Direct transport of newly synthesized HLA-DR from the trans-Golgi network to major histocompatibility complex class II containing compartments (MIICS) demonstrated using a novel tyrosine-sulfated chimera

Assembly of matched alpha/beta subunits to HLA class II peptide receptors

Human antigen presenting cells express three human leukocyte antigen (HLA) class II isotypes (DR, DP, and DQ), which are composed of polymorphic α and β subunits. The combination of polymorphic α- and β-chains results in cis (encoded on the same chromosome) or trans (encoded on different chromosomes) combinations. Since some of the α-β combinations may yield mismatched non-functional α-β heterodimers, it is not entirely clear which type of HLA class II peptide receptors are found on the cell surface of antigen presenting cells. We have developed a combination of biochemical techniques for inspection of the assembly and intracellular transport of isotype matched and mismatched class II heterodimers.

Competition model for upregulation of the major histocompatibility complex class II-associated invariant chain by human immunodeficiency virus type 1 Nef

The human immunodeficiency virus type 1 (HIV-1) Nef protein upregulates the expression of the invariant chain (Ii)/major histocompatibility complex class II (MHC-II) complex at the cell surface. This complex appears to reach the antigen-loading endosomal compartment at least in part via an indirect pathway in which it is internalized from the cell surface via the adaptor protein 2 (AP-2) complex. Here we provide evidence for a competition model to explain how Nef upregulates the expression of Ii at the cell surface. In this model, Nef and Ii compete for binding to AP-2. In support of this model, Nef decreased the rate of internalization of Ii from the cell surface. The AP-binding dileucine motif in Nef, ENTSLL(165), was necessary and sufficient for the upregulation of Ii. In addition, two leucine-based AP-binding motifs in the Ii cytoplasmic tail, DDQRDLI(8) and EQLPML(17), were critical for the efficient upregulation of Ii by Nef. Experiments using Nef variants in which the native dileucine-based sorting motif was replaced with similar motifs from cellular transmembrane proteins allowed modulation of AP-binding specificity. Analysis of these variants suggested that the binding of Nef to AP-2 is sufficient to upregulate Ii at the plasma membrane. Finally, interference with the expression of AP-2 caused an upregulation of Ii at the plasma membrane, and this decreased the effect of Nef. These data indicate that Nef usurps AP-2 complexes to dysregulate Ii trafficking and potentially interfere with antigen presentation in the context of MHC-II.

Chaperoning antigen presentation by MHC class II molecules and their role in oncogenesis

Tumor vaccine development aimed at stimulating the cellular immune response focuses mainly on MHC class I molecules. This is not surprising since most tumors do not express MHC class II or CD1 molecules. Nevertheless, the most successful targets for cancer immunotherapy, leukemia and melanoma, often do express MHC class II molecules, which leaves no obvious reason to ignore MHC class II molecules as a mediator in anticancer immune therapy. We review the current state of knowledge on the process of MHC class II-restricted antigen presentation and subsequently discuss the consequences of MHC class II expression on tumor surveillance and the induction of an efficient MHC class II mediated antitumor response in vivo and after vaccination.

Involvement of clathrin and AP-2 in the trafficking of MHC class II molecules to antigen-processing compartments

Major histocompatibility complex class II (MHC-II) molecules are composed of two polymorphic chains, alpha and beta, which assemble with an invariant chain, Ii, in the endoplasmic reticulum. The assembled MHC-II complexes are transported to the Golgi complex and then to late endosomes/lysosomes, where Ii is degraded and alphabeta dimers bind peptides derived from exogenous antigens. Targeting of MHC-II molecules to these compartments is mediated by two dileucine-based signals in the cytoplasmic domain of Ii. These signals bind in vitro to two adaptor protein (AP) complexes, AP-1 and AP-2, which are components of clathrin coats involved in vesicle formation and cargo sorting. The physiological roles of these proteins in MHC-II molecule trafficking, however, remain to be addressed. Here, we report the use of RNA interference to examine the involvement of clathrin and four AP complexes (AP-1, AP-2, AP-3, and AP-4) in MHC-II molecule trafficking in vivo. We found that depletion of clathrin or AP-2 caused >10-fold increases in Ii expression on the cell surface and a concomitant decrease in Ii localization to endosomal/lysosomal vesicles. In addition, depletion of clathrin or AP-2 delayed the degradation of Ii and reduced the surface expression of peptide-loaded alphabeta dimers. In contrast, depletion of AP-1, AP-3, or AP-4 had little or no effect. These findings demonstrate that clathrin and AP-2 participate in MHC-II molecule trafficking in vivo. Because AP-2 is only associated with the plasma membrane, these results also indicate that a significant pool of MHC-II molecules traffic to the endosomal-lysosomal system by means of the cell surface.

