Defense Date


Document Type


Degree Name

Doctor of Philosophy


Microbiology & Immunology

First Advisor

Jason Carlyon


Anaplasma phagocytophilum is an obligatory intracellular bacterium that infects neutrophils to cause human granulocytic anaplasmosis. Sialyl Lewis x (sLex)-modified P-selectin glycoprotein ligand-1 (PSGL-1) is the confirmed receptor utilized by A. phagocytophilum to bind and invade human neutrophils and myeloid cell lines. As an obligate intracellular pathogen, the binding of A. phagocytophilum to a host cell receptor is a prerequisite step for entry and replication, and thus its survival. However, the bacterial adhesins mediating this process have yet to be identified. In this study, we sought to identify surface proteins of A. phagocytophilum as putative adhesins. A. phagocytophilum undergoes a biphasic developmental cycle, transitioning between a smaller electron dense-cored cell (DC), which has a dense nucleoid, and a larger, pleomorphic electron lucent reticulate cell (RC), which has a dispersed nucleoid. We determined that the respective roles of the A. phagocytophilum DCs and RCs are adherence/infection and vacuolar replication, respectively, which is a finding that is consistent with the life cycles of other obligate intravacuolar pathogens that undergo biphasic development. Most importantly, we demonstrated the A. phagocytophilum DC is responsible for recognizing human PSGL-1. To identify surface proteins as putative adhesins we tested a variety of approaches. Three different computer prediction programs were compared, resulting in identification of 16 to 130 potential membrane proteins. As a more direct means to identify A. phagocytophilum surface proteins as PSGL-1 adhesins, several affinity capture approaches were tested. We used commercially available recombinant human PSGL-1 (rhPSGL-1) to try and capture adhesins by crosslinking and affinity purification. We were unsuccessful, but nevertheless gained insight into the binding properties of A. phagocytophilum. We next chose to take a broader approach to identify outer membrane proteins of the adherent DC by biotinylation. In the process we developed new density-gradient centrifugation approaches which successfully purified an RC-enriched population as well as a mixed population of RC and DC organisms. Results from this work demonstrate that A. phagocytophilum DC organisms are responsible for binding PSGL-1. Additionally, the results obtained thus far of gradient-purified bacteria will serve as a foundation for future experiments in identifying surface and developmental form specific proteins.


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Is Part Of

VCU Theses and Dissertations

Date of Submission

December 2010