Platelets share structural and functional similarities with granulocytes known to participate in antimicrobial host defense. To evaluate the potential antimicrobial activities of platelet proteins, normal human platelets were stimulated with human thrombin in vitro. Components of the stimulated-platelet supernatants were purified to homogeneity by reversed-phase high-performance liquid chromatography. Purified peptides with inhibitory activity against Escherichia coli ML35 in an agar diffusion antimicrobial assay were characterized by mass spectrometry, amino acid analysis, and sequence determination. These analyses enabled the identification of seven thrombin-releasable antimicrobial peptides from human platelets: platelet factor 4 (PF-4), RANTES, connective tissue activating peptide 3 (CTAP-3), platelet basic protein, thymosin β-4 (Tβ-4), fibrinopeptide B (FP-B), and fibrinopeptide A (FP-A). With the exception of FP-A and FP-B, all peptides were also purified from acid extracts of nonstimulated platelets. The in vitro antimicrobial activities of the seven released peptides were further tested against bacteria (E. coli and Staphylococcus aureus) and fungi (Candida albicans and Cryptococcus neoformans). Each peptide exerted activity against at least two organisms. Generally, the peptides were more potent against bacteria than fungi, activity was greater at acidic pHs, and antimicrobial activities were dose dependent. Exceptions to these observations were observed with PF-4, which displayed a bimodal dose-response relationship in microbicidal assays, and Tβ-4, which had greater activity at alkaline pHs. At concentrations at which they were individually sublethal, PF-4 and CTAP-3 exerted synergistic microbicidal activity against E. coli. Collectively, these findings suggest a direct antimicrobial role for platelets as they are activated to release peptides in response to trauma or mediators of inflammation.

https://iai.asm.org/content/iai/70/12/6524.full.pdf

Antimicrobial Peptides from Human Platelets

https://pure.rug.nl/ws/files/3618956/2000JBiolChemKrijgsveld.pdf

Thrombocidins, Microbicidal Proteins from Human Blood Platelets, Are C-terminal Deletion Products of CXC Chemokines

Platelets exhibit archetypal structures and functions enabling their roles as multifunctional host defense cells. Beyond roles in hemostasis and wound repair, platelets sense and respond to pathogens through mechanisms including pattern recognition, early trafficking to sites of infection, direct antimicrobial functions, and potentiation of innate and adaptive immune mechanisms. Insufficient platelet quantity and dysfunctions in platelet quality increase the risk of infections or worsen severity. Moreover, certain pathogens may subvert the host defense functions of platelets, exploiting them in pathogenesis. Here, the immunobiology of platelets in antimicrobial host defense is reviewed from each of these perspectives.

The role of platelets in antimicrobial host defense.

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.

/pdf/platelets-in-defense-against-bacterial-pathogens-1rol0f3alp.pdf

Platelets in defense against bacterial pathogens

Antimicrobial compounds in tears.

(3-Lysin is the name that Pettersson proposed to distinguish this thermostabile bactericidal component of serum from Buchner's alexin or a-lysin (51). He defined (8-lysin as the bactericidal substance found in normal serum that resisted inactivation when heated to 56°C for 30 min. According to this definition, there are at least three different ,3-lysins found in normal serum; these consist of lysozyme, ,B-lysin from platelets, and (8-lysin of nonplatelet origin. The first of these, lysozyme, is a single entity which is well defined in regard to its chemical and biological properties. The other two types of (3lysins are not readily distinguished from each other, and each could be composed of several distinct molecules. The ,B-lysin of platelet origin may be similar or identical to plakin (23). The (8-lysin of nonplatelet origin may include the bactericidal cationic proteins isolated from leukocyte lysosomes (64-66). In this review, attention will be focused on the (3-lysin of platelet origin, which will be referred to as \"platelet (3lysin.\

beta-Lysin of platelet origin.

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beta-Lysin of platelet origin.