Blastogenic responsiveness of spleen cells fromLegionella pneumophila-infected mice immunosuppressed with cyclophosphamide

Although guinea pigs are highly susceptible to experimental infection withLegionella pneumophila, mice are considered resistant. In the present study it was found that, although untreated mice resisted lethal infection with up to 10⁷ L. pneumophila, mice treated with three divided doses of cyclophosphamide became 10–100 times more susceptible. Injection of mice with 150 mg cyclophosphamide/kg body weight 96 and 48h prior to and on the same day as intraperitoneal challenge with graded dose ofL. pneumophila resulted in markedly increased lethality. Approximately half of the mice pretreated with cyclophosphamide succumbed to 10⁶ legionellae within 4–10 days after infection, and all treated animals given 10⁷ bacteria died. Legionellae were readily recovered from spleen, lymph nodes, and liver of surviving mice 4–10 days after infection, but not thereafter. Sensitization of mice with Legionella antigen was evident by the lymphocyte blastogenic test in vitro, by use of spleen cells at various times after infection. Mice given graded doses ofL. pneumophila evinced enhanced responsiveness to either formalin-killed whole cell vaccine, cell-free sonicate, or purified outer membrane antigen when tested in vitro on days 3 and 5. Peak responses generally occurred 20–35 days after infection. Mice given none or one dose of cyclophosphamide and injected with legionellae showed enhanced responses on day 5 of culture in vitro, a time when spleen cells from control nonsensitized animals showed much lower responses. Surviving mice given three doses of cyclophosphamide had lower blastogenic responses, generally as low as that occurring with spleen cells from nonsensitized animals. Thus suppression of immune responses of mice by cyclophosphamide substantially increased susceptibility toL. pneumophila and depressed blastogenic responsiveness.