Major histocompatibility complex class I-binding peptides are recycled to the cell surface after internalization

Syncytia formation by SARS-CoV-2-infected cells

Severe cases of COVID-19 are associated with extensive lung damage and the presence of infected multinucleated syncytial pneumocytes. The viral and cellular mechanisms regulating the formation of these syncytia are not well understood. Here, we show that SARS-CoV-2-infected cells express the Spike protein (S) at their surface and fuse with ACE2-positive neighboring cells. Expression of S without any other viral proteins triggers syncytia formation. Interferon-induced transmembrane proteins (IFITMs), a family of restriction factors that block the entry of many viruses, inhibit S-mediated fusion, with IFITM1 being more active than IFITM2 and IFITM3. On the contrary, the TMPRSS2 serine protease, which is known to enhance infectivity of cell-free virions, processes both S and ACE2 and increases syncytia formation by accelerating the fusion process. TMPRSS2 thwarts the antiviral effect of IFITMs. Our results show that SARS-CoV-2 pathological effects are modulated by cellular proteins that either inhibit or facilitate syncytia formation.

Endocytic Recycling of MHC Class I Molecules in Non-professional Antigen Presenting and Dendritic Cells

Major histocompatibility complex class I (MHC I) molecules are glycoproteins that display peptide epitopes at the cell surface of nucleated cells for recognition by CD8⁺ T cells. Like other cell surface receptors, MHC class I molecules are continuously removed from the surface followed by intracellular degradation or recycling to the cell surface, in a process likely involving active quality control the mechanism of which remains unknown. The molecular players and pathways involved in internalization and recycling have previously been studied in model cell lines such as HeLa. However, dendritic cells (DCs), which rely on a specialized endocytic machinery that confers them the unique ability to “cross”-present antigens acquired by internalization, may use distinct MHC I recycling pathways and quality control mechanisms. By providing MHC I molecules cross-presenting antigens, these pathways may play an important role in one of the key functions of DCs, priming of T cell responses against pathogens and tumors. In this review, we will focus on endocytic recycling of MHC I molecules in various experimental conditions and cell types. We discuss the organization of the recycling pathway in model cell lines compared to DCs, highlighting the differences in the recycling rates and pathways of MHC I molecules between various cell types, and their putative functional consequences. Reviewing the literature, we find that conclusive evidence for significant recycling of MHC I molecules in primary DCs has yet to be demonstrated. We conclude that endocytic trafficking of MHC class I in DCs remains poorly understood and should be further studied because of its likely role in antigen cross-presentation.

Dissociation of β2-microglobulin determines the surface quality control of major histocompatibility complex class I molecules

Major histocompatibility complex class I proteins, which present antigenic peptides to cytotoxic T lymphocytes at the surface of all nucleated cells, are endocytosed and destroyed rapidly once their peptide ligand has dissociated. The molecular mechanism of this cellular quality control process, which prevents rebinding of exogenous peptides and thus erroneous immune responses, is unknown. To identify the nature of the decisive step in endocytic sorting of class I molecules and its location, we have followed the removal of optimally and suboptimally peptide-loaded murine H-2K(b) class I proteins from the cell surface. We find that the binding of their light chain, β2-microglobulin (β2m), protects them from endocytic destruction. Thus, the extended survival of suboptimally loaded K(b) molecules at 25°C is attributed to decreased dissociation of β2m. Because all forms of K(b) are constantly internalized but little β2m-receptive heavy chain is present at the cell surface, it is likely that β2m dissociation and recognition of the heavy chain for lysosomal degradation take place in an endocytic compartment.

Enhancement of T cell-mediated immune responses to whole inactivated influenza virus by chloroquine treatment in vivo

Current influenza vaccines induce poor cross-reactive CD8+ T cell responses. Cellular immunity is generally specific for epitopes that are remarkably conserved among different subtypes, suggesting that strategies to improve the cross-presentation of viral antigens by dendritic cells (DC) could elicit a broadly protective immune response. Previous studies have shown that limited proteolysis within the endocytic pathway can favorably influence antigen processing and thus immune responses. Herein, we demonstrate that chloroquine improves the cross-presentation of non-replicating influenza virus in vitro and T cell responses in mice following a single administration of inactivated HI-X31 virus. CD8+ T cells were also recruited to lymph nodes draining the site of infection and able to reduce viral load following pulmonary challenge with the heterologous PR8 virus. These findings may have implications for vaccination strategies aimed at improving the cross-presentation capacity of DCs and thus the size of effector and memory CD8+ T cells against influenza vaccines.

Constitutive internalization of murine MHC class I molecules

The total number of cell surface glycoprotein molecules at the plasma membrane results from a balance between their constitutive internalization and their egress to the cell surface from intracellular pools and/or biosynthetic pathway. Constitutive internalization is net result of constitutive endocytosis and endocytic recycling. In this study we have compared spontaneous internalization of murine major histocompatibility complex (MHC) class I molecules (K(d), D(d), full L(d), and empty L(d)) after depletion of their egress to the cell surface (Cycloheximide [CHX], brefeldin A [BFA]) and internalization after external binding of monoclonal antibody (mAb). MHC class I alleles differ regarding their cell surface stability, kinetics, and in the way of internalization and degradation. K(d) and D(d) molecules are more stable at the cell surface than L(d) molecules and, thus, constitutively internalized more slowly. Although the binding of mAbs to cell surface MHC class I molecules results in faster internalization than depletion of their egress, it is still slow and, thereby, can serve as a model for tracking of MHC class I endocytosis. Internalization of fully conformed MHC class I molecules (K(d), D(d), and L(d)) was neither inhibited by chlorpromazine (CP) (inhibitor of clathrin endocytosis), nor with filipin (inhibitor of lipid raft dependent endocytosis), indicating that fully conformed MHC class I molecules are internalized via the bulk pathway. In contrast, internalization of empty L(d) molecules was inhibited by filipin, indicating that non-conformed MHC class I molecules require intact cholesterol-rich membrane microdomains for their constitutive internalization. Thus, conformed and non-conformed MHC class I molecules use different endocytic pathways for constitutive internalization.

