Serologies in Crohn’s Disease: Can We Change the Gray Zone to Black and White?
Article Outline
The goal of patients with inflammatory bowel disease and the physicians treating them should be to restore normal health. To that end, therapy for Crohn’s disease and ulcerative colitis continues to improve. Recent data from the Benelux countries and Germany have shown us that the early use of biologics with an immunomodulator, so-called “top-down therapy,” is an effective strategy for rapid induction of remission and maintenance of remission.1 Post-hoc data from the maintenance study of infliximab, ACCENT 1, suggest that maintenance infliximab reduces the need for hospitalization and surgery in patients with Crohn’s disease.2 These exciting observations are tempered by new reports of toxicity with the use of combinations of biologics and immunomodulators. While clinicians can see the potential merits of early use of biologics, we are struggling to identify which patients should receive this therapy. Concerns about toxicity and cost diminish the enthusiasm for over-treating patients whose disease course will be mild.
See article on page 366.
In that context, clinicians are eager to have better ways to predict disease outcome in inflammatory bowel disease. In lieu of a crystal ball, are there any reliable ways to predict which patients with inflammatory Crohn’s disease (small bowel or colon) will go on to develop perforating complications and/or need surgery within a short period of time? Patients who present with extensive small bowel disease, perforating complications, or imminent need for surgery have already manifested an aggressive disease course and represent an obvious group for aggressive intervention. Several approaches are being explored to identify biomarkers that predict disease course. These include genetic, genomic, and proteomic approaches. A group of investigators in Spain have recently reported on an IBD-Chip that can simultaneously detect polymorphisms in 40 genes reported to have an association in inflammatory bowel disease.3 Combinations of these polymorphisms had high specificity but low sensitivity for identifying complicated Crohn’s disease. Genomic approaches would be those in which RNA from tissue (such as biopsies) or peripheral blood mononuclear cells is used to profile gene expression using microarrays. Finally, proteomic approaches use mass spectrometry to look at protein expression in tissue or blood. At present, however, serologies lead the pack for distinguishing between IBD and non-IBD, distinguishing between Crohn’s disease and ulcerative colitis, and stratifying Crohn’s disease phenotypes.
Serologic tests are based on the ability to detect the presence of antibodies in the serum of patients with inflammatory bowel disease. To review the serologies we have available, perinuclear anti-cytoplasmic Ab (pANCA) and anti-Saccharomyces cerevisae Ab (ASCA) are the most studied. Although pANCA is detected as antibody reactivity against neutrophils, it likely represents antibodies against colonic bacteria.4 ASCA are antibodies against oligomannans present on the cell wall of yeast. Data from a variety of centers and across ethnic groups have demonstrated that pANCA is associated with ulcerative colitis, whereas ASCA is associated with Crohn’s disease. Patients with Crohn’s colitis may also express pANCA.
The other serologies that are available include antibodies to the outer membrane porin type C of E coli (anti-OmpC) and antibodies to a protein expressed by Pseudomonas fluorescens, anti-I2.5 The most recently described serologic marker, anti-CBir1, is an antibody against flagellin expressed by a certain Clostridial phylum.6 Patients with Crohn’s disease and a small percentage of healthy controls or ulcerative colitis patients express these antibody markers.7 Using all of the serologic markers reported for Crohn’s disease, the sensitivity for diagnosing Crohn’s disease is greater than 80% and the positive predictive value is over 90% but only when the prevalence of Crohn’s disease is high, 38%.
There is still room for improvement in serologic markers for Crohn’s disease both with respect to identifying patients with Crohn’s disease, as well as predicting disease outcomes. The study by Dotan et al8 provides an important advance in the field of serologies for Crohn’s disease. In this study, Dotan et al8 describe the presence of antibodies to carbohydrate epitopes in patients with Crohn’s disease. Patients with Crohn’s disease express antibodies to laminaribioside (anti-laminaribioside carbohydrate [ALCA]) and chitobioside (anti-chitobioside carbohydrate [ACCA]). These antigens are found in cell walls of pathogenic bacteria and fungi. The identification of these antibodies adds to the weight of evidence that patients with Crohn’s disease have lost tolerance to luminal microbial antigens, likely because of an innate immune defect.
