Fluconazole and platelet microbicidal protein inhibit Candida adherence to platelets in vitro

Recent Advances in the Discovery and Function of Antimicrobial Molecules in Platelets

The conventional function described for platelets is maintaining vascular integrity. Nevertheless, increasing evidence reveals that platelets can additionally play a crucial role in responding against microorganisms. Activated platelets release molecules with antimicrobial activity. This ability was first demonstrated in rabbit serum after coagulation and later in rabbit platelets stimulated with thrombin. Currently, multiple discoveries have allowed the identification and characterization of PMPs (platelet microbicidal proteins) and opened the way to identify kinocidins and CHDPs (cationic host defense peptides) in human platelets. These molecules are endowed with microbicidal activity through different mechanisms that broaden the platelet participation in normal and pathologic conditions. Therefore, this review aims to integrate the currently described platelet molecules with antimicrobial properties by summarizing the pathways towards their identification, characterization, and functional evaluation that have promoted new avenues for studying platelets based on kinocidins and CHDPs secretion.

Platelet immunology in fungal infections

Up to date, perception of platelets has changed from key players in coagulation to multitaskers within the immune network, connecting its most diverse elements and crucially shaping their interplay with invading pathogens such as fungi. In addition, antimicrobial effector molecules and mechanisms in platelets enable a direct inhibitory effect on fungi, thus completing their immune capacity. To precisely assess the impact of platelets on the course of invasive fungal infections is complicated by some critical parameters. First, there is a fragile balance between protective antimicrobial effects and detrimental reactions that aggravate the fungal pathogenesis. Second, some platelet effects are exerted indirectly by other immune mediators and are thus difficult to quantify. Third, drugs such as antimycotics, antibiotics, or cytostatics, are commonly administered to the patients and might modulate the interplay between platelets and fungi. Our article highlights selected aspects of the complex interactions between platelets and fungi and the relevance of these processes for the pathogenesis of fungal infections.

Platelets as immune cells in infectious diseases

Platelets have been shown to cover a broad range of functions. Besides their role in hemostasis, they have immunological functions and thus participate in the interaction between pathogens and host defense. Platelets have a broad repertoire of receptor molecules that enable them to sense invading pathogens and infection-induced inflammation. Consequently, platelets exert antimicrobial effector mechanisms, but also initiate an intense crosstalk with other arms of the innate and adaptive immunity, including neutrophils, monocytes/macrophages, dendritic cells, B cells and T cells. There is a fragile balance between beneficial antimicrobial effects and detrimental reactions that contribute to the pathogenesis, and many pathogens have developed mechanisms to influence these two outcomes. This review aims to highlight aspects of the interaction strategies between platelets and pathogenic bacteria, viruses, fungi and parasites, in addition to the subsequent networking between platelets and other immune cells, and the relevance of these processes for the pathogenesis of infections.

Bidirectional crosstalk between platelets and monocytes initiated by Toll-like receptor: an important step in the early defense against fungal infections?

Monocytes are important in the defense against fungal infections due to their phagocytic and immunoregulatory functions. Platelets also contribute in such immune responses through their release of soluble mediators, including chemokines as well as several other soluble mediators. Both monocytes and platelets express several Toll-like receptors (TLRs) that can recognize fungal molecules and thus initiate intracellular signaling events. TLR ligation on monocytes and platelets may thereby be an early immunological event and function as an initiator of a local proinflammatory crosstalk between platelets and monocytes resulting in (i) monocyte-induced increase of platelet activation and (ii) platelet-associated enhancement of the monocyte activation/function. These effects may have clinical implications both for the efficiency of antifungal treatment and for the predisposition to fungal infections, for example, increased predisposition in patients with thrombocytopenia/monocytopenia due to chemotherapy- or disease-induced bone marrow failure.

