Immunopathogenesis of inflammatory bowel disease

Abstract

Inflammatory bowel disease (IBD) is a group of idiopathic, chronic and relapsing inflammatory conditions of the gastrointestinal tract. Familial and epidemiological studies have stressed the involvement of genetic factors and have also shown the critical role of environmental factors such as sanitation and hygiene in the development of IBD. However, the molecular mechanisms of intestinal inflammation in IBD have long remained unknown. In recent years, the study of susceptibility genes involved in the detection of bacterial components and in the regulation of the host immune response has shed light onto the potential role of intestinal pathogens and gut flora in IBD immunobiology. This review presents current knowledge on intestinal epithelial barrier alterations and on dysfunction of mucosal innate and acquired immune responses in IBD. The data support the etiological hypothesis which argues that pathogenic intestinal bacteria and/or infectious agents initiate and perpetuate the inflammation of the gut through disruption of tolerance towards the commensal microbiota in an individual with genetic vulnerability.

Definition and Epidemiology

Inflammatory bowel disease (IBD) is characterized by severe inflammation of the small bowel and/or the colon leading to recurrent diarrhea and abdominal pain. Crohn disease (CD) and ulcerative colitis (UC) are the two main clinicopathological subtypes of IBD. Despite both being chronic and relapsing inflammatory diseases of the bowel, they can be differentiated by the location of the inflammation in the gastrointestinal tract and by the nature of the histological alterations in the intestinal wall. Anatomically, CD can affect the entire gastrointestinal tract from mouth to anus, although it commonly affects the terminal ileum and colon. UC is restricted to the rectum, colon and caecum. Microscopically, CD is transmural and often discontinuous while UC affects only the intestinal mucosa in a continuous pattern.¹

IBD is a very disabling disease due to the fatigue associated with the inflammatory symptoms and due to the chronic pain suffered by patients. IBD affects quality of life but not life span: the mortality rate of patients is not different from the normal population. The prevalence of IBD is similar in men and women and peaks in young adults. The highest incidences are reported in northern Europe and North America where they range from 12 to 19/100,000/year and from 5 to 29/100,000/year, respectively. Nearly 1.4 million Americans and 2.2 million Europeans are affected.² The prevalence of IBD varies depending on ethnicity or race. Caucasians and African Americans are most affected while IBD is rare among Hispanics and Asians. Ashkenazi Jews have a much higher risk of developing IBD with a 2–4 times higher incidence when compared to non-Jewish Caucasians.³ However, there is evidence to suggest that the prevalence of IBD increases in populations and regions with industrialization. For example, a study demonstrated a similar incidence rate of IBD in African Americans compared to that observed in Americans of European ancestry.⁴

IBD Etiology

The pathogenesis of IBD is only partially understood; various environmental and host (e.g., genetic, epithelial, immune and nonimmune) factors are involved. Complex interactions between the immune system, enteric commensal bacteria/pathogens and host genotype are thought to underlie the development of IBD.^5–7 These relapsing chronic inflammatory disorders appear to be caused by overly aggressive T-cell responses directed against environmental factors and/or a subset of commensal bacteria/pathogens that inhabit the distal ileum and colon of genetically susceptible hosts.

Environmental contribution to IBD.

The lower incidence of IBD in Asia and Africa compared with North America and Europe probably reflects both genetic and environmental factors. The hygiene hypothesis has been invoked to account for the rising prevalence of various autoimmune and inflammatory conditions, which are thought to result from the lack of early exposure to select microbial agents due to stringent sanitation conditions.⁸ Although the precise environmental factors that account for changing IBD prevalence have not been defined, changes in dietary and intestinal microbial milieu probably play a key pathogenic role that is common to CD and UC. One important environmental factor is smoking. The effect of smoking is discordant between CD and UC.⁹ Smoking was found to be protective against UC and, after the onset of the disease, might improve its course, decreasing the need for colectomy.¹⁰ In contrast, smoking increases the risk of developing CD and aggravates its course by approximately twofold in western countries.^10–12 Patients with a high lifetime tobacco exposure (>150 cigarettes/year) suffer small bowel disease more often than do nonsmokers. Smokers also have a higher accumulated number of fistulae and/or abscesses than nonsmokers and are also more vulnerable to CD flare-ups.¹³ In particular, heavy smokers (>10 cigarettes/day) are more likely to require gastric surgery as a result of IBD and suffer a significantly decreased quality of life.^14,15

Susceptibility genes in IBD.

Considering that IBD incidence seems linked to race, a strong genetic component in IBD etiology is very likely. Some epidemiological data support this hypothesis: (1) the heterogeneity in the geographical distribution of IBD, (2) the existence of familial forms of IBD and (3) the high rate of concordance in monozygotic twins. Indeed, population-based studies find that 5–10% of patients have a first-degree family member with IBD, with the relative risk to siblings of affected individuals estimated to be 30- to 40-fold higher for CD and 10- to 20-fold higher for UC, suggesting that some susceptibility genes should be specific for CD and UC.^16,17 Twin studies provide additional evidence for genetic contribution in IBD: the concordance rate is significantly greater in monozygotic than dizygotic twins for both CD (50–58% versus 0–12%) and UC (6–14% versus 0–5%).^18,19 However, the significantly higher monozygotic twin concordance rates for CD than UC suggest that there is a greater contribution of genetic factors in CD.

