Antibody response to the protein antigens of "Salmonella typhi" during typhoid infection and following vaccination with a live oral typhoid vaccine Ty21a.

Two protein antigens, named as S1Da and S1Db, common to many gram-negative bacteria, were prepared from an extract of Salmonella typhi cells. Antisera to S. typhi, Escherichia coli, Klebsiella pneumoniae and Pseudomonas aeruginosa, prepared by immunizing rabbits with acetone-treated cells of the corresponding bacterial species, all reacted against S1Da and S1Db as shown both by counterimmunoelectrophoresis (CIE) and by enzyme-linked immunosorbent assay (ELISA). Forty nine (98%) of the 50 tested sera obtained from typhoid patients while only 4 (8%) of the 50 sera obtained from febrile non-typhoid patients showed antibodies to S1Da and S1Db with ELISA titres above 1.000. S1Da and S1Db were then tested against 80 sera collected from 24 volunteers who had received a live typhoid vaccine Ty21a. Those vaccinees, who had antibodies to the LPS antigens of S. typhi following Ty21a vaccination, also developed antibodies to both the S1Da and S1Db antigens. Antigenically, S1Da and S1Db were different from each other and from the LPS antigens of S. typhi. S1Da and S1Db were basically protein in nature, the reactivity of these antigens against a positive control antiserum was destroyed by pronase, i.e., the addition of pronase after coating the MICROELISA plates with S1Da or S1Db reduced the ELISA titre to a minimum. Treatment with pronase, however, did not affect the ELISA titre when plates coated with the LPS antigens of S. typhi were used as the solid-phase antigen.