Deletion of a C-terminal sequence of the class II-associated invariant chain abrogates invariant chains oligomer formation and class II antigen presentation

The MHC class II-associated invariant chain (Ii) is involved in Ag processing and presentation. Physical association of MHC class II molecules with Ii and an effect of Ii on peptide loading to class II have been demonstrated, but to date these functions have not been related to a particular region of Ii. We investigated luminal deletion mutants of Ii and their role in Ag processing and presentation. IAk-expressing L cells were transfected with deletion mutants of the Ii gene and assayed for their ability to present hen egg lysozyme to three different T cell hybridomas. It is shown that the sequence aa 131-191 of Ii is important for the presentation of native hen egg lysozyme. In addition, this C terminal region is shown to be responsible for Ii oligomer formation. It is therefore conceivable that oligomer formation of Ii is a prerequisite for class II-restricted Ag processing and presentation.

Deletion of a C-terminal sequence of the class II-associated invariant chain abrogates invariant chains oligomer formation and class II antigen presentation

The MHC class II-associated invariant chain (Ii) is involved in Ag processing and presentation. Physical association of MHC class II molecules with Ii and an effect of Ii on peptide loading to class II have been demonstrated, but to date these functions have not been related to a particular region of Ii. We investigated luminal deletion mutants of Ii and their role in Ag processing and presentation. IAk-expressing L cells were transfected with deletion mutants of the Ii gene and assayed for their ability to present hen egg lysozyme to three different T cell hybridomas. It is shown that the sequence aa 131-191 of Ii is important for the presentation of native hen egg lysozyme. In addition, this C terminal region is shown to be responsible for Ii oligomer formation. It is therefore conceivable that oligomer formation of Ii is a prerequisite for class II-restricted Ag processing and presentation.

Mode of entry of morbilliviruses

The morbilliviruses have a restricted host range. This is probably dependent on the use of specific host cell receptors. In the present article, we have reviewed our approach to identify a host cell receptor for one of the morbilliviruses, measles virus and to elucidate the interaction between viral and cellular proteins during virus entry into the host cell.

Glycosyl-phosphatidylinositol-anchored and transmembrane forms of CD46 display similar measles virus receptor properties: virus binding, fusion, and replication; down-regulation by hemagglutinin; and virus uptake and endocytosis for antigen presentation by major histocompatibility complex class II molecules

The CD46 molecule is a receptor for measles virus (MV), CD46, which protects autologous cells from complement-mediated damage, exists in several isoforms which are variably expressed in different human tissues. These isoforms differ in their cytoplasmic and transmembrane regions and in a small portion of their proximal extracytoplasmic regions. To examine the role of the cytoplasmic and transmembrane regions of CD46 in MV infection, mouse M12 B cells stably expressing a transmembrane or a chimeric glycosyl-phosphatidylinositol (GPI)-anchored form of CD46 (CD46-GPI) were used. Both the GPI-anchored and transmembrane CD46 forms were able to mediate MV binding. MV binding mediated by the GPI-anchored form but not that mediated by the transmembrane form was abolished after treatment with phosphatidylinositol phospholipase C. MV infection of both M12.CD46 and M12.CD46-GPI cells but not parental M12 cells resulted in MV replication. Expression of hemagglutinin induced cell surface down-regulation of both CD46 and CD46-GPI. Both M12.CD46 and M12.CD46-GPI cells were able to efficiently capture MV for presentation of viral antigens by major histocompatibility complex class II molecules to T cells. This presentation was blocked by chloroquine, indicating some virus endocytosis. These data imply that the extracytoplasmic region encompassing the four N-terminal invariable short consensus repeat regions of CD46 is sufficient to act as a receptor for MV and that the cytoplasmic and transmembrane regions of CD46 may not play a major role in the signal for the hemagglutinin-induced down-regulation of CD46 and/or endocytosis of MV.

