Is human T-cell lymphotropic virus type I really silent?

The role of the cellular immune response to human T-cell lymphotropic virus type I (HTLV-I) is not fully understood. The low level of HTLV-I protein e xpression in peripheral blood lymphocytes has led to the widely held belief that HTLV-I is transcriptionally silent in vivo. However, most HTLV-I-infe cted individuals mount a strong and persistently activated cytotoxic T-lymp hocyte (CTL) response to the virus; this observation implies that there is abundant chronic transcription of HTLV-I genes. Here we Show that HTLV-I Ta x protein expression rises quickly in freshly isolated peripheral blood lym phocytes, but that expressing cells are rapidly killed by CTLs. Mathematica l analysis of these results indicates that the CTL response is extremely ef ficient and that the half-life of a Tax-expressing cell is less than a day We propose that HTLV-I protein expression in circulating lymphocytes is und etectable by current techniques because of the efficiency of the CTL-mediat ed immune surveillance in vivo.