Chloroquine affects biosynthesis of Ia molecules by inhibiting dissociation of invariant (gamma) chains from alpha-beta dimers in B cells

Hydroxychloroquine Therapy in Sarcoidosis-Associated Uveitis

BACKGROUND/PURPOSE

To assess the efficacy and tolerance of hydroxychloroquine in sarcoidosis-associated uveitis.

METHODS

Retrospective study on all patients with sarcoidosis-associated uveitis who were treated with hydroxychloroquine between 2003 and 2019 in a French university hospital.

RESULTS

Twenty-seven patients with sarcoidosis-associated uveitis received hydroxychloroquine. The mean duration of treatment was 20.0 ± 10.9 months. At the end of the follow-up, hydroxychloroquine success was achieved in 15 (55.6%) patients. Four of them were also on oral corticosteroids, with a prednisone dose ≤5 mg/day. Under treatment, the median prednisone dose decreased from 20.0 (interquartile range (IQR), 7-25) to 5.0 (IQR, 3-6.5) mg/day (p = .02). The incidence rate of flare decreased from 204.6 to 63.8 per 100 person-years (p = .02). Hydroxychloroquine was discontinued in 12 (44.4%) patients during follow-up, including 8 (29.6%) for ineffectiveness, and three who experienced side effects.

CONCLUSION

Hydroxychloroquine appears as an interesting option in sarcoidosis-associated uveitis.Abbreviations: AZA: Azathioprine; BAL: Bronchoalveolar Lavage; BCVA: Best-Corrected Visual Acuity; ENT: Ears, Nose and Throat; HCQ: Hydroxychloroquine; IOP: Intra-Ocular Pressure; IQR: interquartile range; MHC: Major Histocompatibility Complex; MMF: Mycophenolate Mofetil; MTX: Methotrexate; PMSI: Programme de Médicalisation du Système d'Information; SAU: Sarcoidosis-Associated Uveitis; SD: Standard Deviation; SUN: Standard Uveitis Nomenclature.

FDA approved drugs with pharmacotherapeutic potential for SARS-CoV-2 (COVID-19) therapy

In December 2019, a novel SARS-CoV-2 coronavirus emerged, causing an outbreak of life-threatening pneumonia in the Hubei province, China, and has now spread worldwide, causing a pandemic. The urgent need to control the disease, combined with the lack of specific and effective treatment modalities, call for the use of FDA-approved agents that have shown efficacy against similar pathogens. Chloroquine, remdesivir, lopinavir/ritonavir or ribavirin have all been successful in inhibiting SARS-CoV-2 in vitro. The initial results of a number of clinical trials involving various protocols of administration of chloroquine or hydroxychloroquine mostly point towards their beneficial effect. However, they may not be effective in cases with persistently high viremia, while results on ivermectin (another antiparasitic agent) are not yet available. Interestingly, azithromycin, a macrolide antibiotic in combination with hydroxychloroquine, might yield clinical benefit as an adjunctive. The results of clinical trials point to the potential clinical efficacy of antivirals, especially remdesivir (GS-5734), lopinavir/ritonavir, and favipiravir. Other therapeutic options that are being explored involve meplazumab, tocilizumab, and interferon type 1. We discuss a number of other drugs that are currently in clinical trials, whose results are not yet available, and in various instances we enrich such efficacy analysis by invoking historic data on the treatment of SARS, MERS, influenza, or in vitro studies. Meanwhile, scientists worldwide are seeking to discover novel drugs that take advantage of the molecular structure of the virus, its intracellular life cycle that probably elucidates unfolded-protein response, as well as its mechanism of surface binding and cell invasion, like angiotensin converting enzymes-, HR1, and metalloproteinase inhibitors.

Hydroxychloroquine Inhibits the Trained Innate Immune Response to Interferons

Hydroxychloroquine is being investigated for a potential prophylactic effect in severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection, but its mechanism of action is poorly understood. Circulating leukocytes from the blood of coronavirus disease 2019 (COVID-19) patients show increased responses to Toll-like receptor ligands, suggestive of trained immunity. By analyzing interferon responses of peripheral blood mononuclear cells from healthy donors conditioned with heat-killed Candida, trained innate immunity can be modeled in vitro. In this model, hydroxychloroquine inhibits the responsiveness of these innate immune cells to virus-like stimuli and interferons. This is associated with a suppression of histone 3 lysine 27 acetylation and histone 3 lysine 4 trimethylation of inflammation-related genes, changes in the cellular lipidome, and decreased expression of interferon-stimulated genes. Our findings indicate that hydroxychloroquine inhibits trained immunity in vitro, which may not be beneficial for the antiviral innate immune response to SARS-CoV-2 infection in patients.

PBMCs of COVID-19 patients show increased responses to Toll-like receptor ligands

Trained immunity is modeled in vitro using Candida-trained PBMCs

Hydroxychloroquine inhibits changes in lipidome and histone modifications

Hydroxychloroquine dampens the trained response to interferons and viral stimuli

Anti-malarial drugs: possible mechanisms of action in autoimmune disease and prospects for drug development

A wide variety of mechanisms of anti-rheumatic action have been proposed for antimalarial agents. The molecular actions of chloroquine have been most thoroughly studied in vitro and in vivo, but it is likely that hydroxychloroquine works by a similar mechanism. Both agents are weak diprotic bases that can pass through the lipid cell membrane and preferentially concentrate in acidic cyto-plasmic vesicles. The resulting slight elevation of pH within these vesicles in macrophages or other antigen-presenting cells may influence the immune response to autoantigens. We hypothesize that anti-malarial agents influence the association of autoantigenic peptides with class II MHC molecules in the compartment for peptide loading and/or the subsequent processing and transport of the peptide-MHC complex to the cell membrane. This model of anti-malarial action provides a method to test additional drugs for their ability to modulate the immune response.

