Platelet TLR4 activates neutrophil extracellular traps to ensnare bacteria in septic blood

Abstract

It has been known for many years that neutrophils and platelets participate in the pathogenesis of severe sepsis, but the inter-relationship between these players is completely unknown. We report several cellular events that led to enhanced trapping of bacteria in blood vessels: platelet TLR4 detected TLR4 ligands in blood and induced platelet binding to adherent neutrophils. This led to robust neutrophil activation and formation of neutrophil extracellular traps (NETs). Plasma from severely septic humans also induced TLR4-dependent platelet-neutrophil interactions, leading to the production of NETs. The NETs retained their integrity under flow conditions and ensnared bacteria within the vasculature. The entire event occurred primarily in the liver sinusoids and pulmonary capillaries, where NETs have the greatest capacity for bacterial trapping. We propose that platelet TLR4 is a threshold switch for this new bacterial trapping mechanism in severe sepsis.

Platelets regulate leucocyte responses to Toll‐like receptor stimulation

“…Platelets are known to express various TLRs,5 and TLR stimulation can mediate a number of thrombotic7, 8, 21 and inflammatory processes 9, 10. Platelets and leucocytes aggregate in response to TLR stimulation,9, 12 and platelets contribute to vascular inflammation via this interaction with leucocytes. This current study adds to the growing body of evidence which suggests that platelets can actively participate in limiting leucocyte functions, a mechanism which is postulated to prevent uncontrolled inflammation that would otherwise cause host damage 13.…”

“…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.…”

Measuring and interpreting platelet-leukocyte aggregates

“…Microfluidic assays represent another microscopy-based approach to study platelet-leukocyte interactions in real time (11,69–74). Typically, isolated cells or whole blood are perfused through a chamber coated with immobilised proteins or cells, allowing precise control of soluble and cellular input.…”

“…Platelet-leukocyte interactions are further stabilised by crosstalk of numerous additional receptor/ligand pairs that were reviewed previously (1,5) and trigger mutual activation and release of granular content by both platelets and leukocytes, thereby modulating leukocyte function and fine-tuning immune responses (5). Importantly, platelet-leukocyte interactions facilitate leukocyte recruitment and extravasation to sites of inflammation (6,7), promote leukocyte release of pro-inflammatory mediators (8,9), oxidative burst, phagocytosis, release of neutrophil extracellular traps (NETs) (10,11), and may also dampen inflammation under some pathological conditions (12–14). …”

Managing thrombosis in cancer patients

“…Increased endothelial expression of P‐selectin appears to be important for CAT, possibly by promoting leukocyte adhesion 7, 12. Platelet‐leukocyte interactions may also be important because neutrophil extracellular traps (NETs) can both promote platelet aggregation and activate the coagulation cascade 13, 14. At least one ongoing clinical trial (NCT02285738) aims to examine the role of anti‐platelet agents in the prevention of CAT.…”