Platelet factor 4 binding to lipid A of Gram-negative bacteria exposes PF4/heparin-like epitopes

Abstract

The positively charged chemokine platelet factor 4 (PF4) forms immunogenic complexes with heparin and other polyanions. Resulting antibodies can induce the adverse drug effect heparin-induced thrombocytopenia. PF4 also binds to bacteria, thereby exposing the same neoantigen(s) as with heparin. In this study, we identified the negatively charged lipopolysaccharide (LPS) as the PF4 binding structure on Gram-negative bacteria. IntroductionBesides their pivotal role in hemostasis, platelets are involved in host defense against pathogens and in modulation of immune reactions. This function of platelets occurs either indirectly through their interaction with endothelial cells and leukocytes 1,2 or directly by secretion of antimicrobial substances from platelet storage granules and lysosomes. 3,4 Recently, we have shown that the chemokine platelet factor 4 (PF4), which is stored within platelet ␣-granules, plays a role in bacterial host defense by inducing a humoral immune response to PF4-coated bacteria. 5 During bacterial infections, platelets are activated 6,7 and release positively charged PF4, which can bind in a charge-dependent manner to the bacterial surface, thereby inducing neoepitopes. The formation of antigenic PF4 clusters is probably the result of neutralization of the positive charge of PF4 by polyanions, 8 which allows narrowing of the distance between single PF4 tetramers down to 3 to 5 nm. This creates linear, ridge-like complexes and exposes new antigenic epitopes on PF4. 9 Antibodies to PF4/polyanion complexes bind to PF4 on the bacterial surface, leading to opsonization and increased phagocytosis of PF4-coated bacteria. 5 As PF4 is capable of binding to a large variety of bacteria, the antibody response to PF4/polyanion complexes constitutes a very broad reactive defense mechanism and could represent an evolutionary interface between innate and specific immunity. Antibodies induced by PF4 clusters would be an example of antibodies with a limited target antigen repertoire that nevertheless could result in binding to a large variety of bacteria when these bacteria are coated with PF4. 5 In medicine, research on the immune reaction to PF4/polyanion complexes to date has primarily focused on its role in causing an adverse reaction to the anticoagulant heparin as PF4 forms immunogenic complexes with heparin on platelet surfaces. Anti-PF4/polyanion antibodies bind to these PF4/heparin complexcoated platelets and induce Fc-receptor-dependent platelet activation, 10,11 leading to intravascular consumption of platelets, associated potentiation of in vivo thrombin generation, and the prothrombotic syndrome, heparin-induced thrombocytopenia (HIT).We and others have recently demonstrated the prevalence of anti-PF4/heparin antibodies of the IgM class in up to 20% and of the IgG class in up to 6% of the general population and in a slightly lower number of normal blood donors. 5,12 These antibodies are highly significantly associated with periodontitis, one of the most prevalent human infections, often associate...

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