Innate immunity, especially Toll-like receptors, plays a significant role in intestinal homeostasis. There is emerging data on the importance of individual components that comprise innate immunity and their relationship to IBD and cancer, however there is a paucity of information on the role of TLR2. We provide a previously unrecognized link between TLR2 and intestinal tumorigenesis under chronic inflammatory conditions. In the absence of TLR2, we observed greater tumor incidence, tumor numbers and tumor sizes in a model of colitis-associated colon cancer, providing evidence for the critical role TLR2 plays in protection from CAC development and progression. In our model, we observed increased inflammation as measured by increased immune infiltration and production of proinflammatory cytokines in TLR2−/−
mice, which correlated with the increase in tumor numbers and tumor sizes, in support of previously established links between inflammation and colonic tumorigenesis 
. We establish a protective role for TLR2 in epithelial injury by (1) maintenance of epithelial homeostasis, (2) maintenance of regulatory mechanisms in colitis-associated inflammation, and (3) development and progression of CAC.
Rakoff-Nahoum et al investigated the role of MyD88 signaling during CAC formation and observed that MyD88−/− mice had reduced tumor numbers 
. However, that study did not use the AOM-DSS model but used a different experimental model of tumorgenesis, the ApcMin/+
. Additionally, MyD88, which is also critical for proper repair of the intestinal tract, is involved in many critical signaling pathways. Another study conducted by Fukata et al investigated the role of TLR4 during CAC development 
. They also found reduced numbers of tumors in TLR4−/− mice using the same AOM-DSS model we used in the current study. This study suggests the critical role of TLR4. Possibly TLR4 is the required proinflammatory signaling pathway for CAC development while the data presented here indicates TLR2 provides a suppressive environment in the intestine. In our model, the increase in tumor multiplicity in TLR2-deficient mice, particularly in the proximal colon where there is the highest concentration of TLR2 and bacteria, indicates that a pivotal source of inhibition during inflammation is withdrawn when TLR2 is not present. This allows for dysregulated proliferation and a subsequent increase in CAC tumor development.
Recent studies have shown that intact NFkB signaling is important for tumor development 
. Further, several studies have linked IL-6 production with colorectal cancer (reviewed in 
) and levels of serum IL-6 are increased in colorectal carcinoma patients 
, which further correlated with tumor size 
. In this CAC model, we observed more IL-6 produced in the serum, colon homogenates and lamina propria cells of TLR2−/−
mice compared to WT mice. The proliferative and survival effects of IL-6 on IEC are largely mediated by the transcription factor Stat3 
as mice lacking Stat3 in colonic epithelium develop fewer adenomas in spite of the fact that they have more severe colitis following exposure to AOM-DSS 
. In our study we have observed that activated Stat3 was increased within the ACF epithelium of TLR2−/−
mice as compared to WT ACF epithelium. IL-6, activated Stat3 and NFkB are intertwined in the development of CAC. Not only can Stat3 increase NFkB activity in tumors, but activation of Stat3 in immune cells in the tumor is dependent on NFkB and IL-6 which is downstream from NFkB. Interestingly, activated Stat3 inhibits lkB kinases (IKKs) thus reducing NFkB associated TH
1 immunity 
. We observed that mice deficient in TLR2 produced significantly less IFNg and more TGFß, which coupled with the increase in IL-6 production, led us to a TH
17 skew. Indeed, IL-17 is a downstream target of IL-6-induced Stat3 signaling 
. The TH
17 skewing that we have observed in TLR2-deficient mice in the context of colitis-induction in this study is consistent with recent findings linking IL-17 and IL-23 variants with increased susceptibility to IBD 
. Several animal models of colitis-induced colorectal cancer have already linked IL-17 production with increased tumorigenesis 
and epithelial protection in the face of colitis 
. Additionally, Chae et al recently showed that in the Apc(Min/+) mouse model, IL-17A−/− mice had drastically reduced intestinal tumor numbers 
. Hence, activation of Stat3 within the ACF epithelium of TLR2−/−
mice appears to promote a rise in NFkB driven pro-cancer TH
17 inflammatory pathway while suppressing TH
We observed an increase in inflammatory infiltrates in TLR2−/−
colons, consistent with prior studies that used DSS alone and found that TLR2 deficiency may lead to exacerbation of intestinal inflammation (10) and that TLR2 controls and protects mucosal inflammation by regulating tight-junction (TJ) associated barrier integrity 
. However, as in the AOM/DSS model, we observed a decrease in crypt damage in TLR2 deficient colons as compared to WT colons. Further, consistent with the increase in anti-apoptotic ACFs detected in TLR2-deficient, TLR2−/−
crypts showed less cryptitis and ulceration and displayed more signs of regeneration. Conversely, WT colons exhibited more crypt atrophy and regions of ulceration. Thus, in TLR2-deficient mice, tumor inducing circumstances, such as AOM/DSS, may suppress crypt damage and promote proliferation by cytokines such as TGF-ß.
A recent study by Boulard et al 
investigated the role of TLR-2 in IBD where they used both a T-cell transfer model, and a H. hepaticus
model, to investigate the innate and adaptive responses during the induction of chronic colonic inflammation. They found that TLR2 played little to no role in the induction of either innate or adaptive responses during the development of the inflammation. The authors concluded that the role of TLR-2 in IBD might differ depending on the model and previous reports indicated a requirement for TLR2 during acute intestinal inflammation models such as DSS used here in this study 
. Whether there is any difference in the involvement of TLR2 in acute versus chronic models of intestinal inflammation and CAC development is unknown at this time.
The role of TLR2 in colitis-associated colorectal cancer development is not well understood. Our study reveals a protective role for TLR2 in this process. In this study we show in vivo evidence that TLR2 signaling during colonic inflammation is essential in regulating proliferation and apoptosis. Based on our observations that TLR2-deficient mice exhibit unregulated proliferative growth and ignorance of apoptotic signals during colitis, we describe a new and critical role for TLR2-induced protection from epithelial transformation. In the genetically susceptible host, IBD results from a breakdown at the epithelial barrier, followed by inappropriate responses to microbial products. TLR2-commensal signaling provides enhanced epithelial barrier protection and pro-survival signals to intestinal epithelial cells especially in the context of inflammation by playing a critical role in promoting regulatory immune responses and preventing pro-tumor inflammatory skewing, thereby providing a protective role in the development and progression of colitis-associated colorectal cancer. These findings have broad implications towards the pathogenesis as well as treatment of colitis-associated colorectal cancer in patients with IBD.
Future studies similar to those that reported the essential role of epithelial expression of TLR4 impacting expression of proinflammatory mediators within the lamina propria 
utilizing bone marrow chimeras will help dissect the importance of TLR2 in each compartment of intestinal immunity. Also, since supplementation with TLR2 ligands during DSS-induced colitis is shown to promote transepithelial resistance and thereby protect from colitis 
, treatment with TLR2 ligands during the full course of AOM-DSS could result in a reduction of tumor development. These observations could lead to new exciting prophylactic and treatment approaches for colitis-associated colorectal cancer.