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Breast cancer remains a leading cause of cancer-related deaths among women, with metastasis being the primary contributor to mortality. The lungs are a frequent site for breast cancer metastasis, and patients typically face a median survival of less than two years post-diagnosis, highlighting significant clinical challenges. Recent research has shed light on the critical role of the tumor microenvironment (TME) and the metastatic microenvironment (mTME) in supporting and regulating tumor metastasis.

The immune microenvironment, comprising various immune cells, cytokines, and signaling pathways, is particularly crucial in the growth and spread of tumor cells. Among these immune cells, macrophages have garnered significant attention for their diversity and complex functions within the TME. Specifically, TREM2+ macrophages are believed to aid tumor cells in evading the immune system by inhibiting T cell activity and secreting immunosuppressive factors.

A recent study led by Ido and colleagues utilized spatial-temporal single-cell RNA sequencing and other advanced biotechnological methods to investigate the dynamic changes in the immune microenvironment and the functional mechanisms of specific immune cell subsets during lung metastasis of breast cancer. The study focused on the role of TREM2+ macrophages at the metastatic margins.

Researchers employed a spontaneous breast cancer lung metastasis model by injecting tdTomato-EO771 breast cancer cells into the mammary glands of mice, simulating the primary and metastatic processes of breast cancer. For spatial localization analysis, they used photoactivation to label specific regions of cells, combined with two-photon microscopy and high-throughput single-cell RNA sequencing (NICHE-seq). Samples were collected at different time points, and immune cells were isolated using flow cytometry. Fluorescence labeling techniques distinguished between the metastatic core and the invasive edge. Single-cell RNA sequencing was performed on the isolated cells, analyzing 24,020 immune cells, identifying nine subsets of T cells and NK cells, fourteen subsets of monocytes, macrophages, and dendritic cells, and five subsets of neutrophils.

The results revealed that TREM2+ macrophages were enriched with inhibitory genes such as Trem2, Gpnmb, and Cd63, which are associated with immune suppression and lipid metabolism. Cell migration assays demonstrated that factors secreted by cells from metastatic lesions significantly promoted the migration of bone marrow-derived monocytes and neutrophils, indicating that metastatic lesions attract immune cells into the microenvironment through the secretion of chemotactic factors. Co-culture experiments further revealed that TREM2+ macrophages significantly inhibited T cell proliferation and IFNγ secretion, confirming the immunosuppressive role of these cells.

Compared to previous large-scale sequencing techniques, single-cell RNA sequencing (scRNA-seq) allows for detailed analysis at the single-cell level, revealing more complex cellular characteristics and interactions. NICHE-seq technology combines spatial localization information, enabling researchers to uncover changes in gene expression and analyze the spatial distribution of these changes across different regions. This spatial and temporal dynamic analysis is crucial for understanding the functional roles of cells within the microenvironment during breast cancer metastasis.

While the study provides significant insights into lung metastasis of breast cancer, it primarily focuses on this specific metastatic site. The immune microenvironment may vary significantly across different cancer types and metastatic locations. Future research needs to validate whether the mechanisms identified apply to other types of cancer and metastatic sites. Additionally, while the study found that TREM2+ macrophages play a crucial role at the metastatic margins, the specific functional mechanisms of these cells require further investigation.

Moreover, single-cell RNA sequencing provides information at the gene expression level but lacks integrative analysis of proteomics, metabolomics, and other multi-omics data. Future research should incorporate multi-omics data to comprehensively understand the complex regulatory mechanisms within the tumor microenvironment.

Addressing these questions and challenges will not only deepen our understanding of tumor metastasis mechanisms but also drive the development of new technologies and clinical applications. Through multidisciplinary collaboration and technological innovation, future studies are expected to achieve major breakthroughs in improving cancer treatment efficacy and patient survival rates.

Reference:

Park MD, Reyes-Torres I, LeBerichel J, Hamon P, LaMarche NM, Hegde S, Belabed M, Troncoso L, Grout JA, Magen A, Humblin E, Nair A, Molgora M, Hou J, Newman JH, Farkas AM, Leader AM, Dawson T, D'Souza D, Hamel S, Sanchez-Paulete AR, Maier B, Bhardwaj N, Martin JC, Kamphorst AO, Kenigsberg E, Casanova-Acebes M, Horowitz A, Brown BD, De Andrade LF, Colonna M, Marron TU, Merad M. TREM2 macrophages drive NK cell paucity and dysfunction in lung cancer. Nat Immunol. 2023 May;24(5):792-801. doi: 10.1038/s41590-023-01475-4. Epub 2023 Apr 20. PMID: 37081148; PMCID: PMC11088947.