Platelets with antibacterial properties – a case for smaller platelets?

Introduction Platelets differ in size and function. We have previously characterized large and small platelet subpopulations and found that smaller platelets carry increased amounts of immunoglobulins, inflammatory and immunomodulatory proteins compared to large platelets. This raises the question, whether platelets with smaller size could play a special role in host-bacteria immune responses and inflammation. In this study, we aimed to investigate the influence of large and small platelet fractions and their releasates on the growth of Staphylococcus aureus.

Method Large and small platelets of healthy donors (n=8) were isolated from whole blood by differential centrifugation. Releasates were obtained from of thrombin-receptor-activating-peptide-6 and collagen activated large and small platelets. The supernant was fractionated into 3 molecular weight (MW) fractions (low:<10kDa, middle:10-100kDa, and high:>100kDa) using Amicon Ultra centrifugal filter units. Intact platelets, complete and MW-fractionated platelet releasates were incubated with S. aureus (SA 113Δspa) and was cultured on blood agar plates (12h at 37°C). Bacterial growth was assessed by counting colony forming units (CFU). Releasates from large and small platelets were further analysed by quantitative LC-MS/MS (liquid chromatography–mass spectrometry).

Results Large platelets and their releasates did not alter the growth of S. aureus. In contrast,intact small platelets and their releasates reduced bacterial growth by median 12% and 38%, respectively, when CFUs were compared to buffer control (p<0.01). While MW-fractionated releasates of small platelets alone could not reduce the growth of S. aureus, the combination all 3 MW-fractions restored the antibacterial effect. Removal of the high MW fraction completely abolished the antimicrobial effect whereas a combination of low/high or middle/high MW fractions could both inhibit bacterial growth. Proteome analyses identified 151 proteins significantly more abundant in releasates of small compared to large platelets (2-fold difference, p-value<0.05,≥2peptides identified), with a high proportion of plasma proteins including immunoglobulins (appearing in the>100kDa fraction) and complement factors (appearing in the>100kDa and 10-100kDa fractions).

Conclusion Small platelets inhibit the growth of S. aureus by releasing antibacterial proteins. The antibacterial effect requires proteins of low, middle, and high MW, suggesting a role of immunoglobulins and complement factors. Platelets with smaller size could serve as reservoir for antimicrobial plasma proteins and as vehicle for these proteins to sites of infection. Our data support a special role of platelets with smaller size in the host defence against bacteria.

Publication History

Article published online:
26 February 2024