(P 128) INVESTIGATING THE ROLE OF MACROPHAGES IN A MURINE MODEL OF SOFT TISSUE SARCOMA

Objective: Soft tissue sarcomas (STSs) are currently treated with morbid surgical resection and radiation therapy, which has not changed in decades. Immunotherapy is an emerging therapeutic strategy that has been successfully used to combat malignancies. Unfortunately, these therapies have yet to show equivalent therapeutic efficacy in patients with STS, which may be attributed to the lack in understanding of the STS tumour microenvironment (TME). Recent studies have shown that non-malignant cells within the TME, including immune cells, can promote tumour growth. Interestingly, the tumour microenvironment of STSs contains an abundance of macrophages, but their role in STS biology is not fully understood. Macrophages can be associated with pro-tumour, angiogenic, immunosuppressive, and metastatic behaviour in multiple malignancies. Therefore, the purpose of the present study is to investigate the role of macrophages in the TME of a murine model of STS. It is hypothesized that macrophages in STSs display a pro-tumour phenotype involved with vascular development, tumour progression, and immune suppression.

Methods: To study the role of macrophages in STS, we utilized two models of inducing macrophage deficiency that have been used in other murine models of cancer. The first model investigated is a transient macrophage depletion model; the Macrophage Fas-Induced Apopotosis (MaFIA) model. This model has a mutant colony stimulating factor-1 receptor (CSF1R) that can be targeted to induce cell death with a pharmacological agent; AP20187. In giving MaFIA mice AP20187, it leads to CSF1R cell death, which predominantly targets macrophages. The second model utilized in this study was a macrophage recruitment deficiency model; C-C chemokine receptor-2 Knockout (CCR2 KO). These mice are unable to recruit monocytes into inflamed tissue, which has shown diminished tumour macrophages in other studies. Our laboratory has developed a syngeneic murine model of STS on a C57Bl/6 background, which can be synergized with both the MaFIA and CCR2 KO models.
Tumours were assessed 10-days after STS cell engraftment, and assessed for tumour volume, as well as macrophages, lymphocytes (natural killer, CD4, and CD8 T cells), and vascularity using flow cytometry.

Results: In assessing changes to tumour burden, anti-CSF1R treatment (AP20187) in the MaFIA mice showed a ~84% decrease in tumour volume compared to control mice. However, the CCR2 KO mice showed no changes to tumour volume compared to control mice. MaFIA mice had no differences in total macrophages, but showed a significant reduction in CSF1R+ and CSF1R+/CD206+ macrophage subtypes (macrophages with a CD206+ phenotype are associated with pro-tumour activity). Interestingly, the CCR2 KO mice showed a 58% decrease in tumour macrophages, but no changes to macrophage subtypes.
MaFIA mice given anti-CSF1R treatment (AP20187) showed no changes to lymphocytes, whereas CCR2 KO mice showed a significant reduction in natural killer cells, but no changes to CD4 or CD8 T cells. There were no changes to tumour vascularity (CD31+ cells) in either group.

Conclusion: The data in this study suggest that the CCR2 KO model diminishes the macrophage population in UPS tumours, whereas the MaFIA model diminishes subsets of macrophages. However, the MaFIA model was able to diminish tumour growth, which was not seen in the CCR2 KO model. It is possible that macrophages targeted by the MaFIA model are driving tumour growth, whereas the inhibition of macrophages infiltrating with the CCR2/L2 do not play a role in tumour progression in this model. Additionally, it is possible targeting the CSF1R+ and/or CSF1r+/CD206+ subtypes can diminish disease progression without lymphocyte input.
Anti-CSF1R therapy has received clinical investigation in other malignancies. Therefore, this study justifies further investigation of clinically translatable CSF1R inhibitory therapies in this preclinical model of STS.