This has been hypothesized to be due to the impaired cellular immune responses in HTLV-1-infected T-cells mediated by HTLV-1 bZIP factor (HBZ) induced expression of co-inhibitory molecules (such as PD-1, CTLA-4, and TIGIT) and production of inhibitory cytokines like interleukin-10 (IL-10), leading to ineffective resolutions of infections (Kozako et al., 2009; Sawada et al., 2017; Higuchi et al., 2020; Tan et al., 2022). Here, CTLA4 is linked to infection.