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

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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.

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Figure 1: Platelet TLR4 regulates adhesion of platelets to neutrophils, but not platelet P-selectin expression or platelet aggregation.
Figure 2: LPS induces platelet-neutrophil interactions in vivo.
Figure 3: Profound neutrophil degranulation and NET formation as a result of platelet binding.
Figure 4: NETs are formed under shear forces in vitro.
Figure 5: Formation of NETs greatly enhances trapping of bacteria in vitro and in vivo.
Figure 6: LPS-stimulated platelet-neutrophil interactions damage tissue.

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Acknowledgements

We would like to thank C. Gwozd for tissue processing and staining, and P. Colarusso for her technical assistance with microscopy. We would also like to acknowledge D. Brown and D. Knight for their technical support. This work was supported by grants from the Canadian Institutes of Health (CIHR) and a CIHR group grant. S.R.C. is a Heart and Stroke Foundation Canada fellow. P.K. is an Alberta Heritage Foundation for Medical Research (AHFMR) Scientist and a Canadian Research Chair recipient. A.C.M. is an AHFMR student.

Author information

S.R.C., A.C.M., S.A.T., B.M. and P.K. designed the studies. S.R.C., A.C.M., S.A.T. and B.M. performed the bulk of the experiments. P.K. wrote the manuscript, and A.C.M. and S.R.C. contributed to the preparation of the manuscript. Z.G. helped with flow cytometry. M.M.K. and E.M. searched for NETs in patients and mice, respectively. K.D.P. and S.C. completed the zymography assays and G.D.S. ran the aggregometry. F.H.Y.G. and E.M.K. helped with the Richardson microscope, and E.A.-V. and R.D. produced transfected bacteria. C.J.D. provided septic human blood samples.

Correspondence to Paul Kubes.

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The authors declare no competing financial interests.

Supplementary information

Supplementary Fig. 1

NETs within pulmonary microvasculature of an LPS-treated mouse visualized by fluorescence microscopy. (PDF 73 kb)

Supplementary Fig. 2

Sections of lung with acute interstitial pneumonitis, demonstrating intravascular NETs. (PDF 206 kb)

Supplementary Video 1 (MOV 2730 kb)

Supplementary Methods

Time-lapse of NET formation. Using darkfield and fluorescence microscopy one can see two neutrophils (orange — visualized by white light through an orange filter) begin to release DNA (green — dyed with Sytox Green) within 5 min. The DNA from both neutrophils fuses and forms one large NET. (PDF 110 kb)

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Clark, S., Ma, A., Tavener, S. et al. Platelet TLR4 activates neutrophil extracellular traps to ensnare bacteria in septic blood. Nat Med 13, 463–469 (2007) doi:10.1038/nm1565

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