HLA-G histocompatibility antigen, class I, G, also known as human leukocyte antigen G (HLA-G), is a protein that in humans is encoded by the HLA-Ggene.[1]
HLA-G belongs to the HLA nonclassical class I heavy chain paralogues. This class I molecule is a heterodimer consisting of a heavy chain and a light chain (beta-2 microglobulin). The heavy chain is anchored in the membrane. HLA-G is expressed on fetal derived placental cells. The heavy chain is approximately 45 kDa and its gene contains 8 exons. Exon one encodes the leader peptide, exons 2 and 3 encode the alpha1 and alpha2 domain, which both bind the peptide, exon 4 encodes the alpha3 domain, exon 5 encodes the transmembrane region, and exon 6 encodes the cytoplasmic tail.[1] Exon 7 and 8 are not translated due to a stop codon present in exon 6.[2]
HLA-G may play a role in immune tolerance in pregnancy, being expressed in the placenta by extravillous trophoblast cells (EVT), while the classical MHC class I genes (HLA-A and HLA-B) are not.[3] As HLA-G was first identified in placenta samples, many studies have evaluated its role in pregnancy disorders, such as preeclampsia and recurrent pregnancy loss.[4] its downregulation is related to HLA-A and -B downregulation results in protection from cytotoxic T cell responses, but would in theory result in a missing self response by natural killer cells. HLA-G is a ligand for NK cell inhibitory receptor KIR2DL4, and therefore expression of this HLA by the trophoblast defends it against NK cell-mediated death.[5]
However, a large family with several members bearing only "null" HLA-G alleles has been found. None of these homozygous subjects have pregnancy or birth difficulties; nor do they present immunodeficiencies, autoimmune diseases, or tumors.[6][7] It is striking that this "null" allele (HLA-G*01:05N), while it is quite frequent in some populations, like in Iranians, it is almost absent in some Amerindian populations.[8] Also, some higher primates do not show all MHC-G isoforms.[9] In addition, Cercopithecinae middle-sized Old World monkeys do not bear full MHC-G molecules since all of these monkeys present stop codons at MHC-G DNA.[10] All of these anomalies must be studied.
The presence of soluble HLA-G (sHLA-G) in embryos is associated with better pregnancy rates. In order to optimize pregnancy rates, there is significant evidence that a morphological scoring system is the best strategy for the selection of embryos.[11] However, presence of soluble HLA-G might be considered as a second parameter if a choice has to be made between embryos of morphologically equal quality.[11]
↑Lash, G, Robson, S, Bulmer, J. (2010). "Review: Functional role of uterine natural killer (uNK) cells in human early pregnancy decidua". Placenta. 31 (S): 87–92. doi:10.1016/j.placenta.2009.12.022. PMID20061017.CS1 maint: Multiple names: authors list (link)
↑Suárez MB, Morales P, Castro MJ, Fernández V, Varela P, Alvarez M, Martínez-Laso J, Arnaiz-Villena A (1997). "A new HLA-G allele (HLA-G*0105N) and its distribution in the Spanish population". Immunogenetics. 45 (6): 464–5. doi:10.1007/s002510050235. PMID9089111.
↑Casro MJ, Morales P, Rojo-Amigo R, Martinez-Laso J, Allende L, Varela P, Garcia-Berciano M, Guillen-Perales J, Arnaiz-Villena A (September 2000). "Homozygous HLA-G*0105N healthy individuals indicate that membrane-anchored HLA-G1 molecule is not necessary for survival". Tissue Antigens. 56 (3): 232–9. doi:10.1034/j.1399-0039.2000.560305.x. PMID11034559.
↑Arnaiz-Villena A, Enriquez-de-Salamanca M, Areces C, Alonso-Rubio J, Abd-El-Fatah-Khalil S, Fernandez-Honrado M, Rey D (April 2013). "HLA-G(∗)01:05N null allele in Mayans (Guatemala) and Uros (Titikaka Lake, Peru): Evolution and population genetics". Hum. Immunol. 74 (4): 478–82. doi:10.1016/j.humimm.2012.12.013. PMID23261410.
↑Castro MJ, Morales P, Martinez-Laso J, Allende L, Rojo-Amigo R, Gonzalez-Hevilla M, Varela P, Moscoso J, Garcia-Berciano M, Arnaiz-Villena A (November 2000). "Lack of MHC-G4 and soluble (G5, G6) isoforms in the higher primates, Pongidae". Hum. Immunol. 61 (11): 1164–8. doi:10.1016/s0198-8859(00)00189-0. PMID11137222.
↑Castro MJ, Morales P, Fernández-Soria V, Suarez B, Recio MJ, Alvarez M, Martín-Villa M, Arnaiz-Villena A (1996). "Allelic diversity at the primate Mhc-G locus: exon 3 bears stop codons in all Cercopithecinae sequences". Immunogenetics. 43 (6): 327–36. doi:10.1007/bf02199801. PMID8606053.
↑ 11.011.1Rebmann V, Switala M, Eue I, Grosse-Wilde H (May 2010). "Soluble HLA-G is an independent factor for the prediction of pregnancy outcome after ART: a German multi-centre study". Hum Reprod. 25 (7): 1691–8. doi:10.1093/humrep/deq120. PMID20488801.
↑Gao GF, Willcox BE, Wyer JR, Boulter JM, O'Callaghan CA, Maenaka K, Stuart DI, Jones EY, Van Der Merwe PA, Bell JI, Jakobsen BK (May 2000). "Classical and nonclassical class I major histocompatibility complex molecules exhibit subtle conformational differences that affect binding to CD8alphaalpha". J. Biol. Chem. 275 (20): 15232–8. doi:10.1074/jbc.275.20.15232. PMID10809759.
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Carosella ED, Moreau P, LeMaoult J, Rouas-Freiss N (2008). "HLA-G: From biology to clinical benefits". Trends in Immunology. 29 (3): 125–32. doi:10.1016/j.it.2007.11.005. PMID18249584.
Arnaiz-Villena A, Martinez-Laso J, Alvarez M, et al. (1997). "Primate Mhc-E and -G alleles". Immunogenetics. 46 (4): 251–66. doi:10.1007/s002510050271. PMID9218527.
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Langat DK, Hunt JS (2003). "Do nonhuman primates comprise appropriate experimental models for studying the function of human leukocyte antigen-G?". Biol. Reprod. 67 (5): 1367–74. doi:10.1095/biolreprod.102.005587. PMID12390864.
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