In women with RPL, as the number of previous spontaneous miscarriages increased, the LBR was decreased even though the prevalence of genetically normal miscarriages was increased [11]. By the time of five or more spontaneous miscarriages, a live birth rate without any treatment is lower than 50%. In other words, more than half of them would miscarry without any treatment. Several risk factors for RPL have been reported, including thyroid dysfunction, antiphospholipid syndrome, and thrombophilia, such as decreased Factor XII, low protein C or S activities [12]. Indeed, 25.0% of the participants in this study received anti-coagulation treatment using low-dose aspirin due to thrombophilia. Ford et al. stated that 40–50% of women with PRL are unexplained [13]. Since not all pregnancy losses underwent genetic study, the known etiology might include fetal chromosomal abnormalities. According to the previous report [11], the prevalence of miscarriages with a euploid pregnancy might be as high as 50% of women who experienced four spontaneous miscarriages. Hence, half of the women who miscarried a euploid pregnancy might have other underlying etiologies, such as immune etiologies.
It is well-known that the conceptus is considered a semi-allograft to a mother. Transplantation of a semi-allogenic solid organ might be immunologically challenging to the recipient. However, there is no immunological rejection in the maternal-fetal interface during implantation. The mechanism of this paradoxical immune phenomenon has been explored by the sophisticated regulation of Th1, Th2, Th17, and Treg cells, in normal pregnancy [14]. Contrarily, dysregulated Th1 and Th2 cells and Th1/Th2 cell ratios have been detected in both PRL and RIF patients compared to normal pregnant women [5, 6].
Tacrolimus is an immunosuppressive agent commonly utilized to reduce the immunological rejection of solid allogenic organ transplantation, such as kidney or liver, in transplant recipients [15, 16]. Tacrolimus, a macrolide antibiotic calcineurin inhibitor, inhibits calcineurin action through binding to FKBP12 in lymphocyte [17]. Consequently, T cell activation is blocked, and cytokine production is reduced, including INF-ϒ, IL-2, IL-4, and TNF-α.
Previously, we reported that tacrolimus could improve the clinical pregnancy rate among the RIF patients with elevated Th1/Th2 cell ratios [6]. The increased Th1/Th2 cell ratio can be two reasons: the high level of Th1 cell and low level of Th2 cell. The RIF patients with elevated Th1/Th2 cell ratios usually showed a high level of Th1 cells, immunologically reacting against the transferred embryos. In our study, intracellular INF-ϒ+/ IL-4− CD4+ Th1 cells were defined as Th1 cells. Therefore, tacrolimus might inhibit the secretion of INF-ϒ from the T cells and improve embryonic implantation.
Previous reports indicated that the RPL patients showed higher Th1 cells, such as INF-ϒ or TNF-α secreting Th cells, resulting in elevated Th1/Th2 cell ratios [5]. Higher Th1 cell levels indicate an increase of INF-ϒ and TNF-α secretion from the Th cells, which might interfere with placental development. Hence, a similar immune mechanism underlines implantation failure and PRL. The immune effector can be regulated by the interaction between uterine extravillous trophoblasts and immune cells, including T cells, and the dysregulation of immune effectors can lead to adverse results, such as spontaneous miscarriage. The tacrolimus might modulate dysregulated T cells by inhibiting the release of inflammatory cytokines, such as INF-ϒ and TNF-α.
According to the previous report [11], the LBR among the RPL patients who experienced 4–6 spontaneous miscarries was 56.6% (175/309). In this paper, the miscarriage rate in the control group was 55.2%, which was comparable to the previous report. The RPL patients in the previous study did not receive any medical treatments before and after the establishment of pregnancy even though they experienced more than four times of miscarriages. By contrast, the RPL patients with elevated Th1/Th2 cell ratios in the treatment group of this study received tacrolimus like our previous study of the RIF patients with similar immunological conditions. As a result of this intervention, 70.4% of the women gave birth (LBR = 70.4%), and this was statistically higher than that of the controls (44.8%, p < 0.05). Hence, the intervention of underlying immunopathology improved the outcome of patients with the evidence of immunopathology.
Moreover, we evaluated the relationship between the LBR and the tacrolimus dosage in this study. The LBR in the 1mg daily dose group (52.2%) was significantly lower than that of the ≥ 3mg daily group (93.3%, p < 0.05). This result showed a similar trend to our previous report, which indicated that the ongoing pregnancy rate of 1mg daily dose group for the RIF patients was lower than that of the 3mg daily dose group.18 We report that 1mg daily dose of tacrolimus might not be adequate for the maintenance of pregnancy, and higher dosages are needed for women with RPL and Th1/Th2 cell ratio between 10.3 and 13.0.
In conclusion, the immunosuppressive agent, tacrolimus, improved the live-birth rate of women who experienced four or more RPL with elevated Th1/Th2 cell ratios than those without tacrolimus treatment. In women with unexplained RPL, immunological evaluation including Th1/Th2 cell proportions and ratios should be considered. If Th1/Th2 cell-related immune etiologies are defined, tacrolimus treatment can be considered. Currently, the guideline about COVID-19 and immunomodulation treatment for women with PRL and RIF was published [19]. The low dose tacrolimus treatment can be utilized in women with RPL and elevated Th1/Th2 cell ratios during the COVID-19 pandemic with careful follow-up.
This study investigated the clinical efficacy of tacrolimus, a T cell activation inhibitor, for women who experienced consecutive PRLs (≥ 4) with elevated Th1/Th2 cell ratios. However, the study was limited since it was an open-label, prospective cohort study but not a randomized trial. Another limitation was that the number of subjected is still small. A further study is needed on larger scale to confirm these findings.