Effects of Extracranial Priming and Treatment with Immune Checkpoint Blockade in Melanoma Brain Metastases: Insights from a Murine Model

Introduction: Upon diagnosis with metastatic melanoma, 20-40% of patients will already have one or more melanoma brain metastases (MBM), and over 50% will develop MBM at some time in their disease course. It was recently shown that the response rate of treatment-naïve, asymptomatic intracranial metastases to dual immune checkpoint blockade (ICB) with anti-CTLA4 and anti-PD1 is over 50%. However, survival of most patients with MBM treated with ICB continues to hover around a year. To harness ICB’s full potential as a therapy for MBM, we must understand the opposing mechanisms that promote and antagonize the immunologic response to MBM and treatment with dual ICB.

Methods: Twenty C57BL/6 mice were implanted with tumors. Thirteen were metastatic models in which 100,000 B16-F10 melanoma cells were implanted extracranially (EC) followed by intracranial (IC) implantation of 100,000 cells seven days later. Of these, seven were treated with dual anti-PD1-anti-CTLA4, and six were treated with IgG as controls. An additional seven mice received IC implantations only, of which four were ICB-treated and three were IgG-treated. EC tumors were measured every two days. Flow cytometry was performed on blood samples from all twenty mice. Ten of twenty mice lived to the 21-day endpoint, and their blood, spleen, tumor(s), and cervical and inguinal lymph nodes were harvested and stained for flow cytometric analysis.

Results: ICB-treated metastatic models grew larger EC tumors, with the largest EC tumor in the ICB group reaching a volume tenfold larger than that of the largest EC tumor in the IgG treated group. Even with small sample sizes and high variability, a higher number of CD8+ T cells and conventional dendritic cells were observed in the IC-only tumors that were ICB treated, but this difference was not observed in the ICB-treated metastatic models. Additionally, a trend toward higher IFNAR in CD8+ T cells was observed only in the metastatic model that was ICB-treated.

Conclusion : These findings are reflective of the metastatic model being more consistent with patterns of ICB resistance than the IC model alone.