Background: The major histocompatibility complex I (MHCI) is a key molecule for the interaction of mononucleated cells with CD8+T lymphocytes. We previously showed that MHCI is upregulated in the spinal cord microglia and motor axons of transgenic SOD1G93Amice. Methods: To assess the role of MHCI in the disease, we examined transgenic SOD1G93Amice crossbred with β2 microglobulin-deficient mice, which express little if any MHCI on the cell surface and are defective for CD8+T cells. Results: The lack of MHCI and CD8+T cells in the sciatic nerve affects the motor axon stability, anticipating the muscle atrophy and the disease onset. In contrast, MHCI depletion in resident microglia and the lack of CD8+T cell infiltration in the spinal cord protect the cervical motor neurons delaying the paralysis of forelimbs and prolonging the survival of SOD1G93Amice. Conclusions: We provided straightforward evidence for a dual role of MHCI in the peripheral nervous system (PNS) compared to the CNS, pointing out regional and temporal differences in the clinical responses of ALS mice. These findings offer a possible explanation for the failure of systemic immunomodulatory treatments and suggest new potential strategies to prevent the progression of ALS. © 2018 The Author(s).