Bacterial-platelet interactions: virulence meets host defense

Platelets have historically been viewed as cell fragments that only mediate blood coagulation. Yet, platelets have as - or perhaps even more - important roles in tissue remodeling, modulation of inflammation and antimicrobial host defense. It is evident that platelets interact with prokaryotes directly and indirectly through multiple molecular and cellular mechanisms. The important roles of platelets in antibacterial host defense can be exemplified through contemporary themes in platelet immunobiology. Platelets have unambiguous structures and functions of host defense effector cells. Recent discoveries reveal platelet expression of toll-like and purinonergic receptors, which enable detection and response to bacterial infection, degranulation of an array of microbicidal peptides and coordination of other molecular and cellular host defenses. From multiple perspectives, platelets are now increasingly recognized as critical innate immune effector cells that also bridge and facilitate optimization of adaptive immunity. It follows that clinical deficiencies in platelet quantity or quality are now recognized correlates of increased risk and severity of bacterial and other infections. Along these lines, new evidence suggests that certain prokaryotic organisms may be capable of exploiting platelet interactions to gain a virulence advantage. Indeed, certain bacterial pathogens appear to have evolved highly coordinated means by which to seize opportunities to bind to surfaces of activated platelets, and exploit them to establish or propagate infection. Hence, it is conceivable that certain bacterial pathogens subvert platelet functions. From these perspectives, the net consequences of bacterial virulence versus platelet host defenses likely decide initial steps towards the ultimate result of infection versus immunity.

Platelets in defense against bacterial pathogens

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.

Antimicrobial peptides versus invasive infections

It has been estimated that there are more microorganisms within and upon the human body than there are human cells. By necessity, every accessible niche must be defended by innate mechanisms to prevent invasive infection, and ideally that precludes the need for robust inflammatory responses. Yet the potential for pathogens to transcend the integument actively or passively and access the bloodstream emphasizes the need for rapid and potent antimicrobial defense mechanisms within the vascular compartment. Antimicrobial peptides from leukocytes have long been contemplated as being integral to defense against these infections. Recently, platelets are increasingly recognized for their likely multiple roles in antimicrobial host defense. Platelets and leukocytes share many structural and functional archetypes. Once activated, both cell types respond in specific ways that emphasize key roles for their antimicrobial peptides in host defense efficacy: (a) targeted accumulation at sites of tissue injury or infection; (b) direct interaction with pathogens; and (c) deployment of intracellular (leukocyte phagosomes) or extracellular (platelet secretion) antimicrobial peptides. Antimicrobial peptides from these cells exert rapid, potent, and direct antimicrobial effects against organisms that commonly access the bloodstream. Experimental models in vitro and in vivo show that antimicrobial peptides from these cells significantly contribute to prevent or limit infection. Moreover, certain platelet antimicrobial proteins are multifunctional kinocidins (microbicidal chemokines) that recruit leukocytes to sites of infection, and potentiate the antimicrobial mechanisms of these cells. In turn, pathogens pre-decorated by kinocidins may be more efficiently phagocytosed and killed by leukocytes and their antimicrobial peptide arsenal. Hence, multiple and relevant interactions between platelets and leukocytes have immunologic functions yet to be fully understood. A clearer definition of these interactions, and the antimicrobial peptide effectors contributing to these functions, will significantly advance our understanding of antimicrobial host defense against invasive infection. In addition, this knowledge may accelerate development of novel anti-infective agents and strategies against pathogens that have become refractory to conventional antimicrobials.

Susceptibility to thrombin-induced platelet microbicidal protein is associated with increased fluconazole efficacy against experimental endocarditis due to Candida albicans

