CIITA methylation and decreased levels of HLA-DR in tumour progression

Structural aspects of the MHC expression control system

The major histocompatibility complex (MHC) spans innate and adaptive immunity by presenting antigenic peptides to CD4⁺ and CD8⁺ T cells. Multiple transcription factors form an enhanceosome complex on the MHC promoter and recruit transcriptional machinery to activate gene transcription. Immune signals such as interferon-γ (IFN-γ) control MHC level by up-regulating components of the enhanceosome complex. As MHC plays crucial roles in immune regulation, alterations in the MHC enhanceosome structure will alter the pace of rapid immune responses at the transcription level and lead to various diseases related to the immune system. In this review, we discuss the current understanding of the MHC enhanceosome, with a focus on the structures of MHC enhanceosome components and the molecular basis of MHC enhanceosome assembly.

Distinct Molecular Mechanisms of Altered HLA Class II Expression in Malignant Melanoma

BACKGROUND

The human leukocyte antigen (HLA) class II molecules are constitutively expressed in some melanoma, but the underlying molecular mechanisms have not yet been characterized.

METHODS

The expression of HLA class II antigen processing machinery (APM) components was determined in melanoma samples by qPCR, Western blot, flow cytometry and immunohistochemistry. Immunohistochemical and TCGA datasets were used for correlation of HLA class II expression to tumor grading, T-cell infiltration and patients' survival.

RESULTS

The heterogeneous HLA class II expression in melanoma samples allowed us to characterize four distinct phenotypes. Phenotype I totally lacks constitutive HLA class II surface expression, which is inducible by interferon-gamma (IFN-γ); phenotype II expresses low basal surface HLA class II that is further upregulated by IFN-γ; phenotype III lacks constitutive and IFN-γ controlled HLA class II expression, but could be induced by epigenetic drugs; and in phenotype IV, lack of HLA class II expression is not recovered by any drug tested. High levels of HLA class II APM component expression were associated with an increased intra-tumoral CD4+ T-cell density and increased patients' survival.

CONCLUSIONS

The heterogeneous basal expression of HLA class II antigens and/or APM components in melanoma cells is caused by distinct molecular mechanisms and has clinical relevance.

MHC Class II levels and intracellular localization in human dendritic cells are regulated by calmodulin kinase II

Dendritic cells (DC) are professional APC, which activate the adaptive immune response. A Ca2+-calmodulin (CaM)-CaM kinase II (CaMKII) pathway regulates maturation and MHC Class II antigen presentation in human DC. The objective of this study was to characterize the mechanisms by which CaMKII modulates the levels and subcellular distribution of MHC Class II molecules. Inhibition of CaMKII via the highly specific, autoinhibitory peptide derived from the enzyme's regulatory domain resulted in rapid (60 min) and sustained (24 h) reduction of MHC Class II levels in antigen-stimulated, primary, human DC. The initial depletion of intracellular and cell surface MHC Class II was associated with its enhanced lysosomal trafficking and increased activity of specific proteases in the absence of effects on other transmembrane proteins (CD1b and CD34) or a detectable change in lysosomal degradation of exogenous protein. Inhibition of CaMKII also resulted in significant reductions in the level and stability of MHC Class II mRNA and the levels and nucleocytosolic localization of its major transcriptional regulator CIITA. These data support a model in which CaMKII regulates the levels and localization of MHC Class II protein in human DC via transcriptional, post-transcriptional, and post-translational mechanisms. These pathways are likely important to the physiologic regulation of MHC Class II as well as to its dysregulation in disease states associated with altered CaMKII function.