Επιγενετική ρύθμιση της ανοσολογικής απόκρισης: Υποπυρηνική οργάνωση και χρωματίνη

Abstract

Transcription of the Class II genes of the Major Histocompatibility Complex (MHCII) requires a complex of proteins that synergistically bind to the promoter and constitute the MHCII enhanceosome. Although necessary, these proteins are not sufficient to drive transcription. The master coactivator CIITA is required for efficient transcriptional initiation and elongation. The deacetylase inhibitor Trichostatin A (TSA) induces the transcription of the MHCII DRA gene in a CIITA-independent manner. To analyze the molecular mechanisms by which this epigenetic regulator stimulates MHCII expression, chromatin immunoprecipitation (ChIP) assays were employed to monitor the alterations in histone modifications that correlate with DRA transcription after TSA treatment. A dramatic increase in promoter linked histone acetylation and Histone H3 lysine 4 methylation followed by concurrent decrease of lysine 9 methylation were found. Fluorescence recovery after photobleaching (FRAP) experiments showed that TSA treatment increased the mobility of HDAC while decreasing the mobility of the class II enhanceosome factor RFX5. These data, in combination with ChIP experiments, indicate that the TSA-mediated induction of DRA transcription involves HDAC relocation and enhanceosome stabilization. In order to gain a genome-wide view of the genes responding to inhibition of deacetylases, we compared the transcriptome of B cells before and after TSA treatment using Affymetrix microarrays. This analysis showed that in addition to the DRA gene, the entire MHCII family and the adjacent histone cluster, located in chromosome 6p21-22, are strongly induced by TSA. A complex pattern of gene reprogramming by TSA involves immune recognition, antiviral, apoptotic and inflammatory pathways and extends the rationale for using Histone Deacetylase Inhibitors (HDACi) to modulate the immune response. Since many of these genes are direct targets of STAT1 transcription factor we monitored its expression after TSA treatment and found it to be induced. This induction was mediated by the simultaneous down-regulation of c-Myc oncogene expression thus revealing a negative feedback loop between the two genes. Concurrent activation by tyrosine phosphorylation of the produced STAT1 protein prompted for the examination of interferon (IFN) production. Its presence in the culture supernatant was confirmed and was identified as type I IFN. The whole antiviral response of the cell was found to be activated by TSA. The presence of Epstein Barr virus (EBV) was crucial for the initiation of this response. Adaptation is one essential property of the immune response conferred by immunological memory. The question whether IFN-gamma (IFNβ) inducible transcription generates memory that sensitizes cells to a secondary stimulus was addressed. The MHCII gene DRA was found to relocate to Promyelocytic leukemia nuclear bodies (PML-NB) upon induction with IFNβ and this topology was maintained long after transcription shut off. Concurrent interaction of PML protein with Mixed Lineage Leukemia (MLL) generates a prolonged permissive chromatin state on the DRA gene characterized by high promoter histone H3 K4 dimethylation that facilitates rapid expression upon restimulation. Primary signal-induced transcription generates spatial and epigenetic memory that is maintained through several cell generations and endows the immune system with increased responsiveness to future activation signals.