During pregnancy, various immune effectors and molecules participating in the immune-microenvironment establish specific maternal tolerance toward the semi-allogeneic fetus. Activated maternal immune effectors by the trophoblast antigens, such as T helper (Th), T cytotoxic (Tc), T regulatory (Treg), and B cells, are involved in the regulation of adaptive immunity. Recognition of active signal through the T cell receptors stimulate the differentiation of naive CD3⁺CD4⁺ T cells into specific T cell subsets, such as Th1, Th2, Th9, Th17, Th22, and follicular Th cells (Tfh). Each of these subsets has a significant and distinct role in human pregnancy. Th1 immunity, characterized by immune-inflammatory responses, becomes dominant during the peri-implantation period, and the “controlled” Th1 immunity benefits the invading trophoblasts rather than harm. Quickly after the placental implantation, the early inflammatory Th1 immunity is shifted to the Th2 anti-inflammatory immune responses. The predominant Th2 immunity, which overrules the Th1 immunity at the placental implantation site, protects a fetus by balancing Th1 immunity and accommodate fetal and placental development. Moreover, Treg and Th9 cells regulate local inflammatory immune responses, potentially detrimental to the fetus. Th17 cells induce protective immunity against extracellular microbes during pregnancy. However, excessive Th17 immunity may induce uncontrolled neutrophil infiltration at the maternal-fetal interface. Other Th cell subsets such as Tfh cells, also contribute to pregnancy by setting up favorable humoral immunity during pregnancy. However, dysregulation of Th cell immunity during pregnancy may result in obstetrical complications, such as recurrent pregnancy losses (RPL) and preeclampsia (PE). With this review, we intend to deliver a comprehensive overview of CD4⁺ Th cell subsets, including Th1, Th2, Th9, Th17, Th22, and Tfh cells, in human pregnancy by reviewing their roles in normal and pathological pregnancies.

T cells which produce interleukin (IL)-17 are involved in chronic inflammatory processes and regulatory T (Treg) cells are possibly the most important immune regulators. We aimed to investigate peripheral blood IL-17⁺ T and Foxp3⁺ Treg cells in women with idiopathic recurrent pregnancy loss (RPL). The study design is a cross-sectional evaluation of Th1, Th2, IL-17⁺ T and Treg cells in women with idiopathic RPL (n = 42) and age-matched parous controls (n = 24). Flow cytometric analysis was performed to measure IL-17⁺ T and Foxp3⁺ Treg cells, and ratios of Th1/Th2 cells using anti-IL-17A and anti-Foxp3 antibodies, and monoclonal antibodies to tumor necrosis factor (TNF)-α, interferon (IFN)-γ and IL-10. Student's t-test and partial correlations were applied for statistical analysis. TNF-α-/IL-10-producing CD3⁺CD4⁺ T cell ratio was higher in women with RPL than controls (P = 0.048). Levels of IL-17⁺ T cells (P = 0.021) and the IL-17⁺ T/CD4⁺Foxp3⁺ Treg cell ratio (P = 0.001) were increased, whereas Foxp3⁺ (P = 0.035), Foxp3^low (P = 0.032) and CD4⁺Foxp3⁺ T cell (P = 0.037) levels were decreased in women with RPL, compared with controls. Levels of IL-17⁺ T cells were correlated with TNF-α-producing CD3⁺CD4⁺ T cells (r = 0.269, P = 0.033), and with ratios of TNF-α/IL-10 (r = 0.276, P = 0.027) and IFN-γ/IL-10 (r = 0.266, P = 0.035)-producing CD3⁺CD4⁺ cells. Furthermore, the ratio of IL-17⁺ T cells to CD4⁺Foxp3⁺ Treg cells showed a positive correlation with TNF-α-producing CD3⁺CD4⁺ T cells (P = 0.047) and IFN-γ-producing CD3⁺CD4⁺ T cells (P = 0.048) as well as a ratio of IFN-γ/IL-10-producing CD3⁺CD4⁺ T cells (P = 0.037). Enhanced pro-inflammatory immune responses with suppressed immune regulation may be an important immune mechanism involved in RPL.

