Phenotypic and functional complexity of brain-infiltrating T cells in Rasmussen encephalitis.

Objective: To characterize the brain-infiltrating immune cell repertoire in Rasmussen encephalitis (RE) with special focus on the subsets, clonality, and their cytokine profile.

Methods: The immune cell infiltrate of freshly isolated brain tissue from RE was phenotypically and functionally characterized using immunohistology, flow cytometry, and T-cell receptor (TCR) deep sequencing. Identification of clonally expanded T-cell clones (TCCs) was achieved by combining flow cytometry sorting of CD4+ and CD8+ T cells and high-throughput TCR Vβ-chain sequencing. The most abundant brain-infiltrating TCCs were isolated and functionally characterized.

Results: We found that CD4+ , CD8+ , and also γδ T cells infiltrate the brain tissue in RE. Further analysis surprisingly revealed that not only brain-infiltrating CD8+ but also CD4+ T cells are clonally expanded in RE. All 3 subsets exhibited a Tc1/Th1 phenotype characterized by the production of interferon (IFN)-γ and TNF. Broad cytokine profiling at the clonal level showed strong production of IFN-γ and TNF and also secretion of interleukin (IL)-5, IL-13, and granzyme B, both in CD4+ and CD8+ T cells.

Conclusions: CD8+ T cells were until now considered the central players in the immunopathogenesis of RE. Our study adds to previous findings and highlights that CD4+ TCCs and γδ T cells that secrete IFN-γ and TNF are also involved. These findings underline the complexity of T-cell immunity in RE and suggest a specific role for CD4+ T cells in orchestrating the CD8+ T-cell effector immune response.