Antigen presentation mediated by recycling of surface HLA-DR molecules

Uveitogenic epitopes of retinal S-antigen are generated in vivo via an alternative antigen-presentation pathway

We have found that different antigen-processing pathways are involved in the induction of experimental autoimmune uveoretinitis (EAU) by the retinal autoantigens S-antigen and interphotoreceptor retinoid-binding protein (IRBP). Although in vitro T-cell proliferative responses to IRBP were completely inhibited in the presence of irreversible cysteine protease inhibitors, no significant reduction of S-antigen proliferative responses was found. Furthermore, acidic proteolysis of S-antigen by the cysteine protease cathepsin B prior to immunization radically reduced pathogenicity (disease severity). In addition, in vitro processing of S-antigen, but not IRBP, was also found to be resistant to the action of cycloheximide and lysosomotropic agents, inhibition of proliferation only occurring after extended exposure of antigen-presenting cells to methyl amine or high concentrations of chloroquine. These data indicate that an alternative pathway of antigen processing exists for S-antigen, which is independent of processing within the normal endolysosomal pathway and that uveitogenic peptides of naturally processed S-antigen bind to major histocompatibility complex class II antigens either at the cell surface or within very early endosomes where cathepsin B is inactive.

Deficient peptide loading and MHC class II endosomal sorting in a human genetic immunodeficiency disease: the Chediak-Higashi syndrome

The Chediak-Higashi syndrome (CHS) is a human recessive autosomal disease caused by mutations in a single gene encoding a protein of unknown function, called lysosomal-trafficking regulator. All cells in CHS patients bear enlarged lysosomes. In addition, T- and natural killer cell cytotoxicity is defective in these patients, causing severe immunodeficiencies. We have analyzed major histocompatibility complex class II functions and intracellular transport in Epstein Barr Virus-transformed B cells from CHS patients. Peptide loading onto major histocompatibility complex class II molecules and antigen presentation are strongly delayed these cells. A detailed electron microscopy analysis of endocytic compartments revealed that only lysosomal multilaminar compartments are enlarged (reaching 1-2 micron), whereas late multivesicular endosomes have normal size and morphology. In contrast to giant multilaminar compartments that bear most of the usual lysosomal markers in these cells (HLA-DR, HLA-DM, Lamp-1, CD63, etc.), multivesicular late endosomes displayed reduced levels of all these molecules, suggesting a defect in transport from the trans-Golgi network and/or early endosomes into late multivesicular endosomes. Further insight into a possible mechanism of this transport defect came from immunolocalizing the lysosomal trafficking regulator protein, as antibodies directed to a peptide from its COOH terminal domain decorated punctated structures partially aligned along microtubules. These results suggest that the product of the Lyst gene is required for sorting endosomal resident proteins into late multivesicular endosomes by a mechanism involving microtubules.

Transfer of antigen between dendritic cells in the stimulation of primary T cell proliferation

Primary proliferative T cell responses require stimulation with antigen-pulsed dendritic cells (Ag-DC). Here we show that for optimal stimulation, dendritic cells (DC) not exposed directly to antigen are also required. Ag-DC added to DC-depleted T cells caused negligible primary stimulation; adding back DC resulted in stimulation. These effects were seen using the contact sensitizer fluorescein isothiocyanate (FITC), FITC conjugated to ovalbumin (FITC-OVA) or influenza virus as antigens. DC co-cultured with Ag-DC (using FITC or FITC-OVA) acquired antigen indicating that antigen was transferred between DC. DC that acquired antigen secondarily were separated by cell sorting and stimulated primary T cell proliferation directly. DC were also pulsed with FITC, washed thoroughly and incubated overnight. Supernatants contained shed antigen since DC incubated in these supernatants acquired antigen as indicated by flow cytometry. DC acquiring the shed antigen also stimulated T cell proliferation although the stimulation was not as effective as that seen when cell contact between DC and antigen-bearing DC occurred. Thus, in primary stimulation, activation of T cells may occur when there is an antigen gradient between Ag-DC and DC and the mechanisms underlying these effects are now being sought. We propose that this unique interaction between antigen-presenting cells may be a paradigm for self/non-self discrimination.

Cathepsins B and D are dispensable for major histocompatibility complex class II-mediated antigen presentation

Antigen presentation by major histocompatibility complex (MHC) class II molecules requires the participation of different proteases in the endocytic route to degrade endocytosed antigens as well as the MHC class II-associated invariant chain (Ii). Thus far, only the cysteine protease cathepsin (Cat) S appears essential for complete destruction of Ii. The enzymes involved in degradation of the antigens themselves remain to be identified. Degradation of antigens in vitro and experiments using protease inhibitors have suggested that Cat B and Cat D, two major aspartyl and cysteine proteases, respectively, are involved in antigen degradation. We have analyzed the antigen-presenting properties of cells derived from mice deficient in either Cat B or Cat D. Although the absence of these proteases provoked a modest shift in the efficiency of presentation of some antigenic determinants, the overall capacity of Cat B-/- or Cat D-/- antigen-presenting cells was unaffected. Degradation of Ii proceeded normally in Cat B-/- splenocytes, as it did in Cat D-/- cells. We conclude that neither Cat B nor Cat D are essential for MHC class II-mediated antigen presentation.

Antibodies for targeted gene therapy: extracellular gene targeting and intracellular expression

Antibody genes of human origin and human antibodies directed against human proteins have become widely available in recent years. These are valuable reagents for gene therapy applications, in which the use of human proteins and genes allows for increased therapeutic benefit. Engineered human antibodies can be used in gene therapy both as a component of a gene delivery system and as a therapeutic gene. As the targeting moiety of a gene delivery system, the antibody should meet certain criteria that have been previously determined from other clinical applications of antibodies. These include bioavailability, specificity for the target cell, and rapid clearance. In addition, if repeat delivery of therapeutic genes is going to be needed, then gene delivery vectors should be non-immunogenic to allow repeated administration. The use of human antibodies in this application should therefore be superior to approaches which use rodent-derived antibodies. Another application of antibodies in gene therapy is the use of antibodies expressed inside the cell (intrabodies) as therapeutic agents. The power of the immune system to rearrange a limited set of genes to create recognition sites for any known molecule is well documented. The ability to harness this information and use these highly specific binding molecules as medicines to inhibit an unwanted cellular function is a promising advance in the field of molecular medicine, and in particular, in the field of intracellular immunization.