Measles virus receptor properties are shared by several CD46 isoforms differing in extracellular regions and cytoplasmic tails

Human CD46, a member of the family of regulators of complement activation, has been shown recently to act as a measles virus (MV) receptor, interacting with the virus envelope glycoprotein haemagglutinin (HA). Owing to alternative RNA splicing, several CD46 isoforms are co-expressed in all tissues except erythrocytes. The optional exons encode extracellular serine-, threonine- and proline-rich regions of CD46 (designated STP-A, -B and -C) which are located proximal to the plasma membrane, and alternatively cytoplasmic tails (CYT1 or CYT2). The ability of the BC-CYT2, B-CYT2 and BC-CYT1 CD46 isoforms, expressed in rodent Chinese hamster ovary (CHO) cells, to mediate MV infection was tested. Every isoform was recognized by a monoclonal antibody (MAb), MCI20.6, which recognizes the MV-binding site on CD46. CHO cells expressing any of these CD46 isoforms were able to bind MV, the level of binding correlating with the CD46 expression level. Likewise, MV infection induced the cell-cell fusion of all CD46-expressing CHO cells but not of the parental CHO cells. Accordingly, MV replication was observed after infection of CHO cells expressing each CD46 isoform but not after infection of parental CHO cells. Finally, cell surface expression of every isoform was decreased after infection by MV. Altogether these data showed that the specific STP regions of CD46 played no major role in HA-mediated MV binding to CD46, virus infection and virus-induced down-regulation of CD46. Moreover, the CYT1 and CYT2 cytoplasmic tails of CD46 are either functionally similar although having distinct amino acid sequences or are dispensable for interaction with HA of MV.

Efficient MHC class II-restricted presentation of measles virus to T cells relies on its targeting to its cellular receptor human CD46 and involves an endosomal pathway

The role of the measles virus (MV) receptor, human CD46, in the uptake of MV and antigen presentation by Major Histocompatibility Complex (MHC) class II molecules was investigated. Expression of CD46 in murine B cells resulted in cells highly efficient in capturing UV-inactivated MV particles and presenting both envelope hemagglutinin H and nucleoprotein N to specific T cell hybridomas. Although MV fuse with the plasma membrane of its target cells, presentation of both MV-H and -N was sensitive to inhibition by chloroquine but was not affected by a tripeptide which prevents virus-cell fusion. Whereas 50 microM of chloroquine was required to inhibit presentation of MV-H, purified H or soluble N, only a two-fold lower concentration was required to inhibit that of MV-N. This shows that some CD46-mediated captured MV particles are endocytosed, then disrupted and processed in an endosome/lysosome compartment.

Critical residue combinations dictate peptide presentation by MHC class II molecules

Peptides encompassing the core hen egg lysozyme HEL(52-61) peptide elongated or not and substituted or not with natural and unnatural amino acids were used to find a peptide motif for binding to the major histocompatibility complex (MHC) class II I-Ak. Using a T-cell recognition functional assay, nine out of 10 positions were found to be somehow involved in the I-Ak binding, and six out of 10 residues were involved in T-cell recognition. The deleterious effect of single substitutions could be rescued by changing peptide length and/or sequence. Thus, efficient binding to MHC class II molecules requires not only few anchoring residues correctly interspaced, but a complex, nonrandom combination of residues with appropriate orientation of the peptide backbone and some crucial side chains.

