Mechanisms for depleting tumor immunity in AIDS

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Human V¿2V¿2 T cells respond to low molecular weight isoprenoid pyrophosphate antigens and exhibit cytotoxicity against a variety of human tumors. Cells expressing this T cell receptor are depleted early in HIV disease and their loss is associated with increased risk for malignant disease and opportunistic infections in AIDS. Recently, we reported (Alexander, et al., 2008) that the subset of ¿¿ T cells expressing cell surface CD56 is potently cytotoxic against squamous cell carcinoma cell lines and resists TNF¿ or Fas-mediated cellular apoptosis. CD56 is regulated by the Runx1 transcription factor and increases after stimulation by common ¿ chain cytokines, possibly reflecting STAT 5 activation that would free Runx1 for nuclear translocation. Cell surface CD56 on V¿2V¿2 was a costimulatory receptor, promoting phosphorylation of Akt-1 and apoptosis resistance. Based on these and additional data, we proposed a model for the control of ¿¿ T cell levels: Expression of CD56 and the apoptosis resistance phenotype favored accumulation of antigen- experienced cells in the circulating population and higher expression of CD56 was associated with higher baseline V¿2V¿2 levels. In HIV disease, there is specific depletion of V¿2V¿2+ cells that is presumed to occur by indirect mechanisms because the cells do not express CD4 and are not susceptible to HIV infection in vitro. To date, the mechanism for depletion is not known. Our recent studies revealed phenotypic differences in those V¿2V¿2 cells remaining in HIV+ individuals (concentrating on donors with >300 CD4 T cells/mm3), including a significantly decreased capacity for expressing CD56 after cytokine stimulation. Without CD56 we predict lower Akt-1 phosphorylation and an apoptosis-sensitive phenotype. In the pre-apoptotic environment of HIV infection, sensitive cells would be depleted more rapidly. This is a plausible and testable model for the loss of V¿2V¿2 cells during HIV disease. Our proposal defines individual steps in the pathway for V¿2V¿2 T cell activation and expression of the apoptosis-resistant phenotype and compares these mechanisms with cells from control and HIV+ donors. Control cells are manipulated to mimic the behavior of cells from HIV donors and HIV donor cells are altered to increase CD56 expression and apoptosis-resistance. Specific cytokines or alternate costimulatory molecules are substituted for CD56 and IL-2 to search for means to activate and potentially reconstitute V¿2V¿2 cells in HIV+ individuals. Recent studies implicated the Nef protein as an agonist for peroxisome proliferator activated receptor (PPAR) that would decrease Akt-1 activation and potentially decrease apoptosis-resistance in ¿¿ T cells. We will test Nef to determine whether this viral accessory protein has a role in the ¿¿ T cell depletion mechanism.