Antibodies to GP2, the major zymogen granule membrane glycoprotein, in inflammatory bowel diseases
- 1Experimental Laboratory Medicine, Department of Medical Diagnostic Sciences, Katholieke Universiteit Leuven, Belgium
- 2Internal Medicine, University Hospitals Leuven, Leuven, Belgium
- Correspondence to Dr Xavier Bossuyt, Laboratory medicine University Hospitals Leuven Herestraat 49 CDG7 B-3000 Leuven, Belgium;
It was with interest that we read the paper by Roggenbuck et al1 in a recent issue of Gut, where GP2, the major zymogen granule membrane glycoprotein, is demonstrated to be the autoantigen of pancreatic antibodies in Crohn's disease. The authors concluded that quantification of this novel Crohn's disease-specific marker could significantly improve the serological diagnosis of inflammatory bowel disease. A commercial assay for the detection of anti-GP2 antibodies has recently become available (Generic Assays, Dahlewitz/Berlin). In the present paper, we evaluated this assay on a large well-characterised study population.
The patient cohort has been previously described2 and consisted of 164 patients with Crohn's disease, 118 patients with ulcerative colitis, and 75 control patients with other gastrointestinal diseases (non-inflammatory bowel disease diarrhoeal illnesses). One hundred blood donors were included as well. The diagnosis of ulcerative coliltis or Crohn's disease was based on accepted clinical and endoscopic criteria.3 The assay was performed according to the manufacturer's instructions (cut-off 15 U/ml).
The ELISA showed good linearity and only minor interference by haemoglobin, bilirubin and triglycerides (data not shown). The between-run coefficient of variation (n=13) was 7.5% for IgG (73.6 U/ml) and 5.8% for IgA (58.1 U/ml).
The results of IgG and IgA anti-GP2 antibodies in patients with ulcerative colitis, patients with Crohn's disease, controls with other gastrointestinal diseases, and healthy controls are shown in figure 1. The reactivity found in patients with Crohn's disease was significantly higher in these patients than in patients with ulcerative colitis, gastrointestinal controls, and healthy controls (p<0.0003; Mann–Whitney). Of the 164 patients with Crohn's disease, 34 (20.7%) had IgG or IgA anti-GP2 antibodies (16 IgG, 8 IgA, 10 IgG and IgA). Of the 118 patients with ulcerative colitis, 11 (9.3%) had IgG or IgA anti-GP2 antibodies (8 IgG, 3 IgA). Of 75 controls with other gastrointestinal diseases, 3 (4%) had IgA anti-GP2 antibodies. Of 100 healthy controls, 4 (4%) had IgG or IgA anti-GP2 antibodies (3 IgG, 1 IgA). The prevalence of anti-GP2 antibodies was significantly higher in patients with Crohn's disease than in patients with ulcerative colitis (p=0.014; Fisher exact test), controls with gastrointestinal diseases (p=0.0007; Fisher exact test), and healthy controls (p=0.0001; Fisher exact test). The likelihood ratio for Crohn's disease was 5 and for ulcerative colitis was 2.3.
Next, we investigated whether anti-GP2 antibodies were associated with pancreatic antibodies. Table 1 summarises the prevalence of anti-GP2 antibodies as a function of the staining pattern of the pancreatic antibodies (intracellular or extracellular). The pancreatic antibodies were detected by indirect immunofluorescence as previously described.2 Anti-GP2 antibodies were associated with pancreatic antibodies (extracellular staining). 68.8% (22/32) of patients with Crohn's disease with the extracellular pattern had anti-GP2 antibodies, whereas only 0% (0/8) of patients with Crohn's disease with the intracellular pattern and 8% (9/113) of patients negative for pancreatic antibodies had anti-GP2 antibodies.
We evaluated whether specific clinical characteristics of patients with Crohn's disease (gender, age, anti-Saccharomomyces cerevisiae antibodies (ASCA), disease localisation (upper gastrointestinal tract, terminal ileum, ileocolon, colon, anal), disease behaviour (inflammatory, structuring, penetrating), and surgery) were associated with anti-GP2 antibodies. Except for an association of anti-GP2 and upper gastrointestinal tract localisation of Crohn's disease (p=0.042, χ2), no significant associations were found.
Finally, we determined anti-GP2 antibody levels before and 6–44 months after treatment with infliximab (n=28) or adalimumab (n=34). Treatment did not prominently affect antibody titres (data not shown).
In summary, the anti-GP2 ELISA was reproducible, had a good linearity, and showed minor interferences. Anti-GP2 antibodies were found in of 20.7% of patients with Crohn's disease, 9.3% of patients with ulcerative colitis, and 4% of controls. We furthermore showed that anti-GP2 antibodies were associated with the extracellular pancreatic staining on indirect immunofluorescence.
We thank GA Generic Assays GmbH for the generous gift of the reagents to perform this study.
Funding This research has been funded by the Fund for Scientific Research – Flanders. XB is a senior clinical investigator of the Fund for Scientific Research – Flanders.
Competing interests None.
Ethics approval This study was conducted with the approval of the University Hospitals, Leuven.
Provenance and peer review Not commissioned; not externally peer reviewed.