Lack of tyrosine 320 impairs spontaneous endocytosis and enhances release of HLA-B27 molecules

Several lines of evidence suggest that endocytosis of MHC class I molecules requires conserved motifs within the cytoplasmic domain. In this study, we show, in the C58 rat thymoma cell line transfected with HLA-B27 molecules, that replacement of the highly conserved tyrosine (Tyr320) in the cytoplasmic domain of HLA-B27 does not hamper cell surface expression of beta2-microglobulin H chain heterodimers or formation of misfolded molecules. However, Tyr320 replacement markedly impairs spontaneous endocytosis of HLA-B27. Although wild-type molecules are mostly internalized via endosomal compartments, Tyr320-mutated molecules remain at the plasma membrane in which partial colocalization with endogenous transferrin receptors can be observed, also impairing their endocytosis. Finally, we show that Tyr320 substitution enhances release of cleaved forms of HLA-B27 from the cell surface. These studies show for the first time that Tyr320 is most likely part of a cytoplasmic sorting motif involved in spontaneous endocytosis and shedding of MHC class I molecules.

Endolysosomal Processing of Exogenous Antigen into Major Histocompatibility Complex Class I-Binding Peptides

An alternative endolysosomal pathway has recently been suggested for the processing of MHC-I-binding peptides, and peptide/MHC-I complexes have been demonstrated in this compartment. However, it remains unclear where in the antigen-presenting cells such peptides are processed, in the endolysosomes themselves or in the proteasomal complex. Here, we have investigated this using monoclonal antibodies specific for the immunodominant SIINFEKL/Kb complex (25-D1) or for the carbohydrate part of Db- or Kb-binding glycopeptides in combination with inhibitors for classical and endolysosomal MHC-I-processing pathways. Alternative processing was detected in both wt and TAP1(-/-) immature DC (iDC) as the expression of SIINFEKL/Kb complexes on the surface of OVA-treated cells in the presence of Brefeldin A (BFA) or lactacystin and their absence in the presence of the lysosomotropic amines ammonium chloride, chloroquine and methylamine. Internalized Db- and Kb-binding glycopeptides, detected with high specificity using an anti-galabiose (Gal2) monoclonal antibody, were found to appear on the cell surface of BFA-treated cells after intracellular MHC-I-binding. Peptide exchange in Kb was demonstrated as the gradual appearance of SIINFEKL/Kb complexes on BFA-treated cells which earlier had been saturated with another Kb-binding peptide. Our data support the presence of a fully functional endolysosomal processing pathway in iDC guided by the chaperone function of MHC-I molecules.

Degradation of origin recognition complex large subunit by the anaphase-promoting complex in Drosophila

The initiation of DNA synthesis is thought to occur at sites bound by a heteromeric origin recognition complex (ORC). Previously, we have shown that in Drosophila, the level of the large subunit, ORC1, is modulated during cell cycle progression and that changes in ORC1 concentration alter origin utilization during development. Here, we investigate the mechanisms underlying cell cycle-dependent degradation of ORC1. We show that signals in the non-conserved N-terminal domain of ORC1 mediate its degradation upon exit from mitosis and in G1 phase by the anaphase-promoting complex (APC) in vivo. Degradation appears to be the result of direct action of the APC, as the N-terminal domain is ubiquitylated by purified APC in vitro. This regulated proteolysis is potent, sufficient to generate a normal temporal distribution of protein even when transcription of ORC1 is driven by strong constitutive promoters. These observations suggest that in Drosophila, ORC1 regulates origin utilization much as does Cdc6 in budding yeast.

Antigen degradation or presentation by MHC class I molecules via classical and non-classical pathways

Major histocompatibility complex (MHC) class I molecules usually present endogenous peptides at the cell surface. This is the result of a cascade of events involving various dedicated proteins like the peptide transporter associated with antigen processing (TAP) and the ER chaperone tapasin. However, alternative ways for class I peptide loading exist which may be highly relevant in a process called cross-priming. Both pathways are described here in detail. One major difference between these pathways is that the proteases involved in the generation of peptides are different. How proteases and peptidases influence peptide generation and degradation will be discussed. These processes determine the amount of peptides available for TAP translocation and class I binding and ultimately the immune response.

Stability of surface H-2K(b), H-2D(b), and peptide-receptive H-2K(b) on splenocytes

We have used flow cytometry to study the stability and peptide-binding capability of MHC class I (MHC-I) on the surface of normal C57BL/6 mouse T lymphoblasts. The MHC-I molecules on each cell are nearly evenly divided into two populations with mean half-life values of approximately 1 and 20 h. Our observations suggest that members of the later contain peptide bound with medium to high affinity. Cell surface MHC-I molecules capable of binding exogenous peptide (thus, "peptide-receptive") belong almost entirely to the less stable population. Before exogenous peptide can bind, MHC-I must undergo a change, probably loss of a very low affinity peptide. For MHC-I-K(b), we found that the maximum rate for binding of exogenous peptide corresponds to a t(1/2) value of 12 min. To maintain the 50:50 steady-state distribution of long- vs short-lived MHC-I molecules on the cell surface, approximately 20 short-lived molecules must be exported to the cell surface for each long-lived molecule.

