title: Complexes of platelet factor 4 and heparin activate Toll-like receptor 4.
abstract: BACKGROUND
  In some patients, the anticoagulant heparin elicits formation of antibodies that can cause the life and/or limb-threatening syndrome known as heparin-induced thrombocytopenia (HIT). HIT antibodies target complexes formed at specific molar ratios of heparin and platelet factor 4 (PF4). The unpredictable occurrence and the mechanism of this atypical immune response to PF4:heparin complexes are poorly understood.
  
  
  OBJECTIVE
  We investigated whether complexes formed at specific PF4:heparin ratios (PHRs) might resemble molecular patterns associated with host defense responses.
  
  
  METHODS
  We used an in vitro cytokine release assay to determine whether defined PHRs caused cytokine release from human whole blood. Lipopolysaccharide (LPS) was used as a positive assay control, and some experiments included antibodies to block Toll-like receptor 4 (TLR4).
  
  
  RESULTS
  PF4:heparin complexes caused release of the biomarker interleukin 8 in whole blood, and the level of response varied with the stoichiometric ratio of PF4 to heparin. The profile of response to LPS and to PF4:heparin complexes varied among blood donors, and the interleukin 8 response to both LPS and PF4:heparin was inhibited by TLR4-blocking antibodies.
  
  
  CONCLUSIONS
  Specific PF4-heparin complexes can elicit a TLR4-mediated response, suggesting that these complexes can mimic a pathogen-associated molecular pattern, and supporting the suggestion that the HIT immune response represents a misdirected host defense mechanism.
authors.affiliation: Cardiovascular Research Institute, Loyola University Health Sciences Division, Maywood, IL, USA.
authors.forename: M M
authors.initials: MM
authors.lastname: Prechel
authors.forename: J M
authors.initials: JM
authors.lastname: Walenga
mesh_terms: D004305:Dose-Response Relationship, Drug; D006493:Heparin / Q000097:blood* / Q000276:immunology / Q000633:toxicity*; D006801:Humans; D016209:Interleukin-8 / Q000097:blood; D008070:Lipopolysaccharides / Q000494:pharmacology; D010978:Platelet Factor 4 / Q000097:blood* / Q000276:immunology; D015398:Signal Transduction / Q000187:drug effects; D013921:Thrombocytopenia / Q000097:blood / Q000139:chemically induced* / Q000276:immunology; D051197:Toll-Like Receptor 4 / Q000819:agonists* / Q000097:blood / Q000276:immunology
chemical_list: C508606:CXCL8 protein, human; D016209:Interleukin-8; D008070:Lipopolysaccharides; C495345:TLR4 protein, human; D051197:Toll-Like Receptor 4; D010978:Platelet Factor 4; D006493:Heparin
keywords: Toll-like receptors; heparin; innate immune response; lipopolysaccharides; platelet factor 4