Antagonism between two immunogens extracted from Brucella (cell wall peptidoglycan and lipopolysaccharide fractions) and inactivity of the brucellin allergen in immunization of the mouse.

Three fractions extracted from Brucella possessing immunogenic activity were tested either as unique antigens or in combinations using various dose levels. Immunity in the mouse was tested one month after vaccination by the intraperitoneal administration of a virulent strain of B. abortus and the spleen count of Brucella 15 days after challenge. The brucellin protein fraction, the cutaneous hypersensitivity reagent did not immunize the mouse and did not interfere with the immunization produced by the PG (cell wall PG extracted with SDS) and LPS fractions. The PG fractions, extracted either from B. abortus (PG-F8) or from B. melitensis (PG-F7), conferred good dose-dependent immunity used on their own. The LPS fraction (B. melitensis) on its own also conferred good immunity but the dose-response curve was less regular. Immunity induced by either fraction was not affected by the presence of an oil adjuvant. Various dose-ratio combinations of PG and LPS were studied with respect to their interactions in three successive factorial experiments. The interactions were consistently significant and largely negative. According to the dose of each fraction and the ratio, the immunity was either equivalent to that produced by the most active fraction in the combination (no additive effect) or less (antagonism). The antagonism between two vaccinal fractions extracted from the same bacteria pleads in favour of purified vaccines.