AP2 clathrin adaptor complex, but not AP1, controls the access of the major histocompatibility complex (MHC) class II to endosomes

Newly synthesized MHC II alpha- and beta-chains associated with the invariant chain chaperone (Ii) enter the endocytic pathway for Ii degradation and loading with peptides before transport to the cell surface. It is unclear how alphabetaIi complexes are sorted from the Golgi apparatus and directed to endosomes. However, indirect evidence tends to support direct transport involving the AP1 clathrin adaptor complex. Surprisingly, we show here that knocking down the production of AP1 by RNA interference did not affect the trafficking of alphabetaIi complexes. In contrast, AP2 depletion led to a large increase in surface levels of alphabetaIi complexes, inhibited their rapid internalization, and strongly delayed the appearance of mature MHC II in intracellular compartments. Thus, in the cell systems studied here, rapid internalization of alphabetaIi complexes via an AP2-dependent pathway represents a key step for MHC II delivery to endosomes and lysosomes.

Lysosome associated membrane protein 1 (Lamp1) traffics directly from the TGN to early endosomes

The precise trafficking routes followed by newly synthesized lysosomal membrane proteins after exit from the Golgi are unclear. To study these events we created a novel chimera (YAL) having a lumenal domain comprising two tyrosine sulfation motifs fused to avidin, and the transmembrane and cytoplasmic domains of lysosome associated membrane protein 1 (Lamp1). The newly synthesized protein rapidly transited from the trans- Golgi Network (TGN) to lysosomes (t(1/2) approximately 30 min after a lag of 15-20 min). However, labeled chimera was captured by biotinylated probes endocytosed for only 5 min, indicating that the initial site of entry into the endocytic pathway was early endosomes. Capture required export of YAL from the TGN, and endocytosis of the biotinylated reagent, and was essentially quantitative within 2 h of chase, suggesting that all molecules were following an identical route. There was no evidence of YAL trafficking via the cell surface. Fusion of TGN-derived vesicles with 5 min endosomes could be recapitulated in vitro, but neither late endosomes nor lysosomes could serve as acceptor compartments. This suggests that contrary to previous conclusions, most if not all newly synthesized Lamp1 traffics from the TGN to early endosomes prior to delivery to late endosomes and lysosomes.

Human immunodeficiency virus-1 Nef expression induces intracellular accumulation of multivesicular bodies and major histocompatibility complex class II complexes: potential role of phosphatidylinositol 3-kinase

Nef alters the cell surface expression of several immunoreceptors, which may contribute to viral escape. We show that Nef modifies major histocompatibility complex class II (MHC II) intracellular trafficking and thereby its function. In the presence of Nef, mature, peptide-loaded MHC II were down-modulated at the cell surface and accumulated intracellularly, whereas immature (invariant [Ii] chain-associated) MHC II expression at the plasma membrane was increased. Antibody internalization experiments and subcellular fractionation analyses showed that immature MHC II were internalized from the plasma membrane but had limited access to lysosomes, explaining the reduced Ii chain degradation. Immunoelectron microscopy revealed that Nef expression induced a marked accumulation of multivesicular bodies (MVBs) containing Nef, MHC II, and high amounts of Ii chain. The Nef-induced up-regulation of surface Ii chain was inhibited by LY294002 exposure, indicating the involvement of a phosphatidylinositol 3-kinase, whose products play a key role in MVB biogenesis. Together, our results indicate that Nef induces an increase of the number of MVBs where MHC II complexes accumulate. Given that human immunodeficiency virus recruits the MVB machinery for its assembly process, our data raise the possibility that Nef is involved in viral assembly through its effect on MVBs.

