Astrocytes contribute to the pathology of multiple neurological disorders, including the T cell-driven autoimmune disease of the central nervous system (CNS) multiple sclerosis and its mouse model, experimental autoimmune encephalomyelitis1. However, little is known about functional interactions between astrocytes and CD4+ T cells. Here using rabies barcode interaction detection followed by sequencing2, in combination with single-cell RNA sequencing, in vitro co-culture systems and cell-specific in vivo CRISPR–Cas9-based genetic perturbation studies, we established that astrocytes expressing CD40 and MHC-II promote CNS T cell autoimmunity. We harnessed universal labelling immune partnerships by SorTagging intercellular contacts3 to analyse astrocyte-interacting CD4+ T cells, finding that direct astrocyte–CD4+ T cell interactions enhance pathogenic T helper 17 cell responses in experimental autoimmune encephalomyelitis. In addition, we studied the effect of these interactions on astrocytes. Using in vivo subproteomic approaches4 and AlphaFold-Multimer predictions5, we established that CD40 activation in astrocytes by CD40L expressed by CD4+ T cells induces the accumulation of PLIN4-positive lipid droplets, which provide acetyl-CoA to promote p65 acetylation-dependent NF-κB activation and antigen presentation. Finally, we detected CD40+MHC-II+LD+ astrocytes in multiple sclerosis samples by single-nucleus RNA sequencing and immunohistochemistry. In summary, these studies define a previously unrecognized mechanism by which astrocytes promote CNS autoimmunity.