The immune system plays a complex role in tumorigenesis. While full activation of an effective adaptive immune response can lead to tumor eradication, it is clear that unresolved inflammatory responses can promote tumorigenesis through multiple mechanisms, including mutagenic or pro-proliferative effects on the tumor, enhanced angiogenesis, and suppression of cytotoxic T-cell responses (2
). Thus deficiency of innate immune cells frequently attenuates spontaneous or experimental carcinogenesis, while deficiency of adaptive immune cells generally enhances tumorigenesis in mouse models (2
). Interestingly however, although immune suppression in humans is associated with a dramatically enhanced cancer risk for many tumors (38
), it is associated with a somewhat decreased relative risk of breast, prostate, and bladder cancer (38
), suggesting that the adaptive immune response may be weakly tumor promoting for these tumor types.
Here we have shown for the first time that a tumor can subvert the CD8+ arm of the immune surveillance system into promoting tumorigenesis through direct effects on tumor cell survival. Using the 4T1 model of metastatic breast cancer, we showed that the presence of a tumor secreting both IL-6 and TGF-β causes local polarization or expansion of CD8+ T-cells into an IL-17-secreting state (Tc17
). We further showed that IL-17 could directly promote survival of 4T1 tumor cells, both in vitro
and in vivo
, thus providing a plausible explanation for the tumor promoting effect of CD8+ T-cells in this model. Many tumors are already known to evade immune surveillance by compromising the development or activity of tumor-specific cytotoxic T-cells, often in a TGF-β-dependent manner (3
). However, our data suggest that some tumors may go further, creating a local cytokine environment that actively skews local differentiation or expansion of CD8+ T cells to a state where they directly promote tumor growth. This effect is dependent on the presence of TGF-β at one or more steps (see model in ), and turns a key anti-tumor defense mechanism against the host organism.
To our knowledge, ours is the first report of a direct pro-survival effect of IL-17 on tumor cells, though overexpression of an IL-17 receptor-like gene (IL17RL
), activated by an unknown ligand, was recently shown to suppress apoptosis in prostate cancer cell lines (43
). The underlying mechanism is currently not known, but we have found that IL-17 and TGF-β synergize to suppress expression of the death receptor ligand TRAIL in 4T1 cells, which may contribute to this effect (data not shown). The anti-apoptotic effect is not just confined to the 4T1 model, as in a preliminary screen of a small panel of cancer cell lines, we found that IL-17 could promote the in vitro
survival of 3/6 cell lines tested, with a variable requirement for the simultaneous presence of TGF-β. However, in one case (EMT6 mammary carcinoma), IL-17 actually promoted apoptosis, suggesting that as for TGF-β (33
), the nature of the apoptotic response to IL-17 depends on the cellular context, and the specific determinant of the response is not yet known.
IL-17 production is defining for a new lineage of pro-inflammatory T-cells whose role in tumorigenesis has not been extensively explored (22
). In a recent study, it was demonstrated that both IL-17+ CD4+ and IL-17+CD8+ T-cells are present in the tumor microenvironment of several different human and mouse tumor types (44
), suggesting that local production of IL-17 by tumor-infiltrating T-cells may be a relatively widespread phenomenon in tumorigenesis. Indeed we have preliminary evidence, in a small cohort of breast cancer patients, that IL-17 mRNA is significantly upregulated in the tumor when compared with normal breast from the same patient. However, in contrast to Kryczek et al., in our preclinical model we saw a tumor-induced increase in IL-17 production in the locoregional lymph nodes as well as in the tumor, and within the lymph nodes the increase in IL-17 was specific to CD8+ T-cells and not CD4+ T cells. Thus the dominant site of IL-17 production and the originating cell type may vary among tumors.
The production of IL-17 is amplified and sustained by IL-23 in mouse CD4+ T-cells (22
). Recently IL-23 was shown to be overexpressed in several different human cancer types, and to promote tumorigenesis in a preclinical model of chemically-induced skin carcinogenesis, and in transplantable tumors (37
). The tumor-promoting effect of IL-23 in this study was attributed to the upregulation of inflammatory responses, including enhanced matrix metalloproteinase expression and angiogenesis, and to a reduced infiltration of cytotoxic T-cells into the transformed tissue. While IL-17 clearly has the potential to exert similar effects (25
), in our current work we believe that the direct effect of IL-17 on tumor cell survival predominates over alternative mechanisms, since blockade of IL-17 response in the 4T1 tumor cell was as effective as CD8+ cell depletion in suppressing tumorigenesis. However, the relative contributions of direct and indirect effects of IL-17 on tumorigenesis are likely to vary between tumor types, depending at least in part on whether the tumor cell can use IL-17 as a direct pro-survival factor.
Both preclinical and clinical studies support the concept that the adaptive arm of the immune system can be tumor promoting under certain circumstances (reviewed in (2
)). Although this effect is often due to the dominance of immunosuppressive CD4+ regulatory T-cells (Tregs), a number of CD8+ T cell subclasses with the potential to promote tumorigenesis have now been described. Similar to our findings with the 4T1 breast and CT26 colon carcinoma models, Girardi et al have recently shown that CD8+ T-cells can promote skin tumorigenesis induced by a two-step chemical initiation/promotion protocol using DMBA and TPA (41
). They identified a putative tumor promoting CD8+ population of tumor-infiltrating lymphocytes (“T-pro”), that had a phenotype consistent with activated effector memory cells (αβTCR+CD8+CD44+CD62L-) and expressed inflammatory mediators (IFN-γ, TNF-α and COX2), but were deficient in perforin (41
). Since the DMBA/TPA carcinogenic regimen upregulates both TGF-β and IL-6 (45
), and would be predicted to cause Tc17
skewing, it will be interesting to determine whether the T-pros also make IL-17, or whether an additional tumor promoting IL-17+CD8+ subset may also exist in this model. A number of other CD8+ subsets have also been shown to have immunosuppressive activity, including CD8+CD28- cells, CD8+CD45R+ cells, CD8+CD122+, and IL-10-secreting CCR7+CD45RO+CD8+ T-cells (47
). With the exception of the CCR7+CD45RO+CD8+ T cells (50
), these suppressor CD8+ T cells have mostly been identified in the context of autoimmunity and self-tolerance, and roles in promoting tumorigenesis have yet to be demonstrated. In all cases however, the predicted mode of action of these cells is the suppression of primary T-cell responses. The CD8+ tumor promoting mechanism that we have described here is novel in that it involves direct effects on the tumor cell itself.
In summary, here we have shown that presence of TGF-β in a tumor bed contributes to a local cytokine milieu that can direct inappropriate CD8+ T-cell differentiation or expansion down a pro-inflammatory, IL-17-secreting path. In tumor cells that have evolved to use IL-17 as a survival factor, this subversion of the CD8+ T-cell response can promote tumorigenesis directly, converting the adaptive arm of the immune system from a form in which it attacks the tumor into one in which it provides direct trophic support for the tumor, and thus promotes tumor progression.