The murine meninges acquire lymphoid tissue properties and harbour autoreactive B cells during chronic Trypanosoma brucei infection

The meningeal space is an important structure in the brain borders, which provides immunosurveillance for the central nervous system, but the impact of infections on the meningeal immune landscape is far from being fully understood. <em>Trypanosoma brucei</em>, the causative agents of Human African Trypanosomiasis or sleeping sickness, accumulates in the meningeal spaces, inducing severe meningitis and resulting in death if left untreated. Thus, sleeping sickness represents an attractive model to study immunological dynamics in the meninges during infection. Here, combining single cell transcriptomics and mass cytometry by time of flight (CyTOF), coupled with <em>in vivo</em> interventions, we found that chronic <em>T. brucei</em> infection triggers the development of ectopic lymphoid aggregates (ELAs) in the murine meninges during chronic infection. These infection-induced ectopic structures are defined by the presence of ER-TR7⁺ fibroblastic reticular cells (FRCs) and follicular dendritic cells (FDCs) that initiate a signalling cascade driving local T cell activation towards a T follicular helper (T_FH)-like phenotype, as well as B cell class switching. Furthermore, the meningeal-resident plasma cells display activity typically associated with germinal centre reactions and are able to produce antibodies able to recognise mouse brain antigens, which correlates with cortical (and white matter) demyelination. Lastly, we found that systemic lymphotoxin b (LTb) signalling blockade led to a significant depletion of meningeal FDCs and autoreactive B cells, indicating that LTb signalling is critical to induce and maintain local responses in the meninges. Taken together, we provide evidence that the meningeal immune response results in the acquisition of lymphoid tissue-like properties during chronic <em>T. brucei</em> infection. These findings have broader implications for understanding the mechanisms underlying the formation ELAs during chronic inflammation resulting in autoimmunity, as observed in other demyelinating neurodegenerative disorders such as multiple sclerosis.