Mechanism of Action of Antimalarial Drugs: Inhibition of Antigen Processing and Presentation

Recent studies have elucidated the steps involved in the association of antigenic peptides with major histocompatibility complex (MHC) encoded proteins and have suggested how antimalarial compounds might influence this important site of immune activation. These steps of antigen presentation in the macrophage (or other antigen-presenting cells) include: (a) the partial proteolytic degradation of endogenous and exogenous proteins into peptides within the lysosome; (b) the synthesis of MHC class II (i.e. HLA-D associated) α, β, and invariant (Ii) chains in the endoplasmic reticulum; (c) the initial association of α-Ii and β-li chains in the endoplasmic reticulum and the transport of these complexes to the primary endosome; (d) the fusion of lysosomal vacuoles and endosomal vacuoles, allowing the mixtures of lysosomal enzymes, peptides, α–Ii and β–Ii; (e) the displacement of Ii chains by peptides to form α–β–peptide complexes in the endosome; and (f) the migration of α–β–peptide complexes to the macrophage cell surface where they can stimulate CD4 T cells, resulting in release of cytokines. A low pH is required for digestion of the protein by acidic hydrolases in the lysosome, for assembly of the α–β–peptide complex and for its transport to the cell surface. Chloroquine and hydroxychloroquine are weak diprotic bases that can diffuse across the cell membrane and raise the pH within cell vesicles. This background provides the underlying basis for the theory that antimalarials may act to prevent autoimmunity by the following putative mechanism. Antimalarial compounds may: (a) stabilize the α-Ii and β-Ii interactions and prevent low-affinity peptides from forming α–β–peptide complexes; and (b) interfere with the efficient movement of α-Ii, β-Ii and α–β–peptide complexes to the correct locations within the cell cytoplasm or to the cell surfaces. Decreased presentation of autoantigenic peptides by macrophages might then lead to downregulation of autoimmune CD4+ T cells and diminish release of cytokines associated with clinical and laboratory signs of autoimmune disease.

Chloroquine inhibits Ca2+ permeable ion channels-mediated Ca2+ signaling in primary B lymphocytes

BACKGROUND

Chloroquine, a bitter tastant, inhibits Ca²⁺ signaling, resulting in suppression of B cell activation; however, the inhibitory mechanism remains unclear.

RESULTS

In this study, thapsigargin (TG), but not caffeine, induced sustained intracellular Ca²⁺ increases in mouse splenic primary B lymphocytes, which were markedly inhibited by chloroquine. Under Ca²⁺-free conditions, TG elicited transient Ca²⁺ increases, which additionally elevated upon the restoration of 2 mM Ca²⁺. The former were from release of intracellular Ca²⁺ store and the latter from Ca²⁺ influx. TG-induced release was inhibited by 2-APB (an inhibitor of inositol-3-phosphate receptors, IP₃Rs) and chloroquine, and TG-caused influx was inhibited by pyrazole (Pyr3, an inhibitor of transient receptor potential C3 (TRPC3) and stromal interaction molecule (STIM)/Orai channels) and chloroquine. Moreover, chloroquine also blocked Ca²⁺ increases induced by the engagement of B cell receptor (BCR) with anti-IgM.

CONCLUSIONS

These results indicate that chloroquine inhibits Ca²⁺ elevations in splenic B cells through inhibiting Ca²⁺ permeable IP₃R and TRPC3 and/or STIM/Orai channels. These findings suggest that chloroquine would be a potent immunosuppressant.

Processing of viral antigens and presentation to class II-restricted T cells

Some antigens require intracellular processing by antigen presenting cells before being presented to T cells in conjunction with surface major histocompatibility complex antigens. The whole mechanism of these processing events is not known and in this article, Kingston Mills puts forward arguments for alternative routes of antigen processing, with particular reference to recognition of viral proteins by class II-restricted T-cell clones.

Hydroxychloroquine in systemic lupus erythematosus and rheumatoid arthritis and its safety in pregnancy

INTRODUCTION

The antimalarial drug hydroxychloroquine (HCQ) is widely used to treat various rheumatic diseases. Many autoimmune diseases occur in women of child-bearing age who may become pregnant while on therapy, which raises concerns regarding the teratogenicity of HCQ and its effect on the outcome of the pregnancy. There is a lack of data regarding the safety of HCQ during pregnancy.

AREAS COVERED

In this review, the authors attempt to identify relevant publications by searching MEDLINE, Cochrane database, Ovid-Currents Clinical Medicine, Ovid-Embase:Drugs and Pharmacology, EBSCO, Web of Science and SCOPUS using the search terms HCQ and/or pregnancy. A basis for the mechanism of action of HCQ is provided.

EXPERT OPINION

HCQ has been shown by numerous studies over the past 15 years to be efficacious in the treatment of autoimmune diseases, including systemic lupus erythematosus, discoid lupus erythematosus and rheumatoid arthritis. HCQ does not appear to be associated with any increased risk of congenital defects, spontaneous abortions, fetal death, prematurity or decreased numbers of live births in patients with autoimmune diseases. Therefore, in the author's opinion, HCQ is safe for the treatment of autoimmune diseases during pregnancy.

Why functional pre-erythrocytic and bloodstage malaria vaccines fail: a meta-analysis of fully protective immunizations and novel immunological model

BACKGROUND

Clinically protective malaria vaccines consistently fail to protect adults and children in endemic settings, and at best only partially protect infants.

METHODOLOGY/PRINCIPAL FINDINGS

We identify and evaluate 1916 immunization studies between 1965-February 2010, and exclude partially or nonprotective results to find 177 completely protective immunization experiments. Detailed reexamination reveals an unexpectedly mundane basis for selective vaccine failure: live malaria parasites in the skin inhibit vaccine function. We next show published molecular and cellular data support a testable, novel model where parasite-host interactions in the skin induce malaria-specific regulatory T cells, and subvert early antigen-specific immunity to parasite-specific immunotolerance. This ensures infection and tolerance to reinfection. Exposure to Plasmodium-infected mosquito bites therefore systematically triggers immunosuppression of endemic vaccine-elicited responses. The extensive vaccine trial data solidly substantiate this model experimentally.

CONCLUSIONS/SIGNIFICANCE

We conclude skinstage-initiated immunosuppression, unassociated with bloodstage parasites, systematically blocks vaccine function in the field. Our model exposes novel molecular and procedural strategies to significantly and quickly increase protective efficacy in both pipeline and currently ineffective malaria vaccines, and forces fundamental reassessment of central precepts determining vaccine development. This has major implications for accelerated local eliminations of malaria, and significantly increases potential for eradication.

New concepts in antimalarial use and mode of action in dermatology

Although chloroquine, hydroxychloroquine and quinacrine were originally developed for the treatment of malaria, these medications have been used to treat skin disease for over 50 years. Recent clinical data have confirmed the usefulness of these medications for the treatment of lupus erythematosus. Current research has further enhanced our understanding of the pharmacologic mechanisms of action of these drugs involving inhibition of endosomal toll-like receptor (TLR) signaling limiting B cell and dendritic cell activation. With this understanding, the use of these medications in dermatology is broadening. This article highlights the different antimalarials used within dermatology through their pharmacologic properties and mechanism of action, as well as indicating their clinical uses. In addition, contraindications, adverse effects, and possible drug interactions of antimalarials are reviewed.

