Fetal CD103+ IL-17-Producing Group 3 Innate Lymphoid Cells Represent the Dominant Lymphocyte Subset in Human Amniotic Fluid

Amniotic fluid (AF) surrounds the growing fetus, and cells derived from AF are commonly used for diagnosis of genetic diseases. Intra-amniotic infections are strongly linked to preterm birth, which is the leading cause of perinatal mortality worldwide. Surprisingly little is known, however, about mature hematopoietic cells in AF, which could potentially be involved in immune responses during pregnancy. In this study, we show that the dominating population of viable CD45(+) cells in AF is represented by a subset of fetal CD103(+) group 3 innate lymphoid cells (ILCs) producing high levels of IL-17 and TNF. Fetal CD103(+) ILC3s could also be detected at high frequency in second-trimester mucosal tissues (e.g., the intestine and lung). Taken together, our data indicate that CD103(+) ILC3s accumulate with gestation in the fetal intestine and subsequently egress to the AF. The dominance of ILC3s producing IL-17 and TNF in AF suggests that they could be involved in controlling intra-amniotic infections and inflammation and as such could be important players in regulating subsequent premature birth.