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

B-cell receptor dependent phagocytosis and presentation of particulate antigen by chronic lymphocytic leukemia cells

Aim

T-helper cells could play an important role in the pathogenesis of chronic lymphocytic leukemia (CLL), a common B-cell neoplasm. Although CLL cells can present soluble antigens targeted from the B-cell receptor to T-helper cells via major histocompatibility complex (MHC) class II, antigens recognized by some CLL cells may be encountered in a particulate form. Here the ability of CLL cells to internalize and present anti-immunoglobulin M (IgM) beads as a model for the interaction of CLL cells with particulate antigens was investigated.

Methods

The effect of anti-IgM beads on antigen presentation pathways was analyzed using RNA-seq and internalization of anti-IgM beads by primary CLL cells was investigated using confocal microscopy and flow cytometry. Antigen presentation was investigated by analyzing activation of a T-cell line expressing a T-cell receptor specific for a peptide derived from mouse κ light chains after incubating CLL cells with a mouse κ light chain-containing anti-IgM monoclonal antibody. Kinase inhibitors were used to characterize the pathways mediating internalization and antigen presentation.

Results

Stimulation of surface IgM of CLL cells increased expression of the antigen presentation machinery and CLL cells were able to phagocytose anti-IgM beads. Internalization of anti-IgM beads was associated with MHC class II-restricted activation of cognate T-helper cells. Antigen presentation by CLL cells was dependent on activity of spleen tyrosine kinase (SYK) and phosphatidylinositol 3-kinase delta (PI3Kδ) but was unaffected by inhibitors of Bruton's tyrosine kinase (BTK).

Conclusions

CLL cells can internalize and present antigen from anti-IgM beads. This capacity of CLL cells may be particularly important for recruitment of T-cell help in vivo in response to particulate antigens.

The intramembrane proteases signal Peptide peptidase-like 2a and 2b have distinct functions in vivo

We reported recently that the presenilin homologue signal peptide peptidase-like 2a (SPPL2a) is essential for B cell development by cleaving the N-terminal fragment (NTF) of the invariant chain (li, CD74). Based on this, we suggested that pharmacological modulation of SPPL2a may represent a novel approach to deplete B cells in autoimmune disorders. With regard to reported overlapping substrate spectra of SPPL2a and its close homologue, SPPL2b, we investigated the role of SPPL2b in CD74 NTF proteolysis and its impact on B and dendritic cell homeostasis. In heterologous expression experiments, SPPL2b was found to cleave CD74 NTF with an efficiency similar to that of SPPL2a. For in vivo analysis, SPPL2b single-deficient and SPPL2a/SPPL2b double-deficient mice were generated and examined for CD74 NTF turnover/accumulation, B cell maturation and functionality, and dendritic cell homeostasis. We demonstrate that in vivo SPPL2b does not exhibit a physiologically relevant contribution to CD74 proteolysis in B and dendritic cells. Furthermore, we reveal that both proteases exhibit divergent subcellular localizations in B cells and different expression profiles in murine tissues. These findings suggest distinct functions of SPPL2a and SPPL2b and, based on a high abundance of SPPL2b in brain, a physiological role of this protease in the central nervous system.

The intramembrane protease SPPL2a promotes B cell development and controls endosomal traffic by cleavage of the invariant chain

The intramembrane protease SPPL2a cleaves the NTF of invariant chain (CD74), which is essential for normal trafficking of MHC class II–containing endosomes and thus for B cell development and function.

Regulated intramembrane proteolysis is a central cellular process involved in signal transduction and membrane protein turnover. The presenilin homologue signal-peptide-peptidase-like 2a (SPPL2a) has been implicated in the cleavage of type 2 transmembrane proteins. We show that the invariant chain (li, CD74) of the major histocompatability class II complex (MHCII) undergoes intramembrane proteolysis mediated by SPPL2a. B lymphocytes of SPPL2a^−/− mice accumulate an N-terminal fragment (NTF) of CD74, which severely impairs membrane traffic within the endocytic system and leads to an altered response to B cell receptor stimulation, reduced BAFF-R surface expression, and accumulation of MHCII in transitional developmental stage T1 B cells. This results in significant loss of B cell subsets beyond the T1 stage and disrupted humoral immune responses, which can be recovered by additional ablation of CD74. Hence, we provide evidence that regulation of CD74-NTF levels by SPPL2a is indispensable for B cell development and function by maintaining trafficking and integrity of MHCII-containing endosomes, highlighting SPPL2a as a promising pharmacological target for depleting and/or modulating B cells.

