Swift elimination of undesirable cells is an important feature in tumour suppression and immunity. The tumour suppressor p53 and interferon-α and -β (IFN-α/β) are essential for the induction of apoptosis in cancerous cells and in antiviral immune responses, respectively, but little is known about their interrelationship. Here we show that transcription of the p53 gene is induced by IFN-α/β, accompanied by an increase in p53 protein level. IFN-α/β signalling itself does not activate p53; rather, it contributes to boosting p53 responses to stress signals. We show examples in which p53 gene induction by IFN-α/β contributes to tumour suppression. Furthermore, we show that p53 is activated in virally infected cells to evoke an apoptotic response and that p53 is critical for antiviral defence of the host. Our study reveals a hitherto unrecognized link between p53 and IFN-α/β in tumour suppression and antiviral immunity, which may have therapeutic implications.

STAT1 is an essential transcription factor for macrophage activation by IFN-γ and requires phosphorylation of the C-terminal Ser727 for transcriptional activity. In macrophages, Ser727 phosphorylation in response to bacterial lipopolysaccharide (LPS), UV irradiation, or TNF-α occurred through a signaling path sensitive to the p38 mitogen-activated protein kinase (p38 MAPK) inhibitor SB203580 whereas IFN-γ-mediated Ser727 phosphorylation was not inhibited by the drug. Consistently, SB203580 did not affect IFN-γ-mediated, Stat1-dependent transcription but inhibited its enhancement by LPS. Furthermore, LPS, UV irradiation, and TNF-α caused activation of p38 MAPK whereas IFN-γ did not. An essential role for p38 MAPK activity in STAT1 Ser727 phosphorylation was confirmed by using cells expressing an SB203580-resistant p38 MAPK. In such cells, STAT1 Ser727 phosphorylation in response to UV irradiation was found to be SB203580 insensitive. Targeted disruption of themapkap-k2gene, encoding a kinase downstream of p38 MAPK with a key role in LPS-stimulated TNF-α production and stress-induced heat shock protein 25 phosphorylation, was without a significant effect on UV-mediated Ser727 phosphorylation. The recombinant Stat1 C terminus was phosphorylatedin vitroby p38MAPKα and β but not by MAPK-activated protein kinase 2. Janus kinase 2 activity, previously reported to be required for IFN-γ-mediated Ser727 phosphorylation, was not needed for LPS-mediated Ser727 phosphorylation, and activation of Janus kinase 2 did not cause the appearance of STAT1 Ser727 kinase activity. Our data suggest that STAT1 is phosphorylated at Ser727 by a stress-activated signaling pathway either through p38 MAPK directly or through an unidentified kinase downstream of p38MAPK.

We generated a transgenic mouse line that expresses the Cre recombinase under the control of the Ncr1 (p46) promoter. Cre-mediated recombination was tightly restricted to natural killer (NK) cells, as revealed by crossing Ncr1-iCreTg mice to the eGFP-LSLTg reporter strain. Ncr1-iCreTg mice were further used to study NK cell–specific functions of Stat5 (signal transducers and activators of transcription 5) by generating Stat5^f/f Ncr1-iCreTg animals. Stat5^f/f Ncr1-iCreTg mice were largely devoid of NK cells in peripheral lymphoid organs. In the bone marrow, NK-cell maturation was abrogated at the NK cell–precursor stage. Moreover, we found that in vitro deletion of Stat5 in interleukin 2–expanded NK cells was incompatible with NK-cell viability. In vivo assays confirmed the complete abrogation of NK cell–mediated tumor control against B16F10-melanoma cells. In contrast, T cell–mediated tumor surveillance against MC38-adenocarcinoma cells was undisturbed. In summary, the results of our study show that STAT5 has a cell-intrinsic role in NK-cell development and that Ncr1-iCreTg mice are a powerful novel tool with which to study NK-cell development, biology, and function.

Complete activation of macrophages during immune responses results from stimulation with the activating cytokine interferon‐γ (IFN‐γ) and a second stimulus, usually a microbial product. Bacterial infection of macrophages, or treatment with bacterial lipopolysaccharide (LPS), resulted in rapid Stat1 phosphorylation on Ser727 (S727) independently of concomitant tyrosine phosphorylation. IFN‐γ also caused rapid phosphorylation of S727. In both situations, S727 phosphorylation was reduced by pre‐treatment of cells with the serine kinase inhibitor H7. When macrophages were treated sequentially or simultaneously with LPS and IFN‐γ, the pool of molecules phosphorylated on both Tyr701 (Y701) and S727 was strongly increased. Consistently, Stat1‐dependent transcription in response to IFN‐γ was significantly enhanced if the cells were pre‐treated with bacterial LPS. The relative amount of S727‐phosphorylated Stat1 in the non‐tyrosine phosphorylated fraction was considerably smaller than that in the tyrosine‐phosphorylated fraction. No evidence was found for an effect of S727 phosphorylation on the phosphorylation of Y701 by IFN‐γ. Thus, serine and tyrosine phosphorylation of Stat1 are caused independently of each other, but the serine kinase may recognize tyrosine‐phosphorylated Stat1 preferentially in the course of an IFN‐γ response. The data suggest Stat1 to be a convergence point for immunological stimuli in a macrophage proinflammatory response.

Type I interferons (IFNs) are known to mediate antitumor effects against several tumor types and have therefore been commonly used in clinical anticancer treatment. However, how IFN signaling exerts its beneficial effects is only partially understood. The clinically relevant activity of type I IFNs has been mainly attributed to their role in tumor immune surveillance. Different mechanisms have been postulated to explain how type I IFNs stimulate the immune system. On the one hand, they modulate innate immune cell subsets such as natural killer (NK) cells, on the other hand, type I IFNs also influence adaptive immune responses. Here, we review evidence for the impact of type I IFNs in immune surveillance against cancer and highlight the role of NK cells therein.