Antimicrobial Peptides from Human Platelets

Abstract

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 ␤-4 (T␤-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 T␤-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.

The use of autologous platelet–leukocyte gels to enhance the healing process in surgery, a review

“…Furthermore, Yeaman et al [57] and Tang et al [58] support the idea that platelets also are involved in microbicidal activity, suggesting that they play a role in the platelet host defense mechanism by releasing a variety of platelet microbicidal proteins. The platelet microbicidal proteins were shown to be released after platelet acti-vation, demonstrating potent activities against pathogens that have a tendency to enter the bloodstream [59].…”

Platelets regulate leucocyte responses to Toll‐like receptor stimulation

“…Platelets express a range of Toll‐like receptors (TLRs),5, 6 which are responsible for mediating early immune responses to both infection and sterile injury. Activating platelet‐TLRs causes platelet activation and aggregation7, 8 alongside a number of pro‐inflammatory, antimicrobial responses 9, 10, 11. Platelet–leucocyte aggregation is a well‐characterised response to TLR stimulation,9, 12 and these platelet TLR‐mediated antimicrobial responses are facilitated, in the main, by their interaction with leucocytes.…”

The blood platelets contribution to innate host defense – What they have learned from their big brothers

“…They were identified and described in detail in rabbits by Yeaman et al [12] and later on in humans by Krijgsveld et al [13] and Tang et al [14]. But Tang et al reported about conflicting data on bactericidal activities of purified peptides, possibly due to inhibitory effects of sodium phosphate [14], which is also used in our experiments. Moreover, Krijgsveld et al [13] needed 100-fold higher platelet concentrations to extract thrombocidins in bactericidal amounts than we used in our killing model.…”

“…The possibility exist, that thrombin or other extracellular proteases cleave a precursor protein, which contributes to the generation of bactericidal proteins [2]. According to these findings, Tang et al [14] could identify more bactericidal proteins in human platelets, but two of them are only present in stimulated platelets.…”

“…A possible explanation for these conflicting data on bactericidal activity of platelet releasates is presented by Tang et al [14], who supposed an inhibiting effect of sodium phosphate which is used in most buffer systems. …”