Platelet microbicidal proteins (PMPs) are believed to be integral to host defense against endovascular infection. We previously demonstrated that susceptibility to thrombin-induced PMP 1 (tPMP-1) in vitro negatively influences Candida albicans virulence in the rabbit model of infective endocarditis (IE). This study evaluated the relationship between in vitro tPMP-1 susceptibility (tPMP-1s) or resistance (tPMP-1r) and efficacy of fluconazole (FLU) therapy of IE due to C. albicans. Candida IE was established in rabbits with either tPMP-1s or tPMP-1r strains. Treatment groups received FLU (100 mg/kg/day) intraperitoneally for 7 or 14 days; control animals received no therapy. At these time points, cardiac vegetations, kidneys, and spleens were quantitatively cultured to assess fungal burden. At both 7 and 14 days and in all target tissues, the extent of candidal clearance by FLU was greater in animals infected with the tPMP-1s strain than in those infected with the tPMP-1r strain. These differences were statistically significant in the spleen and kidney. Corroborating these in vivo data, FLU (a candidastatic agent), in combination with tPMP-1, exerted an enhanced fungicidal effect in vitro against tPMP-1s and tPMP-1r C. albicans, with the extent of this effect greatest against the tPMP-1s strain. Collectively, these results support the concept that tPMP-1 susceptibility contributes to the net efficacy of FLU against C. albicans IE in vivo, particularly in tissues in which platelets and tPMP-1 likely play significant roles in host defense.

Beneficial influence of platelets on antibiotic efficacy in an in vitro model of Staphylococcus aureus-induced endocarditis

Platelets contribute to antimicrobial host defense against infective endocarditis (IE) by releasing platelet microbicidal proteins (PMPs). We investigated the influence of thrombin-stimulated human platelets on the evolution of simulated IE in the presence and absence of vancomycin or nafcillin. Staphylococcus aureus strains differing in intrinsic susceptibility to PMPs or antibiotics were studied: ISP479C (thrombin-induced PMP-1 [tPMP-1] susceptible; nafcillin and vancomycin susceptible), ISP479R (tPMP-1 resistant; nafcillin and vancomycin susceptible), and GISA-NJ (tPMP-1 intermediate-susceptible; vancomycin intermediate-susceptible). Platelets were introduced and thrombin activated within the in vitro IE model 30 min prior to inoculation with S. aureus. At 0 to 24 h postinoculation, bacterial densities in chamber fluid and simulated endocardial vegetations (SEVs) were quantified and compared among groups. Activated platelets alone, or in combination with antibiotics, inhibited the proliferation of ISP479C in chamber fluid or SEVs over the initial 4-h period (P < 0.05 versus controls). Moreover, nafcillin-containing regimens exerted inhibitory effects beyond 4 h against ISP479C in both model phases. By comparison, activated platelets inhibited GISA-NJ proliferation in SEVs but not in chamber fluid. The combination of platelets plus nafcillin or vancomycin significantly inhibited proliferation of the GISA-NJ strain in SEVs compared to the effect of platelets or antibiotics alone (P < 0.05). In contrast, platelets did not significantly alter the antistaphylococcal efficacies of nafcillin or vancomycin against ISP479R. These data support our hypothesis that a beneficial antimicrobial effect may result from the interaction among platelets, PMPs, and anti-infective agents against antibiotic-susceptible or -resistant staphylococci that exhibit a tPMP-1-susceptible or -intermediate-susceptible phenotype.

Platelets and Platelet Inhibitors in Infective Endocarditis

The pathogenesis of infective endocarditis depends on complex interactions between the causative pathogen, plasma proteins, platelets, and vascular endothelial cells. In addition to being the main target of bacteria in the initial stage of bacterial adherence to the endocardium, platelets now appear to play an important role in antimicrobial host defense against endocarditis through the secretion of so-called platelet microbicidal proteins. In animal models of endocarditis, low-dose aspirin was shown to significantly reduce the vegetation weight, the bacterial density of vegetation, the hematogenous bacterial dissemination, and the frequency of embolic events. However, these facts cannot be extrapolated to clinical care in humans, since to date, there is no definitive proof of the adjunctive benefit of aspirin in human infective endocarditis.

