Abstract
BACKGROUND: Cancer metastasis to the brain is a common complication of advanced disease with limited therapeutic options. Although immune checkpoint inhibition has shown some efficacy in treating brain metastases (BrM), patient response is variable, in part due to the unique composition of the brain tumor immune microenvironment (TIME). Our group has shown that BrM can grow in two distinct patterns, either as minimally invasive (MI) masses with well-defined borders, or as tumors with highly invasive (HI) growth into surrounding brain tissue. Here, we investigate how the TIME differs between MI and HI BrM. MATERIAL AND METHODS: We use Nanostring Digital Spatial Profiling coupled with the Cancer Transcriptome Atlas panel on BrM from 20 patients to quantify 1,825 cancer-specific RNA targets in cancer cells at the tumor-brain interface. Additionally, we perform imaging mass cytometry (IMC) on 119 BrM from 46 patients, encompassing over 350,000 cells. Samples represent BrM from various primary sites (lung, breast, melanoma, other), and include patient-matched samples from the brain-tumor interface and the centre of the metastatic lesion. Additional multiplexed immunohistofluorescence staining was performed on a subset of 18 patients with lung cancer BrM. RESULTS: Gene set enrichment analyses revealed elevated interferon gamma signalling in MI versus HI BrM, which was confirmed by immunohistochemical staining for pSTAT1, consistent with an “immune hot” TIME in MI BrM. IMC revealed that MI BrM had significantly higher frequencies of lymphocytes, including B cells, CD4+, CD8+, CD4- CD8-, and regulatory T cells compared to HI BrM. Pairwise cellular association analyses revealed increased interactions between lymphoid cell populations in MI versus HI BrM, suggestive of specific regions enriched in B and T cells in MI BrM, rather than a generally diffuse infiltrate. Indeed, cellular neighbourhood analyses revealed a lymphoid-rich cellular neighbourhood that was more abundant in MI versus HI BrM and localized to the tumor-brain interface. Multiplexed immunohistofluorescence staining revealed that T cells were more likely to be cytotoxically active and exhausted in MI versus HI BrM. CONCLUSION: These data suggest that HI BrM invade into an immunosuppressed microenvironment while MI BrM are characterized by an active anti-tumor immune infiltrate, predominantly localized to the tumor-brain interface. Together, this work suggests potential immune regulation of BrM invasion. Work is ongoing to identify factors present in the HI BrM environment that enforce an immunosuppressive environment. This research has been supported by grants from the Terry Fox Foundation and the Quebec Breast Cancer Foundation (Grant #: 251427-251690) and the Canadian Institutes of Health Research (CIHR PJT-175066) to P.M.S.