Small Molecule Inhibition of IFN‐γ‐Induced Major Histocompatibility Complex Class II Expression by Thyroid Cells

ABSTRACT We have previously shown that Mycobacterium tuberculosis attenuates cell surface expression of major histocompatibility complex class II molecules in response to gamma interferon (IFN-γ) by a mechanism dependent on intracellular sequestration of α,β dimers. In this study we examined whether intracellular alkalinization due to mycobacterial urease could account for the defect in intracellular trafficking of class II molecules. Phagocytosis of wild-type Mycobacterium bovis BCG was associated with secretion of ammonia intracellularly, which increased substantially upon addition of exogenous urea to the culture medium. Increased intracellular ammonia, due to urea degradation by the bacterium, correlated with inhibition of class II surface expression. Conversely, no ammonia was detected in cells infected with a urease-negative mutant strain of M. bovis BCG, which also displayed a reduced effect on surface expression of class II molecules. A direct cause-effect relationship between urease and class II molecule trafficking was established with experiments where cells ingesting beads coated with purified urease showed an increased ammonia level and decreased surface expression of class II in response to IFN-γ. In contrast to BCG, infection of macrophages with Mycobacterium smegmatis, which expresses relatively greater urease activity in cell-free culture, had a marginal effect on both the intracellular level of ammonia and class II expression. The limited effect of M. smegmatis was consistent with a failure to resist intracellular killing, suggesting that urease alone is not sufficient to resist macrophage microbicidal mechanisms and that this is required for a more distal effect on cell regulation. Our results demonstrate that alkalinization of critical intracellular organelles by pathogenic mycobacteria expressing urease contributes significantly to the intracellular retention of class II dimers.

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Histone Deacetylase 1/mSin3A Disrupts Gamma Interferon-Induced CIITA Function and Major Histocompatibility Complex Class II Enhanceosome Formation

Molecular and Cellular Biology ◽

10.1128/mcb.23.9.3091-3102.2003 ◽

2003 ◽

Vol 23 (9) ◽

pp. 3091-3102 ◽

Cited By ~ 71

Author(s):

Eleni Zika ◽

Susanna F. Greer ◽

Xin-Sheng Zhu ◽

Jenny P.-Y. Ting

Keyword(s):

Major Histocompatibility Complex ◽

Major Histocompatibility Complex Class ◽

Trichostatin A ◽

Class Ii ◽

Gamma Interferon ◽

Complex Class ◽

Major Histocompatibility ◽

Mhc Ii ◽

Histocompatibility Complex ◽

Ifn Γ

ABSTRACT The class II transactivator (CIITA) is a master transcriptional regulator of major histocompatibility complex class II (MHC-II) promoters. CIITA does not bind DNA, but it interacts with the transcription factors RFX5, NF-Y, and CREB and associated chromatin-modifying enzymes to form an enhanceosome. This report examines the effects of histone deacetylases 1 and 2 (HDAC1/HDAC2) on MHC-II gene induction by gamma interferon (IFN-γ) and CIITA. The results show that an inhibitor of HDACs, trichostatin A, enhances IFN-γ-induced MHC-II expression, while HDAC1/HDAC2 inhibits IFN-γ- and CIITA-induced MHC-II gene expression. mSin3A, a corepressor of HDAC1/HDAC2, is important for this inhibition, while NcoR, a corepressor of HDAC3, is not. The effect of this inhibition is directed at CIITA, since HDAC1/HDAC2 reduces transactivation by a GAL4-CIITA fusion protein. CIITA binds to overexpressed and endogenous HDAC1, suggesting that HDAC and CIITA may affect each other by direct or indirect association. Inhibition of HDAC activity dramatically increases the association of NF-YB and RFX5 with CIITA, the assembly of CIITA, NF-YB, and RFX5 enhanceosome, and the extent of H3 acetylation at the MHC-II promoter. These results suggest a model where HDAC1/HDAC2 affect the function of CIITA through a disruption of MHC-II enhanceosome and relevant coactivator-transcription factor association and provide evidence that CIITA may act as a molecular switch to modulate MHC-II transcription by coordinating the functions of both histone acetylases and HDACs.

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