MHC class II regulation by epigenetic agents and microRNAs.

MicroRNAs have been shown to regulate gene expression both transcriptionally and translationally. Here, we examine evidence that various stresses regulate miRNAs which, in turn, regulate immune gene levels. Multiple studies are reviewed showing altered microRNA levels in normal cells under stress and in various disease states, including cancer. Unexpected was the finding that Dicer expression is altered by treatments with several agents, such as interferons and cortisone, employed in the treatment of immune disorders. Potential signal transduction pathways, including JAK/Stat, PI3K and PKR, that may regulate Dicer and microRNA levels in normal and stressed mammalian cells are discussed.

Restoration of immune response gene induction in trophoblast tumor cells associated with cellular senescence.

Trophoblast cells and many cancer cells that harbor foreign antigens may evade immunity by epigenetic silencing of key immune response genes, including MHC class I and II and CD40. Chromatin active agents, such as histone deacetylase inhibitors (HDACi), induce immune response gene expression but often the expression levels are low and the cells lack a robust antigen presentation response. We show here that pre-treatment of trophoblast cells and certain cancer cells with agents that activate stress pathways (Ras oncogene, PMA or H2O2) and induce senescence can substantially enhance the induction of immune response genes (MHC class II, CD40, MICA, MICB) by HDACi and restore a vigorous IFN-gamma response in trophoblast cells and tumor cells. These results could potentially impact the development of novel anti-cancer therapeutic strategies.

Histone deacetylase inhibitors induce TAP, LMP, Tapasin genes and MHC class I antigen presentation by melanoma cells.

Histone deacetylase inhibitors (HDACi), including trichostatin A (TSA) and valproic acid, can alter the acetylation of histones in chromatin and enhance gene transcription. Previously we demonstrated that HDACi-treated tumor cells are capable of presenting antigen via the MHC class II pathway. In this study, we show that treatment with HDACi enhances the expression of molecules (TAP1, TAP2, LMP2, LMP7, Tapasin and MHC class I) involved in antigen processing and presentation via the MHC class I pathway in melanoma cells. HDACi treatment of B16F10 cells also enhanced cell surface expression of class I and costimulatory molecules CD40 and CD86. Enhanced transcription of these genes is associated with a significant increase in direct presentation of whole protein antigen and MHC class I-restricted peptides by TSA-treated B16F10 cells. Our data indicate that epigenetic modification can convert a tumor cell to an antigen presenting cell capable of activating IFN-gamma secreting T cells via the class I pathway. These findings suggest that the abnormalities, observed in some tumors in the expression of MHC class I antigen processing and presentation molecules, may result from epigenetic repression.

MicroRNA targets in immune genes and the Dicer/Argonaute and ARE machinery components.

We studied 613 genes which regulate immunity and, utilizing predictive algorithms, identified 285 genes as microRNA (miRNA or miR) targets. Of these, approximately 250 are newly predicted gene-miR interactions. The frequency of predicted miRNA binding sites in immune gene 3'UTRs indicated preferential targeting of immune genes compared to the genome. Major targets include transcription factors, cofactors and chromatin modifiers whereas upstream factors, such as ligands and receptors (cytokines, chemokines and TLRs), were, in general, non-targets. About 10% of the immune genes were 'hubs' with eight or more different miRNAs predicted to target their 3'UTRs. Hubs were focused on certain key immune genes, such as BCL6, SMAD7, BLIMP1, NFAT5, EP300 and others. NF-kappaB and p53 do not themselves have binding sites for miRNAs but rather these pathways are targeted by miRNAs at downstream sites. MHC class II genes lacked miRNA targets but binding sites were identified in the CIITA gene and were shown experimentally to repress IFN-gamma-induced MHC class II activation. Unexpectedly, factors involved in regulating message stability via AU-rich elements (ARE) were heavily targeted. Moreover, multiple components involved in the generation and effector functions of miRNAs (Dicer and Argonautes) were themselves miRNA targets suggesting that a subset of miRNAs may indirectly control their own production as well as other miRNAs.