Endosomally stored MHC class II does not contribute to antigen presentation by dendritic cells at inflammatory conditions

Major histocompatibility complex (MHC) class II (MHCII) is constitutively expressed by immature dendritic cells (DC), but has a short half-life as a consequence of its transport to and degradation in lysosomes. For its transfer to lysosomes, MHCII is actively sorted to the intraluminal vesicles (ILV) of multivesicular bodies (MVB), a process driven by its ubiquitination. ILV have, besides their role as an intermediate compartment in lysosomal transfer, also been proposed to function as a site for MHCII antigen loading and temporal storage. In that scenario, DC would recruit antigen-loaded MHCII to the cell surface in response to a maturation stimulus by allowing ILV to fuse back with the MVB delimiting membrane. Other studies, however, explained the increase in cell surface expression during DC maturation by transient upregulation of MHCII synthesis and reduced sorting of newly synthesized MHCII to lysosomes. Here, we have characterized the relative contributions from the biosynthetic and endocytic pathways and found that the vast majority of antigen-loaded MHCII that is stably expressed at the plasma membrane by mature DC is synthesized after exposure to inflammatory stimuli. Pre-existing endosomal MHCII contributed only when it was not yet sorted to ILV at the moment of DC activation. Together with previous records, our current data are consistent with a model in which passage of MHCII through ILV is not required for antigen loading in maturing DC and in which sorting to ILV in immature DC provides a one-way ticket for lysosomal degradation.

The Effect of B Cell Receptor Signaling on Antigen Endocytosis and Processing

B cell receptor (BCR)-mediated antigen processing and presentation involves both the BCR-mediated internalization and processing of cognate antigen as well as the formation and expression of antigenic peptide-MHC class II complexes. While BCR signaling is known to result in changes in the biosynthesis and intracellular trafficking of class II molecules, the effect of BCR signaling on the cell biology of antigen endocytosis and processing is less clear. Therefore, the effect of BCR signaling on the cell biology of fluid phase antigen endocytosis, processing and presentation was analyzed in both B cell lines or in normal splenic B cells. The results demonstrate that BCR signaling alters neither the global level of fluid phase antigen endocytosis nor the duration of intracellular persistence of fluid phase internalized antigen. Moreover, while BCR signal does result in an increase in the level of total cell surface MHC class II molecules as well as specific peptide-class II complexes, stimulation failed to alter the fraction of class II molecules loaded with antigen-derived peptide. These results indicate that while BCR-mediated signaling elicits an increase in the expression of antigenic peptide-class II complexes, signaling does not augment antigen presentation by profoundly altering the basic biology of antigen endocytosis and processing. These results also demonstrate that the high efficiency of BCR-mediated antigen processing (when compared to fluid phase antigen processing) is likely to occur independent of BCR signaling-induced global alterations in the biology of endocytosis, processing and presentation. This finding suggests that if BCR signaling augments the efficiency of processing of cognate antigen, it must impact unique aspects of BCR-mediated antigen processing, such as the intracellular persistence of internalized antigen-BCR complexes.

Multivesicular bodies in intestinal epithelial cells: responsible for MHC class II-restricted antigen processing and origin of exosomes

