The regulation mechanism of HLA class II gene expression at the level of mRNA stability

Differential expressions of MHC-DQB1 mRNA in Chinese merino sheep infected with Echinococosus granuclosus

The purpose of the present study was to investigate the dynamic changes of MHC-DQB1 mRNA expression in sheep infected with Echinococosus granuclosus. A total of 14 healthy Chinese merino sheep were experimentally infected with E. granuclosus. The blood samples were collected on days 0 (initiation of the infection), 7, 21, 30, and 60 post-infection, respectively. On day 60 post-infection, when the experiment was terminated, all sheep were euthanized to make a diagnosis of cystic echinococcosis (CE) using routine meat inspection and microscopical examination, respectively. The sheep were then divided into two groups according to the diagnostic results: group A (n = 8) consisted of sheep which were diagnosed as CE infection, while group B (n = 6) comprised sheep diagnosed as self-cured or healthy controls. Blood samples obtained during the period of the study were correspondingly divided into groups A and B. The mRNA expression levels of DQB1 revealed significant alterations detected at different stages of E. granuclosus infection in the two groups. Results showed that in group A, DQB1 mRNA expression underwent a progressive increase from day 0 to day 21 post-infection (P = 0.073), and suddenly, suffered from a dramatic drop until day 30 post-infection, and then jumped rapidly and peaked on day 60 post-infection (P = 0.004). Meanwhile, in group B, DQB1 mRNA expression displayed a sharp increase from day 0 to day 7 post-infection (P = 0.000), which thereafter showed a marked decrease until day 30 post-infection, and experienced a plateau from day 30 to day 60 post-infection, remaining at or above that on day 0. It is concluded that DQB1 mRNA expression levels varied in different stages of E. granuclosus infection in sheep. In addition, it appears that the ability to eliminate the parasites possibly depends, at least in part, on the DQB1 expression in the early stage of infection, especially in the first week post-infection.

EBP1 and DRBP76/NF90 binding proteins are included in the major histocompatibility complex class II RNA operon

Major histocompatibility complex class II mRNAs encode heterodimeric proteins involved in the presentation of exogenous antigens during an immune response. Their 3'UTRs bind a protein complex in which we identified two factors: EBP1, an ErbB3 receptor-binding protein and DRBP76, a double-stranded RNA binding nuclear protein, also known as nuclear factor 90 (NF90). Both are well-characterized regulatory factors of several mRNA molecules processing. Using either EBP1 or DRBP76/NF90-specific knockdown experiments, we established that the two proteins play a role in regulating the expression of HLA-DRA, HLA-DRB1 and HLA-DQA1 mRNAs levels. Our study represents the first indication of the existence of a functional unit that includes different transcripts involved in the adaptive immune response. We propose that the concept of 'RNA operon' may be suitable for our system in which MHCII mRNAs are modulated via interaction of their 3'UTR with same proteins.

The MHC class II transactivator (CIITA) mRNA stability is critical for the HLA class II gene expression in myelomonocytic cells

The human promyelocytic U937 cells express detectable levels of MHC class II (MHC-II) molecules. Treatment with 12-o--tetradecanoyl phorbol 13-acetate (TPA), inducing macrophage-like differentiation, produces a dramatic decrease of MHC-II expression as result of down-modulation of the activation of immune response gene 1 (AIR-1)-encoded MHC-II transactivator (CIITA). This event is specific, as MHC class I remains unaffected. Similar results are observed with U937 cells expressing an exogenous full-length CIITA. Molecular studies demonstrate that TPA treatment affects the stability of CIITA mRNA rather than CIITA transcription. Importantly, cis-acting elements within the distal 650 bp of the 1035-bp 3' untranslated region (3'UTR, nucleotides 3509-4543) are associated to transcript instability. Transcription inhibitors actinomycin D and 5,6-dichlororibofuranosyl benzimidazole, and the translation inhibitor cycloheximide significantly rescue the accumulation of CIITA mRNA in TPA-treated cells. A similar effect is also observed after treatment with staurosporine and the PKC-specific inhibitor GF109203X. The instability of CIITA mRNA produced by TPA in U937 cells is not seen in B cells. These results demonstrate the presence of an additional level of control of MHC-II expression in the macrophage cell lineage depending upon the control of CIITA mRNA stability, most likely mediated by differentiation-induced, 3'UTR-interacting factors which require kinase activity for their destabilizing function.

