An immune switch called NFAT controls whether pregnancy succeeds or fails

University of Alabama at Birmingham

A molecular switch previously known only for its role in T cells turns out to govern whether a pregnancy establishes a healthy blood supply. Researchers at the University of Alabama at Birmingham have identified NFAT (Nuclear Factor of Activated T cells) as the signal that tells uterine natural killer cells to settle into the uterine lining and begin remodeling maternal blood vessels for early placental growth. When that signal is suppressed, fewer killer cells reach the uterus, and complications can follow. The findings were published March 11, 2026, in Science Translational Medicine.

The residency switch no one expected

Early pregnancy depends on precise immune choreography. Before a placenta can nourish a fetus, the embryo must connect with the mother's blood supply. That connection is guided by uterine natural killer cells (uNK cells), a specialized immune population that remodels spiral arteries in the uterine wall to allow adequate blood flow.

Until now, no one had described how uNK cells become "tissue-resident," the necessary step that allows them to stay in the uterine lining rather than circulating through the bloodstream. The UAB team, led by Paige Porrett, used single-cell RNA sequencing to track gene activity across thousands of individual cells in uterine tissue. That granularity revealed NFAT acting as a residency switch: when active, it drives uNK cells to embed in the tissue and carry out their vascular remodeling work. When dampened, the cells fail to take up residence.

NFAT's role in T cells has been studied for decades. Its function in uterine NK cells was entirely unknown.

A transplant lens on universal biology

The discovery came through an unusual avenue. Porrett's team specializes in uterus transplantation, which gave them access to uterine tissue from transplant recipients who were taking immunosuppressive drugs. That clinical context provided a natural experiment: transplant recipients take tacrolimus, a drug that works by suppressing NFAT signaling.

By comparing uterine tissue from transplant recipients with tissue from non-transplant patients, the team could see what happens when NFAT is turned down. The results were clear: fewer tissue-resident NK cells, reduced vascular remodeling, and a biological signature consistent with the pregnancy complications these patients sometimes experience.

But Porrett emphasized that this is not just a transplant story. The NFAT pathway appears to be a fundamental part of human placental development, relevant to any pregnancy. The transplant setting simply made the mechanism visible.

Rethinking tacrolimus in pregnancy

Tacrolimus is taken by nearly all solid-organ transplant recipients. Prior studies have consistently shown it does not increase the risk of birth defects. But the new findings suggest it may raise the risk of placental complications by suppressing the NFAT signal that uNK cells need.

The researchers were careful to note that no medication changes should be made based on this single study. Tacrolimus remains highly effective at preventing organ rejection, and any alternative must be rigorously tested. What the study provides is a mechanistic reason to investigate timing strategies or alternative drugs that could protect both the pregnancy and the transplanted organ.

What the data can and cannot tell us

The study's strength lies in its use of human tissue and single-cell sequencing, which provided cell-by-cell resolution of gene programs that older techniques could not capture. But the data are largely associative. The team observed correlations between NFAT suppression and reduced uNK residency, but proving direct causation will require further work.

The sample comes from transplant recipients, a population that differs from the general pregnant population in multiple ways beyond immunosuppression. Extrapolating to non-transplant pregnancies is biologically plausible but not yet confirmed.

Additionally, the study focused on NK cells. The uterus contains many other immune and non-immune cell types, including stromal and epithelial cells, that may also respond to NFAT signaling. The team plans to map how immunosuppressive drugs affect these broader cell populations.

Porrett described the findings as a foundation, not a conclusion. Validating causality, understanding the broader uterine cellular landscape, and identifying safer alternatives for pregnant patients are the hard work ahead.

Implications beyond transplant medicine

Preeclampsia affects roughly 5 to 8 percent of pregnancies worldwide. Implantation failure and inadequate placental blood flow contribute to early pregnancy losses that remain poorly understood. If NFAT signaling proves to be a common factor in these conditions, it would open new avenues for diagnosis and intervention in the general obstetric population.

The path from this discovery to clinical application is long. But identifying a specific molecular switch that controls a critical step in early pregnancy gives researchers something concrete to target, rather than treating pregnancy complications as a collection of symptoms with unclear origins.