Optimization of CD4+ T lymphocyte response to human cytomegalovirus nuclear IE1 protein through modifications of both size and cellular localization

We have previously reported that the CD4+ T lymphocyte response against nuclear human CMV IE1 protein depends in part on endogenous MHC class II presentation. To optimize presentation by HLA-DR of the nuclear IE1 protein and increase the response by CD4+ T cells, we have constructed two different adenovirus vectors containing mutant versions of IE1, containing a HLA-DR3 epitope, fused to GFP. The first construct consisted of a sequence of 46 aa encoded by exon 4, called GFP-IE1 (86-131). The second construct consisted of the whole IE1 mutated on exon 4 nuclear localization signals, identified in this study, and deleted of already known exon 2 nuclear localization signals (GFP-IE1M). Both of these IE1 vectors expressed proteins with cytoplasmic localization, as evidenced by GFP expression, as opposed to control GFP-IE1, which was nuclear. GFP-IE1 (86-131) induced IE1-specific CD4+ T cell clone response that was >30-fold more potent than that against GFP-IE1 and GFP-IE1M. The CD4+ T cell response was due to endogenous presentation followed by exogenous presentation at later time points. Presentation was dependent on both proteasome and acidic compartments. GFP-IE1 (86-131) was rapidly degraded by the APC, which may account for better presentation. Our data show potentiation of the CD4+ T cell response to a specific epitope through shortening and relocation of an otherwise nuclear protein and suggest applications in vaccination.

Lysosomal cysteine proteases regulate antigen presentation

Antigen presentation by both classical MHC class II molecules and the non-classical MHC class I-like molecule CD1D requires their entry into the endosomal/lysosomal compartment. Lysosomal cysteine proteases constitute an important subset of the enzymes that are present in this compartment and, here, we discuss the role of these proteases in regulating antigen presentation by both MHC class II and CD1D molecules.

Antimalarials in cutaneous lupus erythematosus: mechanisms of therapeutic benefit

Antimalarials are arguably the best modality currently available for treating patients with cutaneous lupus erythematosus (LE). Although antimalarials have been used for decades in treating cutaneous LE, the precise mechanisms by which they provide therapeutic benefit are not well defined. The putative mechanisms by which antimalarials might provide therapeutic benefit to patients with cutaneous LE include a number of interrelated anti-inflammatory and immunosuppressive effects that include photoprotection, lysosomal stabilization, suppression of antigen presentation, and inhibition of prostaglandin and cytokine synthesis. If we had a more precise understanding of how antimalarials provide therapeutic benefit in cutaneous LE we might gain better insight into the pathogenic mechanisms of LE and ways of developing better therapies for afflicted patients.

H2-O inhibits presentation of bacterial superantigens, but not endogenous self antigens

H2-O/HLA-DO are MHC class II accessory molecules that modulate exogenous Ag presentation. Most class II accessory molecules are expressed in all professional APC; however, H2-O is only expressed in B cells and medullary thymic epithelial cells. Because B cells present exogenous Ags and superantigens (SAgs), and medullary thymic epithelial cells are specialized APC for self Ags during negative selection in the thymus, we have hypothesized that H2-O might play a role in MHC class II-restricted SAg and self Ag presentation. In this study, we demonstrate that H2-O expression inhibits presentation of the bacterial SAgs staphylococcal enterotoxins A and B to four SAg-reactive T hybridoma cells. In contrast, H2-O has no effect on presentation of endogenous self Ags, as measured by tumorigenicity in vivo and Ag presentation to three self Ag-specific T hybridoma cells. Additional experiments suggest that H2-O inhibits presentation of exogenous Ags by both newly synthesized and recycling MHC class II molecules. These data suggest H2-O may have a physiological role in tolerance induction and SAg-mediated toxic shock.

MHC class II presentation of endogenous tumor antigen by cellular vaccines depends on the endocytic pathway but not H2-M

We have developed cell-based cancer vaccines that activate anti-tumor immunity by directly presenting endogenously synthesized tumor antigens to CD4+ T helper lymphocytes via MHC class II molecules. The vaccines are non-conventional antigen-presenting cells because they express MHC class II, do not express invariant chain or H-2M, and preferentially present endogenous antigen. To further improve therapeutic efficacy we have studied the intracellular trafficking pathway of MHC class II molecules in the vaccines using endoplasmic reticulumlocalized lysozyme as a model antigen. Experiments using endocytic and cytosolic pathway inhibitors (chloroquine, primaquine, and brefeldin A) and protease inhibitors (lactacystin, LLnL, E64, and leupeptin) indicate antigen presentation depends on the endocytic pathway, although antigen degradation is not mediated by endosomal or proteasomal proteases. Because H2-M facilitates presentation of exogenous antigen via the endocytic pathway, we investigated whether transfection of vaccine cells with H-2M could potentiate endogenous antigen presentation. In contrast to its role in conventional antigen presentation, H-2M had no effect on endogenous antigen presentation by vaccine cells or on vaccine efficacy. These results suggest that antigen/MHC class II complexes in the vaccines may follow a novel route for processing and presentation and may produce a repertoire of class II-restricted peptides different from those presented by professional APC. The therapeutic efficacy of the vaccines, therefore, may reside in their ability to present novel tumor peptides, consequently activating tumor-specific CD4+ T cells that would not otherwise be activated.

Intracellular Formation and Cell Surface Expression of a Complex of an Intact Lysosomal Protein and MHC Class II Molecules

The generation of invariant chain-free MHC class II molecules and their association with endocytically generated peptides are thought to occur in specialized lysosome-like compartments called MIICs (MHC class II compartments). A number of in vitro studies have shown that large denatured proteins can bind to class II molecules, and that class II association can protect the bound segment of protein from proteolytic degradation. In this work, we present what we believe is the first example of an intact endogenous protein (IP30) binding in an allele-dependent fashion to class II molecules in vivo. IP30 is an IFN-γ-inducible 35-kDa glycoprotein that localizes in MIICs. In this study, we show that intact IP30 binds to certain HLA-DR alleles via an N-terminal prosequence. The association takes place in the endocytic pathway following removal of invariant chain from class II molecules and before their cell surface expression. We also show that DR-IP30 complexes are SDS stable. The potential precursor-product relationship between DR-IP30 complexes and the DR-peptide complex is discussed.

