Background: Recurrent spontaneous miscarriage (RSM) is a reproductive complication in which immune dysregulation is thought to contribute to pregnancy loss. Although natural killer (NK) cells have been widely studied, the role of mucosal-associated invariant T (MAIT) cells remains unclear.

Objective: To evaluate the distribution of phenotypically defined MAIT cells, CD3+Vα7.2+CD161dim cells, NK subsets, and natural killer T (NKT)-like cells in the peripheral blood and decidual tissue of women with spontaneous miscarriage and RSM.

Methods: This exploratory hospital-based case-control study included 39 women with first-trimester miscarriage (17 with RSM and 22 with spontaneous miscarriage) and 27 healthy pregnant controls. Peripheral blood was collected from all participants, and decidual tissue from miscarriage cases. phenotypically defined MAIT cells, CD3+Vα7.2+CD161dim cells, NK cell subsets, and NKT-like cells were assessed by flow cytometry.

Results: Compared with healthy controls, women with miscarriage had higher circulating percentages of phenotypically defined MAIT (p = 0.037) and lower CD3+Vα7.2+CD161- (p=0.011). In decidual tissue, phenotypically defined MAIT cells were reduced, whereas CD3+Vα7.2+CD161dim and CD3+Vα7.2+CD161-cell populations were increased. CD56bright NK cells were enriched in the decidua, but CD56dim cells remained the predominant subset. Peripheral CD3+Vα7.2+CD161dim cells positively correlated with the number of previous miscarriages (r = 0.348, p = 0.030), whereas circulating CD56bright NK cells were negatively correlated with decidual MAIT and CD3+Vα7.2+CD161dim cells.

Conclusion: Miscarriage is associated with distinct systemic and decidual immune alterations involving MAIT cells and NK cell subsets. These findings describe immune profiles accompanying pregnancy loss. Further studies are needed to determine their prognostic or therapeutic relevance.