Immunization with wild-type p53 gene sequences coadministered with Flt3 ligand induces an antigen-specific type 1 T-cell response

We examined the ability of immunization with sequential adenovirus/plasmid DNA vectors expressing human wild-type p53 to stimulate a type 1 T-cell res ponse and induce protection against challenge from a metastatic tumor that expresses mutated murine p53. We found that tumor protection and an antigen (Ag)-specific immune response were enhanced by prior injection of FIB liga nd (Flt3L) at a dose and schedule that significantly increased dendritic ce ll (DC) number and frequency. Preliminary studies using enzyme-linked immun ospot and Winn assays suggested that Ag-specific CD8 cells, with their sign ificant increase in IFN-gamma -secreting activity (Tel cells), were respons ible for the tumor protection. The delayed-type hypersensitivity response t o p53 was increased in mice immunized with p53 alone or p53 and Flt3L compa red with a negative control. In contrast, spleen cells from mice immunized with p53 and Flt3L exhibited a higher Ag-specific proliferative response th an mice immunized with p53 alone. The frequencies of Ag-specific IFN-gamma and interleukin (IL)-4-secreting cells were determined using an enzyme-link ed immunospot assay, which demonstrated that the frequency of IFN-gamma sec reting cells was significantly higher in mice immunized with p53 and Flt3L than in mice receiving Flt3L, excipient, or p53 treatment alone. In contras t, the frequency of IL-4-secreting cells did not differ significantly among these groups. We also observed an increased frequency of IL-12 and IFN-gam ma -secreting cells (but not IL-4 or IL-10) in the spleens of mice immediat ely after 10 days of Flt3L treatment, which was also the day of p53 priming . This observation supports the likelihood that there are multiple mechanis ms of Flt3L adjuvant activity, including expansion of DC and type 1 T-cell number. Overall, these results suggest that immunization with p53 genetic s equences after in vivo expansion of DC, using Flt3L, provides a useful stra tegy to induce p53-specific, and protective, type 1 T-cell responses.