In normal conditions intestinal epithelial cells (IECs) constitutively stimulate regulatory CD4(+) T cells. However, in Crohn's disease (CD), this major histocompatibility complex (MHC) class II-restricted antigen presentation results in stimulation of proinflammatory CD4(+) T cells. We hypothesized that these alternative functions might be mediated by differential sorting and processing of antigens into distinct MHC II-enriched compartments (MIICs). Accordingly, we analysed the endocytic pathways of lumenally applied ovalbumin (OVA) in IECs of the jejunum and ileum of wild-type (WT) and TNFDeltaARE/WT mice that develop a CD-resembling ileitis. Using quantitative reverse transcription polymerase chain reaction, we found that messenger RNA levels of interferon-gamma, tumour necrosis factor-alpha, interleukin-17 and interleukin-10 were significantly up-regulated in the inflamed ileum of TNFDeltaARE/WT mice, confirming CD-like inflammation. Fluorescence and immunoelectron microscopy revealed the presence of MHC II and invariant chain throughout the late endocytic compartments, with most molecules concentrated in the multivesicular bodies (MVB). OVA was targeted into MVB and, in contrast to other MIICs, accumulated in these structures within 120 min of exposure. The IEC-specific A33 antigen localized to internal vesicles of MVB and A33/class II-bearing exosomes were identified in intercellular spaces. Remarkably, the expression pattern of MHC II/invariant chain molecules and the trafficking of OVA were independent of mucosal inflammation and the specific region in the small intestine. MVB seem to be principally responsible for class II-associated antigen processing in IECs and to constitute the origin of MHC II-loaded exosomes. The distinctive functions of IECs in antigen presentation to CD4(+) T cells might arise as a result of differential processing within the MVB identified here.

Role of protein tyrosine kinase and Erk1/2 activities in the Toll-like receptor 2-induced cellular activation of murine B cells by neisserial porin

Neisserial porins are potent immune adjuvants and have been demonstrated to stimulate and induce the activation of human and murine B lymphocytes. Their immunopotentiating ability is due largely to the upregulation of the surface expression of the costimulatory ligand CD86 (B7-2) on B cells and other antigen-presenting cells. Porin-induced activation is dependent on the innate immune pattern recognition receptor Toll-like receptor 2 (TLR2). These data have led us to investigate the signal transduction events induced by PorB from Neisseria meningitidis and then, using inhibitors of these pathways, to establish the mechanism by which this bacterial major outer membrane protein induces CD86 upregulation and the proliferation of murine B cells. PorB was able to induce (i) protein tyrosine kinase (PTK) activity, (ii) the phosphorylation of Erk1 and Erk2, and (iii) IkappaB-alpha phosphorylation, leading to NF-kappaB nuclear translocation in B cells in a TLR2-dependent manner. PorB-induced NF-kappaB nuclear translocation was not dependent on either PTK or Erk1/2 activities. However, B-cell proliferation and the induction of increased surface expression of CD86 by PorB were dependent on PTK activity and not Erk1/2 activation. In conclusion, PorB acts through TLR2 as a B-cell mitogen, triggering tyrosine phosphorylation of various cellular proteins that are involved in proliferation and CD86 expression, as well as the phosphorylation of Erk1/2, which is not necessary for CD86 upregulation or the proliferation of B cells.

Dendritic cells regulate exposure of MHC class II at their plasma membrane by oligoubiquitination

Dendritic cells (DCs) initiate adaptive immune responses by activating T cells via cognate interactions between MHC-peptide complexes and T cell receptors. In immature DCs, MHC class II is predominantly stored in late endocytic compartments, where it has a short half-life because of degradation. In contrast, mature DCs recruit MHC class II to the plasma membrane. We here demonstrate that in immature DCs, the beta-chain of MHC class II was oligoubiquitinated after proteolytic processing of the associated invariant chain in endosomes and that this modification was required for efficient endocytosis and sorting into luminal vesicles of multivesicular bodies. Ubiquitination of MHC class II was suppressed in lipopolysaccharide-activated DCs. Mutated MHC class II lacking its ubiquitination site was expressed at the plasma membrane, irrespective of DC maturation. Together, these data provide a molecular basis for the regulation of MHC class II-mediated antigen presentation by DCs.

