Heterologous vaccination targeting prostatic acid phosphatase (PAP) using DNA and<i>Listeria</i>vaccines elicits superior anti-tumor immunity dependent on CD4+ T cells elicited by DNA priming

<b>Background</b>. Sipuleucel T, an autologous cell-based vaccine targeting prostatic acid phosphatase (PAP), has demonstrated efficacy for the treatment of advanced prostate cancer. DNA vaccines encoding PAP and live attenuated <i>Listeria</i> vaccines have entered clinical trials for patients with prostate cancer, and have advantages in terms of eliciting predominantly Th1-biased immunity. In this study, we investigated whether the immunogenicity and anti-tumor efficacy of a DNA and <i>Listeria</i> vaccine, each encoding PAP, could be enhanced by using them in a heterologous prime/boost approach. <b>Methods</b>. Transgenic mice expressing HLA-A2.01 and HLA-DRB1*0101 were immunized alone or with a heterologous prime/boost strategy. Splenocytes were evaluated for MHC class I and II-restricted, PAP-specific immune responses by IFNγ ELISPOTs. Anti-tumor activity to a syngeneic, PAP-expressing tumor line was evaluated. <b>Results</b>. PAP-specific cellular immunity and anti-tumor activity were elicited in mice after immunization with DNA- or listeria-based vaccines. Greater CD4+ and CD8+ responses, and anti-tumor responses, were elicited when mice were immunized first with DNA and boosted with <i>Listeria</i>, but not when administered in the opposite order. This was found to be dependent on CD4+ T cells elicited with DNA priming, and was not due to inflammatory signals by <i>Listeria</i> itself or due to B cells serving as antigen-presenting cells for DNA during priming. <b>Conclusions</b>. Heterologous prime/boost vaccination using DNA priming with <i>Listeria</i> boosting may provide better anti-tumor immunity, similar to many reports evaluating DNA priming with vaccines targeting foreign microbial antigens. These findings have implications for the design of future clinical trials.