Entry of Coxiella burnetii into host cells

The attachment to and entry into L mouse fibroblast cells of viable phase I and phase II Nine Mile Coxiella burnetii was investigated. The use of 32P-labelled rickettsiae showed that phase II C. burnetii attached more readily to L cells than phase I organisms; this probably accounts for the more rapid establishment of infection of host cells by the phase II agents. Two lines of evidence indicated that C. burnetii plays a passive role in both attachment and entry into host cells: (1) inactivation of rickettsiae by either heat or glutaraldehyde did not affect either process, and (2) metabolic inhibitors of L cell phagocytic function--NaF and cytochalasin B and D--abolished rickettsiae uptake. These results indicate that it is an endocytotic event. While the presence of purified phase I lipopolysaccharide (LPS) did not interfere with attachment of rickettsiae to the surface of host cells, it markedly impaired entry of C. burnetii in both phases. This suggests that LPS is not an adhesin and that it is toxic to the host cell. Treatment of L cells with either pronase, subtilisin or subtilopeptidase A significantly reduced the number of C. burnetii that adhered to the host cell surface; this result suggests that proteins are either proximate to or components of the C. burnetii attachment site.