Synergistic Effect Of Stereotactic Ablative Radiotherapy And Immunotherapy With A Xenogeneic Fibroblast Activation Protein-Based Whole Cell Tumor Vaccine

Abstract

Immunotherapies has revolutionized the treatment approach of patients with metastatic cancer, although response rates to such therapies when given alone are disappointingly limited in the majority. Stereotactic ablative radiotherapy (SABR) increases immunogenic cell death, boosts priming of APCs, activates effector T-cell responses, and serves as an ideal partner of immunotherapy. Accounting for a large proportion of cells in the tumor microenvironment (TME), cancer-associated fibroblasts (CAFs), along with their products such as fibroblast activation protein-α (FAPα), greatly affect tumor development and treatment resistance, which make them promising targets for immunotherapy. Therefore, the application of immunotherapy targeting CAFs combined with SABR may have a synergistic effect. We previously reported a xenogeneic FAPα-based whole cell tumor vaccine (WCTV) enhance specific antitumor responses to fight cancer cells and CAFs. This study aimed to test the efficacy and safety of SABR in combination with xenogeneic FAPα-based WCTV. In the present study, we firstly compare the different radiation schemes with a same biologically effective dose, the timing and sequence of the association of irradiation and FAPα-based WCTV in mouse LLC Lewis lung cancer model to investigate the optimal protocol. Mice were monitored for tumor growth, survival and gross measures of health. With the use of mouse H22 liver and 4T1 breast cancer models, we then tested the antitumor effect, safety and anti-fibrosis effect of the optimal combination protocol. The potential immune mechanisms were investigated through adoptive immunotherapy and 51Cr release assay. The distributions of the immune cells in the TME were detected by immunohistochemical staining and flow cytometry. SABR was dramatically more effective with 3x8Gy compared to 1×16.4Gy when associated with FAPα-based WCTV, and the best anti-tumor effect was observed in the group of mice started the vaccination on the 7th day after the last irradiation. No obvious side effects were detected following treatment. Tumor-infiltrating lymphocytes (TILs) ratio increased and products of CAF, such as FAPα, α-SMA and collagen I expression decreased in the tumor tissue. further investigation showed that the anti-tumor effect involves cellular and humoral immune responses. In the combination treatment group, T cells obtained from mice showed higher cytotoxicity against cancer cells and CAFs, and autoantibodies against FAPα were detected in the sera. These findings indicate combination of SABR and xenogeneic FAPα-based WCTV can weaken CAFs-mediated immune suppression, interstitial fibrosis and treatment tolerance to effectively inhabit tumor growth and reduce side effects. This study innovatively associates radiation therapy, anti-fibrosis therapy with immunotherapy, and provide new ideas for developing new treatment strategies for patients with multiple epithelial cancers.