Soluble human leukocyte antigen-G is a potential embryo viability biomarker and a positive predictor of live-births in humans

Association of human leukocyte antigen-G and -F with recurrent miscarriage and implantation failure: A systematic review and meta-analysis

PROBLEM

The immune system plays an essential role in embryonic implantation and pregnancy, but the molecular details remain controversial. In the past four decades, human leukocyte antigen (HLA)-G and -F have garnered significant attention.

METHOD OF STUDY

MEDLINE, EMBASE, Web of Science, and the Cochrane Trials Registry were searched from their inception dates until December 2022. Studies were selected following PRISMA guidelines. Meta-analyses were used to assess the relationship of soluble HLA-G (sHLA-G) and HLA-G 3'-untranslated region polymorphisms with recurrent miscarriage (RM) and recurrent implantation failure (RIF). Narrative synthesis was conducted to determine the association of RM with other single nucleotide polymorphisms (SNPs) and HLA-G protein in tissues and of RIF with HLA-F. Risk-of-bias was assessed using ROBINS-I. Publication bias was assessed using Egger's and Begg's tests.

RESULTS

Finally, 42 articles were eligible for inclusion in the systematic review (32 in the meta-analysis; 13 in narrative synthesis). We found a significant association between the 14-bp ins/del HLA-G polymorphism and RM risk, but no definitive association with RIF risk. Women with RM had lower blood concentrations of sHLA-G during pregnancy and non-pregnancy than did controls. For women in the RIF group, no significant difference was found.

CONCLUSION

HLA-G protein and gene expression levels may be closely related to RM. The relevance of HLA-G to RIF is still being determined. A narrative synthesis of current studies has shown that HLA-F is likely associated with RIF.

Association of parental HLA-G polymorphisms with soluble HLA-G expressions and their roles on recurrent implantation failure: A systematic review and meta-analysis

Introduction

HLA-G plays a central role in immune tolerance at the maternal-fetal interface. The HLA-G gene is characterized by low allelic polymorphism and restricted tissue expression compared with classical HLA genes. HLA-G polymorphism is associated with HLA-G expression and linked to pregnancy complications. However, the association of parental HLA-G polymorphisms with soluble HLA-G (sHLA-G) expression and their roles in recurrent implantation failure (RIF) is unclear. The study aims to systematically review the association of HLA-G polymorphisms with RIF, the association of sHLA-G expression with RIF, and the association of HLA-G polymorphisms with sHLA-G expressions in patients attending in-vitro fertilization (IVF) treatment.

Methods

Studies that evaluated the association of HLA-G polymorphisms with RIF, the association between sHLA-G expression with RIF, and the association between HLA-G polymorphisms with sHLA-G expressions in patients attending IVF treatment were included. Meta-analysis was performed by random-effect models. Sensitivity analysis was performed by excluding one study each time. Subgroup analysis was performed based on ethnicity.

Results

HLA-G 14bp ins variant is associated with a lower expression of sHLA-G in seminal or blood plasma of couples attending IVF treatment. The maternal HLA-G*010101 and paternal HLA-G*010102 alleles are associated with RIF risk compared to other alleles. However, single maternal HLA-G 14bp ins/del polymorphism, HLA-G -725 C>G/T polymorphism, or circulating sHLA-G concentration was not significantly associated with RIF in the general population. HLA-G 14bp ins/ins homozygous genotype or ins variant was associated with a higher risk of RIF in the Caucasian population.

Discussion

Specific HLA-G alleles or HLA-G polymorphisms are associated with sHLA-G expression in couples attending IVF treatment. Several HLA-G polymorphisms may be related to RIF, considering different ethnic backgrounds. A combined genetic effect should be considered in future studies to confirm the association of HLA-G polymorphisms and sHLA-G expressions in relation to RIF.

The impact of soluble HLA-G in IVF/ICSI embryo culture medium on implantation success

The HLA-G molecule is widely accepted as an important factor for pregnancy success. Its expression has been detected in the extravillous trophoblasts. Soluble HLA-G (sHLA-G) was found in the genital tract, pre-implanted embryos as well as in seminal fluid. In this study, we investigated the concentration of sHLA-G (sHLA-G1 and sHLA-G5) in media from 344 single cultured embryos following in vitro fertilization/intracytoplasmic sperm injection (IVF/ICSI). The level of sHLA-G (U/ml) was tested with a sandwich enzyme-linked immunosorbent assay (ELISA) kit. We correlated sHLA-G secretion with ovarian stimulation protocols, the type of embryo transfer (fresh or frozen cycle) and the quality of the embryos. The ovarian stimulation protocol affects the secretion of sHLA-G by the embryo. Embryos obtained from the long agonist protocol secreted more sHLA-G than those originating from the short antagonist protocol (p = 0.0001). Embryos whose transfer resulted in a clinical pregnancy and/or live birth secreted more sHLA-G compared to those whose transfer ended without pregnancy. This was particularly observable in embryos following the long ovarian stimulation protocol and from a frozen embryo cycle. In conclusion, sHLA-G secreted by the embryo has an impact on implantation and live birth and could be a developmental potential marker of the embryo. Its concentration depends on the ovarian stimulation protocol used.

Advances in the study of HLA class Ib in maternal-fetal immune tolerance

The HLA class Ib molecule is an alloantigen that causes transplant rejection on behalf of individual human and plays an important role in maternal-fetal immune tolerance. Early studies on HLA class Ib focused on the mechanism of HLA-G-induced immune escape, but in recent years, studies on the mechanism of HLA-G have deepened and gradually explored the mechanism of HLA-E and HLA-F, which are also HLA class Ib molecules. In the maternal-fetal interface, trophoblast cells express HLA class Ib molecules to protect the fetus from maternal immune cells by binding to inhibitory receptors of decidual immune cells (DICs) and shifting Th1/Th2 balance toward Th2 bias. Further studies on the molecular mechanism of HLA class Ib molecules provide a reference for its application in the field of clinical assisted reproduction.