Title: The Recruitment and Function of Tumor Microenvironment Components During Gastric Carcinogenesis
Description: Helicobacter felis (H. felis) infection has been used for numerous years as a standard model of gastric carcinogenesis and recapitulates the pathogenesis seen with human Helicobacter pylori infection. H. felis induces a a wide array of chronic low grade inflammatory responses, some of which directly contribute to carcinogenesis and some of which do not. To tease out the role of various processes, we have compared and contrasted models that recapitulate H. felis infection or synergize with it, namely, the K19kras transgenic mouse, and diet-induced obesity, respectively. While much attention has been rightly paid to the behavior of transformed cells, some focus has recently shifted to the components of the soil in which tumors develop and grow, commonly referred to as the microenvironment. Here, we investigate the gastric tumor microenvironment and shed light on the role of myofibroblasts, T lymphocytes and myeloid cells. K19kras mice express a mutant K-ras transgene in K19+ cells of the stomach; while K19 (and therefore the transgene) is not expressed in candidate stem/progenitor cells of the gastric corpus (marked by Dcamkl1 expression), a severe expansion and repositioning of Dcamkl1+ cells is observed in this model, and correlates with the accumulation of Î±SMA+ myofibroblasts and recruitment of bone marrow-derived inflammatory cells surrounding glands.

These transgenic mice also progress to high grade dysplasia at a similar rate as H. felis-infected mice, and this correlates with expression of select cytokines and chemokines expressed in both models, namely IL-6, IL-1β, and CXCL1. While H. felis infection had no effect on the phenotype of K19kras mice, epidemiological studies have shown that in humans, obesity increases one's risk of a wide variety of cancers, including gastric cancer, and indeed, we noted a synergy when recapitulating this in the mouse model. Acceleration in dysplasia progression was correlated with elevated levels of serum IL-6 and Leptin, which translated to excessive Stat3 activation in both epithelial and stromal compartments of the gastric epithelium, and excessive production of IL-17A in obese, H. felis-infected mice. As some reports indicate the TH17 response (characterized by IL-17A production) can, in certain contexts, promote tumorigenesis, it may be fundamental to H. felis-induced gastric carcinogenesis, and be a mechanism by which obesity accelerates cancer development.

Another important component of the tumor microenvironment, as described previously in the H. felis and other cancer models are myeloid cells. Namely, a group of phenotypically immature myeloid cells have been termed IMCs (immature myeloid cells), and can both suppress anti-tumor immunity and express mediators that directly promote carcinogenesis. IMCs were recruited from the bone marrow in H. felis-infected mice, although unexpectedly, these cells were intercepted by expanded adipose tissue (in obese mice), where they differentiated into mature macrophages. This led to an overall increase in the number of myeloid cells in the adipose tissue of obese H. felis-infected mice when compared to uninfected obese mice, and most importantly, an increase in adipose CD11b+F4/80+CD11c+ myeloid cells, which are known to be a primary source of insulin resistance-inducing adipokines. Fittingly, obese H. felis-infected mice had significantly higher levels of IL-6, resistin, and PAI-1, and were more glucose intolerant than uninfected obese mice.

Myeloid proliferation was similar between the two groups (as measured by KI67 and BrdU), as well as myeloid chemotaxis from the blood into adipose tissue (as determined by CCL chemokine production). Therefore, excessive myeloid recruitment to the adipose tissue could be due to elevated myeloid cells in the blood, which in turn is due to mobilization from the bone marrow. Indeed, we noted slightly elevated levels of blood IMCs in uninfected obese mice and H. felis-infected lean mice; in an additive fashion, this led to significantly elevated levels in obese H. felis-infected mice. Blood myeloid concentration was correlated with serum levels of the recently identified IMC mobilizer CXCL1, which was produced by H. felis-infected gastric tissue, and obese adipose tissue. Collectively, these results further stress the role of desmoplasia in gastric carcinogenesis. The recruitment of bone marrow derived cells may lead to the engraftment of myofibroblasts in the stem cell niche and permanently alter signals regulating stem cell homeostasis.

This, in turn, may increase the number of long-lived undifferentiated progenitors which can accumulate multiple mutations and initiate tumors. Furthermore, interaction and cytokine production by components of the inflammatory microenvironment, namely IL-6, IL-1β, and IL-17 (Stat3 and NFÎºB activators) initiate and maintain cell transformation and promote their progression. Finally, the recruitment of cells that participate in the tumor microenvironment may have unintended side effects if they are intercepted by other tissues, as exemplified here in the exacerbation of insulin resistance.