This study aimed to immunophenotype peripheral blood mononuclear cell (PBMC) subsets in relapsing remitting multiple sclerosis (RRMS) patients longitudinally and examine the effect lymphopenia has on them.

RRMS is an autoimmune disorder of the central nervous system. Dimethyl Fumarate (DMF) is a disease modifying medication that in some cases may cause lymphopenia.

A characterization of peripheral blood mononuclear cells (PBMC) was done by multi-color panels that allowed us to identify up to 50 PBMC subtypes. These subpopulations were monitored over time. Patients had been on treatment for a span of 45 days to 4 years at the time of sample collection, with 15 patients being followed longitudinally.

32% of DMF-treated patients developed mild lymphopenia. Among monocytes, the classical subtype (CD14+CD16-) was significantly higher in non-lymphopenic (DMF-N) patients over time and not in the lymphopenic patients (DMF-L). Although T cells were significantly reduced in DMF-L versus DMF-N patients, no significant change was observed over time. DMF-L, but not DMF-N, patients who have been treated for longer times had higher CD4+ and lower CD8+ T cells, causing an increased CD4-to-CD8 ratio. Within CD4+, the naïve subtype (CD45RA+CCR7+) decreased in DMF-L patients over time but not in DMF-N patients. This was also observed in CD8+ naïve subtype in DMF-L patients over time but not DMF-N patients. Next, we analysed T helper (Th) cells. Only Th22 had higher levels in DMF-L over time and no change in DMF-N. The overall B cell population and its subtypes were not significantly changed over time, regardless of lymphopenia status.

Our data provide evidence that under the same medication, the immune phenotype and change over time differs in lymphopenic versus non-lymphopenic patients. Monitoring of PBMC subtypes may provide more information for identifying non-responders or patients who are at risk of developing side effects such as opportunistic infections.