Are we ready to start studies of Th17 cell manipulation as a therapy for cancer?

Abstract

From a therapeutic perspective, the bourgeoning literature on Th17 cells should allow us to decide whether to rationally pursue the manipulation of Th17 cells in cancer. The purpose of this review is to attempt a synthesis of a number of contradictory conclusions as to the role that these cells are playing in the process of tumourigenesis in order to provide guidance as to whether our current understanding is sufficient to safely pursue Th17-targeted therapy in cancer at this time. Th17 cells are a highly plastic population and the cytokine drivers for Th17 cell generation and skewing will vary between various cancers and importantly between different sites of tumour involvement in any individual patient. The net impact of the pro-angiogenic IL-17 produced not only by Th17 cells but by other cells particularly macrophages and the anti-tumour effects of Th1/Th17 cells will in turn be determined by the complex interplay of diverse chemokines and cytokines in any tumour microenvironment. Th17 cells that fail to home to tumours may be immunosuppressive. The complexity of IL-17 and Th17 dynamics makes easy prediction of the effects of either enhancing or suppressing Th17 cell differentiation in cancer problematic.

Introduction

The production of IL-17 by human CD4⁺ T cells was first described in 1995 [1] but it was a further decade before the production of IL-17 by a distinct sub-set of T helper cells, Th17 cells, was described [2–4]. Uncommitted (naïve) CD4⁺ T helper cells can be induced to differentiate to specific lineages according to the local cytokine milieu, towards T helper type 1 (Th1), Th2, Th17 and regulatory T cell (Treg) phenotypes in a mutually exclusive manner. Each phenotype is characterised by unique signalling pathways and expression of specific transcription factors, notably T-bet for Th1, GATA-3 for Th2, forkhead box P3 (FoxP3) for Tregs and receptor-related orphan receptor (ROR) alpha and ROR gamma for Th17 cells. Th17 cells are characterised by the production of IL-17A and IL-17F. These two IL-17 family members share the greatest homology, bind the same receptor, IL-17RA (and can bind as a heterodimer [5]) and result on receptor activation in a similar pattern of cytokine secretion from target cells [6]. There is evidence that IL-17A is an autocrine feedback regulator of IL-17F production [7]. The entire IL-17 family is more heterogeneous with members being produced by diverse cell types and subserving different biological functions. For example, although a significant anti-tumourogenic role has recently been described for IL-17E [8], this cytokine is not produced by TH17 cells and actually inhibits the function of Th17 cells.

A review article recently published in this journal summarised the development and transcriptional plasticity of Th17 cells and comprehensively referenced studies dealing with the role of Th17 cells in cancer [9]. This is a rapidly evolving field and our aim is to summarise studies published since then, particularly those which deal with the issue of the plasticity of directly isolated human Th17 cells both in the direction of Th1 cell and FoxP3⁺ Treg skewing and the epigenetic basis of that plasticity [10, 11]. These studies, in conjunction with recently published studies in non-small cell lung cancer [12, 13] which suggest that the role of Th17 cells in any individual cancer will depend critically on the tumour site analysed, are important in trying to answer the key question of whether the tumour-resident Th17 cells are actively promoting or inhibiting tumourigenesis. The particular aim of this article is to attempt a synthesis of a number of apparently contradictory findings on the role of Th17 cells in cancer in order to assess whether at the present time our knowledge of Th17 biology in cancer is sufficient to safely and rationally pursue Th17-directed therapies for cancer.

Th17 cell generation, plasticity and Th1/Th17 polarisation

The presence of TGF-β was initially thought to be essential for the differentiation of naïve murine CD4⁺ cells to Th17 cells, particularly in combination with IL-6: in the absence of IL-6, and following Treg depletion, the combination of TGF-β and IL-21 was also found to cause the differentiation of naïve cells towards a Th17 phenotype [14]. In subsequent work, this laboratory demonstrated that the differentiation of human naïve CD4⁺ cells to Th17 cells also required TGF-β but whilst combination with IL-6 was ineffective in this regard the combination of TGF-β and IL-21 of all the cytokine conditions tested, induced Th17 differentiation [15]. Th17 cells differentiated under these conditions secreted IL17-A but did not secrete IFN-γ. However, other studies demonstrated that TGF-β was either non-essential, or indeed suppressed, Th17 differentiation in humans [16, 17] and recent data has also demonstrated a TGF-β independent pathway of Th17 cell development in mice [18]. This latter work developed from the observation that transgenic mice lacking TGF-β expression, or TGF-β receptor expression still had significant numbers of IL-17 producing CD4⁺ cells in the intestinal lamina propria. In vitro, the combination of IL-6, IL-1β and IL-23 without TGF-β-induced IL-17 expression. These IL-23/IL-6-induced Th17 cells differed significantly from the conventional TGF-β differentiated Th17 cells, with higher expression of IL-2, IL-33 and IL-18r1. In mice, immunised with ovalbumin and adjuvant, the majority of IL-17 producing OT-11 CD4⁺ cells expressed IL-18r1 and produced IL-2.

