The thymus gland is a target and maintainer of amyotrophic lateral sclerosis pathophysiology

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disease characterized by the selective loss of motor neurons in the brain and spinal cord with evidence of local neuroinflammation. Regulatory T cells (Treg) exert neuroprotective effects and correlate with disease progression, but central mechanisms governing Treg development during ALS remain unclear. We investigated the thymic architecture and T cell differentiation in SOD1-G93A (mSOD1) ALS mice at multiple disease stages. Thymocyte and thymic epithelial cells (TEC) subpopulations, as well as stages of Treg differentiation and suppressive capacity, were analyzed by flow cytometry, in situ immunofluorescence, and functional assays. At late disease stage (120 days-old), mSOD1 mice displayed thymic atrophy, reduced thymocyte numbers, and decreased double-positive (DP), CD4 single-positive (SP), and Treg populations. Foxp3 expression per cell was preserved, but Treg progenitors and mature Treg numbers declined, paralleled by reduced suppressive function in vitro. TEC analysis revealed reduced total and mature medullary TEC (CD80^highMHCII^high), despite preserved Aire expression. Foxp3⁺ cells were abnormally localized at cortical sites, away from mTEC, associated with migratory abnormalities and increased apoptosis. TREC evaluation revealed reduced intrathymic sjTREC, consistent with impaired proliferation and differentiation between DN and DP stages, although peripheral sj/βTREC ratios remained stable. Reduced Treg numbers were also observed in lymph nodes draining affected hindlimbs. Our findings identify the thymus as a target organ in ALS, where thymic involution, TEC loss, and Treg dysfunction may compromise immune tolerance. These alterations likely reinforce the role of the thymus in peripheral immune dysregulation and neuroinflammation observed in ALS.