Formalin-killed cells of strain M-24 elicited an antibody-independent protective effect against infection in mice. The effect was observed as early as 6 h after administration and 100% protection was obtained by 48 h. The protective effect could not be attributed to the production of specific antibody. In M-24-treated mice, the bacteria in the peritoneal cavity, blood and liver were eliminated 12 h after infection. This suggested that the protective effect was due to enhanced bacterial elimination. The percentage of macrophages in the peritoneal cavity was increased after M-24 administration. Furthermore, the enhanced bacterial elimination was abrogated by treatment of mice with Coirradiation or carrageenan. These findings suggest the involvement of macrophages in the enhanced bacterial elimination observed. The chemiluminescence of peritoneal exudate cells from M-24-treated mice was markedly increased when compared with that of cells from untreated mice. The ability to kill was also greater in macrophages from mice treated with killed M-24 than in cells from proteose-peptone-treated mice. The M-24-treated mice showed enhanced nonspecific protection against infection with lethal doses of or . However, susceptibility to LPS in mice was not increased by M-24 treatment. These results suggest that macrophage activation without increasing LPS susceptibility was responsible for the antibody-independent protection induced by killed M-24.


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