Human granulocytic anaplasmosis (HGA) is a potentially fatal infection and the second-most common tickborne disease in the United States. Its incidence is increasing at an alarming rate. Indeed, the 39% rise in the number of cases from 2016 to 2017 is greater than for any other tickborne infection. Even more concerning, the true incidence of HGA in this country is estimated to be 10- to 50-fold higher. The causative agent of HGA is Anaplasma phagocytophilum, an obligate intracellular bacterial pathogen that exhibits an unusual tropism for neutrophils. Like all obligate intracellular bacteria, Ap must invade cells to survive and cause disease. Identifying the adhesins and their host cell receptors that facilitate this essential step is fundamental to understanding Ap pathogenesis and preventing HGA. During the prior funding period, we identified three adhesins called Asp14, AipA, and AipB that Ap expresses when it converts to the infectious form and during tick transmission feeding on mammalian hosts. Hence, they are ideal targets for neutralizing infection. The adhesins’ receptor binding domains are Asp14 residues 113-124 (Asp14113-124), AipA residues 9-21 (AipA9-21), and AipB residues 80-94 (AipB80-94). An antibody cocktail against all three domains robustly inhibits Ap infection of host cells, whereas targeting only one or two is less effective. Therefore, they function cooperatively, are compensatory, and maximal blockade of Ap infection is achievable by disrupting the full cohort of adhesin-receptor interactions. Asp14 binds protein disulfide isomerase (PDI) on the neutrophil surface, which brings Ap in sufficient proximity to PDI such that it reduces bacterial surface thiols as a key step in infection. AipB is a PDI substrate that potentially binds CD18, a 2 integrin component and newly discovered Ap receptor. AipA and CD13 are a third Ap adhesin- receptor pair that is critical for invasion. PDI, CD13, and CD18 cluster in lipid rafts, which are essential platforms for Ap entry. Moreover, signaling events that CD13 and CD18 elicit are associated with Ap infection. Mice immunized against Asp14113-124 and/or AipA9-21 are significantly, but incompletely, protected from Ap challenge. Thus, Asp14 and AipA are important for Ap infection in vivo and their receptor binding domains are protective antigens. For our competitive renewal, we will build on these compelling data. We will dissect how the AipA- CD13 and AipB-CD18 interactions orchestrate Ap uptake into host cells and define the relevance of AipB to Ap infection in vivo. We will determine the protective capacity of AipB alone and in cooperation with AipA and Asp14 against syringe-mediated and tick-transmitted Ap challenge. Completing the proposed aims will yield the most comprehensive understanding of adhesin-receptor interactions for any rickettsial agent and one of the most refined models for the molecular basis of infection among intracellular bacterial pathogens. This fundamental knowledg...