LL-37 Triggers Antimicrobial Activity in Human Platelets.

Platelets play a crucial role in hemostasis and the immune response, mainly by recognizing signals associated with vascular damage. However, it has recently been discovered that the antimicrobial peptide LL-37 activates platelets in functions related to thrombus formation and inflammation. Therefore, this work aims to evaluate the effect of LL-37 on the activation of antimicrobial functions of human platelets. Our results show that platelets treated with LL-37 increase the surface expression of receptors (Toll-like receptors (TLRs) 2 and -4, CD32, CD206, Dectin-1, CD35, LOX-1, CD41, CD62P, and αIIbß3 integrins) for the recognition of microorganisms, and molecules related to antigen presentation to T lymphocytes (CD80, CD86, and HLA-ABC) secrete the antimicrobial molecules: bactericidal/permeability-increasing protein (BPI), azurocidin, human neutrophil peptide (HNP) -1, and myeloperoxidase. They also translate azurocidin, and have enhanced binding to Escherichia coli, Staphylococcus aureus, and Candida albicans. Furthermore, the supernatant of LL-37-treated platelets can inhibit E. coli growth, or platelets can employ their LL-37 to inhibit microbial growth. In conclusion, these findings demonstrate that LL-37 participates in the antimicrobial function of human platelets.

Human Platelets Contain, Translate, and Secrete Azurocidin; A Novel Effect on Hemostasis.

Platelets play a significant role in hemostasis and perform essential immune functions, evidenced by the extensive repertoire of antimicrobial molecules. Currently, there is no clear description of the presence of azurocidin in human platelets. Azurocidin is a 37 kDa cationic protein abundant in neutrophils, with microbicidal, opsonizing, and vascular permeability-inducing activity. Therefore, this work aimed to characterize the content, secretion, translation, and functions of azurocidin in platelets. Our results show the presence of azurocidin mRNA and protein in α-granules of platelet and megakaryoblasts, and stimulation with thrombin, ADP, and LPS leads to the secretion of free azurocidin as well as within extracellular vesicles. In addition, platelets can translate azurocidin in a basal or thrombin-induced manner. Finally, we found that the addition of low concentrations of azurocidin prevents platelet aggregation and activation. In conclusion, we demonstrate that platelets contain, secrete, and translate azurocidin, and this protein may have important implications for hemostasis.

Human platelets and megakaryocytes express defensin alpha 1.

Platelets are anucleate cells that have a role in several innate immune functions, including the secretion of proteins with antimicrobial activity. Several studies have demonstrated the ability of platelets to secrete thrombin-induced platelet microbicidal proteins and antimicrobial peptides, like hBD-1. However, the expression and secretion of defensins of the alpha family by platelets have not been fully elucidated. The aim of this study was to characterize the expression of defensin alpha 1 (DEFA1) in human platelets and megakaryocytes. Our data indicate that DEFA1 mRNA and protein are present in peripheral blood platelets and in the megakaryoblastic leukemia cell line (MEG-01). DEFA1 co-localize with α-granules of platelets and MEG-01 cells, and was also detected in cytoplasm of MEG-01 cells. The assay of our in vitro model of platelet-like particles (PLPs) revealed that MEG-01 cells could transfer DEFA1 mRNA to their differentiated PLPs. Furthermore, platelets secreted DEFA1 into the culture medium when activated with thrombin, adenosine diphosphate, and lipopolysaccharide; meanwhile, MEG-01 cells secreted DEFA1 when activated with thrombopoietin. Platelet's secreted DEFA1 can rebind to platelet's surface and have antibacterial activity against the gram-negative bacteria Escherichia coli. In summary, our data indicate that both, human platelets and megakaryocytes, can express and secrete DEFA1. These results suggest a new role of platelets and megakaryocytes in the innate immune response.