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The new corpora lutea (CLs) in pigs are formed from the preovulatory follicles after the luteinizing hormone (LH) surge. However, total autonomy and independence of CLs from LH up to Day 12 of cycle has recently been questioned. Transformation of estrous cycle CL to CL of pregnancy initiated by embryonic signals requires not only the cessation of prostaglandin F2 (PGF_2α) supply to the luteal tissue but also needs the CL to overcome luteolytic acquisition and/or changing its sensitivity to PGF_2α during Days 12–14 of pregnancy. The luteolytic cascade is prevented by inhibition of lymphocyte infiltration and leucocyte recruitment, limitation of cell apoptosis, upregulation of pregnancy-associated genes and an enhanced antiluteolytic role of PGE₂. Our ‘two-signal switch hypothesis’ highlights the importance of post PGF_2α and PGE₂ receptor signaling pathways activation in CLs during luteolysis and rescue. The ‘luteolytic switch’ involves increased expression of many regression mediators and activation of the post PTGFR signaling pathway. The ‘rescue switch’ initiated by embryonic signals – estradiol 17β and PGE₂ – induces post PTGER2/4 pathway, turning the ‘luteolytic switch’ off and triggering activity of genes responsible for CL maintenance. In mid and late pregnancy, CLs are maintained by LH and the synergistic action of metabolic hormones. This paper provides an outline of recent views on CL regression, rescue and maintenance during pregnancy in pigs that conflict with previous paradigms and highlights new findings regarding the actions of prostaglandins, role of microRNAs (miRNA) and immune system and signaling pathways governing the life cycle of porcine CL.

Interferon-tau (IFNT), a maternal recognition of pregnancy (MRP) signals in domestic ruminants, suppresses the release of luteolytic pulses of uterine prostaglandin F2a (PGF2a), thus extending the corpus luteum (CL) life span. We hypothesized that IFNT also exerts anti-luteolytic actions in bovine CL. To examine the direct effects of IFNT on bovine CL, luteal slices and enriched luteal endothelial cells (LECs) were utilized. We found that recombinant ovine IFNT (roIFNT) markedly elevates interferon-associated genes (STAT1, STAT2 and IRF9) and interferon-stimulated genes (ISGs: MX2, ISG15 and OAS1Y) in both models. Furthermore, IFNT time-dependently induced STAT1 phosphorylation in LECs without affecting total STAT1. roIFNT-stimulated viable LECs numbers and the knockdown of protein inhibitor of activated STAT1 (PIAS1) abolished this effect, suggesting that PIAS1 may mediate the proliferative effect of IFNT. IFNT significantly downregulated luteolytic genes such as TGFB1, thrombospondin-1 (THBS1), endothelin-1 (EDN1) and serpin family E member-1 (SERPINE1) in LECs. However, less robust effects were observed in luteal slices. Moreover, PGF2a alone induced THBS1, SERPINE1 and EDN1 mRNA in CL slices whereas in the presence of IFNT, THBS1 and SERPINE1 stimulation was abolished. Collectively, these results indicate that IFNT acts via STAT1- IRF9-dependent and independent pathways and affects diverse luteal functions. Most interestingly, this study suggests the existence of an anti-luteolytic effect of IFNT in bovine CL, namely, inhibiting key PGF2a-induced luteolytic genes. The proliferative effect of IFNT may constitute an additional mechanism that promotes luteal cell survival, thus, extending the luteal life span during early pregnancy in cows.