Distinct trafficking pathways mediate Nef-induced and clathrin-dependent major histocompatibility complex class I down-regulation

The human immunodeficiency virus type 1 Nef protein alters the post-Golgi stages of major histocompatibility complex class I (MHC-I) biogenesis. Presumed mechanisms involve the disclosure of a cryptic tyrosine-based sorting signal (YSQA) located in the cytoplasmic tail of HLA-A and -B heavy chains. We changed this signal for a prototypic sorting motif (YSQI or YSQL). Modified HLA-A2 molecules, termed A2-endo, displayed constitutively low surface levels and accumulated in a region close to or within the Golgi apparatus, a behavior reminiscent of wild-type HLA-A2 in Nef-expressing cells. However, several lines of evidence indicate that the action of prototypic signals on MHC-I trafficking differs from that of Nef. Internalization of surface A2-endo was more rapid and was associated with efficient recycling to the surface. A transdominant-negative mutant of dynamin-1 inhibited A2-endo constitutive internalization and Nef-induced CD4 down-regulation, whereas it did not affect the activity of Nef on MHC-I. Moreover, trafficking of A2-endo was still affected by the viral protein, indicating additive effects of prototypic signals and Nef. Therefore, distinct trafficking pathways regulate clathrin-dependent and Nef-induced MHC-I modulation.

Extension of HLA-A*0201-restricted minimal epitope by N epsilon-palmitoyl-lysine increases the life span of functional presentation to cytotoxic T cells

The delineation of the minimal requirements for efficient delivery of functional cytotoxic epitopes into APC could be a step toward the definition of "minimal length" lipopeptides for the modulation of CTL activity. Several analogues of the HLA-A*0201-restricted HIV-1 polymerase (pol476-484) minimal cytotoxic epitope were obtained by modifying P0, P1, or P10 positions by a single N epsilon-palmitoyl-lysine residue. The use of fluorescent derivatives confirmed the cell-permeating activities and suggested that a P0- and a P1-modified lipopeptide possessing ionizable extremities fulfills the structural requirements for MHC loading. The expressions of HLA-peptide complexes at the surface of TAP-deficient cells incubated with the parent epitope or lipopeptide derivatives were compared, in terms of intensity and stability. Both lipopeptides induced a considerably prolonged expression of conformationally correct complexes, which were dependent on the integrity of the exocytosis pathway, suggesting a dynamic mechanism of formation or reloading of the complexes from an intracellular pool. The agonistic activities of the different HLA-peptide complexes were evaluated using two independent T cell lines from HIV-infected donors. We report that a lipodecapeptide obtained by N-terminal addition of a N epsilon-palmitoyl-lysine to the pol476-484 epitope was able to increase the life span of functional presentation to cytotoxic T cells specific for the parent peptide.

Phosphorylation of class I MHC molecules in the absence of phorbol esters is an intracellular event and may be characteristic of trafficking molecules

Class I Major Histocompatibility Complex (MHC) molecules are displayed at the cell surface where they present antigenic peptides to T lymphocytes. Class I MHC molecules undergo cytoplasmic domain phosphorylation on a serine residue late in their biosynthesis. Here we show that phosphorylation occurs on mature, beta(2)-microglobulin-associated class I MHC molecules in a mouse lymphoid cell line. Both recently synthesized class I MHC molecules and molecules which are at least 3 h old become phosphorylated. Approximately 14% of phosphorylated class I MHC molecules occur at the cell surface. Density gradient analysis indicates that phosphorylated class I MHC molecules also occur in lamp(+) intracellular compartments and in fractions containing rab4, a GTP-binding protein associated with recycling endosomes. Class I MHC molecules are endocytosed and recycled to the cell surface in these cells. Furthermore, the lysosomotropic drug, primaquine, inhibits both class I MHC phosphorylation and its recycling back to the cell surface, suggesting that phosphorylation is related to class I MHC recycling. These observations are intriguing since several studies have shown that class I MHC molecules can acquire antigenic peptides in NH(4)Cl-sensitive compartments. Hence, class I MHC phosphorylation may play a role in regulating intracellular sorting through these compartments.

Alternative pathways for processing exogenous and endogenous antigens that can generate peptides for MHC class I-restricted presentation

The concept of distinct endogenous and exogenous pathways for generating peptides for MHC-I and MHC-II-restricted presentation to CD4+ or CD8+ T cells fits well with the bulk of experimental data. Nevertheless, evidence is emerging for alternative processing pathways that generate peptides for MHC-I-restricted presentation. Using a well characterized, particulate viral antigen of prominent medical importance (the hepatitis B surface antigen), we summarize our evidence that the efficient, endolysosomal processing of exogenous antigens can lead to peptide-loaded MHC-I molecules. In addition, we describe evidence for endolysosomal processing of mutant, stress protein-bound, endogenous antigens that liberate peptides binding to (and presented by) MHC-I molecules. The putative biological role of alternative processing of antigens generating cytotoxic T-lymphocyte-stimulating epitopes is discussed.

Endosomal/lysosomal retention and degradation of major histocompatibility complex class I molecules is induced by myxoma virus

The highly immunosuppressive leporipoxvirus myxoma, previously was shown to promote the loss of cell surface class I major histocompatibility complex (MHC I) molecules. Here, we show that myxoma virus induces the loss of both cell surface and intracellular post-Golgi, beta(2)-microglobulin-associated MHC I. Myxoma-induced loss of these MHC I molecules is abrogated by vacuolar ATPase inhibitors, NH(4)Cl, and leupeptin. Furthermore, immunofluorescence microscopic studies reveal that in myxoma-infected cells, beta(2)-microglobulin-associated MHC I accumulates in Lamp-1(+) vesicular structures, suggesting that myxoma virus targets MHC I for degradation in late endosomes and/or lysosomes. These events are regulated by early gene product or products because they occur unabated in cells infected with myxoma virus in the presence of cytosine arabinoside, an inhibitor of DNA synthesis. Studies with baby green monkey kidney cells transfected with wild-type and tail-less forms of a mouse MHC I molecule, H-2L(d), indicate that the MHC I cytoplasmic tail is required for myxoma-induced localization in Lamp-1(+) organelles. Myxoma-induced endocytosis and degradation of MHC I may provide the virus with a means of dispensing with cell surface MHC I molecules that were loaded with peptides derived from viral proteins synthesized early in infection.

