Elicitation of predictable immune responses by using live bacterial vectors

There is an increasing need for novel vaccines able to stimulate efficient and long-lasting responses, which have also low production costs. To confer protective immunity following vaccination, the adequate type of response should be elicited. Vaccines based on attenuated bacterial carriers have contained production and delivery costs, and are able to stimulate more potent immune responses than non-replicating formulations. The improved knowledge on carrier physiology and host response, the availability of different mutants and highly sophisticated expression tools, and the possibility of co-administering modulators enable to trigger predictable responses according to the specific needs. Recent studies support the use of attenuated bacteria not only as conventional carriers, but also as a delivery system for DNA vaccines against infectious agents and tumors. In this review we discuss the most widely used bacterial carrier systems for either antigens or nucleic acid vaccines, and the strategies which have been successfully exploited to modulate the immune responses elicited.