Immune defenses against Legionella pneumophila in rats

Rats were examined for their susceptibility to infection with Legionella pneumophila. Different strains of L. pneumophila representing serogroups 1 to 4 were tested (Philadelphia 1 and 2, Togus 1, Bloomington 2, Los Angeles 1, and the recently isolated LD-8 from the epidemics in Västeraas, Sweden). When about 9 x 10(8) bacteria, or 6 x 10(8) colony-forming units, were injected intraperitoneally, only rats infected with the LD-8 strain showed signs of illness. They became ill within 4 to 6 h and usually died within 24 h with septicemia and signs of circulatory collapse. Bacteria killed by heating at 100 degrees C for 15 min lost their pathogenicity. Rats that were preimmunized with the Philadelphia 1 strain (same serogroup as LD-8), regardless of the route of immunization (subcutaneous, intraperitoneal, or intravenous), developed resistance to subsequent infection with LD-8. This resistance was serogroup specific and could be passively transferred to nonimmune animals with syngeneic immune serum or immunoglobulin fractions of serum. Antiserum or gamma globulin-enriched serum fractions from sheep with specificity for the Philadelphia 1 or Togus 1 strains also protected rats from the lethal effect of strain LD-8. The kinetics of the antibody and blood leukocyte response to immunization with the Philadelphia 1 strain of L. pneumophila in normal rats was investigated. During week 1 after intravenous injection the rats developed a marked lymphocytosis, which lasted for at least 1 week. The antibody response, as measured by indirect immunofluorescence or by a modification of the microagglutination method which we describe here, reached a peak during week 2 after immunization. Splenectomized rats were also resistant to infection with the Philadelphia 1 strain, but their antibody responses were substantially lower than those of their sham-operated littermates.