Proteasome-mediated reduction in proapoptotic molecule bim renders CD4+CD28null T cells resistant to apoptosis in acute coronary syndrome

Abstract

Background: The number of CD4+CD28null (CD28null) T cells, a unique subset of T lymphocytes with proinflammatory and cell-lytic phenotype, increases markedly in patients with acute coronary syndrome (ACS). ACS patients harboring high numbers of CD28null T cells have increased risk of recurrent severe acute coronary events and unfavorable prognosis. The mechanisms that govern the increase in CD28null T cells in ACS remain elusive. We investigated whether apoptosis pathways regulating T-cell homeostasis are perturbed in CD28null T cells in ACS. Methods and Results: We found that CD28null T cells in ACS were resistant to apoptosis induction via Fas-ligation or ceramide. This was attributable to a dramatic reduction in proapoptotic molecules Bim, Bax, and Fas in CD28null T cells, whereas antiapoptotic molecules Bcl-2 and Bcl-xL were similar in CD28null and CD28+ T cells. We also show that Bim is phosphorylated in CD28null T cells and degraded by the proteasome. Moreover, we demonstrate for the first time that proteasomal inhibition restores the apoptosis sensitivity of CD28null T cells in ACS. Conclusions: We show that CD28null T cells in ACS harbor marked defects in molecules that regulate T-cell apoptosis, which tips the balance in favor of antiapoptotic signals and endows these cells with resistance to apoptosis. We demonstrate that the inhibition of proteasomal activity allows CD28null T cells to regain sensitivity to apoptosis. A better understanding of the molecular switches that control the apoptosis sensitivity of CD28null T cells may reveal novel strategies for targeted elimination of these T cells in ACS patients.