Extracellular immune checkpoint molecules released from HTLV-1-infected cells mount immune suppression in the context of neuroinflammation

Abstract HTLV associated myelopathy/tropical spastic paraparesis (HAM/TSP) is a chronic, progressive neuroinflammatory demyelinating condition of the spinal cord. We have previously shown that aberrant expression and activity of immune checkpoint (ICP) molecules – PD-1/PD-L1, negatively associates with the cytolytic potential of T cells in individuals with HAM/TSP. Interestingly, ICPs can exist in soluble cell-free form and can be carried on extracellular vesicles (EV) and exosomes (small EVs, <200nm) while maintaining their immunomodulatory activity. Therefore, we investigated the role of soluble/EV membrane-bound ICPs in HTLV-1 associated neuroinflammation. We first profiled a range of ICP mediators in sera of HAM/TSP patients, and in purified exosomes from HAM/TSP-derived HTLV-1-producing (OSP2) cells. ICPs, such as BTLA, LAG-3, and PD-L2, were elevated in the plasma samples of patients at the levels comparable to their presence in EVs from OSP2 cells, primarily in exosomal fractions. These exosomal ICPs were found to be functional and adversely affected healthy CD8 T-cell functions. Viral proteins and cytokines (IFNγ) were also found to be present in purified exosomes. IFNγ exposure boosted release of ICP molecules while antiretroviral drugs significantly inhibited this process. HTLV-1 b-Zip protein (HBZ) has been linked to factors that enhance EV release and its knockdown here led to reduced expression of several endosomal sorting complex required for transport (ESCRT) associated genes as well as abrogated release of ICP molecules, suggesting a transcriptional control. Collectively, these findings highlight potential new immunotherapeutic avenues to be explored that may slow or reverse the risk or progression of HAM/TSP. NIH/NINDS: R01 NS097147 to P.J.