Pancreatic ductal adenocarcinoma (PDAC) represents one of the deadliest cancers in the world. PDAC cells activate tumor-specific immune responses but simultaneously trigger a strong immunosuppression. We showed that PDAC cells produce high amount of chronic inflammatory mediators and PDAC tumors build an immunosuppressive cytokine milieu, which correlates with tumor progression. We observed a low frequency of dendritic cells (DC) and a pronounced accumulation of macrophages and myeloid-derived suppressor cells (MDSC) in murine PDAC tumors. A strong accumulation of MDSC has also been demonstrated in the peripheral blood of resected PDAC patients. While DC and macrophages seem not to play a significant role in this PDAC model in the context of immunosuppression, MDSC are highly suppressive, and their accumulation is associated with an increase in intratumoral VEGF concentration during the PDAC progression. Application of the phosphodiesterase-5 inhibitor sildenafil led to a prolonged survival of PDACbearing female mice, which was due to the decrease in MDSC frequencies and in the systemic VEGF level. This led to a restoration of anticancer immune responses, manifested in the recovery of T lymphocyte functions and in an increase in the frequency of conventional CD4 C T cells in tumors and IFNg level in serum of PDAC-bearing mice. Thus, MDSC are strongly involved in the PDAC-associated immunosuppression and that their depletion could create new approaches for therapy of PDAC.

Interferon-α (IFNα) has one of the longest histories of use amongst cytokines in clinical oncology and has been applied for the treatment of many types of cancers. Due to its immune-activating properties, IFNα is also an attractive candidate for combinatory anti-cancer therapies. Despite its extensive use in animal tumor models as well as in several clinical trials, the different mechanisms underlying patient responses and affecting desirable clinical benefits are still under investigation. Here we show that in addition to its immune-activating properties, IFNα induces the expression of a key negative regulator, immunosuppressive PD-L1 molecule, in the majority of the specific immune cell populations, particularly in the dendritic cells (DC). DC can modulate immune responses by a variety of mechanisms, including expression of T-cell regulatory molecules and cytokines. Our results showed that treatment of DC with IFNα-2b led to pronounced up-regulation of surface expression of PD-L1 molecules, increased IL-6 and decreased IL-12 production. Moreover, we present evidence that IFNα-treated DC exhibited a reduced capacity to stimulate interferon-γ production in T cells compared to control DC. This T-cell response after treatment of DC with IFNα was recovered by a pre-treatment with an anti-PD-L1 blocking antibody. Further analyses revealed that IFNα regulated PD-L1 expression through the STAT3 and p38 signaling pathways, since blocking of STAT3 and p38 activation with specific inhibitors prevented PD-L1 up-regulation. Our findings underline the important roles of p38 and STAT3 in the regulation of PD-L1 expression and prove that IFNα induces STAT3/p38-mediated expression of PD-L1 and thereby a reduced stimulatory ability of DC. The augmentation of PD-L1 expression in immune cells through IFNα treatment should be considered by use of IFNα in an anti-cancer therapy.

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Prognosis of patients with carcinoma of the exocrine pancreas is particularly poor. A combination of chemotherapy with immunotherapy could be an option for treatment of pancreatic cancer. The aim of this study was to perform an immunomonitoring of 17 patients with pancreatic cancer from the CapRI-2 study, and tumor-bearing mice treated with combination of chemo (radio) therapies with interferon-2α. Low doses of interferon-2α led to a decrease in total leukocyte and an increase in monocyte counts. Furthermore, we observed a positive effect of interferon-2α therapy on the dendritic cells and NK (natural killer) cell activation immediately after the first injection. In addition, we recorded an increased amount of interferon-γ and IL-10 in the serum following the interferon-2α therapy. These data clearly demonstrate that pancreatic carcinoma patients also show an immunomodulatory response to interferon-2α therapy. Analysis of immunosuppressive cells in the Panc02 orthotopic mouse model of pancreatic cancer revealed an accumulation of the myeloid-derived suppressor cells in spleens and tumors of the mice treated with interferon-2α and 5-fluorouracil. The direct effect of the drugs on myeloid-derived suppressor cells was also registered in vitro. These data expose the importance of immunosuppressive mechanisms induced by combined chemo-immunotherapy.