Genetic loci of Streptococcus mitis that mediate binding to human platelets

The direct binding of bacteria to platelets is a postulated major interaction in the pathogenesis of infective endocarditis. To identify bacterial components that mediate platelet binding by Streptococcus mitis, we screened a Tn916deltaE-derived mutant library of S. mitis strain SF100 for reduced binding to human platelets in vitro. Two distinct loci were found to affect platelet binding. The first contains a gene (pblT) encoding a highly hydrophobic, 43-kDa protein with 12 potential membrane-spanning segments. This protein resembles members of the major facilitator superfamily of small-molecule transporters. The second platelet binding locus consists of an apparent polycistronic operon. This region includes genes that are highly similar to those of Lactococcus lactis phage r1t and Streptococcus thermophilus phage 01205. Two genes (pblA and pblB) encoding large surface proteins are also present. The former encodes a 107-kDa protein containing tryptophan-rich repeats, which may serve to anchor the protein within the cell wall. The latter encodes a 121-kDa protein most similar to a tail fiber protein from phage 01205. Functional mapping by insertion-duplication mutagenesis and gene complementation indicates that PblB may be a platelet adhesin and that expression of PblB may be linked to that of PblA. The combined data indicate that at least two genomic regions contribute to platelet binding by S. mitis. One encodes a probable transmembrane transporter, while the second encodes two large surface proteins resembling structural components of lysogenic phages.

Influence of platelets and platelet microbicidal protein susceptibility on the fate of Staphylococcus aureus in an in vitro model of infective endocarditis

Several lines of evidence indicate that platelets protect against endovascular infections such as infective endocarditis (IE). It is highly likely that a principal mechanism of this platelet host defense role is the release of platelet microbicidal proteins (PMPs) in response to agonists generated at sites of endovascular infection. We studied the ability of platelets to limit the colonization and proliferation of Staphylococcus aureus in an in vitro model of IE. Three isogenic S. aureus strains, differing in their in vitro susceptibility to thrombin-induced platelet microbicidal protein-1 (tPMP), were used: ISP479C (parental strain; highly susceptible to tPMP [tPMP(s)]); ISP479R (transposon mutant derived from ISP479; tPMP resistant [tPMP(r)]); or 757-5 (tPMP(r) transductant of the ISP479R genotype in the ISP479 parental background). Time-kill assays and in vitro IE models were used to examine the temporal relationship between thrombin-induced platelet activation and S. aureus killing. In time-kill studies, early platelet activation (30 min prior to bacterial exposure) correlated with a significant bactericidal effect against tPMP(s) ISP479C (r(2) > 0.90, P < 0.02) but not against tPMP(r) strains, ISP479R or 757-5. In the IE model, thrombin activation significantly inhibited proliferation of ISP479C within simulated vegetations compared to strains ISP479R or 757-5 (P < 0.05). The latter differences were observed despite there being no detectable differences among the three S. aureus strains in initial colonization of simulated vegetations. Collectively, these data indicate that platelets limit intravegetation proliferation of tPMP(s) but not tPMP(r) S. aureus. These findings underscore the likelihood that platelets play an important antimicrobial host defense role in preventing and/or limiting endovascular infections due to tPMP(s) pathogens.

Adherence of platelets to Candida species in vivo

The in vivo interactions of platelets with Candida species yeast cells were investigated in a murine model. Mice were injected intravenously via the lateral caudal vein, and blood drawn by periorbital puncture was collected in phosphate-buffered saline-formaldehyde to avoid in vitro platelet activation. The study of the clearance of blastoconidia of Candida albicans and Candida glabrata showed that these cells disappeared quickly from the bloodstream. Microscopic observation of blood samples, stained by Calcofluor white or May Grunwald Giemsa, demonstrated the rapid attachment of platelets to fungal elements of all the Candida spp. tested. The attachment of murine platelets to C. albicans cells, observed by scanning electron microscopy, revealed morphological changes. The platelets lost their discoid shape, generated pseudopodia, and flattened against the yeast cells. The reversibility of platelet binding to C. albicans by chelating agents suggests a cation-dependent link. In contrast, the fixation of C. glabrata and Candida tropicalis was not modified by chelating agents. The mechanisms involved in the in vivo adherence of platelets to Candida cells may therefore differ according to the species of Candida.

