HLA-G MOLECULES: FROM EMBRYO IMPLANTATION TO OOCYTE MATURATION

Background Assisted reproduction technique (ART) pregnancy rates have not changed in recent years and an increased risk of twins, triplets or higher order pregnancies leads to a perinatal mortality and morbidity. Studies have therefore involved the identification of non-invasive methods to determine the oocyte/embryo quality allowing fewer embryos to be transferred while maintaining or improving pregnancy rates. In order to increase the chance of a successful pregnancy, the most viable embryos must be transfered but current knowledge of suitable biochemical markers that could predict the viability of embryos is extremely limited. The selection of embryos to be transferred is conducted using morphological aspects, cleavage speed and development appearance. This embryo scoring system could help in selecting the best embryo for transfer but it has limited ability to predict the implantation potential of individual embryos. The clinical challenge is to establish a marker of embryo competency that could increase the pregnancy rate following ART and reduce the number of multiple pregnancies. Successful implantation in the human is dependent on the early embryo ability to avoid the maternal immune system. The fetus is considered a semi-allograft but, in normal pregnancies, it is not rejected by the maternal immune system. The presence of a complex signalling system, with molecules passing from the conceptus to the mother throughout pregnancy, is appealing and embryo suppressor factors responsible for early implantation have been proposed. One of the key protective mechanisms is thought to be the expression of non classical HLA class I HLA-G molecules by trophoblasts. Due to its importance in reproductive immunology it has been considered a possible marker for oocyte/embryo selection. The HLA-G gene is located at the telomeric part of the 6p21-3 chromosomal region, near the HLA-A locus. It exhibits the typical structure of a classical HLA class I gene with a similar exon/intron organization. The HLA-G antigen has some characteristics that differentiate it from classical HLA class I antigens. The HLA-G molecule has a restricted tissue distribution, being expressed in physiological conditions by cytotrophoblasts and thymus. The allelic polymorphism is limited to 36 alleles. The HLA-G gene is characterized by a 14 base pair insertion/deletion polymorphism (rs16375) in exon 8 in the 3’ untranslated region (UTR) that is associated with mRNA stability and HLA-G protein expression. The allele with an insertion of 14 bp has been associated with lower levels of HLA-G expression than the allele with the 14 bp deleted. Seven different HLA-G transcriptional isoforms, derived from mRNA splicing, have been described. Four of these encode membrane-bound products (HLA-G1, -G2, -G3, -G4), the other three soluble proteins (HLA-G5, -G6, -G7). It is well known that the biological functions of the classical HLA-class I and class II molecules are related to the complex mechanism of antigen recognition. The high polymorphism of the HLA structures represent a guarantee for the development of an efficient response against different viral and bacterial antigens whilst the elevated number of alleles is responsible for the allogeneic response resulting in the rejection of transplanted organs. HLA class Ia and HLA class II genes are totally unexpressed in cytothrophoblast cells preventing the consequential development of a semiallogenic response of the maternal CD8 positive T cells. However, the absence of HLA-Ia molecules would enhance the natural killer (NK) mediated cell cytotoxicity that is normally inhibited by the presence on target cells of the classical HLA-I determinants. The modulation of HLA-C and the nearly monomorphic HLAG molecules by invasive cytotrophoblasts prevents the allogeneic response and maintain a tolerogenic microenvironment. Membrane-bound HLA-G1 and soluble HLA-G (HLA-G5 and sHLA-G1) molecules exert immunosuppressive effects: (i) inhibit the cytotoxic activity of CD8 positive T lymphocytes (CTL) and NK cells, (ii) induce the apoptosis of NK and activated cytotoxic T cells, (iii) inhibit the allogeneic CD4 positive T-cell proliferation and interfere with naïve CD4 positive T-cell priming, (iv) inhibit antigen presenting cell and B lymphocyte differentiation, (v) induce regulatory T cells. sHLA-G affects angiogenesis interacting with endothelial cells and induces resting NK cells to produce chemokines and cytokines [1]. The functions of HLA-G molecules are due to their ability to act as a ligand for different receptors expressed by immune cells. HLA-G