Subclinical inflammation with increased neutrophil activity in healthy twin siblings reflect environmental influence in the pathogenesis of inflammatory bowel disease

The mechanisms behind increased fecal calprotectin (FC) in healthy relatives of patients with inflammatory bowel disease (IBD) are unknown. Our aims were to explore if there is a subclinical inflammation with increased neutrophil activity in healthy twin siblings in discordant twin pairs with IBD and to assess the influence of genetics in this context. Nuclear factor kappa B (NF-κB) and neutrophil activity, based on myeloperoxidase (MPO) and FC, were analyzed in healthy twin siblings in discordant twin pairs with IBD and compared with healthy controls. NF-κB and MPO were assessed by immunohistochemistry and FC by enzyme-linked immunosorbent assay. In total, 33 of 34 healthy twin siblings were histologically normal. Increased NF-κB was more often observed in healthy twin siblings in discordant twin pairs with Crohn's disease (13/18 [73%]) and with ulcerative colitis (12/16 [75%]) than in healthy controls (8/45 [18%]). MPO was more often increased in healthy twin siblings in discordant pairs with Crohn's disease (12/18 [67%]) than in healthy controls (11/45 [24%]) and FC more often in healthy twin siblings in discordant pairs with ulcerative colitis (14/21 [67%]) than in healthy controls (6/31 [19%]). Interestingly, the observed differences remained when healthy monozygotic and dizygotic twin siblings were analyzed separately. We observed increased NF-κB, MPO, and FC in healthy twins in both monozygotic and dizygotic discordant pairs with IBD. These novel findings speak for an ongoing subclinical inflammation with increased neutrophil activity in healthy first-degree relatives.