The CLIP region of invariant chain plays a critical role in regulating major histocompatibility complex class II folding, transport, and peptide occupancy

Defective removal of invariant chain peptides from MHC class II suppresses tumor antigen presentation and promotes tumor growth

Tumor-draining lymph node dendritic cells (DCs) are poor stimulators of tumor antigen-specific CD4 T cells; however, the mechanism behind this defect is unclear. We now show that, in tumor-draining lymph node DCs, a large proportion of major histocompatibility complex class II (MHC-II) molecules retains the class II-associated invariant chain peptide (CLIP) fragment of the invariant chain bound to the MHC-II peptide binding groove due to reduced expression of the peptide editor H2-M and enhanced activity of the CLIP-generating proteinase cathepsin S. The net effect of this is that MHC-II molecules are unable to efficiently bind antigenic peptides. DCs in mice expressing a mutation in the invariant chain sequence that results in enhanced MHC-II-CLIP accumulation are poor stimulators of CD4 T cells and have diminished anti-tumor responses. Our data reveal a previously unknown mechanism of immune evasion in which enhanced expression of MHC-II-CLIP complexes on tumor-draining lymph node DCs limits MHC-II availability for tumor peptides.

The show and tell of cross-presentation

Cross-presentation is the culmination of complex subcellular processes that allow the processing of exogenous proteins and the presentation of resultant peptides on major histocompatibility class I (MHC-I) molecules to CD8 T cells. Dendritic cells (DCs) are a cell type that uniquely specializes in cross-presentation, mainly in the context of viral or non-viral infection and cancer. DCs have an extensive network of endovesicular pathways that orchestrate the biogenesis of an ideal cross-presentation compartment where processed antigen, MHC-I molecules, and the MHC-I peptide loading machinery all meet. As a central conveyor of information to CD8 T cells, cross-presentation allows cross-priming of T cells which carry out robust adaptive immune responses for tumor and viral clearance. Cross-presentation can be canonical or noncanonical depending on the functional status of the transporter associated with antigen processing (TAP), which in turn influences the vesicular route of MHC-I delivery to internalized antigen and the cross-presented repertoire of peptides. Because TAP is a central node in MHC-I presentation, it is targeted by immune evasive viruses and cancers. Thus, understanding the differences between canonical and noncanonical cross-presentation may inform new therapeutic avenues against cancer and infectious disease. Defects in cross-presentation on a cellular and genetic level lead to immune-related disease progression, recurrent infection, and cancer progression. In this chapter, we review the process of cross-presentation beginning with the DC subsets that conduct cross-presentation, the signals that regulate cross-presentation, the vesicular trafficking pathways that orchestrate cross-presentation, the modes of cross-presentation, and ending with disease contexts where cross-presentation plays a role.

Antigen Presentation and Autophagy in Teleost Adaptive Immunity

Infectious diseases are a burden for aquaculture. Antigen processing and presentation (APP) to the immune effector cells that fight pathogens is key in the adaptive immune response. At the core of the adaptive immunity that appeared in lower vertebrates during evolution are the variable genes encoding the major histocompatibility complex (MHC). MHC class I molecules mainly present peptides processed in the cytosol by the proteasome and transported to the cell surface of all cells through secretory compartments. Professional antigen-presenting cells (pAPC) also express MHC class II molecules, which normally present peptides processed from exogenous antigens through lysosomal pathways. Autophagy is an intracellular self-degradation process that is conserved in all eukaryotes and is induced by starvation to contribute to cellular homeostasis. Self-digestion during autophagy mainly occurs by the fusion of autophagosomes, which engulf portions of cytosol and fuse with lysosomes (macroautophagy) or assisted by chaperones (chaperone-mediated autophagy, CMA) that deliver proteins to lysosomes. Thus, during self-degradation, antigens can be processed to be presented by the MHC to immune effector cells, thus, linking autophagy to APP. This review is focused on the essential components of the APP that are conserved in teleost fish and the increasing evidence related to the modulation of APP and autophagy during pathogen infection.

Efficient Cholesterol Transport in Dendritic Cells Defines Optimal Exogenous Antigen Presentation and Toxoplasma gondii Proliferation

Dendritic cells are the most powerful antigen-presenting cells of the immune system. They present exogenous antigens associated with Major Histocompatibility Complex (MHC) Class II molecules through the classical pathway to stimulate CD4+ T cells, or with MHC-I to activate CD8+ T lymphocytes through the cross-presentation pathway. DCs represent one of the main cellular targets during infection by Toxoplasma gondii. This intracellular parasite incorporates essential nutrients, such as cholesterol, to grow and proliferate inside a highly specialized organelle, the parasitophorous vacuole (PV). While doing so, T. gondii modulates the host immune response through multiple interactions with proteins and lipids. Cholesterol is an important cellular component that regulates cellular physiology at the structural and functional levels. Although different studies describe the relevance of cholesterol transport for exogenous antigen presentation, the molecular mechanism underlying this process is not defined. Here, we focus our study on the inhibitor U18666A, a drug widely used to arrest multivesicular bodies biogenesis that interrupts cholesterol trafficking and changes the lipid composition of intracellular membranes. Upon bone marrow-derived DC (BMDC) treatment with U18666A, we evidenced a drastic disruption in the ability to present exogenous soluble and particulate antigens to CD4+ and CD8+ T cells. Strikingly, the presentation of T. gondii-associated antigens and parasite proliferation were hampered in treated cells. However, neither antigen uptake nor BMDC viability was significantly affected by the U18666A treatment. By contrast, this drug altered the transport of MHC-I and MHC-II molecules to the plasma membrane. Since U18666A impairs the formation of MVBs, we analyzed in T. gondii infected BMDCs the ESCRT machinery responsible for the generation of intraluminal vesicles. We observed that different MVBs markers, including ESCRT proteins, were recruited to the PV. Surprisingly, the main ESCRT-III component CHMP4b was massively recruited to the PV, and its expression level was upregulated upon BMDC infection by T. gondii. Finally, we demonstrated that BMDC treatment with U18666A interrupted cholesterol delivery and CHMP4b recruitment to the PV, which interfered with an efficient parasite replication. Altogether, our results highlight the importance of cholesterol trafficking and MVBs formation in DCs for optimal antigen presentation and T. gondii proliferation.

The transmembrane domain and luminal C-terminal region independently support invariant chain trimerization and assembly with MHCII into nonamers

Background

Invariant chain (CD74, Ii) is a multifunctional protein expressed in antigen presenting cells. It assists the ER exit of various cargos and serves as a receptor for the macrophage migration inhibitory factor. The newly translated Ii chains trimerize, a structural feature that is not readily understood in the context of its MHCII chaperoning function. Two segments of Ii, the luminal C-terminal region (TRIM) and the transmembrane domain (TM), have been shown to participate in the trimerization process but their relative importance and impact on the assembly with MHCII molecules remains debated. Here, we addressed the requirement of these domains in the trimerization of human Ii as well as in the oligomerization with MHCII molecules. We used site-directed mutagenesis to generate series of Ii and DR mutants. These were transiently transfected in HEK293T cells to test their cell surface expression and analyse their interactions by co-immunoprecipitations.