Efficient major histocompatibility complex class II-restricted presentation of measles virus relies on hemagglutinin-mediated targeting to its cellular receptor human CD46 expressed by murine B cells

Measles virus after binding to its cell surface human CD46 receptor fuses with the plasma membrane. This fusion results in envelope hemagglutinin (H) and fusion glycoprotein (F) incorporated into the plasma membrane and injection of the nucleocapsid made of nucleoprotein (NP) into the cytosol. The influence of targeting measles virus (MV) to CD46 in the processing and presentation of MV H and NP to antigen specific MHC class II I-E(d)- and I-A(d)-restricted T cell hybridomas was explored using murine M12-CD46 B cell transfectants. Parent M12 cells, which lack any MV receptor, were unable to present any of these two viral proteins when incubated with MV particles. Incubating M12.CD46 cells with 200 ng and 10 micrograms of MV could strongly stimulate H-specific and NP-specific T cells, respectively. Neosynthesis of MV proteins was not necessary since the efficiency of antigen presentation was similar when using ultraviolet-inactivated MV. Similar enhancing effects (more than 1,000-fold) on antigen presentation were also observed when using purified native H soluble or incorporated into liposomes whereas denaturating H glycoprotein resulted in a poor efficiency in T cell stimulation, M12.CD46 being no more potent than the parental M12 counterpart. MV H and NP presentation efficiency did not depend on MV fusion with plasma membrane as revealed by the lack of effect of specific fusion inhibitors. Both MV H and NP presentations were sensitive to chloroquine inhibition indicating that antigens from CD46-mediated captured MV were likely processed in the endosome/lysosome compartment. Altogether these data indicate that (a) MV targeting via CD46 has a strong effect on the efficiency of antigen presentation by MHC class II, (b) the effect is mediated by the binding of H to CD46, and (c) though MV does fuse with plasma membrane, endocytosis, and processing of virus particles are also occurring. Since, in humans, CD46 is expressed in almost every tissue including professional antigen-presenting cells, such a targeting is likely to play a crucial role in the CD4+ T cell-mediated primary immune response against the pathogen in vivo.

Major histocompatibility complex class II-restricted presentation of secreted and endoplasmic reticulum resident antigens requires the invariant chains and is sensitive to lysosomotropic agents

We have tested the involvement of the invariant chains (Ii) p31 and p41 in the presentation of peptides derived from hen egg lysozyme (HEL) constructs targeted to different intracellular compartments within transfected fibroblasts. The endogenous HEL constructs were either present in the cytosol (HELc), secreted (HELs), or linked to the mammalian (KDEL C-terminal sequence that causes retention of HEL in the endoplasmic reticulum (ER)/pre-Golgi recycling compartment (HELr). Using Ii-negative antigen-presenting cells, the presentation of HELr to a HEL 46-61 specific T cell hybridoma was far less efficient than the presentation of the HELs. High levels of Ii expression enhanced drastically the presentation of the HEL 46-61 determinant derived from both HELr and HELs. HELr and HELs presentation was fully sensitive to lysosomotropic agents such as chloroquine, indicating that the formation of complexes between major histocompatibility complex (MHC) class II molecules and determinants derived from endogenous antigens entering the secretory pathway is taking place in an acidic compartment. The degradation and dissociation of Ii might be a prerequisite for the efficient presentation of endogenously derived determinants by MHC class II molecules, as for the presentation of most exogenous antigens. All our results are compatible with the notion that endogenous molecules being translocated into the lumen of the ER could be presented by class II molecules through a processing pathway involving an acidic compartment in which Ii chains dissociate from class II molecules.

Inhibition by chloroquine of the class II major histocompatibility complex-restricted presentation of endogenous antigens varies according to the cellular origin of the antigen-presenting cells, the nature of the T-cell epitope, and the responding T cell

