Maarten Cozijnsen

86 Discussion By studying the differences in the immune responses of therapy naïve pediatric CD patients to infliximab and prednisolone treatment, we demonstrated that infliximab treatment reduces systemic and intestinal CD inflammation more effectively than prednisolone treatment does. We identified three dominant pathways that associate with infliximab treatment and with endoscopic remission: Firstly, infliximab treatment homogeneously reduces the expression and concentration of Th1 related genes and proteins. Secondly, infliximab treatment reduces the expression of S100 calcium binding proteins and the concentrations of several neutrophil chemo attractants. Thirdly, infliximab treatment reduces the transcription and concentration of proteins involved in tissue remodeling. Moreover, multiple proteins within these pathways positively associate with the degree of endoscopic disease activity. Downregulation of these proteins thus associates with endoscopic remission. In comparison, these pathways were not significantly affected in CD patients treated with prednisolone, due to a more heterogeneous and weaker response. We identified these pathways by comparing patient samples before and after infliximab treatment initiation in newly diagnosed, treatment naïve patients. Therefore, our results are unaffected by effects that previous treatments may have had, and they reveal the most dominant underlying immune mechanisms of infliximab without bias. Furthermore, we identified these pathways in analyses of patient blood rather than in analyses of mucosal biopsies. Thereby we proved that it is possible to monitor immune responses of pediatric CD patients to infliximab treatment in patients’ blood. Downregulation of Th1 related genes by infliximab treatment has been reported earlier in IBD patients. Microarray analyses of mucosal biopsies in adult IBD patients demonstrated that the T-cell chemo attractants CXCL9 , CXCL10 and CXCL11 5 are downregulated as well as CD274 (PD-L1) 3 and IFNG 7 in responders to infliximab treatment (defined as endoscopic remission) but remain high in non-responders. Also, mucosal RNA expression of JAK2 and STAT1 and IL18R1 are reported to be lower in responders than in non-responders. 10 We demonstrated that these genes are also downregulated in peripheral blood of treatment naïve pediatric CD patients after infliximab initiation. Similarly, downregulation of mRNA expression of S100 calcium binding proteins and neutrophil chemo attractants has also been reported in adult IBD patients treated with infliximab. RNA-expression analyses of mucosal biopsies of adult CD patients demonstrated downregulation of S100A8 , S100A9 and S100A12 in responders (defined as endoscopic remission) to infliximab treatment but not in non-responders 3,12 Secondly they demonstrated downregulation of the neutrophil chemo attractants CXCL1 , CXCL5 and CXCL8 in