interacts with NK receptor KIR2DL4 and leukocyte inhibitory receptors (LILRs) / immunoglobulin-like transcripts (ILT) as LILRB1 (LIR-1/ILT2/CD85j), which is highly expressed on T and B lymphocytes and with LILRB2 (LIR-2/ILT4/CD85d), present mainly in monocytes/macrophages. Aim This thesis reports on several studies of the HLA-G molecule and its implication in pregnancy and embryo implantation. Methods and Results The analysis of the soluble HLA-G (sHLA-G) levels in lypopolysaccharide (LPS)- activated peripheral blood mononuclear cell (PBMC) cultures from healthy subjects has revealed no differences between the three HLA-G insertion/deletion 14 bp genotypes (+14/+14 bp, -14/+14 bp, +14/+14 bp), while higher concentrations of interleukin (IL)-10, the main up-modulator of HLA-G production, have been observed in the +14/+14bp LPS-PBMC cultures [2]. Our data support the hypothesis of a feed-back loop mechanism between HLA-G and IL-10 molecules, which sustains their production. The -14/-14 bp and -14/+14 bp HLA-G samples with a - 477 G/G single nucleotide polymorphism (SNP) genotype in the 5’ upstream regulatory region (5’URR) of the HLA-G gene have presented a higher IL-10 concentration in LPS-PBMC cultures. These observations could indicate that the – 477 SNP might have an independent impact on IL-10 concentration and that the differences are not only a consequence of linkage disequilibrium between the G -477 SNP polymorphism and the -14 bp 3’UTR polymorphism. -477 SNP polymorphism is located very close to a putative heat shock element (HSE) and could influence the binding of the heat shock factor 1 (HSF1) leading to differences in IL-10 and sHLAG expression. The levels of sHLA-G are increased in the plasma samples of pregnant women during the first trimester in comparison to non-pregnant women. On the contrary sHLA-G plasma levels decrease during the third trimester while it has an impressive boost at delivery [3]. We have analyzed sHLA-G and IL-10 levels in the plasma samples of 43 women (15 non-allergic, 28 allergic) during third trimester, at delivery and 2 years after pregnancy. A significant increase in sHLA-G and IL-10 levels has been documented at delivery regardless of the allergic status, however, allergic women have shown lower sHLA-G concentrations in comparison with non-allergic women. The reduced sHLA-G levels have not been caused by deficient IL-10 production, as allergic and non-allergic women presented equal amounts at all three time points investigated. This indicates that other factors involved in sHLA-G production and/or regulation differ between these two groups of women. It is possible that the Th2 cytokine microenvironment present in an allergic individual differently influences the sHLA-G secretion. Two years after pregnancy, the two groups have presented equal levels as the allergic women seem to experience a prime during pregnancy that is still evident two years after pregnancy, suggesting the presence of immunological changes imposed by pregnancy and still evident two years after labour. Our data have demonstrated that sHLA-G1 molecules are the most frequent isoform in plasma (75-80%) in both allergic and non-allergic women during labour. As sHLA-G1 molecules are mainly originated by metalloproteinase (MMP)- dependent shedding at post-translational level of the membrane antigens, it could be hypothesized that sHLA-G1 could derive from the placenta disruption during labour that is characterized by an increase in MMP-9 amounts. Several data have suggested an important role for HLA-G molecules in the survival of human embryos. HLA-G expression has been documented not only on trophoblast cells but also in preimplantation human embryos. Jurisicova et al. [4] have shown HLA-G heavy chain specific mRNA in about 40% of the 148 embryos tested. HLAG proteins at 2-cell stage and an increased embryo cleavage rate when compared to the embryos without HLA-G transcripts were detected. These results propose a variable expression of HLA-G during the critical period of preimplantation embryonic development. In order to have an in vitro and non-invasive system to analyze embryo behaviour towards sHLA-G production, an in vitro fertilization protocol was used, where the oocytes are fertilized in vitro and the embryos are transferred to the woman 2-3 days after fertilization. This allowed the analysis of the embryo culture supernatants for sHLA-G presence by a specific immunoenzymatic assay. In 2002 the first in vivo confirmation of the pivotal role of HLA-G molecules in embryo implantation was presented [5]. The presence of sHLA-G molecules in 285 supernatants from cultures containing one to