Do women with recurrent pregnancy losses (RPL) and low vitamin D have increased prevalence of auto- and cellular immune abnormalities when compared with women with RPL who have normal vitamin D, and does vitamin D have any effect on cellular immunity in vitro? A high proportion of women with RPL have vitamin D deficiency and the risk of auto- and cellular immune abnormalities is increased in women with RPL and vitamin D deficiency. Vitamin D deficiency in pregnant women is associated with increased risk of obstetrical complications such as pre-eclampsia, bacterial vaginosis associated preterm delivery, gestational diabetes mellitus and small-for-gestational age births. A retrospective cross-sectional study of 133 women with RPL who were enrolled in a 2-year period, together with laboratory experiments. Women with three or more consecutive spontaneous abortions prior to 20 weeks of gestation who were enrolled at the University clinic. Serum vitamin D level, cellular activity and autoimmune parameters in vivo and in vitro were measured. Sixty-three out of 133 women (47.4%) had low vitamin D (<30 ng/ml). The prevalence of antiphospholipid antibody (APA) was significantly higher in low vitamin D group (VDlow) (39.7%) than in the normal vitamin D group (VDnl) (22.9%) (P< 0.05) and the adjusted odds ratio (OR) for APA in VDlow was 2.22 with the 95% confidence interval (CI) of 1.0–4.7. The prevalence of antinuclear antigen antibody (VDlow versus VDnl; 23.8% versus 10.0%, OR 2.81, 95% CI 1.1–7.4), anti-ssDNA (19.0% versus 5.7%, OR 3.76, 95% CI 1.1–12.4) and thyroperoxidase antibody (33.3% versus 15.7%, OR 2.68, 95% CI 1.2–6.1) was significantly higher in VDlow than those of VDnl (P < 0.05 each). Peripheral blood CD19⁺ B and CD56⁺ NK cell levels and NK cytotoxicity at effector to target cell (E:T) ratio of 25:1 were significantly higher in VDlow when compared with those of VDnl (P < 0.05 each). Reduction (%) of NK cytotoxicity (at E:T ratio of 50:1 and 25:1) by IgG (12.5 mg/dl) was significantly lower in VDlow than those of VDnl (P < 0.05, P < 0.01, respectively). There were no differences in Th1/Th2 ratios between VDlow and VDnl. When vitamin D₃ was added in NK cytotoxicity assay in vitro, NK cytotoxicity at E:T ratio of 50:1 was significantly suppressed with 10 nMol/L (nM) (11.9 ± 3.3%) and 100 nM (10.9 ± 3.7%) of vitamin D₃ when compared with controls (15.3 ± 4.7%) (P < 0.01 each). TNF-α/IL-10 expressing CD3⁺/4⁺ cell ratios were significantly decreased with 100 nM of vitamin D₃ (31.3 ± 9.4, P < 0.05) when compared with controls (40.4 ± 11.3) in vitro. Additionally, INF-γ/IL-10 expressing CD3⁺/4⁺ cell ratio was significantly decreased with 100 nM of vitamin D₃ (12.1 ± 4.0, P < 0.05) when compared with controls (14.8 ± 4.6). IFN-γ and TNF-α secretion from NK cells were significantly decreased (P < 0.01 each), and IL-10, IL-1β, vascular endothelial growth factor and granulocyte colony stimulating factor levels were significantly increased (P < 0.01 each) with vitamin D₃ 100 nM when compared with those of controls. The prevalence of vitamin D deficiency in women with RPL in this study is open to a possible type I error since women with vitamin D supplementation were excluded from this study. Assessment of vitamin D level is recommended in women with RPL. Vitamin D supplementation should be explored further as a possible therapeutic option for RPL. This work was supported by the intramural funding from Department of Microbiology and Immunology, Chicago Medical School at Rosalind Franklin University of Medicine and Science. None of the authors has any conflict of interest to declare. N/A.