Novel Glycosylation of HLA-DRα Disrupts Antigen Presentation Without Altering Endosomal Localization

The HLA-DR hemizygous B lymphoblastoid cell line, 10.24.6, has a DRA mutation (Pro96→Ser) that creates a novel glycosylation site at Asn94. The mutant DR molecules are primarily associated with nested fragments of invariant chain (class II-associated invariant chain peptides), and their interaction with HLA-DM is impaired. Here we further analyzed the defect in 10.24.6 cells. Expressing Ser96 mutant DRA cDNA in DRA-null cells recapitulated the 10.24.6 phenotype, indicating that the mutation causes the Ag presentation defect. A mutation to Ala96α, which does not introduce an extra glycan, generated a normal phenotype; the critical role of the glycan was further supported by experiments in which N-glycosylation was blocked by tunicamycin. We also evaluated whether the 10.24.6 mutation affected DR3 maturation or trafficking. Metabolic labeling and subcellular fractionation showed that assembly, endosomal transport, and invariant chain proteolysis of mutant DR3 molecules were similar to wild-type. A slight delay in export from the endoplasmic reticulum to the Golgi apparatus in 10.24.6 cells probably did not contribute significantly to the Ag presentation defect, because the abundance of DM and mutant DR in peptide-loading compartments was normal at steady state. Our results indicate that proper localization of these molecules does not depend on their interaction.

Interleukin-10 down-regulates MHC class II alphabeta peptide complexes at the plasma membrane of monocytes by affecting arrival and recycling

Interleukin-10 (IL-10) inhibits antigen-specific T cell responses when human monocytes are used as antigen-presenting cells. This is correlated with a down-regulation of MHC class II molecules on the surface of the monocyte. Here we show that IL-10 does not affect MHC class II transcription, polypeptide synthesis, subunit assembly, or antigenic peptide loading. Instead, newly synthesized mature MHC class II molecules are localized to the MHC class II loading compartment but are prevented from reaching the plasma membrane. In addition, treatment of monocytes with IL-10 leads to an accumulation of internalized MHC class II complexes in intracellular vesicles. These results indicate that IL-10 affects antigen presentation by regulating MHC exocytosis and recycling.

Major histocompatibility complex class II-dependent unfolding, transport, and degradation of endogenous proteins

We have analyzed the ability of major histocompatibility (MHC) class II molecules to capture proteins in the biosynthetic pathway and whether this may be associated with MHC class II-dependent antigen processing. When coexpressed with HLA-DR 4 molecules in HeLa cells, influenza hemagglutinin was inhibited from folding and trimerization in the biosynthetic pathway, targeted to endosomal compartments, and rapidly degraded. Due to the interaction with MHC class II molecules, therefore, unfolded forms of hemagglutinin were bypassing the quality control mechanism of the secretory pathway. More important, however, the transport, endocytosis, and rapid degradation of unfolded hemagglutinin in the presence of MHC class II molecules suggest that proteins captured in the endoplasmic reticulum by class II molecules may become substrates for antigen processing and presentation to CD4-positive T cells. In insect cells we show that this phenomenon is not restricted to a few proteins such as hemagglutinin. A highly heterogeneous mixture of proteins from the endoplasmic reticulum including coexpressed hemagglutinin can form stable complexes with soluble HLA-DR alpha and beta chains that were transported into the supernatant. This mechanism may gain biological significance in abnormal situations associated with accumulation of unfolded or malfolded proteins in the endoplasmic reticulum, for example during viral infections.

Decreased endosomal delivery of major histocompatibility complex class II-invariant chain complexes in dynamin-deficient cells

Major histocompatibility complex class II molecules are heterodimeric cell surface molecules which acquire antigenic peptides in the endosomal/lysosomal system. Invariant chain (Ii), a third chain which is associated with class II molecules intracellularly mediates the endosomal targeting, but it is debated whether class II molecules reach the endosomal system mainly from the trans-Golgi network or via the cell surface. Dynamin is a cytosolic GTPase which is necessary for the formation of clathrin-coated vesicles from the plasma membrane, but which is not required for vesicle formation from the trans-Golgi network. Here we have used HeLa cells expressing a dominant negative form of dynamin to show that inhibition of clathrin-mediated uptake from the plasma membrane leads to accumulation of transfected Ii-class II complexes at the cell surface, while delivery of such complexes to endosomes/lysosomes is decreased. Our data therefore suggest that in this experimental system the majority of Ii-class II complexes traverse the cell surface before they reach the endosomal system.

Antigen presentation by MHC class II molecules: invariant chain function, protein trafficking, and the molecular basis of diverse determinant capture

Major histocompatibility complex class II molecules are heterodimeric integral membrane proteins whose primary function is the presentation of antigenic peptides derived from proteins entering the endocytic pathway to CD4+ T lymphocytes. To accomplish this physiologic function, class II molecules must assemble in the secretory pathway without undergoing irreversible ligand association at that site, traffic efficiently to the endocytic pathway, and productively interact with protein ligands in these organelles before their ultimate expression on the plasma membrane. Here we review our work describing how invariant chain promoters the assembly and transport process, the complex itinerary of class II-invariant chain complexes through the endocytic pathway, the role of large protein fragments as substrates for class II binding, and the existence of a second pathway for antigen capture by mature class II molecules that complements that involving newly synthesized dimers. We integrate these observations into a coherent model for the operation of a class II-dependent antigen processing and presentation system able to capture diverse antigenic determinants present in proteins of varying structure.

Expression of beta 2-microglobulin-free HLA class I alpha-chains on activated T cells requires internalization of HLA class I heterodimers

HLA class I molecules on activated T cells are expressed as heterodimers associated with beta 2-microglobulin (beta 2-m) and also beta 2-m-free HLA class I alpha-chains. Mechanisms leading to the expression of the activation associated beta 2-m-free HLA class I alpha-chains are poorly defined, however. Upon enzymatical removal of HLA class I alpha-chains on activated T cells, re-expression is observed within minutes upon reculture, reaching half-maximal levels within 1 hr. This process is independent of de novo protein synthesis and of export of newly synthesized proteins. Inhibition of the formation of coated pits by potassium depletion of cells abrogated the re-expression of HLA class I alpha-chains, suggesting that recycling events of HLA class I heterodimers via endosomal compartments are required for the generation of monoclonal antibody LA45-reactive alpha-chains. Furthermore, the rate of alpha-chain generation seems to be governed by the amount of cell surface-expressed HLA class I heterodimers. Taken together these findings suggest that beta 2-m-free HLA class I alpha-chains are generated during the process of class I heterodimer recycling.

Class II antigen processing compartments and the function of HLA-DM

The DM alpha and DM beta genes encode a nonpolymorphic, class II-like molecule which functions by an, as yet, undefined mechanism in the assembly of Major Histocompatibility Complex class II-peptide complexes. Indeed, mutant cells which express class II molecules but fail to express DM are unable to process and present native protein antigens. A striking phenotype of the mutation is class II molecules that contain almost exclusively a nested set of invariant chain peptides, termed CLIP, for class II associated Ii peptides, instead of the normal array of endogenously and exogenously derived peptides. Thus, DM appears to be required for the correct assembly of processed antigen-class II complexes. Recently, the subcellular compartments that contain DM and in which functional processed antigen-class II complexes are first formed have been described. Here, the evidence for the function of DM in the antigen-processing compartments is reviewed.

