709M yocardial infarction and stroke caused by atherosclerosis are the most frequent cardiovascular causes of death and disability in the world despite the progress in prevention and therapy.1 Recent evidence clearly implicates the innate and adaptive immune system in the pathogenesis of atherosclerosis.2 T lymphocytes, the main orchestrators of adaptive immune responses, have pivotal roles in atherosclerosis. In particular, proinflammatory CD4+ T lymphocytes such as T helper 1 cells have been shown to promote atherosclerosis in animal models and constitute the predominant T cells in human atherosclerotic plaques. [3][4][5] We and others have demonstrated that CD4 + CD28null T cells, a unique subset of T helper 1 lymphocytes characterized by the lack of costimulatory receptor CD28, are present in high numbers in the circulation and atherosclerotic plaques of patients with acute coronary syndrome (ACS), 6-8 but these cells are not present in healthy individuals. Moreover, CD4 + CD28null T cells (from now on abbreviated as CD28 null T cells) produce higher levels of inflammatory cytokines interferon-γ and tumor necrosis factor-α than conventional CD4 + CD28+ (CD28 + ) T cells. 6,9 In stark contrast to CD28 + T lymphocytes, CD28 null T cells release cytotoxic molecules (ie, perforin and granzymes) 6 that are instrumental in killing endothelial and vascular smooth muscle cells in vitro. 10 These properties suggest that CD28 null T cells have harmful effects in atherosclerosis, although a direct causal link is yet to be established, because this subset of T cells is present only in humans with an equivalent population absent in mice. Recently, we were the first to show that the distinguishing feature between CD28 null and conventional CD28 + T cells in ACS patients is a significantly higher expression on CD28 null T cells of alternative costimulatory receptors . We further demonstrated that 4-1BB and OX40 have critical roles in regulating the production of inflammatory cytokines interferon-γ and tumor necrosis factor-α and perforin release from CD28 null T cells in ACS. 6 In addition to their roles in T-cell costimulation, 4-1BB and OX40 are important for the survival of T cells. 11,12 As mentioned, ACS patients are known to harbor significantly higher numbers of CD28 null T cells in comparison with patients who have stable angina (SA) and healthy subjects. 8,13 Importantly, Background-The number of CD4 + CD28 null (CD28 null ) 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 CD28 null T cells have increased risk of recurrent severe acute coronary events and unfavorable prognosis. The mechanisms that govern the increase in CD28 null T cells in ACS remain elusive. We investigated whether apoptosis pathways regulating T-cell homeostasis are perturbed in CD28 null T cells in ACS.
Methods and Results-We found that CD28null T cells in ACS were resistant to apoptosis induction...