Inhibition of Anti-Inflammatory Macrophage Phenotype Reduces Tumour Growth in Mouse Models of Brain Metastasis.

Vasiliki Economopoulos,Helen Scott,Nicola R Sibson,Vanessa A Johanssen,James R Larkin,Kleopatra E Andreou,Maria Pannell

doi:10.3389/fonc.2022.850656

Abstract

Breast cancer brain metastasis is a significant clinical problem and carries a poor prognosis. Although it is well-established that macrophages are a primary component of the brain metastasis microenvironment, the role of blood-derived macrophages (BDM) and brain-resident microglia in the progression of brain metastases remains uncertain. The aim of this study, therefore, was to determine the role, specifically, of pro- and anti-inflammatory BDM and microglial phenotypes on metastasis progression. Initial in vitro studies demonstrated decreased migration of EO771 metastatic breast cancer cells in the presence of pro-inflammatory, but not anti-inflammatory, stimulated RAW 264.7 macrophages. In vivo, suppression of the anti-inflammatory BDM phenotype, specifically, via myeloid knock out of Krüppel-like Factor 4 (KLF4) significantly reduced EO771 tumour growth in the brains of C57BL/6 mice. Further, pharmacological inhibition of the anti-inflammatory BDM and/or microglial phenotypes, via either Colony Stimulating Factor 1 Receptor (CSF-1R) or STAT6 pathways, significantly decreased tumour burden in two different syngeneic mouse models of breast cancer brain metastasis. These findings suggest that switching BDM and microglia towards a more pro-inflammatory phenotype may be an effective therapeutic strategy in brain metastasis.

Highlights

Brain metastasis is a devastating diagnosis for patients with primary breast cancer, and the prognosis is extremely poor; patient survival ranges from 2 to 23 months from diagnosis [1, 2]
Since the phenotype of macrophages exists as a continuum, rather than as discrete classes, the ratio of inducible nitric oxide synthase (iNOS) : arginase 1 (Arg1) was used in addition to individual expression levels to assess shifts in macrophage and microglial phenotype
There is mounting evidence that macrophages/microglia, and their different phenotypes, play an important role in brain metastasis progression, with an increasing number of studies suggesting a role for the anti-inflammatory phenotype

Summary

Introduction

Brain metastasis is a devastating diagnosis for patients with primary breast cancer, and the prognosis is extremely poor; patient survival ranges from 2 to 23 months from diagnosis [1, 2]. Anti-Inflammatory Macrophages Aid Brain Metastasis cells in the lung, and that depletion of myeloid cells in the transgenic CD11b-DTR mouse decreases tumour cell survival and prevents the establishment of micrometastases [3]. Qian et al showed that depletion of the macrophage population, using clodronate liposomes, hinders the establishment of metastases in the lung [4]. Yao et al found that inhibiting anti-inflammatory (M2) macrophage polarization, with the tyrosine kinase inhibitor imatinib, significantly reduced the number of nodules present in the lungs in a lung cancer model, in part due to STAT6 inhibition [5]. Binnemars-Postma et al evaluated STAT6 inhibition in vivo and showed that inhibition in macrophages reduces tumour growth and development of the metastatic niche within the liver in a murine breast cancer model [6]

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