Phagocytic antigen processing and effects of microbial products on antigen processing and T-cell responses

Processing of exogenous antigens and microbes involves contributions by multiple different endocytic and phagocytic compartments. During the processing of soluble antigens, different endocytic compartments have been demonstrated to use distinct antigen-processing mechanisms and to process distinct sets of antigenic epitopes. Processing of particulate and microbial antigens involves phagocytosis and functions contributed by phagocytic compartments. Recent data from our laboratory demonstrate that phagosomes containing antigen-conjugated latex beads are fully competent class II MHC (MHC-II) antigen-processing organelles, which generate peptide:MHC-II complexes. In addition, phagocytosed antigen enters an alternate class I MHC (MHC-I) processing pathway that results in loading of peptides derived from exogenous antigens onto MHC-I molecules, in contrast to the cytosolic antigen source utilized by the conventional MHC-I antigen-processing pathway. Antigen processing and other immune response mechanisms may be activated or inhibited by microbial components to the benefit of either the host or the pathogen. For example, antigen processing and T-cell responses (e.g. Th1 vs Th2 differentiation) are modulated by multiple distinct microbial components, including lipopolysaccharide, cholera toxin, heat labile enterotoxin of Escherichia coli, DNA containing CpG motifs (found in prokaryotic and invertebrate DNA but not mammalian DNA) and components of Mycobacterium tuberculosis.

Crystal structure of an MHC class I presented glycopeptide that generates carbohydrate-specific CTL

T cell receptor (TCR) recognition of nonpeptidic and modified peptide antigens has been recently uncovered but is still poorly understood. Immunization with an H-2Kb-restricted glycopeptide RGY8-6H-Gal2 generates a population of cytotoxic T cells that express both alpha/beta TCR, specific for glycopeptide, and gamma/delta TCR, specific for the disaccharide, even on glycolipids. The crystal structure of Kb/RGY8-6H-Gal2 now demonstrates that the peptide and H-2Kb structures are unaffected by the peptide glycosylation, but the central region of the putative TCR binding site is dominated by the extensive exposure of the tethered carbohydrate. These features of the Kb/RGY8-6H-Gal2 structure are consistent with the individual ligand binding preferences identified for the alpha/beta and gamma/delta TCRs and thus explain the generation of a carbohydrate-specific T cell response.

Temporal Synthesis of Band 3 Oligomers During Terminal Maturation of Mouse Erythroblasts. Dimers and Tetramers Exist in the Membrane as Preformed Stable Species

Band 3, the anion transport protein of the erythrocyte membrane, exists in the membrane as a mixture of dimers (B3D) and tetramers (B3T). The dimers are not linked to the skeleton and constitute the free mobile band 3 fraction. The tetramers are linked to the skeleton by their interaction with ankyrin. In this report we have examined the temporal synthesis and assembly of band 3 oligomers into the plasma membrane during red cell maturation. The oligomeric state of newly synthesized band 3 in early and late erythroblasts was analyzed by size-exclusion high-pressure liquid chromatography of band 3 extracts derived by mild extraction of plasma membranes with the nonionic detergent C12E8 (octaethylene glycol n-dodecyl monoether). This analysis revealed that at the early erythroblast stage, the newly synthesized band 3 is present predominantly as tetramers, whereas at the late stages of erythroid maturation, it is present exclusively as dimers. To examine whether the dimers and tetramers exist in the membrane as preformed stable species or whether they are interconvertible, the fate of band 3 species synthesized during erythroblast maturation was examined by pulse-chase analysis. We showed that the newly synthesized band 3 dimers and tetramers are stable and that there is no interconversion between these species in erythroblast membranes. Pulse-chase analysis followed by cellular fractionation showed that, in early erythroblasts, the newly synthesized band 3 tetramers are initially present in the microsomal fraction and later incorporated stably into the plasma membrane fraction. In contrast, in late erythroblasts the newly synthesized band 3 dimers move rapidly to the plasma membrane fraction but then recycle between the plasma membrane and microsomal fractions. Fluorescence photobleaching recovery studies showed that significant fractions of B3T and B3D are laterally mobile in early and late erythroblast plasma membranes, respectively, suggesting that many B3T-ankyrin complexes are unattached to the membrane skeleton in early erythroblasts and that the membrane skeleton has yet to become tightly organized in late erythroblasts. We postulate that in early erythroblasts, band 3 tetramers are transported through microsomes and stably incorporated into the plasma membrane. However, when ankyrin synthesis is downregulated in late erythroblasts, it appears that B3D are rapidly transported to the plasma membrane but then recycled between the plasma membrane and microsomal compartments. These observations may suggest novel roles for membrane skeletal proteins in stabilizing integral membrane protein oligomers at the plasma membrane and in regulating the endocytosis of such proteins.

The SH3 domain-binding surface and an acidic motif in HIV-1 Nef regulate trafficking of class I MHC complexes

Nef, a regulatory protein of human and simian immunodeficiency viruses, downregulates cell surface expression of both class I MHC and CD4 molecules in T cells by accelerating their endocytosis. Fibroblasts were used to study alterations in the traffic of class I MHC complexes induced by Nef. We found that Nef downregulates class I MHC complexes by a novel mechanism involving the accumulation of endocytosed class I MHC in the trans-Golgi, where it colocalizes with the adaptor protein-1 complex (AP-1). This effect of Nef on class I MHC traffic requires the SH3 domain-binding surface and a cluster of acidic amino acid residues in Nef, both of which are also required for Nef to downregulate class I MHC surface expression and to alter signal transduction in T cells. Downregulation of class I MHC complexes from the surface of T cells also requires a tyrosine residue in the cytoplasmic domain of the class I MHC heavy chain molecule. The requirement of the same surfaces of the Nef molecule for downregulation of surface class I MHC complexes in T cells and for their accumulation in the trans-Golgi of fibroblasts indicates that the two effects of Nef involve similar interactions with the host cell machinery and involve a molecular mechanism regulating class I MHC traffic that is common for both of these cell types. Interestingly, the downregulation of class I MHC does not require the ability of Nef to colocalize with the adaptor protein-2 complex (AP-2). We showed previously that the ability of Nef to colocalize with AP-2 correlates with the ability of Nef to downregulate CD4 expression. Our observations indicate that Nef downregulates class I MHC and CD4 surface expression via different interactions with the protein sorting machinery, and link the sorting and signal transduction machineries in the regulation of class I MHC surface expression by Nef.