Results

Our results showed that the TRIM domain is not essential for Ii trimerization nor for intracellular trafficking with MHCII molecules. We also gathered evidence that in the absence of TM, TRIM allows the formation of multi-subunit complexes with HLA-DR. Similarly, in the absence of TRIM, Ii can assemble into high-order structures with MHCII molecules.

Conclusions

Altogether, our data show that trimerization of Ii through either TM or TRIM sustains nonameric complex formation with MHCII molecules.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12865-021-00444-6.

AE37: a HER2-targeted vaccine for the prevention of breast cancer recurrence

INTRODUCTION

HER2 is a prevalent growth factor in a variety of malignancies, most prominently breast cancer. Over-expression has been correlated with the poorest overall survival and has been the target of successful therapies such as trastuzumab. AE37 is a novel, HER2-directed vaccine based on the AE36 hybrid peptide (aa776-790), which is derived from the intracellular portion of the HER2 protein, and the core portion of the MHC Class II invariant chain (the Ii-Key peptide). This hybrid peptide is given with GM-CSF immunoadjuvant as the AE37 vaccine.

AREAS COVERED

This article describes in detail the preclinical science leading to the creation of the AE37 vaccine and examines use of this agent in multiple clinical trials for breast and prostate cancer. The safety profile of AE37 is discussed and opinions on the potential of the vaccine in breast and prostate cancer patient subsets along with other malignancies, are offered.

EXPERT OPINION

Future trials utilizing the AE37 vaccine to treat other HER2-expressing malignancies are likely to see similar success, and this will be enhanced by combination immunotherapy. Ii-Key modification of other peptides of interest across oncology and virology could yield impressive results over the longer term.

MIF-Dependent Control of Tumor Immunity

Initially identified as a T lymphocyte-elicited inhibitor of macrophage motility, macrophage migration inhibitory factor (MIF) has since been found to be expressed by nearly every immune cell type examined and overexpressed in most solid and hematogenous malignant cancers. It is localized to both extracellular and intracellular compartments and physically interacts with more than a dozen different cell surface and intracellular proteins. Although classically associated with and characterized as a mediator of pro-inflammatory innate immune responses, more recent studies demonstrate that, in malignant disease settings, MIF contributes to anti-inflammatory, immune evasive, and immune tolerant phenotypes in both innate and adaptive immune cell types. This review will summarize the studies describing MIF in tumor-specific innate and adaptive immune responses and attempt to reconcile these various pleiotropic functions in normal physiology.

Immune stimulation of rainbow trout reveals divergent regulation of MH class II-associated invariant chain isoforms

Major histocompatibility complex (MHC) class II-associated invariant chain is a chaperone responsible for targeting the MHC class II dimer to the endocytic pathway, thus enabling the loading of exogenous antigens onto the MHC class II receptor. In the current study, in vivo and in vitro methods were used to investigate the regulation of the rainbow trout invariant chain proteins S25-7 and INVX, upon immune system activation. Whole rainbow trout and the macrophage/monocyte-like cell line RTS11 were treated with PMA at concentrations shown to induce IL-1β transcripts and homotypic aggregation of RTS11. S25-7 transcript levels remained unchanged in the gill, spleen, and liver and were found to be significantly decreased in head kidney beginning 24 h post-stimulation. Meanwhile, INVX transcript levels remained unchanged in all tissues studied. Both S25-7 and INVX proteins were produced in gill and spleen tissues but their expression was unaffected by immune system stimulation. Surprisingly, neither INVX nor S25-7 protein was detected in the secondary immune organ, the head kidney. Analysis of RTS11 cultures demonstrated that both INVX and S25-7 transcript levels significantly increased at 96 h and 120 h following PMA stimulation before returning to control levels at 168 h. Meanwhile, at the protein level in RTS11, S25-7 remained unchanged while INVX had a significant decrease at 168 h post-stimulation. These results indicate that neither INVX nor S25-7 is upregulated upon immune system activation; thus, teleosts have evolved a system of immune regulation that is different than that found in mammals.

Gene profiling of antimicrobial peptides, complement factors and MHC molecules from the skin transcriptome of Channa striatus and its expression pattern during Aeromonas hydrophila infection

Channa striatus is one of the economically important freshwater fish with high demand in Southeast Asia for its nutritional and medicinal values. The unique composition of skin mucus of murrel provides immunity against pathogens; however, they are susceptible to few bacterial pathogens especially Aeromonas hydrophila. Although few immune molecules such as antimicrobial peptides have already been identified from the murrel mucus, there is no report on the complete gene profile of the skin and mucosal immunity. Therefore, in this study we applied transcriptome approach to identify the mRNA sequences of various immune molecules such as antimicrobial peptides, complement factors and adaptive immune molecules from the skin tissue. Transcriptome wide search revealed unique mRNA sequences of 13 antimicrobial peptides, 11 complement components, 2 major histocompatibility complex proteins and its receptor, 6 butyrophilins, 2 leptins and its receptor. Brief bioinformatics analysis of the identified mRNA sequences and their respective putative protein sequences were performed to understand molecular information of those immune components. Further, we analysed the differential expression pattern of selected 13 mRNA sequences representing each immune group using qRT-PCR technique which highlighted the role of those genes during A. hydrophila challenge. Overall, this study revealed the complex immune response of murrel skin and the involvement of various innate and adaptive immune molecules against A. hydrophila infection.

Role of membrane environment and membrane-spanning protein regions in assembly and function of the Class II Major Histocompatibility complex

Class II Major Histocompatibility complex (MHC-II) is a polymorphic heterodimer that binds antigen-derived peptides and presents them on the surface of antigen presenting cells. This mechanism of antigen presentation leads to recognition by CD4 T-cells and T-cell activation, making it a critical element of adaptive immune response. For this reason, the structural determinants of MHC-II function have been of great interest for the past 30 years, resulting in a robust structural understanding of the extracellular regions of the complex. However, the membrane-localized regions have also been strongly implicated in protein-protein and protein-lipid interactions that facilitate Class II assembly, transport and function, and it is these regions that are the focus of this review. Here we describe studies that reveal the strong and selective interactions between the transmembrane domains of the MHC α, and invariant chains which, when altered, have broad reaching impacts on antigen presentation and Class II function. We also summarize work that clearly demonstrates the link between membrane lipid composition (particularly the presence of cholesterol) and MHC-II conformation, subsequent peptide binding, and downstream T-cell activation. We have integrated these studies into a comprehensive view of Class II transmembrane domain biology.