Chloroquine treatment of antigen-presenting cells (APC) was explored as a tool to investigate the processing pathway for major histocompatibility complex (MHC) class II-restricted presentation of the endogenous secreted hen egg lysozyme (HEL) and transmembrane measles virus haemagglutinin (HA). A 72-hr pretreatment of the APC with 25 microM chloroquine blocked the presentation of the HEL(52-61) T-cell epitope generated from endogenous HEL to the I-Ak-restricted 3A9 T-cell hybridoma by MHC class II-transfected L cells expressing the invariant chain (Ii). The presentation of exogenously added HEL peptides was not affected. Under the same conditions, no inhibition of the presentation of HEL(106-116) to the I-Ed-restricted G28 high-avidity T-cell hybridoma, nor of HA when synthesized by L cells, was observed. When B-lymphoid APC were used, inhibition was observed in every case with a low number of B APC pretreated for 48 hr with chloroquine prior to the T-cell stimulation test. Moreover, addition of chloroquine to untreated B APC during the T-cell stimulation assay was sufficient to inhibit completely the presentation of HEL(106-116) to the B10.D24.42 low avidity T-cell hybridoma. Altogether these studies suggest that an apparent resistance of endogenous Ag presentation to chloroquine inhibition may not necessarily indicate the existence of a non-endosomal pathway but may be due to the nature of the T-cell epitope, to the use of 'non-professional' APC such as L cells, to the use of T cells of high avidity, and to high amounts of pre-existing MHC class II-peptide complexes expressed by the APC. We demonstrate here that, at least in conventional APC such as B cells, class II-restricted presentation of both endogenous secreted HEL and transmembrane HA involves an endosomal pathway.

Human membrane cofactor protein (CD46) acts as a cellular receptor for measles virus

A monoclonal antibody (MCI20.6) which inhibited measles virus (MV) binding to host cells was previously used to characterize a 57- to 67-kDa cell surface glycoprotein as a potential MV receptor. In the present work, this glycoprotein (gp57/67) was immunopurified, and N-terminal amino acid sequencing identified it as human membrane cofactor protein (CD46), a member of the regulators of complement activation gene cluster. Transfection of nonpermissive murine cells with a recombinant expression vector containing CD46 cDNA conferred three major properties expected of cells permissive to MV infection. First, expression of CD46 enabled MV to bind to murine cells. Second, the CD46-expressing murine cells were able to undergo cell-cell fusion when both MV hemagglutinin and MV fusion glycoproteins were expressed after infection with a vaccinia virus recombinant encoding both MV glycoproteins. Third, M12.CD46 murine B cells were able to support MV replication, as shown by production of infectious virus and by cell biosynthesis of viral hemagglutinin after metabolic labeling of infected cells with [35S]methionine. These results show that the human CD46 molecule serves as an MV receptor allowing virus-cell binding, fusion, and viral replication and open new perspectives in the study of MV pathogenesis.

Measles virus haemagglutinin induces down-regulation of gp57/67, a molecule involved in virus binding

A surface glycoprotein (gp57/67) was previously shown to be involved in measles virus (MV) binding and characterized in our laboratory. Here, we described down-regulation of cell surface gp57/67 after infection with MV. This effect is specific for MV since cells infected with canine distemper virus, closely related to MV, did not down-regulate gp57/67. The decrease in cell surface gp57/67 correlated with expression of MV glycoproteins and more particularly with the expression of MV haemagglutinin (MV-H). Indeed, expression of MV-H after infection with a vaccinia virus recombinant coding for MV-H was necessary and sufficient to induce down-regulation of gp57/67. Kinetics of cell surface expression of MV-H and gp57/67 showed that the degree of down-regulation was correlated with the amount of MV-H expressed by infected cells. Experiments using antibody-prelabelled gp57/67 and indirect immunofluorescence microscopy allowed us to follow the fate of gp57/67 and showed that down-regulation was occurring by rapid internalization of gp57/67 from the cell surface. These results provide additional evidence that the gp57/67 molecule is closely associated with the pathway of MV infection and also reveal a phenomenon which may be related to viral pathogenesis and persistence.

