IL-9 at the Maternal-Fetal Interface Drives a Novel Immunological Mechanism of Preterm Labor and Birth

Introduction/Rationale: Preterm birth (PTB) – the leading cause of neonatal morbidity and mortality - frequently arises from spontaneous preterm labor (PTL) without an identifiable infectious trigger, which is categorized as idiopathic (iPTL). Here, we uncover a previously unrecognized immunological mechanism driving a major subset of iPTL.

Methods: We integrated single-cell, spatial, and flow cytometric profiling of human decidual tissues from women with idiopathic or inflammation-associated PTL. A novel ultrasound-guided murine model was developed to deliver IL-9 at the maternal-fetal interface (MFI) and was compared to established models of PTB. In vitro and in vivo assays assessed IL-9’s role in inducing the common pathway of labor, and bulk and single-cell analyses assessed cellular homeostasis. Neutralization of IL-9 was tested to confirm causality, and vitamin D3 supplementation was evaluated for PTB prevention.

Results: IL-9–producing T cells were enriched at the MFI in women with iPTL but not in inflammation-associated cases. While systemic (maternal) and intra-amniotic (fetal) delivery of IL-9 did not cause adverse outcomes, IL-9 at the MFI induced PTB and neonatal mortality in mice. Mimicking iPTL, IL-9 at the MFI did not cause systemic or intra-amniotic inflammation, progesterone withdrawal, or fetal growth restriction. IL-9 induced myometrial contractility, membrane activation, and cervical shortening. Single-cell analyses revealed that IL-9 disrupted macrophage–stromal signaling and suppressed TGF-β–mediated immune regulation. Neutralization of IL-9 or vitamin D₃ supplementation prevented PTB.

Conclusion: IL-9 is key immune effector driving non-infectious PTL through localized immune dysregulation at the MFI and represents a modifiable pathway for immune-based prevention of prematurity.