During early pregnancy in the mouse, nidatory estrogen (E2) stimulates endometrial receptivity by activating a network of signaling pathways that is not yet fully characterized. Here, we report that bone morphogenetic proteins (BMPs) control endometrial receptivity via a conserved activin receptor type 2 A (ACVR2A) and SMAD1/5 signaling pathway. Mice were generated to contain single or double conditional deletion of SMAD1/5 and ACVR2A/ACVR2B receptors using progesterone receptor (PR)-cre. Female mice with SMAD1/5 deletion display endometrial defects that result in the development of cystic endometrial glands, a hyperproliferative endometrial epithelium during the window of implantation, and impaired apicobasal transformation that prevents embryo implantation and leads to infertility. Analysis of Acvr2a-PRcre and Acvr2b-PRcre pregnant mice determined that BMP signaling occurs via ACVR2A and that ACVR2B is dispensable during embryo implantation. Therefore, BMPs signal through a conserved endometrial ACVR2A/SMAD1/5 pathway that promotes endometrial receptivity during embryo implantation.

Immunosuppressive elements within the tumor microenvironment, such as tumor-associated macrophages (TAM), can present a barrier to successful anti-tumor responses by cytolytic T cells. Here we employed preclinical syngeneic p53 null mouse models of triple-negative breast cancer (TNBC) to develop a treatment regimen that harnessed the immunostimulatory effects of low-dose cyclophosphamide coupled with the pharmacologic inhibition of TAMs using either a small molecule CSF1R inhibitor or an anti-CSF1R antibody. This therapeutic combination was effective in treating several highly aggressive TNBC murine mammary tumor and lung metastasis models. Single cell RNA sequencing characterized tumor-infiltrating lymphocytes (TIL) including helper T cells and antigen-presenting B cells that were highly enriched in responders to combination therapy. In one model that exhibited long-term post-treatment tumor regression, high dimensional imaging techniques identified the close spatial localization of B220+/CD86+-activated B cells and CD4+ T cells in tertiary lymphoid structures that were present up to 6 weeks post-treatment. The transcriptional and metabolic heterogeneity of TAMs was also characterized in two closely related claudin-low/mesenchymal subtype tumor models with differential treatment responses. A murine TAM signature derived from the T12 model was highly conserved in human claudin-low breast cancers, and high expression of the TAM signature correlated with reduced overall survival in breast cancer patients. This TAM signature may help identify human claudin-low breast cancer patients that will benefit from the combination of cyclophosphamide and anti-CSF1R therapy. These studies illustrate the complexity of the tumor immune microenvironment and highlight different immune responses that result from rational immunotherapy combinations.

Tumor dormancy is a stage in which residual cancer cells remain inactive, but regrowth of dormant cancer cells contributes to recurrence. The complex ecosystem in cancer that promotes cell survival and the factors that eventually overcome growth constraints and result in proliferation remain to be fully elucidated. Doing so may provide new insights and help identify novel strategies to prolong cancer dormancy and prevent disease recurrence. To dissect the molecular pathways and the microenvironments involved in regulation of dormancy, we utilized a novel immunocompetent transgenic model to study minimal residual disease and relapse. This model revealed a significant reorganization of cancer cell structures, stroma, and immune cells, with cancer cells showing dormant cell signatures. Single-cell RNA sequencing uncovered remodeling of myeloid and lymphoid compartments. In addition, the Jagged-1/Notch signaling pathway was shown to regulate many aspects of tumorigenesis, including stem cell development, epithelial-to-mesenchymal transition, and immune cell homeostasis during minimal residual disease. Treatment with an anti–Jagged-1 antibody inhibited the Jagged-1/Notch signaling pathway in tumor cells and the microenvironment, delaying tumor recurrence. These findings uncover a cascade of regulatory changes in the microenvironment during dormancy and identify a therapeutic strategy to undercut these changes. Significance: Single-cell RNA-sequencing analysis reveals dormancy-associated changes in immune and stromal cells and demonstrates a rationale to pursue Jagged-1/Notch pathway inhibition as a viable therapeutic strategy to reduce disease recurrence.

Polo-like kinase (PLK) family members play important roles in cell-cycle regulation. The founding member PLK1 is oncogenic and preclinically validated as a cancer therapeutic target. Paradoxically, frequent loss of chromosome 5q11–35, which includes PLK2, is observed in basal-like breast cancer. In this study, we found that PLK2 was tumor suppressive in breast cancer, preferentially in basal-like and triple-negative breast cancer (TNBC) subtypes. Knockdown of PLK1 rescued phenotypes induced by PLK2 loss both in vitro and in vivo. We also demonstrated that PLK2 directly interacted with PLK1 at prometaphase through the kinase but not the polo-box domains of PLK2, suggesting PLK2 functioned at least partially through the interaction with PLK1. Furthermore, an improved treatment response was seen in both Plk2-deleted/low mouse preclinical and patient-derived xenograft (PDX) TNBC models using the PLK1 inhibitor volasertib alone or in combination with carboplatin. Reexpression of PLK2 in an inducible PLK2-null mouse model reduced the therapeutic efficacy of volasertib. In summary, this study delineates the effects of chromosome 5q loss in TNBC that includes PLK2, the relationship between PLK2 and PLK1, and how this may render PLK2-deleted/low tumors more sensitive to PLK1 inhibition in combination with chemotherapy. Significance: The tumor-suppressive role of PLK2, and its relationship with oncogene PLK1, provide a mechanistic rationalization to use PLK1 inhibitors in combination with chemotherapy to treat PLK2-low/deleted tumors. TNBC, and other cancers with low PLK2 expression, are such candidates to leverage precision medicine to identify patients who might benefit from treatment with these inhibitors.