Exosomes contain ubiquitinated proteins

Multivesicular bodies (MVB) are endosomal compartments that contain multiple vesicles, which derive from a delimiting membrane by inward budding. Incorporation of membrane proteins into the luminal vesicles requires, at least for some model proteins, monoubiquitination of their cytoplasmic domain. The ubiquitin tags are recognized by a sorting machinery, of which some components are also monoubiquitinated. The ubiquitin tags and the sorting machinery are both removed before the vesicles bud into the MVB lumen. MVB vesicles are therefore not expected to contain monoubiquitinated proteins. The MVB content is degraded upon fusion of MVB with lysosomes. In many cell types, however, MVB can also fuse with the plasma membrane, resulting in secretion of their luminal vesicles into the extracellular milieu. Such secreted vesicles are termed exosomes, and their protein composition should, due to their origin, be identical to that of MVB luminal vesicles. We here demonstrate that exosomes contain polyubiquitinated proteins, many of which are not integrated into the membrane and relatively enriched as compared to total cell lysates. These results suggest that a subset of polyubiquitinated cytoplasmic proteins is incorporated into the MVB pathway. The potential cell biological relevance of this observation is discussed. Furthermore, these data indicate that ubiquitinated proteins can serve as markers for exosomes.

Phosphoinositide 3-kinase activation by Igbeta controls de novo formation of an antigen-processing compartment

Antigens that bind B cell antigen receptor (BCR) are preferentially and rapidly processed for antigen presentation. The BCR is a multimeric complex containing a signaling module composed of Igalpha and Igbeta. Signaling pathways implicated in antigen presentation through the BCR are ill defined. Here we demonstrate that phosphoinositide 3-kinase (PI3K) inhibitors preclude antigen presentation induced by BCR or Igbeta but not Igalpha. Unraveling the mechanisms responsible for this inhibition, we show that PI3K inhibitors block neither antigen internalization nor degradation. Rather PI3K inhibitors block de novo formation of a multivesicular antigen processing compartment, which is induced by triggering of the BCR or Igbeta. Strikingly, we found using fluorescent probes binding specifically to PI3K products that BCR and Igbeta but not Igalpha induce PI3K activation in endocytic compartments wherein antigen is transported. Altogether, these results strongly suggest that Igbeta couples the BCR to PI3K activation that is instrumental for de novo formation of the antigen processing compartment and efficient antigen presentation.

Receptor-facilitated antigen presentation requires the recruitment of B cell linker protein to Igalpha

Ags that cross-link the B cell Ag receptor are preferentially and rapidly delivered to the MHC class II-enriched compartment for processing into peptides and subsequent loading onto MHC class II. Proper sorting of Ag/receptor complexes requires the recruitment of Syk to the phosphorylated immunoreceptor tyrosine-based activation motif tyrosines of the B cell Ag receptor constituent Igalpha. We postulated that the Igalpha nonimmunoreceptor tyrosine-based activation motif tyrosines, Y(176) and Y(204), contributed to receptor trafficking. Igalpha(YDeltaF(176,204))/Igbeta receptors were targeted to late endosomes, but were excluded from the vesicle lumen and could not facilitate the presentation of Ag to T cells. Subsequent analysis demonstrated that phosphorylation of Y(176)/Y(204) recruited the B cell linker protein, Vav, and Grb2. Reconstitution of Igalpha(YDeltaF(176,204))/Igbeta with the B cell linker protein rescued both receptor-facilitated Ag presentation and entry into the MHC class II-enriched compartment. Thus, aggregation accelerates receptor trafficking by recruiting two separate signaling modules required for transit through sequential checkpoints.

Dynamics of major histocompatibility complex class II compartments during B cell receptor-mediated cell activation

Antigen recognition by clonotypic B cell receptor (BcR) is the first step of B lymphocytes differentiation into plasmocytes. This B cell function is dependent on efficient major histocompatibility complex (MHC) class II-restricted presentation of BcR-bound antigens. In this work, we analyzed the subcellular mechanisms underlying antigen presentation after BcR engagement on B cells. In quiescent B cells, we found that MHC class II molecules mostly accumulated at the cell surface and in an intracellular pool of tubulovesicular structures, whereas H2-M molecules were mostly detected in distinct lysosomal compartments devoid of MHC class II. BcR stimulation induced the transient intracellular accumulation of MHC class II molecules in newly formed multivesicular bodies (MVBs), to which H2-M was recruited. The reversible downregulation of cathepsin S activity led to the transient accumulation of invariant chain-MHC class II complexes in MVBs. A few hours after BcR engagement, cathepsin S activity increased, the p10 invariant chain disappeared, and MHC class II-peptide complexes arrived at the plasma membrane. Thus, BcR engagement induced the transient formation of antigen-processing compartments, enabling antigen-specific B cells to become effective antigen-presenting cells.