Invariant chain expression similarly controls presentation of endogenously synthesized and exogenous antigens by MHC class II molecules

I-Ak- and I-Ed-transfected L fibroblasts were supertransfected with cDNA coding for hen egg lysozyme (HEL) or measles virus hemagglutinin (HA). Well-defined cell culture conditions allowed us to obtain L cells with either no detectable endogenous Ii mRNA or a high level of endogenous Ii mRNA induced by serum starvation. Cells supertransfected with mouse Ii chain gene stably expressing a high level of Ii chain were also used as APC in parallel experiments. Class II presentation of endogenously secreted HEL or an ER-retained form of HEL to the HEL-specific I-Ak-restricted 3A9 T cell hybridoma was found to be strongly enhanced in cell transfectants expressing Ii chain. Similar results were obtained with the presentation of transmembrane HA to the HA-specific I-Ed-restricted TH5.143 T cell hybridoma. These findings correlate with those obtained with the presentation of exogenous HEL and HA. In addition, as reported to be the case for exogenous antigen, expression of a large amount of endogenous HA by the APC supplants the requirement for Ii chain expression in the enhancement of antigen presentation. These data demonstrate that presentation by MHC class II molecules of a given antigen, either exogenously provided or endogenously synthesized, is controlled in a similar manner by the Ii chain.

Can one predict antigenic peptides for MHC class I-restricted cytotoxic T lymphocytes useful for vaccination?

The cytotoxic T-lymphocyte (CTL) response can be crucial for efficient immunological control of intracellular pathogens and the MHC class I-restricted CTL have a major role to play in this process. They recognize complexes associating antigen-derived peptides with MHC class I molecules expressed on infected target cells. The characterization of these antigenic peptides is thus a key issue for developing vaccines efficient in inducing specific CTL. Recently, by sequencing the whole set of self-peptides eluted from a given MHC class I molecule, Falk and colleagues have found that they have a homogeneous 8-10 residue length and contain allele-specific peptidic motifs with two conservative dominant anchor residues. The existence of consensus motifs opens the way for a strategy to predict the MHC class I-restricted T-cell epitopes and here we discuss such an approach using hen egg lysozyme (HEL) as an antigenic model. Two HEL peptides corresponding to allele-specific motifs were found, HEL(49-56) and HEL(70-78) peptides, which can associate with MHC class I H-2Kb and H-2Db molecules, respectively. The HEL peptide HEL(70-78) was found to be able to induce HEL-specific CTL in H-2b mice also. Moreover, using an empiricial approach, we have also characterized the N-terminal HEL(1-17) peptide as an immunodominant antigenic peptide in the H-2k haplotype. This peptide presented by H-2Kk molecules neither contained the corresponding allele-specific binding motif nor fitted the expected 8-10 residue length.(ABSTRACT TRUNCATED AT 250 WORDS)

A monoclonal antibody recognizes a human cell surface glycoprotein involved in measles virus binding

Measles virus (MV) has a very limited host range, humans being the only natural reservoir of the virus. This restriction may be due to the absence of an MV receptor on the surface of non-primate cells. We have studied the MV-binding ability of several cell lines and attempted to characterize the receptor by studying the binding of 35S-labelled MV and by a rosette formation technique. We confirmed that all the human cell lines examined (HeLa, Raji and Jurkat) bound MV and that the murine cell lines (BW and L) did not. The glycoprotein nature of the receptor activity was demonstrated by the fact that it could be removed from the cell membrane using proteolytic enzymes and by its failure to be re-expressed in the presence of a protein synthesis inhibitor or an N-glycosylation inhibitor. A monoclonal antibody isolated after immunization of mice with Raji cells specifically inhibited MV binding and infection of human cells, and recognized human and simian but not murine cells. Depending on the cell line (HeLa, Raji, Jurkat or Vero), this antibody immunoprecipitated one or two glycoproteins with apparent M(r)s of 57K and/or 67K from human and simian cells, but not from murine cells.

