Invasion of specialized fetal cells, the ‘extravillous trophoblast’ (EVT) into the uterine decidua is critical for the establishment of pregnancy. This presents an immunological paradox – why do invasive EVTs escape rejection?

One hypothesis is that decidual leukocytes are ‘specialized’ compared to peripheral leukocytes with pregnancy-specific functions. However, the mechanisms leading to the differentiation of decidual-specific leukocyte populations are not well understood. The local endometrial environment likely regulates leukocyte differentiation and activation. Prior to implantation endometrial stromal cells (ESC) differentiate to become ‘decidualized’ and decidual leukocyte numbers increase such that during the 1^st trimester, the period of maximal EVT invasion, around 30-40% of cells in the endometrium are leukocytes, predominantly uterine Natural Killer cells and macrophages. Abberant leukocyte activation and impaired decidualization are features of recurrent miscarriage and preeclampsia. We aimed to investigate how interactions between decidual cells and the invading EVTs might influence maternal tolerance.

Cytokine secretion by primary human decidual biopsies (n=3) was identified by Luminex. Primary human ESC were decidualized in vitro and conditioned media (CM) collected from non-decidualized and decidualized ESC. Primary human EVT (n=6) were treated with pooled (n=14) ESC CM and secreted proteins <30kD identified by mass spectrometry and validated by immunohistochemistry and Western blot.

Decidual biopsies secreted high levels of G-CSF, GM-CSF, IL6, IL8 and MCP1. The presence of EVTs within this tissue elevated IL1RA secretion and reduced IL6, IL15 and MCP1 secretion.  EVTs exposed to decidualized CM expressed cell surface factor CD59, which inhibits formation of the complement membrane attack complex. In contrast, EVTs exposed to non-decidualized CM secreted pro-inflammatory proteases including Annexin A2, which can activate macrophages via TLR4 and DPP1, which activates granzymes.

Overall, our data suggests decidualization and appropriate interactions between ESC and EVTs are critical for maternal tolerance. Impaired decidualization may lead to EVT expression of factors which aberrantly activate maternal leukocytes.