Elevated uterine natural killer cell density is associated with severity of recurrent implantation failure in frozen embryo transfer cycles

Previous studies have demonstrated an increase in the number of uterine natural killer (uNK) cells in women with recurrent implantation failure (RIF), compared with fertile control groups. This study investigates the association of uNK cell density with the severity of RIF (i.e., the number of failed frozen embryo transfer cycles) during frozen-thawed embryo transfer (FET) cycles, rather than establishing a diagnostic threshold compared to fertile women. This retrospective cohort study enrolled a total of 94 women, including 24, 34 and 36 women who failed to conceive after 2, 3, or 4 or more FETs, respectively. All endometrial tissue biopsies were collected precisely on the putative day of blastocyst transfer during a mock non-conception hormone replacement therapy (HRT) cycle, which is five days after initiation of progesterone ( P + 5). Endometrial tissue sections underwent immunohistochemical staining for CD56 to identify uNK cells. Cell counting was performed using a standardized protocol, and results were expressed as the percentage of CD56 + uNK cells relative to total stromal cells. The median uNK cell density was 1.8% (1.4%-3.3%), 1.8% (1.4%-3.6%) and 3.9% (1.9%-5.8%) in women who failed to conceive after 2, 3, and ≥ 4 FETs, respectively. The median uNK cell density in women with 4 or more FET failures was significantly higher than that in women who had 2 or 3 FET failures. We found that uNK cell density was significantly higher in women with 4 or more FET failures, highlighting a notable increase specifically in this high-frequency failure group compared to those with 2 or 3 failures.