Haemophilus (Actinobacillus) pleuropneumoniae Serotypes 5 and 7 capsular antigens (CA-1) were precipitated from culture supernatants with N-cetyl-N,N,N,-trimethylammonium bromide (Cetavlon). CA-1 contained a carbohydrate to protein ratio of 2:1 for Serotype 5 and 3:1 for Serotype 7. Glucosamine and uronic acid were detected in CA-1 from both serotypes suggesting that the capsule contained hyaluronic acid. All mice immunized intraperitoneally with CA-1 vaccine were protected from death when challenged with 10 × LD50 of the homologous but not the heterologous serotype. Oil adjuvants and the use of young (6 h) cultures were necessary for CA-1 vaccines to be protective. Deproteinization of CA-1 with chloroform and butanol followed by pronase treatment resulted in failure to protect mice from death. The protective capsular protein antigen in CA-1 vaccine may not originate from the outer membrane (OM) since repeated washing of the OM to elute the capsular protein antigen rendered the OM vaccine completely nonprotective for mice. Vaccines prepared from cell-wall lipopolysaccharide also were nonprotective for mice. Passive immunization of mice with anti-CA-1 antibody produced in rabbits to Serotype 5 was highly protective (P < 0.01) for mice when challenged with 10 × the LD50 of the homologous serotype.
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