Vascular colonisation by Bartonella henselae may cause vaso-proliferative tumour growth with clumps of bacteria found in close association with proliferating endothelial cells. By using B. henselae-infected human umbilical vein endothelial cells as an in vitro model for endothelial colonisation, we report here on a novel mechanism of cellular invasion by bacteria. First, the leading lamella of endothelial cells establishes cellular contact to sedimented bacteria and mediates bacterial aggregation by rearward transport on the cell surface. Subsequently, the formed bacterial aggregate is engulfed and internalised by a unique host cellular structure, the invasome. Completion of this sequence of events requires 24 hours. Cortical F-actin, intercellular adhesion molecule-1 and phosphotyrosine are highly enriched in the membrane protrusions entrapping the bacterial aggregate. Actin stress fibres, which are anchored to the numerous focal adhesion plaques associated with the invasome structure, are typically found to be twisted around its basal part. The formation of invasomes was found to be inhibited by cytochalasin D but virtually unaffected by nocodazole, colchicine or taxol, indicating that invasome-mediated invasion is an actin-dependent and microtubuli-independent process. Bacterial internalisation via the invasome was consistently observed with several clinical isolates of B. henselae, while a spontaneous mutant obtained from one of these isolates was impaired in invasome-mediated invasion. Instead, this mutant showed increased uptake of bacteria into perinuclear localising phagosomes, suggesting that invasome-formation may interfere with this alternative mechanism of bacterial internalisation. Internalisation via the invasome represents a novel paradigm for the invasion of bacteria into host cells which may serve as a cellular colonisation mechanism in vivo, e.g. on proliferating and migrating endothelial cells during Bartonella-induced vaso-proliferative tumour growth.

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