NOS2 and myeloid sarcoma: In addition to oligodendroglial loss as a cause of demyelination, failure of remyelination in MS lesions is attributed to extrinsic immune factors in the inflammatory environment, whereby oligodendroglial progenitor cell differentiation is impaired in the presence of CD4+ T cells, interferon-gamma, and the M1-polarized microglia cytokines IL-6, CXCL10, and TNFα, while remyelination in mixed lesions is decreased in the presence of greater proportions of TMEM119+ microglia and inducible nitric oxide synthase (iNOS)+ myeloid cells [204,205].