Adding combination immunotherapy consisting of cancer vaccine, anti-PD-1 and anti-CSF1R antibodies to gemcitabine improves anti-tumor efficacy in murine model of pancreatic ductal adenocarcinoma

Background: Immunotherapy can take advantage of the immunogenic response that chemotherapy elicits in tumors. Gemcitabine is a standard agent used in the treatment of pancreatic cancer, with known effects on the tumor immune microenvironment. The combination immunotherapy of the GVAX cancer vaccine, anti-PD-1 antibody and anti-CSF-1R antibody has been shown to improve survival in a murine model of metastatic pancreatic adenocarcinoma. This combination regimen also increased the infiltration of CD8+ T-cells that expressed both PD1 and CD137, and these T-cells were shown to express high levels of interferon-gamma, a marker of cytotoxic effector CD8+ T-cells. The effect of the addition of gemcitabine to this promising immunotherapy regimen has not been investigated.

Methods: Mice with liver-metastatic pancreatic adenocarcinoma were followed for 120 days to determine if adding immunotherapy, which comprised of varying combinations of GVAX, anti-PD-1 antibody and anti-CSF-1R antibody, to gemcitabine improved survival. Tumor-infiltrating CD8+ T-cells and myeloid cells, harvested after the mice were treated for 2 weeks, were analyzed with flow cytometry to characterize the effect the chemo-immunotherapy regimen had on the tumor microenvironment (TME).

Results: Adding combination immunotherapy after gemcitabine improved survival compared to gemcitabine treatment alone (gemcitabine/GVAX/anti-PD1, P<0.001; gemcitabine/anti-PD1/anti-CSF-1R, P<0.05; gemcitabine/GVAX/anti-PD1/anti-CSF-1R, P<0.01). However, there was no difference in survival between the three chemo-immunotherapy treatment regimens. Compared to gemcitabine-only treatment, the chemo-immunotherapy regimens also increased the percentage of tumor-infiltrating CD8+ T-cells that expressed interferon-gamma (gemcitabine/GVAX/anti-PD1, P<0.0001 and gemcitabine/GVAX/anti-PD1/anti-CSF-1R, P<0.0001). The chemo-immunotherapy regimens also increased the number of tumor-infiltrating PD1+CD137+CD8+ T-cells and interferon-gamma-expressing PD1+CD137+CD8+ T-cells, but these increases were not statistically significant. Anti-CSF-1R antibody decreased the infiltration of myeloid cells and myeloid-derived suppressor cells caused by GVAX (P<0.05), and trended towards decreasing tumor-associated macrophages (TAMs) (P=0.18).

Conclusions: The addition of anti-PD1 antibody with GVAX and/or anti-CSF-1R antibody to gemcitabine improved the survival of mice with liver-metastatic pancreatic ductal adenocarcinoma (PDA). Gemcitabine with GVAX and anti-PD1 with or without anti-CSF-1R also improved the infiltration of effector CD8+ T-cells, and the presence of anti-CSF-1R in the chemo-immunotherapy regimens decreased the infiltration of myeloid cells. The overlapping mechanisms of the components in the chemo-immunotherapy regimens may explain the lack of survival difference between the various regimens, and this remains to be explored.