Fig. 2.

Switch of HTLV-1 expression in vivo and in vitro. a HTLV-1 antigens in primary ATL cells isolated from the peripheral blood are undetectable before culture (day 0), but are induced after in vitro culture (day 1 and 3) [20]. b The spontaneous induction of HTLV-1 expression in primary ATL cells in the first 24 h of culture is suppressed by exogenously added IFNα [81]. c Transient suppression and re-induction of HTLV-1 expression can be reproduced in vitro using IL-2-dependent HTLV-1-infected cells derived from ATL patients. When the HTLV-1-infected cells (purple) are co-cultured with stromal cells (blue), the viral expression becomes undetectable in 24 h but recovers again when the infected cells are re-isolated and cultured alone over the next 48 h [83]