Dendritic cells pulsed with RNA are potent antigen-presenting cells in vitro and in vivo

Immunization with defined tumor antigens is currently limited to a small number of cancers where candidates for tumor rejection antigens have been identified. In this study we investigated whether pulsing dendritic cells (DC) with tumor-derived RNA is an effective way to induce CTL and tumor immunity. DC pulsed with in vitro synthesized chicken ovalbumin (OVA) RNA were more effective than OVA peptide-pulsed DC in stimulating primary, OVA- specific CTL responses in vitro. DC pulsed with unfractionated RNA (total or polyA⁺) from OVA-expressing tumor cells were as effective as DC pulsed with OVA peptide at stimulating CTL responses. Induction of OVA-specific CTL was abrogated when polyA⁺ RNA from OVA-expressing cells was treated with an OVA- specific antisense oligodeoxynucleotide and RNAse H, showing that sensitization of DC was indeed mediated by OVA RNA. Mice vaccinated with DC pulsed with RNA from OVA-expressing tumor cells were protected against a challenge with OVA-expressing tumor cells. In the poorly immunogenic, highly metastatic, B16/F10.9 tumor model a dramatic reduction in lung metastases was observed in mice vaccinated with DC pulsed with tumor-derived RNA (total or polyA⁺, but not polyA RNA). The finding that RNA transcribed in vitro from cDNA cloned in a bacterial plasmid was highly effective in sensitizing DC shows that amplification of the antigenic content from a small number of tumor cells is feasible, thus expanding the potential use of RNA-pulsed DC- based vaccines for patients bearing very small, possibly microscopic, tumors.