In recent years, genome-wide association (GWA) studies have provided a broad view of the relative contributions of various genomic loci and have substantially improved the understanding of the molecular pathways leading to IBD. GWA have stressed an association between IBD and specific chromosomal loci^20,21 and thereby have led to the identification of candidate genes which are involved in the development of inflammatory processes (Table 1). In general, the main genetic associations can be divided into genes that contribute to the innate immune response and those that contribute to the acquired immune response. In the innate immune arm, the association of CD with polymorphisms in NOD2^22,23 and the two autophagy-related genes ATG16L1 and IRGM^24–26 suggests that alterations in the recognition and intracellular processing of bacterial components may have a role in the immunopathogenesis of the disease.²⁷ A link between nucleotide-binding oligomerization domain (NOD) protein and the autophagy protein ATG16L1 has been recently reported by showing that these two proteins form a complex at the cytoplasmic membrane.²⁸ This highlights the hypothesis of autophagy deficiency in IBD patients since mutations in NOD or the ATG16L1 protein result in diminished and ineffective autophagic response to intracellular pathogens.

Table 1.

Chromosomal loci of susceptibility for IBD

Genetic marker	Locus	Candidate gene(s)	Proposed role(s) in inflammation	Related disease	Associated polymorphism(s)	Reference(s)	PMID
IBD1	16q12	NOD2/CARD15, CD19, sialophorin, CD11 integrin cluster, IL4R	Detection of cytosolic bacterial components	CD; UC?	SNPs D16S409 and D16S419	Hugot, et al. 1996	11385576
					SNPs D16S409 and D16S753	Cavanaugh, et al. 1998	9924607
					SNP D16S3117	Zouali, et al. 2001	11781683
IBD2	12p13.2-q24.1	STAT6, VDR, IFNγ	Development of T-cell response	CD + UC	SNP D12S83	Duerr, et al. 1998	9634527
IBD3	6p21.3	MHC HLA-DQA1, HLA-DRA, HLA-DRB5 and HLA-DRB1, TNFalpha, TNFbeta, IL10, BTLN2	Detection of non-self components	CD + UC	SNPs D6S289 and D6S276	Hampe, et al. 1999	10053016
IBD4	14q11-q12	MMP14, DAD1, TCR	Development of T-cell response	CD	SNP D14S261	Duerr, et al. 2000	10747815
IBD5	5q31	IL3, IL4, IL5 and IL13, OCTN1, OCTN2, CSF2, SLC22A5	Regulation of inflammation	CD	SNPs rs6596075 and rs2188962	Rioux, et al. 2000–2001	11586304, 10777714
IBD6	19p13	ICAM-1, C3, TBXA2R, LTB4H	Diapedesis, stimulation of neutrophil response	CD + UC	ns	Van Heel, et al. 2003	12928481
IBD7	1p36	PADI4	ns	CD + UC	SNPs D1S1597, D1S507 and D1S1628	Cho, et al. 1998–2000	9636179, 10814724
IBD8	16p	ns	ns	UC	SNP D16S408	Annese, et al. 2003	14627676
IBD9	3p26	DEC1	Anti-neoplasia	CD + UC	SNP D3S1297	Duerr, et al. 2002	12354785
IBD10	2q37.1	ATG16L1	Autophagy	CD	SNP rs2241880	Hampe, et al. 2007	17200669
IBD11	7q22	MUC3A	Protection of the epithelial barrier	CD + UC	SNP D7S669	Satsangi, et al. 1996	8841195
IBD12	3p21	MST1, BSN, GNAI2	Regulation of proinflammatory mediators expression	CD + UC	SNP D3S2432	Paavola, et al. 2001	11378820
IBD13	7q21.1	ABCA1	Regulation of glucocorticosteroids production?	CD + UC	SNP D7S669	Satsangi, et al. 1996	8841195
IBD14	7q32	IRF5	Regulation of proinflammatory mediators expression	CD + UC	CGGGG insertion in the promoter of the gene IRF5	Dideberg, et al. 2007	17881657
IBD15	10q21	ZNF365	Regulation of proinflammatory mediators expression?	CD + UC	SNP rs224136	Rioux, et al. 2007	17435756
					SNP rs10761659	Fisher, et al. 2008	18438406
					SNP rs10995271	Barrett, et al. 2008	18587394
IBD16	9q32	TNFSF15	Costimulation of activated T CD4+ cells	CD + UC	SNP D9S2157	Cho, et al. 1998	9636179
					SNPs rs3810936, rs6478108, rs6478109, rs7848647 and rs7869487	Yamakasi, et al. 2005	16221758
					SNP rs4263839	Barrett, et al. 2008	18587394
IBD17	1p31.1	IL23R, PTGER4	Generation and maintenance of Th17 cells; prostaglandins signaling	CD + UC	R381Q substitution in IL23R (rs11209026)	Libioulle, et al. 2007	17447842
					SNP rs7517847	Rioux, et al. 2007	17435756
					SNP rs4613763	Barrett, et al. 2008	18587394
IBD18	5p13.1	Gene modulating PTGER4?	Prostaglandins signaling?	CD + UC	SNP rs1373692	Libioulle, et al. 2007	17447842
					SNP rs4613763	Barrett, et al. 2008	18587394
					SNP rs1992660	Franke, et al.	2007 18438405
IBD19	5q33.1	IRGM, IL12B	Autophagy	CD	SNP IL12B rs6887695	Parkes, et al. 2007	17554261
					SNP IL12B rs6556416	Fisher, et al. 2008	18438406
					SNPs IRGM rs4958847 and rs4958427	Franke, et al. 2007	18438405
					SNPs IRGM rs11747270 and rs10045431	Barrett, et al. 2008	18587394
IBD20	10q23-q24	DLG5, NKX2-3	Maintaining epithelial integrity	CD + UC	SNPs rs11190140 and rs10883365	Franke, et al. 2008	18438405
					SNPs D10S547 and D10S20	Hampe, et al. 1999	10053016
					SNP rs10883365	Parkes, et al. 2007	17554261
IBD21	18p11	NDUFV2, VAPA, PACAP, PTPN2	TCR signaling	CD + UC	SNP PTPN2 rs2542151	Parkes, et al. 2007	17554261
IBD22	17q21	STAT3, ORMDL3	Development of T cell response	CD	SNP ORMDL3 rs2872507	Barrett, et al. 2008	18587394
					SNP STAT3 rs744166	Franke, et al. 2008	18438405
IBD23	1q32	IL10	Regulation of inflammatory mediators production	CD + UC	SNP-1082G	Fowler, et al. 2005	15937090
					SNP rs17419032	Franke, et al. 2008	18438405
					SNP rs3024505	Franke, et al. 2008	18836448
IBD24	20q13	TNFRSF6B or DCR3	Costimulation of activated T CD4+ cells	CD + UC	SNPs rs2315008 and rs4809330	Kugathasan, et al. 2008	18758464
IBD25	21q22	PSMG1	Autophagy	CD + UC	SNP rs2836878	Kugathasan, et al. 2008	18758464
IBD26	12q15	ns	ns	UC	SNPs rs1558744, rs7134599, rs2870946	Silverberg, et al. 2009	19122664
IBD27	13q13.3	ns	ns	CD	SNPs rs20411	Shugart, et al. 2008	18246054
				UC	SNPs rs17085007	Asano, et al. 2009	19915573
IBD28	21q22	IL10RA	Regulation of inflammation	CD + UC	SNPs rs2836878	Glocker, et al. 2009	19890111