The effect of the new triazole, voriconazole (UK-109,496), on the interactions of Candida albicans and Candida krusei with endothelial cells

In this study, we investigated how voriconazole affects specific endothelial cell interactions utilizing both fluconazole-susceptibles and resistantR Candida albicans strains (C. albicansS and C. albicansR, respectively) as well as Candida krusei. Our data show that exposing C. albicansS to voriconazole significantly reduced its adherence to endothelial cells (p <0.001). The adherence of C. albicansR to endothelial cells was not affected by treatment with either antifungal agent. Exposure of C. albicans to both agents inhibited germ tube formation; however, voriconazole showed higher ability in inhibiting germination as compared with fluconazole. The effect of antifungals on germination was also tested during co-incubation of yeast cells with endothelial cells. Pretreated C. albicansS cells germinated on endothelial cells in the presence of voriconazole or fluconazole. However, the degree of germination was reduced by 81% and 16%, respectively. Similar results were observed with C. albicansR. Our data demonstrate that voriconazole treatment reduced the median germ tube length of C. albicansS and C. albicansR by approximately 60%, whereas fluconazole reduced the germ tube length of these strains by 27% and 63%, respectively (P < 0.0001 for each comparison). We compared the efficacy of voriconazole and fluconazole in protecting endothelial cells against damage caused by C. albicansS, C. albicansR, and C. krusei. Voriconazole and fluconazole reduced C. albicans-mediated endothelial cell injury by about 90% and 40%, respectively (P < 0.01 for each comparison). Additionally, voriconazole treatment significantly reduced C. krusei-mediated injury to endothelial cells by 69% (P < 0.01), whereas fluconazole did not exhibit significant protection (P < 0.6). These results demonstrate that voriconazole, in addition to its direct inhibitory activity against fungi, may act against Candida spp. by interfering with critical host/parasite interactions, such as adherence and endothelial cell damage, as well as germination. Therefore, this triazole represents a new and promising agent for the treatment of disseminated candidal infections caused by both fluconazole-susceptible and -resistant species.

Diminished platelet binding in vitro by Staphylococcus aureus is associated with reduced virulence in a rabbit model of infective endocarditis

The direct binding of platelets by bacteria is a postulated central mechanism in the pathogenesis of endocarditis. To address the role of binding more definitively, we employed Tn551 insertional mutagenesis of Staphylococcus aureus parental strain ISP479 to generate an isogenic variant (strain PS12) that bound platelets minimally. As compared with the binding of ISP479, the binding of PS12 to platelet monolayers was reduced by 67.2%. Similarly, the binding of PS12 to platelets in suspension was reduced by 71.3%, as measured by flow cytometry. The low-binding phenotype was transducible into both ISP479 and S. aureus Newman. Southern blotting indicated that a single copy of Tn551 was inserted within the chromosomes of PS12 and the transductants. When tested in a rabbit model, animals inoculated with PS12 were significantly less likely to develop endocarditis and had lower densities of organisms (CFU per gram) within vegetations and a decreased incidence of renal abscess formation, as compared with animals inoculated with the parental strain. The diminished virulence of PS12 was not attributable to a reduction in the initial attachment of organisms to the damaged endocardium, since 30 min after inoculation, PS12-infected animals had microbial densities on the valve surface comparable to those seen with the parental strain. These results indicate that the direct binding of Staphylococcus aureus to platelets is a major determinant of virulence in the pathogenesis of endocarditis. Staphylococcus-platelet binding appears to be critical for pathogenetic events occurring after the initial colonization of the valve surface, such as vegetation formation and septic embolization.

