text: Immortalization of CD4(+) and CD8(+) T lymphocytes by human T-cell leukemia virus type 1 Tax mutants expressed in a functional molecular clone. 
  The human T-cell leukemia virus type 1 (HTLV-1) transcriptional trans-activator Tax has been demonstrated to have transforming activity in multiple cell culture and transgenic-mouse models. In addition to activating transcription from the viral long terminal repeat (LTR) through the cyclic AMP response element binding protein/activating transcription factor (CREB/ATF) family of transcription factors, Tax activates the expression of multiple cellular promoters through the NF-kappaB pathway of transcriptional activation. The Tax mutants M22 and M47 have previously been demonstrated to selectively abrogate the ability of Tax to activate transcription through the NF-kappaB or CREB/ATF pathway, respectively. These mutations were introduced in the tax gene of the ACH functional molecular clone of HTLV-1, and virus produced from the mutant ACH clones was examined for the ability to replicate and immortalize primary human lymphocytes. While virus derived from the clone containing the M47 mutation retained the ability to immortalize T lymphocytes, the M22 mutant lost the ability to immortalize infected cells. These results indicate that activation of the CREB/ATF pathway by Tax is dispensable for the immortalization of T cells by HTLV-1, whereas activation of the NF-kappaB pathway may be critical. 