Mechanisms underlying the lack of endogenous processing and CLIP-mediated binding of the invariant chain by HLA-DP84Gly

While the principles of classical antigen presentation via MHC class II are well-established, the mechanisms for the many routes of cross-presentation by which endogenous antigens become associated with class II molecules are not fully understood. We have recently demonstrated that the single amino acid polymorphism HLA-DPβ^84Gly (DP^84Gly) is critical to abrogate class II invariant chain associated peptide (CLIP) region-mediated binding of invariant chain (Ii) to DP, allowing endoplasmic reticulum (ER)-resident endogenous antigens to constitutively associate with DP^84Gly such as DP4. In this study, we demonstrate that both the CLIP and N-terminal non-CLIP Ii regions cooperatively generate an Ii conformation that cannot associate with DP^84Gly via the CLIP region. We also demonstrate the ability of DP4 to efficiently process and present antigens encoded in place of CLIP in a chimeric Ii, regardless of wild type Ii and HLA-DM expression. These data highlight the complex interplay between DP polymorphisms and the multiple Ii regions that cooperatively regulate this association, ultimately controlling the presentation of endogenous antigens on DP molecules. These results may also offer a mechanistic explanation for recent studies identifying the differential effects between DP^84Gly and DP^84Asp as clinically relevant in human disease.

Grass carp (Ctenopharyngodon idellus) invariant chain of the MHC class II chaperone protein associates with the class I molecule

The invariant chain (Ii) is an important immune molecule, as it assists major histocompatibility complex (MHC) class II molecules to present antigenic peptides. The relationship between the Ii and MHC molecules in teleosts remains poorly understood. This study focused on the molecular structure of grass carp Ii (gIi), its organ distribution, correlations with gene transcription, and the association with MHC. gIi cDNA was cloned using designed degenerate primers and the rapid amplification of cDNA ends method (RACE). The gIi sequence was 92%-96% similar to that of other teleosts, but only 52%-67% similar to that of mammals, respectively. The gIi gene was distributed in all 12 organs examined by PCR. The gIi gene transcription levels were markedly higher in organs enriched with immune cells than in other organs (P < 0.01). Moreover, positive correlations were detected between transcription levels of the gIi and gMhcI or II genes in different organs (r = 8.415-8.523, P = 0.001). The gIi co-localized on endomembrane systems with either class I or II molecules in co-transfected cells observed by a laser confocal. Further testing confirmed that the gIi bound gMHCI and II molecules. Taken together, these results indicate that the gIi is associated with MHC class I and II molecules, suggesting homology of both MHC molecules.

Variations in MHC Class II Antigen Processing and Presentation in Health and Disease

MHC class II (MHC-II) molecules are critical in the control of many immune responses. They are also involved in most autoimmune diseases and other pathologies. Here, we describe the biology of MHC-II and MHC-II variations that affect immune responses. We discuss the classic cell biology of MHC-II and various perturbations. Proteolysis is a major process in the biology of MHC-II, and we describe the various components forming and controlling this endosomal proteolytic machinery. This process ultimately determines the MHC-II-presented peptidome, including cryptic peptides, modified peptides, and other peptides that are relevant in autoimmune responses. MHC-II also variable in expression, glycosylation, and turnover. We illustrate that MHC-II is variable not only in amino acids (polymorphic) but also in its biology, with consequences for both health and disease.

Novel immunotherapeutic approaches for the treatment of acute leukemia (myeloid and lymphoblastic)

There have been major advances in our understanding of the multiple interactions between malignant cells and the innate and adaptive immune system. While the attention of immunologists has hitherto focused on solid tumors, the specific immunobiology of acute leukemias is now becoming defined. These discoveries have pointed the way to immune interventions building on the established graft-versus-leukemia (GVL) effect from hematopoietic stem-cell transplant (HSCT) and extending immunotherapy beyond HSCT to individuals with acute leukemia with a diversity of immune manipulations early in the course of the leukemia. At present, clinical results are in their infancy. In the coming years larger studies will better define the place of immunotherapy in the management of acute leukemias and lead to treatment approaches that combine conventional chemotherapy, immunotherapy and HSCT to achieve durable cures.

A conserved WW domain-like motif regulates invariant chain-dependent cell-surface transport of the NKG2D ligand ULBP2

Malignant cells expressing NKG2D ligands on their cell surface can be directly sensed and killed by NKG2D-bearing lymphocytes. To ensure this immune recognition, accumulating evidence suggests that NKG2D ligands are trafficed via alternative pathways to the cell surface. We have previously shown that the NKG2D ligand ULBP2 traffics over an invariant chain (Ii)-dependent pathway to the cell surface. This study set out to elucidate how Ii regulates ULBP2 cell-surface transport: We discovered conserved tryptophan (Trp) residues in the primary protein sequence of ULBP1-6 but not in the related MICA/B. Substitution of Trp to alanine resulted in cell-surface inhibition of ULBP2 in different cancer cell lines. Moreover, the mutated ULBP2 constructs were retained and not degraded inside the cell, indicating a crucial role of this conserved Trp-motif in trafficking. Finally, overexpression of Ii increased surface expression of wt ULBP2 while Trp-mutants could not be expressed, proposing that this Trp-motif is required for an Ii-dependent cell-surface transport of ULBP2. Aberrant soluble ULBP2 is immunosuppressive. Thus, targeting a distinct protein module on the ULBP2 sequence could counteract this abnormal expression of ULBP2.

The human-specific invariant chain isoform Iip35 modulates Iip33 trafficking and function

The invariant chain (Ii) is a multifunctional protein, which has an essential role in the assembly and transport of major histocompatibility complex class II (MHC II) molecules. From a single gene, Ii is synthesized as four different isoforms: Iip33, Iip35, Iip41 and Iip43. Iip35 and Iip43 are specific to humans, and are formed due to an upstream alternative translation site, resulting in an N-terminal extension of 16 amino acids. This extension harbors a strong endoplasmic reticulum (ER) retention motif. Consequently, Iip35 or Iip43 expressed alone are retained in the ER, whereas Iip33 and Iip41 rapidly traffic to the endosomal pathway. Endogenously expressed, the four isoforms form mixed heterotrimers in the ER; however, mainly due to the absence of the Iip35/p43 isoforms in mice, little is known about how they influence general Ii function. In this study, we have co-expressed Iip33 and Iip35 in human cells with and without MHC II to gain a better understanding of how Iip35 isoform influences the cellular properties of Iip33. We find that Iip35 significantly affects the properties of Iip33. In the presence of Iip35, the transport of Iip33 out of the ER is delayed, its half-life is dramatically prolonged and its ability to induce enlarged endosomes and delayed endosomal maturation is abrogated.

MHC class II antigen presentation by dendritic cells regulated through endosomal sorting

For the initiation of adaptive immune responses, dendritic cells present antigenic peptides in association with major histocompatibility complex class II (MHCII) to naïve CD4(+) T lymphocytes. In this review, we discuss how antigen presentation is regulated through intracellular processing and trafficking of MHCII. Newly synthesized MHCII is chaperoned by the invariant chain to endosomes, where peptides from endocytosed pathogens can bind. In nonactivated dendritic cells, peptide-loaded MHCII is ubiquitinated and consequently sorted by the ESCRT machinery to intraluminal vesicles of multivesicular bodies, ultimately leading to lysosomal degradation. Ubiquitination of newly synthesized MHCII is blocked when dendritic cells are activated, now allowing its transfer to the cell surface. This mode of regulation for MHCII is a prime example of how molecular processing and sorting at multivesicular bodies can determine the expression of signaling receptors at the plasma membrane.

