HTLV-1 bZIP Factor Impairs Anti-viral Immunity by Inducing Co-inhibitory Molecule, T Cell Immunoglobulin and ITIM Domain (TIGIT)

<div><p>Human T-cell leukemia virus type 1 (HTLV-1) infects CD4⁺ T cells and induces proliferation of infected cells <i>in vivo</i>, which leads to the onset of adult T-cell leukemia (ATL) in some infected individuals. The HTLV-1 bZIP factor (<i>HBZ</i>) gene, which is encoded in the minus strand of HTLV-1, plays critical roles in pathogenesis. In this study, RNA-seq and ChIP-seq analyses using HBZ transduced T cells revealed that HBZ upregulates the expression and promoter acetylation levels of a co-inhibitory molecule, T cell immunoglobulin and ITIM domain (<i>TIGIT</i>), in addition to those of regulatory T cells related genes, <i>Foxp3</i> and <i>Ccr4</i>. TIGIT was expressed on CD4⁺ T cells from HBZ-transgenic (HBZ-Tg) mice, and on ATL cells and HTLV-1 infected CD4⁺ T cells of HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP) <i>in vivo</i>. Expression of Blimp1 and IL-10 was upregulated in TIGIT⁺CD4⁺ cells of HBZ-Tg mice compared with TIGIT^-CD4⁺ T cells, suggesting the correlation between TIGIT expression and IL-10 production. When CD4⁺ T cells from HBZ-Tg mice were stimulated with TIGIT’s ligand, CD155, their production of the inhibitory cytokine IL-10 was enhanced. Furthermore, dendritic cells from HBZ-Tg mice produced high levels of IL-10 after stimulation. These data suggest that HBZ alters immune system to suppressive state via TIGIT and IL-10. Importantly, TIGIT suppressed T-cell responses to another HTLV-1 virus protein, Tax, <i>in vitro</i>. Blocking of TIGIT and PD-1 slightly increased anti-Tax T-cell activity in some HAM/TSP patients. These results suggest that HBZ-induced TIGIT on HTLV-1 infected cells impairs T-cell responses to viral antigens. This study shows that HBZ-induced TIGIT plays a pivotal role in attenuating host immune responses and shaping a microenvironment favorable to HTLV-1.</p>