Immune responses to Legionella pneumophila in athymic nude rats

Athymic nude rats (rnu) were immunized intraperitoneally with the attenuated Phil I strain (serogroup 1) of Legionella pneumophila. These rats developed a humoral immune response, including the IgG class, that was of comparable magnitude to that seen in their non-nude littermates. When the rats were challenged ip with a dose of the virulent strain LD-8 (also serogroup 1) that proved to be lethal in non-immune rnu-rats, none of them developed any signs of disease, but survived well for the observation period of 2 weeks. However, it was impossible by adoptive transfer of lymphocytes from immunized inbred PVG donors to protect non-immune syngeneic rats from the lethality of the LD-8. These results together with previous data (16) indicate that specific antibodies, present in the animal at the time of infection, protect effectively against a lethal dose of L. pneumophila in this experimental system.