Resistance to platelet microbicidal protein results in increased severity of experimental Candida albicans endocarditis

Thrombin-induced platelet microbicidal protein (tPMP) exerts potent in vitro microbicidal activity against pathogens commonly found in the bloodstream, including Staphylococcus aureus, Staphylococcus epidermidis, and Candida albicans. Localized platelet release of tPMP may be important in defense against infections involving the vascular endothelium caused by tPMP-susceptible organisms. In contrast, pathogens capable of surviving in the presence of tPMP could then exploit the platelet as an adhesive surface for attachment to damaged endothelium. To examine these hypotheses, we derived a tPMP-resistant (tPMP(r)) C. albicans strain from its tPMP-sensitive (tPMP(s)) parental strains were equivalent in vitro as assessed by genotyping (electrophoretic karyotype and restriction endonuclease analysis of genomic DNA), biotyping, germination, platelet aggregation, adherence to vascular endothelial cells, and growth characteristics. In addition, the tPMP(r) phenotype was stable following multiple in vitro and in vivo passages. We then investigated the in vivo relevance of tPMP susceptibility on endovascular infection using a rabbit model of endocarditis and hematogenous dissemination. Rabbits with transaortic catheters (n = 15 in each group) were challenged with either the tPMP(s) or tPMP(r) C. albicans strain. All rabbits developed C. albicans-induced endocarditis, as determined by the presence of infected vegetations. In rabbits challenged with tPMP(s) strain (P < 0.001). These results were seen in the absence of differences in either initial adherence of strains to cardiac valves or vegetation weights. Furthermore, although these C. albicans strains induced equivalent rates and extent of hematogenous renal infection, only the tPMP(r) strain disseminated hematogenously to the spleen (15 of 15 rabbits) versus 0 of 15 [tpmp(s) strain]; P < 0.0001). Thus, tPMP(r) C. albicans caused more-severe endocarditis and produced greater metastatic sequelae than the tPMP(s) counterpart.

Platelet microbicidal protein alone and in combination with antibiotics reduces Staphylococcus aureus adherence to platelets in vitro

Bacterial adherence to platelets on the cardiac valve surface is believed to be critical in the induction of infective endocarditis. Recent studies have confirmed that thrombin-activated platelets secrete platelet microbicidal protein (PMP), which can both kill and exert nonlethal antiadherence effects against endovascular pathogens. In the present study, we quantified the influence of antibiotic and/or PMP exposures on in vitro platelet adherence of two Staphylococcus aureus strains, identical by DNA restriction and cell wall protein profiles, that differed in their susceptibility to PMP-induced killing (PMPs or PMPr, respectively). Adherence assays were performed by flow cytometry in the presence of sublethal PMP concentrations (1 to 2.5 micrograms/ml) alone or in combination with ampicillin (AMP) alone, sulbactam (SUL) alone, or AMP plus SUL (AMP-SUL), at levels achievable in serum. Exposure of the PMPs and PMPr S. aureus strains to antibiotics (for 2 h at 37 degrees C) prior to flow cytometry resulted in no substantive changes in the percent adherence to platelets compared with that for S. aureus cells not exposed to antibiotics, except for modestly increased adherence of both PMPs and PMPr cells exposed to AMP-SUL (18.5 and 15.8% increases, respectively). Addition of PMP to antibiotic-S. aureus mixtures (final 30 min) caused a significant decrease in S. aureus adherence to platelets, for both the PMPs and PMPr S. aureus strains, compared with antibiotic exposure alone (e.g., reduction in platelet adherence from 57.9 +/- 8.2% to 12.2 +/- 3.6% for PMPs cells exposed to AMP-SUL and PMP [P = 0.01]). Moreover, addition of PMP following exposure of the PMPs and PMPr strains to AMP-SUL reversed the enhanced bacterium-platelet adherence observed with such antibiotic exposures alone (P < or = 0.005). These data demonstrate that PMP exerts a potent antiplatelet adherence effect which is independent of its microbicidal capacity, rendering S. aureus cells less adherent to platelets in the presence or absence of antibiotics. Reduction of microbial adherence to platelets by PMP alone or with antibiotics provides further insight into the mechanism(s) that may be involved in host defense and antibiotic prophylaxis of infective endocarditis and other endovascular infections.