The dynamics of brain T cell populations during the course of rasmussen encephalitis: from expansion to exhaustion.

Abstract

Rasmussen Encephalitis (RE) is a chronic, unilateral epileptic disorder mostly found in children. Neuropathologically, it is characterized by T lymphocyte infiltration targeting neurons and leading to microglia activation, astrogliosis, and cortical degeneration. Within a patient's brain, distinct pathological stages are found that offer a unique opportunity to study T cell dynamics in situ. Using quantitative multiplex fluorescence imaging, we analyzed CD103⁺ and CD69⁺ Tissue-resident memory T cells (T_RM) across different disease stages. This analysis revealed that T_RM were more abundant in the parenchyma than in the perivascular space, suggesting that their differentiation occurs locally after antigen encounter. Further, part of the T_RM expressed Granzyme-B (GrB) and frequently were attached to neurons, suggesting that they are actively involved in neuronal destruction. While T_RM showed a stage-dependent increase in older lesions, the proportions of these cells did not correlate with disease duration, indicating that their accumulation may be more dependent on the local environment in the lesion than on the length of the disease. In addition, we found that T cells using the γδ T cell receptor comprised up to 66%. Like CD8⁺ T cells, the γδ T cells could develop a T_RM phenotype and, while expressing GrB⁺ granules, they were seen attached to neurons, suggesting that they are involved in neuronal destruction. Finally, analysis of exhaustion- and T_RM-associated immune checkpoint control markers PD-1 and LAG-3 revealed a significant stage-dependent increase in PD-1 expression in the oldest lesions. In contrast, LAG-3 expression did not show any stage-specific pattern, pointing towards a distinct regulatory mechanism. The study demonstrates a dynamic and one-way T cell response throughout the course of RE at a given spot in the CNS: from the establishment of T cell residence after entry into the CNS, the killing of neurons, and eventually T cell exhaustion. It further suggests an important role of γδ T-cells in the propagation of disease and lesions.