X box-like sequences in the MHC class II region maintain regulatory function

Area of Science:

• Immunogenetics
• Molecular Biology
• Epigenetics

Background:

• The human major histocompatibility complex (MHC) class II (MHC-II) genes are crucial for immune responses.
• Regulatory elements like the X box are essential for MHC-II gene transcription.
• The role of novel regulatory sequences in MHC-II gene expression remains an active area of research.

Purpose of the Study:

• To identify and characterize novel regulatory elements homologous to the MHC class II X box within the human MHC-DR subregion.
• To investigate the binding of transcription factors and transcriptional activity of these identified sequences.
• To elucidate the associated histone modifications and their role in regulating MHC-II gene expression.

Main Methods:

• Sequence homology analysis to identify X box-like (XL) sequences.
• Electrophoretic mobility shift assays (EMSAs) to assess transcription factor binding (RFX, CIITA).
• Chromatin immunoprecipitation (ChIP) assays to determine histone modifications.
• Analysis of gene expression in CIITA-positive and -negative B cell lines.
• Chromatin looping assays to investigate DNA interactions.

Main Results:

• Identification of X box-like (XL) sequences within the HLA-DR subregion with homology to canonical MHC-II X box elements.
• XL sequences bind MHC class II transcription factors RFX and CIITA and exhibit transcriptional activity.
• CIITA-specific histone modifications were identified at active XL boxes, showing similarities to the HLA-DRA promoter.
• Differences in histone modification magnitudes distinguished the HLA-DRA promoter from XL boxes, particularly concerning general transcription factor assembly.
• IFN-gamma stimulation led to increased acetylation of XL box-associated histones, correlating with CIITA expression and MHC-II regulation.
• Chromatin looping assays demonstrated an interaction between an XL box and the HLA-DRA promoter.

Conclusions:

• Certain XL box sequences contribute to increased chromatin accessibility in the HLA-DR region.
• These XL sequences are involved in the regulation of MHC-II genes in B lymphocytes, particularly in response to IFN-gamma.
• The findings support a role for XL sequences in the cell type-specific and inducible expression of MHC-II genes through epigenetic mechanisms.