Immune cells in pregnant uterine mucosa : functional properties, cellular composition and tissue organization

Abstract: The pregnant uterus mucosa - decidua - is an "immunologically privileged" site. A semiallogeneic embryo is allowed to survive, develop, and grow while the same tissue implanted outside the uterus will be rejected. The decidua basalis, which participates in the placenta formation, is a tissue rich in lymphoid cells. We have studied decidua associated mononuclear cells (DMC) from normal early pregnancies in humans. The cells were investigated with respect to surface marker profiles, ultrastructure, organization in the tissue, and functional properties. In addition, we have studied the expression of receptors for the iron-binding protein lactoferrin on these cells, and characterized the receptor (Lf-R).Ten to fiftee percent of all cells in decidua belong to the lymphoid cell lineage. They are present in aggregates [lymphoid cell clusters (LCC) mainly located in the vicinity of decidual/endometrial glands] and as individual cells, intra- or subepithelially along the glands (IEL) , and in the stroma. The LCC appear to be centres of immune reactivity. They occur at a frequency of 0.40.2/mm2 tissue and are composed of different population of activated T cells and NK cells in close contact with each other. Interestingly, B cells are not present in the LCC. DMC consist of four major lymphocyte subpopulation of similar sizes: TCRγδ+/CD56+cells, TCRγδ+/CD56-cells, TCRγδ-/CD56+cells and TCRαβ+/CD8+cells. TCRαβ+/CD4+ cells and monocytes are also present. Most DMC have long, thick processes, microvilli, and cytoplasmic granules. They are in intimate contact with surrounding lymphoid, epithelial and stromal cells. Signs of cellular movement and excretion of granules are also seen.About half of the T cells are TCRαβ cells. These cells lack CD4 and CD8. A large fraction of them are CD56+, a rare phenotype at other sites. Most of the TCRγδ+ cells express activation/memory markers (CD45RO, the Kp43 antigen, transferrin receptor, and MHC class II antigens), and many cells express the mucosa homing receptor HML-1. Morphologically these cells either display features characteristic for cytotoxic cells or contain unique nuclear inclusions.More than half of TCRαβ cells are CD8+, but CD4+ cells are also found. These cells also display activation markers.DMC use both transferrin and lactoferrin for their iron supply. The Lf-R on activated lymphocytes appears to be made of two peptides of 47 and 65 kD MW.Freshly isolated DMC respond poorly or not at all to activation through the TCR/CD3 complex, probably due to the low surface density of the complex. However, the TCR/CD3 complex can be up-regulated by stimulation with PMA/Ionomycin in vitro, suggesting that the lymphocytes are suppressed in vivo. Glandular epithelial cells produce immunosuppressive factor(s) that act on CD8+, TCRγδ+, and CD56+ cells. The proximity between the LCC and the glands indicates that this factor(s) may play a role in local immunosuppression. The identity of the factor(s) is presently unknown. The cytokine mRNA profile of DMC, as determined by RT-PCR, reveals IFN-γ, IL-8 and TGF-β1 mRNA in all samples, and IL-1β, IL-2, IL-10, TNF-α and GM-CSF mRNA in some samples. The cytokine profile is compatible with down-regulation of CTL activity.The demands on the immune system in pregnant uterus mucosa are unique. On one hand, a genetically incompatible fetus must be accepted, the development of the placenta must be allowed, and the uteral mucosal tissue must be remodelled. On the other hand, the invasiveness of the trophoblast must be controlled, and the fetomaternal unit must be protected against infections. Our studies indicate that this is achieved through a highly regulated process involving different types of activated lymphoid cells interacting with each other and with glandular epithelial cells.