Pathogenicity island 2 mutants of Salmonella typhimurium are efficient carriers for heterologous antigens and enable modulation of immune responses

The potential use as vaccine delivery system of Salmonella typhimurium stra ins harboring defined mutations in the sseC (HH104) and sseD (MvP101) genes , which encode putative effector proteins of the type III secretion system of Salmonella pathogenicity island 2, was evaluated and compared with that of the well-characterized aroA mutant strain SL7207 by using beta-galactosi dase (beta-Gal) as a model antigen. When orally administered to immune-comp etent or gamma interferon-deficient (TFN-gamma(-/-)) BALB/c mice, both muta nts were found to be highly attenuated (50% lethal dose, >10(9) bacteria). Both strains were also able to efficiently colonize and persist in Peyer's patches. Immunization with HH104 and MvP101 triggered beta-Gal-specific ser um and mucosal antibody responses equivalent to or stronger than those obse rved in SL7207-immunized mice. Although immunoglobulin G2 (IgG2) serum anti bodies were dominant in all groups, IgG1 was also significantly increased i n mice vaccinated with MvP101 and SL7207. Comparable beta-Gal-specific IgA and Ige antibodies were detected in intestinal lavages from mice immunized with the different strains. Antigen-specific CD4(+) T-helper cells were gen erated after vaccination with all vaccine prototypes; however, responses we re significantly more efficient when HH104 and MvP101 were used (P < 0.05). Significantly higher levels of IFN-gamma were produced by restimulated spl een cells from mice immunized with HH104 than from those vaccinated with th e MvP101 or SL7207 derivatives (P less than or equal to 0.05). Interestingl y, the three strains induced major histocompatibility complex class I-restr icted CD8(+) cytotoxic T cells against beta-Gal; however, cytotoxic T-lymph ocyte responses were significantly stronger after immunization with HH104 ( P < 0.05). These novel S. typhimurium attenuated strains constitute promisi ng delivery systems for vaccine antigens. The qualitative differences obser ved in the obtained responses with different carriers may be useful for tho se applications in which a targeted immunomodulation is required.