Heterologous expression of Brucella abortus GroEL heat-shock protein in Lactococcus lactis

Brucella abortus is a facultative intracellular pathogen that mainly infects cattle and humans. Current vaccines rely on live attenuated strains of B. abortus, which can revert to their pathogenic status and thus are not totally safe for use in humans. Therefore, the development of mucosal live vaccines using the food-grade lactic acid bacterium, Lactococcus lactis, as an antigen delivery vector, is an attractive alternative and a safer vaccination strategy against B. abortus. Here, we report the construction of L. lactis strains genetically modified to produce B. abortus GroEL heat-shock protein, a candidate antigen, in two cellular locations, intracellular or secreted.

Only the secreted form of GroEL was stably produced in L. lactis, suggesting a detrimental effect of GroEL protein when intracellularly produced in this bacterium. Only trace amounts of mature GroEL were detected in the supernatant fraction of induced lactococcal cultures, and the GroEL precursor remained stacked in the cell fraction. Attempts to raise the secretion yields were made, but even when GroEL was fused to a synthetic propeptide, secretion of this antigen was not improved.

We found that L. lactis is able to produce, and to secrete, a stable form of GroEL into the extracellular medium. Despite the low secretion efficiency of GroEL, which suggest that this antigen interacts with the cell envelope of L. lactis, secretion seems to be the best way to achieve both production and protein yields, regardless of cellular location. The L. lactis strain secreting GroEL has potential for in vivo immunization.