Transient surface delivery of invariant chain-MHC II complexes via endosomes: a quantitative study

Newly synthesized major histocompatibility complex class II needs to be directed to late endocytic compartments to combine with peptide antigens. Efficient transport requires complexes of major histocompatibility complex class II and invariant chain (alphabetaIi). Since such complexes have been detected on the plasma membrane in human cells, this compartment was proposed as the primary destination for alphabetaIi exiting the trans-Golgi network. Here, I have used density gradient electrophoresis and selective biotinylation to investigate the trafficking route of alphabetaIi quantitatively. Density gradient electrophoresis analysis showed that alphabetaIi was transported from the trans-Golgi network to endosomes at approximately 1.7% min-1. Surface delivery of alphabetaIi was delayed relative to endosome transport by approximately 10 min and showed slower kinetics ( approximately 0.4% min-1), suggesting that alphabetaIi reached the plasma membrane only after arrival in endosomes. A biotinylation assay revealed that 20-40% of endosomal alphabetaIi was delivered to the plasma membrane at steady state, suggesting that surface alphabetaIi was entirely derived from endosomes. Surface alphabetaIi was rapidly re-internalized and either returned to the cell surface or accessed degradative compartments. Peptide loading commenced approximately 30 min after delivery to endosomes. Thus alphabetaIi directly traffics from trans-Golgi network to endosomes and enters an endosome-plasma membrane 'carousel' until transport to peptide-loading compartments ensues.

Trafficking of MHC class II molecules in the late secretory pathway

Antigen presenting cells (APCs) alert the immune system to attack by extracellular organisms; APCs achieve this via internalization, degradation, and display of antigenic fragments on the cell surface by MHC class II molecules. These class II molecules bind to an accessory protein, termed the invariant chain, that ensures proper folding of the molecules. Invariant-chain binding also directs class II molecules to lysosomes, which are probably the most important sites for antigen loading. Endosomes are intermediates in the transport of class-II-invariant chain complexes to antigen-processing compartments, whereas trafficking of class II-peptide complexes to the membrane (and beyond) is less-well understood. Unlike other APCs, dendritic cells alter their capacity to present peptides via MHC class II molecules during differentiation, revealing a complex level of regulated antigen-presentation by this APC subtype.

Enhanced IkappaB kinase activity is responsible for the augmented activity of NF-kappaB in human head and neck carcinoma cells

The nuclear transcription factor kappaB (NF-kappaB) plays an important role in the development and progression of cancers. However, the mechanism by which cancer cells in the head and neck region acquire high NF-kappaB activity has not yet been clarified. In this study, we examined the NF-kappaB binding activity and the expression of the signal-transduction-related proteins of NF-kappaB in head and neck carcinoma cell lines. These cancer cells showed significantly higher NF-kappaB binding activity than normal oral epithelial and salivary gland cells. We also demonstrated the increased phosphorylation and degradation of IkappaB-alpha protein in cancer cells. Thus, enhanced NF-kappaB activity in cancer cells is attributable to the rapid phosphorylation and degradation of IkappaB-alpha protein. To further elucidate the mechanism involved in this phenomenon, we analyzed both the expression levels of upstream kinases (IkappaB kinase- (IKK-) alpha, IKK-beta, IKK-gamma, and NF-kappaB-inducing kinase (NIK)) and the IKK activity in cells. Although there was no significant difference in the expression levels of NIK, IKK-beta, or IKK-gamma in cancer cell lines compared to those in normal cells, increased expression of IKK-alpha protein was observed in cancer cells. In addition, IKK activity was significantly augmented in cancer cells as compared to normal cells. Thus, our results suggest that enhanced NF-kappaB activity in head and neck cancer cells may be due to the augmentation of IKK activity.