High efficiency of endogenous antigen presentation by MHC class II molecules

MHC class II molecules are involved in the presentation of both exogenous and endogenous antigens to CD4 T cells. Using the trans-membrane hemagglutinin (HA) from measles virus and the secreted hen egg lysozyme (HEL) as antigen models, we have compared the efficiency of MHC class II presentation by naive antigen presenting cells (APCs) pulsed with exogenous antigen with that of their transfected counterparts synthesizing endogenous antigen. B cells expressing even a very low amount of trans-membrane HA were found to present endogenous HA to I-Ed restricted T cell hybridomas with a high efficiency whereas their naive counterparts required to be pulsed with a comparatively high amount of exogenous HA. Similarly, MHC class II presentation of endogenous secreted HEL was found to be much more efficient when compared with that of exogenous HEL. Biochemical studies did not reveal any enhanced intracellular degradation of endogenous HEL. As expected, HEL was released in the surrounding medium within < 1 h. MHC class II presentation of endogenous HEL could not be explained by re-uptake by bystander APCs of HEL secreted in the surrounding medium. No sensitization of naive APCs could be observed either when co-cultured with HEL secreting cells or when cultured for 10 days with a sub-threshold amount of exogenous HEL. At the cell surface, I-Ed molecules immunoprecipitated from HEL secreting cells were found to be slightly enriched in SDS-resistant forms. These data raised the question of how peptides derived from endogenous transmembrane and secreted antigens can so efficiently reach an MHC class II loading compartment.

Correlation between invariant chain expression level and capability to present antigen to MHC class II-restricted T cells

In this study we investigated the role of the invariant chain (li) in the presentation of hen egg lysozyme (HEL) and measles virus hemagglutinin (HA) antigens to MHC class II-restricted T hybridoma cells. Fibroblastic cells transfected with Ed or Ak genes, and supertransfected or not with the li gene, were used as antigen-presenting cells (APC). For every APC pair analysed, the amount of exogenous antigen needed to obtain a T-cell response was inversely correlated with the level of li expression. Exogenously provided HEL was efficiently presented by li-supertransfected APC at doses of 10 micrograms/ml or below. In contrast, non-li transfected fibroblastic cells, which express a low level of endogenous li, required at least 10 times more HEL to stimulate most of the T hybridoma cells. Analogous results were also obtained using exogenous HA. Finally, two different experiments suggest that basal li expressed in fibroblastic cells is involved in the presentation of exogenous antigen. In the first one, we showed that li/class II ratio was increased in high-density grown fibroblastic cells and that this increase correlates with the ability of the cells to present exogenous antigen. In the second, treating high-density grown cells with an antisense li oligodeoxynucleotide could impair their ability to present exogenous HEL. We also examined the presentation of endogenously-synthesized HEL or HA after introduction of the antigen into the biosynthetic pathway of the APC by transfection of HEL and HA cDNAs. There was no apparent difference in the capability of high density grown fibroblastic cells, transfected or not with li gene, to present endogenous HEL or HA.(ABSTRACT TRUNCATED AT 250 WORDS)

Cytosolic targeting of hen egg lysozyme gives rise to a short-lived protein presented by class I but not class II major histocompatibility complex molecules

A way to study the role of intracellular trafficking of an antigen in its presentation to T cells is to target the antigen to various cell compartments of the antigen-presenting cells (APC) and compare the nature of the complexes associating major histocompatibility complex (MHC) molecules and antigenic peptides, expressed on the cell surface. MHC class I+ and MHC class II+ mouse L fibroblasts secreting hen egg lysozyme (HELs cells) or expressing HEL in their cytosol (HELc cells) were obtained after transfection with HEL cDNA and signal sequence-deleted HEL cDNA, respectively. HEL was evidenced in both HELs- and HELc-transfected cells and the former type of transfectant secreted a large amount of HEL. However, HEL produced in the cytosol exhibited a short half-life of less than 5 min. HEL-derived peptides could not be shown biochemically either in HELc- nor in HELs-transfected cells. We then studied the capacity of these cells to present HEL to HEL-specific class I- and class II-restricted T cells. Both cell types could be recognized by the HEL-specific MHC class I-restricted CTL clones. In contrast, MHC class II-HEL peptide complexes, recognized by HEL-specific helper T cell hybridomas, could be detected on MHC class II+ HELs- but not HELc-transfected cells. In vivo experiments showed, however, that HELc-transfected cells could provide host APC with HELc-derived peptides able to associate with MHC class II molecules. This was inferred from the capacity of MHC class II-HELc-transfected cells, unable by themselves to elicit any anti-HEL antibody response, to prime syngeneic and allogeneic mice against HEL. The priming was revealed by the induction of an antibody response after a boost with an amount of HEL unable itself to elicit an antibody response.

