Ultrastructural analysis of the invasion of tick cells by Lyme disease spirochetes (Borrelia burgdorferi) in vitro

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Abstract
The association of Lyme disease spirochetes, Borrelia burgdorferi, with tick cell cultures was characterized by electron microscopy. These cells were active in endocytosis and intracellular digestion, containing coated vesicles, pits, and phagosomes. Borrelia burgdorferi in tick cell cultures resembled those described in tick tissues. In RAE25 cultures, isolated from Rhipicephalus appendiculatus embryos, invasion of cells was mediated by coated pits, indicating active host cell participation. The invading, tapered end of B. burgdorferi contained an electron-dense body that persisted throughout invasion. A host-derived membrane surrounded invading and intracellular borreliae as observed by transmission electron microscopy of cross and longitudinal sections, whereas degenerating ones lay inside secondary lysosomes. Borrelia burgdorferi cocultivated with cell lines from Ixodes scapularis were mainly found at the cell surface or within lysosomes. The differences seen in invasion of these cell lines are interpreted to reflect differences in cell types rather than species. Gemmae, indicative of degenerative changes in the spirochetes, were observed extra- and intra-cellularly. Membrane blebs were liberated by the spirochetes into the medium and onto the cells, and were avidly endocytosed. Tick cell cultures are a useful tool to elucidate spirochete – vector cell interactions that may be obscured in vivo.

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