**2.4 Immunotherapy**

Immunotherapy is a treatment that eliminates tumor cells by reactivating and enhancing the anti-tumor immune response of tumor patients. Much excitement has been generated immunotherapy in tumors that are refractory to traditional treatment, as well as resistance to traditional agents. Moreover, cancer immunotherapy has gone all the way from a promising preclinical application to a clinical reality. A variety of tumor associated antigens are high expression in pancreatic cancer, such as mucin1 (MUC-1), carcinoembryoni-cantigen (CEA), prostate stem cell antigen (PSCA), vascular endothelial growth factor (VEGF), mesoth-elin (MSLN) and K-ras mutation. Unfortunately, the use of immunotherapy alone has encountered disappointing results in clinical trials in pancreas cancer, with response rates only [10]. Immunotherapy includes the following methods: **(1) Active immunotherapy.** Active immunotherapy refers to immunizing tumorassociated antigens to stimulate tumor-specific immune response of the body to eliminate tumors. Tumor associated antigens (TAAs) have been widely explored as cancer vaccines for treatment of pancreatic cancer in both mouse models and clinical trial [11]. Due to a variety of the tumor associated antigens expressing on the pancreatic cancer cells, several vaccines can be explored for the pancreatic cancer active immunotherapy, such as GVAX vaccine. K-ras gene has a high mutation rate in pancreatic cancer and K-ras vaccine has become an important target for immunotherapy of pancreatic cancer. Studies have found that the cationic nanoencapsulated K-ras peptide vaccine has a significant therapeutic effect on pancreatic cancer xenograft mice, and can significantly prolong the survival of mice [12]. In a I phase of clinical trial, following inoculated with MUC-1 peptide-loaded DC vaccine in 7 pancreatic cancer patients, the number of mature DC cells in 2 of them increased significantly and peripheral blood lymphocytes were activated and produced large amounts of IL-12p40 and IFN-γ [13]. **(2) Passive immunotherapy.** Passive immunotherapy refers to substances with immune effects are modified in vitro and then injected into human body to enhance anti-tumor immune response and eliminate tumors. At present, passive immunotherapeutic strategies used for pancreatic cancer including: 1) Antibody-mediated passive immunotherapy. Antibody-mediated passive immunotherapy involves targeting tumors using

