Immune cells in the corpus luteum: Friends or foes?

The corpus luteum produces progesterone, which is essential for the maintenance of pregnancy. In the absence of a viable embryo, the corpus luteum must regress rapidly to allow for development of new ovulatory follicles. In many species, luteal regression is initiated by uterine release of PGF_2α, which inhibits steroidogenesis and may launch a cascade of events leading to the ultimate demise of the tissue. Immune cells, primarily macrophages and T lymphocytes, are present in the corpus luteum, particularly at the time of luteolysis. The macrophages are important for ingestion of cellular remnants that result from the death of luteal cells. However, it has also been hypothesized that immune cells are involved directly in the destruction of luteal cells, as well as in the loss of steroidogenesis; this hypothesis is reviewed in the first part of this article. An alternative hypothesis is also presented, namely that immune cells serve to abate an inflammatory response generated by dead and dying luteal cells, in effect, preventing a response that would otherwise damage surrounding ovarian tissues. Finally, the changes in immune cells that accompany maternal recognition of pregnancy and rescue of the corpus luteum are discussed briefly. Inhibition of immune cells in the corpus luteum during early pregnancy may be due to embryonic or uterine signals, or to maintenance of high progesterone concentrations within the luteal tissue.