Lymphocyte subtypes in relapsing–remitting multiple sclerosis patients treated with dimethyl fumarate

BACKGROUND Recent data suggest that lymphopenia is more prevalent than reported in relapsing-remitting multiple sclerosis (RRMS) patients taking dimethyl fumarate (DMF). OBJECTIVE The objective of this study was to investigate the effect of DMF on lymphocyte subtypes in RRMS patients with and without lymphopenia. METHOD A retrospective study compared lymphocyte subtypes in DMF-treated RRMS patients with low (G1, n = 35) and normal lymphocyte counts (G2, n = 24). RESULTS Fifty-nine patients were identified, with mean age 49, 71.2% females, and average DMF duration 20 months. Age, sex, baseline white blood count, disease and treatment durations were similar between groups. Prior interferon therapy and baseline lower normal lymphocyte counts were more frequent in G1. Mean lymphocyte counts were 0.8 ± 0.2 × 109/L in G1 and 1.6 ± 0.3 × 109/L in G2. CD3+, CD4+, and CD8+ T cell mean counts were lower (p < 0.0001), while CD4/CD8 ratio higher (p = 0.03) in G1 than G2. Mean CD19 + B cell counts were normal; however, values were lower in G1 (p = 0.04). After adjusting for confounders, significantly positive correlations were noted between lymphocyte counts and CD3 + , CD4+, CD8+ T, and B cell counts. Negative correlation was observed between lymphocyte counts and CD4/CD8 ratio driven by low CD8+ T cell counts. CONCLUSION DMF treatment predominantly impacts T cells, in particular CD8+ subtype. This finding may have implications in this population's immunocompetence.