ns, not specified; ABC, ABC transporter; ATG16L1, autophagy-related protein 16-1; BSN, Bsn nuclease; C3, complement C3; CD, cluster of differentiation or Crohn disease; CSF, colony stimulating factor; DAD, daughters against ddp protein; DCR, decoy receptor; DEC, differentially expressed in chondrocytes; DLG, disc large homolog; GNAI, guanine nucleotide binding protein, alpha inhibiting; IBD, inflammatory bowel disease; ICAM, intercellular adhesion molecule; IL, interleukin; IFN, interferon; IRF, interferon regulatory factor; IRGM, immunity-related GTPase family, M protein; HLA, human leukocyte antigen; LTB4, leukotriene B4; MHC, major histocompatibility complex; MMP, matrix metalloprotease; NFκB, nuclear factor kappaB; MST, microtubule stabilizing; MUC, mucin; NDUFV, NADH dehydrogenase ubiquinone flavoprotein; NKX, homeodomain protein NKX; NOD2/CARD15, nucleotide-binding oligomerization domain/caspase-activating recruitment domain 15; OCTN, organic cation transporter; ORMDL, orosomucoid (ORM)-like protein family; PACAP, pituitary adenylate cyclase-activating polypeptide; PADI, peptidylarginine deiminase; PSMG, proteasome assembly chaperone; PTGER, G-protein coupled, E-series prostaglandin receptor; PTPN, protein tyrosine-phosphatase; SLC, chemokine CCL21; SNP, single nucleotide polymorphism; STAT, signal transducers and activator of transcription; TBXA2, thromboxane receptor A2; TCR, T-cell receptor; TGF, transforming growth factor; TLR, toll-like receptor; TNF, tumor necrosis factor; TNFSF, tumor necrosis factor superfamily; UC, ulcerative colitis; VAPA, vesicle-associated protein A; VDR, vitamin D receptor; ZNF, zinc finger protein. Reference publication: Wellcome Trust Case Control Consortium: Genome-wide association study of 14,000 cases of seven common diseases and 3,000 shared controls. Nature 2007; 447: 661–78; PMID: 17554300.

