OMG and myeloid sarcoma: To mimic the situation in MS patients who have intrathecal lipid-specific IgM in their CNS before JCV disseminates to the CNS, rat myelinated cultures were pre-treated for 24 h with O4 or isotype control A4CD, an IgM mAb specific to myelin oligodendrocyte glycoprotein (MOG) peptide.