At the incipient stages of the development of adult T-cell leukemia, T-cells infected with human T-cell leukemia virus type 1 (HTLV-1) suffer disregulation in cell growth caused by aberrant expression of host genes by the HTLV-1 transactivator protein Tax (Tax1). Tax1-mediated growth promotion is thought to result from, at least in part, up-regulation of genes for growth factors and their receptors that induce T-cell growth. In the present study, we demonstrate that Tax1 transactivates the interleukin-21 (IL-21) and its receptor (IL-21R) genes in human T-cells. Introduction of Tax1 via recombinant adenoviruses induced expression of endogenous IL-21 and IL-21R. Isolated promoters of the IL-21 and IL-21R genes were activated by Tax1 in reporter assays, which further revealed that there were at least two Tax1-responsive elements in either the IL-21 promoter or the IL-21R promoter. Chromatin immunoprecipitation assay and gel mobility shift assay exhibited that the IL-21 promoter elements bound transcription factors AP-1 and NF-kappaB, and the IL-21R promoter elements were associated with AP-1 and interferon regulatory factor. Collectively, Tax1-dependent activation of these transcriptional factors presumably contributes to expression of the IL-21 gene and its receptor gene. The related virus HTLV-2 with Tax2 similar to Tax1 is known not to be pathogenic. Tax2 exhibited little, if any, or no induction of the IL-21 transcription in CD4+ T-cells, in contrast to Tax1. The study suggests insights into cytokine-dependent aberrant growth of HTLV-1-infected T-cells and the molecular basis of different pathogenicity between HTLV-1 and HTLV-2.