Structure of the murine Ia-associated invariant (Ii) chain as deduced from a cDNA clone

The transmembrane domain and luminal C-terminal region independently support invariant chain trimerization and assembly with MHCII into nonamers

Background

Invariant chain (CD74, Ii) is a multifunctional protein expressed in antigen presenting cells. It assists the ER exit of various cargos and serves as a receptor for the macrophage migration inhibitory factor. The newly translated Ii chains trimerize, a structural feature that is not readily understood in the context of its MHCII chaperoning function. Two segments of Ii, the luminal C-terminal region (TRIM) and the transmembrane domain (TM), have been shown to participate in the trimerization process but their relative importance and impact on the assembly with MHCII molecules remains debated. Here, we addressed the requirement of these domains in the trimerization of human Ii as well as in the oligomerization with MHCII molecules. We used site-directed mutagenesis to generate series of Ii and DR mutants. These were transiently transfected in HEK293T cells to test their cell surface expression and analyse their interactions by co-immunoprecipitations.

Results

Our results showed that the TRIM domain is not essential for Ii trimerization nor for intracellular trafficking with MHCII molecules. We also gathered evidence that in the absence of TM, TRIM allows the formation of multi-subunit complexes with HLA-DR. Similarly, in the absence of TRIM, Ii can assemble into high-order structures with MHCII molecules.

Conclusions

Altogether, our data show that trimerization of Ii through either TM or TRIM sustains nonameric complex formation with MHCII molecules.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12865-021-00444-6.

The invariant chain p35 isoform promotes formation of nonameric complexes with MHC II molecules

Four different isoforms of the human invariant chain (Ii) have been described (p33, p35, p41 and p43). These heterotrimerize in the endoplasmic reticulum (ER) before associating with MHC class II molecules (MHCIIs). However, the final stoichiometry of the Ii/MHCII complex remains debated. This is particularly interesting as both p35 and p43 include a di-arginine motif that requires masking by MHCII to allow ER egress. Here, to functionally address the requirement for stoichiometric interactions, we used a recombinant DR heterodimer bearing its own cytoplasmic di-lysine ER-retention motif (DRKKAA). When coexpressed with p33 and a control myc-tagged DR (DRmyc), DRKKAA was retained in the ER but had little impact on surface expression of DRmyc. However, when coexpressed with p35, DRKKAA restricted the surface expression of DRmyc, indicating that Ii trimers can be loaded with more than one MHCII. Similar results were obtained using HLA-DQ instead of DRmyc, showing that a single trimeric Ii scaffold can include distinct MHCII isotypes. Altogether, these results demonstrate that the subunit stoichiometry of oligomeric Ii/MHCII complexes is influenced by p35.

Glycosylation signals that separate the trimerization from the mhc class II-binding domain control intracellular degradation of invariant chain

Invariant chain (Ii) serves as a chaperone for folding and intracellular transport of major histocompatibility complex class II (MHCII) molecules. Early in biosynthesis, Ii associates with MHCII molecules and directs their intracellular transport to endocytic compartments where vesicular proteinases sequentially release Ii from the MHCII heterodimer. The detachment of Ii makes the MHCII groove susceptible for binding of antigenic peptides. We investigated the role of N-linked glycosylation in the controlled intracellular degradation of Ii. Motifs for asparagine-linked glycosylation were altered, and mutated Ii (IiNmut) was transiently expressed in COS cells. The half-life of IiNmut was strongly reduced compared with wild-type Ii although the sensitivity of the N glycan-free polypeptide to in vitro proteinase digestion was not substantially increased. Inhibition of vesicular proteinases revealed endosomal degradation of IiNmut. Intracellular proteolysis of IiNmut is substantially impaired by serine proteinase inhibitors. Thus, a considerable amount of IiNmut is degraded in nonacidic intracellular compartments. The data suggest that N-linked glycosylation of Ii hinders premature proteolysis in nonacidic vesicles resulting in Ii degradation in acidic MHC class II-processing compartments.

