2.1.3. S. typhimurium Delivers DNA Vaccine
DNA vaccines encoding psaA and pspA have been shown to be effective in inducing antibody responses and Th1 immunity , which are important against pneumococcal infection . However, preparation and characterization of DNA vaccines need complex procedures . These procedures increase the cost of final products. DNA vaccines also induce poor mucosal responses in the nasopharynx. Zhang et al. used Salmonella to deliver multi-antigen-encoding DNA vaccines encoding psaA and pspA genes . They modified the DNA vector by replacing the selection marker from ampicillin to Asd to better maintain the vector and reduce the safety concern due to the use of antibiotic selection markers. They also eliminated the neomycin-resistance selection marker for the same concern. The modified vector was used to clone psaA and pspA genes. Salmonella delivering DNA vaccines encoding pspA or psaA, either alone or mixed together, significantly reduced S. pneumoniae colonization in nasal washes compared with control. Mice orally immunized with RASV carrying multi-antigen DNA vaccines significantly reduced nasal colonization by S. pneumoniae strain D39 compared to immunization with DNA vaccines administered intramuscularly (i.m.). These findings are related to the high level of sIgA in the nasal washer, as well as systemic IgG antibodies and a shift toward a Th1-mediated immune response