Differences between human sperm and somatic cell DNA in CpG methylation within the HLA class I chromosomal region

HLA-G transactivation by cAMP-response element-binding protein (CREB). An alternative transactivation pathway to the conserved major histocompatibility complex (MHC) class I regulatory routes

The expression of HLA-G in extravillous cytotrophoblast cells coincides with a general lack of classical major histocompatibility complex (MHC) class I expression in this tissue. This differential expression of HLA-G and classical HLA class I molecules in trophoblasts suggests a tight transcriptional control of MHC class I genes. Transactivation of the classical MHC class I genes is mediated by two groups of juxtaposed cis-acting elements that can be viewed as regulatory modules. Both modules are divergent in HLA-G, rendering this gene unresponsive to NF-kappaB, IRF1, and class II transactivator (CIITA)-mediated induction pathways. In this study, we searched for alternative regulatory elements in the 1438-bp HLA-G promoter region. HLA-G was not responsive to interferon-alpha (IFNalpha), IFNbeta, or IFNgamma, despite the presence of an upstream ISRE binding IRF1 in vitro. However, the HLA-G promoter contains three CRE/TRE elements with binding affinity for CREB/ATF and Fos/Jun proteins both in vitro and in vivo. In transient transfection assays, it was shown that HLA-G transactivation is regulated by CREB, CREB-binding protein (CBP), and p300. Moreover, immunohistochemical analysis demonstrated that HLA-G is co-expressed with CREB and CBP in extravillous cytotrophoblasts, revealing the in vivo relevance of this transactivation pathway. This implies a unique regulation of HLA-G transcription among the MHC class I genes.

Major histocompatibility complex expression on human, male germ cells: a review

PROBLEM

The male reproductive compartment is an immunologically privileged site. The expression pattern of human leukocyte antigens (HLAs) may play an important role in the maintenance of immune tolerance toward differentiating gametogenic cells. This review presents current knowledge about HLA gene expression on human, male germ cells, on mRNA and protein levels, and on their methylation status.

METHOD OF STUDY

Different techniques were applied to study HLA gene expression in human testis: (a) protein: e.g., cytotoxicity test, fluorescent labeling techniques, enzyme-linked immunosorbent assay, and confocal microscopy; (b) mRNA: reverse transcriptase-polymerase chain reaction, Northern blot hybridization, and in situ hybridization; and (c) methylation status.

RESULTS

In normal testicular tissue we observe a lack of HLA-class I (classical) antigens expression and inversely related expression pattern of HLA class I classical and nonclassical genes. HLA-A, -B, -C, and -E loci are likewise methylated in somatic and germ cells, whereas -F and -G genes are less methylated in sperm precursors.

CONCLUSIONS

Immunologic tolerance in human testis is actively maintained by the specific expression pattern of HLA genes regulated by hormones and growth factors.

HLA expression on immature and mature human germ cells

Human leukocyte antigens (HLA) are membrane-bound glycoproteins encoded by the human major histocompatibility complex located on chromosome 6. They are known to function in immnunologic recognition and, with regard to reproduction, a number of non-immune functions have been proposed. Although the expression patterns of the major histocompatibility antigens have been extensively studied at the maternal fetal interface, there are still controversial reports on the expression of these molecules by human gametes and preimplantation stages. This brief review focuses on recent studies where the expression and distribution of HLA on human spermatogenic cells (spermatogonia, primary and secondary spermatocytes, spermatids, spermatozoa), primary and secondary oocytes, and preimplantation embryos have been investigated. These results, and their possible implications for the fertilization process and further embryonic development, will be presented.

CpG methylation patterns in the 5' part of the nonclassical HLA-G gene in peripheral blood CD34+ cells and CD2+ lymphocytes

A dominant goal of research focused on the nonclassical human leukocyte antigen G (HLA-G) gene is to understand the molecular mechanism involved in its limited expression. In the present report, we examined DNA methylation as a potential regulatory mechanism of HLA-G transcription in two cell types of the adult lymphomyeloid lineage: CD2+ lymphocytes express several mRNA isoforms while transcripts are undetectable in CD34+ hematopoietic cells. The methylation status of 63 CpG sites in the promoter and in the 5' CpG island was established using bisulfite-treated genomic DNA sequencing. Methylation was first analyzed by the direct sequencing of bisulfite-treated and amplified products. The general patterns of CpG methylation in the 5' part of the gene were found to be similar for CD34+ cells and CD2+ lymphocytes: the distribution of methylation was not uniform across the 63 CpG sites. In the promoter region, both CpG dinucleotides were partially or fully methylated whereas in the CpG island, several CpG sites were totally demethylated. Unexpectedly, in HLA-G positive CD2+ lymphocytes, a great number of CpG dinucleotides displayed a higher frequency of methylation relative to that found in CD34+ cells. However, the sequence analysis of cloned products revealed that the molecules have different methylation patterns which suggests that the HLA-G gene is differentially expressed in CD2+ cells. Our results suggest that methylation is not the sole mechanism that achieves the repression of HLA-G transcription in immature CD34+ cells.

