Transmission of Anaplasma phagocytophilum from endothelial cells to peripheral granulocytes in vitro under shear flow conditions

Anaplasma phagocytophilum (Ap) is a gram-negative, obligate intracellular bacterium that is able to infect different animal species and humans worldwide. Based on DNA sequencing, Ap has newly been reallocated from the genus Ehrlichia to the genus Anaplasma in the family Anaplasmataceae (DUMLER et al. 2001). In humans and animals, the clinical signs of Ap infection vary from mild symptoms to severe clinical outcomes, including death. However, the disease generally presents as undifferentiated fever accompanied by leucopenia, thrombocytopenia and increased serum transaminase activities (DUMLER et al. 2005; DUMLER et al. 2007; RIKIHISA 2011). Hard-bodied ticks of the genus Ixodes (family Ixodidae) are the main vectors for Ap dissemination. Compared to other pathogens such as Neorickettsia and Wolbachia spp., which can be transmitted from adult ticks to their offspring, Anaplasma and Ehrlichia spp. are the only Rickettsiales that are not transmitted transovarially (RIKIHISA 2011). Thus, ticks need to acquire Ap through blood feeding from infected hosts to complete the life cycle of Ap. During attachment of the tick, the bacterium is released by salivary secretion and is transmitted to the host. It is known that Ap multiplies within membrane-bound vacuoles (or called ‘morulae’) in the cytoplasm of peripheral granulocytes. The binding and infection of bacteria depends on the tetrasaccharide sialyl Lewisx (sLex or CD15s) of P-selectin glycoprotein ligand 1 (PSGL-1) on the surface of host cells, a factor expressed on peripheral granulocytes and HL-60 cells (GOODMAN et al. 1999; HERRON et al. 2000; RENEER et al. 2006; RENEER et al. 2008). Only little information is known about the transmission pathway of Ap after tick bite in the very early stage of infection. It is described that Ap is able to evade and replicate within microvascular endothelial cells in vitro (MUNDERLOH et al. 2004), while endothelial cells lining the inner lumen of blood vessels allow them to easily interact with any circulating blood cells. Since granulocytes do not return back to the blood stream after extravasation, it is reasonable to postulate that Ap evades and replicates within microvascular endothelial cells in the initial transmission, and subsequently transmits into peripheral granulocytes for ongoing dissemination. Therefore, the objective of the study was to establish a flow culture model that mimics the physiological environment in the blood vessel to study the possible transmission pathway of Ap between endothelial cells and polymorphonuclear leukocytes (PMNs). For this purpose, a novel ex vivo flow culture system was established. For experimental setup, human microvascular endothelial cell line (HMEC-1) and primary human dermal microvascular endothelial cells (HDMEC) were used. Under static conditions, Ap evades endothelial cells within 24 h, supporting the hypotheses that endothelial cells might be the first infection site of the pathogen in the host. Thereby a high level of interleukin-8, a chemokine that is known to recruit PMNs, secreted by Ap-infected endothelial cells was detected. Using the investigated flow culture model, it was shown for the first time, that Ap is able to translocate from endothelial cells to PMNs under dynamic flow conditions. Furthermore, under defined shear stress, an increased binding of PMNs to Ap-infected endothelial cells monolayer was observed, resulting from the elevated expression of adhesion molecules associated with PMNs recruitment on endothelial cells. The flow culture model investigated in this study can be used to study the interaction between Ap-infected endothelial cells and PMNs under physiological flow conditions, and is therefore helpful to study the infection mechanism in the early stage of Ap dissemination in the host