Mouse resident peritoneal macrophages partially control in vitro infection with Coxiella burnetii phase II

Coxiella burnetii, the agent of Q fever in man and of coxiellosis in other species, is a small, dimorphic, obligate intracellular bacterium, sheltered within large, acidified, and hydrolase-rich phagosomes. Although several primary and established cell lines, macrophage-like cells, and primary macrophages from other species have been infected with C. burnetii, the infection of mouse primary macrophages has not been sufficiently characterized. In this report quantification of DAPI (4', 6-diamino-2-phenylindole) fluorescence images acquired by confocal microscopy, and transmission electron microscopy were used to compare the infection of three mouse-derived cells, L929 fibroblasts, J774 macrophage-like cells, and resident peritoneal macrophages, with a phase II clone of C. burnetii known to be non-virulent for mammals. Infected peritoneal phagocytes differed from L929 or J774 cells in that: (a) large vacuoles took longer to appear (3-5 d instead of 2), and were only found in a subset (20-30%) of macrophages, as opposed to in more than 70% of the other cells; (b) total and vacuole-associated relative bacterial loads in L929 and J774 cells were several-fold higher than in peritoneal macrophages; (c) estimated doubling times of the bacteria were about 68 h in the primary macrophages, 18 h in J774 and 22 h in L929 cells. Thus, mouse resident peritoneal macrophages control both the formation of the large vacuoles and the intracellular proliferation of C. burnetii phase II.