Unlike TGF-β-induced Th17 cells, IL23-induced Th17 cells co-expressed Rorc and T-bet and these cells when cultured in IL-12 had a significantly enhanced ability to produce IFN-γ. Both T-bet and IL-23 are crucial for the development of experimental autoimmune encephalomyelitis (EAE), an autoimmune condition in which Th17 cells are pathogenically important. In mice immunised with myelin oligodendrocyte glycoprotein (MOG), 25–60% of RORγt⁺, IL-17⁺, CD4⁺ cells in the CNS expressed T-bet, and significantly more T-bet⁺ than T-bet⁻ Th17 cells were found in the CNS. The IL-23-induced cells produced much more severe disease than TGFβ-induced Th17 cells upon adoptive transfer, and this was associated with significant elevations of IL17⁺ and IFN-γ⁺ T cells in the CNS, whereas TGF-β-induced cells were weakly pathogenic. Thus, in the setting of murine autoimmunity, Th17 cells generated by TGF-β and IL-6 were non-pathogenic, whereas Th17 cells generated in the presence of IL-23 were essential for the development of the disease.

T-bet⁺ Th17 cells are also seen in multiple sclerosis patient lesions [19], and IL-23-dependent, TGF-β-independent production of IFN-γ expressing Th17 cells has been demonstrated in humans [16]. In the latter study, psoriatic skin lesions harboured IL-23-producing dendritic cells and lesions were enriched with Th17 cell-derived cytokines, which provoked the production of anti-microbial peptides by human keratinocytes. Thus, in both human and murine Th17-driven disease, particularly autoimmune disease, the Th17 cells with greatest pathogenic potential are those which co-express T-bet, and which differentiate via an IL-23-dependent pathway independently of TGF-β which serves to suppress pathogenicity. As will be described below, it is the Th1-skewed Th17 cells that co-express RORC and T-bet and produce IFN-γ, which are the putative anti-tumour Th17 cell effectors.

Non-Th1 polarised Th17 cells directly isolated from mice can be polarised towards a Th1/Th17 phenotype [20]. Directly isolated Th17 cells were shown to express fivefold less IL-12 Rβ2 transcripts than in vitro polarised Th17 cells; 95% of murine splenic in vivo Th17 cells lacked a functional IL-12 receptor. However, IFN-γ stimulation restored IL-12 responsiveness in directly isolated Th17 cells by inducing IL-12 Rβ2 expression; these cells were capable of T-bet up-regulation and 50% subsequently expressed intracellular IFN-γ. The cells continued to express RORγτ. Thus, non-polarised in vivo generated Th17 cells could be polarised to Th1/Th17 cells by IFN-γ-mediated up-regulation of the IL-12 Rβ2 and subsequent IL-12 stimulation. This process has strong overlaps with the two-loop model proposed for the sequential polarisation and imprinting of Th1 cells from naïve murine CD4⁺ T cells [21]. The differentiation of Th1 cells occurs in two phases: an early phase induced by TCR signalling that causes the induction of T-bet expression synergistically with IFN-γ but which represses activation of the IL-12 pathway, and a later imprinting phase that occurs after the termination of antigenic stimulation and which is critically dependent on IL-12 signalling and the maintenance of T-bet expression through a positive feedback loop of IL-12R and T-bet. When cells are primed with IFN-γ in the absence of IL-12, T-bet expression in the late imprinting phase was strongly reduced, and subsequent IFN-γ cytokine memory was reduced by around 80%. IFN-γ was important in competent polarisation of Th1 cells. IFN-γ accelerated activation of the T-bet-IL12R feedback loop, IFN-γ stimulus without CD3⁺/CD28 stimulation resulted in around a fourfold higher IL12Rβ2 MRNA expression and finally, in IFN-γ receptor-deficient cells, T-bet expression was reduced in the late phase of priming, and the polarisation efficiency of these cells reduced by approximately 50% compared with wild type cells.

