Natural killer cells are required for the recruitment of CD8+ T cells and the efficacy of immune checkpoint blockade in melanoma brain metastases

This study investigated how the depletion of natural killer (NK) cells in mice treated with a combined PD-1/CTLA-4 blockade affects the molecular profiles of intracranial tumors in a two-site B16-OVA melanoma brain metastases model. This model contains concomitant intracranial and extracranial tumors, to mimic the presence of extracranial metastases in melanoma patients with brain metastases, and intracranial responses to the combined PD-1/CTLA-4 blockade that are observed in the clinic can be reproduced in this model. Overall design: C57Bl6 mice were implanted with B16-OVA murine melanoma cells on the flank under the skin, and 3 days later B16-OVA/Fluc melanoma cells were implanted into the brain (striatum). Mice were treated with IgG control antibodies (IgG_B group) or a combined anti-PD-1 / anti-CTLA-4 therapy on days 5 and 7 post-intracranial cancer cell implantation, and intracranial tumors were isolated on day 8 post-intracranial cancer cell implantation. Mice in the therapy group were further split into those with high (PCcd8H_B group) and low (PCcd8L_B group) intra-tumoral CD8+ T cell abundance, corresponding to therapeutic responders and non-responders, respectively. In the fourth group, NK cells were depleted throughout the experiment using anti-Asialo-GM2 antibody concomitant to therapy with a combined anti-PD-1 / anti-CTLA-4 therapy (PCdNK_B group). mRNA was isolated from bulk tumors obtained from 3 separate mice per group (replicates 1-3) and subjected to mRNAseq and data analysis.