by ecancer reporter Clare Sansom

Despite the rapid development of targeted anti-cancer agents, cytotoxic chemotherapy drugs still play an important part in cancer treatment, and are likely to continue to do so for many years to come.

The immune system is closely involved in cancer development, and immune responses can both promote and prevent tumour growth; all anti-cancer drugs, including cytotoxic ones, can modulate these responses.

Two important cytotoxic chemotherapy drugs, gemcitabine and 5-fluorouracil, have been shown to be able to both enhance and suppress immune responses to tumour growth, the former through selective depletion of a type of immature myeloid cell known as a MDSC that suppresses T-cell activation.

François Ghiringhelli from Institut National de la Santé et de la Recherche Médicale (INSERM), Dijon, France and her co-workers have now elucidated a mechanism through which these two agents also activate an immune response that limits their efficacy and promotes tumour growth.

Following on from earlier results showing that gemcitabine (Gem) and 5-fluorouracil (5FU) could induce apoptosis in MDSCs, Ghiringhelli and her colleagues first demonstrated that these drugs induced the activation of caspase-1 in these cells in a way that suggested that this response was independent of cell death.

Caspase-1 was also shown to be activated in MSDCs taken from patients with solid tumours who had been treated with regimens including Gem and 5FU, but not from those treated with other cytotoxic drugs.

The activation of this protease is known to be required for the induction of interleukin-1b (IL-1b) and it was therefore not surprising that MSDCs treated with Gem or 5FU were found to secrete active IL-1b in vitro and in vivo.

Activation of caspase-1 and thence IL-1b induction by endogenous signals is often mediated through a complex of proteins termed the Nlrp3 inflammosome.

The researchers showed that treatment of wild type mice, but not transgenic mice in which the Nlrp3 inflammosome had been deleted (Nlrp3^-/- mice) with either of these drugs induced IL-1b, further implicating this protein complex in the immune response.

Cathepsin B, which is known to be one of the potential triggers of Nlrp3 activation, was found to be present in MSDCs that had been treated with Gem or 5FU, and reduced caspase-1 activation was detected when these cells were treated with an inhibitor of cathepsin B.

The release of cathepsin B into the cytosol on treatment with Gem or 5FU was confirmed using confocal microscopy.

Furthermore, mouse cathepsin B was shown to co-precipitate with Nlrp3 in vitro, suggesting a direct interaction between the proteins that was confirmed using surface plasmon resonance.

Tumours in mice in which either Nlrp3 or the caspase-1 gene Casp1 had been inactivated were more sensitive to treatment with Gem or 5FU than similar tumours in wild type mice, implicating this immune response, and specifically the production of IL-1b, in resistance to the chemotherapeutic agents.

Interleukin-1b is a pro-inflammatory cytokine that produces its response through inducing the secretion of interleukin-17 (IL-17) from pro-inflammatory CD4⁺ cells.

The researchers cultured MSDCs isolated from tumour-bearing wild type, Nlrp3^-/- and Casp1^-/- mice that had been treated with these drugs with immature T cells, and found that only the T cells cultured with wild type MSDCs differentiated rapidly into IL-17 producing CD4⁺ T cells.

Interleukin-17 release in the tumour micro-environment has also been implicated in angiogenesis and therefore in tumour progression.

Taken together, these results suggest a mechanism through which the cytotoxic drugs induce a tumourigenic immune response that limits their own activity: they induce cathepsin B release leading to activation of Nlrp3 and caspase-1, and thence secretion of the inflammatory cytokines IL-1b and IL-17.

Ghiringhelli and her co-workers therefore conclude that administering an inhibitor targeting this inflammosome pathway alongside gemcitabine or 5-fluorouracil might increase the efficacy of these cytotoxic drugs.

Reference

 Bruchard, M., Grégoire Mignot, G., Derangère, V. and 12 others (2012).

 Chemotherapy-triggered cathepsin B release in myeloid-derived suppressor cells activates the Nlrp3 inflammasome and promotes tumor growth. Nature Medicine, published online ahead of print 2 December 2012. doi: 10.1038/nm.2999