Cytotoxic CD8 ⁺ T lymphocytes expressing ALS-causing SOD1 mutant selectively trigger death of spinal motoneurons

Significance

CD8 ⁺ T lymphocytes, which are typically devoted to eliminate malignant and infected cells, have been described in the central nervous system (CNS) of patients and mice with amyotrophic lateral sclerosis (ALS). However, their role in ALS pathogenesis has yet to be unraveled. Here, we show that ablation of CD8 ⁺ T cells in ALS mice increased the number of surviving motoneurons. CD8 ⁺ T cells expressing the ALS-causing superoxide dismutase-1 mutant protein recognize and selectively kill motoneurons in vitro. To exert their cytotoxic function, mutant CD8 ⁺ T cells required presentation of the antigen-MHC-I complex at the surface of the motoneurons. Analysis of T cell receptor diversity supports the evidence that self-reactive CD8 ⁺ T lymphocytes infiltrate the CNS of ALS mice to exert cytotoxic function.

Abstract

Adaptive immune response is part of the dynamic changes that accompany motoneuron loss in amyotrophic lateral sclerosis (ALS). CD4 ⁺ T cells that regulate a protective immunity during the neurodegenerative process have received the most attention. CD8 ⁺ T cells are also observed in the spinal cord of patients and ALS mice although their contribution to the disease still remains elusive. Here, we found that activated CD8 ⁺ T lymphocytes infiltrate the central nervous system (CNS) of a mouse model of ALS at the symptomatic stage. Selective ablation of CD8 ⁺ T cells in mice expressing the ALS-associated superoxide dismutase-1 (SOD1) ^G93A mutant decreased spinal motoneuron loss. Using motoneuron-CD8 ⁺ T cell coculture systems, we found that mutant SOD1-expressing CD8 ⁺ T lymphocytes selectively kill motoneurons. This cytotoxicity activity requires the recognition of the peptide-MHC-I complex (where MHC-I represents major histocompatibility complex class I). Measurement of interaction strength by atomic force microscopy-based single-cell force spectroscopy demonstrated a specific MHC-I-dependent interaction between motoneuron and SOD1 ^G93A CD8 ⁺ T cells. Activated mutant SOD1 CD8 ⁺ T cells produce interferon-γ, which elicits the expression of the MHC-I complex in motoneurons and exerts their cytotoxic function through Fas and granzyme pathways. In addition, analysis of the clonal diversity of CD8 ⁺ T cells in the periphery and CNS of ALS mice identified an antigen-restricted repertoire of their T cell receptor in the CNS. Our results suggest that self-directed immune response takes place during the course of the disease, contributing to the selective elimination of a subset of motoneurons in ALS.