Vol. 72, No. 11
Combined Vaccine Regimen Based on Parenteral Priming with a DNA Vaccine and Administration of an Oral Booster Consisting of a Recombinant Salmonella enterica Serovar Typhimurium Vaccine Strainfor Immunization against Infection with Human-Derived Enterotoxigenic Escherichia coli Strains
Marcio O. Lasaro,1† Wilson B. Luiz,1 Maria E. Sbrogio-Almeida,2 Lucilia S. Nishimura,3 ´ Beatriz E. C. Guth,3 and Luis C. S. Ferreira1*
Department of Microbiology, Biomedical Sciences Institute, University of Sao Paulo,1 Department of Microbiology, ˜ Immunology and Parasitology, Federal University of SaoPaulo,3 and Division of Technological ˜ Development and Production, Butantan Institute,2 Sao Paulo, Brazil ˜
Received 21 January 2004/Returned for modiﬁcation 12 March 2004/Accepted 6 May 2004
Repeated evidence has demonstrated that combined primer-booster immunization regimens can improve both secreted and humoral immune responses to antigens derived from viral, bacterial, and parasiticpathogens. For the present work, we evaluated the synergic serum immunoglobulin G (IgG) and fecal IgA antibody responses elicited in BALB/c mice who were intramuscularly primed with a DNA vaccine, pRECFA, followed by oral boosting with an attenuated Salmonella enterica serovar Typhimurium vaccine (HG3) strain, with both vaccines encoding the structural subunit (CfaB) of the CFA/I ﬁmbriae produced byhuman-derived enterotoxigenic Escherichia coli (ETEC) strains. The immunological properties of the vaccine regimen were evaluated according to the order of the administered vaccines, the nature of the oral antigen carrier, the age of the vaccinated animals, the interval between the priming and boosting doses, and the amount of injected DNA. The production of gamma interferon and the IgG2a subclassin serum indicated that mice immunized with the primer-booster regimen developed prevailing type 1 T-cell-dependent immune responses. The synergic effect of the vaccine regimen on the induced antibody responses was also revealed by its ability to block the adhesive properties of CFA/I ﬁmbriae expressed by live bacteria, as shown by the inhibition of Caco-2 cell and human erythrocyte binding.Moreover, DBA2 newborn mice were protected from lethal challenges with a CFA/I ETEC strain after the incubation of live bacteria with serum samples harvested from mice who were subjected to the primer-booster regimen. We propose, therefore, that the DNA primer-Salmonella booster regimen represents an alternative for the development of vaccines requiring both mucosal and systemic antibody responses forimmunological protection. The stimulation of mammalian immune systems by the administration of DNA vaccines encoding heterologous antigens has been repeatedly demonstrated to efﬁciently activate humoral and cellular immune responses to various infectious agents and tumors (9, 19). Nonetheless, it is also well known that one of the main limitations of genetic vaccines is their limited capacity toinduce speciﬁc secreted antibody responses at intestinal or respiratory epithelia of animals that are immunized via parenteral routes. Consequently, several approaches have been designed to circumvent the limited mucosal immunogenicity of DNA vaccines, such as direct delivery of DNA to mucosal sites (23, 26, 30), incorporation of DNA into liposomes or biodegradable polymers (22, 25),coadministration of plasmids expressing cytokine or costimulatory molecules (12, 42), in vivo transfection mediated by attenuated bacterial vectors (8, 13), and primer-booster immunization regimens (11, 41). So far, most primer-booster immunization strategies based on DNA vaccines have targeted the induction of cellular and humoral systemic immune responses and have usually employed a DNA vaccine for priming...