Complexes of platelet factor 4 and heparin activate Toll‐like receptor 4

Summary 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 ( PF 4). The unpredictable occurrence and the mechanism of this atypical immune response to PF 4:heparin complexes are poorly understood. Objective We investigated whether complexes formed at specific PF 4:heparin ratios ( PHR s) might resemble molecular patterns associated with host defense responses. Methods We used an in vitro cytokine release assay to determine whether defined PHR s 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 ( TLR 4). Results PF 4:heparin complexes caused release of the biomarker interleukin 8 in whole blood, and the level of response varied with the stoichiometric ratio of PF 4 to heparin. The profile of response to LPS and to PF 4:heparin complexes varied among blood donors, and the interleukin 8 response to both LPS and PF 4:heparin was inhibited by TLR 4‐blocking antibodies. Conclusions Specific PF 4–heparin complexes can elicit a TLR 4‐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.