Immune checkpoint LAG-3 governs stage-dependent and disease-associated microglial modules in ALS model mice

Immune checkpoint molecules, inhibitory receptors originally characterized in T cell biology, have recently emerged as regulators of microglial function in neurodegeneration, yet their roles in amyotrophic lateral sclerosis (ALS) remain unexplored. Here, we investigated LAG-3, an inhibitory immune checkpoint receptor, in microglial regulation during ALS pathogenesis using SOD1^G93A mice. LAG-3 expression was progressively upregulated in spinal cord microglia during disease progression, and LAG-3-high microglia exhibited a disease-associated microglia (DAM) transcriptional signature. Genetic deletion of LAG-3 produced a biphasic phenotype, with accelerated disease onset but significantly prolonged disease duration. LAG-3 deficiency enhanced inflammatory microglial responses at the early disease stage, whereas at the late stage it suppressed inflammatory signaling while selectively preserving phagocytic effector gene expression, demonstrating that LAG-3 dissociates the inflammatory and phagocytic modules within the DAM program in a stage-dependent manner. These transcriptional changes translated into enhanced phagocytic capacity in primary microglia and amelioration of the spinal cord environment through suppression of inflammatory pathways and restoration of oxidative phosphorylation. Our findings identify LAG-3 as a stage-dependent regulator of microglial functional states in ALS and support the concept that immune checkpoint molecules constitute a class of module-level regulators of microglial function in neurodegeneration.