Vaccination against Legionella pneumophila: serum antibody correlates with protection induced by heat-killed or acetone-killed cells against intraperitoneal but not aerosol infection in guinea pigs.

An aerosol model of Legionella infection has been established in guinea pigs. Infected animals showed growth of Legionella in their lungs, dissemination of organisms to the spleen, development of pneumonia and fever, and weight loss. Vaccination studies using heat-killed or acetone-killed cells were carried out, and guinea pigs were challenged intraperitoneally or by using the aerosol model of infection. Both vaccines were shown to give moderately high levels of protection against intraperitoneal challenge (28 to 145 50% lethal doses). Protection was found to be dose dependent and correlated with antibody levels as measured by enzyme-linked immunosorbent assay to an outer membrane antigen and by indirect immunofluorescence to heat-killed cells. In contrast, the same vaccination regimens that protected against intraperitoneal challenge failed to protect guinea pigs against aerosol challenge with comparable doses of Legionella, despite the presence of serum antibody. The results are discussed in terms of the possible requirements for immunity to aerosolized Legionella, including secretory immunoglobulin or cell-mediated immunity.