In situ tumor destruction: towards in vivo modulation of immune responses by dendritic cells.

Dendritic cells (DC's) are professional antigen presenting cells that play a critical role in initiation of immune responses. In recent years, it has become evident that tumor antigens presented by ex vivo generated DC can evoke tumor-specific responses in cancer patients. Although promising results have been obtained with tumor antigen loaded DC vaccination, many questions regarding the most effective tumor antigens and DC migration remain unanswered. Moreover, ex vivo generation of DC's is time consuming and costly. In vivo loading and maturation of DC's would therefore greatly improve the applicability of DC vaccination. For in vivo loading of DC's we pioneered on a novel mouse model that makes use of in situ tumor ablation (cryo and radiofrequency ablation) to create an antigen source for uptake by phagocytes. In this model we could show that after ablation, antigens were preferentially taken up by dendritic cells in the draining lymph node. This, however, induced only a minor anti-tumor immunity that was mainly cell-mediated and tumor-specific. We and others showed that the maturation and activation status of DC's is crucial for the outcome of vaccination. Thus, to enhance immune responses after in vivo loading we combined the ablative treatments with peri-tumoral injection of CpG ODN containing unmethylated CG motifs. We were able to trace back antigen positive DC's in the lymph nodes that were increasingly matured after CpG, compared to antigen negative DC's. This was accompanied by an increase in cross presentation of antigens and subsequent induction of specific CD8+ T cells. Accordingly, this strategy decreased the basal level of primary site recurrences. Furthermore, when a re-challenge with B16-OVA cells was given forty days after ablation we could obtain 100% protection from outgrowth of this re-challenge. The results presented in this thesis suggest that immunomodulation can be a powerful additive treatment modality to combine with in situ tumor ablation.