The absence of IL-12 Rβ2 message in directly isolated, non-immunised, murine splenic Th17 cells has been confirmed [10]. In this study, such cells had significantly higher levels of H3K27me3 (a histone mark associated with gene repression) at the IL-12 Rβ2 promoter, compared with in vitro generated cells which had higher levels of the H3K4me3 mark associated with actively transcribed genes. IFN-γ and IL-12 treatment of directly isolated cells was again shown to result in the appearance of IFN-γ/IL-17 double-positive cells; there was a corresponding enrichment of H3K4me3 at the IL-12 Rβ2 locus. At an epigenetic level, the plasticity of Th17 cells was suggested by the presence of bivalent marks at the Tbx 21 promoter permissive of subsequent T-bet expression. Thus, the in vivo generation of polyfunctional Th1/Th17 effectors (with likely differing biological effects from non-polarised Th17 cells) will be highly dependent on local IFN-γ and IL-12 concentrations. These are likely to vary widely according to tumour type, tumour site, and spatially and temporally within any individual tumour microenvironment.

Th1/Th17 cells in cancer

The presence of Th1/Th17 cells expressing both IL-17 and IFN-γ have been demonstrated in a range of human cancers, including ovarian, colon, pancreatic and renal cancers, hepatocellular carcinoma and melanoma [22]. The phenotype and functionality of these cells were extensively investigated in ovarian cancer. There was no increase in Th17 cells in peripheral blood or draining lymph nodes, but the proportion of Th17 cells was higher in cancer tissue than in these other compartments. The cells expressed high levels of CXCR4, CCR6 and CD161. These molecules are likely to be important in Th17 cell migration towards and retention within tumour microenvironments. There was minimal co-expression of PD-1, FoxP3 or IL-10, but there were higher levels of TNFα, IL-2 and IFN-γ. Th17 induction was mediated by tumour-associated macrophage-mediated production of IL-1β, and Tregs suppressed Th17 cells and T cell IL-17 production in a dose-dependent manner, via ectonucleotidase-mediated conversion of ATP to adenosine.

The importance of the co-expression of IFN-γ by Th17 cells was twofold: firstly, IFN-γ and IL-17 synergistically induced the production of CXCL9 and CXCL10 by ovarian cancer cells, which are potent anti-angiogenic cytokines. Secondly, levels of IL-17 were positively correlated with levels of tumour-infiltrating CD8⁺ T cells, presumably via the increased CXCL9 and 10 expression: CD8⁺ T cells highly express the receptor for these chemokines, CXCR3. Thus, in ovarian cancer, Th1-polarised Th17 cells expressing IFN-γ in addition to IL-17-induced tumour cell CXCL9 and 10 expression, which reduced angiogenesis and enhanced CD8⁺ T cell migration.

A positive impact of IFN-γ-expressing Th17 cells on adaptive anti-cancer immunity has been demonstrated in mice [23]. Using a model of MHC class II-restricted cell immunity specific for a self-antigen (Trp1) antigen-specific CD4⁺ cells were cultured in different conditions to polarise them towards the Th1, Th17 or Th0 lineages. B16-bearing mice were then adoptively transferred with these three cell populations after the tumour had been allowed to grow for 10–12 days. Only the Th17 skewed cells mediated significant anti-tumour response and this effect was greater than that achieved by the adoptive transfer of Th1 cells. Critically, the in vivo neutralisation of IL-17a and IL-23 had no significant effects on the ability of these Th17 cells to reject tumours, but rejection was completely inhibited upon anti-IFN γ treatment. Thus, both in humans and mice, IFN-γ-expressing Th17 cells mediate potent anti-tumour immunity, and this suggests that the preferential expansion of CD4 cells in vitro under Th17 polarising conditions, and subsequent adoptive transfer, may be an effective therapy for cancer.

