Oligodendrocyte Precursor Cells Are Co-Opted by the Immune System to Cross-Present Antigen and Mediate Cytotoxicity
Oligodendrocyte precursor cells (OPCs) are abundant in the adult CNS and can be recruited to form new oligodendrocytes and myelin in response to injury or disease. However, in multiple sclerosis (MS), oligodendrocyte regeneration and remyelination are often incomplete, suggesting that recruitment and maturation of OPCs is impaired. MS and the rodent model experimental autoimmune encephalomyelitis (EAE) are characterized by infiltration of activated T-cells into the CNS. To investigate the mechanisms by which this neuroinflammatory process influences OPC mobilization, we performed in vivo fate tracing in an inflammatory demyelinating animal model. Results of our studies showed that the OPC differentiation and myelin production are inhibited by either adoptive transfer of CNS infiltrating cytokine producing effector T-cells or CNS production of interferon gamma (IFN{gamma}), using an astrocyte specific IFN{gamma} transgene model. In both systems, IFN{gamma} changes the profile of OPCs by inducing functional expression of the immunoproteasome and upregulation of MHC class I. OPCs exposed to IFN{gamma} are shown to cross present exogenous antigen to cytotoxic CD8 T-cells, which then produce proteases and FasL that results in subsequent caspase 3/7 activation and OPC death, both in vitro and in vivo. Cross presentation by OPCs is dependent on the cytosolic processing pathway and can be inhibited by small molecules targeting MHC class I antigen processing and the immunoproteasome subunits. Finally, the immunoproteasome subunit, PSMB8, is shown to be markedly increased on Sox10+ oligodendrocyte lineage cells only in the demyelinated white matter lesions from patients with MS. These findings support the notion that OPCs have multiple functions beyond differentiation into myelinating cells and adapt to their microenvironment by responding to local cues. In MS, OPCs may be co-opted by the immune system to perpetuate the autoimmune response. Strategies aimed at inhibiting the aberrant immune activation pathways in OPCs may allow more efficient remyelination in MS.