NEFL and tropical spastic paraparesis: To ascertain the importance of HTLV-1–infected T cells among HAM-PBMCs in causing neuronal cell damage, we measured the NF-L concentration in the medium of NB-1 cells cocultured with HAM-PBMCs in the presence or absence of an anti-CCR4 antibody (mogamulizumab), which is known to deplete HTLV-1–infected T cells (12, 13).