Effect of endocytosis inhibitors on Coxiella burnetii interaction with host cells.

The obligate intracellular rickettsia Coxiella burnetii has previously been reported to reach the intravacuolar compartment of host cells by phagocytosis. With the aim to further examine the mechanisms of C. burnetii internalisation, macrophage monolayers were treated with well characterised inhibitors of endocytosis. The treatment with two general inhibitors, colchicine and methylamine, resulted in a pronounced dose-dependent decrease of radiolabelled phase II rickettsiae retained from the intracellular fraction. A third inhibitor used, amiloride, has been reported to reduce effectively clathrin-independent pinocytic pathways. The internalisation of C. burnetii was shown to be substantially reduced also by amiloride and the effect was dependent on its concentration. The passive role of C. burnetii in the internalisation was verified by using heat-killed C. burnetii. Host cells treated with either of the three inhibitors (amiloride, colchicine and methylamine) showed a similar reduction of intracellular C. burnetii after exposure to killed as well as live organisms. The data presented indicate that different endocytic mechanisms, pinocytosis as well as phagocytosis, may mediate the uptake of C. burnetii by a host cell.