<i>Anaplasma phagocytophilum</i> invasin AipB interacts with the β2 integrin β-subunit CD18 to orchestrate infection

<i>Anaplasma phagocytophilum</i> is an obligate intracellular bacterium that invades neutrophils to cause the potentially severe infection, human granulocytic anaplasmosis. The <i>A. phagocytophilum</i> invasin-host cell receptor interactions that facilitate bacterial uptake and hence disease are incompletely defined. Here, we show that EPHNCH_1611, which we refer to as <i>A. phagocytophilum</i> invasion protein B (AipB), mediates bacterial entry but not adhesion. The AipB host cell-interacting domain was narrowed down to amino acids 109 to 129 and is conserved among <i>Anaplasma</i> and <i>Ehrlichia</i> species. AipB_109-123 antisera reduced <i>A. phagocytophilum</i> infection of host cells by 28%. Combining anti-AipB_109-123 with antisera specific for the receptor binding domains of previously identified invasins AipA, Asp14, and OmpA reduced infection by 93%, suggesting a synergistic relationship among these surface proteins. Immunizing mice against AipB_109-123 alone yielded partial protection from <i>A. phagocytophilum</i> challenge that rivaled the protection achieved by immunizing against it together with other invasin binding domains. A yeast two-hybrid screen identified the N-terminal domain of CD18, the β2 integrin β-subunit found on the neutrophil surface, as an AipB binding partner. The interaction was validated by co-immunoprecipitation. siRNA knockdown of CD18 reduced permissiveness to <i>A. phagocytophilum</i> infection. Antibodies targeting the CD18 N-terminus inhibited <i>A. phagocytophilum</i> infection of, but not adherence to, HL-60 cells and human neutrophils. These results demonstrate that the AipB-CD18 interaction is critical for <i>A. phagocytophilum</i> infection <i>in vitro</i> and <i>in vivo</i>, further dissecting the multifaceted mechanism by which <i>A. phagocytophilum</i> invades host cells.IMPORTANCE<i>Anaplasma phagocytophilum</i> causes human granulocytic anaplasmosis, an emerging tick-borne infection for which there is no vaccine and limited treatments. Although <i>A. phagocytophilum</i> absolutely relies on invading host neutrophils to survive and cause disease, the microbe-host cell interactions that predicate infection are inadequately defined. We found that the bacterium uses its outer surface protein AipB to engage CD18 as a critical invasion step. An AipB peptide that is conserved among its homologs in other <i>Anaplasmataceae</i> pathogens is responsible for binding CD18 and can be targeted by antibody to inhibit infection <i>in vitro</i> and <i>in vivo</i>. However, blocking of <i>A. phagocytophilum</i> infection is most effectively achieved by targeting this AipB peptide together with the functional domains of other invasins. AipB is the first rickettsial invasin identified to interact with a β2 integrin, is key for pathogenesis, and could be targeted to protect against diseases caused by <i>A. phagocytophilum</i> and other <i>Anaplasmataceae</i> bacteria.