ArticlesDefective acute inflammation in Crohn's disease: a clinical investigation
Introduction
Crohn's disease is a chronic inflammatory disorder, the incidence of which rose greatly in the latter part of the 20th century.1 It primarily affects the bowel, but can also involve the musculoskeletal system, skin, and eyes. The origin remains an enigma, although many diverse causes have been proposed.2 The diagnosis is currently established on the basis of clinical, endoscopic, radiological, and histological findings. Characteristic pathological appearances include the formation of skip lesions (discrete regions of inflamed bowel separated by uninvolved mucosa), aphthous ulceration, and fistulation; these signs relate to the presence of an underlying granulomatous transmural inflammation. The search for a microbial cause has been extensive because of the similarity between features of Crohn's disease and those produced by infection with organisms such as mycobacteria.3 The other main theory is that inflammation arises as a primary autoimmune process, for which both humoral and cellular mechanisms have been implicated.4
Detailed genome-wide linkage analyses in affected families have identified several susceptibility loci (IBD1–IBD8) for Crohn's disease.5 The strongest association is attributable to the CARD15 gene located within the IBD1 locus.6, 7, 8 The risk of Crohn's disease is increased two to four times in the presence of a single disease-associated polymorphism; it is increased 20–40 times if two variant alleles are present.
The mechanism of this predisposition remains unclear. CARD15 encodes a cytoplasmic protein, NOD2, which is expressed predominantly in mononuclear phagocytes. The NOD2 protein is implicated in recognition of muramyl dipeptide (MDP), a component of peptidoglycan that is present in cell walls of gram-positive and gram-negative bacteria9, 10 and is found in high concentrations in the bowel lumen.11 Binding of MDP activates the transcription factor nuclear factor (NF) κB,9, 10 leading to induction of genes for proinflammatory cytokines.12 Consequently, the polymorphisms associated with Crohn's disease that abolish the response to MDP13 should attenuate inflammation. This prediction contrasts both with the pathology of Crohn's lesions, which are obviously inflamed and contain proinflammatory cytokines,14 and with the therapeutic efficacy of immunosuppressive drugs.15 The situation is further confused by observations from studies in transgenic mice with Nod2 mutations that produce a truncated protein.16 Although this mutation was believed to be identical to one of the human polymorphisms, the effect seemed to be opposite: MDP induced greater activation of NFκB and more efficient processing and secretion of interleukin 1β. This finding supported a proinflammatory mechanism.
Despite the robust association, CARD15 is not strongly mechanistically related to the causation of Crohn's disease.17, 18 Polymorphisms show very limited penetrance6 and occur in only 40% of patients (predominantly those with ileal disease19) as well as in 15% of healthy individuals. The possibility exists that polymorphisms in CARD15 are not in themselves causal, but modify the immune response in lesions that are elicited by some other mechanism.
An alternative theory20 is that the acute inflammatory response fails in Crohn's disease, leading to delayed or incomplete removal of bacteria and other bowel contents that breach the mucosal barrier. Subsequent persistence of this foreign material within the tissues could provoke a granulomatous reaction and produce a secondary chronic inflammation. We undertook to examine acute inflammatory responses in Crohn's disease and to relate them to the CARD15 genotype.
Section snippets
Patients
Patients who met inclusion criteria were identified in the gastroenterology outpatient clinics at University College London Hospitals (UCLH) and invited to participate in the study. Controls were identified through the gastroenterology and rheumatology outpatient clinics at UCLH or the Department of Medicine, University College London (UCL). Patients were recruited by DJBM, MWNH, SB, or AWS. Details of patients and controls included in each set of experiments are provided in the webtable and
Results
Figure 1 shows the effects of trauma on the intestinal mucosa. After rectal biopsies in nine controls, an acute inflammatory response ensued, with a large increase in numbers of neutrophils and interleukin 8-positive cells (figure 1). Accumulation of both cell types was substantially lower in traumatised rectal mucosa from eight patients with Crohn's disease (79% reduction [184·2 cells/hpf], n=8, p=0·0003 for neutrophils; 63% reduction [74·9 cells/hpf], n=8, p=0·003 for interleukin 8-positive
Discussion
Our investigations identified defective innate immunity in Crohn's disease. The findings showed reduced neutrophil accumulation and interleukin 8 production, not only at sites of acute inflammation in the bowel, but also in the skin, indicative of a general constitutional abnormality. That a systemic defect existed was confirmed by the deficient responses of patients' macrophages, cultured in vitro for 5 days, to acute inflammatory mediators.
What causes this abnormality, and what does it mean
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