Establishment of methylation-sensitive-representational difference analysis and isolation of hypo- and hypermethylated genomic fragments in mouse liver tumors

Methylation of CpG sites in the genome, which is generally conserved during cell replication, is considered to play important roles in cell differentiation and carcinogenesis. However, investigations on changes in methylation status have been limited to known genes. To make a genome-wide search for differentially methylated genes, we developed a methylation-sensitive-representational difference analysis (MS-RDA) method. The representation of the genome was prepared using the methylation-sensitive restriction enzyme HpaII, and the mixture ratio of tester and driver DNAs was optimized to detect differences in methylation status of a single copy per diploid mammalian genome. By performing comparative MS-RDA of one hepatocellular carcinoma and of background liver tissue of one mouse treated with a food carcinogen (2-amino-3,4-dimethylimidazo[4,5-f] quinoline), we were able to identify (i) extensive hypomethylation of long interspersed nuclear element repetitive sequences in a number of hepatocellular carcinomas, (ii) reduction of the gene dosage of their mitochondrial DNA, and (iii) a hypermethylated DNA fragment of unknown origin. Furthermore, by adding the clones obtained in the first MS-RDA to the driver DNA [MS-RDA with elimination of excessive clones (MS-RDA-WEEC)], nine DNA fragments that could not be detected at the first MS-RDA were isolated as differentially methylated DNA fragments. MS-RDA, combined with MS-RDA-WEEC, is thus a promising approach to identify DNA fragments differentially methylated in two DNA sources.

HLA-G transcription studies during the different stages of normal and malignant hematopoiesis

Specific expression of the non classical class I HLA-G gene on trophoblasts, the only fetal tissue in contact with maternal cells which lack MHC class I antigens, may indicate a role of this gene in fetal-maternal tolerance. We recently reported HLA-G transcription in peripheral blood leukocytes. In this work, we have investigated HLA-G transcription in hematopoietic stem cells, in different hematopoietic lineages and in malignant cells by using a RT-PCR technique. PCR amplification with primers specific to the exon 2 and the 3' untranslated region has enabled to detect HLA-G transcription in B and T cell populations. No transcription was found in CD34+ cells, in thymocytes, in polynuclear cells, in monocytes and in natural killer cells. Among the malignancies analyzed, HLA-G is transcribed in 2 of 13 cases of acute leukemia characterized by a monocytic contingent, in 3 of 6 CLL and in all the cases of B-NHL (n = 6). No HLA-G transcription was detected in myeloma (n = 2). The splicing type does not seem to be linked to a lymphocyte subpopulation nor to a malignant proliferation stage. These results suggest that HLA-G is a marker of mature lymphoid cells and may play an immunological function as a peptide presenting molecule. HLA-G transcription in some cases of malignancy might indicate a contribution to the tumoral progression by blocking natural killing reaction.

HLA-G transcription studies during the different stages of normal and malignant hematopoiesis

Specific expression of the non classical class I HLA-G gene on trophoblasts, the only fetal tissue in contact with maternal cells which lack MHC class I antigens, may indicate a role of this gene in fetal-maternal tolerance. We recently reported HLA-G transcription in peripheral blood leukocytes. In this work, we have investigated HLA-G transcription in hematopoietic stem cells, in different hematopoietic lineages and in malignant cells by using a RT-PCR technique. PCR amplification with primers specific to the exon 2 and the 3' untranslated region has enabled to detect HLA-G transcription in B and T cell populations. No transcription was found in CD34+ cells, in thymocytes, in polynuclear cells, in monocytes and in natural killer cells. Among the malignancies analyzed, HLA-G is transcribed in 2 of 13 cases of acute leukemia characterized by a monocytic contingent, in 3 of 6 CLL and in all the cases of B-NHL (n = 6). No HLA-G transcription was detected in myeloma (n = 2). The splicing type does not seem to be linked to a lymphocyte subpopulation nor to a malignant proliferation stage. These results suggest that HLA-G is a marker of mature lymphoid cells and may play an immunological function as a peptide presenting molecule. HLA-G transcription in some cases of malignancy might indicate a contribution to the tumoral progression by blocking natural killing reaction.

Placental expression of HLA class I genes

This article presents an overview of the more recent data dealing with the constitutive, transcriptional, and translational expression of classical class Ia and nonclassical HLA-E and -G class Ib products in the different trophoblast cell subpopulations that constitute the maternofetal interface during human pregnancy. Of particular interest is the expression of alternatively spliced HLA-G transcriptional isoforms that may be translated in membrane-bound or soluble protein products. Molecular regulatory mechanisms that may control the differential expression of class Ia and class Ib molecules, according to the cell types, state of differentiation, and stages of gestation are also examined. They may operate at the levels of transcription, translation and/or transport of proteins to the cell surface. Functional significance of the absence of detectable cell surface expression of class Ia molecules in all trophoblast cell subpopulations, and of the presence of membrane-bound HLA-G products in extravillous cytotrophoblast cells is finally questioned.