Role of IL-21 in HTLV-1 infections with emphasis on HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP).

Interleukin-21 (IL-21) enhances the survival and cytotoxic properties of cytotoxic T cells (CTLs) and exhibits essential roles in controlling chronic viral infections. HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP) is a chronic progressive inflammatory disease of the nervous system. The main determinant of disease progression is efficiency of the CTL response to Human T lymphotropic virus types I (HTLV-1). In this study, the expression of host IL-21 and HTLV-I Tax and proviral load (PVL) was evaluated to understand the role and mechanism of IL-21 in HTLV-1 infections and the subsequent development of HAM/TSP. A cross-sectional study was carried out on 20 HAM/TSP patients, 20 asymptomatic HTLV-1 carriers (ACs) and 20 healthy controls (HCs) to evaluate the expression of IL-21 and Tax and PVL in non-activated and phorbol myristate acetate (PMA)-ionomycin-activated peripheral blood mononuclear cells (PBMCs). The mean mRNA expression of IL-21 in the non-activated and activated PBMCs was higher (by 5-13 times) in the HAM/TSP patients than in ACs and HCs (p < 0.05); however, there was no significant difference between ACs and HCs. In contrast to the IL-21 mRNA expression, the serum level of the IL-21 protein was significantly lower in the HAM/TSP patients than in ACs and HCs (p < 0.05). Furthermore, higher expression of Tax and PVL was observed in the HAM/TSP subjects than ACs (p < 0.05). In addition, Tax gene expression was positively correlated with PVL (R = 0.595, p = 0.000) and IL-21 gene expression (R = 0.395, p = 0.021) in the HTLV-1-infected subjects. In conclusion, the increase in IL-21 mRNA expression may reflect the attempt of infected T cells to induce an appropriate antiviral response, and the decrease in IL-21 protein expression may reflect the inhibition of IL-21 mRNA translation by viral factors in favour of virus evasion and dissemination.