Manipulation of TNFalpha delays the loss of CD28 in human CD8 T cells via the caspase-3 pathway

S.T. Parish, R.B. Effros
University of California, Los Angeles, Los Angeles, CA

Increased proportions of CD8 T cells lacking expression of the CD28 co-stimulatory receptor and having short telomeres are associated with numerous deleterious clinical outcomes in the elderly, including decreased vaccine responsiveness and early mortality. Cells with a similar phenotype arise in cell culture following multiple rounds of antigen-driven proliferation. In addition to the cell cycle arrest and loss of CD28 expression, senescent CD8 T cells secrete high titers of the pro-inflammatory cytokine, TNFalpha.
Since TNFalpha has been shown to bind one of the CD28 promoter regions, we tested whether antibody-mediated neutralization of the TNFalpha present in the culture supernatant would affect CD28 expression. Human T cells that were repeatedly stimulated in vitro with antigen in the presence of anti-TNFalpha showed prolonged expression of both the CD28 receptor and CD28 transcript, as well as increased proliferative potential. Consistent with the role of CD28 signal transduction in T cell telomerase upregulation, the prolonged expression of CD28 was associated with increased telomerase activity. Finally, based on reports showing that nuclear localization of caspase-3 leads to CD28 down-regulation, we evaluated the expression of caspase-3 in the anti-TNFalpha-treated cultures. Indeed, prolonged CD28 expression induced by anti-TNFalpha was associated with lower constitutive levels of caspase-3 activity. The inverse association between CD28 expression and caspase-3 was also seen in CD8 T cells tested immediately ex vivo: We separated CD8+CD28+ and CD8+CD28- from peripheral blood samples of healthy donors, and measured the constitutive caspase-3 activity levels in each population. The caspase-3 activity was significantly greater in the CD8+CD28- population, as compared to their CD28+ counterparts.
Our data suggest that loss of CD28 expression is caused by increased caspase-3 activity that can be induced by TNFalpha. These studies help elucidate the pathway involved in the senescence-associated loss of CD28 expression, and also suggest possible strategies for retarding the generation of senescent CD8 T cells during aging in vivo. (Supported by NIH AG 023720 & AI 060362)

Keywords: Senescence, CD28, TNFalpha, Caspase-3