Pseudomonas aeruginosa is the most common bacterium of postburn
infection. In the present study we investigated the immune mechanism of susceptibility to this type of postburn
infection and also examined the efficacy of
IL-18 treatment. C57BL/6 mice were challenged with P. aeruginosa on day 7 after
burn injury. Although the
burn-injured mice showed a poor survival rate after bacterial challenge, they retained their IFN-gamma production. The burned mice showed lower serum
IgM levels and a poor
IgM response following P. aeruginosa challenge in comparison with the
sham mice, whereas
IL-18 treatment after
burn injury (alternate day
injections for 1 wk) greatly improved the serum
IgM levels, which are P. aeruginosa-independent natural
IgM before bacterial challenge, thereby increasing the survival rate after the challenge.
IL-18 treatment also induced specific
IgM to P. aeruginosa in the sera 5 days after bacterial challenge in the burned mice. Interestingly, CD43(+)CD5(-)CD23(-)B220(dim) cells, namely B-1b cells, increased in the liver after the
IL-18 treatment and were found to actively produce
IgM in vitro without any additional stimulation. Furthermore, the
IL-18 treatment up-regulated the neutrophil count and the C3a levels in the blood as a result of the increased
IgM level, which may thus play a critical role in the opsonization and elimination of any invading bacteria.
IL-18 treatment for the burned mice and their resultant natural
IgM production were thus found to strengthen the host defense against P. aeruginosa
infection.