Platelet factor 4 binds to bacteria, inducing antibodies cross-reacting with the major antigen in heparin-induced thrombocytopenia

Abstract

A clinically important adverse drug reaction, heparin-induced thrombocytopenia (HIT), is induced by antibodies specific for complexes of the chemokine platelet factor 4 (PF4) and the polyanion heparin. Even heparin-naive patients can generate anti-PF4/heparin IgG as early as day 4 of heparin treatment, suggesting preimmunization by antigens mimicking PF4/heparin complexes. These antibodies probably result from bacterial infections, as (1) PF4 bound charge-dependently to various bacteria, (2) human heparin-induced anti-PF4/heparin antibodies cross-reacted with PF4-coated Staphylococcus aureus and Escherichia coli, and (3) mice developed anti-PF4/heparin antibodies during polymicrobial sepsis without heparin application. Thus, after binding to bacteria, the endogenous protein PF4 induces antibodies with specificity for PF4/polyanion complexes. These can target a large variety of PF4-coated bacteria and enhance bacterial phagocytosis in vitro. The same antigenic epitopes are expressed when pharmacologic heparin binds to platelets augmenting formation of PF4 complexes. Boosting of preformed B cells by PF4/ heparin complexes could explain the early occurrence of IgG antibodies in HIT. We also found a continuous, rather than dichotomous, distribution of anti-PF4/heparin IgM and IgG serum concentrations in a cross-sectional population study (n ‫؍‬ 4029), indicating frequent preimmunization to modified PF4. PF4 may have a role in bacterial defense, and HIT is probably a misdirected antibacterial host defense mechanism. (Blood. 2011;117(4): 1370-1378) IntroductionThe chemokine platelet factor 4 (PF4, CXCL4) is stored within platelet ␣-granules 1 and released during platelet activation. Although its biologic role is poorly understood, PF4 commands attention in clinical medicine because it binds charge-dependently to the anticoagulant heparin, one of the most frequently used anticoagulants in clinical medicine, thereby neutralizing heparin's anticoagulant effect, 2,3 but also forming highly antigenic multimolecular complexes. [4][5][6] The resulting antibody response 7 induces the most frequent immune-mediated adverse drug reaction involving human blood cells, heparin-induced thrombocytopenia (HIT). 8 The pathogenic antibodies bind to PF4/heparin complexes at the platelet surface, and the resulting immune complexes induce Fcreceptor-mediated platelet activation and enhanced thrombin generation. 8 In a subset of patients, this causes thrombocytopenia and triggers paradoxical thrombosis, which is aggravated by continuation of heparin treatment.The immune response of HIT has several atypical features. Even in patients who receive heparin for the first time, there is rapid induction (as early as 4 days) of anti-PF4/heparin antibodies of IgG isotype. Moreover, IgG antibodies are the predominant class of immunoglobulins formed. 9,10 We and others discovered that up to 50% of patients after cardiopulmonary bypass surgery and 20% to 30% of orthopedic surgery patients (many of whom have not been previously exposed to heparin) devel...

Anti–protamine-heparin antibodies: incidence, clinical relevance, and pathogenesis

“…[35][36][37] Protamine seems to be another example of such a mechanism, which may indicate that this type of rapid, but transient, immune reaction toward conformationally altered proteins might be more common than anticipated.…”

A case of heparin-induced thrombocytopenia (HIT) triggered by methicillin-susceptible Staphylococcus aureus (MSSA) bacteremia

“…This theory may explain the formation of HIT IgG after approximately 5 days in the early period. In rare cases, HIT was recently shown to develop incidentally among patients with no history of heparin administration [8]. These patients had auto-immune diseases such as SLE and/or recent microbial infection.…”

“…When bacterial infection occurs in the human body, platelets are activated and release positively charged PF4. PF4 binds charge dependently to various bacteria including Staphylococcus aureus [8]. Especially in gram-negative bacteria, PF4 combines with lipid A and forms a PF4/bacteria complex [9].…”

Relationship between the 4Ts scoring system and the antiplatelet factor 4/heparin antibodies test in critically ill patients

“…[31][32][33] Production of HIT antibodies is reportedly associated with periodontal disease 34 and induced by complex formation between PF4 generated after bacterial infection and invading bacterial cells. 32 Cases of "spontaneous HIT," characterized by manifestation of clinical symptoms of HIT and positivity for HIT antibodies without prior heparin exposure, have also been reported. 35 Platelet factor 4/heparin Ab positive individuals may include those developing spontaneous HIT.…”