The important role of the human adaptive immune system in controlling cancer is now widely understood and has been exploited in the development of effective targeted immunotherapies for cancer.

Many of these therapies are cytostatic rather than cytotoxic: that is, they arrest the growth of cancer cells rather than eliminating them.

Several studies, including clinical trials, have implicated the cytokines interferon-g (IFN-g) and tumour necrosis factor (TNF), and the CD4⁺ helper T cells (T_H1 cells) that produce IFN-g, in controlling cancer growth.

A group of researchers led by Martin Röcken of Eberhard Karls University, Tuebingen, Germany has now investigated the mechanism through which these cells inhibit cancer cell proliferation.

Röcken and his co-workers used a transgenic mouse model in which the tumour antigen Tag, expressed under the control of the rat insulin promoter, caused the development of invasive beta-cell cancers by partial or complete silencing of the tumour suppressors p53 and Rb.

They found that CD4⁺ T_H1 cells that secreted TNF and IFN-g, but no other T cells were able to suppress the growth of these tumours and thus double the lifespan of the mice. 

Tumour cells isolated from mice that had been treated with T_H1 cells were unable to proliferate in vitro, whereas cells isolated from similar mice that had not been so treated (“sham-treated” mice) did so.

The researchers then cultured beta-cancer cells from the sham-treated mice either with TNF and IFN-g or with control medium, and found that the cytokine-treated cells were arrested in the G1/G0 phase of the cell cycle after three days.

In contrast, over 25% of the cells that had not been treated with cytokines were found to be in the S phase, which explained their high proliferation rate.

Arrest of tumour cells in G1/G0 phase is characteristic of the state known as cellular senescence.

The researchers therefore sought to investigate whether the cytokines IFN-g and TNF could cause permanent tumour senescence and growth arrest.

They washed out cells that had been treated for five days with the cytokines and then cultured them in medium for another two weeks.

These cells remained senescent for the whole two weeks, whereas similar cells that had not been treated with cytokines proliferated rapidly during this period.

Changes in gene expression patterns and epigenetic changes that are known to be associated with cell senescence were also observed in the cells that had been treated with cytokines.

Expression of so-called early senescence marker genes, such as pHP1c, was observed after three days of IFN-g and TNF treatment, whereas expression of late senescence genes such as senescence associated b-galactosidase (SA-b-gal) and stable growth arrest required at least four days’ cytokine incubation.

Neither IFN-g nor TNF alone was sufficient to induce permanent growth arrest, although each caused some of the associated changes in gene expression.

This state of cytokine-induced senescence was found to require signalling by the transcription factor STAT1 and the TNF receptor TNFR1 (also known as CD120), which is thought to activate p16INK4a, a downstream target of JUNB.

Interferon-g and TNF were further found to induce p16INK4a expression, leading to enhanced phosphorylation of the tumour suppressor protein Rb at position Ser 795.

This combination of changes is known to induce senescence in beta-cancer cells through stabilizing the p16INK4a–Rb pathway.

The researchers found that beta cancer cells deficient in either STAT1 or TNFR1 did not enter senescence when treated with IFN-g  and TNF.

Cytokine-induced senescence was also observed in several other mouse and human cell lines and in cells from spontaneously regressing human melanomas, indicating that this observation may be of broad significance in tumour immunology.

To test whether T_H1 cell cytokines can induce tumour senescence in vivo, Röcken and his colleagues implanted the different beta-cancer cell lines into immunodeficient mice.

Beta-cancer cells from mice treated with Tag-T^H1 cells failed to proliferate after transplantation, whereas similar cells with no TNFR1 expression (Tnfr^-/- cells) grew aggressively even in mice that expressed this gene.

Taken together, these results suggest that IFN-g  and TNF secreted by T_H1 cells drive a mechanism of tumour cell senescence through the p16INK4a–Rb signalling pathway, which may explain the clinical effectiveness of treatments that stimulate tumour-specific T_H1 immunity in some tumour types.

Reference

Braumüller, H., Wieder, T., Brenner, E. and 27 others (2013).  T-helper-1-cell cytokines drive cancer into senescence. Nature, published online ahead of print 3 February 2013. doi: 10.1038/nature11824