Background: A common finding in amyotrophic lateral sclerosis (ALS) is the infiltration of CNS by CD4+ and CD8+ T cells, demonstrating the important role of adaptive immunity in ALS. Recent studies also showed a significant alteration in CD4+ T cell counts in patients with ALS. Furthermore, the progression rate of ALS is inversely correlated with the total number of Treg cells, effector Foxp3+ Treg cells, as well as CD4+ T cells. Objective: To characterize the heterogeneous gene expression profile of immune cells(mainly T cells) in ALS and better understand the disease etiology and identify proper therapeutic targets for ALS. Methods: We will include 3 ALS patients with rapid progression and 3 ALS patients with slow progression as the cases, 3 patients with multiple sclerosis and 3 patients with normal pressure hydrocephalus as controls, and characterize the gene expression profiles of T cells and TCR repertoire from the CSF samples by single cell RNA sequencing. Single cells will be run at 10X Genomics platform and processed following the manufacturer’s instructions. Approximately 5000 cells per each sample will be sequenced. Significance: Our study will comprehensively investigate the role of immune responses in ALS, aiming to resolve to large extent the ongoing debate on the causality between neuroinflammation and neurodegeneration in ALS and provide novel insight for disease-modifying effect of T cells in patients with ALS.