Platelets and Infections – Complex Interactions with Bacteria

Abstract

Platelets can be considered sentinels of vascular system due to their high number in the circulation and to the range of functional immunoreceptors they express. Platelets express a wide range of potential bacterial receptors, including complement receptors, FcγRII, Toll-like receptors but also integrins conventionally described in the hemostatic response, such as GPIIb–IIIa or GPIb. Bacteria bind these receptors either directly, or indirectly via fibrinogen, fibronectin, the first complement C1q, the von Willebrand Factor, etc. The fate of platelet-bound bacteria is questioned. Several studies reported the ability of activated platelets to internalize bacteria such as Staphylococcus aureus or Porphyromonas gingivalis, though there is no clue on what happens thereafter. Are they sheltered from the immune system in the cytoplasm of platelets or are they lysed? Indeed, while the presence of phagolysosome has not been demonstrated in platelets, they contain antimicrobial peptides that were shown to be efficient on S. aureus. Besides, the fact that bacteria can bind to platelets via receptors involved in hemostasis suggests that they may induce aggregation; this has indeed been described for Streptococcus sanguinis, S. epidermidis, or C. pneumoniae. On the other hand, platelets are able to display an inflammatory response to an infectious triggering. We, and others, have shown that platelet release soluble immunomodulatory factors upon stimulation by bacterial components. Moreover, interactions between bacteria and platelets are not limited to only these two partners. Indeed, platelets are also essential for the formation of neutrophil extracellular traps by neutrophils, resulting in bacterial clearance by trapping bacteria and concentrating antibacterial factors but in enhancing thrombosis. In conclusion, the platelet–bacteria interplay is a complex game; its fine analysis is complicated by the fact that the inflammatory component adds to the aggregation response.

Platelet Lysate-Loaded Photocrosslinkable Hyaluronic Acid Hydrogels for Periodontal Endogenous Regenerative Technology

“…Then, 25 µL of me-HA solution were injected into a circular (5mm diameter) PDMS mold and exposed to a UV light (Omnicure series 2000 EXFO S2000-XLA, Omnicure, Canada) to trigger the photocrosslinking, producing disk-shaped hydrogels. The produced formulations, incorporating 0, 50 and 100% PL, were designated PL 0 , PL 50 , and PL 100 , respectively. …”

“…Protein release from PL 0 , PL 50 and PL 100 was quantified after 30min, 1h, 4h, 8h, 1, 7, 14 and 21 days of incubation in PBS at 37°C. For this purpose, at each time point, a volume of supernatant was collected and stored at -20°C.…”

“…Thus, formulations of hydrogels with increasing concentrations of PL (PL 0 , PL 50 and PL 100 ), were prepared into disc-shaped samples of 5mm in diameter and 1 mm thickness, as above described, and placed in 24 wells plate.…”

“…The hPDLFs (ScienCell Research Laboratories) at passage 3 were seeded on disc-shaped (5 mm diameter) samples of the formulations PL 0 , PL 50 and PL 100 produced as previously described, at a cell density of 5×10 4 cm -2 . A 50 µL drop of a cellular suspension containing 1×10 4 cells was seeded on the surface of each sample, previously placed in a 24 wells plate, and allowed to adhere for 1h.…”

Cellular Response Mechanisms in Porphyromonas gingivalis Infection

“…However, they are also involved in the immune system and express a broad repertoire of immune cell features such as TLRs, the immunoglobulin γ-receptor FcγRIIA, complement receptors, inflammatory mediators, as well as microbicidal activities, for example, throm bocidins [99,100]. Furthermore, platelets bind to and encapsulate bacteria, release ROS and recruit and activate leukocytes and regulate inflammatory processes of the vessel wall [101]. These characteristics make it possible for platelets to recognize and respond to pathogens, such as P. gingivalis, and engage other immune cells for enhanced bacterial clearance and inflammatory response.…”