The first interesting aspect of this study is the methodology used to identify these antibodies. Glycans are composed of saccharide building blocks and have great combinatorial diversity based on the number of different linkages possible between saccharides. These investigators developed a glycan array consisting of mono- and oligosaccharides bound to a solid support by way of a long linker.9 This set-up permits the glycan to be presented to antibodies in an orientation found in native glycoconjugates, for example in bacterial cell walls. Using this glycan array, they first identified which glycan moieties were recognized by antibodies present in the serum of patients with inflammatory bowel disease. In addition to the 2 studies described previously, Crohn’s disease patient sera also reacted with mannans such as those used in the ASCA assay and provided internal validity for this approach. They next developed enzyme-linked immunosorbent assay (ELISA) assays for laminaribioside and chitobioside to permit larger-scale analyses.
The percentage of Crohn’s disease patients positive for ALCA and ACCA were 37.5% and 36%, respectively. ASCA testing was performed using a commercial ASCA ELISA kit and 65% of patients were positive for ASCA. Importantly, ALCA or ACCA were positive in 44% of patients in whom ASCA was negative, so it could help detect additional patients not identified by ASCA as a stand alone test. By contrast, <10% of patients with ulcerative colitis or miscellaneous gastrointestinal conditions were positive for any of these markers. The investigators did not test these assays in indeterminate colitis but did find that 22% of patients with celiac disease were positive for ACCA but the number of patients tested was small (n = 27).
ALCA was better for distinguishing Crohn’s disease from ulcerative colitis. The sensitivity of any one Ab test being positive is 77% with a positive predictive value of 90% and a negative predictive value of 77%. The positive predictive value for distinguishing between Crohn’s disease and inflammatory or healthy controls is 76%. As expected, combinations of anti-glycan Abs result in greater specificity and positive predictive value when distinguishing Crohn’s disease from ulcerative colitis and Crohn’s disease from controls. We are not told the estimates for disease prevalence in these calculations so the positive and negative predictive values of these serologic tests in patient populations with a low prevalence of inflammatory bowel disease will be lower than these values.
They next turned their attention to the association of anti-glycan antibodies with Crohn’s disease phenotypes. As qualitative tests, ALCA tends to be more frequently positive in patients with penetrating disease compared with inflammatory disease (34% vs 25%) and ACCA is more commonly positive in stricturing disease compared with inflammatory disease (29% vs 18%) but the differences are small. Like Mow et al, the investigators performed quartile analyses to investigate whether qualitative differences in the level of Ab titers helped clarify phenotypic associations.10 The 3 Abs ASCA, ACCA, and ALCA were assigned a score between 1–4 for a range of results from 3 to 12. They found that high titers, quartile sums of 9–12 were positively associated with small bowel disease, >90%, compared with 55% for quartile sums of 3–4. High sum scores were also positively associated with penetrating or stricturing disease (69 vs 33%). Penetrating disease and stricturing disease were not separated in their study. These numbers are in the same general range as combinations of ASCA, anti-I2, and anti-OmpC.10 We are also not given data with respect to time from diagnosis to development of penetrating/stricturing complications or need for surgery. No association was seen for need for small bowel surgery but the rates of surgery were low across all the groups and were between 22% and 38%.
The currently available serologies, ASCA, anti-OmpC, anti-I2, and anti-CBir already provide pretty good information with respect to the extremes of disease severity. A patient with Crohn’s disease that is serologically negative or expressing one low titer serology is more likely to have inflammatory disease whereas a patient expressing multiple serologic markers at a high titer is more likely to have complicated small bowel disease (stricture ± perforation) and greater likelihood of surgery. Unfortunately for clinicians trying to interpret these tests, these extremes on the serologic spectrum are found in the minority of patients. For example, 26% of Crohn’s disease patients are simultaneously positive for ASCA, anti-I2, and anti-OmpC.9 Similarly, if one looks at the subset of patients with the highest quartile sums in the 10–12 range, this represents approximately 26% of the Crohn’s disease patients, and they are mirrored by 26% of patients in the lowest quartiles (sum scores 3–5). If one adds seroreactivity to CBir flagellin, one captures approximately 40% of the ASCA, anti-I2, anti-OmpC negative patients, and this further increases the association of serologies with perforating or stricturing disease phenotypes.7 One thing that is not known is how many patients with high serologies at the time of diagnosis have not already declared themselves to have an aggressive disease phenotype obviating the need fore serologies. Nevertheless, about half the patients with Crohn’s disease will have an intermediate phenotype based on these sets of serologies. The hope is that for this gray zone of patients additional markers will clarify phenotypic aspects that may guide therapy.
