Abstract B107: Dormant breast tumor cells avoid the adaptive immune system through regulation of the immune and tumor microenvironment

Abstract

Abstract Many cancer survivors respond to early treatment with complete remission only to succumb to the disease after the tumor returns, sometimes decades later, in a distant metastatic site. Tumor cells can exist in a nonproliferative state of dormancy (quiescence) in specific, perivascular niches in the lungs, brain, and especially bone marrow. In particular, dormant breast tumors can recur after long periods of time, even over 15 years after treatment of the primary tumor. Why the immune system is ineffective at eliminating disseminated/dormant tumor cells before growing into new metastatic lesions is currently unknown. Therefore, in the current study we hypothesized that dormant tumor cells actively elicit a reduced adaptive immune response compared to proliferative tumor cells, ultimately aiding in long-term survival and persistence of dormant cells. We utilized the established syngeneic D2 series of cell lines, which comprises independent clones from a spontaneous mammary tumor in a female Balb/c mouse with varying degrees of dormancy—D2A1 cells grow rapidly upon injection while D2.1 remain dormant. Mammary fat pad (MFP) injections (n=8/group) revealed that proliferative D2A1 tumors rapidly grew in the absence of an adaptive immune system in SCID mice and reached terminal endpoints after 2 to 3 weeks. In comparison, D2A1 tumor growth was significantly reduced in syngeneic Balb/c mice (p<0.0005). Surprisingly, the dormant D2.1 tumors grew at the same rate in Balb/c and SCID mice and persisted post implantation up to 10 weeks and beyond in both, suggesting that these tumors are able to avoid the effects of the adaptive immune system despite very slow growth. To determine potential mechanisms of this differential response, MFP-implanted tumors in Balb/c mice were evaluated via flow cytometry 5 weeks post implantation (n=5/group). Interestingly, the dormant D2.1 tumors contained significantly more Tregs than D2A1 tumors (25.2% vs. 12.6%; p=0.0009), as well as a lower CD4:CD8 ratio (0.45:1 vs. 2.74:1; p=0.032) and enhanced PD-L1 expression on infiltrated myeloid cells (MFI = 1080.75 vs. 1729.25; p = 0.0146). Furthermore, analysis of D2A1 and D2.1 tumor cells revealed that D2.1 cells express significantly more immunomodulatory cytokines and matricellular proteins including IL-10, TGFβ, tenascin-c, and periostin. Taken together, our data illustrate that dormant tumors survive and persist long term despite the immune system, as evidenced by similar growth kinetics of D2.1 cells in the absence or presence of an adaptive immune system. The presence of increased Tregs and higher PD-L1 expression of infiltrated immune cells, along with the altered expression profiles, suggest that dormant tumors elicit an immunosuppressive and protective microenvironment to enable long-term survival. Studies to determine temporal changes in the tumor microenvironment, how dormant cells recruit immunosuppressive cells, and antigenicity of dormant versus proliferative tumor cells are ongoing. Citation Format: Timothy N. Trotter, Cong-Xiao Liu, Tao Wang, H. Kim Lyerly, Zachary C. Hartman. Dormant breast tumor cells avoid the adaptive immune system through regulation of the immune and tumor microenvironment [abstract]. In: Proceedings of the AACR Special Conference on Tumor Immunology and Immunotherapy; 2019 Nov 17-20; Boston, MA. Philadelphia (PA): AACR; Cancer Immunol Res 2020;8(3 Suppl):Abstract nr B107.