Abstract A103: Intracellular immunotherapy of disseminated pancreatic cancer activates potent IFNγ and CD8+ T cell dependent anti-tumor responses
Authors: Sanders Kiah L. , Fox Barbara A. , Bzik David J. 
Published: October 2015

Innovative therapies with the ability to activate effective cellular-based immune mechanisms in conjunction with current treatment strategies have potential to combat highly aggressive solid and metastatic tumors such as pancreatic adenocarcinoma, which remains one of the most deadly cancers with an overall 5-year survival rate of 6% following diagnosis. In this study, we explored the immunotherapeutic mechanisms triggered by an avirulent and nonreplicating obligate intracellular parasite Toxoplasma gondii (cps strain), which preferentially invades myeloid cell types, in the disseminated pancreatic tumor environment. We hypothesized the strong induction of Th1 responses in the tumor environment triggered by cps may stimulate significant mechanisms that could halt or eliminate established pancreatic cancer.

Pan02 cells were injected intraperitoneally into syngeneic C57BL/6 mice. Seven days later, these mice were treated with 2, 3, or 5 doses of cps or PBS, as a treatment control. Pan02 tumor-bearing mice treated with cps had a significant survival advantage over untreated mice. Additionally, treatment with heat-killed cps, which is unable to actively invade, led to loss of treatment efficacy. Following a single cps treatment CD11b+CD11c+ dendritic cell populations increased and CD11b+F4/80+CD11c- macrophage populations decreased in the peritoneum. Cps-invaded macrophages and dendritic cells in the tumor environment upregulated surface expression of CD80 and CD86, and there was an increase in local production of IL-12 up to 7 days post-treatment. Pan02 tumor-bearing mice unable to produce IL-12 lost protection mediated by cps treatment. Seven days post-cps treatment, both CD4+ and CD8+ T cells were significantly higher than within the tumor microenvironment of untreated mice. Local T cells had lower expression of CD62L and increased expression of CD44 and CD69 indicating activation of recruited T cells. Six and seven days post-cps treatment, local IFNγ production in Pan02 tumor-bearing mice increased and the numbers of IFNγ+ CD4+ and IFNγ+ CD8+ T cells were higher. Similar to IL-12, loss of IFNγ within the tumor microenvironment led to a loss of treatment efficacy. A lack of CD8+ T cells prior to and during therapy resulted in a loss of treatment efficacy, whereas CD4+ T cells and NK cells were not required for treatment efficacy. CD8+ T cells isolated from the spleens of treated animals 10 days after cps treatment robustly produced IFNγ ex-vivo when stimulated with Pan02 cells. This response appears to be tumor specific as stimulation with irrelevant tumor antigen did not induce an IFNγ response from the same CD8+ T cells. Cps-mediated activation of tumor-associated myeloid cells leads to increases in local IFNγ+ CD8+ T cells, which drive specific anti-tumor responses. Cps induction of host Th1 responses elicited in the tumor microenvironment demonstrates the capability of cps to be broadly applied as an immunotherapeutic against pancreatic tumors.

Citation Format: Kiah L. Sanders, Barbara A. Fox, David J. Bzik. Intracellular immunotherapy of disseminated pancreatic cancer activates potent IFNγ and CD8+ T cell dependent anti-tumor responses. [abstract]. In: Proceedings of the AACR Special Conference on Pancreatic Cancer: Innovations in Research and Treatment; May 18-21, 2014; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2015;75(13 Suppl):Abstract nr A103.