The Anaplasma phagocytophilum invasin AipB promotes host cell invasion

Anaplasma phagocytophilum is a tick-transmitted obligate intracellular bacterium that causes human granulocytic anaplasmosis (HGA). Since HGA became reportable in 2000, cases have increased significantly. Currently, the treatment of choice is doxycycline. There is no vaccine available. Because A. phagocytophilum must invade host cells to propagate, infection can be disrupted by inhibiting host-pathogen interactions. Three bacterial adhesins and invasins and their host cell receptors have been identified. Outer membrane protein A (OmpA) interacts with sialyl Lewis x (sLex)-capped P-selectin glycoprotein ligand-1 (PSGL-1). 14-kDa A. phagocytophilum surface protein (Asp14) interacts with protein disulfide isomerase (PDI). Through a series of in vitro and in vivo experiments it has been demonstrated that these adhesins and invasins work cooperatively to facilitate infection by exploiting host cell signaling pathways. A. phagocytophilum undergoes a biphasic developmental cycle, transitioning between the infectious dense-cored form (DC) and the non-infectious replicating reticulate cell (RC). A proteomic analysis of A. phagocytophilum during infection of HL-60 cells revealed a protein that is abundantly expressed on the infectious DC form. A. phagocytophilum invasion protein B (AipB) was identified as a potential invasin and its binding domain has been narrowed down to a linear stretch of 21 amino acids. A yeast-two hybrid analysis using AipB as bait identified a putative host cell interacting partner. The relevance of AipB and the interacting partner to A. phagocytophilum infection was confirmed through a series of in vitro and ex vivo experiments. These data help contribute to understanding how A. phagocytophilum establishes infection and presents new targets that may be useful in developing a vaccine