title: Epigenetic mechanisms regulate MHC and antigen processing molecules in human embryonic and induced pluripotent stem cells.
abstract: BACKGROUND
  Human embryonic stem cells (hESCs) are an attractive resource for new therapeutic approaches that involve tissue regeneration. hESCs have exhibited low immunogenicity due to low levels of Mayor Histocompatibility Complex (MHC) class-I and absence of MHC class-II expression. Nevertheless, the mechanisms regulating MHC expression in hESCs had not been explored.
  
  
  RESULTS
  We analyzed the expression levels of classical and non-classical MHC class-I, MHC class-II molecules, antigen-processing machinery (APM) components and NKG2D ligands (NKG2D-L) in hESCs, induced pluripotent stem cells (iPSCs) and NTera2 (NT2) teratocarcinoma cell line. Epigenetic mechanisms involved in the regulation of these genes were investigated by bisulfite sequencing and chromatin immunoprecipitation (ChIP) assays. We showed that low levels of MHC class-I molecules were associated with absent or reduced expression of the transporter associated with antigen processing 1 (TAP-1) and tapasin (TPN) components in hESCs and iPSCs, which are involved in the transport and load of peptides. Furthermore, lack of beta2-microglobulin (beta2m) light chain in these cells limited the expression of MHC class I trimeric molecule on the cell surface. NKG2D ligands (MICA, MICB) were observed in all pluripotent stem cells lines. Epigenetic analysis showed that H3K9me3 repressed the TPN gene in undifferentiated cells whilst HLA-B and beta2m acquired the H3K4me3 modification during the differentiation to embryoid bodies (EBs). Absence of HLA-DR and HLA-G expression was regulated by DNA methylation.
  
  
  CONCLUSIONS
  Our data provide fundamental evidence for the epigenetic control of MHC in hESCs and iPSCs. Reduced MHC class I and class II expression in hESCs and iPSCs can limit their recognition by the immune response against these cells. The knowledge of these mechanisms will further allow the development of strategies to induce tolerance and improve stem cell allograft acceptance.
authors.affiliation: Histocompatibility and Transplantation Unit, Hospital Universitario Central de Asturias, Oviedo, Spain.
authors.forename: Beatriz
authors.initials: B
authors.lastname: Suárez-Alvarez
authors.forename: Ramón M
authors.initials: RM
authors.lastname: Rodriguez
authors.forename: Vincenzo
authors.initials: V
authors.lastname: Calvanese
authors.forename: Miguel A
authors.initials: MA
authors.lastname: Blanco-Gelaz
authors.forename: Steve T
authors.initials: ST
authors.lastname: Suhr
authors.forename: Francisco
authors.initials: F
authors.lastname: Ortega
authors.forename: Jesus
authors.initials: J
authors.lastname: Otero
authors.forename: Jose B
authors.initials: JB
authors.lastname: Cibelli
authors.forename: Harry
authors.initials: H
authors.lastname: Moore
authors.forename: Mario F
authors.initials: MF
authors.lastname: Fraga
authors.forename: Carlos
authors.initials: C
authors.lastname: López-Larrea
mesh_terms: D017951:Antigen Presentation; D053595:Embryonic Stem Cells / Q000276:immunology*; D044127:Epigenesis, Genetic; D006649:Histocompatibility Antigens; D015395:Histocompatibility Antigens Class I; D000949:Histocompatibility Antigens Class II; D006801:Humans; D007109:Immunity; D057026:Induced Pluripotent Stem Cells / Q000276:immunology*
chemical_list: D006649:Histocompatibility Antigens; D015395:Histocompatibility Antigens Class I; D000949:Histocompatibility Antigens Class II