Abstract 113: Lyve-1 expressing macrophages direct the expansion of pericytes within the perivascular niche to support angiogenesis in cancer
Authors: Opzoomer James W. , Anstee Joanne E. , Dean Isaac , Hill Emily J. , Bouybayoune Ihssane , Caron Jonathan , Muliaditan Tamara , Gordon Peter , Nuamah Rosamond , Pinder Sarah E. , Ng Tony , Dazzi Francesco , Kordasti Shahram , Withers David R. , Lawrence Toby , Arnold James N. 
Published: July 2021

Abstract
Despite the many pro-tumoral functions that have been described for the various stromal populations in cancer, it is becoming apparent that these cells are not working alone, but instead in concert with one another as part of a wider network of cross-communication to facilitate disease progression. Identifying non-redundant signaling pathways within the stromal network is desirable, as therapeutically targeting these signals could result in an unraveling of the stromal support network upon which the tumor cells rely. Tumor associated macrophages (TAMs) have been implicated in a variety of pro-tumoral processes and are polarized by signals in the tumor microenvironment (TME) to adopt a range of functionally distinct subsets to perform these tasks. Single-cell RNA-sequencing of the F4/80+ TAMs from the MMTV-PyMT murine model of mammary adenocarcinoma identified one subset selectively expressing Lyve-1 and adopting a CD206hiMHCIIlo phenotype. This population was identified to spatially reside in the niche proximal to blood vasculature within the tumor. We demonstrate that selective depletion of the Lyve-1+ TAM population using liposome-based approaches controlled tumor growth and was associated with a loss of perivascular alpha smooth muscle actin (αSMA)-expressing stromal cells. The αSMA+ stromal cells were characterized ex vivo and expressed a surface phenotype associated with pericytes (CD90+CD34-NG2+), highlighting a possible role for Lyve-1+ TAMs in maintaining the population. To exclude a role of Lyve-1+ TAMs in recruiting a pericyte-progenitor cell to the tumor, we selectively photo-labelled the TME using MMTV-PyMT mice ubiquitously expressing the photo-convertible green/red Kaede protein. Labelling the TME Kaede-red through exposure to ultraviolet light, demonstrated that pericytes and CD90+ mesenchymal cells more broadly, were not recruited from a peripheral site but instead suggested these populations must rely on local proliferation to maintain their prevalence in the TME. Indeed, we demonstrate through Edu incorporation, that loss of Lyve-1+ TAMs from the perivascular niche renders pericytes senescent. Using computational approaches, we characterized the signaling cross-talk in the perivascular niche between Lyve-1+ TAMs, pericytes and endothelial cells and identified a specific interaction involving platelet derived growth factor-CC (PDGF-CC) expression by Lyve-1+ TAMs and PDGF-receptor alpha (PDGFRα) on the pericytes. Using blocking antibodies to PDGF-CC in tumor bearing mice we demonstrate that blocking this axis of stromal crosstalk can render pericytes senescent. These data highlight that local expansion of pericytes in cancer is not an autonomous event, nor regulated by the endothelium alone, but tightly regulated by the perivascular Lyve-1+ TAM population, which dictates the success of angiogenesis in cancer.
Citation Format: James W. Opzoomer, Joanne E. Anstee, Isaac Dean, Emily J. Hill, Ihssane Bouybayoune, Jonathan Caron, Tamara Muliaditan, Peter Gordon, Rosamond Nuamah, Sarah E. Pinder, Tony Ng, Francesco Dazzi, Shahram Kordasti, David R. Withers, Toby Lawrence, James N. Arnold. Lyve-1 expressing macrophages direct the expansion of pericytes within the perivascular niche to support angiogenesis in cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 113.