Generation of hen egg lysozyme-specific and major histocompatibility complex class I-restricted cytolytic T lymphocytes: recognition of cytosolic and secreted antigen expressed by transfected cells

Syngeneic cells exogenously supplied with hen egg lysozyme (HEL) or endogenously synthesizing HEL were used as antigen-presenting cells to induce major histocompatibility complex class I-restricted cytotoxic T lymphocytes (CTL). Immunization of C57BL/6 mice followed by repeated stimulation of their splenocytes in vitro with trypsinized HEL peptides led to the generation of CTL lines specific for trypsinized HEL peptides and restricted by H-2K. Immunization of C3H mice with a mixture of soluble native HEL and irradiated syngeneic spleen cells followed by in vitro stimulation of immune spleen cells with soluble HEL could in a few cases result in HEL-specific CTL able to kill syngeneic transfectant L cells secreting HEL (HELs) or expressing cytosol-targeted HEL (HELc). The use of HELs or HELc transfectant L cells as in vivo and in vitro immunogens was a potent way for eliciting HEL-specific polyclonal CTL. These CTL and two CD8+ clones were found to be H-2K restricted and specific for the 1-17 N-terminal HEL peptide. In addition, the anti-HEL CTL could also exhibit a significant cross-reactivity against unsensitized and HEL-untransfected targets expressing the K restriction element. This cross-reactivity was likely due to recognition of unidentified HEL mimicking peptides (self-derived?) presented by the MHC class I (H-2K or H-2K) molecule used as the restriction element for the specific recognition of HEL. The CTL raised after immunization with HELs or HELc transfectant cells were found to recognize both the HELs and HELc transfectant cells even though HEL was not detected in the latter after a 2- or 5-min radiolabeling pulse. Recognition of both HELs and HELc transfectant cells by a given CTL clone suggests that HEL subjected to two separate processing pathways, each depending on the initial subcellular localization, can ensure the generation of similar MHC class I peptide complexes.

Antigen processing--from cell biology to molecular interactions

Antigen presentation is now a central theme in molecular cell biology. A recent meeting focused on the developments in the field and their implications for future manipulation of the immune system. The following report was written from a round-table discussion involving a number of participants'.

Characterization of N-linked oligosaccharides of an HLA-DR molecule expressed in different cell lines

In order to determine the factors that influence the glycosylation of an integral membrane protein, we investigated the N-glycosylation of a molecule of the human major histocompatibility complex (MHC) class II, the HLA-DR antigen. This glycoprotein was studied in a human Epstein-Barr-virus-transformed B cell line and in a mouse fibroblastic cell line co-transfected with DR alpha and DR beta genes. We observed that the HLA-DR-antigen glycosylation pattern depends on the cell line in which processing takes place and is closely related to the glycosylation pattern of the overall cellular glycoproteins. Furthermore, when comparing the glycosylation of the separated alpha- and beta-chains, differences were noticed within the same molecule, showing the importance of the individual peptide backbone for the glycosylation process.

Working principles in the immune system implied by the "peptidic self" model

The hypothesis that self as well as foreign proteins are processed into peptides and presented by major histocompatibility complex antigens leads to a set of working principles that could govern cellular interactions in immune responses. In particular, "idiopeptides," derived from immunoglobulins and T-cell receptors and recognized by appropriate T cells, are expected to play an important regulatory role. We show here that these speculations fit into a consistent view of the immune system.