However, there are a number of caveats to this. The plasticity of Th17 cells is not solely in the direction of Th1 skewing with IFN-γ expression. In a transfer model of colitis whilst Th17 precursors gave rise to IFN-γ-expressing cells which mediated the inflammation, the majority of the recovered cells expressed neither IL-17 nor IFN-γ [24]. Other groups have demonstrated the same phenomenon with directly isolated Th17 cells: over a quarter of the cells treated with initial IFN-γ and subsequent IL-12 expressed neither IFN-γ nor IL-17 [20]. The phenotypic characteristics and functionality of these cell populations are currently unknown, and it is unwise to assume that they have the characteristics of an effector cell population. For example, recent data demonstrate that human tumour-infiltrating Th17 cells can differentiate into immunosuppressive Tregs [11]. Th17 clones were generated from human tumour-infiltrating lymphocytes and all primary clones expressed RORγτ and IRF-4 with minimal T-bet, GATA-3 and FoxP3 expression, as expected. When these clones were subsequently expanded with irradiated allogeneic PBMCs, OKT3 and IL-2, the IL-17-producing cell numbers dropped, and along with an increase in IFN-γ expression, there was also a significant increase in the percentage of cells expressing FoxP3 after repeated rounds of expansion. Methylation of the Treg-specific demethylated region decreased significantly with increasing rounds of stimulation and expansion of the Th17 clones. TCR engagement was critical in FoxP3 expression. Using proliferation assays primary Th17 clones promoted the proliferation of naïve CD4⁺ cells and thus showed an effector phenotype but after three rounds of stimulation, all clones strongly suppressed proliferation showing that they had become functional Tregs. These cells produced IL-10 and TGFβ, and unlike natural Tregs were resistant to re-conversion to Th17 cells after treatment with IL-1β, ΙL-6 and IL-23. Thus, not only there is a reciprocal interaction between natural Tregs and Th17 cells in the tumour microenvironment [22], but under certain polarisation conditions Th17 cells can be converted to inducible Tregs. Treg population numbers and the conditions suitable for the polarisation of Th17 cells to Tregs are again likely to vary from cancer to cancer, from one tumour site to another and are likely to change over time both with disease progression and as a result of treatment. The most important caveat, however, is that there is a coherent body of data that links the production of IL-17 with tumour progression, particularly via the process of angiogenesis.

IL-17 and angiogenesis

Studies that have carefully distinguished between Th17 cells and IL-17-producing cells per se have demonstrated an association between Th17 cells and microvessel density. In hepatocellular carcinoma, where around 20–40% of IL-17-producing cells were CD4⁻ and where around half of the tissue Th17 cells were double-positive for IL-17 and IFN-γ, there was a significant correlation between levels of Th17 cells in the tumour-infiltrating lymphocytes and microvessel density, with no such correlation between vascularisation and the levels of Th1 cells [25]. Although a multivariate analysis for survival was performed using intratumoural IL-17-producing cell density, rather than Th17 cell density, the IL-17+ cell number was an independent negative prognostic factor for overall survival. A similar picture was found in gastric cancer [26]. Using double immunofluorescence, most of the CD4⁺ cells in tumour tissue were stained with anti-IL-17 antibody. IL-17 mRNA expression increased with increasing depth of invasion and stage of disease, and the number of vascular endothelial cells was significantly higher in tumours expressing high IL-17 mRNA than in tumours expressing low IL-17 mRNA.

Other studies have confirmed the association between high Il-17 levels and increased vessel density but fail to conclusively demonstrate that the IL-17 is being produced solely or principally by Th17 cells. In colorectal cancer, a significant correlation between IL-17 expression and VEGF expression, and between IL-17 expression and microvessel density was again demonstrated [27]. IL-17 treatment of a range of colorectal cancer cell lines resulted in increased VEGF expression and VEGF protein production in all lines tested and high IL-17 expression conferred a worse outcome in multivariate analyses. Although the link between high IL-17 expression and VEGF production, increased vessel density and poor outcome seems clear the exact source of the IL-17 was not ascertained; the correlations were made with IL-17 levels rather than Th17 cell density. Using double immunostaining, both IL-17⁺/CD4⁺ cells and IL-17⁺/CD68⁺ cells were demonstrated in colorectal cancer, showing that both CD4-positive lymphocytes and macrophages produce IL-17 in this disease. Breast cancer is a good example of a malignancy where IL-17 expression is associated with poor outcome [28]. IL-17 promoted significant invasion of breast cancer cells and in 8/19 breast cancer samples, the presence of IL-17-positive cells was documented. More than 98% of these IL-17 expressing cells exhibited classical macrophage morphology and seemed to co-localise with CD68 expression; there was very little evidence of IL-17 expression by T cells further highlighting the fact that caution must be taken in ascribing IL-17 production to Th17 cells.

