Immunosuppressive elements within the tumor microenvironment, such as tumor-associated macrophages (TAM), can present a barrier to successful antitumor 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 including Th cells and antigen-presenting B cells that were highly enriched in responders to combination therapy. In one model that exhibited long-term posttreatment 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 posttreatment. 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 patients with breast cancer. This TAM signature may help identify human patients with claudin-low breast cancer 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. Significance: Immunostimulatory chemotherapy combined with pharmacologic inhibition of TAMs results in durable treatment responses elicited by Th cells and B cells in claudin-low TNBC models.

The word myeloid is derived from the Greek word muelós which means “marrow”. Therefore, myeloid cells are described as cells that arise in the bone marrow. They can be distinguished from lymphoid cells based on their different differentiation trajectories—Lymphoid cells (B and T cells) are usually born in the bone marrow, but they need to migrate to lymphoid organs to mature and differentiate usually in response to antigens produced due to infections and diseases like cancer. On the other hand, myeloid cells do not follow this differentiation trajectory. They arise from the bone marrow, and do not need an encounter with antigens to gain their functionality. Thus, while lymphoid cells are a part of the adaptive immune system, myeloid cells are a part of the innate immune system. Hematopoiesis gives rise to two progenitor cells—the common myeloid progenitor (CMP) and the common lymphoid progenitor (CLP). The CMP can give rise to megakaryocytes, erythrocytes, mast cells and myeloblasts. Myeloblasts in turn lead to the formation of basophils, neutrophils, eosinophils and monocytes that can further differentiate into macrophages. This review will focus on macrophages as well as the phenotypes they acquire with the tumor immune microenvironment (TIME) in triple-negative breast cancer (TNBC). It will address how cancer cells in the tumor microenvironment (TME) recruit macrophages and may switch to recruiting neutrophils upon depletion of these tumor-associated macrophages (TAMs). Finally, it will also shed light on past and current treatment options that specifically target these cells and how those affect patient outcomes in TNBC.

Bone metastases can disseminate to secondary sites and promote breast cancer progression, creating additional clinical challenges. The mechanisms contributing to secondary metastasis are barely understood. Here, we evaluate the prediction power of HER2-expressing (HER2E) circulating tumor cells (CTC) after analyzing over 13,000 CTCs from a cohort of 137 patients with metastatic breast cancer with initial HR+/HER2− status and use preclinical models of bone metastasis (BM) to validate the role of HER2E CTCs in multiorgan metastases. Although HER2 expression was higher in patients with BM, experimental analyses revealed that HER2E CTCs derived from bone lesions were more dependent on HER2 activity and more susceptible to anti-HER2 therapy. Targeting the bone-mediated HER2 induction reduces CTC detection and abrogates secondary metastasis from the bone. Overall, we elucidate that HER2E CTCs can serve as a noninvasive biomarker for BM formation with high therapeutic benefit for patients with HR+ metastatic breast cancer. Significance: Given the urgent need for alternative strategies to block metastasis progression, we demonstrate that blocking HER2-mediated secondary metastasis improves clinical outcome and establish HER2 as a biomarker for bone metastasis in patients with initial HR+/HER2− breast cancer, which represents ∼70% of all cases.

Supplementary Data from Chemotherapy Coupled to Macrophage Inhibition Induces T-cell and B-cell Infiltration and Durable Regression in Triple-Negative Breast Cancer

Supplementary Data from Chemotherapy Coupled to Macrophage Inhibition Induces T-cell and B-cell Infiltration and Durable Regression in Triple-Negative Breast Cancer