Live recombinant Salmonella oral vaccine against avian influenza viruses.

In 1997, the avian influenza virus (AIV) subtype H5N1 was first reported to infect humans.1 Up to 3 March 2015, the cumulative number of confirmed human cases of H5N1 was 784, with 429 deaths. There is no evidence that H5N1 viruses can transmit efficiently from person to person.2 Therefore, the key to prevention of a human H5N1 pandemic is to first control the H5N1 virus in birds. Traditionally, inactivated influenza vaccines have been used prophylactically against pandemic influenza. However, their cost is high and yield is low, and they are difficult to administer on a large scale. The use of live, attenuated Salmonella vaccines has been extensively studied. Salmonella is a Gramnegative, intracellular bacterium.3 It has advantages as a live antigen delivery vector.4 First, it can be modified to express a wide range of antigens. Second, live Salmonella vaccine is easy to administer; it can be mixed with food and given to birds orally. Third, after oral administration, Salmonella can penetrate the Peyer’s patches via M cells in the gastrointestinal tract and colonise the mesenteric lymph nodes that contain various antigen-presenting cells.3 This can generate a range of immune responses, including mucosal and systemic immunity. Fourth, live Salmonella vaccine can be easily and cheaply produced.5 This study constructed analogous sets of live recombinant Salmonella Typhimurium vaccine strains expressing a model antigen—enhanced green fluorescent protein (EGFP). The advantages and deleterious or synergistic effects of these vaccine strains were identified by comparing their immunogenicity in mice.