Among a myriad of genes pinpointed by genetic linkage studies, alterations in genes of the innate immune system, such as NOD2, ATG16L1 and IRGM are specific to patients with CD and are not observed in those with UC. By contrast, multiple genes implicated in the IL-23 pathway, including IL23R (encoding a subunit for IL-23 receptor) and IL12B (encoding the p40 subunit of IL-12 and IL-23), STAT3 (encoding signal transducer and activator of transcription 3) and the homeodomain-containing transcription factor NKX2-3 have been shown to be associated with both UC and CD.^25,29–33 On the other hand, mutations in ECM1 (extracellular matrix protein 1) are specific to patients with UC.^21,31,32

Microbiota in IBD.

The importance of gut flora in IBD is directly supported by studies in a variety of murine models in which ‘spontaneous’ chronic colitis seems to be entirely dependent on the presence of a luminal flora.^34,35 Thus, colitis cannot develop when these lines are maintained in a germ-free gnotobiotic state, but rapidly emerges when they are reconstituted with bacteria that are considered normal constituents of luminal flora.^36,37 At least three not necessarily mutually exclusive theories can be proposed concerning the implication of bacteria in the aetiopathogenesis of IBD: (1) an involvement of a persistent pathogen; (2) an abnormally permeable mucosal barrier leading to excessive bacterial translocation; and (3) a breakdown in the balance between putative “protective” versus “harmful” intestinal bacteria (“dysbiosis”) which can promote inflammation.³⁸

The possibility that IBD is an infection has been debated since the first description of CD. Many infectious agents (viral, bacterial or parasitic agents; Table 2) have been suspected in IBD etiology but the strongest evidence comes from studies pointing to the role of bacteria. In the intestinal mucosa of patients with IBD, bacteria that can potentially be pathogenic such as Escherichia coli, Listeria monocytogenes, Yersinia enterocolitica and Mycobacterium avium paratuberculosis are found in excess.³⁹ Mycobacterium avium paratuberculosis was the first organism to be suggested as an IBD pathogen, and it has been argued that it fulfils Koch's postulates⁴⁰ and could be designated the cause of CD. Other organisms have been postulated as possible IBD pathogens, including various Helicobacter species, one of which has been identified in primate colitis while others are widely used in animal models of IBD.

Table 2.

Targeted search for micro-organisms in tissues of patients with IBD

Virus	Bacteria	Parasite
Adenovirus	Brucella spp.	Borrelia spp.
Epstein-Barr virus	Chlamidia spp.	Toxoplasma gondii
Cytomegalovirus	Campylobacter spp.	Treponema spp.
Flu virus	Escherichia coli
Coronavirus	Helicobacter spp.
Rotavirus	Legionella spp.
Measles virus	Mycobacterium spp.
Respiratory syncytial virus	Pseudomonas spp.
Para-influenza type 2 and 3 virus	Salmonella spp.
Bovine viral diarrhea virus	Staphylococcus spp.
	Streptococcus spp.
	Shigella spp.
	Yersinia spp.
	Enterococcus
	Listeria monocytogenes
	Bacteroides vulgatus
	Bacteroides burgdorferi

Ssp, subspecies.

Escherichia coli appear specific to ileal CD and have been shown to induce the release of TNFα, a key cytokine in IBD inflammation.^41,42 Increased numbers of mucosa-associated E. coli forming a biofilm on the surface of the gut mucosa are observed in patients with IBD.^43–49 Bacteria colonizing the gut mucosa have the ability (1) to strongly adhere to intestinal epithelial cells (IEC),^44,46 (2) to invade IEC by a mechanism involving actin polymerisation and microtubule recruitment,^50,51 (3) to survive and replicate inside macrophages inducing the release of large amounts of TNFα and (4) to induce granuloma formation in vitro.^42,52,53 Based on the pathogenic characteristics of CD-associated E. coli, an additional pathogenic group of E. coli was defined and named AIEC for Adherent-Invasive E. coli. Such pathogenic strains are achieving increasing relevance since various studies performed in France,⁵⁴ the United Kingdom,⁴⁶ Spain⁵⁵ and the USA^56–58 have reported them to be more prevalent in CD patients than in controls. The presence of AIEC in healthy subjects suggests that AIEC are facultative pathogens that cause disease in susceptible hosts. AIEC strains were found to be highly associated with ileal mucosa in CD patients, suggesting that there are specific alterations to the ileal epithelial cells in patients with CD that allow AIEC to adhere to a greater extent. The receptor involved in AIEC colonization and abnormally expressed on ileal mucosa in 35% of CD patients was characterized as being the carcinoembryonic antigen-related cell adhesion molecule 6 (CEACAM6).^59,60 Recently, the endoplasmic reticulum (ER) stress protein Gp96 was found to be overexpressed on the apical surface of ileal epithelial cells in CD patients and to act as a host cell receptor for outer membrane vesicles, promoting AIEC invasion.⁶¹

