Tumor rejection induced by CD70-mediated quantitative and qualitative effects on effector CD8+ T cell formation.

In vivo priming of antigen-specific CD8+ T cells results in their expansion and differentiation into effector T cells followed by contraction into a memory T cell population that can be maintained for life. Recent evidence suggests that after initial antigenic stimulation, the magnitude and kinetics of the CD8+ T cell response are programmed. However, it is unclear to what extent CD8+ T cell instruction in vivo is modulated by costimulatory signals. Here, we demonstrate that constitutive ligation of the tumor necrosis factor receptor family member CD27 by its ligand CD70 quantitatively augments CD8+ T cell responses to influenza virus infection and EL-4 tumor challenge in vivo by incrementing initial expansion and maintaining higher numbers of antigen-specific T cells in the memory phase. Concomitantly, the quality of antigen-specific T cells improved as evidenced by increased interferon (IFN)-gamma production and a greater cytotoxic potential on a per cell basis. As an apparent consequence, the superior effector T cell formation induced by CD70 protected against a lethal dose of poorly immunogenic EL4 tumor cells in a CD8+ T cell- and IFN-gamma-dependent manner. Thus, CD70 costimulation enhances both the expansion and per cell activity of antigen-specific CD8+ T cells.

Blockage of transcription as a trigger for p53 accumulation by 2-acetylaminofluorene DNA-adducts.

The hepatocarcinogen 2-acetylaminofluorene is one of the most studied experimental carcinogens. We have shown previously that normal rat hepatocytes accumulate the tumour suppressor p53 after exposure to this compound while preneoplastic rat hepatocytes do not. We suggested that the lack of p53 response may confer a growth advantage on preneoplastic hepatocytes and may be an important factor in hepatic tumor promotion by 2-acetylaminofluorene and other genotoxic compounds. Inhibition of RNA polymerase II driven transcription by DNA lesions may constitute one of the mechanisms leading to accumulation of the tumour suppressor p53. We have investigated the accumulation of p53 by structurally different DNA lesions of 2-acetylaminofluorene for which the rate of nucleotide excision repair (NER) and inhibition of transcription are known. Experiments were performed with NER proficient human fibroblasts as well as repair deficient xeroderma pigmentosum group A (XPA) cells, XPC cells [only transcription coupled repair (TCR)] and Cockayne syndrome (CS)B cells [only global genome repair (GGR)]. The cells were exposed to N-acetoxy-acetylaminofluorene (NAAAF) in the presence or absence of paraoxon inducing dG-C8-AAF or dG-C8-AF adducts respectively. Both treatments led to accumulation of p53 in all cells. However, dG-C8-AAF adducts produced greater p53 induction than dG-C8-AF adducts. The percentage p53-positive cells was highest and the threshold for p53 accumulation was lowest in XPA and CSB cells. Our results further demonstrate that both the potency of a lesion to inhibit transcription as well as the restoration of RNA synthesis determines the magnitude of p53 induction.

CD27-CD70 interactions sensitise naive CD4+ T cells for IL-12-induced Th1 cell development.

Stimulation of CD27, a member of the tumour necrosis factor receptor family, by its ligand CD70 induces expansion of IFNgamma secreting CD4+ and CD8+ T cells in vivo. We here analysed the mechanisms through which CD27 mediates this effect. CD27 co-stimulation induced cell division but did not directly instruct naive CD4+ T cells to differentiate into IFNgamma-producing Th1 cells. Rather, in concert with signals delivered through the TCR-CD3 complex, CD27 co-stimulation enhanced the Th1-specific transcription factor T-bet and caused up-regulation of the IL-12Rbeta2 chain. Consequently, CD27-costimulated T cells yielded vast numbers of IFNgamma-secreting cells in response to IL-12. Additionally, CD27 ligation induced a strong up-regulation of Bcl-xL, but not of related anti-apoptotic molecules. Thus, CD27-CD70 interactions may promote Th1 formation by permitting naive T cells to respond to differentiation signals and by promoting survival of activated effector T cells.