A live avirulent mutant Legionella pneumophila vaccine induces protective immunity against lethal aerosol challenge

We have examined the capacity of a live avirulent mutant form of the intracellular bacterial pathogen Legionella pneumophila to induce immune responses and protective immunity in guinea pigs. The mutant L. pneumophila is nonlethal to guinea pigs and does not revert to virulence with passage through guinea pigs. In contrast, exposure of guinea pigs to aerosols containing wild-type L. pneumophila induces a pneumonic illness that clinically and pathologically resembles Legionnaires' disease in humans. Guinea pigs immunized by aerosol exposure to mutant L. pneumophila developed a strong humoral immune response to wild-type L. pneumophila antigens with reciprocal antibody titers of 32-512 (median 256) by the indirect fluorescent antibody assay, compared with titers of less than 2 for control (sham immunized) guinea pigs. Mutant immunized but not control guinea pigs also developed strong cell-mediated immune responses to wild-type L. pneumophila antigens, as demonstrated in assays of cutaneous delayed-type hypersensitivity and in vitro splenic lymphocyte proliferation. Mutant immunized guinea pigs developed strong protective immunity to lethal aerosol challenge with wild-type L. pneumophila. In four independent experiments 40-83% of mutant immunized guinea pigs survived compared with 0% of control guinea pigs. Overall, 13 of 21 (62%) mutant immunized guinea pigs survived compared with 0 of 21 (0%) control guinea pigs (P = 0.00002, Fisher's exact test, two-tailed). Mutant immunization induced protection comparable to wild-type immunization in these studies; 8 of 14 (57%) guinea pigs immunized by wild-type L. pneumophila survived. This study demonstrates that guinea pigs immunized with a live avirulent mutant L. pneumophila vaccine (a) develop a strong humoral immune response to wild-type L. pneumophila antigens; (b) develop a strong cell-mediated immune response to wild-type L. pneumophila antigens; and (c) develop protective immunity to lethal aerosol challenge with wild-type L. pneumophila. This study demonstrates the feasibility of a vaccine against Legionnaires' disease.