Detection of biotinylated cell surface receptors and MHC molecules in a capture ELISA: a rapid assay to measure endocytosis

Cell surface receptors and antigens, such as TfR and MHC molecules, are endocytosed and subsequently redisplayed on the plasma membrane. The internalization and recycling of MHC molecules is thought to play an important role in antigen presentation, but studying this process has been hindered due to the lack of a rapid and easily quantitated assay. The combination of a cleavable biotin reagent to label surface molecules and a capture ELISA to detect these molecules of interest, allows for the quantitation of their cell surface expression, endocytosis and recycling. The endocytosis of TfR and MHC II molecules was readily quantitated in B cell lines using this procedure with results nearly identical to previously published data using more laborious radioactive methods. Evidence for the recycling of class II antigens and TfR back to the plasma membrane was obtained by monitoring the exit of these molecules from endosomes. Exposing cells to hypertonic media blocks clathrin-dependent endocytosis and was found to inhibit the internalization of MHC class II proteins on B cells. This flexible assay to capture and quantitate the cell surface expression and endocytosis of MHC molecules and other surface antigens offers a sensitive and non-radioactive alternative to study the intracellular trafficking of diverse membrane proteins.

Impaired spontaneous endocytosis of HLA-G

HLA-G is a class Ib (non-classical) major histocompatibility complex (MHC) protein expressed at the maternal-fetal interface that inhibits natural killer (NK) cell-mediated lysis in an allotype-independent manner. Here we report that the spontaneous endocytosis of HLA-G is severely reduced because of its short cytoplasmic tail. Class I (classical) MHC proteins on the surface of B cell transfectants detected by primary and secondary antibodies underwent endocytosis at a moderate rate, whereas HLA-G, chimeric proteins consisting of the extracellular domains of HLA-C with the C-terminal sequence of HLA-G, or glycophosphatidylinositol-tailed HLA-C proteins, were not efficiently internalized. In addition, a mutant of beta 2-microglobulin (Ser88Cys) that could be specifically labeled with Texas red (or other fluorescent probes) and exchanged into class I or class Ib MHC proteins was employed to study spontaneous internalization of MHC proteins by a non-perturbative method independent of an antibody ligand. These data are discussed in terms of both the role of HLA-G expressed on the fetal trophoblast and the function of the cytoplasmic tail in class I MHC proteins.

Priming of cytotoxic T lymphocytes by five heat-aggregated antigens in vivo: conditions, efficiency, and relation to antibody responses

Mice were immunized i.p. with soluble or heat-denatured protein antigens [ovalbumin, beta-galactosidase, or recombinant E7 protein of human papilloma virus type 16 (HBV)]. Heat-denatured (100 degrees C) preparations of these proteins were able to induce cytotoxic T lymphocytes (CTL) that recognize cells expressing the respective genes, whereas native protein was either inefficient or required up to 30-fold higher doses. If the heat-treated proteins were separated into aggregated and soluble fractions by ultracentrifugation, only the aggregated fractions were able to induce specific CTL; this is probably because of the easier access to one of the major histocompatibility complex class I loading pathways for exogenous antigen. Addition of the adjuvant aluminium hydroxide (alum) to aggregated proteins abolished their ability to induce CTL; thus, a condition leading to a strong antibody response appeared to inhibit CTL induction. Interestingly, immunization with heat-denatured ovalbumin plus alum increased the IgM/IgG1 ratio compared to immunization with native ovalbumin and alum. Immunization of B6 mice transgenic for an HLA-A2/H-2K(b) hybrid gene with heat-denatured, recombinant HPV 16-E7 protein induced D(b)-restricted CTL specific for the peptide 49-57 of E7, indicating that this epitope is immunodominant over any A2-restricted E7 epitope in these mice. A whole influenza virus preparation heated to 100 degrees C or even autoclaved was still able to induce virus-specific CTL and BALB/c spleen cells heated to 100 degrees C could still cross-prime minor H-specific CTL in B6 mice, although with lower efficiency than fresh spleen cells. Thus, aggregated proteins can be considered as components for future vaccines.

Stress protein (hsp73)-mediated, TAP-independent processing of endogenous, truncated SV40 large T antigen for Db-restricted peptide presentation

Transporter associated with antigen processing (TAP)-competent and TAP-deficient cell lines were transfected with expression plasmids encoding either the wild-type (wt) large tumor antigen (T-Ag) of SV40, or a truncated cytoplasmic variant (cT-Ag) of this viral protein. Stable expression of comparable levels of both forms of the viral protein was observed in different transfectants. The truncated cT-Ag variant, but not the wtT-Ag was stably associated with the constitutively expressed, cytosolic heat shock protein (hsp)73 chaperone. Two Db-binding peptides and one Kb-binding peptide of T-Ag were presented to cytotoxic T lymphocyte lines (CTLL) by TAP-competent transfectants expressing either wtT-Ag or cT-Ag. TAP-deficient transfectants expressing the wtT-Ag did not present any of these epitopes to CTLL. In contrast, TAP-deficient transfectants expressing the truncated hsp73-associated cT-Ag, presented the two Db-binding epitopes, but not the Kb-binding T-Ag epitope to CTLL. Regurgitation of peptides by transfectants was not detectable. The described data indicate that a pool of post-Golgi Db molecules is available for 2-3 h in TAP-deficient transfectants for loading with peptides released during endolysosomal processing of hsp73-associated, endogenous antigen.

