RELIEF OF CYCLIN-A GENE TRANSCRIPTIONAL INHIBITION DURING ACTIVATION OF HUMAN PRIMARY T-LYMPHOCYTES VIA CD2 AND CD28 ADHESION MOLECULES

Cyclin A transcription is cell cycle regulated and induced by cell pro liferative signals. To understand the mechanisms underlined in this re gulation in normal human cells, we have analysed in vivo protein-DNA i nteractions at the Cyclin A locus in primary T lymphocytes. Stimulatio n of purified T lymphocytes by a combination of monoclonal antibodies directed at CD2 and CD28 adhesion molecules gives rise to a long lasti ng proliferation in the absence of accessory cells. Cyclin A was obser ved after 4 days of costimulation with anti CD2+ CD28 whereas stimulat ion by anti CD2 or anti CD28 alone was not effective. In vivo genomic DMS footprinting revealed upstream of the major transcription initiati on sites, the presence of at least three protein binding sites, two of which were constitutively occupied. They bind in vitro respectively A TF-1 and NF-Y proteins. The third site was occupied in quiescent cells or in cells stimulated by anti CD2 or anti CD28 alone. The mitogenic combination of anti CD2+ anti CD28 released the footprint as cells wer e committed to proliferation. Consistent with theses results, nuclear extracts prepared from quiescent cells formed a specific complex with this element, whereas extracts prepared from cells treated with anti C D2+ anti CD28 failed to do so after cells entered-a proliferative stat e.