Steady-state memory-phenotype conventional CD4+ T cells exacerbate autoimmune neuroinflammation in a bystander manner via the Bhlhe40/GM-CSF axis

Memory-phenotype (MP) CD4 + T cells are a substantial population of conventional T cells that exist in steady-state mice, yet their immunological roles in autoimmune disease remain unclear. In this work, we unveil a unique phenotype of MP CD4 + T cells determined by analyzing single-cell transcriptomic data and T cell receptor (TCR) repertoires. We found that steady-state MP CD4 + T cells in the spleen were composed of heterogeneous effector subpopulations and existed regardless of germ and food antigen exposure. Distinct subpopulations of MP CD4 + T cells were specifically activated by IL-1 family cytokines and STAT activators, revealing that the cells exerted TCR-independent bystander effector functions similar to innate lymphoid cells. In particular, CCR6 high subpopulation of MP CD4 + T cells were major responders to IL-23 and IL-1β without MOG 35-55 antigen reactivity, which gave them pathogenic Th17 characteristics and allowed them to contribute to autoimmune encephalomyelitis. We identified that Bhlhe40 in CCR6 high MP CD4 + T cells as a key regulator of GM-CSF expression through IL-23 and IL-1β signaling, contributing to central nervous system (CNS) pathology in experimental autoimmune encephalomyelitis. Collectively, our findings reveal the clearly distinct effector-like heterogeneity of MP CD4 + T cells in the steady state and indicate that CCR6 high MP CD4 + T cells exacerbate autoimmune neuroinflammation via the Bhlhe40/GM-CSF axis in a bystander manner. Autoimmunity: Bystander T cells fuel inflammation in the CNS An antigen independent function of T cells, known as bystander-activated T cell, contributes to autoimmune disease in the CNS (Central Nervous System), and could be targeted therapeutically to help suppress neuroinflammation. A team led by Je-Min Choi from Hanyang University in Seoul, South Korea, showed in mice that different subgroups of memory-phenotype T cells naturally arising without immunization. These cells become activated in response to different immune-stimulating cytokines, even without antigen-specific immune recognition. The researchers identified one particular subgroup that is a key driver of inflammatory signaling and works with other T cells to exacerbate an experimental model of multiple sclerosis. The findings provide important insights into potential therapeutic approaches that drugs designed to block bystander-activated T cells may offer relief for people with immune-inflammatory diseases of the central nervous s