One hypothesis could be that higher serum β-NGF in HAM/TSP patients is related to more effective control of inflammatory events leading to the development of neurological lesions, independently of the control of infected T-cell populations in the periphery, since the HTLV-1 PVL in PBMCs was similar between HAM/TSP patients with normal and elevated NfL. Here, NEFL is linked to tropical spastic paraparesis.