Mac-1 regulates disease stage-specific immunosuppression via the nitric oxide pathway in autoimmune disease

Integrin Mac-1 plays a critical role in the development of multiple sclerosis (MS); however, the underlying mechanism is not fully understood. Here, we developed a myeloid-specific Mac-1-deficient mouse. Using an experimental autoimmune encephalomyelitis (EAE) mouse model of MS, we report that Mac-1 on myeloid cells is key to disease development. Our data reveal that myeloid-specific Mac-1 significantly increases EAE severity and hinders disease regression. Loss of Mac-1 increases Gr-1⁺ cells in peripheral tissues and the CNS and preferably accelerates the transition of Ly6C^hi monocytes from a pro-inflammatory to an immunosuppressive phenotype in a disease stage-dependent manner. Mechanistically, our results demonstrate that Mac-1 suppresses interferon-γ production and prevents monocytes from acquiring immunosuppressive functions by reducing the expression of iNOS, IDO, and CD84. Administration of a NOS-specific inhibitor in Mac-1-deficient EAE mice abolishes disease regression. These insights could help develop Mac-1-targeting strategies for better treatment of MS.