SUMMARY: In the assessment of immunity to the encapsulated virulent strain of and as avirulent mutant defective for capsular polysaccharide (CPS), killed bacterial vaccine of both strains could protect mice equally against challenge with 100 x LD of encapsulated wild strain. Antisera to each strain conferred the same level of protection on naive mice upon transfer; the protective anti-mutant serum was highly capable of opsonizing the encapsulated bacteria. In addition to the common antigenic components shared by both strains, the wild strain had antigen(s) unrelated to the mutant since the protective capacity of the anti-wild serum was not affected by preabsorption with the mutant strain; the protection conferred by the anti-mutant serum was mediated by antibodies against non-capsular antigens since the antiserum did not contain antibodies against purified CPS detectable by ELISA. As possible candidates among the non-capsular antigens, outer-membrane proteins (OMPs) extracted from the mutant strain were examined for their immunogenicity. Immunoblotting of the protein-containing fraction and ELISA using LPS-free OMP suggested that a number of proteins were involved in the immune response evoked by Furthermore, mice immunized with OMP or anti-OMP serum could overcome a lethal challenge with the wild strain. These results indicated that OMPs of are implicated as the protective antigens and may pave the way for the development of non-capsular, proteinaceous vaccines.


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