CD Markers, Ratios of NK Cells and T Cells in Relation to Habitual Abortion
Habitual abortion, also known as recurrent spontaneous abortion (RSA) or recurrent miscarriage (RM), is often linked to immunological dysregulation at the maternal-fetal interface. Natural killer (NK) cells and T cells play critical roles in immune tolerance during pregnancy, and aberrations in their subsets, marked by cluster of differentiation (CD) antigens, as well as their ratios, have been associated with increased RM risk. These cells are evaluated in peripheral blood (pNK, pT) and uterine/decidual tissues (uNK, uT). Below is a discussion of key findings from studies, highlighting differences in CD markers and ratios.
NK Cells in Recurrent Miscarriage
NK cells are innate immune effectors expressing CD56 (a neural cell adhesion molecule) and CD16 (Fcγ receptor IIIa), which define their subsets and functions. In normal pregnancies, uNK cells (predominantly CD56bright CD16-) promote trophoblast invasion, angiogenesis, and immune tolerance via cytokine secretion (e.g., IL-10, TGF-β). In contrast, pNK cells are mostly CD56dim CD16+ and cytotoxic.
- Elevated NK Cell Levels and Activation: Women with RM often show increased proportions of total NK cells (CD56+) in both peripheral blood and endometrium/decidua compared to fertile controls. For instance, meta-analyses indicate higher endometrial CD56+ uNK levels in RM (standardized mean difference [SMD] 0.49) and recurrent implantation failure (SMD 0.49). Activated states are common, with upregulated killer-cell immunoglobulin-like receptors (KIRs) and increased cytotoxicity contributing to fetal rejection.
- Subset Ratios: A key imbalance is a decreased ratio of regulatory CD56bright CD16- (bright/dim <1) to cytotoxic CD56dim CD16+ NK cells in RM, favoring cytotoxicity over tolerance. Studies report higher CD56dim CD16+ in peripheral blood of RM patients, associated with pro-inflammatory cytokines (e.g., IFN-γ, TNF-α) and lower inhibitory receptors. Conversely, some RM cases show reduced CD56bright NK cells, correlating with lower IL-4, IL-10, and TGF-β. Preconception high pNK (e.g., >17% CD56+) predicts miscarriage in subsequent pregnancies. However, not all studies find consistent differences in CD56+ CD16+ ratios between primary and secondary RM.
- Prognostic Implications: A decrease in pNK levels during early pregnancy is associated with live births in unexplained RM, while persistently high levels (>10-15%) predict loss. No strong correlation exists between pNK and uNK levels in RM.
T Cells in Recurrent Miscarriage
T cells, marked by CD3 (pan-T cell marker), include helper (CD4+) and cytotoxic/suppressor (CD8+) subsets. In pregnancy, a shift toward Th2 (anti-inflammatory) dominance supports tolerance, while Th1 (pro-inflammatory) skewing promotes rejection.
- Elevated and Activated T Cells: RM is linked to upregulated activated CD3+ T cells, with higher proportions in peripheral blood as a predictive factor (alongside NK). Activated CD4+ and CD8+ T cells are increased, often with downregulated regulatory T cells (Tregs, CD4+ FoxP3+), leading to immune intolerance. Endometrial CD8+ tissue-resident memory T cells (CD8+ CD103+) show altered phenotypes in RM, with reduced inhibitory markers and impaired regulation.
- Subset Ratios: The Th1/Th2 ratio is elevated in RM, with higher Th1 (IFN-γ, TNF-α-producing) to Th2 (IL-4, IL-10-producing) cells in decidua and blood. CD4/CD8 ratio may be inverted or decreased due to higher CD8+ cytotoxicity. Some studies note low TGF-β+ T cells in RM, correlating with high NK and NKT cells. However, endometrial distributions of CD3+ T, Th2/Tc2 ratios do not always predict outcomes.
Interactions Between NK and T Cells
NK cells influence T cell responses; high NK proportions correlate with CD3+ T cell dysregulation in RM. For example, cytotoxic NK (CD56dim CD16+) may enhance Th1 skewing, while low regulatory NK reduces T cell tolerance. Combined high NK and activated T cells predict URSA (unexplained RSA).
In summary, RM is associated with elevated cytotoxic NK subsets (high CD56dim CD16+, low CD56bright/CD56dim ratio) and pro-inflammatory T cell shifts (high Th1/Th2, activated CD4+/CD8+). These imbalances disrupt immune tolerance, but findings vary across studies, and clinical testing (e.g., via flow cytometry) should be interpreted cautiously with professional guidance. Therapies like IVIG or steroids target these, but evidence is mixed.