Early pregnancy loss is one of the most severe problems restricting the development of the dairy industry(Wiltbank et al., 2016), and embryo implantation failure is one of the leading causes of early pregnancy loss. Successful implantation requires not only a well-developed embryo but also a well-receptive endometrium. Endometrial receptivity refers to a series of morphological and functional properties acquired by the endometrium during the window of implantation (WOI) that promote adhesion, implantation, and normal embryo development. The establishment of endometrial receptivity requires the coordination of various substances, including steroid hormones (e.g., estrogen and progesterone), cytokines (e.g., leukemia inhibitory factor), homeobox genes (e.g., HOXA10), and cell adhesion molecules (e.g., integrin αvβ3) (Achache and Revel, 2006, Zhang et al., 2013). The receptive endometrial epithelium also undergoes plasma membrane transformation (PMT), characterized by the loss of cell polarity and cell–cell adhesion, as well as the loss of microvilli at the apical plasma membrane (Kumro et al., 2020, Murphy, 2004, Oghbaei et al., 2022). Molecules like E-cadherin and N-cadherin have been widely used to evaluate endometrial receptivity (Zhang et al., 2023a, Heng et al., 2021, Uchida et al., 2012).

Endometrial receptivity is mainly regulated by estrogen E2 and progesterone P4 in humans and mice (Neykova et al., 2022), while in ruminants, endometrial receptivity can also be regulated by interferon-τ (IFN-τ). IFN-τ, a type I interferon, is a pregnancy recognition signal unique to ruminants. It is secreted by embryo trophoblast cells as early as on Day 7 to 8 and throughout the entire peri-implantation period (Passaro et al., 2018, Sponchiado et al., 2017). Its classical role is to prevent luteal lysis by inhibiting the pulsatile release of endometrial PGF2α, thereby maintaining pregnancy. IFN-τ acts on the cell surface receptor IFNAR1/2, which activates the classical interferon downstream JAK/STAT pathway. This activation leads to the expression of numerous classical interferon-stimulated genes (ISGs) and also the expression of extensive non-classical ISGs through other STAT1-independent pathways that are conducive to embryo extension and implantation (Dorniak et al., 2013, Hansen et al., 2017). However, the specific molecular mechanism of IFN-τ regulating bovine endometrial receptivity remains to be further investigated.

Interferon regulatory factor 1 (IRF1), an evolutionarily conserved protein in vertebrates, plays an essential role in mediating host constitutive and inducible antiviral abilities and mainly functions as a transcription factor (Feng et al., 2021). IRF1 binds to the IRF-binding elements (IRF-E, "GTGAAA") in the gene promoter to regulate the transcription of a range of genes. Type I IFNs can regulate IRF1 by activating the JAK/STAT pathway. After activation of the JAK/STAT pathway, STAT1 is phosphorylated and forms a p-STAT1 homodimer (interferon γ activator, GAF) translocating into the nucleus, and then it binds to the γ-activated sequence (GAS, "TTCN [2-4] GAA") to initiate gene transcription. Of note, the promoter region of IRF1 contains a GAS sequence (Feng et al., 2021). As mentioned above, IFN-τ is secreted by the conceptus spatiotemporally and acts on the endometrial epithelium in a paracrine manner. Based on this, we speculate that IFN-τ can induce the expression of IRF1 in the endometrial epithelium of dairy cows during the pre-implantation state. However, the specific role of IRF1 in bovine endometrial receptivity remains unclear.

Leukemia inhibitory factor receptor (LIFR) is a transmembrane receptor of leukemia inhibitory factor (LIF). LIFR, together with interleukin 6 cytokine family signal transducer (IL6ST) on the cell membrane, forms a heterodimer to mediate the downstream signaling pathways including JAK/STAT3, MAPK, and PI3K after LIF binds to it. LIF is a cytokine belonging to the IL-6 family secreted by the endometrial gland epithelium before implantation begins. Furthermore, it is a widely considered key factor in regulating embryo implantation in mammals. It is critical to enter the endometrial epithelium into the receptive phase (Rosario et al., 2014). However, little is known about how deeply is LIFR involved in bovine endometrial receptivity.

In this study, bioinformatic analysis predicted that the promoter of bovine LIFR contained several IRF-Es. Based on the above findings, we speculated that IFN-τ may enhance bovine endometrial receptivity through the interaction between IRF1 and LIFR. Our results demonstrate that IFN-τ can induce IRF1 expression in bovine endometrial epithelial cells, and IRF1 upregulates LIFR expression by binding to LIFR promoter, contributing to the enhancement of bovine endometrial receptivity.