Dysbiosis could be a key factor in the immunopathogenesis of IBD by disrupting the host immune defences to commensal flora microbes at the mucosal border. Arguments in favour of dysbiosis are: (1) the low proportion of Faecalibacterium prausnitzii, i.e., bacteria with anti-inflammatory properties, on ileal CD mucosa,^39,62 and (2) the abnormal colonization of the ileal mucosa by AIEC. In line with this hypothesis, a recent twin study revealed specific imbalances in the mucosa-associated microbiota.⁶³ Large interindividual variation in the composition of the intestinal microbiota between unrelated individuals has made it challenging to identify specific aspects of dysbiosis that lead to CD. The microbial compositions at all biopsy locations for each individual were similar, regardless of disease state, but differences between individuals were observed. In particular, individuals with predominantly ileal CD had a dramatically lower abundance of Faecalibacterium prausnitzii and increased abundance of Escherichia coli compared to healthy co-twins and those with CD localized in the colon. This dysbiosis was significantly correlated to the disease phenotype rather than genotype.⁶³ Dysbiosis can promote the growth of invasive pathogenic bacteria and also can facilitate bacterial translocation through the intestinal mucosal barrier to the mesenteric lymph nodes. These two phenomena contribute to the permeabilization process of the epithelial barrier, which is the prerequisite for the activation of the mucosal immune response.

Immunopathogenesis of IBD

In IBD, the immune defense against intestinal microbes fails at two levels: (1) the epithelial mucosal barrier is impaired and (2) the innate and acquired host immune responses are altered. The immunopathogenesis of IBD occurs in three temporally distinct stages: (1) penetration of luminal contents into underlying tissues which may be facilitated by environmental factors such as infection or inherent defects in mucosal barrier, (2) impaired clearance of foreign material from the bowel wall which may be due to defective secretion of pro-inflammatory cytokines by macrophages and (3) a compensatory acquired immune response which leads to a chronic inflammatory response and gives rise to characteristic IBD lesions.

Alterations of the mucosal epithelial barrier.

The intestinal barrier (Fig. 1) consists of a bacterial biofilm, a layer of mucus and an intestinal epithelium in which lie the innate immune system (dendritic cells, Paneth cells, macrophages and neutrophils). The intestinal epithelial barrier normally protects the body from potential bacterial threats but in IBD each of these defences is impaired. Indeed, the number of goblet cells secreting mucins, which constitute the protective mucus in the intestinal epithelium, is decreased in IBD. In addition, the use of microarrays has revealed that genes encoding mucins are underexpressed in the ileum and colon of patients.⁶⁴ The cohesion and tightness of the intestinal mucosa depend on the cell junctions of IEC. Defects in the structure and function of apical junctional complexes (AJC) are implicated in both animal models of IBD and in IBD patients.^65,66 For example, deregulated epithelial barrier function, specifically tight junctions, precedes histological evidence of ileitis in SAMP mice.⁶⁷ Moreover, barrier dysfunction precedes relapse of CD in asymptomatic patients as proteins forming the tight junctions of enterocytes (occludin, cadherins and catenins) are underexpressed⁶⁸ and studies demonstrate a specific distribution pattern of the tight junction proteins claudin 2, 3, 4, 5 and 8 in IBD patients.^69–71

Figure 1.

Most important mechanisms of defence in the intestinal epithelial barrier. Invasion of the mucosa is prevented by physical defences (mucus layer secreted by goblet cells, cellular barrier formed by epithelial cells) and by epithelial immune cells (Paneth cells and M cells). Recognition of pathogenic bacterial components is performed via TLR and NOD receptors. The inflammatory reaction involves mucosal immune system activation (mononuclear cells, lymphocytes and dendritic cells of the subepithelial dome). Inflammation is mediated by cytokines (TNF, IL) secreted by these different immune cells. EGF, epithelial growth factor; GM-CSF, granulocyte macrophage-colony stimulating factor; FAE, follicle-associated epithelium; ITF, intestinal trefoil factor; IL, interleukin; NOD2, nucleotide oligomerization domain 2; SED, subepithelial dome of the Peyer patch; Th, T helper lymphocyte; TLR, toll-like receptor; TNF, tumor necrosis factor. Source: Med Sci (Paris) 2010; 26:405–10.

Intestinal permeability is significantly increased in 36% of CD patients⁷² and bacterial translocation of E. coli, Enterococcus spp., Clostridium perfringens has been observed in mesenteric nodes in 30–50% of CD patients versus 5–15% in healthy controls.^73,74 This could be due to: (1) bacterial uptake via Peyer's patches and (2) bacterial translocation due to abnormal tight junctions. Increased intestinal permeability has been shown in CD patients carrying NOD2 polymorphisms.⁷⁵ Consistently, abnormal development of Peyer's patches as well as increased permeability and bacterial translocation were observed in NOD2 deficient mice.⁷⁶ The findings that increased E. coli translocation occurs in follicle-associated epithelium or in Peyer's patches isolated from CD patients compared with non-IBD patients^77,78 and that increased numbers of Peyer's patches are found in CD, represent novel insights into CD pathophysiology by a previously unrecognized defect of intestinal barrier function in ileal CD.

