Immunization against Anaplasma phagocytophilum adhesin binding domains confers protection against infection in the mouse model

Anaplasma phagocytophilum causes granulocytic anaplasmosis, a debilitating infection that can be fatal in the immunocompromised. It also afflicts animals, including dogs, horses, and sheep. No granulocytic anaplasmosis vaccine exists. Because A. phagocytophilum is an obligate intracellular bacterium, inhibiting microbial-host cell interactions that facilitate invasion can disrupt infection. The binding domains of A. phagocytophilum adhesins AipA, Asp14, and OmpA are essential for optimal bacterial entry into host cells, but their relevance to infection in vivo are undefined. In this study, C57BL/6J mice were immunized with a cocktail of keyhole limpet hemocyanin-conjugated peptides corresponding to the AipA, Asp14, and OmpA binding domains in alum followed by challenge with A. phagocytophilum The bacterial peripheral blood burden was pronouncedly reduced in immunized mice compared to controls. Examination of pre- and post-challenge sera from these mice revealed that immunization elicited antibodies against AipA and Asp14 peptides, but not OmpA peptide. Nonetheless, pooled sera from pre- and post-challenge groups, but not from control groups inhibited A. phagocytophilum infection of HL-60 cells. Adhesin domain immunization also elicited IFNγ-producing CD8+ T cells. A follow up study confirmed that immunization against only the AipA or Asp14 binding domain was sufficient to reduce the bacterial peripheral blood load in mice following challenge and elicit antibodies that inhibit A. phagocytophilum cellular infection in vitro These data demonstrate that AipA and Asp14 are critical for A. phagocytophilum to productively infect mice and immunization against their binding domains elicits a protective immune response.