Preprint Club
A cross-institutional Journal Club Initiative

Keywords

• Macrophage heterogeneity

• Tumour microenvironment (TME)

• Cancer-associated fibroblasts (CAF)

Main Findings

The local signals which regulate diverse tumour-associated macrophage (TAM) phenotypes and therefore different clinical outcomes in breast cancer are not well understood. As such, Ben-Chetrit N et al. investigate the molecular mechanisms which regulate macrophage heterogeneity in solid tumours. Here, Ben-Chetrit N et al use novel organotypic tumour microenvironment (oTME) and mouse models of breast cancer to demonstrate that a subset of F4/80high Sca1+ macrophages require both type 1 interferon (IFN) and Notch4 signalling from cancer associated fibroblasts (CAF) for self-renewal. Aligned with this, Ben-Chetrit N et al demonstrate that a similar stroma-associated macrophage (SAM) population is associated with both high tumour histologic grade and decreased survival in human breast cancer patient datasets. These results demonstrate the importance of macrophage-fibroblast crosstalk in promoting cancer progression and highlight the need for therapeutic intervention targeting tumour stroma.

Limitations

• The authors leverage scRNA-seq analysis in the oTME model to demonstrate that Notch4 signalling is critical for the self-renewing F4/80high Sca1+ SAM phenotype. In addition, they demonstrate that anti-Notch4 antibodies reduced tumour volume in a mouse model of breast cancer. Despite this, the specificity of the interaction between SAM and CAF is not definitively shown. The authors may consider 1) tracking tumour progression in transgenic mouse models which specifically abrogate Notch4 production in fibroblasts or 2) in silicotracking of cell-to-cell interactions in their scRNA-seq dataset with R-based packages such as CellChat to highlight increase the specificity of the proposed molecular mechanism.

• The novel ex vivo organotypic model of breast cancer allows the authors to reconstitute the major non-immune cells (e.g., tumour epithelial cells and CAF) to recapitulate the architecture of mammary tumours. As primary macrophages are introduced after stomal cells, the authors may consider introducing macrophage lacking the relevant receptors for type I IFNs or Notch4 signalling ex vivoto demonstrate that these signals are critical for self-renewal of the SAM population.

• The authors demonstrate that both type I IFN and Notch4 signalling regulate F4/80highSca1+ SAM self-renewal using both scRNA-seq and pharmacological inhibitor-based approaches ex vivo. Despite this, the relationship between type I IFN and Notch4 signalling is not discussed or investigated.

• The authors demonstrate that a similar SAM phenotype is conserved in the oTME ex vivo organotypic model, as well as mouse models of cancer and human patients with breast cancer. The authors may consider harmonizing the markers used in each dataset to fully elucidate the translational aspects of SAM biology.

Significance/Novelty

In summary, Ben-Chetrit N et al definitively demonstrate that macrophage phenotype is shaped by the spatial organization of tumours. Aligned with this, they demonstrate that a population of self-renewing SAM are maintained by Notch4 and/or type I IFN signalling by CAF. As a result, this preprint identifies molecular mechanisms that may be exploited to generate new therapeutics targeting macrophage-fibroblast interactions to limit tumour progression.

Credit

Reviewed by Anthony Altieri as part of a cross-institutional journal club between the Icahn School of Medicine at Mount Sinai, the University of Oxford, the Karolinska Institute and the University of Toronto.

The author declares no conflict of interests in relation to their involvement in the review.