Dormant mammary tumors persist longterm despite adaptive immunity by establishing a Treg-dominated niche via DKK3

Abstract A significant portion of breast cancer survivors eventually experience locoregional or metastatic recurrence. It is clear that the adaptive immune system controls proliferative tumor outgrowth, but how dormant tumor cells escape to eventually form new lesions remains unclear. Previous studies showed that proliferative D2A1 or D2.OR tumors are restricted by adaptive immunity in Balb/c versus SCID mice whereas dormant D2.1 tumors are largely unaffected. Further, D2.1 tumors are highly infiltrated with T cells, particularly CD4+ cells and Tregs. Thus, the aim of the present study was to determine the extent to which dormant D2.1 tumors evade antigen-specific immunity and how these tumors shape the T cell niche. Experiments were performed using Balb/c syngeneic D2 mammary tumor cells – D2A1 cells grow rapidly upon orthotopic injection, D2.OR cells proliferate but to a lesser degree than D2A1, and D2.1 cells remain dormant long term. Cells were also modified to express enhanced green fluorescent protein (eGFP) for use with Just eGFP Death Inducing (JeDI) T cells containing a T cell receptor (TCR) targeted to eGFP200-208. Transfer of JeDI T cells caused complete rejection of D2A1-eGFP tumors in the mammary fat pad (MFP) while no difference was observed in D2.1-eGFP tumors. In contrast, treatment with anti-CTLA4 alone, anti-PD-1 alone, or combined anti-CTLA4/PD-1 reduced D2.1 tumor progression compared to isotype control. Whole JeDI splenocytes were also stimulated in vitro with eGFP200-208 peptide in the presence of D2A1 or D2.1 conditioned medium (CM). Surprisingly, while CD8 cells proliferated in both D2A1 and D2.1 CM, CD4+ T cells also proliferated in D2.1 CM and the Treg fraction of CD4+ cells was significantly elevated in D2.1 CM compared to D2A1 CM or control medium. RNA-sequencing confirmed substantial immune activity in D2.1 tumors compared to D2A1. D2.1 tumors also displayed a hybrid epithelial/mesenchymal (E/M) phenotype and expressed genes associated with both mammary epithelium (Krt8, Krt14, Cdh1) and mesenchymal cells (Itga5, Snai2, Notch1, Zeb1). Moreover, Gene Set Enrichment Analysis (GSEA) showed an enrichment of mammary stem and luminal progenitor cell signatures in D2.1 tumors. Of the top upregulated genes, Dickkopf WNT Signaling Pathway Inhibitor 3 (DKK3) was chosen for further investigation. Interestingly, increased CD8 proliferation and IFNγ production, but decreased CD4 and Treg abundance, was observed in DKK3 shRNA knockdown (k/d) D2.1 CM compared to shScramble control. In vivo, overexpressing DKK3 in D2A1 cells significantly increased MFP tumor growth in Balb/c but not SCID mice, altered T cell activation locally and systemically, and decreased eGFP-specific antibodies. Similar results were observed in DKK3-overexpressing D2.OR tumors. Conversely, DKK3 k/d in D2.1 cells resulted in ~60% complete tumor rejection. Together, these data indicate that CD4 cells, particularly Tregs, are crucial in establishing a protective niche for long-term tumor dormancy with tumor-derived DKK3 emerging as a mediator of the Treg niche. Citation Format: Timothy N. Trotter, Delila Serra, Carina Dagotto, Tao Wang, Xiao Yang, Junping Wei, Gangjun Lei, H. Kim Lyerly, Zachary C. Hartman. Dormant mammary tumors persist long-term despite adaptive immunity by establishing a Treg-dominated niche via DKK3 [abstract]. In: Proceedings of the AACR Special Conference: Cancer Metastasis; 2022 Nov 14-17; Portland, OR. Philadelphia (PA): AACR; Cancer Res 2022;83(2 Suppl_2):Abstract nr PR015.