Regulation of MHC class II antigen presentation by sorting of recycling HLA-DM/DO and class II within the multivesicular body.

MHC class II molecules bind antigenic peptides in the late endosomal/lysosomal MHC class II compartments (MIIC) before cell surface presentation. The class II modulatory molecules HLA-DM and HLA-DO mainly localize to the MIICs. Here we show that DM/DO complexes continuously recycle between the plasma membrane and the lysosomal MIICs. Like DMbeta and the class II-associated invariant chain, the DObeta cytoplasmic tail contains potential lysosomal targeting signals. The DObeta signals, however, are not essential for internalization of the DM/DO complex from the plasma membrane or targeting to the MIICs. Instead, the DObeta tail determines the distribution of both DM/DO and class II within the multivesicular MIIC by preferentially localizing them to the limiting membrane and, in lesser amounts, to the internal membranes. This distribution augments the efficiency of class II antigenic peptide loading by affecting the efficacy of lateral interaction between DM/DO and class II molecules. Sorting of DM/DO and class II molecules to specific localizations within the MIIC represents a novel way of regulating MHC class II Ag presentation.

Modulation of the major histocompatibility complex class II-associated peptide repertoire by human histocompatibility leukocyte antigen (HLA)-DO.

Antigen presentation by major histocompatibility complex class II molecules is essential for antibody production and T cell activation. For most class II alleles, peptide binding depends on the catalytic action of human histocompatibility leukocyte antigens (HLA)-DM. HLA-DO is selectively expressed in B cells and impedes the activity of DM, yet its physiological role remains unclear. Cell surface iodination assays and mass spectrometry of major histocompatibility complex class II-eluted peptides show that DO affects the antigenic peptide repertoire of class II. DO generates both quantitative and qualitative differences, and inhibits presentation of large-sized peptides. DO function was investigated under various pH conditions in in vitro peptide exchange assays and in antigen presentation assays using DO(-) and DO(+) transfectant cell lines as antigen-presenting cells, in which effective acidification of the endocytic pathway was prevented with bafilomycin A(1), an inhibitor of vacuolar ATPases. DO effectively inhibits antigen presentation of peptides that are loaded onto class II in endosomal compartments that are not very acidic. Thus, DO appears to be a unique, cell type-specific modulator mastering the class II-mediated immune response induced by B cells. DO may serve to increase the threshold for nonspecific B cell activation, restricting class II-peptide binding to late endosomal compartments, thereby affecting the peptide repertoire.

HLA-DO is a negative modulator of HLA-DM-mediated MHC class II peptide loading.

BACKGROUND: Class II molecules of the major histocompatibility complex become loaded with antigenic peptides after dissociation of invariant chainderived peptides (CLIP) from the peptide-binding groove. The human leukocyte antigen (HLA)-DM is a prerequisite for this process, which takes place in specialised intracellular compartments. HLA-DM catalyses the peptide-exchange process, simultaneously functioning as a peptide 'editor', favouring the presentation of stably binding peptides. Recently, HLA-DO, an unconventional class II molecule, has been found associated with HLA-DM in B cells, yet its function has remained elusive. RESULTS: The function of the HLA-DO complex was investigated by expression of both chains of the HLA-DO heterodimer (either alone or fused to green fluorescent protein) in human Mel JuSo cells. Expression of HLA-DO resulted in greatly enhanced surface expression of CLIP via HLA-DR3, the conversion of class II complexes to the SDS-unstable phenotype and reduced antigen presentation to T-cell clones. Analysis of peptides eluted from HLA-DR3 demonstrated that CLIP was the major peptide bound to class II in the HLA-DO transfectants. Peptide exchange assays in vitro revealed that HLA-DO functions directly at the level of class II peptide loading by inhibiting the catalytic action of HLA-DM. CONCLUSIONS: HLA-DO is a negative modulator of HLA-DM. By stably associating with HLA-DM, the catalytic action of HLA-DM on class II peptide loading is inhibited. HLA-DO thus affects the peptide repertoire that is eventually presented to the immune system by MHC class II molecules.

Introduction of oxygen into the alkyl chain of N-decyl-dNM decreases lipophilicity and results in increased retention of glucose residues on N-linked oligosaccharides.

N-Alkylation of the alpha-glucosidase inhibitor 1-deoxynojirimycin (dNM) dramatically increases its inhibitory potency (Tan et al., J. Biol. Chem., 266, 14504-14510, 1991). However, the possibility of extending the alkyl chain to N-decyl-dNM is limited by an increase of detergent-like (amphiphilic) properties of long-chain alkylated dNM derivatives. Substitution of methylene groups in the N-decyl chain by oxygen reduced the amphiphilicity of N-decyl-dNM derivatives, while retaining their superior inhibitory properties. In intact HepG2 cells, the compound N-7-oxadecyl-dNM was found to result in the most pronounced retention of glucose residues on N-linked glycans. Permeabilization of the plasma membrane with the bacterial toxin Streptolysin O improves the inhibitory properties of the derivatives N-3,6,9-trioxadecyl-, N-7,10,13-trioxatetradecyl-, N-3-oxadecyl- and N-7-oxadecyl-dNM, but not those of dNM. These observations suggest differences in the mode of entry of the oxygen-substituted dNM derivatives in comparison with dNM. We observed that the dNM derivative N-3,6,9-trioxadecyl-dNM, devoid of inhibitory activity in intact cells, was inhibitory in Streptolysin O-permeabilized cells. Thus, the permeability barriers posed by plasma membrane and endoplasmic reticulum membrane are not equivalent. The use of a permeabilized cell system thus allows the elaboration of inhibitory principles for novel bioactive compounds where study of the isolated enzymes may not be possible, and where intact cells are not a suitable target due to permeability barriers.

Lack of correlation between levels of MHC class I antigen and susceptibility to lysis of small cellular lung carcinoma (SCLC) by natural killer cells.

Small cellular lung carcinoma (SCLC) cell lines are susceptible to lysis by NK cells. SCLC, normally negative for MHC class I Ag, were rendered positive for HLA-A and -B Ag by two methods: treatment with IFN-gamma or transfection with HLA class I genes. Exposure to IFN-gamma induced high levels of class I Ag and reduced susceptibility to NK-mediated lysis. However, transfection with either HLA-A2, HLA-B27, or HLA-B27 with beta 2m did not result in reduced susceptibility to NK cells. These transfectants expressed amounts of HLA class I Ag comparable to those in IFN-gamma-treated, untransfected cells. Transfection with the beta 2m gene or plasmid alone neither influenced levels of surface class I Ag nor resulted in reduced susceptibility to lysis by NK cells. Thus, the effects of IFN-gamma on NK susceptibility can be dissociated from the induction of class I Ag.