Modulation of antigen trafficking to MHC class II-positive late endosomes of enterocytes

BACKGROUND & AIMS

Oral tolerance is recognized as a central immunoregulatory phenomenon. The mechanisms of its induction remain unclear, and the role of the intestinal epithelial cells that are able to present antigens to T lymphocytes is poorly understood. In this report, we analyze under in vivo conditions the intracellular targeting of mucosally administered ovalbumin (OVA) to major histocompatibility complex (MHC) class II antigen containing compartments of enterocytes and compare these pathways between BALB/c and SCID mice, the latter being unable to generate a transferable tolerogenic moiety after a feed of OVA.

METHODS

OVA, lysosome-associated membrane proteins (LAMP-1), and MHC class II antigens were localized in jejunal biopsy specimens of BALB/c and SCID mice at 0, 5, 10, 20, 40, 60, and 120 minutes after a single feed with OVA by fluorescence and electron microscopy.

RESULTS

Ten minutes after oral administration, OVA was transported to the proximity of MHC class II antigens within LAMP-1-positive vacuoles and to the basolateral membrane of enterocytes from BALB/c strain mice. However, in SCID mice, OVA reached the paracellular spaces during the same time period through LAMP-1-negative vacuoles of enterocytes, which lacked MHC class II antigens.

CONCLUSIONS

Orally administered OVA is rapidly targeted to late endosomes containing LAMP-1 and MHC class II antigens in enterocytes of BALB/c mice but not in SCID mice bred on a BALB/c background. We suggest that this targeting process within the enterocytes is one of the requirements for the induction of oral tolerance.

The Invariant Chain Gene Intronic Enhancer Shows Homology to Class II Promoter Elements

Coordinate expression of MHC class II proteins and the class II-associated invariant chain (Ii) is important for proper MHC class II functioning in Ag processing and presentation. The coordinate regulation of these genes results, in part, from the sharing of transcriptional regulatory regions between MHC class II and Ii genes; the Ii has previously been shown to have an upstream enhancer closely related to the essential class II promoter elements. We report here the characterization of a second enhancer in the Ii gene, located within the first intron. This intronic enhancer is contained within a 155-bp region, enhances transcription from the Ii minimal promoter, and also contains elements that are homologous to class II promoter elements X1, X2, and Y boxes.

Different endocytic compartments are involved in the tight association of class II molecules with processed hen egg lysozyme and ribonuclease A in B cells

The processing of exogenous antigens and the association of peptides with class II molecules both occur within the endocytic pathway. 2A4 B lymphoma cells of the H-2k haplotype were grown in the presence or the absence of two different exogenous antigens (hen egg lysozyme and ribonuclease A) internalized by fluid-phase endocytosis. Using subcellular fractionation techniques, we demonstrate that, in the presence of hen egg lysozyme, newly synthesized SDS-stable class II molecules are detected in a dense endocytic compartment which does not have the characteristics of neither early and late endosomes nor lysosomes. In contrast, no SDS-stable class II molecules are observed between ribonuclease A and newly synthesized class II molecules. Interestingly, when class II molecules are analyzed at steady state, SDS-stable class II molecules induced by ribonuclease A are found in a compartment cosedimenting with late endosomes. These results suggest that the tight associations between ribonuclease A or hen egg lysozyme with class II molecules occur in distinct endocytic compartments and that these associations may depend on the sensitivity of antigens to proteolysis.

Immunological significances of invariant chain from the aspect of its structural homology with the cystatin family

The primary structure of p31 of invariant chain (Ii-chain) shows about 50% homology with those of the cystatin family which are endogenous cysteine protease inhibitors. The binding domains between Ii-chain and HLA-DR-7 were estimated from the structural homology between cystatin and Ii-chain and also between cathepsins and DR-7, respectively. The QL64-71 and GS76-88 of Ii-chain were estimated to be the binding domains with GG45-51 and VS57-63 of HLA-DR7, respectively. The purified human Ii-chain from spleen is capable of forming four molecular forms from monomer to tetramer by redox-potential dependent disulfide bond formation. The Ii-chain inhibits cathepsin L and H competitively as a dimer and the K(i) value for cathepsin L was 4.1 x 10(-8) M, but cathepsin B was not inhibited at all. The Ii-chain showed mainly a dimer (60 kDa) under the assay condition of cathepsins with cysteine and was not degraded by these cathepsins. The Ii-chain may play an important role in the regulation of antigenic peptide presentation to MHC class II.

