Anaplasma phagocytophilum, a member of the family Anaplasmataceae, is the tick-transmitted obligate intracellular bacterium that causes
human granulocytic anaplasmosis. The life cycle of A. phagocytophilum is biphasic, transitioning between the noninfectious reticulate cell (RC) and infectious dense-cored (DC) forms. We analyzed the bacterium's DC surface
proteome by selective biotinylation of
surface proteins,
NeutrAvidin affinity purification, and mass spectrometry. Transcriptional profiling of selected outer
membrane protein candidates over the course of
infection revealed that aph_0248 (designated asp14 [14-kDa A. phagocytophilum
surface protein]) expression was upregulated the most during A. phagocytophilum cellular invasion. asp14 transcription was induced during transmission feeding of A. phagocytophilum-infected ticks on mice and was upregulated when the bacterium engaged its receptor,
P-selectin glycoprotein ligand 1. Asp14 localized to the A. phagocytophilum surface and was expressed during in vivo
infection. Treating DC organisms with Asp14 antiserum or preincubating mammalian host cells with
glutathione S-transferase (GST)-Asp14 significantly inhibited
infection of host cells. Moreover, preincubating host cells with GST-tagged forms of both Asp14 and
outer membrane protein A, another A. phagocytophilum invasin, pronouncedly reduced
infection relative to treatment with either
protein alone. The Asp14 domain that is sufficient for cellular adherence and invasion lies within the C-terminal 12 to 24
amino acids and is conserved among other Anaplasma and Ehrlichia species. These results identify Asp14 as an A. phagocytophilum
surface protein that is critical for
infection, delineate its invasion domain, and demonstrate the potential of targeting Asp14 in concert with OmpA for protecting against
infection by A. phagocytophilum and other Anaplasmataceae pathogens.