Modulation of uterine responses in dogs: The divergent effects of semen and embryonal signals on steroidogenic receptors and selected immune system-related factors

Abstract

Although dogs lack the classical pregnancy recognition via embryo-derived anti-luteolytic signaling, precise embryo-maternal communication remains essential in establishing pregnancy. While previous studies confirmed uterine responses to pre-attachment embryos versus semen-exposed non-pregnant controls, they did not include non-mated, non-pregnant controls. Following the hypothesis that embryo-derived signaling differs from the responses induced by semen, the present project included three groups of dogs (d. 10–12 of pregnancy/corresponding dioestrus): non-mated, mated and pregnant, and mated but non-pregnant (semen-exposed). The expression of nuclear hormone receptors and selected immunomodulatory factors was detected using semi-quantitative PCR and immunohistochemistry. While transcript levels of the progesterone (P4) nuclear receptor (PGR) were significantly elevated in response to embryo presence, its staining across the endometrial layers indicated apparent effects of semen exposure, further potentiated by the presence of embryos. ESR1/ERα showed a clear embryo-driven response, whereas ESR2/ERβ remained unchanged. The decreased availability of FOXP3 transcripts in response to semen, and the consistent expression of CD4 and CD25, suggests an increased presence of T-helper (Th) cells. Semen exposure also resulted in higher levels of CD163 and CD206, and decreased expression of CD8. Embryos had a stimulatory effect on the joint expression of T-cell markers (FOXP3, CD4, and CD25), pointing toward T-regs activity. Embryos also increased DLA-DRA and NCR1 expression. This study underscores the importance of differentiating between seminal plasma- and embryo-driven signals in the canine uterus. While exposure to semen may help prepare the uterus for implantation by modulating the immune response, embryo-derived signals further modulate these effects.