monoclonal antibodies, antibody-drug conjugates, antibody fragments, or radioimmunotherapy conjugates to inhibit oncogenic signaling, immune suppression, or immune checkpoints [11]. Combination anti-CD40 antibody with gemcitabine showed an effective tumor inhibition. 2) Passive T cell mediated immunotherapy. Passive T cell mediated immunotherapy includes adoptive T-cell transfer and chimeric antigen receptors (CAR) T-cells therapy. **(3) Immune checkpoint blocking therapy.** Immune checkpoint blocking therapy is an immunotherapy method that can reverse the immunosuppressive signal by blocking the immunoregulatory molecules and enhance the anti-tumor immune response. Pancreatic cancer tumor cells overexpress immunosuppressive ligands, such as B7-1, B7-2 and PD-L1, which bind to surface inhibitory receptors CTL-4 and PD-1 of T cells to suppress effector T cell activity and evade immune surveillance. In 2011, the FDA approved Ipilimumab, the first humanized monoclonal antibody targeting CTL-4, for the treatment of patients with advanced melanoma. Its effect on pancreatic cancer has entered the clinical trial stage. Studies have shown that Ipilimumab can promote the proliferation of T cells and the secretion of Th1 cytokines, and enhance the killing effect of CD8+ T cells on Colo356/FG pancreatic cancer cells [14]. Two PD-1 blocking antibodies pembrolizumab and nivolumab are FDA approved for use in the treatment of metastatic non-small cell lung cancer, melanoma, renal cell cancer, and head and neck cancer [15]. In mouse model of pancreatic cancer, anti-PD-1 or PD-L1 blocking treatment promoted the generation of CD8+ T cells to tumor invasion and anti-tumor immune response. In a number of clinical trials, no objective tumor remission was observed in pancreatic cancer patients treated with anti-PD-1 or anti-PD-L1 blocking alone, suggesting that PD-1 or PD-L1 blocking alone does not have a good therapeutic effect on pancreatic cancer. **(4) CAR-T therapy.** Chimeric antigen receptor (CAR) is composed of the single chain variable fragment (ScFv) of monoclonal antibody, the hinge region and the transmembrane region of TCR receptor, and the intracellular signal transduction region in series. They form the chimeric antigen receptor by viral infection or electrical transformation on the surface of T cells. CAR-T can recognize antigens on the surface of tumor cells directly without being restricted by HLA molecules. Therefore, CAR-T has a broader application prospect in tumor immunotherapy. Target specific CAR-T cells were designed to target the highly expressed tumor-associated antigens of pancreatic cancer, making the treatment more specific. Tn-MUC-1 CAR-T: Posey et al. designed CAR-T cells targeting the Tn/STn glycopeptide phenotype on MUC-1 [16]. When CEA + C15A3 pancreatic cancer cells were transfected to mice, the cancer cells were quickly cleared and serum levels of IL-1β and IL-5 were significantly increased [17]. PSCA CAR-T: PSCA is also a tumor-associated antigen highly expressed in pancreatic cancer, and CAR-T targeting PSCA has a significant anti-tumor effect on xenograft mice of human pancreatic cancer after treatment, in which 40% of the tumors in mice have completely subsided [18]. MSLN CAR-T: Hingorani et al. designed CD8+ CAR-T cells targeting MSLN, and found that MSLN CAR-T cells could specifically kill KPC tumor cells and produce a large amount of IFN-γ in vitro. The metastasis rate dropped from 64% to 46%, and overall survival increased from 54 days to 96 days [19].

Due to unique tumor microenvironment of pancreatic cancer, both traditional treatment and single immunotherapy is not ideal. Although tumor vaccine can induce the activation of effector T cells, the activation degree is very limited and only a few effector T cells and NK cells exist in the tumor microenvironment and peripheral blood of pancreatic cancer patients. Although immune checkpoint blocking therapy can block the inhibitory effect of effector T cells, there are still many soluble immunosuppressants inhibiting effector T cells in the tumor microenvironment of pancreatic cancer. For CAR-T treatment, in addition to being

*Advance in Pancreatic Cancer Diagnosis and Therapy DOI: http://dx.doi.org/10.5772/intechopen.94413*

influenced by immunosuppressive factors in the tumor microenvironment, the fibrous stroma layer around pancreatic cancer cells can prevent the infiltration of CAR-T into the tumor and further its efficacy. Therefore, a deep understanding the characteristics of the immune microenvironment of pancreatic cancer and its impact on immunotherapy and/or other traditional treatment will greatly improve the treatment effect. The combination of immune checkpoint blocking therapy with radiotherapy, chemotherapy and tumor vaccines can enhance the function of tumor-specific T cells and promote lymphocyte infiltration into the tumor site. Anti-CD40 agonist can reverse the resistance of pancreatic cancer mice to PD-1 and CTLA-4 blocking antibodies, and improve the therapeutic effect of blocking antibodies. Pembrolizumab (PD-1 blocking antibody) and nivolumab (CTLA-4 blocking antibody) combined with radiotherapy can significantly prolong the survival of mice with pancreatic adenocarcinoma [20]. In order to design a reasonable immunotherapy strategy according to the characteristics of pancreatic cancer microenvironment and improve the effect of pancreatic cancer immunotherapy, we should start from the following aspects: (1) destroy the fibrous matrix layer of pancreatic cancer and increase the infiltration of effector T cells into the tumor; (2) remove excessive immunosuppressive cells such as Tregs and MDSCs, and reverse the immunosuppressive microenvironment; (3) recruit more T cells to the tumor site to enhance the anti-tumor immune response of effector T cells; (4) enhance the targeting and killing of effector T cells.
