Maarten Cozijnsen

87 Chapter 4 Infliximab impacts leukocyte RNA expression and serum inflammatory proteins responders but not in non-responders. 5,10 By neutralizing TNFα, infliximab treatment reduces the migration of neutrophils to inflamed mucosa and suppresses their production of proinflammatory cytokines in CD patients. 13 Our results confirm that these proteins are also downregulated in newly diagnosed pediatric CD patients upon infliximab treatment initiation. Moreover, our results show that the downregulation of mRNA expression in peripheral blood leukocytes of S100 calcium binding proteins associates with reduced endoscopic disease activity. Furthermore, our results demonstrate that infliximab treatment reduces neutrophil concentrations in peripheral blood of therapy naïve CD patients. Thirdly, previous publications also reported downregulation of MMP1 and TGFA in RNA of mucosal biopsies of adult IBD responders (endoscopic remission) to infliximab treatment but not in non-responders. 4,7 Reports on the effect of infliximab treatment on MMP9 and IL6 expression are somewhat contradictory: one trial demonstrated downregulation of MMP9 in responders but not in non-responders 4,7 , whereas another reported downregulation of MMP9 regardless of response. 3 The latter trial also reported downregulation of IL6 in mucosal biopsies regardless of response, while yet another trial reported increased IL6 expression in RNA of mucosal biopsies of non-responders versus responders. 10 Our results indicate that both MMP9 RNA expression and IL-6 protein concentrations are significantly downregulated in RNA of peripheral blood leukocytes or reduced in serum upon infliximab treatment initiation, and that reduced of IL-6 serum concentrations associates with reduced endoscopic disease activity and thus response to infliximab. Lastly, a positive association between OSM expression in RNA of mucosal biopsies and endoscopic disease activity was previously reported in adult IBD patients. 14 Our results now show that this association is also true for OSM serum concentrations, and in pediatric CD patients. Our pilot results are unique. We had extensive, high quality clinical data because of the prospective RCT design, with two fairly comparable treatment groups because of randomization. There were also some limitations: Due to the pilot setting, the number of patients included in each treatment arm was low, which prevented us from drawing conclusions from subgroup analysis. We therefore could not exclude patients that had started additional treatment from our analysis. Moreover, the two treatment groups had an unequal sample size, in favor of the group receiving infliximab. Lastly, we missed some SESCD scores, as not all patients consented to an endoscopic examination at week 10. In summary, infliximab treatment had a strong effect on mRNA expression and protein concentrations. It reduced Th1 and neutrophil signatures, and tissue remodeling proteins. This effect was not seen in CD patients treated with prednisolone. Our findings provide insight into the effect of infliximab treatment and help unravel the underlying immune mechanisms by which infliximab treatment restores homeostasis.