Epstein-Barr virus transactivates the human endogenous retrovirus HERV-K18 that encodes a superantigen

Superantigens (SAgs) are proteins produced by pathogenic microbes to elicit potent, antigen-independent T cell responses that are believed to enhance the microbes' pathogenicity. Here we show that the human lymphotropic herpesvirus Epstein-Barr virus (EBV) transcriptionally activates the env gene of an endogenous retrovirus, HERV-K18, that possesses SAg activity. SAg activity was demonstrated by MHC class II dependent preferential activation of TCRVB13 T cells in response to murine B cells transfected with the HERV-K18 env gene. This is a unique demonstration of a pathogen inducing a host-encoded Sag and accounts for the previously described EBV associated Sag activity. The T cell activation elicited by the Sag could play a central role in EBV infection and associated diseases.

Reorganization of multivesicular bodies regulates MHC class II antigen presentation by dendritic cells

Immature dendritic cells (DCs) sample their environment for antigens and after stimulation present peptide associated with major histocompatibility complex class II (MHC II) to naive T cells. We have studied the intracellular trafficking of MHC II in cultured DCs. In immature cells, the majority of MHC II was stored intracellularly at the internal vesicles of multivesicular bodies (MVBs). In contrast, DM, an accessory molecule required for peptide loading, was located predominantly at the limiting membrane of MVBs. After stimulation, the internal vesicles carrying MHC II were transferred to the limiting membrane of the MVB, bringing MHC II and DM to the same membrane domain. Concomitantly, the MVBs transformed into long tubular organelles that extended into the periphery of the cells. Vesicles that were formed at the tips of these tubules nonselectively incorporated MHC II and DM and presumably mediated transport to the plasma membrane. We propose that in maturing DCs, the reorganization of MVBs is fundamental for the timing of MHC II antigen loading and transport to the plasma membrane.

Presentation of antigens by MHC class II molecules: getting the most out of them

The function of MHC class II molecules is to bind peptides derived from antigens that access the endocytic route of antigen presenting cells and display them on the plasma membrane for recognition by CD4(+) T cells. Formation of the MHC II-peptide complexes entails the confluence of the antigens and the MHC II molecules in the same compartments of the endocytic route. There, both the antigens and the MHC II molecules undergo a series of orchestrated changes that involve proteases, other hydrolases and chaperones, culminating in the generation of a wide repertoire of MHC II-peptide combinations. All the events that lead to formation of MHC II-peptide complexes show a considerable degree of flexibility; this lack of strict rules is advantageous in that it provides T cells with the maximum amount of information, ensuring that pathogens do not go undetected.

The control and facilitation of MHC class II antigen processing by the BCR

Antigens capable of cross-linking the BCR are preferentially captured, processed and presented to MHC-class-II-restricted T cells. Cross-linking antigens initiate tyrosine-kinase-dependent pathways that accelerate the delivery of antigen-receptor complexes to specialized late-endocytic processing compartments. Accelerated trafficking is mediated by the recruitment of signaling molecules required for transience through specific checkpoints along the endocytic pathway.

Early endosomal maturation of MHC class II molecules independently of cysteine proteases and H-2DM

Major histocompatibility complex (MHC) class II molecules bind and present to CD4(+) T cells peptides derived from endocytosed antigens. Class II molecules associate in the endoplasmic reticulum with invariant chain (Ii), which (i) mediates the delivery of the class II-Ii complexes into the endocytic compartments where the antigenic peptides are generated; and (ii) blocks the peptide-binding site of the class II molecules until they reach their destination. Once there, Ii must be removed to allow peptide binding. The bulk of Ii-class II complexes reach late endocytic compartments where Ii is eliminated in a reaction in which the cysteine protease cathepsin S and the accessory molecule H-2DM play an essential role. Here, we here show that Ii is also eliminated in early endosomal compartments without the intervention of cysteine proteases or H-2DM. The Ii-free class II molecules generated by this alternative mechanism first bind high molecular weight polypeptides and then mature into peptide-loaded complexes.