MHC class I antigen processing pathways

The multistep process that culminates in major histocompatibility complex (MHC) class I presentation of foreign of self-peptides begins in the last phases of protein catabolism. Although the individual roles of many key molecules-such as proteasomes, the transporter associated with antigen processing, and various endoplasmic reticulum chaperones-have recently been elucidated, there still remain many questions regarding processing of proteins into MHC class I bound peptides. This review summarizes the recent developments in antigen processing for MHC class I molecules, with a focus on how proteins are believed to be sampled and selected for degradation.

External glycopeptide binding to MHC class-I in relation to expression of TAP transporters, beta 2-microglobulin and to pH

MHC class-I binding glycopeptides are easily visualized on the cell surface by carbohydrate specific monoclonal antibodies. By comparing the staining intensity between anti-carbohydrate and anti-MHC class-I specific monoclonal antibodies, an estimation of the fraction of peptide accessible 'empty' sites on the cell surface of MHC class-I molecules can be made. This system was used to analyze glycopeptide binding to MHC class-I molecules in relation to transporter associated with antigen processing (TAP) peptide transporters and beta 2-M expression, using gene targeted mice, and in relation to pH. Approximately 15, 40, and 95% 'empty' Db molecules were found on activated T cells from normal, beta 2-M-/- and TAP -/- mice, respectively. The ASN9-6h-Gal2 glycopeptide also bound to transfected 'empty' Db molecules on T1-Db, T2-Db and T3-Db cells with a preference for T2-Db cells, lacking TAP peptide transporters. The stability of glycopeptide binding to H-2Db is also highest on T2-Db cells. pH was found to influence binding either positively or negatively, using four different glycopeptides, binding either to Db or Kb. We conclude that external glycopeptide binding may reflect important functional properties in the MHC class-I system and that pH in different processing compartments might influence the expressed peptide repertoire.

'Empty' Ld molecules capture peptides from endocytosed hepatitis B surface antigen particles for major histocompatibility complex class I-restricted presentation

Peptides recognized by CD8+ cytotoxic T lymphocytes in the context of major histocompatibility complex (MHC) class I molecules are usually derived from endogenous proteins synthesized within the cell. Exogenous 22-nm hepatitis B surface antigen (HBsAg) particles are taken up by many cells, and are processed in a novel peptide-transporter-independent, endosomal or lysosomal pathway for class I (Ld)-restricted epitope presentation. Here, we present evidence that 'empty' Ld molecules derived from the cell surface are involved in presenting antigenic peptides from endocytosed HBsAg particles. Intracellular assembly of presentation-competent, trimeric Ld molecules required endocytosis of the exogenous antigen and 'empty' Ld molecules. These data assign a functional role to surface-associated, 'empty' MHC class I molecules.

Major histocompatibility complex class I presentation of ovalbumin peptide 257-264 from exogenous sources: protein context influences the degree of TAP-independent presentation

Peritoneal macrophages from C57BL/6 mice process antigens from bacteria or coated on polystyrene beads for presentation by major histocompatibility complex (MHC) class I molecules. To investigate this antigen processing pathway, peritoneal macrophages from homozygous TAP1-/- mice, which lack the transporter associated with antigen processing (TAP) and are defective in presenting endogenous antigens on MHC class I, were used. TAP1-/- or C57BL/6 macrophages were co-incubated with either bacteria or polystyrene beads containing the 257-264 epitope from ovalbumin [OVA(257-264)], which binds the mouse class I molecule Kb. The source of the OVA(257-264) epitope was either the Crl-OVA(257-264) (Crl-OVA) fusion protein, the maltose binding protein (MBP)-Crl-OVA fusion protein, native OVA or bacterial recombinant OVA (rOVA); Crl-OVA, MBP-Crl-OVA and rOVA were each expressed in bacteria, and Crl-OVA and MBP-Crl-OVA purified from bacterial lysates and native egg OVA were coated onto polystyrene beads. The data reveal that peritoneal macrophages from C57BL/6 and TAP1-/- mice can process bacteria expressing Crl-OVA, MBP-Crl-OVA and rOVA as well as beads coated with native OVA, purified Crl-OVA, and purified MBP-Crl-OVA and present OVA(257-264) for recognition by OVA(257-264)/Kb-specific T hybridoma cells, albeit with different relative processing efficiencies. The processing efficiency of TAP1-/- macrophages co-incubated with bacteria or beads containing Crl-OVA or MBP-Crl-OVA was reduced approximately three to five times compared to C57BL/6 macrophages, but OVA(257-264) was presented 100 times less efficiently when the source of OVA(257-264) was full-length OVA. Chloroquine inhibition studies showed a differential requirement for acidic compartments in C57BL/6 versus TAP1-/- macrophages, which also depended upon the source of the OVA (257-264) epitope (Crl-OVA versus full-length OVA). These data suggest that TAP1-/- and C57BL/6 macrophages may process Crl-OVA and full-length OVA in different cellular compartments and that the protein context of the OVA(257-264) epitope influences the extent of TAP-independent processing for MHC class I presentation.