A structural transition in class II major histocompatibility complex proteins at mildly acidic pH

Peptide binding by class II major histocompatibility complex proteins is generally enhanced at low pH in the range of hydrogen ion concentrations found in the endosomal compartments of antigen-presenting cells. We and others have proposed that class II molecules undergo a reversible conformational change at low pH that is associated with enhanced peptide loading. However, no one has previously provided direct evidence for a structural change in class II proteins in the mildly acidic pH conditions in which enhanced peptide binding is observed. In this study, susceptibility to denaturation induced by sodium dodecyl sulfate (SDS) detergent or heat was used to probe the conformation of class II at different hydrogen ion concentrations. Class II molecules became sensitive to denaturation at pH 5.5-6.5 depending on the allele and experimental conditions. The observed structural transition was fully reversible if acidic pH was neutralized before exposure to SDS or heat. Experiments with the environment-sensitive fluorescent probe ANS (8-anilino-1-naphthalene-sulfonic acid) provided further evidence for a reversible structural transition at mildly acidic pH associated with an increase in exposed hydrophobicity in class II molecules. IAd conformation was found to change at a higher pH than IEd, IEk, or IAk, which correlates with the different pH optimal for peptide binding by these molecules. We conclude that pH regulates peptide binding by influencing the structure of class II molecules.

The invariant chain derived fragment CLIP is an efficient in vitro inhibitor of peptide binding to MHC class II molecules

The invariant chain derived peptide CLIP inhibits association of peptides to the class II peptide binding site. Two DR3 specific peptides, the microbacterial heat shock protein 65 derived peptide hsp3-13 and the naturally occurring invariant chain derived peptide Ii131-149 were employed to study binding inhibition by CLIP (Ii82-102) in a series of combinations. Incubation of detergent solubilized DR polypeptides from Ii-free cells with 500 microM of synthetic CLIP almost completely prevents binding of 50 microM subsequently added DR3-specific peptides. When CLIP and the peptides were added simultaneously to DR3 molecules, binding of hsp3-13 was abolished, whereas binding of Ii131-149 was only partially blocked. This indicates apparent affinity differences of the peptides. The addition of CLIP to preformed DR-peptide complexes substantially reduced binding of hsp3-13,while there was little effect on the DR associated Ii131-149. The profound inhibitory ability of CLIP, which in vivo would diminish binding of antigenic peptides, suggests an intracellular mechanism that abrogates the persistence of the CLIP-DR complex. The HLA-DM molecules have been suggested as candidates for this function. The strong in vitro binding of the naturally occurring peptide Ii131-149 to DR3 may suggest that only limited amounts of this peptide are available in vivo for competition of exogenous peptide binding to class II molecules.

Inhibition of invariant chain (Ii)-calnexin interaction results in enhanced degradation of Ii but does not prevent the assembly of alpha beta Ii complexes

Calnexin is a resident protein of the endoplasmic reticulum (ER) that associates with nascent protein chains. Among the newly synthesized integral membrane proteins known to bind to calnexin is invariant chain (Ii), and Ii release from calnexin coincides with proper assembly with major histocompatibility complex (MHC) class II heterodimers. Although calnexin association with several membrane glycoproteins depends on interactions involving N-linked glycans, we previously reported that a truncation mutant of mouse Ii (mIi1-107) lacking both N-glycosylation sites was highly effective in associating with MHC class II heterodimers and escorting these dimers through the secretory pathway. This could indicate that calnexin, despite binding to both Ii and class II, is not necessary for the proper interaction of these proteins, or that in contrast to most membrane glycoproteins, the N-linked glycans of Ii are not critical to its interaction with this chaperone. To examine this issue, we have directly explored the binding of calnexin to both Ii truncation mutants lacking the typical sites of N-glycosylation or Ii produced in cells treated with tunicamycin to prevent glycan addition. These experiments revealed that either method of eliminating N-linked carbohydrates on Ii also inhibited association with calnexin. A lumenally truncated form of Ii (mIi1-131) that still has N-linked carbohydrates showed a decreased affinity for calnexin compared with intact Ii, however, indicating that calnexin-Ii binding is not determined solely by the sugar moieties. All forms of Ii lacking N-linked sugars and showing defective association with calnexin also had enhanced rates of preendosomal degradation. Despite this effect on degradation rate, tunicamycin treatment did not inhibit the association of class II with glycan-free Ii. These data support the view that calnexin is not an absolute requirement for the proper assembly of class II-Ii nonamers, but rather acts primarily to retain Ii in the ER and to inhibit its degradation. These two properties of calnexin-Ii interaction may help ensure that sufficient intact Ii is available for efficient inactivation of the binding sites of newly synthesized class II molecules, while limiting the ability of excess free Ii to alter the transport properties of the early endocytic pathway.

Regulation and targeting of T-cell immune responses by IgE and IgG antibodies

A set of chimeric antibodies with identical F(ab')2 fragments specific for the hapten 5-iodo-4-hydroxyl-3-nitrophenacetyl (NIP), but with different human Fc parts (gamma 1, gamma 2, gamma 3, gamma 4, epsilon), was used to compare the role of IgG and IgE antibodies in antigen presentation by human Epstein-Barr virus (EBV) B cells. Two or three molecules of NIP were coupled to one molecule of Der pI (Der pI-(3)NIP), a major allergen of Dermatophagoides pteronyssinus. Both monomeric IgG and performed complexes of various Der pI/IgG ratios failed to bind significantly to the Fc receptor for IgG on B cells (Fc gamma RII; CD32). Binding of IgG3 (> IgG1)-containing complexes (optimal ratio of antigen to antibody = 1:1) could be enhanced by increasing the number of haptens per Der pI molecule to nine or more. However, antigen presentation mediated by IgG and CD32 was not seen with either pulsed B cells or B cells that were allowed to capture the IgG complexes during the whole stimulation period. IgE binding to CD23 and subsequent IgE-mediated antigen presentation was seen under all conditions tested. Even monomeric immune complexes (IC) (Der pI-(3)NIP/IgE), in the absence of CD23 cross-linking, induced an immune response. As the number of natural epitopes for human antibodies on Der pI was less than five, we conclude that, in vivo, complexes consisting of Der pI/IgG will be directed to antigen-presenting cells expressing the high-affinity receptor for IgG (CD64), whereas IgE will allow antigen presentation by CD23-expressing cells, including B cells.