HLA-DO as the optimizer of epitope selection for MHC class II antigen presentation

Processing of antigens for presentation to helper T cells by MHC class II involves HLA-DM (DM) and HLA-DO (DO) accessory molecules. A mechanistic understanding of DO in this process has been missing. The leading model on its function proposes that DO inhibits the effects of DM. To directly study DO functions, we designed a recombinant soluble DO and expressed it in insect cells. The kinetics of binding and dissociation of several peptides to HLA-DR1 (DR1) molecules in the presence of DM and DO were measured. We found that DO reduced binding of DR1 to some peptides, and enhanced the binding of some other peptides to DR1. Interestingly, these enhancing and reducing effects were observed in the presence, or absence, of DM. We found that peptides that were negatively affected by DO were DM-sensitive, whereas peptides that were enhanced by DO were DM-resistant. The positive and negative effects of DO could only be measured on binding kinetics as peptide dissociation kinetics were not affected by DO. Using Surface Plasmon Resonance, we demonstrate direct binding of DO to a peptide-receptive, but not a closed conformation of DR1. We propose that DO imposes another layer of control on epitope selection during antigen processing.

Intracellular localization and association of MHC class I with porcine invariant chain

The objective was to investigate the intracellular localization and association of pig major histocompatibility complex (MHC) class I subunits with invariant chain (Ii). Pig MHC class I subunit cDNAs were cloned by RT-PCR and eukaryotic expression plasmids of α and β2m were constructed with fusions to red or enhanced green fluorescent protein (pDsRed2-N1-α, pEGFP-N1-α, pDsRed2-N1-β2m, and pEGFP-N1-β2m). A pig Ii mutant with a deleted CLIP region (DCLIP-Ii) was constructed by overlap extension PCR. Wild-type Ii and mutant Ii were cloned into pEGFP-C1 (pEGFP-C1-Ii, pEGFP-C1-DCLIP-Ii). The recombinant plasmids of MHC I subunits and pEGFP-C1-Ii (pEGFP-C1-ΔCLIP-Ii) were transiently cotransfected into COS-7 cells with Lipofectamine 2000. Immunofluorescence microscopy was performed to detect expression and intracellular localization of Ii and MHC I subunits, and immunoprecipitation was used to analyze their association. Our results indicated that pig Ii associates with integrated MHC I subunits to form oligomers, but cannot associate with single MHC I subunits. Furthermore, deletion of the Ii CLIP sequence blocks association with integrated MHC I subunits. Thus, pig Ii cannot associate with a single MHC I molecule, the α or β2m chain, but Ii and the integrated MHC I molecule can form complexes that colocalize in the endomembranes of COS-7 cells. The Ii of CLIP plays a key role in assembly of Ii and MHC I.

GILT expression in B cells diminishes cathepsin S steady-state protein expression and activity

MHC class II-restricted Ag processing requires protein degradation in the endocytic pathway for the activation of CD4(+) T cells. Gamma-interferon-inducible lysosomal thiol reductase (GILT) facilitates Ag processing by reducing protein disulfide bonds in this compartment. Lysosomal cysteine protease cathepsin S (CatS) contains disulfide bonds and mediates essential steps in MHC class II-restricted processing, including proteolysis of large polypeptides and cleavage of the invariant chain. We sought to determine whether GILT's reductase activity regulates CatS expression and function. Confocal microscopy confirmed that GILT and CatS colocalized within lysosomes of B cells. GILT expression posttranscriptionally decreased the steady-state protein expression of CatS in primary B cells and B-cell lines. GILT did not substantially alter the expression of other lysosomal proteins, including H2-M, H2-O, or CatL. GILT's reductase active site was necessary for diminished CatS protein levels, and GILT expression decreased the half-life of CatS, suggesting that GILT-mediated reduction of protein disulfide bonds enhances CatS degradation. GILT expression decreased the proteolysis of a CatS selective substrate. This study illustrates a physiologic mechanism that regulates CatS and has implications for fine tuning MHC class II-restricted Ag processing and for the development of CatS inhibitors, which are under investigation for the treatment of autoimmune disease.

Boosting immune response with the invariant chain segments via association with non-peptide binding region of major histocompatibility complex class II molecules

BACKGROUND

Based on binding of invariant chain (Ii) to major histocompatibility complex (MHC) class II molecules to form complexes, Ii-segment hybrids, Ii-key structure linking an epitope, or Ii class II-associated invariant chain peptide (CLIP) replaced with an epitope were used to increase immune response. It is currently unknown whether the Ii-segment cytosolic and transmembrane domains bind to the MHC non-peptide binding region (PBR) and consequently influence immune response. To investigate the potential role of Ii-segments in the immune response via MHC II/peptide complexes, a few hybrids containing Ii-segments and a multiepitope (F306) from Newcastle disease virus fusion protein (F) were constructed, and their binding effects on MHC II molecules and specific antibody production were compared using confocal microscopy, immunoprecipitation, western blotting and animal experiments.

RESULTS

One of the Ii-segment/F306 hybrids, containing ND (Asn-Asp) outside the F306 in the Ii-key structure (Ii-key/F306/ND), neither co-localized with MHC II molecules on plasma membrane nor bound to MHC II molecules to form complexes. However, stimulation of mice with the structure produced 4-fold higher antibody titers compared with F306 alone. The two other Ii-segment/F306 hybrids, in which the transmembrane and cytosolic domains of Ii were linked to this structure (Cyt/TM/Ii-key/F306/ND), partially co-localized on plasma membrane with MHC class II molecules and weakly bound MHC II molecules to form complexes. They induced mice to produce approximately 9-fold higher antibody titers compared with F306 alone. Furthermore, an Ii/F306 hybrid (F306 substituting CLIP) co-localized well with MHC II molecules on the membrane to form complexes, although it increased antibody titer about 3-fold relative to F306 alone.

CONCLUSIONS

These results suggest that Ii-segments improve specific immune response by binding to the non-PBR on MHC class II molecules and enabling membrane co-localization with MHC II molecules, resulting in the formation of relatively stable MHC II/peptide complexes on the plasma membrane, and signal transduction.

Peptide linkage to the α-subunit of MHCII creates a stably inverted antigen presentation complex

Class II proteins of the major histocompatibility complex (MHCII) typically present exogenous antigenic peptides to cognate T cell receptors of CD4-T lymphocytes. The exact conformation of peptide-MHCII complexes (pMHCII) can vary depending on the length, register and orientation of the bound peptide. We have recently found the self-peptide CLIP (class-II-associated invariant chain-derived peptide) to adopt a dynamic bidirectional binding mode with regard to the human MHCII HLA-DR1 (HLA, human leukocyte antigen). We suggested that inversely bound peptides could activate specific T cell clones in the context of autoimmunity. As a first step to prove this hypothesis, pMHC complexes restricted to either the canonical or the inverted peptide orientation have to be constructed. Here, we show that genetically encoded linkage of CLIP and two other antigenic peptides to the HLA-DR1 α-chain results in stable complexes with inversely bound ligands. Two-dimensional NMR and biophysical analyses indicate that the CLIP-bound pMHC(inv) complex (pMHC(inv), inverted MHCII-peptide complex) displays high thermodynamic stability but still allows for the exchange against higher-affinity viral antigen. Complemented by comparable data on a corresponding β-chain-fused canonical HLA-DR1/CLIP complex, we further show that linkage of CLIP leads to a binding mode exactly the same as that of the corresponding unlinked constructs. We suggest that our approach constitutes a general strategy to create pMHC(inv) complexes. Such engineering is needed to create orientation-specific antibodies and raise T cells to study phenomena of autoimmunity caused by isomeric pMHCs.

