The role of Interleukin-18 in recurrent early pregnancy loss

•Interleukin-18 (IL-18) can induce either Th1 or Th2 immune response depending on the local cytokine environment.•The TaqMan® Human Cytokine Network Array was carried out with placental tissue of patients with healthy pregnancies and recurrent miscarriage in order to investigate differences in IL-18 mRNA expression.•Gene expression analysis revealed a 4,9-times higher expression of IL-18 in recurrent miscarriage patients when compared to the control group..•IL-18 protein expression was investgated in the placenta of patients with healthy pregnancies, spontaneous and recurrent miscarriage.•A significant upregulation of IL-18 in the decidua was restricted to patients with recurrent miscarriage and therefore might be interesting as a diagnostic marker. A successful pregnancy is a unique and complex immunological state. Cytokines seem to be crucial for the implementation of a tolerogenic environment at the feto-maternal interphase towards the semi-allogenic fetus. Importantly, the switch from a Th1- to a Th2 cytokine profile might play a key role. Interestingly, Interleukin-18 (IL-18) can induce either Th1 or Th2 immune response depending on the local cytokine environment. Therefore, this study investigates the expression of IL-18 in early pregnancy loss. The TaqMan® Human Cytokine Network Array was carried out with placental tissue of patients with healthy pregnancies (n = 15) and recurrent miscarriage (n = 15) in order to investigate differences in IL-18 mRNA expression. Immunohistochemical staining was applied to examine the IL-18 protein expression in the syncytiotrophoblast and decidua of healthy pregnancies (n = 15), spontaneous (n = 12) and recurrent miscarriage (n = 9). The characterization of IL-18 expressing cells in the decidua was evaluated by double-immunofluorescence. Correlation analysis between IL-18 protein expression and clinical data of the study population was performed via spearman correlation coefficient. Gene expression analysis revealed a 4,9-times higher expression of IL-18 in recurrent miscarriage patients. IL-18 protein expression was significantly upregulated only in the decidua in the recurrent miscarriage group (p = 0.031). We did not observe significant changes of IL-18 protein expression in spontaneous miscarriage specimens when compared to healthy controls (p = 0.172). Double-immunofluorescence identified decidual stroma cells as IL-18 expressing cells. Correlation analysis showed a significant negative correlation of IL-18 pro