four embryos obtained from ART has been analyzed. Although no clinical differences have been observed between the women, positive embryo implantation occurred only in women with sHLA-G molecules in embryo culture supernatants (p= 2.56×10–3, Fisher’s exact p test). This is the first observation made in humans to prove the importance of HLA-G expression in embryo implantation. In 2004 the analysis of sHLA-G molecules in supernatants from 318 single embryo cultures was presented [6]. We have confirmed a significant relationship between the secretion of these molecules by an early embryo and a higher implantation rate (p= 0.045, Mann-Whitney U test). These data propose the sHLA-G analysis in embryo supernatants as a useful marker, together with morphological characterization, for the selection of embryos to be transferred. Since 2002 up to six thousand supernatants from single ART procedure embryos have been analyzed for sHLA-G presence. Discrepancies in the embryo culture protocol and the sHLA-G detection systems have not yet allowed the importance of sHLA-G as an embryo quality marker to be confirmed and studies are still needed to standardize the procedures to sustain the data obtained [7,8]. No hypotheses have yet been advanced on the absence of HLA-G expression in a percentage of early embryos obtained by ART. The presence of germinal defects or an impaired IL-10 secretion can be hypothesized. The presence of sHLA-G in the supernatants of single embryo cultures from couples admitted to a second fertilization procedure has been analyzed. These couples have previously shown a complete absence of sHLA-G in the first cycle embryo supernatants (0/31) [9]. The results obtained in the second in vitro fertilization cycle have shown some embryo supernatants positive for HLA-G (14/40), suggesting that the previous lack of antigen modulation is independent of germinal defects. The levels of IL-10 in the same embryo culture supernatants have been also investigated. No associations have been observed between the presence of IL-10, the production and levels of sHLA-G and pregnancy outcome. These results indicate that the lack of sHLA-G production in some early embryos is not related to germinal defects or IL-10 impairment and suggest a gestational programming of sHLA-G secretion. Several ethical and legislative problems are increasing the necessity to reduce also the number of fertilized oocytes. Nowadays the oocyte selection is mainly performed by intra and extracytoplasmic morphological characteristics, but no data documents a clear association between the morphology and implantation outcome. The oocyte ability to mature, be fertilized and to develop into a viable embryo starts with oocyte growth during the first steps of follicular development and goes on until the final “oocyte capacitation” that seems to rely on the storage of messenger RNAs and proteins that will support early stages of embryo development, before full activation of embryonic genome. It is known that in the early developmental stage of the fertilized oocytes the transcription is silenced and the activation of the human embryonic genome starts between the 4- and 8-cell stages, approximately 70 hours after fertilization. Follicular fluid (FF) represents a specific microenvironment for oocyte maturation and a possible relationship has been proposed between specific FF components and ART outcome. 50 FFs were analyzed for sHLA-G molecule presence [10] and detectable sHLA-G molecules were observed in 31.2% FFs. To investigate the possible functional significance of sHLA-G molecules in FFs, we have related the sHLA-G in FFs and in the corresponding 4-8-cell early embryos. This analysis has shown a significant relationship between sHLA-G presence in FFs and in the corresponding embryo culture supernatants (p= 1.3x10-6; Fisher exact p test). These results could suggest the analysis of sHLA-G in FFs as a reliable and non-invasive tool for oocytes selection to obtain embryos with an elevated ability to modulate HLA-G expression and consequently a higher implantation rate. Granulosa cells and the polymorphonuclear population have been identified as sHLA-G producers but because of ethical problems it was not possible to characterize the oocyte. In order to confirm that sHLA-G is involved in oocyte maturation, 152 in vitro maturated oocytes were analyzed of which culture supernatants could be characterized for sHLA-G presence without the influence of the maternal microenvironment [11]. Our results have demonstrated that the cumulus-oocyte complex (COC), characterized by the surrounding granulosa cells and the oocyte, produces sHLA-G. The sHLA-G molecules