In this study, we have established human IL-1β transgenic mice by targeting expression of hIL-1β to the stomach using the H/K-ATPase promoter. The IL-1β transgenic mice provide an in vivo
model of inflammation-related cancer that closely mimics development of human cancer. Our results indicate that these IL-1β transgenic mice develop step-wise spontaneous inflammation, metaplasia, dysplasia and carcinoma of the stomach, and that IL-1β activation of NF-κB in MDSCs contribute to the development of gastric inflammation and initiation of carcinogenesis. H. felis
infection resulted in more rapid progression to gastric atrophy and cancer in IL-1β transgenic mice. Our current results provide support and validation for clinical studies that suggested a strong link between SNPs in the IL-1β gene and the risk of gastric cancer in the setting of H. pylori
infection (El-Omar et al., 2001
). In addition, the data offer direct evidence that elevation of a single proinflammatory cytokines, IL-1β, is sufficient for the induction of gastric dysplasia/carcinoma, and thus establish a crucial etiological role for IL-1β in gastric carcinogenesis.
Previous studies have pointed to roles for MDSCs in cancer. Accumulating data have shown that MDSCs can contribute to suppression of tumor immunity, resistance to immunotherapy, tumor angiogenesis and metastasis (Shojaei et al., 2007
; Yang et al., 2008
). In addition, it has been demonstrated that overexpression of IL-1β by cancer cells themselves can accelerate tumor progression and its spread in part by mobilization and recruitment of MDSCs in tumor tissues (Bunt et al., 2006
; Song et al., 2005
). Our study suggests for a link between IL-1β and MDSCs in carcinogenesis. Several findings in our study suggest a direct contribution of MDSCs to IL-1β induced gastric inflammation and carcinoma. (1) The mobilization of MDSCs into the blood, and the recruitment of MDSCs to the stomach, occurred at the very earliest stage of gastric inflammation in IL-1β transgenic mice. (2) IL-1β directly activated MDSCs in vivo
and in vitro
to induce secretion of pro-inflammatory cytokines (IL-6 and TNF-α) and chemokines (SDF1) in IL-1β mice at this early stage. (3) The IL-1β;Rag2-/-
mice developed spontaneous gastritis and dysplasia in the absence of T cells. (4) Antagonism of IL-1 receptor signaling by IL-1RA inhibited gastric inflammation and carcinoma and also suppressed MDSC mobilization and recruitment. Interestingly, inhibitory effects of IL-1RA were also observed in H. felis
-infected wild type mice, suggesting that the effects were not limited to the transgenic mice. However, given that MDSCs were not specifically ablated in our model, a possible role for other cell types (including neutrophils, macrophages, dendritic cells, myofibroblasts, endothelial cells, etc.) cannot be excluded.
One important finding in this study is that IL-1β can directly activate MSDCs through an IL-1β/ IL-1RI / NF-κB pathway. In prior studies, such a direct link was not well established and other possibilities were considered, including a role for IL-6 in MDSC activation (Bunt et al., 2006
). IL-1R-deficient mice have a delayed accumulation of MDSC, which was partially restored by IL-6, indicating that IL-6 is a downstream mediator of the IL-1β-induced expansion of MDSC (Bunt et al., 2007
). However, we found that while the levels of expression were low in MDSCs, IL-1RI is clearly expressed in these cells, and IL-1β could directly activate MDSCs. IL-1β stimulation of MDSCs led to increased NF-κB activity, both in vitro
and in vivo
, and increased secretion of IL-6 and TNF-α. Using an IL-1β;NF-κBEGFP
mouse model, we found that overexpression of IL-1β directly activated NF-κB in both epithelial cells and immune cells, but the greatest activation occurred in MDSCs. We thus have linked IL-1β directly to NF-κB activation in MDSCs, and to the NF-κB downstream targets genes, IL-6 and TNF-α.
Activation of the transcription factor, NF-κB, is a key molecular link between inflammation and cancer (Karin and Greten, 2005
). Activation of NF-κB occurs downstream of both the TLR and IL-1RI pathways through a complex involving IKK-beta. We have previously shown that Helicobacter
infection activates the innate immune system through a TLR pathway (Mandell et al., 2004
) leading to induction of NF-κB signaling and cytokine production (such as IL-1β, TNF-α and IL-6) by macrophages. Knockout of IKK-β in myeloid cells has been shown to downregulate the innate immune system and suppress both murine hepatocellular and colon carcinoma (Greten et al., 2004
; Maeda et al., 2005
; Pikarsky et al., 2004
). However, in these conditional knockouts of IKK-β in myeloid cells, the relevant cell population targeted has not been completely defined, although IL-6 levels are consistently downregulated in the knockout animals. In our IL-1β transgenic mice, the levels of IL-6 and TNF-α were significantly increased in both the stomach and the serum, and correlated well with MDSC mobilization and recruitment. IL-6 in particular, which is frequently elevated in patients with cancer, is an important inducer of tumor promotion and progression, and may account for gender differences in cancer susceptibility (Heikkila et al., 2008
; Naugler et al., 2007
). Thus, IL-6 and TNF-α may be major secreted products of MDSCs and as such serve as direct downstream target of IL-1β and NF-κB that amplify the inflammatory immune response and promote carcinogenesis (Balkwill, 2006
; Naugler et al., 2007
Clinical and epidemiologic studies have suggested a strong association between chronic inflammation and cancer (Coussens and Werb, 2002
). Specific polymorphisms in proinflammatory cytokine genes, such as IL-1β, can now be linked to MDSCs. Previous studies have demonstrated that MDCSs have immunosuppressive properties, and we have confirmed that MDSCs from our IL-1β mice can inhibit T and B cell proliferation. However, the development of gastric preneoplasia has in the past been shown to be related to the host immune response to infection, and largely dependent on CD4+ T cells, which we have again confirmed in this study. Rag2-/-
were resistant to Helicobacter
-induced gastric atrophy, while Rag2-/-
mice reconstituted with whole splenocytes or CD4+ T cells developed severe atrophic gastritis after H. felis
infection. However, IL-1β transgenic mice showed an early and marked infiltration of the gastric mucosa with MDSCs at a stage when very few T cells were present. More importantly, IL-1β mice crossed into a Rag2-/-
background (i.e. IL-1β;Rag2-/-
mice) still developed spontaneously gastritis and dysplasia, in association with a marked infiltrate of MDSCs, indicating that IL-1β-induced gastric inflammation may be mediated by MDSCs independent of a T cell-mediated Th1 immune response. IL-1β is able to induce gastric atrophy and mobilize MDSCs in lymphocyte deficient animals. While the data fall short of demonstrating that MDSCs are the primary mediator of carcinogenesis in our model, they do suggest an early role for MDSCs in cancer that does not dependent on an immunosuppressive role, but is instead more consistent with a pro-inflammatory role.
In conclusion, our study strengthens the link between IL-1β and gastric cancer, and implicates MDSCs as being important in the early stages of gastric carcinogenesis. This observation could lead to a more general understanding of the role of inflammation in carcinogenesis and provide a model for test the efficacy of anti-IL-1β therapies in cancer prevention.