The gram-negative bacteria of the genus Brucella are facultative intracellular parasites that cause
brucellosis, a world wide-distributed zoonotic disease that represents a serious problem for animal and
human health. There is no human-to-human contagion and, since there is no human vaccine, animal
vaccination is essential to control brucellosis. However, current vaccines (all developed empirically) do
not provide 100% protection and are infectious in humans. Attempts to generate new vaccines by
obtaining mutants lacking the lipopolysaccharide O-polysaccharide, in purine metabolism or in Brucella
type IV secretion system have not been successful. Here we propose a new approach to develop
brucellosis vaccines based on the concept that Brucella surface molecules evade efficient detection by
innate immunity, thus delaying protective Th1 responses and opening a time window to reach sheltered
intracellular compartments. We showed recently that a branch of the core oligosaccharide section of
Brucella lipopolysaccharide hampers recognition by TLR4-MD2. Mutation of glycosyltransferase WadC,
involved in the synthesis of this branch, results in a lipopolysaccharide that, while keeping the
O-polysaccharide essential for optimal protection, shows a truncated core, is more efficiently recognized
by MD2 and triggers an increased cytokine response. In keeping with this, the wadC mutant is attenuated
in dendritic cells and mice. In the mouse model of brucellosis vaccines, the Brucella abortus wadC mutant
conferred protection similar to that provided by S19, the best cattle vaccine available. The properties of
the wadC mutant provide the proof of concept for this new approach and open the way for more effective
brucellosis vaccines.