Moreover, epithelial cells are the first line of defence against invading pathogens. In constant interaction with the intraluminal flora, they are able to identify pathogenic bacterial components via extracellular receptors to bacterial products like TLR (toll-like receptors) and via intracellular receptors like NLR (NOD-like receptors), in particular NOD2 receptor (or NOD2/CARD15 receptor, Nucleotide-binding Oligomerization Domain 2/Caspase-Activating Recruitment Domain 15 receptor). Once activated through TLR and NOD2 receptors, epithelial cells produce antimicrobial peptides (human β-defensins, HBD) and express MHC molecules to trigger the mucosal acquired immune response. In IBD, there is a functional defect of the immune barrier formed by antimicrobial peptides.⁷⁹ Indeed, UC is associated with a very high production of HBD2, 3 and 4.⁸⁰ However, the induction of these HBD is low in CD. The lack of induction of HBD2 in patients suffering from colonic CD could be explained by a reduced copy number of the HBD gene.⁸¹ Paneth cells, which are specialized cells located in intestinal crypts, also secrete antimicrobial peptides (human α-defensins, HD) following the binding of pathogenic bacterial components to TLR and NOD receptors. In UC, the production of HD5 and 6 is greatly increased whereas in ileal CD, a sharp decrease in the production of HD is observed.^82–84 The lack of HD5 synthesis in ileal CD could result from a defect in the biology of Paneth cells caused by mutations in NOD2. Besides, NOD2-deficient animals are susceptible to Listeria monocytogenes infection via the oral route but not through intravenous or peritoneal delivery. Expression of defensin-related cryptdin 4 (Defcr4) and Defcr-related sequence 10 (Defcr-rs10) was very low in NOD2^-/- mice and was further reduced after infection in NOD2^-/- animals relative to wild-type mice.⁸⁵ Cryptdins are antimicrobial peptides that are preferentially produced in intestinal Paneth cells, and their antimicrobial activity is important in suppressing infection with pathogenic bacteria, including L. monocytogenes⁸⁶ and Mycobacterium paratuberculosis, an organism implicated in CD.^87,88 Overall, current data suggest that, in UC, changes in expression of defensins are secondary to inflammation while they could cause impairment of innate mucosal immunity in ileal CD.

Innate immune response deregulation.

The weakening of mucosal defences promotes intestinal epithelium permeabilization resulting in more frequent contacts between the commensal flora and the mucosal immune system. In CD, these interactions would still be facilitated through a lack of bacterial clearance by macrophages in which secretion of inflammatory cytokines is defective.⁸⁹ An excess of such interactions would cause a loss of tolerance to commensal flora by activating mucosal dendritic cells, the sentinel cells of innate immunity (Fig. 1).

Dendritic cells (DC) are at the interface between IEC and T cells. They present antigens to naive CD4⁺ helper T cells (Th0) and ensure tolerance to commensal flora by promoting their differentiation into regulatory T cells (Treg). In case of infection, DC activated by their TLR and NOD2 receptors produce pro-inflammatory cytokines and promote the differentiation of effector T cells Th1, Th2 and Th17 (Fig. 2), which triggers a local persistent inflammation.⁹⁰ The interplay between secreted cytokines determines the balance between the different types of CD4⁺ effector T cells.⁹¹ In IBD, an overactivation of DC was observed at sites of inflammation.⁹² DC overactivation induces a strong differentiation of effector lymphocytes (CD4⁺ and CD8⁺) and of other effector cells such as natural killer (NK) and NK T (NKT) cells while abolishing the production of regulatory cells. The absence of Treg lymphocytes contributes to the development of an immune response to commensal bacteria normally tolerated by the mucosal immune system (peripheral tolerance). Loss of peripheral tolerance then perpetuates inflammation.

Figure 2.

Differentiation of naive T cells (Th0). CD, cluster of differentiation; Th, T helper lymphocyte; IFNγ, interferon gamma; IL, interleukin; TGFβ, transforming growth factor β. Source: Med Sci (Paris) 2010; 26:405–10.

