Abstract
HTLV-1 is a human retrovirus that is associated with the neuroinflammatory disorder HTLV-1 associated myelopathy/ tropical spastic paraparesis (HAM/TSP). Much work has been undertaken to understand the mechanisms that lead to the development of this chronic progressive neuroinflammatory disorder in subsets of infected individuals. Similar to other inflammatory disorders, HAM/TSP appears to involve interaction between the immune system, viral/environmental triggers, and host genetics. When focusing on those individuals who develop HAM/TSP, certain factors are prominent. In particular, HTLV-1 is primarily found in the CD4+CD25+ T cell subset (Regulatory T cells: Tregs), which is responsible for peripheral immune tolerance and is known to be dysfunctional in HAM/TSP. This can be assessed in a number of ways. For example, recent evidence suggests that FoxP3, a lineage marker and master regulator of Tregs, expression and function is determined epigenetically through DNA demethylation in the Treg-specific demethylated region (TSDR). Additionally, Tregs have been shown to suppress activation through the production of microvesicles termed exosomes, which measure 30-150nm in size. Therefore, we analyzed the methylation state of the FoxP3 TSDR in peripheral blood mononuclear cells (PBMCs), CD4+ T cells, and CD4+CD25+ T cells from normal healthy donors (NDs) and HAM/TSP patients. We demonstrated that there is decreased demethylation in analyzed PBMCs and CD4+CD25+ T cells from HAM/TSP patients as compared to NDs. Furthermore, decreased TSDR demethylation was associated with decreased functional suppression by Tregs and increased HTLV-1 Tax expression in HAM/TSP PBMC culture correlated with a concomitant decline in FoxP3 TSDR demethylation. Additionally, we assessed the content of exosomes produced by ND and HAM/TSP patient PBMCs and found that HAM/TSP exosomes contained HTLV-1 Tax protein, tax mRNA, and HTLV-1 bZIP (HBZ) protein. Furthermore, Tax containing exosomes were present in cell-free, virus-free HAM/TSP patient cerebrospinal fluid (CSF) supernatant. Importantly, HAM/TSP patient derived exosomes were able to sensitize targets for Tax-specific cytotoxic T lymphocyte (CTL) lysis. We extended our studies into the potential consequences of Treg dysfunction into other immune subsets and found preliminary evidence to suggest that B cell receptors (BCRs) sequenced from HAM/TSP patient CSF recognize HTLV-1 specific antigens, which may potentially be present in isolated exosomes. Overall, we suggest that HTLV-1 infection leads to neuroinflammation in HAM/TSP through HTLV-1 mediated epigenetic modification leading to Treg dysfunction and through incorporation of HTLV-1 products in secreted exosomes, leading to antigen sensing and subsequent destruction of target cells by HTLV-1 specific CTLs and potentially HTLV-1 specific antibodies in the CNS. All of these factors may therefore contribute to HAM/TSP disease pathogenesis and warrant further investigation with the goal of developing clinical strategies that target these mechanisms.
