Spinal cord injury (SCI) causes the release of danger signals by stressed and dying cells, a process that leads to neuroinflammation.Evidence suggests that inflammation plays a role in both the damage and repair of injured neural tissue. We show that microglia at sites of SCI rapidly express the alarmin interleukin (IL)-1␣, and that infiltrating neutrophils and macrophages subsequently produce IL-1␤. Infiltration of these cells is dramatically reduced in both IL-1␣Ϫ / Ϫ and IL-1␤ Ϫ / Ϫ mice, but only IL-1␣ Ϫ / Ϫ mice showed rapid (at day 1) and persistent improvements in locomotion associated with reduced lesion volume. Similarly, intrathecal administration of the IL-1 receptor antagonist anakinra restored locomotor function post-SCI. Transcriptome analysis of SCI tissue at day 1 identified the survival factor Tox3 as being differentially regulated exclusively in IL-1␣ Ϫ / Ϫ mice compared with IL-1␤ Ϫ / Ϫ and wild-type mice. Accordingly, IL-1␣Ϫ / Ϫ mice have markedly increased Tox3 levels in their oligodendrocytes, beginning at postnatal day 10 (P10) and persisting through adulthood. At P10, the spinal cord of IL-1␣ Ϫ / Ϫ mice showed a transient increase in mature oligodendrocyte numbers, coinciding with increased IL-1␣ expression in wild-type animals. In adult mice, IL-1␣ deletion is accompanied by increased oligodendrocyte survival after SCI. TOX3 overexpression in human oligodendrocytes reduced cellular death under conditions mimicking SCI. These results suggest that IL-1␣-mediated Tox3 suppression during the early phase of CNS insult plays a crucial role in secondary degeneration.