Exploring the mechanisms of antigen processing by cell fractionation

It is becoming increasingly clear that most of the intracellular compartments that contain MHC class II products in antigen-presenting cells simply represent the conventional endosomes and lysosomes that are expressed in all cell types. Data from recent cell fractionation studies, however, predominantly those using electrophoresis techniques, show that a population of class-II-containing vesicles exists that may comprise a class of endosomes that are specialized for antigen processing. Strong support for this possibility comes from the observation that such specialized structures, designated class II vesicles (CIIV), are particularly abundant in mature dendritic cells.

Distinct Peptide Loading Pathways for MHC Class II Molecules Associated with Alternative Ii Chain Isoforms

Mutant mouse strains expressing either p31 or p41 Ii chain appear equally competent with respect to their class II functional activities including Ag presentation and CD4+ T cell development. To further explore possibly divergent roles provided by alternative Ii chain isoforms, we compare class II structure and function in double mutants also carrying a null allele at the H2-DM locus. As for DM mutants expressing wild-type Ii chain, AαbAβb dimers present in DM-deficient mice expressing either Ii chain isoform appear equally occupied by class II-associated Ii chain-derived peptides (CLIP). Surprisingly, in functional assays, these novel mouse strains exhibit strikingly different phenotypes. Thus, DM-deficient mice expressing wild-type Ii chain or p31 alone are both severely compromised in their abilities to present peptides. In contrast, double mutants expressing the p41 isoform display markedly enhanced peptide-loading capabilities, approaching those observed for wild-type mice. The present data strengthen evidence for divergent class II presentation pathways and demonstrate for the first time that functionally distinct roles are mediated by alternatively spliced forms of the MHC class II-associated Ii chain in a physiologic setting.

The tyrosine-containing cytoplasmic tail of CD1b is essential for its efficient presentation of bacterial lipid antigens

CD1b is an antigen-presenting molecule that mediates recognition of bacterial lipid and glycolipid antigens by specific T cells. We demonstrate that the nine-amino acid cytoplasmic tail of CD1b contains all of the signals required for its normal endosomal targeting, and that the single cytoplasmic tyrosine is a critical component of the targeting motif. Mutant forms of CD1b lacking the endosomal targeting motif are expressed at high levels on the cell surface but are unable to efficiently present lipid antigens acquired either exogenously or from live intracellular organisms. These results define the functional role of the CD1b targeting motif in a physiologic setting and demonstrate its importance in delivery of this antigen-presenting molecule to appropriate intracellular compartments.

Peptide loading onto recycling HLA-DR molecules occurs in early endosomes

Presentation of exogenous antigens to MHC class II-restricted T cells can follow two different processing pathways. The classical pathway requires newly synthesized MHC class II molecules, invariant chain and HLA-DM expression, whereas the alternative pathway is independent of protein synthesis, invariant chain and HLA-DM. In both cases, MHC class II molecules associate with peptides derived from exogenous antigens that have been processed in endocytic compartments. Different endosomal/prelysosomal compartments where peptide/MHC class II complexes and HLA-DM molecules accumulate have been described. We show here that the alternative pathway uses an earlier compartment than the classical pathway. Experiments with chemically cross-linked antigen suggest that recycling MHC class II molecules present rapidly degraded antigens, leading to a rapid immune response to exogenously added influenza virus proteins.

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.

Protein sorting within the MHC class II antigen-processing pathway

Major histocompatibility complex (MHC) class II molecules are required for the presentation of antigenic peptides that are derived predominantly from internalized proteins. The assembly of MHC class II/peptide complexes occurs within endosomal compartments of antigen-presenting cells (APCs). Therefore, for assembly to occur, MHC class II molecules, foreign proteins, and accessory molecules must be sorted to appropriate intracellular sites. My laboratory is trying to understand how proteins are sorted to various antigen-processing compartments as well as to conventional endosomal organelles. Using chimeric marker proteins and a variety of biochemical and genetic approaches, we are addressing the specificity of protein sorting and the mechanisms by which sorting signals are deciphered. By using a similar chimeric protein approach to target endogenous proteins to distinct compartments, we hope to address the role of processing events in each compartment in the generation of MHC class II ligands.

Type II and III receptors for immunoglobulin G (IgG) control the presentation of different T cell epitopes from single IgG-complexed antigens

T cell receptors on CD4(+) lymphocytes recognize antigen-derived peptides presented by major histocompatibility complex (MHC) class II molecules. A very limited set of peptides among those that may potentially bind MHC class II is actually presented to T lymphocytes. We here examine the role of two receptors mediating antigen internalization by antigen presenting cells, type IIb2 and type III receptors for IgG (FcgammaRIIb2 and FcgammaRIII, respectively), in the selection of peptides for presentation to T lymphocytes. B lymphoma cells expressing recombinant FcgammaRIIb2 or FcgammaRIII were used to assess the presentation of several epitopes from two different antigens. 4 out of the 11 epitopes tested were efficiently presented after antigen internalization through FcgammaRIIb2 and FcgammaRIII. In contrast, the 7 other epitopes were efficiently presented only when antigens were internalized through FcgammaRIII, but not through FcgammaRIIb2. The capacity to present these latter epitopes was transferred to a tail-less FcgammaRIIb2 by addition of the FcgammaRIII-associated gamma chain cytoplasmic tail. Mutation of a single leucine residue at position 35 of the gamma chain cytoplasmic tail resulted in the selective loss of presentation of these epitopes. Therefore, the nature of the receptor that mediates internalization determines the selection of epitopes presented to T lymphocytes within single protein antigens.

Different endosomal proteolysis requirements for antigen processing of two T-cell epitopes of the M5 protein from viable Streptococcus pyogenes

We studied endosomal proteolysis of the surface fibrillar M5 protein from viable Streptococcus pyogenes as an essential step involved in major histocompatibility complex class II-restricted antigen processing of two immunodominant CD4(+) T-cell epitopes (17-31/Ed and 308-319/Ad). Intracellular proteolysis of viable streptococci for presentation of 17-31, bound by serine proteinase cleavage sites, was mediated by serine proteinases, whereas processing of soluble recombinant M5 protein required in addition cysteine proteinases. Furthermore, processing of 17-31 was resistant to ammonium chloride and thus was not dependent on endosome acidification. Cysteine and serine proteinase cleavage sites were located adjacent to 308-319, and its processing was dependent on serine, cysteine, and aspartic proteinases, as well as on endosomal acidification. The data suggest that antigen processing of two major T-cell epitopes on streptococcal M5 protein occurred in different endosomal compartments by different classes of intracellular proteinases.

