The engagement of CD4 surface antigen in the HEL haemopoietic cell line up-regulates the transforming growth factor-β1 (TGF-β1) promoter activity

In order to investigate the effect of CD4 engagement on the transforming growth factor β1 (TGF-β1) promoter activity in haemopoietic progenitors, HEL cells were transiently transfected with a plasmid vector containing - 453/+11 nucleotides of the TGF-β1 promoter fused with the bacterial chloramphenicol acetyltransferase (CAT) gene and then treated with various agonists. Both cross-linked CD4 mAb and envelope gp120 were able to significantly up-regulate CAT activity with respect to the levels of activation observed in HEL cells treated with cross-linked CD8 mAb or p24. By using deletion mutants of the TGFβ1 promoter, we found that the minimal DNA sequence still responsive to cross-linked CD4 mAb or gp120 was located between nucleotides -453/- 323 of the TGF-β1 promoter, which contains two activating protein 1 (AP1) binding sites. In electromobility shift assays (EMSA) we could demonstrate that CD4 engagement of HEL cells induced a significant increase of AP1 binding activity at the nuclear level. Furthermore, the steady-state mRNA of endogenous TGF-β1 showed a small but reproducible increase when HEL cells were treated with cross-linked CD4 mAb or gp120. Altogether, these findings suggest that the engagement of CD4 in HEL cells modulates TGF-β1 expression, acting predominantly at the transcriptional level.