Legionella pneumophila: denizen of defenders

Legionella pneumophila, the causative agent of legionellosis, is an intracellular parasite of human monocytic cells and neutrophils. The life cycle of Legionella within phagocytic cells is distinct from that of other bacterial pathogens. Adherence of L pneumophila to phagocytes is mediated by attachment of complement proteins to the Legionella cell surface, followed by binding to complement receptors of phagocytes. Opsonized Legionella also may enter phagocytes after engagement of the Fc receptors. Within the host cell, the parasites reside in a membrane-bound vacuole that does not fuse with lysosomes. Activation of mononuclear phagocytes by the cell-mediated immune system serves to limit intracellular bacterial growth. Polymorphonuclear leukocytes are better at killing L pneumophila than are macrophages. However, Legionella also can invade and parasitize granulocytes. Although significant progress has been made in understanding some aspects of the pathogenesis of legionellosis, we know very little about the mechanisms by which these facultative intracellular parasites avoid killing by host defense mechanisms. This is an important area for future research and should lead to a better understanding of host-parasite interactions.