DC overactivation is mainly the consequence of abnormalities in TLR and NOD receptor function and the consecutive inability to detect bacterial components. Indeed, in IBD, alterations in epithelial mechanisms of innate immunity could be caused by changes in TLR expression patterns. The hypothesis of a TLR-dependent dysfunction of the immune response to bacteria in the intestinal lumen is supported by the description of polymorphisms in genes coding TLR1, TLR2, TLR4 and TLR6 which are associated with increased risk of developing IBD.⁹³ The existence of polymorphisms of the NOD2 gene is another argument which suggests a defective mechanism in antigen recognition by TLR and NOD2. In fact, 10–15% of CD patients are homo or heterozygous carriers of one of the three major NOD2 mutations (Gly908Arg, Arg702Trp and Leu1007fs).^22,23 “Knock-in” mice were used to evaluate the functions of the NOD2 Leu1007fs variant and showed that the susceptibility to develop colitis induced by dextran sodium sulphate (DSS) is higher in mice with such variant to the mouse NOD2 locus than in wild-type mice. DSS-induced colitis is associated with an overactivation of the nuclear factor kappaB (NFκB) pathway and increased secretion of IL1β.⁹⁴ These results, which are contradictory with clinical data that correlate NOD2 mutations to decreased NFκB activity in patients, could be explained by the existence of a negative regulation exerted by NOD2 on the TLR2 signalling pathway. When the NOD2 mutation Leu1007fs is present, inhibition of the TLR-NFκB pathway by NOD2 would not take place, leading to increased production of pro-inflammatory cytokines in response to contact with commensal bacteria and/or pathogens. The TLR2 signalling disruption and the development of a Th1 immune response with excessive production of pro-inflammatory cytokines reported in these “knock-in” mice give weight to this hypothesis.⁹⁵ Thus, in patients who carry these NOD2 variants, the production of proinflammatory cytokines following immune cell stimulation by bacterial components and the elimination of intracellular microbes are very likely to be altered.⁹⁶

Acquired immune response deregulation.

In IBD, alterations in innate immune responses disrupt antigen recognition and presentation to effector cells (Fig. 1). Consequently, the epithelial barrier is more vulnerable to infections and activation of the mucosal immune system is deregulated. When IBD is active, there is an imbalance in the number of Th and Treg cells.⁹⁷ In CD, Th1 lymphocytes, characterized by elevated production of IL2, IL12 and IFNγ, are predominant.⁹⁸ In contrast, in the mucosa of patients with UC, there is mostly an infiltration of atypical Th2 lymphocytes which are T cells characterized by the production of IL5, IL13 and TGFβ.⁹⁹

MICA, a MHC-related class Ib molecule expressed on IEC, could be involved in the activation of mucosal lymphocytes. Under normal conditions, MICA expression is restricted to intestinal and thymic epithelium. This expression can be induced by stress in different epithelial cells and is upregulated upon exposure to intracellular pathogens,^100–102 in particular E. coli.¹⁰³ MICA is a ligand of the NKG2D activating receptor preferentially expressed on CD8⁺ T cells, γδ T cells and NK cells.^104,105 A subset of CD4⁺ T cells expressing NKG2D was increased in the lamina propria from patients with CD compared to controls or UC.¹⁰⁶ CD4⁺NKG2D⁺ T cells with Th1 cytokine profile and expressing perforin were increased in the periphery and in the mucosa in CD and mediate inflammatory and cytotoxic responses through MICA interaction.¹⁰⁶

Moreover, others studies have pointed to a new T cell population defined as Th17 lymphocytes that contribute to the shift towards effector T cells over regulator T cells populations in IBD.¹⁰⁷ Th17 lymphocytes (CD4⁺ CD25⁻) are T cells which produce the proinflammatory cytokine IL17, especially in response to the presence of extracellular bacteria. Th17 differentiation of naive Th0 cells is induced by the coexpression of IL23 and TGFβ¹⁰⁸ whose role in determining the balance between Treg lymphocytes (anti-inflammatory) and Th17 lymphocytes (proinflammatory) is essential (Fig. 2). Genetic association studies showing an involvement of the IL23R gene in IBD pathogenesis^29,31 have thus led naturally to suspicions that effector subpopulation of T cells which possess strong inflammatory activity promote the activation and local accumulation of neutrophils at the site of tissue inflammation and induce the production of antimicrobial peptides as HBD.¹⁰⁹ The current assumption is that there is a contribution of the IL23R signaling pathway to inflammation via impaired synthesis of antimicrobial peptides and by promotion of the Th17 pro-inflammatory state. IL23 stimulates the development and expansion of pathogenic memory T cells and ensures the survival and clonal expansion of Th17.¹¹⁰

Besides the role of cytokines such as IL23, changes in the pattern of expression of chemokines (or chemo-attractive cytokines) can also explain the abnormal influx of immune effector cells in the intestinal mucosa. The expression of many chemokines (in particular IL8; granulocyte chemotactic protein-2, GCP2; growth regulated protein α and β, GROα and GROβ; epithelial neutrophil activating protein 78, ENA-78; etc.,) and the expression of their respective receptors (CXCR1, CXCR2, etc.) are increased during the active phase of IBD (Table 3).¹¹¹ In IBD, a defect in chemokine production or a defect in the regulation of chemokine signal transduction pathways could contribute to the loss of epithelial integrity by inducing local production of free radicals^112,113 and by inducing an influx of leukocytes in the mucosa by means of neo-angiogenesis.¹¹⁴

Table 3.

