Abstract PR12: Targeting FSTL1 augments therapeutic activities of immune checkpoint inhibitors
Authors: Kudo-Saito Chie , Ogiwara Yamato , Hennmi Marina , Aoki Kazunori 
Published: November 2016

Abstract
Purpose: Tumor heterogeneity is composed of genetic and phenotypic changes in tumor itself, and also of the consequence of reciprocal evolution together with the tumor microenvironment containing a variety of cells such as stromal cells, vascular cells and immune cells in the host. Imbalanced immunity is one of the major problems supporting tumor progression and then amplification of the heterogeneity. Many inhibitors specific for immune checkpoint molecules interfering induction of anti-tumor immunity have been recently developed in clinical settings, and the clinical responses have convinced us of its efficaciousness. However, good results are limited to a part of the treated patients, and additional but another strategy is needed for treating patients more efficiently and effectively. For the purpose, we have been investigating the complex network in the tumor milieu and the systemic responses in the whole body comprehensively and collectively, and previously identified a BMP family FSTL1, which is significantly secreted from Snail+ cancer stem-like cells, as a key effector molecule in the cancer-associated immune suppression and dysfunction. FSTL1 expands pluripotent mesenchymal stem/stromal cells (MSCs), which generate immunoregulatory cells such as Tregs and tolerogenic DCs, and functionally impaired CD8-low T cells. FSTL1 and its induced MSCs also confer higher invasive and metastatic properties on tumor cells leading to further promotion of tumor growth and metastasis. To prevent such FSTL1-caused vicious circulation, we established anti-FSTL1 blocking mAbs, and evaluated the anti-tumor efficacy in combination with immune checkpoint inhibitors (ICIs) using mouse FSTL1+ tumor models.
Experimental Procedures: We generated monoclonal antibodies (mAbs) specific for both mouse and human FSTL1 utilizing the chicken immunization system followed by constructing cDNA phage library. After in vitro screening, the established anti-FSTL1 mAbs were tested for the in vivo anti-tumor activity using two syngeneic tumor models: a bone metastatic tumor model that C57BL/6 was implanted with snail-transduced melanoma B16-F10 cells, and a lung metastatic tumor model that BALB/c was implanted with colon cancer intrinsic FSTL1+ CT26 cells. The mAbs (10mg/kg) were intraperitoneally injected into these mice twice on day 4 and day 7 after tumor implantation. On day 14, immunological analysis was conducted using tumor-infiltrating cells, spleen cells and bone marrow cells obtained from the mice.
Results: We established chicken/mouse chimeric anti-FSTL1 mAb capable of inhibiting the FSTL1-elicited events including MSC expansion, immunoregulatory cell induction, and tumor invasion in vitro. In both bone and lung metastatic tumor models, treatment with the anti-FSTL1 mAb induced tumor-specific CD8+ T cells with higher CTL activities by suppressing increase of MSCs, but not Tregs and MDSCs, and significantly suppressed subcutaneous tumor growth and metastasis in the treated mice as compared to those of the control mice. The ICI mono-therapy was insufficient to suppress increase of MSCs, although significantly reduced immunoregulatory cells, and the anti-tumor efficacy was almost similar. Combination with the anti-FSTL1 mAb synergistically and significantly enhanced the ICI mono-therapeutic efficacy, particularly anti-PDL1 mAb, by inducing potent tumor-specific CTLs, resulting in tumor-free in some of the treated mice.
Conclusions: Blocking FSTL1 reprograms and properly induces anti-tumor immunity by switching the tumor-supportive immune directivity to the active mode against cancer. Anti-FSTL1 mAb having totally different molecular mechanisms from the conventional ICIs may be a promising partner for radically correcting cancer-manipulated immunity in clinical settings.
Citation Format: Chie Kudo-Saito, Yamato Ogiwara, Marina Hennmi, Kazunori Aoki. Targeting FSTL1 augments therapeutic activities of immune checkpoint inhibitors [abstract]. In: Proceedings of the Second CRI-CIMT-EATI-AACR International Cancer Immunotherapy Conference: Translating Science into Survival; 2016 Sept 25-28; New York, NY. Philadelphia (PA): AACR; Cancer Immunol Res 2016;4(11 Suppl):Abstract nr PR12.