Antigens and toxic components of Legionella in pathogenesis and immunity.

Legionellae are opportunistic facultative intracellular pathogens with several antigens and toxic components which may function synergistically in contribution to pathogenesis and immunity. The serogroup specific antigens are associated with endotoxic activities of the bacteria and are potent adjuvants in inducing hypersensitivity and potentiating antibody responses to various antigens. Two protein toxins interfere with oxidative metabolism of host cells and are potential inhibitors of intracellular bactericidal activities. Tissue damage in the host appears to be caused by lysis of infected cells with release of toxic substances of host and bacterial origins. In host defense, the serogroup antigens provide primarily group-specific immunity, and the acid soluble protein toxin induces cross-protection among different serogroups. The combined effects of antibodies to these antigens in promoting phagocytosis and abrogating toxicity would enhance intracellular killing of the invading bacteria. The serogroup antigen complexes may also function as a potent adjuvant to modulate host responses to the invading bacteria.