A more recent study in colorectal cancer has again demonstrated the association of high tumoural IL-17 with worse outcome [29]. Low IL-17/RORC gene expression and low IL-17 immunostaining were associated with a better outcome. However, again double staining for IL-17 and CD4 was not performed. The other gene that the investigators showed was co-modulated with RORC, and IL-17a (thus composing a cluster associated with Th17 genes) was the chemokine CCL24; using double staining, RORC-positive or IL-17-positive cells were found to be CCL24-positive. Again, this in itself does not directly prove that the IL-17 association with worse outcome is due to IL-17 production from Th17 cells. IL-17 can be expressed by some epithelial cells, NK cells, iNKT cells, αβ and γδ-T cells, neutrophils and, importantly, macrophages. Whilst wild type murine macrophages produce very little IL-17, high levels of IL-17 produced after LPS stimulation of macrophages from IL-10 receptor-deficient mice was paralleled by an increase in RORγτ mRNA expression [30]. Both RORγτ and IL-17 expression in IL-10 knock out mice could be inhibited by the addition of endogenous IL-10. Furthermore, CCL24 expression can be up-regulated in macrophages by treatment with IL-4 [31]. Thus, macrophages also appear to demonstrate plasticity with conversion to Th17-like phenotype with IL-17 expression, CCL24 expression and RORC expression. Thus, from the point of view of the biological impact of Th17 cells, it is important to distinguish the presence of Th17 cells that will clearly produce IL-17 from IL-17 production per se which will also include a variable contribution from non-Th17 cells especially macrophages. IL-17 produced from diverse sources will be equally pro-angiogenic but the net effect of its production will be critically dependant upon the context of that production.

Thus, is it possible to square these findings with the data obtained in ovarian cancer, where the only cell type expressing IL-17 was Th17 cells, and where high IL-17 was associated with a better outcome [22]? In that analysis, there was no association between IL-17 and VEGF production; however, IFN-γ and IL-17 synergistically increased CXCL9 and CXCL10 expression by ovarian cancer cells, both potent anti-angiogenic cytokines. Thus, in this cancer, there was no evidence of any pro-angiogenic effect of IL-17 produced by Th17 cells to counteract the anti-tumourigenic effect of the polyfunctional effector cells generated upon Th1 polarisation of the Th17 cells. This balance between Th1/Th17 cell effector activity and the angiogenic effect of IL-17 which would be mediated not only by Th17 cells, but by other IL-17-producing cells in any given microenvironment will be critical in determining the net effect of Th17 cells. As with the liberation of cytokines involved in the polarisation of Th17 cells, this balance is likely to be temporally and spatially dynamic as well as tumour site-dependent. The ovarian cancer tissue that was used for the above analysis was ascites and this microenvironment will not replicate the complex intercellular relationships found in solid cancer tissue; the balance between Th1 skewing and angiogenesis is likely to be very different in highly vascularised, complex tumour masses compared with malignant effusions.

Contradictory findings with regard to Th17 functionality in non-small cell lung cancer (NSCLC) support this concept. Initial data showed that high IL-17 mRNA expression was associated with increased vascular density in primary resection specimens [32]. More recently, high IL-17 expression by immunohistochemistry has been correlated with increased lymphatic vessel density (lymphangiogenesis is mediated principally via VEGF-C and VEGF-D) [12]. In the latter study, tumour-infiltrating T cells, especially CD4⁺ cells, were found to be the predominant source of IL-17, and there were no IL-17-producing macrophages. In multivariate analyses, high IL-17 expression was associated with worse overall survival and disease-free survival in resected patients. These findings largely concur with those in gastrointestinal cancer. This study did not examine the polarisation status of the Th17 cells.

However, in a separate study, the Th17 cell number in the pleural effusions of patients with newly diagnosed lung cancer was positively correlated with outcome; individuals with low Th17 cells had a 4.5-fold increased risk of death, compared with those with high pleural fluid Th17 cells [13]. These findings concur with those in ovarian cancer ascites. It is noteworthy that, whereas in ovarian cancer IFN-γ and IL-17 synergistically increased expression by cancer cells of the anti-angiogenic chemokines CXCL9 and CXCL10, and in colorectal cancer, IL-17 up-regulated VEGF release by cancer cells, IL-17 stimulation of NSCLC cell lines did not increase expression of any of these three molecules, but instead augmented the release of the angiogenic chemokines CXCL1, CXCL5 and CXCL8. This is further evidence that it is unwise to extrapolate from data pertaining to the biology of Th17 cells in one cancer to the likely functional effects of Th17 cells in a different cancer type.

