Aggressive Herpes Zoster in Young Patients With Multiple Sclerosis Under Dimethyl Fumarate: Significance of CD8+ and Natural Killer Cells.

Dimethyl fumarate (DMF), approved for relapsing-remitting multiple sclerosis (RRMS), exerts immune-mediated mechanisms crucial for T-cell survival and migration, preferentially reducing CD8 + T cells. 1 Although baseline absolute lymphocyte count (ALC) is considered the most critical predictor of developing lymphopenia, 2 it was recently concluded that lymphocyte subset monitoring is not required for safety vigilance because T-cell subset reduction does not increase risks for serious infections. 3 We present 2 young patients with RRMS, under DMF treatment, negative for HIV and SARS-CoV-2 (by RT-PCR in nasal swab) and with normal follow-up white blood cell (WBC)/ALC counts, who developed severe herpes zoster (HZ) infection with normal ALC but low CD8 + and high CD56 bright natural killer (NK) cells, and discuss the potential signi fi cance of T-cell immunophenotyping in HZ manifestation.