Infection of endothelial cells with Anaplasma marginale and A. phagocytophilum

Anaplasma marginale and A. phagocytophilum are obligate intracellular, tick-borne pathogens that target erythrocytes and neutrophil granulocytes, respectively. Because ticks do not directly tap blood vessels, an intermediate tissue may mediate infection of blood cells. We considered that vascular endothelium interacts with circulating blood cells in vivo, and could be involved in pathogenesis and dissemination of the organisms. We used light and electron microscopy and immune labeling to show that A. phagocytophilum invaded rhesus (RF/6A), human (HMEC-1, MVEC), as well as bovine (BCE C/D-1b) endothelial cell lines, whereas A. marginale infected rhesus and bovine endothelial cells. A. marginale formed large intracellular inclusions that appeared smooth and solid at first, and subsequently coalesced into discrete granules. A. phagocytophilum formed numerous smaller inclusions in each cell. Within 1-3 weeks, the monolayers were destroyed, and lysed cultures were diluted onto fresh monolayers. Electron microscopy demonstrated uneven distribution of A. marginale inside large inclusions, with reticulated forms grouped more tightly than denser cells, whereas in A. phagocytophilum individual organisms appeared more evenly spaced. Specific polyclonal and monoclonal antibodies both labeled A. marginale and A. phagocytophilum in endothelial cells, and oligonucleotide primers complimentary to either A. marginale or A. phagocytophilum amplified their expected target from these cultures. In conclusion, we demonstrate that relevant microvascular endothelium is susceptible to anaplasmas in vitro and may present a link that could explain development of the immune response and persistent infection.