CD4 and tropical spastic paraparesis: These observations suggest that persistent T cell activation in HAM/TSP patients triggered by viral antigens, e.g., HTLV-1 tax, that are associated with Foxp3 downregulation may result in decline of the CD4+CD25+Foxp3+ Treg cell population and accumulation of CD4+CD25+Foxp3− T cells that lack suppressive function but are capable of exacerbating the disease process.