HBZ and tropical spastic paraparesis: Since the immune system plays a major role in HAM/TSP progression and since HBZ mRNA expression was correlated with disease severity (Saito et al., 2009), Satou et al. (2011) used the HBZ transgenic mouse model to study the CD4+Foxp3+ Treg population specifically that ATL cells were shown to functionally and phenotypically resemble Foxp3+CD25+CD4+ Treg cells, which control immune responses against self- and non-self-antigen (Sakaguchi et al., 2008).