Intracellular traffic to compartments for MHC class II peptide loading: signals for endosomal and polarized sorting

In this review we focus on the traffic of MHC class II and endocytosed antigens to intracellular compartments where antigenic peptides are loaded. We also discuss briefly the nature of the peptide loading compartment and the sorting signals known to direct antigen receptors and MHC class II and associated molecules to this location. MHC class II molecules are expressed on a variety of polarized epithelial and endothelial cells, and polarized cells are thus potentially important for antigen presentation. Here we review some cell biological aspects of polarized sorting of MHC class II and the associated invariant chain and the signals that are involved in the sorting process to the basolateral domain. The molecules involved in sorting and loading of peptide may modulate antigen presentation, and in particular we discuss how invariant chain may change the cellular phenotype and the kinetics of the endosomal pathway.

Phosphorylation of the Invariant Chain by Protein Kinase C Regulates MHC Class II Trafficking to Antigen-Processing Compartments

The invariant chain (Ii) plays a critical role in the transport of newly synthesized class II molecules to endosomal Ag-processing compartments. Of the two major isoforms of human Ii, only Ii-p35 is phosphorylated in vivo, and inhibiting Ii phosphorylation inhibits the trafficking of newly synthesized class II molecules to Ag-processing compartments. We now report that a member of the protein kinase C family of serine/threonine kinases is responsible for the constitutive phosphorylation of 50% of the total cellular pool of Ii-p35 in a wide variety of APCs, including B lymphocytes, PBMC, immature dendritic cells, and mature dendritic cells. Stimulation of protein kinase C activity in APCs significantly enhanced the kinetics of degradation of class II-associated Ii in Ag-processing compartments and the binding of antigenic peptides to these class II molecules. In cells expressing an Ii-phosphorylation mutant, trafficking of class II molecules to endosomes was impaired and Ii proteolysis was inhibited, demonstrating a direct effect of Ii phosphorylation on MHC class II trafficking. These results demonstrate that phosphorylation of Ii in APCs alters the kinetics of trafficking of newly synthesized class II molecules to lysosomal Ag-processing compartments.

Constitutive cell surface association between CD4 and CCR5

HIV-1 entry into cells involves formation of a complex between gp120 of the viral envelope glycoprotein (Env), a receptor (CD4), and a coreceptor. For most strains of HIV, this coreceptor is CCR5. Here, we provide evidence that CD4 is specifically associated with CCR5 in the absence of gp120 or any other receptor-specific ligand. The amount of CD4 coimmunoprecipitated with CCR5 was significantly higher than that with the other major HIV coreceptor, CXCR4, and in contrast to CXCR4 the CD4-CCR5 coimmunoprecipitation was not significantly increased by gp120. The CD4-CCR5 interaction probably takes place via the second extracellular loop of CCR5 and the first two domains of CD4. It can be inhibited by CCR5- and CD4-specific antibodies that interfere with HIV-1 infection, indicating a possible role in virus entry. These findings suggest a possible pathway of HIV-1 evolution and development of immunopathogenicity, a potential new target for antiretroviral drugs and a tool for development of vaccines based on Env-CD4-CCR5 complexes. The constitutive association of a seven-transmembrane-domain G protein-coupled receptor with another receptor also indicates new possibilities for cross-talk between cell surface receptors.

Presentation of Antigens Internalized Through the B Cell Receptor Requires Newly Synthesized MHC Class II Molecules

Exogenous Ags taken up from the fluid phase can be presented by both newly synthesized and recycling MHC class II molecules. However, the presentation of Ags internalized through the B cell receptor (BCR) has not been characterized with respect to whether the class II molecules with which they become associated are newly synthesized or recycling. We show that the presentation of Ag taken up by the BCR requires protein synthesis in splenic B cells and in B lymphoma cells. Using B cells transfected with full-length I-Ak molecules or molecules truncated in cytoplasmic domains of their α- or β-chains, we further show that when an Ag is internalized by the BCR, the cytoplasmic tails of class II molecules differentially control the presentation of antigenic peptides to specific T cells depending upon the importance of proteolytic processing in the production of that peptide. Integrity of the cytoplasmic tail of the I-Ak β-chain is required for the presentation of the hen egg lysozyme determinant (46–61) following BCR internalization, but that dependence is not seen for the (34–45) determinant derived from the same protein. The tail of the β-chain is also of importance for the dissociation of invariant chain fragments from class II molecules. Our results demonstrate that Ags internalized through the BCR are targeted to compartments containing newly synthesized class II molecules and that the tails of class II β-chains control the loading of determinants produced after extensive Ag processing.

