Binding of A. phagocytophilum (Ap) strains HZ, HZA2, HGE1, and HGE1A2 to HL-60 and HL-60 A2 cells. Host cell-free A. phagocytophilum organisms were incubated with HL-60 or HL-60 A2 cells. After 30 min, unbound bacteria were removed and host cells were examined by immunofluorescence microscopy. (A and C) Percentages of HL-60 and HL-60 A2 cells with bound A. phagocytophilum organisms; (B and D) mean (± standard deviation) numbers of bound A. phagocytophilum organisms per cell. Representative results of one of three experiments are shown. Statistically significant (**, P < 0.01; ***, P < 0.001) values are indicated.