Dendritic cell maturation and antigen presentation in the absence of invariant chain

In immature dendritic cells (DCs), major histocompatibility complex class II molecules accumulate in peptide-loading compartments and, during DC maturation, are exported to the cell surface in response to inflammatory stimuli. Moreover, it has recently been proposed that DCs have specific mechanisms of antigen uptake and delivery into major histocompatibility complex class II-loading compartments. B cells bearing a genetically disrupted invariant chain gene (Ii -/-) show alterations in the transport and function of class II molecules. We herein report that DCs derived from Ii -/- H2(k) but not Ii -/- H2(b) mice undergo normal maturation in response to tumor necrosis factor alpha and show a high degree of class II surface expression. Class II molecules are accumulated in cathepsin D- and H2-M-positive compartments in immature Ii -/- DC and, during DC maturation, are exported to the cell membrane as compact dimers. Ii -/- DCs present putative Ii-dependent hen egg lysozyme-derived epitopes to T cells. These data support the existence of Ii-independent molecular requirements for class II transport and peptide loading in DCs.

T-cell response to myelin basic protein and lipid-bound myelin basic protein in patients with multiple sclerosis and healthy donors

An autoimmune T-cell response to myelin proteins is thought to be involved in the pathogenesis of multiple sclerosis (MS) and myelin basic protein (MBP) is the most widely studied potential target antigen. We investigated the T-cell response to MBP in MS patients and controls using two different molecular forms of the protein: the classical hydrophilic MBP (lipid-free MBP, LF-MBP) and a lipid-bound, native-like preparation of MBP isolated in a molecular form retaining the binding to all myelin lipids (lipid-bound-MBP, LB-MBP). Short term T-cell lines (TCL) were generated using either LF- or LB-MBP and tested for their reactivity to the in vitro stimulating antigen. No differences were detected between MS patients and healthy donors in the percentage of T-cell cultures responsive to the LF-MBP. In contrast, the number of LB-MBP reactive cultures was higher in MS patients than in controls. This difference was almost entirely due to the presence of high numbers of LB-MBP-specific TCL in MS patients which did not cross-react with LF-MBP and were not present in healthy subjects. LB-MBP may represent a novel antigen worth to be investigated in MS.

Invariant chain-independent function of H-2M in the formation of endogenous peptide-major histocompatibility complex class II complexes in vivo

Efficient loading of major histocompatibility complex class II molecules with peptides requires the invariant chain (Ii) and the class II-like molecule H-2M. Recent in vitro biochemical studies suggest that H2-M may function as a chaperone to rescue empty class II dimers. To test this hypothesis in vivo, we generated mice lacking both Ii and H-2M (Ii-/-M-/-). Antigen presenting cells (APCs) from Ii-/-M-/- mice, as compared with APCs from Ii-/- mice, exhibit a significant reduction in their ability to present self-peptides to a panel of class II I-Ab-restricted T cells. As a consequence of this defect in the loading of self peptides, CD4(+) thymocyte development is profoundly impaired in Ii-/-M-/- mice, resulting in a peripheral CD4(+) T cell population with low levels of T cell receptor expression. These findings are consistent with the idea that H-2M functions as a chaperone in the peptide loading of class II molecules in vivo.

MHC Class II Expression in Double Mutant Mice Lacking Invariant Chain and DM Functions

Invariant (Ii) chain and DM functions are required at distinct stages during class II maturation to promote occupancy by diverse peptide ligands. The class II molecules expressed by mutant mouse strains lacking Ii chain or DM activities display discrete structural and functional abnormalities. The present report describes the cellular and biochemical characteristics of Ii−DM− doubly deficient mice. As for Ii chain mutants, their mature AαbAβb dimers similarly exhibit reduced mobilities in SDS-PAGE, and in functional assays these molecules behave as if empty or occupied by an easily displaced peptide. Additionally, the present experiments demonstrate that the production of floppy AαbAβb dimers is TAP independent. In comparison with Ii chain mutants, Ii−DM− doubly deficient cell populations exhibit increased peptide binding activities and consistently greater presentation abilities in T cell stimulation assays. These functional differences appear to reflect higher class II surface expression associated with their increased representation of B lymphocytes. We also observe defective B cell maturation in mice lacking Ii chain or DM expression, and interestingly, B cell development appears more severely compromised in Ii−DM− double mutants. These mutant mice lacking both Ii chain and DM activities should prove useful for analyzing nonconventional class II Ag presentation under normal physiological conditions in the intact animal.

The majority of HLA-DR3 alloreactive T cells is peptide specific, but does not recognize known DR3-bound sequences

Rejection of transplants is frequently caused by activation of alloreactive T cells that recognize HLA/peptide differences between patient and graft. This T-cell response can be directed towards the HLA molecule, the HLA-bound peptide or towards a combination. More insight in the involvement of peptides in this process may help to find ways to avoid rejection using for example antagonist peptides. In recent years many naturally processed HLA-bound peptides have been identified. This raises the question of whether these, presumably abundant, peptides are involved in class II-specific allorecognition. To investigate this, we first determined the proportion of peptide-specific alloreactive T cells in the alloresponse against HLA-DR3. For this purpose we have tested a panel of DR3-specific alloreactive T-cell clones against a DM-mutant (i.e. peptide loading deficient) cell line. We found that 59 out of 64 alloreactive T-cell clones were dependent upon the presence of DM for an optimal response. However, only 2 DM-dependent T-cell clones recognize known peptide sequences. Thus we conclude that most DR3-specific alloreactive T-cell clones are peptide specific and that the currently known DR3-bound peptides are not the main target for allorecognition. Finally, we identified 4 T-cell clones that recognized the DM-mutant better than the wild-type cell line. The response against the wild-type cell line could not be restored with invariant chain derived peptides (CLIP). This provides additional evidence that DM can negatively select self-peptides other than CLIP, which can result in selection against peptides involved in allorecognition.

