An elevated fetal interleukin-6 concentration can be observed in fetuses with anemia due to Rh alloimmunization: Implications for the understanding of the fetal inflammatory response syndrome

Objective. The fetal inflammatory response syndrome (FIRS) has been described in the context of preterm labor and preterm prelabor rupture of the membranes and is often associated with intra-amniotic infection/inflammation. This syndrome is characterized by systemic fetal inflammation and operationally defined by an elevated fetal plasma interleukin (IL)- 6. The objective of this study was to determine if FIRS can be found in fetuses with activation of their immune system, such as the one observed in Rh alloimmune-mediated fetal anemia. Methods. Fetal blood sampling was performed in sensitized Rh-D negative women with suspected fetal anemia (n = 16). Fetal anemia was diagnosed according to reference range nomograms established for the assessment of fetal hematologic parameters. An elevated fetal plasma IL-6 concentration was defined using a cutoff of >11 pg/ml. Concentrations of IL-6 were determined by immunoassay. Non-parametric statistics were used for analysis. Results. (1) The prevalence of an elevated fetal plasma IL-6 was 25% (4/16); (2) there was an inverse relationship between the fetal hematocrit and IL-6 concentration - the lower the hematocrit, the higher the fetal IL-6 (r = -0.68, p = 0.004); (3) fetuses with anemia had a significantly higher plasma IL-6 concentration than those without anemia (3.74 pg/ml, interquartile range (IQR) 1.18-2.63 vs. 1.46 pg/ml, IQR 1.76-14.7; p = 0.02); (4) interestingly, all fetuses with an elevated plasma IL-6 concentration had anemia (prevalence 40%, 4/10), while in the group without anemia, none had an elevated fetal plasma IL-6. Conclusions. An elevation in fetal plasma IL-6 can be observed in a subset of fetuses with anemia due to Rh alloimmunization. This observation suggests that the hallmark of FIRS can be caused by non-infection-related insults. Further studies are required to determine whether the prognosis of FIRS caused by intra-amniotic infection/inflammation is different from that induced by alloimmunization.