Characterization of expression at the human XIST locus in somatic, embryonal carcinoma, and transgenic cell lines

X inactivation requires XIST, a functional RNA that is expressed exclusively from, and localizes to, the inactive X in female somatic cells. In mouse, low-level unstable transcription of Xist is observed prior to the time of inactivation, and an antisense transcript, Tsix, is a critical regulator of early Xist expression. To examine the presence and impact of an antisense transcript in humans we have characterized the extent of sense and antisense transcription in human somatic, transgenic, and embryonal carcinoma (EC) cell lines. Downstream antisense expression at the human XIST locus was not detected in somatic cells, but was detected in the EC line N-Tera2D1 and in somatic cells with an ectopic XIST locus. Presence of the antisense did not disrupt the stability or localization of the sense transcript. We have also identified additional sense transcripts in EC and female somatic cells and demonstrate that the 5' flanking JPX/ENOX gene is expressed from both the active and the inactive X chromosome in somatic cell hybrids, delimiting the extent of inactive X-specific transcriptional control in somatic cells. These analyses reveal similarities to and differences from the murine Xist and Tsix transcripts and generate a complex picture of developmentally regulated transcription through the region.

Pathways for the regulation of interferon-gamma-inducible genes by iron in human monocytic cells

To elucidate iron-regulated interferon-gamma (IFN-gamma) effector functions, we investigated three IFN-gamma-inducible genes [intercellular adhesion molecule-1 (ICAM-1), human leukocyte antigen (HLA)-DR, guanosine 5'-triphosphate-cyclohydrolase I (GTP-CH)] in primary human monocytes and the cell line THP-1. IFN-gamma increased the surface expression of ICAM-1 and HLA-DR and stimulated GTP-CH activity. Addition of iron before cytokine stimulation resulted in a dose-dependent reduction of these pathways, and iron restriction by desferrioxamine (DFO) enhanced ICAM-1, HLA-DR, and GTP-CH expression. Iron neither affected IFN-gamma binding to its receptor nor IFN-gamma receptor surface expression. IFN-gamma-inducible mRNA expression of ICAM-1, HLA-DR, and GTP-CH was reduced by iron and increased by DFO by a transcriptional mechanism. Moreover, ICAM-1 and to a lesser extent, GTP-CH and HLA-DR mRNA expression were regulated post-transcriptionally, as iron pretreatment resulted in shortening the mRNA half-life compared with cells treated with IFN-gamma alone. Thus, iron perturbations regulate IFN-gamma effector pathways by transcriptional and post-transcriptional mechanisms, indicating that iron rather interferes with IFN-gamma signal-transduction processes.

Maturation of dendritic cells is accompanied by rapid transcriptional silencing of class II transactivator (CIITA) expression

Cell surface expression of major histocompatibility complex class II (MHCII) molecules is increased during the maturation of dendritic cells (DCs). This enhances their ability to present antigen and activate naive CD4(+) T cells. In contrast to increased cell surface MHCII expression, de novo biosynthesis of MHCII mRNA is turned off during DC maturation. We show here that this is due to a remarkably rapid reduction in the synthesis of class II transactivator (CIITA) mRNA and protein. This reduction in CIITA expression occurs in human monocyte-derived DCs and mouse bone marrow-derived DCs, and is triggered by a variety of different maturation stimuli, including lipopolysaccharide, tumor necrosis factor alpha, CD40 ligand, interferon alpha, and infection with Salmonella typhimurium or Sendai virus. It is also observed in vivo in splenic DCs in acute myelin oligodendrocyte glycoprotein induced experimental autoimmune encephalitis. The arrest in CIITA expression is the result of a transcriptional inactivation of the MHC2TA gene. This is mediated by a global repression mechanism implicating histone deacetylation over a large domain spanning the entire MHC2TA regulatory region.