Although intracerebral concentrations of PD-1 inhibitors required to produce effects on brain tumor microenvironments remain unknown, activated T cells in the CSF can traffic into the brain by extravasating from meningeal vessels and then crossing the pia mater.15 The finding that pembrolizumab concentrations in the CSF are sufficient to activate endogenous T cells suggests a mechanism through which systemically administered PD-1 antibodies could produce a local effect in the brain. This evidence concerns the gene PDCD1 and brain neoplasm.