The extrusion of DNA traps contributes to a key mechanism in which innate immune cells clear pathogens or induce sterile inflammation. Here we provide evidence that CD4⁺ T cells, a critical regulator of adaptive immunity, release extracellular threads of DNA on activation. These DNA extrusions convey autocrine costimulatory signals to T lymphocytes and can be detected in lymph nodes isolated during the priming phase of experimental autoimmune encephalomyelitis (EAE), a CD4⁺ T cell-driven mouse model of multiple sclerosis. Pharmacologic inhibition of mitochondrial reactive oxygen species (mtROS) abolishes the extrusion of DNA by CD4⁺ T cells, reducing cytokine production in vitro and T cell priming against myelin in vivo. Moreover, mtROS blockade during established EAE markedly ameliorates disease severity, dampening autoimmune inflammation of the central nervous system. Taken together, these experimental results elucidate a mechanism of intrinsic immune costimulation mediated by DNA threads released by activated T helper cells, and identify a potential therapeutic target for such disorders as multiple sclerosis, neuromyelitis optica, and CD4⁺ T cell-mediated disorders.