In the first funding period we observed that graft-versus-host disease (GVHD) causes profound activation of microglia. RNA-sequencing revealed that microglia cells upregulate MHC-II and TNF upon GVHD induction. By using Cre/lox based deletion of TNF or TAK1 in microglia, we could show a functional role of these molecules for the impaired neurocognitive function during GVHD. This prompted us to develop a novel therapeutic approach using TAK1 inhibition which reversed neurocognitive impairment. Our preliminary studies, using immune checkpoint blockade (ICB) therapy with anti-PD1 antibodies in melanoma bearing and naive mice, indicate microglia activation and increased T cell infiltration into the CNS. Based on these preliminary findings we will set out to understand the biology of neurological defects observed after ICB immunotherapy. Firstly, we plan to characterize the distinct signaling /transcriptional events in microglia of mice treated with anti-PD1, anti-CTLA4 or anti-TIM3. Secondly, we will use genetic loss-of-function approaches deleting MHC-II, CD40, CD115 or PD-L1 in microglia to address whether these molecules contribute or inhibit ICB-induced neuroinflammation. The results of the signaling analysis will be the basis for pharmacological approaches targeting the newly identified pathways, as well as promising predefined targets (MEK/ERK or TAK1). Efficacy will be validated by CNS analysis as well as neurocognitive function tests of the mice. To test for a potential translation into the clinic, we will assess the phenotype and function of human microglia cells in patients who died after immune-related adverse events following anti-PD1 immunotherapy. Overall we plan to characterize the role of microglia in tumor-immunotherapy-induced neuroinflammation.