Tolerance towards fetal alloantigens in the maternal immune system is essential for the maintenance of pregnancy, and dysfunction of the maternal–fetal immune tolerance can lead to pregnancy loss or other severe complications, such as preeclampsia or preterm deliveries. Therefore, understanding the mechanism of the maternal–fetal immune tolerance is important for successful pregnancy.

Myeloid-derived suppressor cells (MDSCs) are immature myeloid cells characterized by their ability to suppress immune activity and are classified into two subsets, namely monocytic MDSCs (MO-MDSCs) and granulocytic (or polymorphonuclear) MDSCs (PMN-MDSCs). The role of MDSCs in the tumor microenvironment has been investigated. MDSCs strongly inhibit antitumor immunity and mediate metastasis and tumor growth (Gabrilovich et al., 2017; Kobayashi et al., 2019; Umansky et al., 2016). In addition, recent human and mouse studies have revealed a new role for MDSCs in that they contribute to maternal–fetal immune tolerance by inhibiting maternal immunity (Köstlin-Gille et al., 2020; Köstlin et al., 2014; Ostrand-Rosenberg et al., 2017; Pan et al., 2016a; Zhang et al., 2016). In canines, MDSCs, represented as CD11b+ MHC class II- CD14+ cells (MO-MDSCs) and CD11b+ MHC class II- CD14- cells (PMN-MDSCs) (Goulart et al., 2012, Hutchison et al., 2019; Jackson et al., 2021), also suppress immune activity and contribute to tumor growth (Goulart et al., 2012). In canine uteri, transcriptional analysis has revealed reduced levels of markers of Th cells and pro-inflammatory M1 macrophages at implantation compared to those at pre-implantation, suggesting that implantation modulates uterus immunity to promote anti-inflammatory conditions (Tavares Pereira et al., 2021). Therefore, maternal–fetal immune tolerance may also occur in canines. However, whether MDSCs are involved in canine maternal–fetal immune tolerance is unknown.

Previous mouse studies have shown that MDSCs have the potential to regulate maternal–fetal immune tolerance, and clarifying the induction mechanism of pregnancy-derived MDSCs is helpful for developing methods to prevent miscarriage. Since dramatic hormonal changes occur during pregnancy, pregnancy-related hormones are thought to potentially induce MDSCs. In particular, several studies investigated estradiol, which is increased in human pregnancy, and showed that estradiol induced MDSCs in peripheral blood (Hu et al., 2019, Pan et al., 2016a) and in bone marrow (Kozasa et al., 2012). Although successful induction of MDSCs by estradiol has been reported, other study has revealed that the inducing ability of estradiol cannot be confirmed (Pan et al., 2016b). The hormones that induce MDSCs remains unclarified. Hormonal dynamics differ between canine and human pregnancies. Estradiol levels increase during human pregnancy, however, rarely in canine pregnancy (Hadley et al., 1975; Hoffmann et al., 1994). Therefore, by analyzing the relationship between pregnancy-related hormones and MDSCs in pregnant canines, it is possible to identify a hormone that induces MDSCs, instead of estradiol.

In this study, we investigated the role of MDSCs in pregnant canine immunity using the peripheral blood. In addition, to clarify the induction mechanism of MDSCs, we analyzed the relationship between pregnancy-related hormone levels (estradiol, progesterone, and relaxin) and the ratio of MDSCs. For hormones that were found to be related, their ability to induce MDSCs was analyzed.