G1 arrest and high expression of cyclin kinase and apoptosis inhibitors in accumulated activated/memory phenotype CD4+ cells of older lupus mice

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Abstract
A general characteristic of lupus‐prone mice (and humans) is the expedited accumulation of large numbers of presumably self‐reactive activated/memory phenotype T cells. The mechanism by which these cells escape apoptosis has not been defined. We used activated/memory phenotype CD4+ cells from male BXSB mice with early‐life severe lupus‐like disease to investigate cell cycle status and apoptosis susceptibility, and to determine the role of corresponding genes in survival of these cells. In vitro acridine orange staining indicated that most of the rapidly accumulating memory phenotype CD4+ T cells of 4‐month‐old male BXSB mice are G1 arrested. Long‐term bromodeoxyuridine in vivo labeling also showed that with advanced age, there was a shift of the CD4+ CD44hi male cells from predominantly cycling to predominantly noncycling. Moreover, the CD4+ CD44hi cells of older males were refractory to anti‐CD3‐induced proliferation and apoptosis. Using a multiprobe RNase protection assay encompassing riboprobe panels for cell cycle and apoptosis‐related genes, we found that these cells exhibited high expression of certain members of the Ink4 (p18Ink4C) and Cip/Kip (p21Cip1) families of cyclin kinase inhibitors as well as of the apoptosis‐inhibiting Bcl‐xL gene. Western blot analysis confirmed increased levels of Bcl‐xL and p21Cip1, and also identified increases in another cyclin kinase inhibitor, p27Kip1. We propose that in autoimmunity, self‐reactive CD4+ cells are subjected to successive rounds of activation/division that eventually lead to a build‐up in cyclin‐dependent kinase inhibitors. Once high levels of such inhibitors are reached, they cause refractoriness to further activation, impaired cell cycle entry and resistance to apoptosis, a situation akin to replicative senescence.