Distinct intracellular compartments involved in invariant chain degradation and antigenic peptide loading of major histocompatibility complex (MHC) class II molecules

Major histocompatibility complex (MHC) class II molecules are transported to intracellular MHC class II compartments via a transient association with the invariant chain (Ii). After removal of the invariant chain, peptides can be loaded onto class II molecules, a process catalyzed by human leukocyte antigen-DM (HLA-DM) molecules. Here we show that MHC class II compartments consist of two physically and functionally distinct organelles. Newly synthesized MHC class II/Ii complexes were targeted to endocytic organelles lacking HLA-DM molecules, where Ii degradation occurred. From these organelles, class II molecules were transported to a distinct organelle containing HLA-DM, in which peptides were loaded onto class II molecules. This latter organelle was not directly accessible via fluid phase endocytosis, suggesting that it is not part of the endosomal pathway. Uptake via antigen-specific membrane immunoglobulin resulted however in small amounts of antigen in the HLA-DM positive organelles. From this peptide-loading compartment, class II-peptide complexes were transported to the plasma membrane, in part after transit through endocytic organelles. The existence of two separate compartments, one involved in Ii removal and the other functioning in HLA-DM-dependent peptide loading of class II molecules, may contribute to the efficiency of antigen presentation by the selective recruitment of peptide-receptive MHC class II molecules and HLA-DM to the same subcellular location.

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.

Characterizing immunodominant and protective influenza hemagglutinin epitopes by functional activity and relative binding to major histocompatibility complex class II sites

In the present study the analysis of functional activity and major histocompatibility complex (MHC) binding of two adjacent MHC class II-restricted epitopes, located in the C-terminal 306-329 region of human influenza A virus hemagglutinin 1 subunit (HA1) conserved with subtype sequences and not affected by antigenic drift, was undertaken to explore the hierarchy of local immunodominance. The functional activity of two T cell hybridomas of the memory/effector Th1 phenotype in combination with in vivo immunization studies provided a good tool for investigating the functional characteristics of the T cell response. The in vitro binding assays performed with a series of overlapping, N-terminal biotinylated peptides covering the 306-341 sequence enabled us to compare the relative binding efficiency of peptides, comprising two distinct epitopes of this region, to I-Ed expressed on living antigen-presenting cells. Our studies revealed that (i) immunization of BALB/c mice with the 306-329 H1 or H2 peptides resulted in the activation and proliferation of T cells recognizing both the 306-318 and the 317-329 epitopes, while the 306-329 H3 peptide elicits predominantly 306-318-specific T cells, (ii) the 317-329 HA1 epitope of the H1 and H2 but not the H3 sequence is recognized by T cells and is available for recognition not only in the 317-329 peptide but also in the extended 306-329 or 306-341 peptides, (iii) the 306-318 and the 317-329 hemagglutinin peptides encompassing the H1, H2 but not the H3 sequence bind with an apparently similar affinity to and therefore compete for I-Ed binding sites, and (iv) the 317-341, the 317-329 peptides and their truncated analogs show subtype-dependent differences in MHC binding and those with lower binding capacity represent the H3 subtype sequences. These results demonstrate that differences in the binding capacity of peptides comprising two non-overlapping epitopes located in the C-terminal 306-329 region of HA1 of all three subtype-specific sequences to MHC class II provide a rationale for the local and also for the previously observed in vivo immunodominance of the 306-318 region over the 317-329 epitope in the H3 but not in the H1 or H2 sequences. In good correlation with the results of the binding and functional inhibition assays, these data demonstrate that in the H1 and H2 subtypes both regions are available for T cell recognition, they compete for the same restriction element with an apparently similar binding efficiency and, therefore, function as co-dominant epitopes. Due to the stabilizing effect of the fusion peptide, peptides comprising the 306-341 or 317-341 H1 sequences are highly immunogenic and elicit a protective immune response which involves the production of antibodies and interleukin-2 and tumor necrosis factor producing effector Th1 cells both directed against the 317-329 region. Based on the similarity of the I-Ed and HLA-DR1 peptide binding grooves and motifs, these results suggest that amino acid substitutions inserted to the H3 subtype sequence during viral evolution can modify the relative MHC binding capacity and invert the local hierarchy of immunodominance of two closely situated epitopes that are able to bind to the same MHC class II molecule.

Inefficient peptide binding by cell-surface class II MHC molecules

The efficiency of peptide loading onto surface class II MHC molecules in intact APC was investigated, using a previously defined europium immunoassay as well as a simplified Western blot procedure. Conditions normally employed for peptide loading in T cell stimulation assays were suboptimal for peptide binding, which is enhanced at low pH, in the presence of protease inhibitors, and the absence of competing serum proteins. In contrast to some earlier reports, our results indicate that the rate of peptide loading by class II molecules is not enhanced in the environment of the plasma membrane. Peptide association rates were similar for purified and cell-surface class II molecules. As previously reported, rapid peptide binding can be achieved by reconstituting class II molecules into total cellular membranes. We report that this activity is due solely to HLA-DM (which is not present at the cell surface), since it can be specifically removed by immunodepletion with an anti-DM mAb. Thus, we find no evidence for additional cellular cofactors capable of catalyzing peptide binding to class II molecules.

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.

Major histocompatibility complex class II compartments in human and mouse B lymphoblasts represent conventional endocytic compartments

In most human and mouse antigen-presenting cells, the majority of intracellular major histocompatibility complex (MHC) class II molecules resides in late endocytic MHC class II compartments (MIICs), thought to function in antigen processing and peptide loading. However, in mouse A20 B cells, early endocytic class II-containing vesicles (CIIVs) have been reported to contain most of the intracellular MHC class II molecules and have also been implicated in formation of MHC class II-peptide complexes. To address this discrepancy, we have studied in great detail the endocytic pathways of both a human (6H5.DM) and a mouse (A20.Ab) B cell line. Using quantitative immunoelectron microscopy on cryosections of cells that had been pulse-chased with transferrin-HRP or BSA-gold as endocytic tracers, we have identified up to six endocytic subcompartments including an early MIIC type enriched in invariant chain, suggesting that it serves as an important entrance to the endocytic pathway for newly synthesized MHC class II/invariant chain complexes. In addition, early MIICs represented the earliest endocytic compartment containing MHC class II- peptide complexes, as shown by using an antibody against an abundant endogenous class II-peptide complex. The early MIIC exhibited several though not all of the characteristics reported for the CIIV and was situated just downstream of early endosomes. We have not encountered any special class II-containing endocytic structures besides those normally present in nonantigen-presenting cells. Our results therefore suggest that B cells use conventional endocytic compartments rather than having developed a unique compartment to accomplish MHC class II presentation.