Efficient endosomal localization of major histocompatibility complex class II-invariant chain complexes requires multimerization of the invariant chain targeting sequence

During biosynthesis, MHC class II-invariant chain complexes are transported into endosomal compartments where invariant chain (Ii) is degraded and class II encounters antigenic peptides. One of the signals that determines this intracellular transport route has been localized to the cytosolic domain of Ii. Deletion of this signal disrupts endosomal targeting and results in the stable expression of class II-Ii complexes at the surface. In this paper we have examined the role of Ii trimerization on the generation of this endosomal localization signal. In L cell transfectants expressing class II and both wild type Ii and a truncated form of Ii that lacks this endosomal localization signal, Ii was found to form multimers which could contain both wild type and truncated Ii. The multimers were not large aggregates but were found to be discrete complexes, probably the nine molecule class II-Ii complex that has been observed in human B cells. The co-expression of truncated Ii allowed for cell surface expression of a subset of wild type Ii. This surface-expressed wild type Ii associated with truncated Ii in multimers at a 2:1 ratio, indicating that these trimers contain two truncated and one wild type Ii molecule. These data suggest a division in trafficking of Ii trimers: if two wild type Ii molecules are present, the complex is transported to and rapidly degraded in endosomes, whereas the presence of only one wild type Ii results in trafficking and expression of the heterotrimer on the cell surface. Following surface arrival, complexes containing only a single wild type Ii molecule are internalized more rapidly and have a shorter half-life than complexes containing only truncated Ii molecules. These data suggest that although a single Ii cytosolic domain can function as a plasma membrane internalization signal, multimerization of Ii is required for efficient Golgi complex to endosome targeting of class II-Ii complexes.

In the absence of major histocompatibility complex class II molecules, invariant chain is translocated to late endocytic compartments by autophagy

It has been suggested that the cytoplasmic amino-terminal tail of invariant chain (Ii) contains a sorting signal that directs trafficking of the major histocompatibility complex (MHC) class II: Ii oligomeric complex to endocytic compartments. This model is based, in part, on the observation that in the absence of MHC class II molecules, Ii is detectable in lysosomal structures, a phenotype that is dependent on an intact NH2 terminus. However, the route by which Ii gains access to endosomal compartments in the absence of class II molecules remains uncertain. Here we report a mechanism that localizes Ii in lysosomal compartments independently of class II. We show that murine Ii can be detected by immunofluorescence within late endocytic compartments of stably transfected Ltk- mouse fibroblasts. Immunochemical studies indicate that degradation of Ii in these cells is sensitive to the lysosomotropic agent ammonium chloride, yet the majority of Ii that undergoes this apparent lysosomal degradation is sensitive to the enzyme endoglycosidase H. This finding suggests that Ii may reach the lysosomal compartment by a route that bypasses the Golgi complex. Consistent with this possibility, we found that in contrast to Ii which is complexed to class II molecules, transport of free Ii to lysosomes is prevented by 3-methyladenine, an inhibitor of the autophagic pathway of protein degradation, a process which involves direct transport from the endoplasmic reticulum to lysosomes. These data suggest the route of transport that leads to endosomal localization of Ii in the absence of class II is distinct from that taken when expressed with class II. This forces a re-evaluation of the concept that the cytosolic tail of Ii contains a dominant Golgi-to-endosomal sorting signal.

Extensive trafficking of MHC class II-invariant chain complexes in the endocytic pathway and appearance of peptide-loaded class II in multiple compartments

Major histocompatibility complex class II molecules bind and present to T cells fragments of protein antigens entering the endocytic pathway. Using normal B lymphoblasts, we have combined metabolic pulse-chase labelling, high resolution organelle fractionation, and immunoprecipitation to examine class II trafficking and antigen loading in a physiological model system. Most newly synthesized class II-invariant chain complexes first entered early endosomes, then accessed multiple discrete endocytic subcompartments cofractionating with late endosomes and immature lysosomes. Invariant chain was removed and peptide-loaded class II molecules appeared in each of these latter distinct organelles. These findings suggest that class II molecules traffic through much of the endocytic pathway, permitting capture of distinct determinants made available under differing conditions of pH and proteolytic activity.

In vivo and in vitro formation and dissociation of HLA-DR complexes with invariant chain-derived peptides

HLA-DR molecules associated with class II-associated invariant chain peptides (CLIP) are generated in vivo as an intermediate in class II maturation. Such complexes can be produced in vitro by proteolytic digestion of DR alpha beta I complexes, suggesting that CLIP is a residual fragment that remains associated with class II molecules following I chain degradation. In vitro, CLIP dissociation from DR alpha beta dimers occurs at different rates depending on the allele, and is facilitated by low pH and by detergents containing 8-10 carbon unbranched hydrocarbons, or by primary aliphatic amines or carboxylic acids. The accumulation of DR alpha beta CLIP complexes in HLA-DM-negative antigen-processing mutant cells argues that a functionally similar mechanism, dependent on HLA-DM expression, catalyzes in vivo CLIP dissociation and generation of normal class II-peptide complexes.

Transport and intracellular distribution of MHC class II molecules and associated invariant chain in normal and antigen-processing mutant cell lines

We have compared the intracellular transport and subcellular distribution of MHC class II-invariant chain complexes in a wild-type HLA-DR3 homozygous cell line and a mutant cell line, T2.DR3. The latter has a defect in antigen processing and accumulates HLA-DR3 molecules associated with an invariant chain-derived peptide (CLIP) rather than the normal complement of peptides derived from endocytosed proteins. We find that in the wild-type cells, CLIP is transiently associated with HLA-DR3 molecules, suggesting that the peptide is a normal class II-associated intermediate generated during proteolysis of the invariant chain. In the mutant cell line proteolysis of the invariant chain is less efficient, and HLA-DR3/CLIP complexes are generated much more slowly. Examination of the mutant cell line by immunoelectronmicroscopy shows that class II-invariant chain complexes accumulate intracellularly in large acidic vesicles which contain lysosomal markers, including beta-hexosaminidase, cathepsin D, and the lysosomal membrane protein CD63. The markers in these vesicles are identical to those seen in the class II-containing vesicles (MIICs) seen in the wild-type cells but the morphology is drastically different. The vesicles in the mutant cells are endocytic, as measured by the internalization of BSA-gold conjugates. The implication of these findings for antigen processing in general and the nature of the mutation in particular are discussed.

