The chronic inflammatory bowel diseases (IBD), ulcerative colitis (UC) and Crohn's disease (CD), are complex diseases caused by an interplay between genetic and environmental factors .
The recent years have brought much progress regarding the genetics in IBD and the number of confirmed IBD associated loci and genes have risen dramatically [2–7]. Yet, still, only part of the genetic contribution to disease risk may be explained by the identified genes [8, 9]. Northern European populations, including the Danish, generally have low frequencies of the CD risk-associated variants of CARD15 [3, 10], and it is therefore of interest to search for more genetic determinants in these populations. Less progress has been achieved in the identification of environmental risk factors and gene-environmental interactions. Differences in environmental exposures and genetic heterogeneity between ethnic groups may have complicated the search for genetic and gene-environmental determinants.
The emerging picture of IBD pathogenesis is focused on the sequential occurrence of pivotal events leading to the initiation and subsequent perpetuation of inflammation [11, 12]. First, the initial interaction between luminal constituents and intestinal epithelial cells leads to activation of the innate immune system . The recognition of highly conserved pathogen structures such as lipopolysaccharide (LPS), the main constituent of Gram-negative bacteria, by Toll-like receptors and other pattern recognition receptors on the epithelial and other immunologically active cells in the intestine, initiates the release of various cytokines and enzymes, including interleukins (IL) and heme oxygenase-1 [13, 14]. Second, the inflammation will eventually become chronic due to defective regulation of the immune response. Therefore, polymorphisms in genes encoding cytokines and other molecules involved in the innate immune system, may affect the course of the inflammatory cascade and thereby the risk of developing IBD.
Activation of the pro-inflammatory IL-1β leads to production of prostaglandin E2 (PGE2) and nitric oxide (NO) via the induction of cyclo-oxygenase 2 (COX-2) and inducible nitric oxide synthase (iNOS) among others . IL-1β knock-out mice have no spontaneous abnormalities, however, on challenge with LPS, a less pronounced acute phase response is observed, suggesting that IL-1β is required for an adequate immune response . In both CD and UC patients, high levels of IL-1β are found in the intestinal mucosa  and stimulation by IL-1β leads to a more pronounced inflammatory response in CD immune cells compared to cells from healthy controls . The variant alleles of two IL-1β promoter polymorphisms, IL-1β T-31C and IL-1β C-511T, have been found to be in almost complete linkage disequilibrium , and the haplotypes encompassing the IL-1β T-31C variant conferred higher transcription of IL-1β compared to the wild type haplotype . The role of IL-1β polymorphisms in IBD has been explored in several studies [19–24]. These studies did not find any association with IL-1β, however, the studies were rather small.
IL-10 is an anti-inflammatory cytokine, which leads to dampening of the activated immune system. IL-10 knock-out mice develop colitis if they are not kept in germ-free environment , and the administration of IL-10 ameliorates the inflammation in animal and in vitro models . In patients, an impaired IL-10 production has been fund in severe cases of CD and UC [27, 28]. Recently, a strong association between the marker rs3024505 immediately downstream of the IL-10 gene and adult UC was found in a genome-wide association study . This study also found a modest association between this marker and CD risk . However, no association was found for rs3024493, a linked polymorphism located in an intron in the IL-10 gene, in a case-control study of paediatric onset CD . The IL-10 promoter is polymorphic and genetic variation may account for different levels of cytokine production . The IL-10 promotor polymorphisms G-1082A, C-819T, and C-592A have been most extensively studied. They are in tight linkage disequilibrium  and the haplotype encompassing these three polymorphisms is associated with low IL-10 protein production in lymphocytes in vitro  and low of levels of circulating IL-10 protein in Kenyan children  probably because the A allele of the IL-10 promoter polymorphism C-592A leads to the formation of a binding site for the ETS family of transcription factors . Studies on the IL-10 promoter polymorphisms and IBD susceptibility have been inconsistent [22, 29–31, 35–40].
Heme oxygenase-1 (HO-1) is involved in the degradation of heme, thereby reducing oxidative stress and protecting against acute and chronic inflammation . Animal models of IBD have confirmed the anti-inflammatory effect of HO-1 . Hence, blockade of HO-1 activity results in exacerbation of experimental colitis whereas increased HO-1 pathway activity ameliorates experimental murine colitis [43–45]. Carbon monoxide (CO) is one of the main metabolite of the HO-1 pathway and CO administration has been shown to ameliorate chronic colitis in IL-10 deficient mice [42, 46]. In UC patients, HO-1 expression and protein levels have been reported to be increased in inflamed colon compared to normal mucosa from patients with UC . Studies using luciferase reporter assays of a functional promoter polymorphism, HO-1 A-413T, indicated that the A allele promoter had significantly higher activity than the T allele promoter . The AA genotype of this polymorphism has been associated with a reduced incidence of ischemic heart disease . Another promoter polymorphism, the HO-1 (GT)N dinucleotide repeat polymorphism, was not associated with risk of inflammatory bowel disease . Interestingly, smoking affects the risk of IBD differentially, increasing the risk of CD and reducing the risk of UC . The mechanisms by which smoking affects risk of IBD is not clear and as tobacco smoke contains more than 3000 chemicals, several different mechanisms may be involved. Nicotine has been reported to modulate the immune balance in a Th1-dominant direction  in accordance with the beneficial effect of smoking on UC. Moreover, smoking increases the production of certain pro-inflammatory cytokines, but, on the other hand, smoking is a source of carbon monoxide (CO). Thus, another possible mechanisms may involve interactions between smoking and IL-1β, IL-10 and HO-1 activity in relation to intestinal inflammation .
In this study we wanted to assess the role of polymorphisms in IL-1β, IL-10, and HO-1 together with smoking in relation to risk of developing IBD in a Danish case-control study of 336 CD, 498 UC and 779 healthy controls, respectively.