Immunostimulation by Legionella pneumophila antigen preparations in vivo and in vitro

Injection of Legionella pneumophila antigen, either killed vaccine or soluble sonicate thereof, resulted in an enhanced antibody response by mouse spleen cells to sheep erythrocytes as determined by the hemolytic plaque assay. Enhancement was dose dependent and reached a peak response at a concentration of 10(7) bacteria or 50 micrograms of sonicate per animal. Larger doses of antigen were less stimulatory or even depressed the antibody response. Similar enhancement of antibody formation by normal spleen cell cultures to sheep erythrocytes in vitro occurred in the presence of graded amounts of L. pneumophila vaccine or sonicate. In addition, the L. pneumophila antigen stimulated enhanced background antibody formation in vitro in the absence of sheep erythrocytes or specific antigen. It appeared likely that the immunoenhancing activity of the L. pneumophila extract may be unrelated to the presence of lipopolysaccharide since boiling the antigen preparation eliminated much of the antibody-enhancing properties of the extract. A large-molecular-weight surface component from L. pneumophila was also immunomodulatory in vitro. Immunostimulation appeared to be related to effects on macrophages since adherent spleen cell populations rich in macrophages, when derived from spleen cell suspensions incubated with L. pneumophila antigen in vitro, stimulated enhanced antibody formation by normal mouse spleen cells in coculture experiments. Further investigations concerning the mechanism of immunomodulation by L. pneumophila antigen in vivo and in vitro appear to be warranted.