Promiscuous binding of invariant chain-derived CLIP peptide to distinct HLA-I molecules revealed in leukemic cells

Antigen presentation by HLA class I (HLA-I) and HLA class II (HLA-II) complexes is achieved by proteins that are specific for their respective processing pathway. The invariant chain (Ii)-derived peptide CLIP is required for HLA-II-mediated antigen presentation by stabilizing HLA-II molecules before antigen loading through transient and promiscuous binding to different HLA-II peptide grooves. Here, we demonstrate alternative binding of CLIP to surface HLA-I molecules on leukemic cells. In HLA-II-negative AML cells, we found plasma membrane display of the CLIP peptide. Silencing Ii in AML cells resulted in reduced HLA-I cell surface display, which indicated a direct role of CLIP in the HLA-I antigen presentation pathway. In HLA-I-specific peptide eluates from B-LCLs, five Ii-derived peptides were identified, of which two were from the CLIP region. In vitro peptide binding assays strikingly revealed that the eluted CLIP peptide RMATPLLMQALPM efficiently bound to four distinct HLA-I supertypes (-A2, -B7, -A3, -B40). Furthermore, shorter length variants of this CLIP peptide also bound to these four supertypes, although in silico algorithms only predicted binding to HLA-A2 or -B7. Immunization of HLA-A2 transgenic mice with these peptides did not induce CTL responses. Together these data show a remarkable promiscuity of CLIP for binding to a wide variety of HLA-I molecules. The found participation of CLIP in the HLA-I antigen presentation pathway could reflect an aberrant mechanism in leukemic cells, but might also lead to elucidation of novel processing pathways or immune escape mechanisms.

Host glycans and antigen presentation

The cell-mediated adaptive immune response depends upon the activation of T cells via recognition of antigen in the context of a major histocompatibility complex (MHC) molecule. Although studies have shown that alterations in T cell receptor glycosylation reduces the activation threshold, the data on MHC is far less definitive. Here, we discuss the data on MHC glycosylation and the role the glycans might play during the adaptive host response.

Roles for major histocompatibility complex glycosylation in immune function

The major histocompatibility complex (MHC) glycoprotein family, also referred to as human leukocyte antigens, present endogenous and exogenous antigens to T lymphocytes for recognition and response. These molecules play a central role in enabling the immune system to distinguish self from non-self, which is the basis for protective immunity against pathogenic infections and disease while at the same time representing a serious obstacle for tissue transplantation. All known MHC family members, like the majority of secreted, cell surface, and other immune-related molecules, carry asparagine (N)-linked glycans. The immune system has evolved increasing complexity in higher-order organisms along with a more complex pattern of protein glycosylation, a relationship that may contribute to immune function beyond the early protein quality control events in the endoplasmic reticulum that are commonly known. The broad MHC family maintains peptide sequence motifs for glycosylation at sites that are highly conserved across evolution, suggesting importance, yet functional roles for these glycans remain largely elusive. In this review, we will summarize what is known about MHC glycosylation and provide new insight for additional functional roles for this glycoprotein modification in mediating immune responses.

Expression, purification and assembly of soluble multimeric MHC class II-invariant chain complexes

Major histocompatibility class (MHC) II molecules are essential for running adaptive immune response. They are produced in the ER and targeted to late endosomes with the help of invariant chain (Ii) trimers. Ii trimerization may be induced by the Ii TM domain. To enable mechanistic and structural studies of MHC class II-Ii assembly, soluble forms of the complexes were expressed. We show that Ii trimerizes in the absence of the transmembrane part, prior to binding of α/β chains. The biochemical analysis supports the suggestion that the MHC class II-Ii complexes are not necessarily trimers of trimers, but that the Ii trimer can also be occupied by one or two MHC class II complexes.

Shark class II invariant chain reveals ancient conserved relationships with cathepsins and MHC class II

The invariant chain (Ii) is the critical third chain required for the MHC class II heterodimer to be properly guided through the cell, loaded with peptide, and expressed on the surface of antigen presenting cells. Here, we report the isolation of the nurse shark Ii gene, and the comparative analysis of Ii splice variants, expression, genomic organization, predicted structure, and function throughout vertebrate evolution. Alternative splicing to yield Ii with and without the putative protease-protective, thyroglobulin-like domain is as ancient as the MHC-based adaptive immune system, as our analyses in shark and lizard further show conservation of this mechanism in all vertebrate classes except bony fish. Remarkable coordinate expression of Ii and class II was found in shark tissues. Conserved Ii residues and cathepsin L orthologs suggest their long co-evolution in the antigen presentation pathway, and genomic analyses suggest 450 million years of conserved Ii exon/intron structure. Other than an extended linker preceding the thyroglobulin-like domain in cartilaginous fish, the Ii gene and protein are predicted to have largely similar physiology from shark to man. Duplicated Ii genes found only in teleosts appear to have become sub-functionalized, as one form is predicted to play the same role as that mediated by Ii mRNA alternative splicing in all other vertebrate classes. No Ii homologs or potential ancestors of any of the functional Ii domains were found in the jawless fish or lower chordates.

Epstein-Barr virus evades CD4+ T cell responses in lytic cycle through BZLF1-mediated downregulation of CD74 and the cooperation of vBcl-2

Evasion of immune T cell responses is crucial for viruses to establish persistence in the infected host. Immune evasion mechanisms of Epstein-Barr virus (EBV) in the context of MHC-I antigen presentation have been well studied. In contrast, viral interference with MHC-II antigen presentation is less well understood, not only for EBV but also for other persistent viruses. Here we show that the EBV encoded BZLF1 can interfere with recognition by immune CD4⁺ effector T cells. This impaired T cell recognition occurred in the absence of a reduction in the expression of surface MHC-II, but correlated with a marked downregulation of surface CD74 on the target cells. Furthermore, impaired CD4⁺ T cell recognition was also observed with target cells where CD74 expression was downregulated by shRNA-mediated inhibition. BZLF1 downregulated surface CD74 via a post-transcriptional mechanism distinct from its previously reported effect on the CIITA promoter. In addition to being a chaperone for MHC-II αβ dimers, CD74 also functions as a surface receptor for macrophage Migration Inhibitory Factor and enhances cell survival through transcriptional upregulation of Bcl-2 family members. The immune-evasion function of BZLF1 therefore comes at a cost of induced toxicity. However, during EBV lytic cycle induced by BZLF1 expression, this toxicity can be overcome by expression of the vBcl-2, BHRF1, at an early stage of lytic infection. We conclude that by inhibiting apoptosis, the vBcl-2 not only maintains cell viability to allow sufficient time for synthesis and accumulation of infectious virus progeny, but also enables BZLF1 to effect its immune evasion function.

