The adjuvant effect of pertussis endotoxin protein in modulating the immune response to cholera toxoid in mice.

Endotoxin protein represents a group of immunobiologically active p polypeptides which are associated with the lipopolysaccharide endotoxin in the outer membrane of Gram-negative bacteria. To study the adjuvant effect of endotoxin protein, CF-1 mice were immunized intraperitoneally with graded doses of glutaraldehyde-inactivated cholera toxoid with and without endotoxin protein prepared from Bordetella pertussis, Salmonella typhi or Vibrio cholerae. Immune responsiveness was assessed by measuring resistance to intravenous challenge with cholera enterotoxin and by serum antitoxin responses. The results showed that endotoxin protein from S. typhi can enhance the 50% protective dose (PD50) of cholera toxoid five to 12-fold, the endotoxin protein from V. cholerae enhances the PD50 six to seven fold at most, but the endotoxin protein from B. pertussis can enhance the PD50 some 27-fold. Furthermore, within the variability of both the mouse protection test and the rabbit intracutaneous assay of toxin induced vascular permeability, mouse serum neutralizing antitoxin levels correlated with the greater degree of resistance of the mice to the toxin challenge. The adjuvant effect also has been demonstrated by measuring the appearance of antitoxin specific plaque forming cells derived from mouse lymphocyte cultures. After seven days of culture in the presence of endotoxin protein and cholera toxoid, the number of plaque forming cells to cholera toxin coated sheep erythrocytes was enhanced some 28 times as compared to the cultures exposed to the cholera toxoid alone.