Separation of subcellular compartments containing distinct functional forms of MHC class II

Antigen processing in B lymphocytes entails initial binding of antigen to the surface Ig and internalization of the antigen into acidic compartments where the antigen is degraded, releasing peptides for binding to major histocompatibility complex class II molecules. Using subcellular fractionation techniques we show that functional, processed antigen-class II complexes capable of activating antigen-specific T cells in vitro are first formed in dense vesicles cosedimenting with lysosomes which are distinct from early endosomes and the bulk of late endosomes. With time, processed antigen-class II complexes appear in vesicles sedimenting with early endosomes and finally cofractionate with plasma membrane. A separate compartment is identified which contains major histocompatibility complex class II receptive to peptide binding but which does not have access to processed antigen in the B cell. These class II molecules are in the so-called "floppy" form in contrast to the class II molecules in the very dense vesicles which are in the "compact" form. These results demonstrate a correlation between the floppy and compact forms of class II molecules and their association with processed antigen and show that floppy and compact forms of class II reside in distinct and physically separable subcellular compartments.

Endosomal aspartic proteinases are required for invariant-chain processing

Immunogenic peptides are displayed in the context of class II histocompatibility proteins on the surface of antigen-presenting cells. Class II alpha and beta subunits bind the invariant chain (I-chain), a transmembrane glycoprotein which must dissociate prior to peptide presentation. Proteolytic release of I-chain in an acidic compartment is followed by class II alpha beta surface expression. Two distinct proteinases sequentially catalyze I-chain dissociation in B-lymphoblastoid cell lines. An aspartic proteinase initiates processing whereas a cysteine proteinase catalyzes the final stages of I-chain release. Inactivation of these enzymes prevents class II alpha beta maturation, demonstrating that acidic proteinases are essential for the generation of functional class II complexes. I-chain processing was localized to a dense endosomal compartment, suggesting this is the first site where class II alpha beta become accessible to peptides. I-chain fragments complexed with class II alpha beta accumulate in dense endosomes of B-lymphoblastoid cells treated with cysteine proteinase inhibitors. A signal for endosomal retention/targeting present in the cytoplasmic tail of these fragments may sequester class II alpha beta in this compartment until I-chain processing is complete.

Processing and presentation of tetanus toxin by antigen-presenting cells from patients with chronic granulomatous disease (CGD) to human specific T cell clones are not impaired

The capacity of peripheral blood lymphocytes (PBL) or Epstein-Barr virus (EBV)-transformed B cell lines from CGD patients to process and present tetanus toxin (tt)-specific epitopes was assessed using various tt preparations and human tt-specific T cell clones. PBL from all of the donors were able to process and present either native tt and/or denatured tt to human T cell clones specific for various tt epitopes. Furthermore, no difference was found in the antigen requirement when normal or CGD EBV-B cell lines were used as antigen-presenting cells (APC). These results suggest that the deficiency in oxygen metabolism in CGD cells does not affect tt processing and presentation.

The invariant chain induces compact forms of class II molecules localized in late endosomal compartments

The invariant chain (Ii) binds to newly synthesized major histocompatibility complex (MHC) class II molecules and is targeted to an acidic compartment where it is degraded. To evaluate its role on the conformation and the subcellular distribution of murine MHC class II molecules we have established stable L cell transfectants expressing class II IAk heterodimers alone or in conjunction with p31 and p41 Ii chains. In these cells, class II molecules were present under three forms: alpha beta heterodimers bearing high mannose carbohydrate moieties, and fully glycosylated alpha beta heterodimers that are sensitive or resistant to sodium dodecyl sulfate dissociation at 20 degrees C. The latter class II molecules called compact heterodimers, were here highly induced in Ii-positive cells. Using in situ iodination of endosomal compartments, class II heterodimers were detected in late endosomal compartments essentially as compact forms in Ii-positive cells, and as non-compact forms in Ii-negative cells. Using confocal microscopy, IAk molecules were located in compartments distinct from early endosomes labeled with transferrin, but partially coincident with vesicles containing fluid-phase markers, and highly coincident with compartments containing large amounts of cathepsins B, D, H, and L in Ii-positive and Ii-negative cells. At the ultrastructural level, class II molecules were mostly present in multivesicular bodies, even without Ii expression. But Ii chains were needed to induce an efficient presentation of the hen egg lysozyme antigen and were sufficient to promote a major conformational change of the late endosomal, and/or lysosomal resident, class II molecules. Ii molecules are presumably playing a chaperoning function favoring the association of peptides with class II molecules in endosomal compartments.

Epitope-specific enhancement of antigen presentation by invariant chain

The MHC class II-associated invariant chain (Ii) is involved in the intracellular sorting of class II molecules to the endocytic pathway where peptides from processed exogenous antigens are bound, and thereby Ii is thought to enhance antigen presentation. Here we demonstrate that presentation of only one out of five epitopes of a given antigen is augmented by Ii. We have compared the presentation of five different epitopes derived from hen egg white lysozyme (HEL) to Ak-restricted T hybridomas by rat-2 fibroblasts transfected with A alpha k and A beta k (RKK) and RKK cells supertransfected with the mouse invariant chain (RKKI). Only the presentation of the HEL epitope 46-61 was enhanced whereas the presentation of the HEL epitopes 25-43, 34-45, 112-124, and 116-129 was unchanged or even slightly diminished in RKKI cells. The presentation of the epitopes 25-43 and 34-45 was virtually insensitive to the lysosomotropic reagent chloroquine. Brefeldin A (BFA), which inhibits protein egress from the endoplasmic reticulum, blocked the presentation of all epitopes tested in RKKI cells. In contrast, in Ii-negative RKK cells only the presentation of the epitope HEL(46-61) was inhibited by BFA and the presentation of the epitopes 25-43 and 34-45 was only slightly impaired. These findings suggest that Ii may target class II molecules to selected endosomal subcompartments involved in the processing of different peptides derived from an endocytosed antigen. As a result, the enhancement of the class II-restricted presentation in Ii expressing cells appears to be epitope specific rather than antigen specific.

Mechanism of action of hydroxychloroquine as an antirheumatic drug

The antimalarial agents chloroquine and hydroxychloroquine have been used widely for the treatment of rheumatoid arthritis and systemic lupus erythematosus. These compounds lead to improvement of clinical and laboratory parameters, but their slow onset of action distinguishes them from glucocorticoids and nonsteroidal antiinflammatory agents. Chloroquine and hydroxychloroquine increase pH within intracellular vacuoles and alter processes such as protein degradation by acidic hydrolases in the lysosome, assembly of macromolecules in the endosomes, and posttranslation modification of proteins in the Golgi apparatus. It is proposed that the antirheumatic properties of these compounds results from their interference with "antigen processing" in macrophages and other antigen-presenting cells. Acidic cytoplasmic compartments are required for the antigenic protein to be digested and for the peptides to assemble with the alpha and beta chains of MHC class II proteins. As a result, antimalarials diminish the formation of peptide-MHC protein complexes required to stimulate CD4+ T cells and result in down-regulation of the immune response against autoantigenic peptides. Because this mechanism differs from other antirheumatic drugs, antimalarials are well suited to complement these other compounds in combination drug therapy.