Chemokines and receptors with a suspected role in IBD pathophysiology

Name	Gene	Other names	Receptor	Uniprot	Related disease	Target cells
CC chemokines
CCL2	Scya2	MCP1	CCR2	P13500	CD + UC	Th/mono/baso
CCL3	Scya3	MIP1a	CCR1	P10147	CD + UC	Tc
CCL4	Scya4	MIP1b	CCR1, CCR5	P13236	CD + UC	Th
CCL5	Scya5	RANTES	CCR5	P13501	CD + UC	Th/mono/baso
CCL7	Scya7	MARC, MCP3	CCR2	P80098	CD + UC	mono/baso
CCL8	Scya8	MCP2	CCR1, CCR2B, CCR5	P80075	CD + UC	neutro/baso/mono
CCL25	Scya25	TECK, Ckb15	CCR9	O15444	CD + UC	Th17
CXC chemokines
CXCL1	Scyb1	Gro-a, GRO1, NAP3, KC	CXCR2	P09341	CD + UC	neutro/baso
CXCL2	Scyb2	Gro-b, GRO2, MIP2a	CXCR2	P19875	CD; UC?	neutro/baso
CXCL5	Scyb5	ENA78	CXCR2	P42830	CD + UC	neutro
CXCL6	Scyb6	GCP2	CXCR1, CXCR2	P80162	CD + UC	neutro/baso
CXCL8	Scyb8	IL8, NAP1, MDNCF, GCP1	CXCR1, CXCR2	P10145	CD + UC	neutro/baso
CXCL10	Scyb10	IP10, CRG2	CXCR3	P02778	CD + UC	T/mono
CXCL12	Scyb12	SDF1, PBSF	CXCR4	P48061	CD + UC	T/B
CX3C chemokines
CX3CL1	Scyd1	Fractalkine, neurotactin, ABCD3	CX3CR1	P78423	CD + UC	mono/DC

CD, Crohn disease; DC, dendritic cell; Th, T helper cell; Tc, T cytotoxic cell; mono, monocyte; baso, basophil; neutro, neutrophil; UC, ulcerative colitis.

Finally, UC and CD are associated with a humoral response characterized by infiltration of plasma B cells.¹¹⁵ In the mucosa, the levels of G₁ immunoglobulins (IgG1), IgG2, IgM and IgE are increased while the secretory IgA concentration is decreased.^116–118 In addition, overactivation of B cells in IBD results in a high production of mucosal IgG auto-antibodies directed against commensal bacterial antigens from the intestinal lumen.^118–120 These data suggest a shift from antibodies of the protector subtype (IgA) to the aggressive subtype (IgG) leading to mucosal inflammation.

Conclusion and New Therapies

In IBD, the existence of a genetic vulnerability leads to disrupted identification and presentation of intestinal antigens to effector cells. The subsequent inappropriate adaptive immune response results in loss of tolerance to commensal flora and to amplification and maintenance of the inflammatory response to intestinal pathogens, especially in CD where there is a weakness of the immune system. In parallel to inflammation, infiltration of immune cells in the intestinal mucosa and in the proximity of nerve endings leads to enteric neuro-immune direct contacts. These interactions cause the activation of visceral afferents which is the first step to the development of chronic abdominal pain consecutive to inflammation.

The main goal of therapy for IBD is to induce a clinical remission and then maintain it for a long period of time, in order to realize the best attainable quality of life. Choice of therapy depends on the severity and location of disease, as well as side effects and other adverse events. Recent discoveries have led to the development of novel therapeutic drugs that regulate activation of the cellular immune response and inflammation (antibodies against cytokines and their receptors, antibodies blocking T-cell costimulatory molecules, etc.,) as a complement to the standard anti-inflammatory treatments based on corticosteroids and 5-amino-salicylic acids. In CD, new therapeutic avenues with immunostimulation strategies are now envisaged to overcome the primary immunodeficiency in patients and to reduce complications caused by immunosuppressive drugs and by antibodies against pro-inflammatory cytokines.⁹¹ Several new drugs have been introduced in the last decade or are under investigation for the treatment of IBD. They include anti-TNFα agents, anti-adhesion molecules, anti-IL12/23, anti-IL6R and others. A novel therapeutic target for IBD could be the interaction between MICA and NKG2D since blockade of NKG2D signaling prevents the development of murine CD4⁺ T-cell mediated colitis¹²¹ and inhibition of NKG2D receptor function using antibodies attenuates transfer-induced colitis in SCID mice.¹²² To conclude, current therapies generally involve a combination of pharmacologic agents and dietary manipulation. Newer agents including monoclonal antibodies targeted to specific pro-inflammatory cytokines and adhesion molecules as well as the induction of anti-inflammatory cytokines and T-cell activation, have emerged and provided clinical benefit in the treatment and relapse of IBD.

Abbreviations

AIEC

adherent-invasive E. coli

CEACAM6

carcinoembrionic antigen-related cell adhesion molecule 6

CD

Crohn disease

DC

dendritic cell

HBD

human β-defensin

HD

human α-defensin

IBD

inflammatory bowel disease

IL

interleukin

MHC

major histocompatibility complex

NFκB

nuclear factor kappaB

NOD2/CARD15

nucleotide-binding oligomerization domain 2/caspase-activating recruitment domain 15

TLR

toll-like receptor

UC

ulcerative colitis

Authorship

Wrote the paper: J.M. and N.B. Made the tables and the illustrations: J.M. Supervision and final approval of the manuscript: D.A.