Endosomal proteases and antigen processing

T cells are activated by fragments of antigenic proteins bound to major histocompatibility complex (MHC) molecules and displayed on the cell surface. MHC class II proteins scavenge processed protein antigens from within endosomal compartments. The antigenic peptides are generated within these and other intracellular compartments using the array of proteolytic enzymes normally involved in terminal protein degradation. Antigen-presenting cells use different mechanisms to exploit and control the activity of these enzymes so as to ensure the generation of a wide variety of peptides, while preventing the destruction of antigenic epitopes by excessive proteolysis.

Inflammatory stimuli induce accumulation of MHC class II complexes on dendritic cells

Dendritic cells have the remarkable property of presenting any incoming antigen. To do so they must not only capture antigens with high efficiency and broad specificity, but must also maximize their capacity to load class II molecules of the major histocompatibility complex (MHC) with antigenic peptides in order to present a large array of epitopes from different proteins, each at a sufficient copy number. Here we show that formation of peptide-MHC class II complexes is boosted by inflammatory stimuli that induce maturation of dendritic cells. In immature dendritic cells, class II molecules are rapidly internalized and recycled, turning over with a half-life of about 10 hours. Inflammatory stimuli induce a rapid and transient boost of class II synthesis, while the half-life of class II molecules increases to over 100 hours. These coordinated changes result in the rapid accumulation of a large number of long-lived peptide-loaded MHC class II molecules capable of stimulating T cells even after several days. The capacity of dendritic cells to load many antigenic peptides over a short period of initial exposure to inflammatory stimuli could favour presentation of infectious antigens.

Functionality of major histocompatibility complex class II molecules in mice doubly deficient for invariant chain and H-2M complexes

By combining two previously generated null mutations, Ii degrees and M degrees , we produced mice lacking the invariant chain and H-2M complexes, both required for normal cell-surface expression of major histocompatibility complex class II molecules loaded with the usual diverse array of peptides. As expected, the maturation and transport of class II molecules, their expression at the cell surface, and their capacity to present antigens were quite similar for cells from Ii degrees M degrees double-mutant mice and from animals carrying just the Ii degrees mutation. More surprising were certain features of the CD4(+) T cell repertoire selected in Ii degrees M degrees mice: many fewer cells were selected than in Ii+M degrees animals, and these had been purged of self-reactive specificities, unlike their counterparts in Ii+M degrees animals. These findings suggest (i) that the peptides carried by class II molecules on stromal cells lacking H-2M complexes may almost all derive from invariant chain and (ii) that H-2M complexes edit the peptide array displayed on thymic stromal cells in the absence of invariant chain, showing that it can edit, in vivo, peptides other than CLIP.

Degradation of mouse invariant chain: roles of cathepsins S and D and the influence of major histocompatibility complex polymorphism

Antigen-presenting cells (APC) degrade endocytosed antigens into peptides that are bound and presented to T cells by major histocompatibility complex (MHC) class II molecules. Class II molecules are delivered to endocytic compartments by the class II accessory molecule invariant chain (Ii), which itself must be eliminated to allow peptide binding. The cellular location of Ii degradation, as well as the enzymology of this event, are important in determining the sets of antigenic peptides that will bind to class II molecules. Here, we show that the cysteine protease cathepsin S acts in a concerted fashion with other cysteine and noncysteine proteases to degrade mouse Ii in a stepwise fashion. Inactivation of cysteine proteases results in incomplete degradation of Ii, but the extent to which peptide loading is blocked by such treatment varies widely among MHC class II allelic products. These observations suggest that, first, class II molecules associated with larger Ii remnants can be converted efficiently to class II-peptide complexes and, second, that most class II-associated peptides can still be generated in cells treated with inhibitors of cysteine proteases. Surprisingly, maturation of MHC class II in mice deficient in cathepsin D is unaffected, showing that this major aspartyl protease is not involved in degradation of Ii or in generation of the bulk of antigenic peptides.

Specific T cell recognition of kinetic isomers in the binding of peptide to class II major histocompatibility complex

Helper T cells are triggered by molecular complexes of antigenic peptides and class II proteins of the major histocompatibility complex. The formation of stable complexes between class II major histocompatibility complex proteins and antigenic peptides is often accompanied by the formation of a short-lived complex. In this report, we describe T cell recognition of two distinct complexes, one short-lived and the other long-lived, formed during the binding of an altered myelin basic protein peptide to I-Ak. One myelin basic protein-specific T cell clone is triggered by only the short-lived complex, and another is triggered by only the stable complex. Thus, a single peptide bound to a particular class II molecule can activate different T cells depending on the conditions of the binding reaction.

Intestinal epithelial cells use two distinct pathways for HLA class II antigen processing

Intestinal epithelial cells express a low level of HLA class II molecules constitutively, with elevated levels seen in the setting of mucosal inflammation including inflammatory bowel disease. The ability of intestinal epithelial cells to act as antigen presenting cells for alphabeta CD4(+) T lymphocytes was examined through a molecular analysis of the HLA class II antigen processing pathway. We have shown that intestinal epithelial cells contain abundant constitutive levels of the cathepsin proteases proven to function in HLA class II mediated antigen presentation. Activation of these cells by gamma-IFN induced the expression of invariant chain and HLA-DM alphabeta, thus facilitating the formation of compact, SDS-stable HLA- DR alphabeta heterodimers. Using HLA-DR-restricted T cells and retroviral mediated gene transfer of HLA-DR alleles into the intestinal epithelial cell lines HT-29 and T84, we demonstrated efficient antigen processing and presentation to CD4(+) T lymphocytes in the presence of the proinflammatory cytokine gamma-IFN. The class II processing pathway and presentation in the presence of gamma-IFN was indistinguishable from that observed with a conventional antigen presenting cell. Antigen processing also occurred in intestinal epithelial cells in the absence of gamma-IFN, and in contrast to that seen after stimulation with gamma-IFN, required high concentration of antigen and was not inhibited by the protease inhibitor leupeptin. These data suggest the use of two distinct pathways of HLA class II antigen processing in enterocytes with differential immunomodulatory properties in the presence or absence of mucosal inflammation.

