Microglia in normal appearing white matter of multiple sclerosis are alerted but immunosuppressed

Little is known about the functional phenotype of microglia in normal appearing white matter (NAWM) of multiple sclerosis (MS),although it may hold valuable clues about mechanisms for lesion development. Therefore, we studied microglia from NAWMobtained post-mortem from controls (n525) and MS patients (n521) for their phenotype ex vivo and their immune responsive-ness in vitro, using a microglia isolation method that omits culture and adherence. By flow cytometry, microglia in MS NAWM dis-played elevated CD45 levels and increased size and granularity but were distinct from autologous choroid plexus macrophagesby absent or low expression of additional markers, in particular CD206. Flow cytometric analysis of microglia from NAWM ofthree controls and four MS patients showed alterations in levels of Fc-gamma receptors in MS. In primary microglia from a biggersample of subjects, analysis of Fc-gamma receptors by quantitative PCR indicated a significant increase in mRNA levels of theinhibitory CD32b isoform in MS NAWM. Despite their changed activation status, microglia from MS NAWM were unresponsiveto lipopolysaccharide in vitro. Notably, culture with dexamethasone led to an impaired induction of the inflammation-limitingcytokine CCL18 in microglia from MS NAWM compared with those from control NAWM. Together, these data demonstrate thatmicroglia in MS NAWM are in an alerted state, but display features of immunosuppression. Thus, the activation status of micro-glia in NAWM of MS patients likely reflects a response to ongoing neuroinflammation, which coincides with upregulation ofimmunoregulatory molecules to prevent full activation and damage to the vulnerable milieu.