Antigen processing and presentation by the class I major histocompatibility complex

Major histocompatibility complex (MHC) class I molecules bind peptides derived from cellular proteins and display them for surveillance by the immune system. These peptide-binding molecules are composed of a heavy chain, containing an antigen-binding groove, which is tightly associated with a light chain (beta 2-microglobulin). The majority of presented peptides are generated by degradation of proteins in the cytoplasm, in many cases by a large multicatalytic proteolytic particle, the proteasome. Two beta-subunits of the proteasome, LMP2 and LMP7, are inducible by interferon-gamma and alter the catalytic activities of this particle, enhancing the presentation of at least some antigens. After production of the peptide in the cytosol, it is transported across the endoplasmic reticulum (ER) membrane in an ATP-dependent manner by TAP (transporter associated with antigen presentation), a member of the ATP-binding cassette family of transport proteins. There are minor pathways for generating presented peptides directly in the ER, and some evidence suggests that peptides may be further trimmed in this location. The class I heavy chain and beta 2-microglobulin are cotranslationally translocated into the endoplasmic reticulum where their assembly may be facilitated by the sequential association of the heavy chain with chaperone proteins BiP and calnexin. The class I molecule then associates with the lumenal face of TAP where it is retained, presumably awaiting a peptide. After the class I molecule binds a peptide, it is released for exocytosis to the cell surface where cytotoxic T lymphocytes examine it for peptides derived from foreign proteins.

Immunization with glycosylated Kb-binding peptides generates carbohydrate-specific, unrestricted cytotoxic T cells

Cytotoxic T cells (CTL) recognize target proteins as short peptides presented by major histocompatibility complex (MHC) class I restriction elements. However, there is also evidence for peptide-independent T cell receptor (TCR) recognition of target proteins and non-protein structures. How such T cell responses are generated is presently unclear. We generated carbohydrate (CHO)-specific, MHC-unrestricted CTL responses by coupling di- and trisaccharides to Kb- or Db-binding peptides for direct immunization in mice. Four peptides and three CHO have been analyzed with the CHO either in terminal or central position on the carrier peptide. With two of these glycopeptides, with galabiose (Gal alpha 1-4Gal; Gal2) bound to a homocysteine (via an ethylene spacer arm) in position 4 or 6 in a vesicular stomatitis virus nucleoprotein-derived peptide (RGYVYQGL binding to Kb), CTL were generated which preferentially killed target cells treated with glycopeptide compared to those treated with the core peptide. Polyclonal CTL were also found to kill target cells expressing the same Gal2 epitope in a glycolipid. By fractionation of CTL, preliminary data indicate that glycopeptide-specific Kb-restricted CTL and unrestricted CHO-specific CTL belong to different T cell populations with regard to TCR expression. The results demonstrate that hapten-specific unrestricted CTL responses can be generated with MHC class I-binding carrier peptides. Different models that might explain the generation of such responses are discussed.

Major histocompatibility complex class I binding glycopeptides for the estimation of 'empty' class I molecules

Different forms of major histocompatibility complex (MHC) class I heavy chains are known to be expressed on the cell surface, including molecules which are functionally 'empty'. Direct peptide binding to cells is obvious during sensitization of target cells in vitro for cytotoxic T lymphocyte killing and 'empty' MHC-I molecules are comparatively abundant on TAP-1/2 peptide transporter mutant cells. In the present work we have estimated the fraction of 'empty' MHC class I molecules using glycosylated peptides and cellular staining with carbohydrate specific monoclonal antibodies. Synthetic Db and Kb binding peptides were coupled at different positions with different di- or trisaccharides, using different spacing between the carbohydrate and the peptide backbone. Binding of sugar specific mAbs was compared in ELISA and cellular assays. An optimal Db binding glycopeptide was used for comparative staining with anti-Db and anti-carbohydrate monoclonal antibodies to estimate fractions of 'empty' molecules on different T lymphoid cells. On activated normal T cells, a large fraction of Db molecules were found to be 'empty'. The functional role of such 'empty' MHC class I molecules on T cells is presently unclear. However, on antigen presenting cells they might participate in the antigen presentation process.

Characterization of TAP-independent and brefeldin A-resistant presentation of Sendai virus antigen to CD8+ cytotoxic T lymphocytes

H-2Kb-transfected T2 cells, which lack both TAP1/2 and LMP2/7 genes, are able to efficiently process and present Sendai virus Antigen to Kb-restricted Sendai virus-specific CTL. This presentation is not inhibited by Brefeldin A (BFA). Here we extend our analysis of this novel antigen presentation pathway. We show that presentation of Sendai virus antigen was not due to sensitization of T2Kb cells by peptides in the virus preparation or peptides released from virus infected cells. Also, the ability to present Sendai virus in a BFA resistant fashion was specific for cells of the T2 lineage. Re-expression of TAP1/2 genes in T2Kb cells did not alter the capability to present antigen in a BFA resistant fashion, i.e. the presence of a functional TAP transporter complex did not relocate (all) peptides to the classical pathway for antigen processing and presentation. We found that co-infection of T2Kb cells with either Sendai virus plus influenza virus or Sendai virus plus VSV did not relocate presentation of influenza or VSV antigen to the TAP independent BFA resistant antigen presentation pathway. Peptide elution experiments and studies with peptide-specific CTL firmly demonstrated that the antigen presented by T2Kb cells after infection with Sendai virus was the natural Sendai virus epitope NP324-332. The same epitope, when expressed as a minigene in vaccinia virus, could be presented also by T2Kb cells but this presentation could be blocked by BFA. Thus, the TAP independent BFA resistant presentation of antigen seem cell (T2 lineage) and virus (Sendai virus) specific, but not epitope specific. The ability of T2Kb cells to present Sendai virus antigen in a TAP independent BFA resistant fashion was only partially blocked by lysosomal inhibitors such as methylamine, ammonium chloride and chloroquine. These findings demonstrate that TAP1/2-independent and BFA-resistant class I processing is only expressed in certain cell types, in parallel with classical MHC class I processing, and that Sendai virus selectively can enter this pathway. Hypothetical models for the TAP-independent class I processing are discussed.