Acceleration of intracellular targeting of antigen by the B-cell antigen receptor: importance depends on the nature of the antigen-antibody interaction

The B-cell antigen receptor (BCR) internalizes bound antigen such that antigen-derived peptides become associated with emigrating major histocompatibility complex (MHC) class II molecules for presentation to T cells. Experiments with B-cell transfectants reveal that BCR confers a specificity of intracellular targeting since chimeric antigen receptors which internalize antigen by virtue of a heterologous cytoplasmic domain do not necessarily give rise to presentation. In contrast, however, previous studies have shown that antigen binding to irrelevant cell surface molecules (e.g. transferrin receptor, MHC class I) can ultimately lead to presentation. The solution to this paradox appears to be that the intracellular targeting by BCR actually reflects an acceleration of antigen delivery. Depending on the nature of the BCR-antigen interaction, this accelerated targeting can be essential in determining whether or not internalization leads to significant presentation. Physiologically, the accelerated delivery of antigen by BCR could prove of particular importance early in the immune response when antigen-BCR interaction is likely to be poor.

Late events in the intracellular sorting of major histocompatibility complex class II molecules are regulated by the 80-82 segment of the class II beta chain

The molecular mechanisms that regulate sorting of major histocompatibility complex (MHC) class II molecules into the endocytic pathway are poorly understood. For many proteins, access to endosomal compartments is regulated by cytosolically expressed sequences. We present evidence that a sequence in the lumenal domain of the MHC class II molecule regulates a very late event in class II biogenesis. Class II molecules containing single amino acid changes in the highly conserved 80-82 region of the beta chain were introduced into invariant chain (Ii)-negative fibroblasts with wild-type alpha chain, and the derived transfectants were analyzed biochemically. Using an endosomal isolation technique, we have quantified the level of class II molecules expressed in endocytic compartments and found that in the absence of Ii, approximately 15% of total cellular class II molecules can be isolated from endosomal compartments. Mutation at position 80 enhances this localization, while changes at positions 81 and 82 ablate class II expression in endosomal compartments. In addition, we have evaluated whether the induced changes in intracellular distribution of class II molecules were due to alterations in early biosynthetic events, indicative of misfolding of the molecules, or to modulation of later trafficking events more likely to be a consequence of the modulation of a specific transport event. Despite the dramatic effects on endosomal localization induced by the mutations, early biosynthetic events and maturation of class II were unaffected by the mutations. Collectively, our data argue that late trafficking events that control the ability of the class II molecule to access antigens is regulated by the 80-82 segment of the MHC class II beta chains.

Intracellular signaling and endosomal trafficking of immunoreceptors. Shared effectors underlying MHC class II-restricted antigen presentation

The cells of the immune system express a wide variety of receptors, defined as immunoreceptors because they are involved in antigen recognition. B and T lymphocytes express clonally distributed receptors which recognize either soluble antigens, through B-cell receptors (BcR), or peptides associated to major histocompatibility complex (MHC) molecules, through T-cell receptors (TcR). Many lymphoid or myeloid cells, such as B lymphocytes, macrophages or dendritic cells, express receptors for antigen-antibody complexes, which recognize the Fc portion of immunoglobulins (FcR). Although their ligands are different, immunoreceptors share both structural and functional homologies. The BcRs, TcRs and most FcRs, are multichain complexes composed of a ligand binding module, including one or two chains which determine the specificity of antigen recognition and a transducing module, which includes two to six chains containing a conserved motif in their cytoplasmic tail (A.D. Keegan and W.E. Paul, Immunol. Today 13 (1992) 63-68). This motif, called ITAM for immunoreceptor tyrosine-based activation motif (M.G. Reth, Nature 338 (1989) 383-384 and J.C. Cambier, Immunol. Today 16 (1995) 110-114) consists of five conserved amino acid residues precisely spaced over an amino acid sequence (D2xY2xL7x2xL). ITAMs couple receptors to intracellular effectors which induce a cascade of events leading to both cell activation and to down regulation of the receptors. This review focuses on recent data supporting the involvement of cytosolic effectors of cell activation in the endosomal transport of immunoreceptors. The possible role of these cytosolic factors in lysosomal transport and MHC class II restricted antigen presentation is discussed.

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.

Peptide-MHC complexes assembled following multiple pathways: an opportunity for the design of vaccines and therapeutic molecules

Antigen degradation and peptide loading to major histocompatibility complex class I and class II molecules are described with special emphasis on "noncanonical" pathways. Examples of specific peptide loading for measles proteins are provided. In addition, characterization of defined epitopes presented to T cells can lead to the design of products of special interest in medicine and, in particular, in development of vaccines.

How HLA-DM affects the peptide repertoire bound to HLA-DR molecules

Considerable progress has been made in the field of major histocompatibility complex (MHC) class II-restricted antigen presentation. The analysis of mutant cell lines defective in antigen presentation revealed a central role for the nonclassical MHC class II molecule HLA-DM. Cell biological and biochemical characterization of HLA-DM provided deeper insight into the molecular mechanisms underlying the loading process: HLA-DM accumulates in acidic compartments, where it stabilizes classical class II molecules until a high-stability ligand occupies the class II peptide binding groove. Thus, HLA-DM prevents empty alpha beta dimers from functional inactivation at low endosomal/lysosomal pH in a chaperone-like fashion. In the presence of peptide ligands, HLA-DM acts as a catalyst for peptide loading by releasing CLIP, the residual invariant chain-derived fragment by which the invariant chain is associated with the class II molecules during transport from the endoplasmic reticulum to the loading compartments. Finally, there is accumulating evidence that HLA-DM functions as a peptide editor that removes low-stability ligands, thereby skewing the class II peptide repertoire toward high-stability alpha beta: peptide complexes presentable to T cells.

