Novel Anti-bacterial Activities of β-defensin 1 in Human Platelets: Suppression of Pathogen Growth and Signaling of Neutrophil Extracellular Trap Formation

Abstract

Human β-defensins (hBD) are antimicrobial peptides that curb microbial activity. Although hBD's are primarily expressed by epithelial cells, we show that human platelets express hBD-1 that has both predicted and novel antibacterial activities. We observed that activated platelets surround Staphylococcus aureus (S. aureus), forcing the pathogens into clusters that have a reduced growth rate compared to S. aureus alone. Given the microbicidal activity of β-defensins, we determined whether hBD family members were present in platelets and found mRNA and protein for hBD-1. We also established that hBD-1 protein resided in extragranular cytoplasmic compartments of platelets. Consistent with this localization pattern, agonists that elicit granular secretion by platelets did not readily induce hBD-1 release. Nevertheless, platelets released hBD-1 when they were stimulated by α-toxin, a S. aureus product that permeabilizes target cells. Platelet-derived hBD-1 significantly impaired the growth of clinical strains of S. aureus. hBD-1 also induced robust neutrophil extracellular trap (NET) formation by target polymorphonuclear leukocytes (PMNs), which is a novel antimicrobial function of β-defensins that was not previously identified. Taken together, these data demonstrate that hBD-1 is a previously-unrecognized component of platelets that displays classic antimicrobial activity and, in addition, signals PMNs to extrude DNA lattices that capture and kill bacteria.

Platelets: bridging hemostasis, inflammation, and immunity

“…Importantly, recent work has suggested that there are additional molecules, such as defensins, which are not located in any of the three aforementioned granules but rather exist as either cytosolic molecules or as constituents of an as yet unidentified granule [11].…”

“…This aggregation progresses until the bacteria are completely encased within the platelet aggregate, a situation that is reminiscent of the response of hemocytes to foreign particles [3,4]. Studies have shown that encapsulation by platelets actively attenuates bacterial growth, possibly through the activity of platelet-derived mediators such as b-defensin [11].…”

“…Interestingly, PF4 has also been shown to bind to the Duffy-antigen receptor (Fy), resulting in the death of infected red blood cells [28]. Moreover, human platelets, upon activation, release b-defensins that have direct and potent antimicrobial effects on Staphylococcus aureus [11]. Furthermore, peptides derived from a number of other platelet mediators (NAP-2, CTAP-III, IL-8) have also been demonstrated to have antimicrobial activity [5].…”

Platelets as immune-sensing cells

“…[13][14][15][16][17][18][19] Activated platelets were shown to inhibit the growth of the bacterium Staphylococcus aureus through encapsulating the bacteria and secreting the anti-microbial peptide b-defensin to promote the formation of neutrophil extracellular traps (NETs). 20 Alternatively, bacterial trapping of methicillin-resistant S aureus and Bacillus cereus has also been shown to occur on hepatic Kupffer cells through a mechanism dependent on engagement of the platelet adhesion receptor glycoprotein (GP)1b. 21 In addition, during sepsis, platelet TLR4 was shown to activate neutrophils causing them to release NETs, thereby trapping bacteria in hepatic and pulmonary blood vessels.…”

Characterization of Filifactor alocis and its immune evasion strategies employed against human neutrophils.

“…NETs operate through two major mechanisms to perform their antimicrobial functions, trapping bacteria and preventing their spread both locally and systemically and killing through the use of embedded antimicrobial peptides (149,150). Their fibrous structure is known to contain histones, antimicrobial granular and cytoplasmic proteins, which can trap and kill microbes (72).…”