The Human Humoral Immune Response to Salmonella typhi Ty2la

The short-term kinetics and the effects of different dose regimens and formulations on the humoral immune response induced in human subjects by the live attenuated typhoid vaccine Salmonella typhi Ty21a were examined. Antibody responses in jejunal fluid and serum and by specific antibody production in vitro by peripheral blood lymphocytes to S. typhi lipopolysaccharide were determined. A short vaccination schedule of three doses of 1011 live organisms over 5 days induced significantly greater intestinal 19A antityphoid antibody responses than did two comparable doses 21 days apart. The humoral immune response was dose dependent with 1010 and 1011 live organisms stimulating greater intestinal immune responses than did 1011 killed organisms. No responses were evident with either 109 viable organisms or with an enteric-coated preparation. In the continued development and assessment of oral typhoid vaccines, the effects of different doses and formulations and the timing of sampling on the humoral immune response should be considered.