Abstract
Multipotent human mesenchymal stem cells (MSCs) harbor clinically relevant immunomodulation, and HLA-G, a non-classical MHC class I molecule with highly restricted tissue expression, is one important molecule involved in these processes. Understanding of the natural regulatory mechanisms involved in expression of this elusive molecule has been difficult, with near exclusive reliance on cancer cell lines. We therefore studied the transcriptional control of HLA-G in primary isolated human bone marrow- (BM), human embryonic stem cell-derived (hE-), as well as placenta-derived MSCs (P-MSCs), and found that all 3 types of MSCs express 3 of the 7 HLA-G isoforms at the gene level; however, fibroblasts did not express HLA-G. Protein validation using BM- and P-MSCs demonstrated expression of 2 isoforms including a larger HLA-G-like protein. Interferon-γ (IFN-γ) stimulation upregulated both gene and protein expression in MSCs but not the constitutively expressing JEG-3 cell line. Most interestingly in human MSCs and placental tissue, hypomethylation of CpG islands not only occurs on the HLA-G proximal promoter but also on the gene body as well, a pattern not seen in either of the 2 commonly used choriocarcinoma cell lines which may contribute to the unique HLA-G expression patterns and IFN-γ-responsiveness in MSCs. Our study implicates the importance of using normal cells and tissues for physiologic understanding of tissue-specific transcriptional regulation, and highlight the utility of human MSCs in unraveling the transcriptional regulation of HLA-G for better therapeutic application.
Highlights
Mesenchymal stem cells (MSCs) are somatic progenitors with multilineage differentiation capacity as well as strong immunomodulatory properties [1,2]
We found that bone marrow (BM)- and placenta-derived MSCs (P-MSCs) express HLA-G1/G5 as well as a larger protein, approximately 70 kDa, which was detected in JEG-3 cells (Figure 1B)
Levels of DNA methylation within the promoter region correlate inversely with human leukocyte antigen G (HLA-G) gene expression levels: lack of methylation correlate with high expression levels as seen with JEG-3 cells, partial methylation correlate with low expression levels as seen in the normal, primary MSCs, whereas near-complete methylation correlate with no HLA-G expression as seen in JAR cells
Summary
Mesenchymal stem cells (MSCs) are somatic progenitors with multilineage differentiation capacity as well as strong immunomodulatory properties [1,2]. MSC immunomodulation in particular has become increasingly important for clinical application since these properties have been shown to be broad, affecting multiple populations of leukocytes ranging from adaptive immune cells such as CD4 and CD8 T lymphocytes, to innate immune cells including dendritic cells, monocytes, macrophages, and myeloid-lineage cells [1,4,5]. These clinical trials have not yielded consistent results and have fueled continued interest in elucidating the mechanisms involved in MSC immunodulation to better tailor use of these progenitors to specific disease entities [6,7]. HLA-G involvement in MSC immunodulation has been reported for adaptive/CD4 T cell responses [16,17,18,19]; developmentally early sources of MSCs have been found to express HLA-G protein that have functional relevance in suppressing innate leukocyte responses [20,21]
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