PD-L1 blockade enhances brain resident memory T cell function and stimulates brain to lymph node crosstalk

Resident memory T cells (T RM) are a unique subset of memory T cells that persist within non-lymphoid tissues. Previous studies have identified T RMwithin the brain (bT RM) after peripheral infection and vaccination in mice, with more recent appreciation in human brain tissue. While bT RMare important for protection of the CNS against reinfection, their heterogeneity at the single cell level and overall involvement in shaping the neuroimmune landscape remains unknown. Here, we defined viral-specific bT RMheterogeneity by single cell RNA sequencing and identified unique subsets dependent on local antigen encounter. These data also confirmed previous findings that bT RMexpress markers associated with T cell exhaustion, such as PD-1. Despite this, intracranial delivery of cognate viral peptide led to robust bT RMreactivation and initiated a cascade of immune activation and accumulation within the brain, including rapid activation of microglia, NK cells and T cells, DC maturation, and infiltration of macrophages and monocyte derived DCs. In the presence of PD-L1 blockade, despite observing higher effector molecule production from reactivated bT RM, we found no apparent difference in downstream immune activation in the brain. Interestingly, however, we found a significant increase in DC and B cell maturation in the CNS draining deep cervical lymph nodes (dcLN) only in the presence of PD-L1 blockade. These data indicate a potential unique crosstalk between bT RMand APCs in the dcLN normally suppressed by PD-1/PD-L1 signaling. These studies provide insight into brain T RMfunctions and potential mechanisms for strengthening immune activation that can guide immunotherapies for immunologically cold brain tumors. Supported by grants from the NIH (K22AI148508-02, T32 AI007363)