Peripheral Th17 cells versus homing Th17 cells

Th1/Th17 cells appear to be potent polyfunctional effectors and thus augmentation of their numbers may be a useful immunotherapy in cancer. However, there is a further level of complexity with regards Th17 biology in cancer. High levels of circulating Th17 cells predicted for a lack of immune response and worse outcome in patients with hormone-resistant prostate cancer vaccinated with a mixture of three allogeneic lines supplemented with BCG [33]. No such association with outcome was found with Treg levels.

The poor outcome was specifically associated with circulating Th17 cells with low expression of CCR4 and CCR6; there was no correlation with CCR4⁺ IL-17⁺ T cells and clinical outcome. CCR4 and CCR6 have been implicated in the homing of Th17 cells to the tumour microenvironment and in ovarian cancer, tumour-infiltrating Th17 cells showed a high expression of CCR6 [22]. This suggests that circulating Th17 cells with low levels of CCR4 and CCR6 do not have the ability to home to the tumour microenvironment and thus are maintained in the peripheral compartment; it is these cells which confer the worse outcome and a reduced immunological response to cancer vaccines. The mechanism of this negative impact is unknown, but this clearly adds another layer of complexity: expansion of Th17 cells may potentially lead to a worse outcome in trials attempting to augment any immune response to cancer vaccines if this is also leads to a concomitant increase in peripheral Th17 cells. These findings require validation, and the functional impact of these cells requires further investigation.

Concluding remarks

Conflicts in the literature concerning whether Th17 cells actively enhance or inhibit the progression of tumours are more likely to be apparent than real. The biological impact of Th17 cells will depend critically on the cancer analysed, and the source of tissues under analysis and will be the resultant of two principal opposing effects. The first is that Th17 cells in vivo are capable of significant levels of polarisation and this is well established in the case of Th1/Th17 cells; these cells appear to promote an anti-tumour effect by enhancing adaptive immunity against tumour antigens. Polarisation to FoxP3-positive Tregs has also been demonstrated in vitro, but the extent to which this occurs in human cancers is currently unknown. The second effect is the involvement of IL-17 in the promotion of angiogenesis; in some cancers, the principal source of that IL-17 is Th17 cells. The degree to which this promotion of angiogenesis occurs will be dependent upon the generation of anti-angiogenic cytokines by Th1-skewed Th17 cells and the complex intercellular relationships within any tumoural microenvironment between Th17 cells, cancer cells, stromal cells, Tregs and endothelial precursors. The spatial and temporal complexity of these inter-relationships and the fluxes in the cytokines critical to Th17 cell polarisation such as IL-1 β, IL-2, IL-12, IL-23 and IFN γ means that generalisation from one cancer to the next, and even from one site to another within any individual cancer to another, is unwise. Figure 1 summarises schematically these interactions.

Fig. 1.

The net effect of the pro- and anti-tumourigenic effects of Th17 cells is dependant firstly upon the balance between the angiogenic effects of IL-17 and anti-angiogenic factors secreted under the influence of Th1/Th17 cells and, secondly, the degree of polarisation of Th17 cells either in the direction of Th1/Th17 cells or in the direction of inducible Tregs (iTregs). This latter balance is determined by various cytokines in the tumour microenviroment, levels of which will vary between different tumour types, from site to site in patients and within individual tumour nodules. MVD mean vascular density, TAM tumour-associated macrophage, TCR T cell receptor

A recent paper has considered the prospects for the manipulation of Th17 cell reactivity in acute myeloid leukaemia [34]. Enhancement of Th17 activity may be a future option for patients with high-risk disease and no significant anti-leukaemic graft versus host activity. Here, the challenge will be to avoid the subsequent development of severe graft versus host disease and the authors conclude that before such approaches can be tested clinically there needs to be a standardised assessment of pre-transplant risk factors and validated biomarkers for the risk of post-transplant complications. Extreme care in selection of patients for such an approach will be mandatory.

Whilst it appears that the current contradictions in the literature can be synthesised to give a coherent view of Th17 biology in cancer, it seems that it is too early to confidently move forward to the direct testing of Th17-directed therapies in patients with cancer. Whether robust, validated predictors of the likely outcome of Th17 cell manipulation in any individual cancer and any individual cancer site can be designed through a systems biology approach that can account for the dynamism of the Th17 system remains an open question.