Glycosylphosphatidylinositol-linked Db does not induce an influenza-specific cytotoxic T lymphocyte response or recycle membrane-bound peptides

Major histocompatibility complex (MHC) class I molecules, as well as MHC class I-bound peptides, are known to recycle between the cell surface and an undefined, endosomal-like compartment. Little is known about the functional significance of this process. We have explored this using two different forms of the H-2Db molecule expressed in transgenic mice, either transmembranous (Db-tm) or with a glycophosphatidylinositol (GPI)-lipid anchor (Db-GPI). The recycling capacity of peptides bound to Db-tm and Db-GPI was investigated using glycosylated Db-binding glycopeptides, which were detected by flow cytometry. Only the tm form of Db was found to readily internalize and recycle glycopeptides to the cell surface. When transgenic mice were immunized with influenza A virus (PR8) strain and tested for cytotoxic T lymphocyte (CTL) responses against an immunedominant nucleoprotein epitope (366-374, ASNENMETM), only Db-tm mice were found to generate specific CTL responses. The results support the idea that membrane recycling of MHC class I-bound peptides on antigen-presenting cells may be important for the generation of certain CTL responses.

Hepatitis B virus small surface antigen particles are processed in a novel endosomal pathway for major histocompatibility complex class I-restricted epitope presentation

We investigated the major histocompatibility complex (MHC) class I-restricted presentation of an epitope of the hepatitis B virus small surface (S) antigen particle to cloned murine cytotoxic T lymphocytes (CTL). Efficient Ld-restricted presentation of the S28-39 epitope to CTL is observed in cells of different tissue origin pulsed in vitro, either with the antigenic S28-39 12-mer S-peptide, or with particulate S-antigen. The kinetics of epitope presentation differ in S-peptide-pulsed and in S-particle-pulsed cells: while a 15-min pulse with the antigenic peptide sensitizes targets for class I-restricted CTL lysis, presentation of S-particles requires 30-60 min to sensitize cells for CTL lysis. Uptake of antigenic material and active metabolism of the presenting cell are required for processing of S-particles, but not for sensitizing targets with S-peptides. Intracellular processing and presentation of S-particles is blocked in cells treated with chloroquine, NH4Cl, primaquine, or leupeptin, but not by treatment with cycloheximide or brefeldin A. This processing pathway operates efficiently in peptide-transporter-deficient, Ld-transfected T2 cells, revealing a novel endosomal/lysosomal processing pathway for class I-restricted presentation of peptides derived from exogenous S-particles.

Effect of TAP on the generation and intracellular trafficking of peptide-receptive major histocompatibility complex class I molecules

Using a fluorescein-conjugated antigenic peptide, peptide-receptive H-2Kb MHC class I molecules were found throughout the secretory pathways of RMA cells and peptide transporter (TAP)-deficient derivative cells (RMA/S). RMA/S cells displayed higher levels of intracellular peptide-receptive molecules, while, surprisingly, RMA cells expressed 3- to 5-fold more cell surface peptide-receptive molecules. Metabolic radiolabeling of Kb-associated oligosaccharides with [1-3H]galactose demonstrated that despite a large difference in the fraction of Kb molecules in native conformation in detergent extracts, Kb transport rates from the trans-Golgi complex to the surfaces of RMA and RMA/S cells were similar. Thus, although considerable numbers of class I alpha chains reach the RMA/S cell surface, they are a less productive source of peptide-receptive molecules than class I molecules synthesized by TAP-expressing RMA cells, suggesting paradoxically that TAP functions to increase the amount of peptide-receptive molecules at the cell surface.

Delivery of protein antigen to the major histocompatibility complex class I-restricted antigen presentation pathway

Major histocompatibility complex (MHC) class I-restricted antigen presentation normally requires a protein antigen to be synthesized in the cytosol of the antigen presenting cell (APC). Exogenous protein antigen could gain access to the class I presentation pathway if the protein is introduced into the cytosolic compartment of the APC. Approaches which release the protein antigen from endocytic vesicles have been employed to deliver protein antigen for the recognition by class I-restricted cytotoxic T lymphocytes (CTL). These include osmotic shock, electroporation, cationic and pH-sensitive liposomes. An alternative approach is to deliver a gene that encodes the protein antigen. In this case, the APC is transfected with a gene which synthesizes the "exogenous protein" in the cytosol. Delivery of protein antigen targeted for CTL induction in vivo follows a different strategy and generally requires an antigen carrier of lipidic/membranous nature, such as liposomes, immunostimulating complexes, and/or lipid conjugates. Macrophages that are responsible for scavenging the antigen play an important role in CTL induction. An optimal CTL inductive vaccine must contain other immuno-modulatory activities in addition to its activity in delivering antigen to the class I pathway. Attempts to attenuate viral infection and to improve anti-tumor immunity have been successful by delivering the exogenous antigen entrapped in liposomes. These animal model studies should be of great value in the development of potential vaccine formulation.

Peptide transporter-independent, stress protein-mediated endosomal processing of endogenous protein antigens for major histocompatibility complex class I presentation

The peptide transporter-defective cell line RMA-S expressing the wild-type simian virus 40 large T antigen (wtT-Ag) from a transfected gene did not present two well-defined, H-2 class I (Db)-restricted epitopes of T-Ag to cytotoxic T lymphocytes (CTL). Hence, "endogenous" processing and presentation of the wtT-Ag depended on a functional peptide transporter heterodimer. In contrast, both T-Ag epitopes were efficiently presented to CTL by transfected RMA-S cells expressing a truncated, cytoplasmic T-Ag variant (cT-Ag) or a karyophilic, amino-terminal 272-amino acid T-Ag fragment. Transporter-independent "endogenous" processing of mutant T-Ag molecules correlated with their association with the constitutively expressed heat shock protein 73 (hsp73). Class I-restricted presentation of both epitopes processed from these hsp73-associated protein antigens was sensitive to NH4Cl and chloroquine. These data indicate that selected intracellular proteins access an alternative, hsp73-mediated pathway for class I-restricted presentation that operates independent of peptide transporters in an endosomal compartment.