ANTIGENIC-COMPETITION IN A MULTIVALENT FOOT ROT VACCINE

The antigenic competition that occurs when pilus antigens of different serogroups are combined in multivalent vaccines for foot rot has been investigated using recombinant pilus antigens. Our prototype vaccine contains pill from nine serogroups of Dichelobacter nodosus which are expressed in Pseudomonas aeruginosa. Sheep inoculated with this multiv alent vaccine were not as well protected against foot rot as those giv en the monovalent vaccine. Levels of agglutinating and total antibody specific far any particular pill serogroup were found to be significan tly reduced in sheep vaccinated with six or more closely related pill. This effect was more pronounced for agglutinating antibody, which is thought to mediate protection, but was also observed with total antibo dy levels measured by ELISA. The antigenic competition was not associa ted with the total antigen load as a tenfold higher dose of monovalent pill induced high titres of antibody. Furthermore, distributing the v accine to four sites, each draining to a different lymph node, failed to overcome the competition. Experiments with mixtures of monospecific sera indicate that the phenomenon is unlikely to be due to blocking o f serogroup-specific protective antibodies by an excess of cross-react ive non-protective antibody elicited by heterologous pili.