Activated Monocytes Inhibit Legionella Multiplication

Legionella pneumophila, the agent of Legionnaires' disease, is a facultative intracellular pathogen (1). The bacter ium multiplies intracellularly in human monocytes, and under tissue culture conditions, multiplication is exclusively intraeellular (1). In previous studies, we investigated the role of humoral immuni ty in Legionnaires ' disease (2, 3). We found that virulent in vivo grown L. pneumophila bacteria are completely resistant to the bacteriocidal effects of h u m a n serum even in the presence of high-titer anti-L, pneumophila ant ibody (2). The bacteria also resist killing by freshly explanted h u m a n polymorphonuclear leukocytes and monocytes even in the presence of specific ant ibody and complement; under optimal conditions, only 0.5 log of an inoculum is killed by these phagocytes (2, 3). Most importantly, L. pneumophila bind more avidly to monocytes in the presence of specific ant ibody and complement, but the bacteria multiply intracellularly as rapidly as when they enter monocytes in the absence of ant ibody (3). These results indicate that humoral immunity, at least by itself, does not play a decisive role in host defense against Legionnaires ' disease. In this study, we investigate the potential role of cell-mediated immuni ty in Legionnaires ' disease. We demonstrate that (a) in vitro activated h u m a n monocytes have the capaci ty to inhibit the multiplication of virulent L. pneumophila; (b) cytokines produced by mitogen-sensitized h u m a n mononuclear cell cultures have the capacity to activate the monocytes such that they can inhibit L. pneumophila multiplication; and (c) activated monocytes inhibit L. pneumophila multiplication both by decreasing phagocytosis and by decreasing the rate of intracellular multiplication.