Influence of Acanthamoeba castellanii on intracellular growth of differentLegionella species in human monocytes

Previous studies using a murine model of coinhalation of Legionella pneumop hila and Hartmannella vermiformis have shown a significantly enhanced intra pulmonary growth of L. pneumophila in comparison to inhalation of legionell ae alone (J, Brieland, M, McClain, L, Heath, C, Chrisp, G. Huffnagle, M, Le Gendre, M, Hurley, J, Fantone, and C, Engleberg, Infect. Immun, 64:2449-245 6, 1996), In this study, we introduce an in vitro coculture model of legion ellae, Mono Mac 6 cells (MM6) and Acanthamoeba castellanii, using a cell cu lture chamber system which separates both cell types by a microporous polyc arbonate membrane impervious to bacteria, amoebae, and human cells. Whereas L. pneumophila has shown a maximal 4-log-unit multiplication within MM6, w hich could not be further increased by coculture with Acanthamoeba castella nii, significantly enhanced replication of L. gormanii, L, micdadei, L. ste igerwaltii, L, longbeachae, and L, dumoffii was seen after coculture with a moebae. This effect was seen only with uninfected amoebae, not with Legione lla-infected amoebae. The supporting effect for intracellular multiplicatio n in MM6 could be reproduced in part by addition of a cell-free coculture s upernatant obtained from a coincubation experiment with uninfected A. caste llanii and Legionella-infected MM6, suggesting that amoeba-derived effector molecules are involved in this phenomenon. This coculture model allows inv estigations of molecular and biochemical mechanisms which are responsible f or the enhancement of intracellular multiplication of legionellae in monocy tic cells after interaction with amoebae.