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Melanoma-derived brain metastases (MBM) represent an unmet clinical need because central nervous system progression is frequently an end stage of the disease. Immune checkpoint inhibitors (ICI) provide a clinical opportunity against MBM; however, the MBM tumor microenvironment (TME) has not been fully elucidated in the context of ICI. To dissect unique elements of the MBM TME and correlates of MBM response to ICI, we collected 32 fresh MBM and performed single-cell RNA sequencing of the MBM TME and T-cell receptor clonotyping on T cells from MBM and matched blood and extracranial lesions. We observed myeloid phenotypic heterogeneity in the MBM TME, most notably multiple distinct neutrophil states, including an IL8-expressing population that correlated with malignant cell epithelial-to-mesenchymal transition. In addition, we observed significant relationships between intracranial T-cell phenotypes and the distribution of T-cell clonotypes intracranially and peripherally. We found that the phenotype, clonotype, and overall number of MBM-infiltrating T cells were associated with response to ICI, suggesting that ICI-responsive MBMs interact with peripheral blood in a manner similar to extracranial lesions. These data identify unique features of the MBM TME that may represent potential targets to improve clinical outcomes for patients with MBM.

Supplementary Figure from Microenvironmental Landscape of Human Melanoma Brain Metastases in Response to Immune Checkpoint Inhibition

Supplementary Data from Microenvironmental Landscape of Human Melanoma Brain Metastases in Response to Immune Checkpoint Inhibition

Immune checkpoint inhibitors (ICI) have revolutionized oncologic treatment for metastatic melanoma. With improved systemic control, there has been increasing prevalence of patients with brain metastases. Recent evidence has demonstrated intracranial responses in a subset of these patients treated with ICI. We hypothesize that the response to ICI in melanoma brain metastases (MBM) is reflective of unique features within the tumor microenvironment of the brain. A cohort of 27 patients, encompassing 8 pre- and 19 post-immunotherapy MBM underwent single cell RNA sequencing (Smart-Seq2). The cohort includes patients with longitudinal cranial resections and simultaneously resected, spatially distinct tumors. Each tumor underwent unsupervised transcriptomic analysis, differential gene expression, inferred copy number variation, and T-cell receptor (TCR) clonotyping. Published extracranial melanoma single cell datasets were used to compare the tumor microenvironment of the brain and periphery in response to ICI. A total of 14,027 cells (6,189 malignant, 7,838 non-malignant) were sequenced. Brain metastases demonstrated a heterogeneous distribution of macrophage states. Intracranial macrophages were found to be more tumor-supportive than their extracranial counterparts. MBM also included a distribution of reactive neutrophils and astrocytes. Analysis across pre- and post-treatment MBM demonstrated an increase in clonally expanded T cells in patients responding to ICI. Across longitudinal brain metastases collected from the same patients, there was evidence of identical T cell clones across timepoints and locations. Single cell sequencing of MBM provides insights into the cellular composition of the tumor and microenvironment. Our data suggest the cellular heterogeneity within MBM is unique when compared to extracranial disease. Additionally, T cell clonal expansion is found following ICI and T cells of the same clonotype infiltrate spatially and temporally separated brain metastases. These findings raise potential therapeutic implications as we learn to target the differential features of the innate and adaptive immune system within brain metastases and their extracranial counterparts.

Reversal of immune cell exhaustion through immune checkpoint blockade (ICB) has become a first-line approach for patients with metastatic melanoma due to success in controlling extracranial tumors. However, patients frequently experience discordant responses, with extracranial response and intracranial progression. We hypothesize that ICB exerts selective pressure leading to the clonal evolution of treatment resistant clones that ultimately culminate in disease progression. We collected a cohort of 97 patients, encompassing 312 pre- and post-immunotherapy melanoma tumors, for whole exome sequencing (WES) and included primary, extracranial, or intracranial samples. Each tumor was analyzed for somatic mutations, copy number alterations, neoantigen profile, and patient specific phylogenetic trees were constructed encompassing a tumor’s genetic subclones. Heterogeneity of the tumor microenvironment was evaluated using high multiplicity single-cell immunofluorescent staining (CycIF). Single cell sequencing was performed on fresh tissue from 4 pre-treatment and 14 post-immunotherapy melanoma brain metastases using the Smart-Seq2 protocol. WES of pre- and post-immunotherapy tumors yielded distinct patterns of clonal evolution and immunoediting within brain metastases compared to their extracranial counterparts, including mutations in B2M. In paired pre- and post-immunotherapy samples, CycIF demonstrated decreased in CD8 infiltration and increased CD45RO, FOXP3, and PD-L1 staining suggesting less cytotoxic, terminally differentiated T cells in resistant tumors. Single cell sequencing analysis of 3,974 tumor and immune cells demonstrated patient-specific tumor clustering and gene expression profiles mediating resistance to ICB. In conclusion, we document, for the first time, evidence of ongoing branched evolution during immunotherapy in brain metastases with divergence compared to systemic sites of disease. Next-generation sequencing provides novel insights into clonal evolution mediating discordant responses of intra- and extracranial sites and immunosuppressive features of the intracranial tumor microenvironment. Targeting these mechanisms of resistance provide potential therapeutic avenues for patients with progressive intracranial disease.

e14041Background: The genetic alterations that define PCNSL are beginning to be identified, and the genomic evolution that conveys susceptibility or resistance to treatment in recurrent disease is ...