INVOLVEMENT OF THE INK4 PROTEINS P16 AND P15 IN T-LYMPHOCYTE SENESCENCE

Little is known about the molecular background to senescence in T-lymp hocytes. In fibroblast systems replicative senescence has been shown t o correlate with a number of changes in the expression of the proteins normally regulating progression through the G1 phase of the cell cycl e, and recently the Ink4 inhibitor p16 was implicated as a central reg ulator of replicative senescence in human fibroblasts. It has, however , been claimed that p16 is not expressed in T-lymphocytes, In the pres ent study we have analysed G1 regulating proteins in ageing human T-ly mphocytes, We show that PHA and IL-2 stimulated T-lymphocytes cease to proliferate after around 20 population doublings, these cells can not thereafter be restimulated to growth, and were also found to exhibit markers for senescence, We found that T-lymphocytes accumulate p16 and p15 protein during successive population doublings and display high l evels of these proteins as they enter into replicative senescence. The re was also an increased binding of p16 to the Cdk6 kinase in senescen t cells, and a decreased Cdk6 as well as Cdk2 kinase activity. The lev els of other G1 regulating proteins were also altered in the senescent cells, such as slightly elevated levels of p21/WAF1, and downregulati on of Cdk2 and cyclinD3, The levels off p27/Kipl is down regulated in proliferating cells but rise to approximately 15% of the levels in un- stimulated quiescent cells. As a high proportion of T-cell childhood a cute lymphoblastic leukaemias have deletions of both p15 and p16, our data suggest that inactivation of these genes makes it possible for le ukemic cells to avoid senescence.