Platelets in Inflammation and Resolution

Neutrophils engulf and kill bacteria when their antimicrobial granules fuse with the phagosome. Here, we describe that, upon activation, neutrophils release granule proteins and chromatin that together form extracellular fibers that bind Gram-positive and -negative bacteria. These neutrophil extracellular traps (NETs) degrade virulence factors and kill bacteria. NETs are abundant in vivo in experimental dysentery and spontaneous human appendicitis, two examples of acute inflammation. NETs appear to be a form of innate response that binds microorganisms, prevents them from spreading, and ensures a high local concentration of antimicrobial agents to degrade virulence factors and kill bacteria. Show more

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. Show more

Platelets are critical for hemostasis, thrombosis, and inflammatory responses 1,2 , yet the events leading to mature platelet production remain incompletely understood 3 . The bone marrow (BM) is proposed to be a major site of platelet production although indirect evidence points towards a potential pulmonary contribution to platelet biogenesis 4-7 . By directly imaging the lung microcirculation in mice 8 , we discovered that a large number of megakaryocytes (MKs) circulate through the lungs where they dynamically release platelets. MKs releasing platelets in the lung are of extrapulmonary origin, such as the BM, where we observed large MKs migrating out of the BM space. The lung contribution to platelet biogenesis is substantial with approximately 50% of total platelet production or 10 million platelets per hour. Furthermore, we identified populations of mature and immature MKs along with hematopoietic progenitors that reside in the extravascular spaces of the lung. Under conditions of thrombocytopenia and relative stem cell deficiency in the BM 9 , these progenitors can migrate out of the lung, repopulate the BM, completely reconstitute blood platelet counts, and contribute to multiple hematopoietic lineages. These results position the lung as a primary site of terminal platelet production and an organ with considerable hematopoietic potential. Show more