Engagement of B Cell Receptor Regulates the Invariant Chain-Dependent MHC Class II Presentation Pathway

The intracellular sites in which Ags delivered by the B cell receptor (BCR) are degraded and loaded onto class II molecules remain poorly defined. To address this issue, we generated wild-type and invariant chain (Ii)-deficient H-2k mice bearing BCR specific for hen egg lysozyme. Our results show that, 1) unlike Ags taken up from the fluid phase, Ii is required for presentation of hen egg lysozyme internalized through the BCR in a manner independent of the peptide analyzed; 2) BCR ligation induces intracellular accumulation of MHC class II molecules only in Ii-positive B cells; and 3) these class II molecules reach intracellular compartments where BCR targets exogenous Ag. No differences in expression of adhesion and costimulatory molecules or in the presentation of soluble peptides were detectable between Ii-positive and -negative B cells. Therefore, the BCR delivers its ligand to compartments containing MHC class II-Ii complexes and bypasses the Ii-independent presentation pathway. The linked roles of Ag internalization and B cell activation of the BCR leads to potent Ii-dependent presentation in splenic B cells.

Efficient Presentation of Multivalent Antigens Targeted to Various Cell Surface Molecules of Dendritic Cells and Surface Ig of Antigen-Specific B Cells

To study the relation between the form of an Ag and the response to it, we compared presentation in vitro with hen egg lysozyme (HEL)-specific T cells from TCR transgenic mice of free HEL and liposome-encapsulated HEL by different APC. HEL-specific splenic B cells or bone marrow-derived dendritic cells were incubated with free HEL or HEL-containing liposomes targeted by Ab to either surface Ig, the Fc receptor, or MHC class I and II molecules. Ag presentation by HEL-specific B cells was at least 100-fold more efficient for HEL in surface Ig-targeted liposomes than free HEL taken up by the same receptor or HEL in liposomes targeted to class I or II molecules. Ag presentation by dendritic cells from Fc receptor-targeted vesicles was augmented 1,000–10,000-fold compared with free Ag or nontargeted liposomes, but presentation was also efficient when Ag was targeted to class I or II molecules. These results indicate that Ag-specific B cells and dendritic cells can be equally efficient in stimulating IL-2 production by Ag-specific T cells from unimmunized TCR transgenic mice when the Ag is multivalent and taken up by appropriate receptors. In contrast to B cells, which require engagement of surface Ig for optimal presentation, dendritic cells may present Ag by means of several different cell surface molecules.

Actin microfilaments control the MHC class II antigen presentation pathway in B cells

Newly synthesised major histocompatibility complex class II molecules associate with invariant chains (Ii) to form nonameric complexes. These complexes are transported to endosomes, where proteolytic enzymes generate alphabeta class II dimers associated with nested Ii-derived peptides. These peptides are then exchanged with antigen peptide, and mature class II molecules reach the cell surface. The role of the actin cytoskeleton in the transport and maturation of class II molecules has not been studied. We show here that upon treatment with cytochalasin D (cyto D), the rate of Ii degradation is drastically reduced in B cells. Cyto D treatment also leads to a delayed appearance of stable forms of class II molecules, and a reduced presentation efficiency of antigen determinants requiring newly synthesised class II molecules. Under such conditions, we found that invariant chain fragments and class II molecules are accumulated in early and late endosomal compartments, whereas the leupeptin protease inhibitor induces their accumulation in lysosomal compartments. The addition of cyto D to leupeptin blocks the delivery of class II/invariant chain complexes to lysosomes, and further inhibits degradation of Ii. The dynamics of the actin cytoskeleton can therefore control the meeting point between newly synthesised class II molecules and lysosomal proteases, involved in Ii degradation and antigen peptide loading.

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.

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.