Non-specific stimulation of immunoglobulin synthesis in mice by capsular polysaccharide of Klebsiella pneumoniae

Intramuscular injection of a non-immunogenic dose (1 mg) of the capsular polysaccharide of Klebsiella pneumoniae (CPS-K) into mice caused a marked and prolonged increase in the amount of the serum IgM and IgG. The increase of IgM after this injection was over 100 times greater than that of antigen-specific IgM antibodies produced by injection of an immunogenic dose of CPS-K (5 μg) or sheep red blood cells (SRBC) (2 X 10⁸). The increase in the amount of immunoglobulins was not related to induction of immunological paralysis to the major subcomponent of CPS-K (acidic CPS-K) by a large dose of antigen, but was induced by non-antigenic stimulation by the minor subcomponent of CPS-K (neutral CPS-K), which was non-immunogenic at any dose. In mice injected with a non-immunogenic dose of CPS-K, antibody levels in the serum to three kinds of non-cross-reacting antigens, SRBC, bovine serum albumin (BSA) and a possibly autochthonous IgG (IgM—IgG mixed type cryoglobulin) were also increased. The number of plaque-forming cells (PFC) for SRBC in the spleens of mice was also markedly increased after CPS-K injection. These antibodies to SRBC and BSA and PFC for SRBC were exclusively of IgM type and increased to as high levels as those found after specific antigenic stimulation. This marked increase in IgM antibody was neither followed by an increase in IgG antibody nor by immunological memory for a secondary IgG response.