Epstein-Barr virus (EBV) is a herpesvirus and an important human pathogen that can cause diseases ranging from non-malignant proliferative disease to fully malignant cancers of lymphocytes and epithelial cells. The persistence of EBV in healthy individuals relies on the balance between host immune responses and viral immune evasion. As CD4⁺ immune T cell responses include both helper and cytotoxic functions, viral mechanisms for interfering with MHC class II antigen presentation to CD4⁺ T cells have the potential to greatly influence the outcome of viral infections. Our work on Epstein-Barr virus provides a new paradigm for viral immune evasion of MHC-II presented antigen by targeting CD74. CD74 is a dual function protein; it serves as a surviving receptor as well as a chaperone for MHC-II antigen presentation. Therefore, downregulation of CD74 as a T cell evasion strategy comes at the cost of potentially inducing cell death. However, EBV also encodes a vBcl-2 to attenuate the toxicity associated with reduced CD74, thus enabling the immune-impairment function to be effected. We expect that future studies will identify other viruses utilizing a similar strategy to evade CD4⁺ immune T cell responses.

Stoichiometry of HLA class II-invariant chain oligomers

BACKGROUND

The HLA gene complex encodes three class II isotypes, DR, DQ, and DP. HLA class II molecules are peptide receptors that present antigens for recognition by T lymphocytes. In antigen presenting cells, the assembly of matched α and β subunits to heterodimers is chaperoned by invariant chain (Ii). Ii forms a homotrimer with three binding sites for class II heterodimers. The current model of class II and Ii structure states that three αβ heterodimers bind to an Ii trimer.

METHODOLOGY/PRINCIPAL FINDINGS

[corrected] We have now analyzed the composition and size of the complexes of class II and Ii using epitope tagged class II subunits and density gradient experiments. We show here that class II-Ii oligomers consist of one class II heterodimer associated with one Ii trimer, such that the DR, DQ and DP isotypes are contained within separate complexes with Ii.

CONCLUSION/SIGNIFICANCE

We propose a structural model of the class II-Ii oligomer and speculate that the pentameric class II-Ii complex is bent towards the cell membrane, inhibiting the binding of additional class II heterodimers to Ii.

Bidirectional binding of invariant chain peptides to an MHC class II molecule

T-cell recognition of peptides bound to MHC class II (MHCII) molecules is a central event in cell-mediated adaptive immunity. The current paradigm holds that prebound class II-associated invariant chain peptides (CLIP) and all subsequent antigens maintain a canonical orientation in the MHCII binding groove. Here we provide evidence for MHCII-bound CLIP inversion. NMR spectroscopy demonstrates that the interconversion from the canonical to the inverse alignment is a dynamic process, and X-ray crystallography shows that conserved MHC residues form a hydrogen bond network with the peptide backbone in both orientations. The natural catalyst HLA-DM accelerates peptide reorientation and the exchange of either canonically or inversely bound CLIP against antigenic peptide. Thus, noncanonical MHC-CLIP displays the hallmarks of a structurally and functionally intact antigen-presenting complex.

Conformational heterogeneity of MHC class II induced upon binding to different peptides is a key regulator in antigen presentation and epitope selection

T cells bearing alphabeta receptors recognize antigenic peptides bound to class I and class II glycoproteins encoded in the major histocompatibility complex (MHC). Cytotoxic and helper T cells respond respectively to peptide antigens derived from endogenous sources presented by MHC class I, and exogenous sources presented by MHC II, on antigen presenting cells. Differences in the MHC class I and class II structures and their maturation pathways have evolved to optimize antigen presentation to their respective T cells. A main focus of our laboratory is on efforts to understand molecular events in processing of antigen for presentation by MHC class II. The different stages of MHC class II-interactions with molecular chaperons involved in folding and traffic from the ER through the antigen-loading compartments, peptide exchange, and transport to the cell surface have been investigated. Through intense research on biophysical and biochemical properties of MHC class II molecules, we have learned that the conformational heterogeneity of MHC class II induced upon binding to different peptides is a key regulator in antigen presentation and epitope selection, and a determinant of the ability of MHC class II to participate in peptide association or dissociation and interaction with the peptide editor HLA-DM.

Alternative Ii-independent antigen-processing pathway in leukemic blasts involves TAP-dependent peptide loading of HLA class II complexes

During HLA class II synthesis in antigen-presenting cells, the invariant chain (Ii) not only stabilizes HLA class II complexes in the endoplasmic reticulum, but also mediates their transport to specialized lysosomal antigen-loading compartments termed MIICs. This study explores an alternative HLA class II presentation pathway in leukemic blasts that involves proteasome and transporter associated with antigen processing (TAP)-dependent peptide loading. Although HLA-DR did associate with Ii, Ii silencing in the human class II-associated invariant chain peptide (CLIP)-negative KG-1 myeloid leukemic cell line did not affect total and plasma membrane expression levels of HLA-DR, as determined by western blotting and flow cytometry. Since HLA-DR expression does require peptide binding, we examined the role of endogenous antigen-processing machinery in HLA-DR presentation by CLIP(-) leukemic blasts. The suppression of proteasome and TAP function using various inhibitors resulted in decreased HLA-DR levels in both CLIP(-) KG-1 and ME-1 blasts. Simultaneous inhibition of TAP and Ii completely down-modulated the expression of HLA-DR, demonstrating that together these molecules form the key mediators of HLA class II antigen presentation in leukemic blasts. By the use of a proteasome- and TAP-dependent pathway for HLA class II antigen presentation, CLIP(-) leukemic blasts might be able to present a broad range of endogenous leukemia-associated peptides via HLA class II to activate leukemia-specific CD4(+) T cells.

Impaired antigen presentation in neoplasia: basic mechanisms and implications for acute myeloid leukemia

During onset, treatment and progression of acute myeloid leukemia (AML), inadequate immune responses against certain myeloid leukemic blasts might be associated with the occurrence of minimal residual disease and subsequent relapse. Several studies on this subject have demonstrated that, in general, solid tumor cells are able to avoid CD8+ cytotoxic T-cell recognition by downregulating HLA class I-restricted presentation of tumor-associated antigens. In tumor cells that can express HLA class II molecules, such as myeloid leukemic blasts, abnormalities in the processing pathways of endogenous antigens could also result in impaired HLA class II-restricted tumor-associated antigen presentation to CD4+ T helper cells. More insight into impaired tumor-associated antigen presentation by myeloid leukemic blasts could explain their escape from immune recognition and might be crucial for selecting appropriate strategies to improve whole-cell or dendritic cell-based tumor vaccine efficacy in the treatment of AML patients.

Class II-associated invariant chain peptide down-modulation enhances the immunogenicity of myeloid leukemic blasts resulting in increased CD4+ T-cell responses

BACKGROUND

Disease recurrence in patients with acute myeloid leukemia may be partially explained by the escape of leukemic blasts from CD4(+) T-cell recognition. The current study investigates the role of aberrant HLA class II antigen presentation on leukemic blasts by determining both the clinical and functional impact of the class II-associated invariant chain peptide (CLIP).

