Natural killer (NK) cell therapies for breast cancer brain metastasis

Breast cancer brain metastasis (BCBM) is a devastating stage of breast cancer with no effective treatment options. There is a growing interest in immunotherapies to treat brain metastasis, given that immune cells can enter the brain while many conventional therapies are excluded. Currently, there are no approved immunotherapies specific for BCBM and there have been limited clinical trials. Immune checkpoint inhibitors (ICIs) and CAR-T cell therapies are showing promising intracranial responses in other brain malignancies, but they are often associated with significant toxicities, which is particularly relevant in the brain. Natural Killer (NK) cell-based therapies represent a compelling alternative, since they demonstrate excellent cytotoxic capacity but greater safety profiles. We have investigated the role of Type 1 innate lymphoid cells (ILCs) in BCBM, which include tissue-resident ILC1s and circulating NK cells. Our experiments using mouse models of experimental metastasis have shown that the Type 1 ILC response in the brain is very limited compared to other tissues like the liver. Using single-cell RNA-sequencing (scRNA-seq) and flow cytommetry, we found that the liver contains much larger numbers of effector ILC1 and NK cell subsets than the brain, which predominantly contains immature and inhibited cells. Depletion of all Type 1 ILCs (ILC1s and NK cells) using anti-NK1.1 antibodies, or specifically ILC1s using Hobit knockout animals shows that both cell types are critical to control metastasis in the liver but show limited activity in the brain. Interestingly, we find that IL15 and IL15Ralpha expression are low to absent in healthy and metastatic brain tissues, while highly expressed in the liver. IL15/IL15Ralpha is a cytokine complex that binds the IL-2Rbeta/gammac receptor on Type 1 ILCs and is essential to induce their maturation and activation. We tested the hypothesis that the defective Type 1 ILC response that we observe in the brain is due to the limited availability of IL15/IL15Ralpha, likely a consequence of the naturally immunosuppressive environment of the CNS. Remarkably, these experiments showed that IL15/IL15Ralpha treatment results in a 5-fold reduction in metastatic burden, and a dramatic expansion of effector ILC1 and NK cell subsets like observed in the liver. Finally, we are currently testing the efficacy of human iPS-derived NK cells engineered to express their own soluble IL15, which are a derivative of cells that our group is currently testing in clinical trials (NCT05334329) to treat non-small cell lung cancer (NSCLC). Preliminary experiments show promising results, where we find that intracranial treatment with iPS-NK-IL15 cells results in a 5-fold reduction in tumor burden. These data clearly establish IL15/IL15Ralpha signaling as an essential factor for the Type 1 ILC response in the CNS, and highlight the potential of targeting this pathway in treatment strategies for brain metastasis.