HLA-DR-restricted presentation of purified myelin basic protein is independent of intracellular processing

Antigen presentation to CD4+ T cells involves intracellular antigen processing and loading of peptides onto newly synthesized major histocompatibility complex (MHC)-class II molecules. Some antigens, such as the lipid-bound, native form of myelin basic protein (LB-MBP) can also be presented by recycling of cell surface MHC class II molecules. The data reported here demonstrate that a preparation of highly purified, delipidated MBP (HP-MBP) follows yet another presentation pathway. Similar to LB-MBP, presentation of HP-MBP to HLA-DR1-restricted T cells was independent of HLA-DM, of newly synthesized proteins, and of invariant chain expression. However, in contrast to LB-MBP, presentation of HP-MBP was also independent of internalization of surface HLA-DR molecules. The different requirements for the presentation of the two molecular forms of MBP were further confirmed by use of the protease inhibitor E64: presentation of LB-MBP but not of HP-MBP was inhibited after treatment of target cells with E64. Furthermore, intact HP-MPB bound to isolated HLA-DR molecules in vitro with an association rate that was considerably faster than that of short peptides. These results show that presentation of HP-MBP is independent of intracellular processing and suggest that it may be presented to T cells by direct binding to surface HLA-DR molecules.

Presentation of a T cell receptor antagonist peptide by immunoglobulins ablates activation of T cells by a synthetic peptide or proteins requiring endocytic processing

T cell receptor (TCR) antagonism is being considered for inactivation of aggressive T cells and reversal of T cell-mediated autoimmune diseases. TCR antagonist peptides silence aggressive T cells and reverse experimental allergic encephalomyelitis induced with free peptides. However, it is not clear whether free antagonist peptides could reverse natural disease where the antigen is presumably available for endocytic processing and peptides gain access to newly synthesized class II MHC molecules. Using an efficient endocytic presentation system, we demonstrate that a proteolipid protein (PLP) TCR antagonist peptide (PLP-LR) presented on an Ig molecule (Ig-PLP-LR) abrogates the activation of T cells stimulated with free encephalitogenic PLP peptide (PLP1), native PLP, or an Ig containing PLP1 peptide (Ig-PLP1). Free PLP-LR abolishes T cell activation when the stimulator is free PLP1 peptide, but has no measurable effect when the stimulator is the native PLP or Ig-PLP1. In vivo, Ig-PLP1 induces a T cell response to PLP1 peptide. However, when coadministered with Ig-PLP-LR, the response to PLP1 peptide is markedly reduced whereas the response to PLP-LR is normal. Free PLP-LR coadministered with Ig-PLP1 has no effect on the T cell response to PLP1. These findings indicate that endocytic presentation of an antagonist peptide by Ig outcompete both external and endocytic agonist peptides whereas free antagonist hinders external but not endocytic agonist peptide. Direct contact with antagonist ligand and/or trans-regulation by PLP-LR-specific T cells may be the operative mechanism for Ig-PLP-LR-mediated downregulation of PLP1-specific T cells in vivo. Efficient endocytic presentation of antagonist peptides, which is the fundamental event for either mechanism, may be critical for reversal of spontaneous T cell-mediated autoimmune diseases where incessant endocytic antigen processing could be responsible for T cell aggressivity.

MHC class II molecules: transport pathways for antigen presentation

During biosynthesis, MHC class II molecules travel through the endocytic pathway and interact with antigenic peptides before their stable insertion in the plasma membrane. The process of class II association with these peptides and their final deposition at the cell surface are essential steps in boosting specific antibody responses. Therefore, the study of class II molecules is important in understanding how cell-biological events can direct an immune response.

Selective modulation of the major histocompatibility complex class II antigen presentation pathway following B cell receptor ligation and protein kinase C activation

We noticed that B cell receptor ligation or phorbol 12-myristate 13-acetate treatment induced intracellular vesicles containing major histocompatibility complex (MHC) class II and invariant chain (Ii), and increased the amount of transmembrane p12 Ii fragments coimmunoprecipitated with class II molecules. To determine the influence of protein kinase C activation on the MHC class II presentation pathway, we analyzed the subcellular distribution of Ii, the induction of SDS-stable forms of class II molecules, and their ability to present different antigens. Ii chains visualized with luminal and cytoplasmic directed antibodies appeared in early endosomal compartments accessible to transferrin in response to phorbol 12-myristate 13-acetate treatment, whereas transmembrane Ii degradation products equivalent to the p12 Ii fragments were colocalized with the B cell receptors internalized after cross-linking. Protein kinase C activation delayed in parallel the formation of SDS-stable forms of class II molecules and reduced the presentation of antigenic determinants requiring newly synthesized class II alphabeta-Ii complexes. These data indicate that B cell activation affects Ii processing and MHC class II peptide loading in endosomal compartments intersecting the biosynthetic pathway.

Related leucine-based cytoplasmic targeting signals in invariant chain and major histocompatibility complex class II molecules control endocytic presentation of distinct determinants in a single protein

Leucine-based signals in the cytoplasmic tail of invariant chain (Ii) control targeting of newly synthesized major histocompatibility complex class II molecules to the endocytic pathway for acquisition of antigenic peptides. Some protein determinants, however, do not require Ii for effective class II presentation, although endocytic processing is still necessary. Here we demonstrate that a dileucine-based signal in the cytoplasmic tail of the class II beta chain is critical for this Ii-independent presentation. Elimination or mutation of this signal reduces the rate of re-entry of mature surface class II molecules into the endocytic pathway. Antigen presentation controlled by this signal does not require newly synthesized class II molecules and appears to involve determinants requiring only limited proteolysis for exposure, whereas the opposite is true for li-dependent determinants. This demonstrates that related leucine-based trafficking signals in li and class II control the functional presentation of protein determinants with distinct processing requirements, suggesting that the peptide binding sites of newly synthesized versus mature class II molecules are made available for antigen binding in distinct endocytic compartments under the control of these homologous cytoplasmic signals. This permits capture of protein fragments produced optimally under distinct conditions of pH and proteolytic activity.

How HLA-DM edits the MHC class II peptide repertoire: survival of the fittest?

Loading of classical major histocompatibility complex (MHC) class II molecules with antigen-derived peptides is fast, efficient and highly selective in vivo, quite in contrast to in vitro findings with isolated class II proteins and synthetic peptides. Do accessory proteins speed up the loading process in antigen-presenting cells? Here, a model is presented in which the nonclassical MHC class II molecule HLA-DM plays a pivotal role as a chaperone, catalyst and editor during epitope selection.

