Effect of capsular polysaccharide of Klebsiella pneumoniae on host resistance to bacterial infections. I. Induction of increased susceptibility to infections in mice.

When Klebsiella pneumoniae capsular polysaccharide (CPS-K) from type 1, Kasuya strain, was injected intraperitoneally (i.p.) immediately before i.p. bacterial challenge, the survival time of mice infected with Salmonella enteritidis NUB 1 (virulent strain) was shortened and the mortality rate for mice infected with S. enteritidis NUB 31 (avirulent strain) was enhanced. The promotion of infection with S. enteritidis NUB 1 by CPS-K depended upon its dose, the effect of CPS-K being demonstrable up to as little as 0.2 mug per mouse. In the case of S. enteritidis NUB 31, the effect of CPS-K was detectable only when more than 20 mug per mouse was injected. As a result of enumeration of bacterial populations in the peritoneal washing, blood, liver and spleen, it was revealed that CPS-K promoted in vivo growth of S. enteritidis NUB 1 and NUB 31. In addition, CPS-K enhanced the mortality rate in mice infected with Streptococcus pyogenes or Streptococcus pneumoniae. The peak CPS-K effect on infection with S. enteritidis NUB 1 was seen when given immediately before bacterial challenge. The active substance responsible for the infection-promoting effect of CPS-K was neutral CPS-K, which is distinct from the O antigen and from acidic CPS-K (the type-specific capsular antigen). Preparations of neutral CPS-K isolated from the other three strains of K. pneumoniae exhibited a marked infection-promoting effect comparable with that of preparations from the Kasuya strain. Neutral CPS-K, with identical antigenicity to that from the Kasuya strain, has already been found to exert a strong adjuvant effect on antibody responses to various antigens in mice. No parallelism exists between infection-promoting activity and adjuvant activity of neutral CPS-K.