A potential role of viral infections in ALS has long been suspected, but it remains uncertain. Numerous common viruses are neurotropic and can cause neurological diseases, including encephalitis, meningitis, and acute flaccid paralysis, but attempts to demonstrate viral infections in individuals with ALS have given inconsistent results. Recent experimental work has demonstrated that viral infections may disrupt the normal distribution and metabolism of RNA binding proteins, including TDP43. For example, infection with Coxsackievirus B3 causes segregation of TDP43 in the cytoplasm and its cleavage into two fragments, one of which tends to aggregate and has a dominant negative effect on TDP43 function. The formation of cytoplasmic TDP43 inclusions is a consistent pathological feature in sporadic ALS, as well as in many cases of frontotemporal dementia. It has therefore been hypothesized that viral infections can trigger a process of disruption and aggregation of TDP43, which could then propagate from cell to cell, even in the absence of the virus. Numerous other effects of viral infection on neuronal and glial cells could also contribute to ALS pathology. We propose therefore to conduct an exploratory study of the entire human virome (> 400 species and strains) in repeated blood samples collected years before the onset of the first symptoms of ALS. We will use a recently developed phage display library expressing 481,966 overlapping peptides derived from all vertebrate and arboviruses (VirScan). By exploring the virome of healthy young adults who years later developed ALS, we can identify viral infections that are associated with an increased risk. Further, we will measure serum concentrations of neurofilament light chain (NfL), a sensitive marker of neurodegeneration to investigate the relation between viral infections and NfL elevations. Such a unique study is made possible by the databases maintained by the Armed Forces Health Surveillance Branch and the over 60 million serum samples archived in the Department of Defense Serum Repository. These samples have been collected from over 11 million young men and women who served in the US Army, Navy, Marines, and Air Force for screening for HIV infection. Each individual has contributed on average with a sample every two years starting at the time of entry into active duty. By profiling the virome in repeated samples, it is possible to identify past and new infections in individuals who developed ALS and compare this pattern with that of controls matched by age, sex, and ethnicity who remained healthy. Based on our preliminary work, we expect to be able to identify over 150 incident ALS cases with available serum samples for the proposed investigation. Our team has been conducting research based on the Department of Defense Serum Repository for over 15 years and has thus a long history of collaboration with military institutions for this purpose. This study will be conducted in collaboration with inve...