MHC class II restricted antigen presentation

Presentation of antigenic peptides by MHC class II molecules to CD4(+) T cells requires many events in both the biosynthetic and endocytic pathways that must all occur in a controlled and coordinated fashion. In recent years the roles of two important chaperones, the invariant chain and the HLA-DM dimer, in promoting the acquisition of peptides by MHC class II molecules have largely been elucidated. The different compartments within the endosomal/lysosomal pathway that are involved in peptide loading are now being characterized. In addition to the specialized MHC class II compartments that exist in antigen-presenting cells, other intracellular compartments may also be involved in peptide loading. The precise mechanisms and intracellular sites of MHC class II peptide loading appear to dictate the nature of the T-cell epitopes presented by the antigen-presenting cell.

Origin, maturation and antigen presenting function of dendritic cells

Dendritic cells are cells specialized for antigen capture, migration and T cell stimulation. Recent advances have been made in understanding their origin, their heterogeneity, the mechanism of antigen uptake, and the signals that induce their migration and maturation into immunostimulatory antigen-presenting cells. Dendritic cells represent the natural adjuvants for T cell responses and their therapeutic exploitation in the near future is foreseen.

T helper subset differentiation in the absence of invariant chain

The outcome of murine infection with Leishmania major is regulated by major histocompatibility complex class II-restricted T helper cells. Invariant chain-deficient (Ii -/-) mice have impaired ability to present major histocompatibility complex class II-restricted antigens, and reduced numbers of CD4+ T cells. Despite these deficits, C57BL/6 Ii -/- mice controlled L. major infection comparably to wild-type mice. As assessed by mRNA analysis and in vitro antigen restimulation for IFN-gamma, Ii -/- mice had normal induction of Th1 subset differentiation even though antigen-dependent proliferation of their lymph node cells was substantially compromised. In addition, BALB/c Ii -/- mice exhibited a progressive course of infection and Th2 effector cell development that were comparable to that seen in wild-type BALB/c mice. We wished to determine whether this unexpected efficiency of T helper subset induction despite inefficient T cell stimulation could be modeled in vitro. In the presence of rIL-12 or rIL-4 naive parasite-specific transgenic T cells could mature into IFN-gamma-or IL-4-secreting T helper cells, respectively, even when antigen presentation was suboptimal or antigen dose was submitogenic. These experiments demonstrate that activation of T helper cells to a threshold required for IL-2 production or proliferation is not required to achieve induction of disease-regulating T helper cell effector functions, and that pathogen-associated secondary activation signals may facilitate the full differentiation of T helper subsets during limiting presentation of antigenic peptides.

CD21 augments antigen presentation in immune individuals

CD21 (complement receptor 2, CR2) binds the C3 degradation products, iC3b and C3d, which are covalently linked to antigen or immune complexes in the process of complement activation. The ability of antigen-nonspecific B cells to present immune complexes containing high titers of acquired antibodies was tested. Influenza virus was incubated with serum from immune donors to create complement-containing complexes. These bound specifically to CD21 on transfected fibroblasts and B cell lines, as measured by microcytofluorimetry. Binding of immune complexes was ablated by inactivation of serum complement. In addition, the immunoglobulin in immune human serum blocked influenza binding to cells in the absence of complement, implying a minimal role for immunoglobulin-Fc receptor interactions in this system. Significant immune complex binding required a threshold level of CD21 expression, suggesting that only those cells with the highest levels of CD21 are likely to participate in the processing of macromolecular antigens. B cells pulsed with complement-influenza complexes elicited an augmented response from a panel of influenza-specific, class II-restricted T cell clones, as compared with those which had bound immunoglobulin-influenza complexes lacking complement. This enhanced response did not require CD35. In addition, B cell lines expressing higher levels of CD21 were more efficient in processing antigen than those with lower levels. These findings suggest that presentation of antigen by B cells in immune individuals is dependent on the binding of complement-antigen immune complexes to CD21.

Capture and processing of exogenous antigens for presentation on MHC molecules

Class I and class II MHC molecules bind peptides during their biosynthetic maturation and provide a continuously updated display of intracellular and environmental protein composition, respectively, for scrutiny by T cells. Receptor-mediated endocytosis, phagocytosis, and macropinocytosis all contribute to antigen uptake by class II MHC-positive antigen-presenting cells. Capture of antigenic peptides by class II MHC molecules is facilitated because antigen catabolism and class II MHC maturation take place in the same compartments or in communicating compartments of the endosome/lysosome system. These class II MHC-rich, multivesicular endosomes receive incoming antigen and can support not only antigen processing and class II MHC peptide loading but also the export of peptide/class II MHC complexes to the cell surface. A balance between production and destruction of antigenic peptides is achieved by the activity of local proteases and may be influenced by binding of antigen to other proteins both prior to the onset of processing (e.g. antibodies) and during antigen unfolding (e.g. MHC molecules). T cell determinants that can be released for MHC binding without a substantial processing requirement may be able to utilize a distinct minor population of cell surface class II MHC molecules that become available during peripheral recycling. Although peptides derived from exogenous protein sources are usually excluded from presentation on class I MHC molecules, recent evidence shows that this embargo may be lifted in certain professional antigen-presenting cells to increase the spectrum of antigens that may be displayed on class I MHC.

Presentation of a cytosolic antigen by major histocompatibility complex class II molecules requires a long-lived form of the antigen

Class I and II molecules of the major histocompatibility complex present peptides to T cells. Class I molecules bind peptides that have been generated in the cytosol by proteasomes and delivered into the endoplasmic reticulum by the transporter associated with antigen presentation. In contrast, class II molecules are very efficient in the presentation of antigens that have been internalized and processed in endosomal/lysosomal compartments. In addition, class II molecules can present some cytosolic antigens by a TAP-independent pathway. To test whether this endogenous class II presentation pathway was linked to proteasome-mediated degradation of antigen in the cytosol, the N-end rule was utilized to produce two forms of the influenza virus matrix protein with different in vivo half-lives (10 min vs. 5 h) when expressed in human B cells. Whereas class I molecules presented both the short- and the long-lived matrix proteins, class II molecules presented exclusively the long-lived form of antigen. Thus, rapid degradation of matrix protein in the cytosol precluded its presentation by class II molecules. These data suggest that the turnover of long-lived cytosolic proteins, some of which is mediated by delivery into endosomal/ lysosomal compartments, provides a mechanism for immune surveillance by CD4+ T cells.