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Contents
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Regulation of MHC Class I Expression by a T Lymphocyte–Specific Enhanceosome
MHC class I molecules bind peptide fragments derived from degraded intracellular proteins and present them on the cell surface. Complementary receptors on T lymphocytes recognize foreign, non-self, peptide cargo (such as from degraded viral or transformed proteins) bound to MHC class I molecules. Whereas the immune system is tolerant to “self” peptides, recognition of “non-self” results in a peptide-specific immune response to eliminate cells bearing the foreign peptide signature. Thus, expression of MHC class I molecules is essential for immune surveillance and destruction of virally infected and transformed cells. Regulation of MHC class I expression is mediated through a series of promoter proximal elements. Over the past several years, work from a number of laboratories has demonstrated that constitutively high levels of MHC class I expression in B lymphocytes is attributable to assembly of a B lymphocyte–specific enhanceosome complex in the proximal promoter region. The B-lymphocyte enhanceosome represents the coordinated recruitment of multiple, widely expressed DNA binding transcription factors anchored by a coactivator, the class II transactivator (CIITA), which is constitutively expressed only in B lymphocytes and other antigen-presenting cells. In most other tissues, CIITA can be induced by gamma-interferon (IFNγ) to dynamically upregulate MHC class I expression. Although T lymphocytes express high levels of MHC class I, they do not normally express CIITA. Prior to the described studies, the molecular basis for high constitutive MHC class I expression in T lymphocytes was not known. However, it was known that T lymphocyte–specific genes, such as those encoding the T-cell receptor, were regulated by a T lymphocyte–specific enhanceosome (TCE). The TCE consists of the widely expressed coactivator Aly, T cell–specific transcription factors RUNX1 and LEF1, and the RUNX1 cofactor CBFβ. Aly, like CIITA and other coactivators, does not bind proximal promoter DNA sequences directly, but is recruited by DNA-bound LEF-1 and RUNX1/CBFβ transcription factors to form the TCE, which potently increases expression of cognate genes. In a recent study, we examined the ability of T-lymphocyte enhanceosome components, RUNX1/CBFβ, LEF1, and Aly to mediate T lymphocyte–specific MHC class I expression. The first evidence of a role for the TCE in regulating MHC class I gene expression came from our finding that disrupting RUNX1 in the Jurkat T-lymphocyte cell line resulted in a 50% reduction in endogenous MHC class I expression. In complementary experiments, we reconstituted the T-lymphocyte enhanceosome in HeLa epithelial cells and determined its effects on MHC class I expression. Partial reconstitution of the TCE with RUNX1/CBFβ and LEF1 enhanced the activity of an MHC class I promoter reporter construct 3 to 5 fold; neither RUNX1/CBFβ nor LEF1 alone had an effect. However, complete reconstitution of the TCE with RUNX1/CBFβ, LEF1, and Aly resulted in profound increases, both in exogenous MHC class I promoter activity and transcription of endogenous class I genes. Furthermore, chromatin immunoprecipitation from splenic lymphocytes demonstrated the association of the T-cell enhanceosome component RUNX1 with MHC class I proximal promoter sequences, providing evidence that the TCE directly regulates MHC class I gene expression in vivo. The above findings establish that the TCE—minimally composed of RUNX1/CBFβ, LEF1, and Aly—functions as part of the “intrinsic” pathway regulating MHC class I gene expression. This enabled us to examine the mechanism by which regulatory signals of the tissue-specific “intrinsic” pathways are integrated with dynamic “extrinsic” signals (such as those induced by hormones and cytokines) on a common promoter region. To this end, we examined the effect of the IFNγ-inducible CIITA “extrinsic” pathway on “intrinsic” pathway T lymphocyte–specific class I expression in HeLa epithelial cells. Interestingly, assembly of both “extrinsic” (CIITA) and “intrinsic” T-lymphocyte (RUNX1/CBFβ, LEF1, and Aly) enhanceosomes synergistically activated MHC class I transcription, suggesting that the “intrinsic” and “extrinsic” pathways are distinct and complementary, and target distinct promoter elements. These findings provide both a molecular basis for the constitutively high levels of MHC class I in T lymphocytes and their further synergistic induction by IFNγ.
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ccurate tissue-specific gene expression is, in general, the result of diverse transcriptional reflexes in response to both pre-set “intrinsic” and dynamic “extrinsic” stimuli. Many genes, including members of the major histocompatibility complex (MHC) class I family, are regulated by complex and overlapping developmental, tissue-specific, and inducible stimuli. Although MHC class I genes are ubiquitously expressed, distinct pre-set “intrinsic” stimuli lead to dramatically different tissue-specific levels of expression. For example, the lowest MHC class I levels are observed in neural tissues and germ-line cells, whereas the highest levels occur in lymphoid tissues, especially among T and B lymphocytes. Further, numerous “extrinsic” stimuli, in particular hormones and cytokines, are capable of dynamically modulating pre-set tissue-specific MHC class I expression patterns. The molecular mechanisms driving tissue-specific and dynamically modulated modes of MHC class I transcription are only partially understood and our ongoing efforts to understand these processes help us to understand how complexly regulated genes achieve proper levels of expression.