PF4 binds to bacteria found in periodontal lesions and PF4/bacteria complexes are recognized by anti-PF4/heparin IgG from sera of patients with HIT. (A) The anaerobic bacteria A actinomycetemcomitans (smooth and rough colonies; Aa smooth [n = 4] and Aa rough [n = 4]) and P gingivalis (Pg [n = 5]; both clinical isolates from periodontal pockets) were incubated with human biotinylated PF4. All 3 strains bound PF4 with saturation at approximately 20 μg/mL as measured by flow cytometry and expressed as geometric mean fluorescence intensity (GMFI) multiplied with the percentage of labeled bacteria. Data represent mean ± SD of at least 4 independent experiments. (B) From sera of 3 patients with HIT, known to contain anti-PF4/heparin Abs, IgG Abs were affinity purified using hu-PF4–precoated bacteria. Symbols represent reactivities of Abs affinity purified by PF4-coated Aa smooth (■) or rough colonies (▴) and Pg (●); means are presented as horizontal lines. Reactivity of the purified Abs with hu-PF4/heparin complexes (first column) was inhibited by excess heparin (100 IU/mL UFH, column 2), which disrupts PF4/heparin complexes. Abs did not react with hu-PF4 alone (column 3). Non-PF4–coated bacteria served as control for unspecific binding of the Abs to bacteria alone (column 4).