Pro-inflammatory T helper 17 directly harms oligodendrocytes in neuroinflammation

T helper (Th)17 cells are considered to contribute to inflammatory mechanisms in diseases such as multiple sclerosis (MS). However, the discussion persists regarding their true role in patients. Here, we visualized central nervous system (CNS) inflammatory processes in models of MS live in vivo and in MS brains and discovered that CNS-infiltrating Th17 cells form prolonged stable contact with oligodendrocytes. Strikingly, compared to Th2 cells, direct contact with Th17 worsened experimental demyelination, caused damage to human oligodendrocyte processes and increased cell death. Importantly, we found that in comparison to Th2 cells, both human and murine Th17 cells express higher levels of the integrin CD29, which is linked to glutamate release pathways. Of note, contact of human Th17 cells with oligodendrocytes triggered release of glutamate. As we found that Th17-polarized cells release glutamate in contact with oligodendrocytic cells, we then assessed the impact of glutamate itself on human oligodendrocytes in primary culture. We found that 12-24h application of glutamate - but not pro-inflammatory cytokines TNFα or IFN-γ - strongly decreased the area and complexity of human oligodendrocyte processes in vitro without affecting oligodendrocyte survival. To further investigate the impact of glutamate on human oligodendrocytes we used single cell RNA sequencing (scRNAseq) of the human oligodendrocytes in primary culture, unstimulated (vehicle) and stimulated with glutamate. Following exposure to glutamate we observed induced cell stress and changes in biosynthesis of cholesterol and lipids in mature human oligodendrocytes. Finally, exposure to glutamate decreased myelination whereas blockade of CD29 preserved oligodendrocyte processes from Th17-mediated injury. Our data provide first evidence for direct and deleterious attack of Th17 cells on the myelin compartment and show the potential for new therapeutic opportunities in MS. Overall design: Two different batches, corresponding to two different donors in total. Each batch consisted of human oligodendrocytes in primary culture derived from the same brain sample (resection path of surgery for epilepsy) exposed to glutamate 1mM vs. vehicle for 12h.