Formalin-fixed tumor cells effectively induce antitumor immunity both in prophylactic and therapeutic conditions

Background: Autologous whole tumor cell-based vaccinations would seem to be ideal since such vaccinations, in contrast to vaccination with a single defined antigen, have the potential to elicit a broad type of T-cell immune response to tumor-associated antigens. Objective: We modified formaldehyde (formalin)-fixed mouse melanoma cells and investigated the utility of those cells as sources of tumor antigens for immunotherapy. Methods: C57BL/6 or the proteasome activator PA28α-knockout mice were intradermally inoculated with 1% formalin-fixed B16 cells three times at weekly intervals either before or after tumor challenge. Simultaneously, interleukin-12 gene was transferred into the skin around immunization sites using gene gun technology. The effects were evaluated by tumor growth, antigen-specific interferon-γ production in splenic lymphocytes, and activation of dendritic cells. Results: Fixed cells directly induced production of tumor necrosis factor-α in dendritic cells more effectively than did frozen and thawed cells. More than 60% of the mice immunized with fixed cells and interleukin-12 rejected the challenged B16 tumor. CD4⁺ T cells from those mice produced a significant amount of interferon-γ in response to melanoma cells. Furthermore, this combined treatment showed antitumor immunity initiated by CD8⁺ and CD4 ⁺ T cells in the therapeutic experiments. PA28α/β appeared not to be required for the development of CD8⁺ T cells, although it is known to be essential for the development of CD8⁺ T cells specific for tyrosinase-related protein-2, one of melanocyte-lineage differentiated antigens. Conclusion: These results suggest that formalin-fixed autologous melanoma cells have a potential to function as effective antigen sources for immunotherapy.