Crohn’s disease is a complex genetic and immunologic disorder with diverse clinical manifestations. Serologic markers offer a window into the pathogenesis of Crohn’s disease. They are a convenient read out of potential microbial antigens to which immunologic tolerance is lost. The association of Crohn’s disease with polymorphisms in innate immune genes such as Nod2/CARD15 and toll-like receptors (TLRs) suggest that dysfunction in the ability to achieve a robust innate immune response to luminal bacteria may elicit a secondary adaptive immune response. The fact that antibodies are generated to only certain commensal-associated molecular structures may speak to the interaction between these structures and specific pattern recognition receptors such as TLRs, differential uptake of microbes by antigen-presenting cells, or inherent differences in the potential for pathogenicity and tissue invasion by microbes.
It stands to reason that one biomarker will never be sufficient to diagnose Crohn’s disease or predict its disease course. The article by Dotan adds to our armamentarium of serologic tests that can help characterize patients with Crohn’s disease. These novel antibodies should not necessarily be compared with existing serologies. Ideally, the next step should be to combine all of the available serologic tests to determine which panel of antibodies provides the best operator characteristics for diagnosis or phenotyping. Indeed, the panel of antibodies that may be best for distinguishing Crohn’s disease from ulcerative colitis may be different from that which is useful for predicting Crohn’s disease course. Serologies can also be combined with genetic, genomic, or proteomic tests currently under development to further improve their usefulness. Prospective studies are needed to define the utility of these tests in truly predicting disease course. Clinicians caring for patients with inflammatory bowel disease should look forward to a time when their clinical acumen will be bolstered by sophisticated tests that help identify which patients needs aggressive early intervention and which patients can be treated expectantly.
References
- . Management of recent onset Crohn’s disease (a controlled, randomized trial comparing step-up and top-down therapy). Gastroenterology. 2005;129:371
- . Comparison of scheduled and episodic treatment strategies of infliximab in Crohn’s disease. Gastroenterology. 2004;126:402–413
- . IBDChip (A new strategy to predict clinical course and development of complications in patients with inflammatory bowel disease). Gastroenterology. 2006;130:A346
- . pANCA represents a cross-reactivity to enteric bacterial antigens. J Clin Immunol. 1998;18:153–160
- . Selected loss of tolerance evidenced by Crohn’s disease-associated immune responses to auto- and microbial antigens. Gastroenterology. 2002;123:689–699
- . Bacterial flagellin is a dominant antigen in Crohn disease. J Clin Invest. 2004;113:1296–1306
- . Antibodies to CBir1 flagellin define a unique response that is associated independently with complicated Crohn’s disease. Gastroenterology. 2005;128:2020–2028
- . Antibodies against laminaribioside and chitobioside are novel serologic markers in Crohn’s disease. Gastroenterology. 2006;131:366–378
- . A new kind of carbohydrate array, its use for profiling antiglycan antibodies, and the discovery of a novel human cellulose-binding antibody. Glycobiology. 2003;13:749–754
- . Association of antibody responses to microbial antigens and complications of small bowel Crohn’s disease. Gastroenterology. 2004;126:414–424
PII: S0016-5085(06)01432-6
doi:10.1053/j.gastro.2006.06.040
© 2006 American Gastroenterological Association Institute. Published by Elsevier Inc. All rights reserved.
Refers to article:
- Antibodies Against Laminaribioside and Chitobioside Are Novel Serologic Markers in Crohn’s Disease