were present in 19% of mature COC culture supernatants. On the contrary no sHLA-G molecules have been detected in the culture supernatants from immature COCs (p= 8.4 x 10-5; Fisher exact p test). These results show, for the first time, the ability of mature COCs to produce sHLA-G antigens that seem to be a marker for oocyte maturation. Conclusions Further research on HLA-G and pregnancy to evaluate the possible correlation between the oocyte and the corresponding embryo sHLA-G production and to confirm the value of sHLA-G as a marker of oocyte/embryo competency is required. Work is also necessary to improve standardization of sHLA-G detection in order to obtain comparable results prior to use HLA-G as an oocyte/embryo selection marker. The sHLA-G molecules is a research response to the need for a rational basis to select few and possibly a single competent oocyte/embryo each time, while maintaining optimal ART success rates. The future of ART foresees the combination of morphologic evaluations with a biochemical assessment of molecules that represent a marker of embryo competency. Future identification of additional molecular markers of oocyte/embryo competency and health can improve these non-invasive methods and their research and therapeutic potential. The culture supernatants of 39 immature and 73 mature COCs and the corresponding preimplantation embryos for the presence of proteins involved in inflammation, including several cytokines, chemokines (IL-1b, IL-1ra, IL-2, IL-4, IL-5, IL-6, IL-7, IL-8, IL-9, IL-10, IL-12 (P70), IL-13, IL-15, IL-17, Basic FGF, Eotaxin, G-CSF, GM-CSF, IFN-g, IP-10, MCP-1 (MCAF), MIP-1a, MIP-1b, PDGFBB, RANTES, TNF-, VEGF) and soluble intercellular adhesion molecule 1 (sICAM-1) have been analyzed [12]. The proteins present in the supernatants were sICAM-1 and IL-1r, however, only sICAM-1 was expressed at high levels. The sICAM-1 release is very high in immature COCs, decreases in mature COCs (p < 0.0001, Student t Test) and become even lower in preimplantation embryos (p < 0.0001, Student t Test). No significant differences have been observed in sICAM-1 levels between immature oocytes with different morphological characteristics. On the contrary, the high grade mature COCs have presented the lower sICAM levels. sICAM-1 seems to have a clear tendency to decrease from immature to mature COCs and to fertilized embryos and it could be a possible biochemical marker for COC maturation and grading. In the future ART laboratories may be able to use morphologic parameters and these non-invasive biochemical markers for single embryo transfer, so reducing the risk of multiple gestation and increasing the pregnancy rate. References 1. Baricordi OR, Stignani M, Melchiorri L, Rizzo R. HLA-G and inflammatory diseases Inflamm. Allergy Drug Targets 7(2), 67-74 (2008). 2. Hviid TV, Rizzo R, Melchiorri L, Stignani M, Baricordi OR. Polymorphism in the 5’ upstream regulatory and 3’ untranslated regions of the HLA-G gene in relation to soluble HLA-G and IL-10 expression. Hum. Immunol. 67, 53–62 (2006). 3. Rizzo R, Stignani M, Amoudruz P, et al. Allergic women have reduced sHLAG plasma levels at delivery. Submitted to Am. J. Immunol. (2008). 4. Jurisicova A, Casper RF, MacLusky NJ, Librach CL. Embryonic human leukocyte antigen-G expression: possible implications for human preimplantation development. Fertil. Steril. 65, 997-1002 (1996). 5. Fuzzi B, Rizzo R, Criscuoli L, et al. HLA-G expression in early embryos is a fundamental prerequisite for the obtainment of pregnancy. Eur. J. Immunol. 32, 311-315 (2002). 6. Noci I, Fuzzi B, Rizzo R, et al. Embryonic soluble HLA-G as a marker of developmental potential in embryos. Hum. Reprod. 20(1), 138-146 (2005). 7. Rizzo R, Baricordi OR. HLA-G expression and regulation in early embryos. Am. J. Reprod. Immunol. 56(1), 17 (2006). 8. Rizzo R, Melchiorri L, Stignani M, Baricordi OR. HLA-G expression is a fundamental prerequisite to pregnancy. Human Immunol. 68(4), 244-250 (2007). 9. Criscuoli L, Rizzo R, Fuzzi B, et al. Lacking of HLA-G expression in early embryos is not related to germinal defects or impairment in IL-10 embryos production. Gynecological Endocrinology 20(5), 264-269 (2005). 10. Rizzo R, Fuzzi B, Stignani M, et al. Soluble HLA-G molecules in follicular fluid: a tool for oocyte selection in IVF? J. Reprod. Immunol. 74(1-2), 133-42 (2007). 11. Rizzo R, Dal Canto MB, Stignani M, et al. Production of sHLA-G molecules by “in vitro” matured cumulus-oocyte complex. Submitted to J. Reprod. Immunol. (2008). 12. Borgatti M, Rizzo R, Canto MB, et al. Release of sICAM-1 in oocytes and in vitro fertilized human embryos. PLoS ONE 3(12), e3970 (2008).