DESIGN AND METHODS

The levels of expression of CLIP and HLA-DR on blood and bone marrow samples from 207 patients with acute myeloid leukemia were correlated with clinical outcome. Irradiated CLIP(-) and CLIP(+) leukemic blasts were compared for their ability to induce CD4(+) T cells during mixed leukocyte reactions. To discriminate between these blasts, we down-modulated CLIP expression on myeloid leukemic cell lines by RNA interference of the invariant chain, a chaperone protein critically involved in HLA-DR processing, and performed flow cytometric sorting for their isolation from primary acute myeloid leukemia samples.

RESULTS

We found that patients with leukemic blasts characterized by a high amount of HLA-DR occupied by CLIP (relative amount of CLIP) had a significantly shortened disease-free survival. The clear reductions in amount of HLA-DR occupied by CLIP on blasts of the THP-1 and Kasumi-1 myeloid leukemic cell lines after treatment with invariant chain short interfering RNA resulted in enhanced rates of allogeneic CD4(+) T-cell proliferation. Similar findings were obtained in an autologous setting, in which there were strong increases in proliferation of remission CD4(+) T cells stimulated with CLIP(-)-sorted leukemic blasts from HLA-DR(+) acute myeloid leukemia patients, in contrast to CLIP(+)-sorted leukemic blasts from the same patients.

CONCLUSIONS

These data highlight the relevance of CLIP expression on leukemic blasts and the potential of CLIP as a target for immunomodulatory strategies to enhance HLA class II antigen presentation and CD4(+) T-cell reactivity in acute myeloid leukemia.

The immune system and cancer

Cancer patients mount adaptive immune responses against their tumor. However, while tumor-infiltrating lymphocytes and natural-killer (NK) cells try to detect and eliminate malignant cells, they eventually fail when these malignant cells develop mechanisms to evade effective immunosurveillance. First, malignant cells produce immunosuppressive cytokines and prostaglandins that skew the immune response toward a Th2 response, resulting in a humoral response with significantly less antitumor capacities, generating a low interleukin-2 environment blocking NK cell division, T-helper cell proliferation, and T-cytotoxic cell proliferation and function. Second, immunoresistant malignant cell variants emerge through selection of major histocompatibility class I and II and antigen-processing mutants reducing antigenicity. Finally, malignant cells may actively eliminate T-cells via activation-induced cell death or by mounting a counterattack through Fas ligand expression.

MHC II and the endocytic pathway: regulation by invariant chain

The major histocompatibility complex (MHC) class I and II molecules perform vital functions in innate and adaptive immune responses towards invading pathogens. MHC class I molecules load peptides in the endoplasmatic reticulum (ER) and display them to the T cell receptors (TcR) on CD8(+) T lymphocytes. MHC class II molecules (MHC II) acquire their peptides in endosomes and present these to the TcR on CD4+ T lymphocytes. They are vital for the generation of humoral immune responses. MHC II assembly in the ER and trafficking to endosomes is guided by a specialized MHC II chaperone termed the invariant chain (Ii). Ii self-associates into a trimer in the ER, this provides a scaffold for the assembly of three MHC II heterodimers and blocks their peptide binding grooves, thereby avoiding premature peptide binding. Ii then transports the nascent MHC II to more or less specialized compartment where they can load peptides derived from internalized pathogens.

Dendritic cell-based immunotherapy in myeloid leukaemia: translating fundamental mechanisms into clinical applications

Immunotherapy for leukaemia patients, aiming at the generation of anti-leukaemic T cell responses, could provide a new therapeutic approach to eliminate minimal residual disease (MRD) cells in acute myeloid leukaemia (AML). Leukaemic blasts harbour several ways to escape the immune system including deficient MHC class II expression, low levels of co-stimulatory molecules and suppressive cytokines. Therapeutic vaccination with dendritic cells (DC) is now recognized as an important investigational therapy. Due to their unique antigen presenting capacity, immunosuppressive features of the leukaemic blasts can be circumvented. DC can be successfully cultured from leukaemic blasts in 60-70% of patients and show functional potential in vivo. Alternatively, monocyte derived DC obtained at time of complete remission loaded with leukaemia-specific antigens can be used as vaccine. Several sources of leukaemia-associated antigen and different methods of loading antigen onto DC have been used in an attempt to optimize antitumour responses including apoptotic cells, necrotic cell lysates and tumour-associated pep-tides. Currently, the AML-derived cell line MUTZ-3, an immortalized equivalent of CD34(+) DC precursor cells, is under investigation for vaccination purposes. For effective DC vaccination the intrinsic tolerant state of the patient must be overcome. Therefore, the development of efficient and safe adjuvants in antigen specific immunotherapeutic programs should be encouraged.

Sea bass (Dicentrarchus labrax) invariant chain and class II major histocompatibility complex: sequencing and structural analysis using 3D homology modelling

The present manuscript reports for the first time the sequencing and characterisation of sea bass (sb) MHCII alpha and beta chains and Ii chain cDNAs as well as their expression analysis under resting state. 3D homology modelling, using crystal structures from mammalian orthologues, has been used to illustrate and support putative structural homologies of the sea bass counterparts. The sbIi cDNA consists of 96 bp of 5'-UTR, a 843 bp open reading frame (ORF) and 899 bp of 3'-UTR including a canonical polyadenylation signal 16 nucleotides before the polyadenylation tail. The ORF was translated into a 280 amino acid sequence, in which all characteristic domains found in the Ii p41 human form could be identified, including the cytoplasmic N-terminus domain, the transmembrane (TM) region, the CLIP domain, the trimerization domain and the thyroglobulin (Tg) type I domain. The trimerization and Tg domains of sbIi were successfully modelled using the human counterparts as templates. Four different sequences of each class II alpha and beta MHCII were obtained from a single fish, apparently not derived from a single locus. All the characteristic features of the MHCII chain structure could be identified in the predicted ORF of sea bass alpha and beta sequences, consisting of leader peptide (LP), alpha1/beta1 and alpha2/beta2 domains, connecting peptide and TM and cytoplasmic regions. Furthermore, independently of the HLA-DR crystal structure used as template in homology modelling, a similar predicted 3D structure and trimeric quaternary architecture was obtained for sbMHC, with major deviations occurring only within the sea bass MHCII alpha1 domain.

HLA-DM targets the hydrogen bond between the histidine at position beta81 and peptide to dissociate HLA-DR-peptide complexes

The peptide editor HLA-DM (DM) mediates exchange of peptides bound to major histocompatibility (MHC) class II molecules during antigen processing; however, the mechanism by which DM displaces peptides remains unclear. Here we generated a soluble mutant HLA-DR1 with a histidine-to-asparagine substitution at position 81 of the beta-chain (DR1betaH81N) to perturb an important hydrogen bond between MHC class II and peptide. Peptide-DR1betaH81N complexes dissociated at rates similar to the dissociation rates of DM-induced peptide-wild-type DR1, and DM did not enhance the dissociation of peptide-DR1betaH81N complexes. Reintroduction of an appropriate hydrogen bond (DR1betaH81N betaV85H) restored DM-mediated peptide dissociation. Thus, DR1betaH81N might represent a 'post-DM effect' conformation. We suggest that DM may mediate peptide dissociation by a 'hit-and-run' mechanism that results in conformational changes in MHC class II molecules and disruption of hydrogen bonds between betaHis81 and bound peptide.

