GENE-TRANSFER IN DENDRITIC CELLS, INDUCED BY ORAL DNA VACCINATION WITH SALMONELLA-TYPHIMURIUM, RESULTS IN PROTECTIVE IMMUNITY AGAINST A MURINE FIBROSARCOMA

A live attenuated AroA(-) auxotrophic mutant of Salmonella typhimurium (SL7207) has been used as carrier for the pCMV beta vector that conta ins the beta-galactosidase (beta-gal) gene under the control of the im mediate early promoter of Cytomegalovirus (CMV). We tested whether ora lly administered bacterial carrier could enter and deliver the transge ne to antigen-presenting cells (APCs) through the natural enteric rout e of infection and whether beta-gal expression could generate a protec tive response against an aggressive murine fibrosarcoma transduced wit h the beta-gal gene (F1.A11) that behaves operationally as a tumor-ass ociated antigen. After three courses, at 15-day intervals, mice develo ped both cell-mediated and systemic humoral responses to P-gal. Mice v accinated with the Salmonella harboring pCMV beta, but not with plasmi d-less carrier, showed resistance to a challenge with F1.A11 cells. Th ese experiments suggest that Salmonella-based DNA immunization allows us to specifically target antigen expression in vivo to APCs. To prove that the transgene is actually expressed by APCs as a function of an eukaryotic promoter, the green fluorescent protein (GFP) was placed un der the control of either the eukariotic CMV or a prokaryotic promoter . Using cytofluorometric analysis, GFP was detected only in splenocyte s of mice receiving a Salmonella carrier harboring GFP under the CWV p romoter. These results indicate that transgene expression occurs becau se of a Salmonella-mediated gene transfer to eukaryotic cells, Finally , approximately 19% of the splenocytes expressed GFP. Among them, F4/8 0(+) macrophages and CD11c(bright) dendritic cells (DCs) were scored a s positive for GFP expression. Extensive work has been performed tryin g to optimize the way to transfect DCs, ex vivo, with genes coding for relevant antigens. We show here, for the first time, that DCs can be directly and specifically transduced in vivo such to induce DNA vaccin ation against tumors. (C) 1998 by The American Society of Hematology.