator (agr) activation and the outcome of vancomycin treatment in an experimental model of infective endocarditis (IE) due to MRSA strains with different agr and clonal complex (CC) types.

Methods

Twelve isolates with the four most common MRSA CC and agr types (CC5-agr II, CC8-agr I, CC30-agr III and CC45-agr I) were evaluated for heterogeneous vancomycin-intermediate S. aureus (hVISA), agr function, agrA and RNAIII transcription, agr locus sequences, virulence and response to vancomycin in the IE model.

Results

Early agr RNAIII activation (beginning at 2 h of growth) in parallel with strong δ-haemolysin production correlated with persistent outcomes in the IE model following vancomycin therapy. Importantly, such treatment failures occurred across the range of CC/agr types studied. In addition, these MRSA strains: (i) were vancomycin susceptible in vitro; (ii) were not hVISA or vancomycin tolerant; and (iii) did not evolve hVISA phenotypes or perturbed δ-haemolysin activity in vivo following vancomycin therapy. Moreover, agr locus sequence analyses revealed no common point mutations that correlated with either temporal RNAIII transcription or vancomycin treatment outcomes, encompassing different CC and agr types.

Conclusions

These data suggest that temporal agr RNAIII activation and agr functional profiles may be useful biomarkers to predict the in vivo persistence of endovascular MRSA infections despite vancomycin therapy.