Tofacitinib extends survival in a mouse model of ALS through NK cell-independent mechanisms
Background Amyotrophic lateral sclerosis (ALS) is a lethal neurodegenerative disease with few treatment options, rendering the development of new, effective therapeutics of critical importance. The immune system plays a substantial role in ALS pathology, with multiple cell populations implicated in disease progression. Natural killer (NK) cells are innate immune cells that accumulate in the brain and spinal cord during ALS, increasing neuroinflammation and killing motor neurons. Depleting NK cells extends survival in mouse models of ALS. Tofacitinib, an FDA-approved janus kinase (Jak) and signal transducer and activator (STAT) pathway inhibitor, reduces NK cytotoxicity and decreases overall levels in peripheral blood and may represent a potential ALS therapy. Therefore, we aimed to evaluate the effects of tofacitinib treatment on survival and phenotype in an ALS mouse model. Additionally, we sought to determine the impact of dose and regimen on efficacy.  Methods SOD1G93A mice, the most used rodent model of ALS, were treated with low- (5 mg/kg) and high-dose (30 mg/kg) tofacitinib following a prevention regimen, an intervention regimen, or a drug-cycling regimen, with survival being the primary outcome. Symptom onset was assessed via body weight, agility, and grip strength measurements. At end-stage disease (i) motor neurons and neuromuscular junctions were counted, (ii) immune populations were quantified via flow cytometry in peripheral blood and spinal cord, (iii) microglial surface marker expression was quantified to assess neuroinflammation, and (iv) bulk RNA-seq was performed on spinal cord.  Results Low-dose, but not high-dose, tofacitinib significantly increased survival and delayed weight loss. Notably, beginning treatment before symptom onset (prevention) did not offer any survival advantage over the intervention nor cycling regimen; further analyses were pooled by dose. There were no differences in motor neuron or neuromuscular junction counts. Peripheral NK and CD8+ T cells were decreased dose-dependently. Interestingly, spinal cord infiltrating NK cells increased with low-dose tofacitinib, though no other changes in neuroinflammation were observed. RNA-seq revealed that low-dose tofacitinib treatment reversed the dysregulation of multiple immune and metabolic pathways.  Conclusions These data support the repurposing of tofacitinib as a potential ALS treatment.