Antibodies against Mycobacterium paratuberculosis in Crohn’s disease

Until recently the investigation of serological responses to mycobacteria in patients with Crohn's disease has been hindered by the considerable degree of cross-reactivity between antigens of M. paratuberculosis, and other mycobacterial subspe-cies. We evaluated the serological response of Crohn's disease patients to a recently identified species-specific 18 kDa protease-resistant antigen corresponding to M. paratuberculosis bacterioferri-tin. The 18 kDa antigen was purified from M. paratu-berculosis as previously described. Serum was obtained from 40 patients with Crohn's disease, 15 with ulcerative colitis, 25 coeliac patients, and 21 normal blood donors. Antibody levels were meas-ured by enzyme-linked immunosorbent assay (ELISA), with anti-human IgA and IgG alkaline phos-phatase conjugate. Antibody titres were expressed as the dilution giving 1/3 of the plateau binding value of a standard positive serum (MT/3). Disease activity of the Crohn's disease cases was assessed using the Harvey-Bradshaw index. There was no statistically significant elevation of the mean IgG or IgA MT/3 titres of Crohn's disease patients over controls. No patients had antibody titres greater than two standard deviations above the mean con-trol MT/3 titres, and there was no significant correla-tion between Crohn's disease activity and level of antibody titres. These findings make it unlikely that M. paratuberculosis is of primary pathogenic impor-tance in Crohn's disease