Relationship between invariant chain expression and major histocompatibility complex class II transport into early and late endocytic compartments

Invariant chain (Ii), which associates with major histocompatibility complex (MHC) class II molecules in the endoplasmic reticulum, contains a targeting signal for transport to intracellular vesicles in the endocytic pathway. The characteristics of the target vesicles and the relationship between Ii structure and class II localization in distinct endosomal subcompartments have not been well defined. We demonstrate here that in transiently transfected COS cells expressing high levels of the p31 or p41 forms of Ii, uncleaved Ii is transported to and accumulates in transferrin-accessible (early) endosomes. Coexpressed MHC class II is also found in this same compartment. These early endosomes show altered morphology and a slower rate of content movement to later parts of the endocytic pathway. At more moderate levels of Ii expression, or after removal of a highly conserved region in the cytoplasmic tail of Ii, coexpressed class II molecules are found primarily in vesicles with the characteristics of late endosomes/prelysosomes. The Ii chains in these late endocytic vesicles have undergone proteolytic cleavage in the lumenal region postulated to control MHC class II peptide binding. These data indicate that the association of class II with Ii results in initial movement to early endosomes. At high levels of Ii expression, egress to later endocytic compartments is delayed and class II-Ii complexes accumulate together with endocytosed material. At lower levels of Ii expression, class II-Ii complexes are found primarily in late endosomes/prelysosomes. These data provide evidence that the route of class II transport to the site of antigen processing and loading involves movement through early endosomes to late endosomes/prelysosomes. Our results also reveal an unexpected ability of intact Ii to modify the structure and function of the early endosomal compartment, which may play a role in regulating this processing pathway.

Nonspecific regulatory mechanism of contact sensitivity: nonspecific suppressor factor (NSF)-treated intermediate cells produce a second nonspecific suppressor factor (NSFint)

Nonspecific suppressor factor (NSF), which inhibits the passive transfer of contact sensitivity (CS), is produced spontaneously from macrophage-like suppressor cells induced by intravenous administration of oxazolone (Ox)-conjugated spleen cells. NSF binds selectively to Ia-positive, cyclophosphamide (CY)-sensitive, and plastic-adherent cells (named intermediate cells) present in the normal spleen. NSF-treated intermediate cells acquire the ability to suppress the passive transfer of CS nonspecifically. In this study, NSF-treated intermediate cells were found to release a second nonspecific suppressor factor (NSFint) during a 2-hr culture, while retaining their suppressor activity. Investigation of the relationship between these two factors showed that both NSF and NSFint were trypsin-sensitive, nondialyzable proteins. However, gel chromatography revealed that NSF was about 43 kDa, while NSFint was about 20 kDa. NSF was released from macrophage-like suppressor cells after RNA-dependent protein synthesis. In contrast, production of NSFint was energy dependent but did not require protein synthesis. Intermediate cells pretreated with lysosomotropic agents, such as ammonium chloride or chloroquine, did not acquire suppressor activity nor release suppressor factors due to NSF treatment. These observations suggest that NSFint is an altered form of NSF released by the intermediate after having undergone some modification; the biochemical mechanism is not known. This study showed that the intermediate cells play an active role in the suppressor cascade of NSF.

Cultured human Langerhans cells process and present intact protein antigens

Epidermal Langerhans cells (LC) undergo profound phenotypic and functional alterations when cultured for 2 to 3 d. To determine whether the in vitro culture of human LC modulates their capacity to process and present intact protein antigens, we compared the ability of freshly isolated LC (fLC) and cultured LC (cLC) to stimulate in vitro T-cell proliferative responses to recall antigens. We found that human fLC and cLC were able to process and present recall antigens to primed T cells, inducing significant proliferative responses. For tetanus toxoid and Candida albicans extract, T-cell proliferative responses at 6 d to antigen-pulsed fLC were slightly greater than responses to antigen-pulsed cLC. For live influenza A virus, the T-cell responses induced by antigen-pulsed cLC were comparable or slightly greater compared with fLC. Allogeneic T-cell proliferation for both LC preparations were also comparable. The exogenous pathway of antigen processing was demonstrated by chloroquine inhibition.

MHC class II molecules and immunoglobulins on peripheral blood lymphocytes of the bottlenosed dolphin, Tursiops truncatus

The immune system of marine mammals is of comparative interest because of its adaptation to the aquatic environment. Little information, however, is available on its cellular and molecular components. Here, we used a cross-reactive antibody to MHC class II molecules and an immunoglobulin-specific antiserum for identifying these molecular species on lymphocytes of the bottlenosed dolphin, Tursiops truncatus. Limited structural analyses indicated that class II molecules and immunoglobulins of dolphin closely resemble those of other vertebrates. In the peripheral blood of most land mammals both class II and immunoglobulins are usually found on B but not T lymphocytes. Expression of immunoglobulins on dolphin peripheral blood lymphocytes suggests a ratio of B cells to T cells comparable to that of land mammals. However, unlike the majority of land mammals, virtually 100% of the peripheral T cells display pronounced expression of class II molecules, generally considered an indication of T cell activation. It is therefore possible that the physiology of T cell activation has unusual attributes in the dolphin. It is especially interesting that some land mammals, namely swine (ungulates) and dogs and cats (carnivores), also express class II molecules on peripheral blood T lymphocytes. Since ungulates and carnivores are thought to share a common distant ancestry with toothed whales, the evolutionary history may be more relevant than the environmental history in determining these unusual attributes.

Intracellular transport of MHC class II molecules

MHC class II molecules associate, during biosynthesis, with peptides derived from endocytosed antigen. Here, Jacques Neefjes and Hidde Ploegh describe the intracellular transport of MHC class II molecules and its relationship to the binding of peptides in endosomal compartments. They discuss alternative routes for the delivery of antigen to sites at which peptides associate with MHC class II molecules and raise the possibility of cell type-specific differences in the handling of MHC class II molecules, and hence in antigen presentation.

Invariant chain — a regulator of antigen presentation

MHC class II molecules present internalized antigens to the immune system. They have long been known to associate with a polypeptide called the invariant chain. Recent findings have revealed that this polypeptide performs two functions. First, it prevents class II molecules from binding antigenic peptides at the site of synthesis of class II molecules in the endoplasmic reticulum.Second, it targets class II molecules to their destination in the endocytic pathway, where they pick up antigenic peptides derived from endocytosed antigens. Short sequences in the cytoplasmic portion of the invariant chain serve as subcellular address labels. The functions of the invariant chain help to explain how the immune system divides its defence against foreign pathogens between cytotoxic T cells and antibodies.