Ii-Key/MHC class II epitope hybrids: a strategy that enhances MHC class II epitope loading to create more potent peptide vaccines

Life-threatening diseases, such as cancer and pandemic influenza, demand new efforts towards effective vaccine design. Peptides represent a simple, safe and adaptable basis for vaccine development; however, the potency of peptide vaccines is insufficient in most cases for significant therapeutic efficacy. Several methods, such as Ligand Epitope Antigen Presentation System and ISCOMATRIX, have been developed to enhance the potency of peptide vaccines. One way of increasing the loading of MHC class II peptides occurs through the use of Ii-Key technology. Ii-Key (LRMK), a portion of the MHC class II-associated invariant chain (Ii), facilitates the direct loading of epitopes to the MHC class II molecule groove. Linking the Ii-Key moiety via a simple polymethylene bridge to an MHC class II epitope, to generate an Ii-Key/MHC class II epitope hybrid, greatly enhances the vaccine potency of the tethered epitope. The combination of such Ii-Key/MHC class II epitope hybrids with MHC class I epitope-containing peptides might generate a potent peptide vaccine for malignancies and infectious diseases. The Ii-Key hybrid technology is compared with other methods that enhance the potency of a peptide vaccine.

A novel domain on HLA-DRbeta chain regulates the chaperone role of the invariant chain

The human lymphocyte antigen (HLA) class II region encodes highly polymorphic peptide receptors, which associate in the ER to the chaperone invariant chain (Ii). Ii facilitates assembly of class II subunits to functional peptide receptors. We searched for a conserved structure on HLA-DR polypeptides that mediates contact to a previously identified proline-rich class-II-binding sequence of Ii. Major histocompatibility complex (MHC) class II beta chain sequences exhibit two conserved tryptophan residues separated by 22 amino acids. Inspection of this motif in the X-ray structure of DR3 showed TrpTyr residues in the vicinity of the Ii-derived fragment CLIP. Five DRbeta mutants were produced. Mutation at Tyr123, Trp153 and Asp152 residues abolished interaction to the proline-rich sequence of Ii. All mutants formed heterodimers with DRalpha, were capable of binding an antigenic sequence and were expressed on the cell surface of transfected cells. In the presence of endogenous DRbeta chain however, the TyrAspTrp mutant was not cell-surface exposed and did not co-isolate with Ii or DRalpha. The competition of the mutant with the endogenous DRbeta for binding to DRalpha indicates that a structure on DRbeta chain regulates assembly of DR subunits. Hence, the chaperone function of Ii is mediated through a conserved region on the beta2 domain of class II.

Feasibility of clinical dendritic cell vaccination in acute myeloid leukemia

Dendritic cells (DC) are increasingly being utilized for anti-cancer therapy. Acute myeloid leukemia (AML) blasts are able to differentiate towards leukemia-derived DC enabling efficient presentation of known and unknown leukemic antigens. Advances in culture techniques and AML-DC characterization justify clinical application. However, clinical trials using AML-DC are hampered by patient inclusion criteria which allow selective entering of patients in second complete remission. Clinical relevant responses to DC-based immunotherapy are likely to only occur in non-end-stage patients. Application in early stage disease is mandatory to permit ultimate proof of clinical benefit of AML-DC vaccination strategy.

MHC class II molecules in tumour immunology: prognostic marker and target for immune modulation

MHC class II molecules presenting MHC class II restricted antigens play an important role in the activation of CD4+ T cells, which are the central orchestrating cells of an immune response. This review focuses on the particular role of MHC class II molecules in tumour immunology. The MHC class II antigen presentation pathway and the expression of MHC class II molecules on tumour cells related to clinical outcome is discussed. Improving the MHC class II tumour antigen presentation pathway, for instance by downregulation of the invariant chain or modulation of HLA-DO expression, offers many opportunities for developing new modalities of immunotherapy.

Gene expression profiling of CD34+ cells identifies a molecular signature of chronic myeloid leukemia blast crisis

Despite recent success in the treatment of early-stage disease, blastic phase (BP) of chronic myeloid leukemia (CML) that is characterized by rapid expansion of therapy-refractory and differentiation-arrested blasts, remains a therapeutic challenge. The development of resistance upon continuous administration of imatinib mesylate is associated with poor prognosis pointing to the need for alternative therapeutic strategies and a better understanding of the molecular mechanisms underlying disease progression. To identify transcriptional signatures that may explain pathological characteristics and aggressive behavior of BP blasts, we performed comparative gene expression profiling on CD34+ Ph+ cells purified from patients with untreated newly diagnosed chronic phase CML (CP, n=11) and from patients in BP (n=9) using Affymetrix oligonucleotide arrays. Supervised microarray data analysis revealed 114 differentially expressed genes (P<10(-4)), 34 genes displaying more than two-fold transcriptional changes when comparing CP and BP groups. While 24 of these genes were downregulated, 10 genes, especially suppressor of cytokine signalling 2 (SOCS2), CAMPATH-1 antigen (CD52), and four human leukocyte antigen-related genes were strongly overexpressed in BP. Expression of selected genes was validated by real-time-polymerase chain reaction and flow cytometry. Our data suggest the existence of a common gene expression profile of CML-BP and provide new insight into the molecular phenotype of blasts associated with disease progression and high malignancy.

A three-amino-acid-long HLA-DRbeta cytoplasmic tail is sufficient to overcome ER retention of invariant-chain p35

The p35 isoform of the human invariant chain (Iip35) contains an N-terminal RXR endoplasmic-reticulum (ER) retention signal that becomes nonfunctional only after assembly with MHC-class-II molecules. We have previously shown that the MHC-class-II beta-chain cytoplasmic tail is crucial for the maturation of class-II/Iip35 complexes. In order to shed some light on the molecular determinants involved in shielding the RXR motif, we performed site-directed mutagenesis of the DRbeta chain and Ii cytoplasmic domains. Chimeric beta chains with irrelevant cytoplasmic tails allowed the efficient transport of Iip35 out of the ER in transiently transfected HEK 293T cells. An alanine scan of the cytoplasmic tail of HLA-DRbeta confirmed that no specific motif is required to overcome ER retention. Surprisingly, a beta chain with a three-amino-acid-long cytoplasmic tail (Tyr-Phe-Arg) was sufficient to overcome the Iip35 RXR motif. Moreover, replacement of residues F231 and R232 with alanines created a cytoplasmic tail (Tyr-Ala-Ala) that allowed ER egress. Given the limited length of this tail, steric hindrance would only be possible if the Ii ER retention motif was close to the membrane in the first place. However, this is not likely because an Ii molecule with an internal cytoplasmic deletion bringing the RXR motif closer to the membrane is not retained in the ER, even in the absence of class-II molecules. These results suggest that MHC-class-II molecules overcome ER retention and prevent COPI binding to the Iip35 RXR motif through a mechanism distinct from steric hindrance by its beta chain.