Modulation of accessory cell function of immortalized bone marrow-derived macrophages by granulocyte/macrophage colony-stimulating factor

To generate cloned macrophage populations with sensitivity towards granulocyte/macrophage colony-stimulating factor (GM-CSF), bone marrow-derived macrophages (BMM phi) were immortalized by transformation with SV40. A panel of transformed clones was established. The majority of clones represented independently derived transformants, as evidenced by restriction fragment length polymorphism using genomic DNA digested with EcoRI and TaqI and the 5.2 kb SV40 DNA for hybridization analysis. The cells belong to the macrophage lineage according to several criteria, e.g. the presence of nonspecific esterase, their phagocytic capacity and their morphology. Many clones were potent antigen-presenting cells (APC), without exogenous stimulation. Two clones, which did not act efficiently as APC when used untreated, were positively responsive to GM-CSF. GM-CSF stimulation of both clones resulted in potent APC capacity. I-A alpha, I-A beta and gamma chain-specific transcripts were observed upon stimulation with GM-CSF, corresponding to detectable levels of class II surface display as revealed by cytofluorometric analysis. Thus the macrophage clones established will allow dissection of the differential effects of GM-CSF on the parameters of antigen presentation.

Analysis of class II MHC structure in thymic nurse cells

During the course of thymocyte maturation, the processes of positive selection and tolerance induction are mediated by interactions between thymocyte T-cell receptors and MHC molecules on thymic stromal cells. The means by which these seemingly contrary processes can be mediated by interactions between the same molecules has long been a source of controversy. One idea which has been put forward is that the MHC molecules in different microenvironments of the thymus are not the same. We have tested this hypothesis by examining class II transcripts derived from thymic cortical epithelial cells known as thymic nurse cells, reasoning that alternative splicing of primary transcripts might give rise to a positively selecting MHC molecule. However, we found no evidence for alternative splicing of these transcripts. These results are presented and discussed with regard to implications for possible mechanisms of positive selection.

Class II antigen-associated invariant chain mRNA in mouse small intestine

MHC class II antigen-associated invariant (Ii) chain mRNA appears in mouse small intestine during postnatal development. Ii chain cDNA hybridizes to RNA from epithelial sheets dissociated from the lamina propria with EDTA. Of several mouse organs tested, only bone marrow and spleen contain higher levels of Ii chain mRNA than small bowel. Ii chain mRNA is not detected in stomach, colon, duodenum, testis, liver, submandibular gland, or L-cell RNA; brain contains a cross-reactive but uncharacterized sequence. cDNA amplification using primers specific for both Ii31 and Ii41 chain mRNAs showed that both forms occur in small intestine. These results support the conclusion that regulation of the class II Ii chain gene is associated with the ontogeny of intestinal immunity.

MHC class II-associated invariant chain contains a sorting signal for endosomal compartments

The invariant chain (Ii) is a transmembrane protein that associates with the MHC class II molecules in the endoplasmic reticulum. Expression of Ii in MHC class II-negative CV1 cells showed that it acquired complex-type oligosaccharide side chains and was retained in endosomal compartments. To search for a sorting signal, we made progressive deletions from the cytoplasmic N-terminus of Ii. Deleting 11 amino acid residues resulted in a protein that was still sorted and retained in endosomal vesicles, whereas deletion of 15 or more amino acid residues resulted in a protein that became resident in the plasma membrane. Amino acids 12-15 are thus essential for intracellular transport to endosomal compartments. As Ii is intracellularly associated with the MHC class II molecules, it is proposed that Ii determines the intracellular transport route of these molecules.

The function of the invariant chain in antigen presentation by MHC class II molecules

The existence of different pathways of antigen presentation by class I and class II molecules raises two fundamental questions. (1) Why are class II, but not class I molecules, transported to the endosomal compartment where the exogenous antigen is met? (2) What are the mechanisms that prevent class II molecules from being occupied and blocked by endogenous peptides before they reach endosomes? These and other questions were discussed at the recent 7th International HLA/H-2 Workshop. In round table discussions particular attention was paid to the class-II-associated invariant chain, because it was considered that this molecule was a likely candidate to explain some of the differences of class I and class II molecules in antigen presentation.

The staphylococcal enterotoxins and their relatives

Staphylococcal enterotoxins and a group of related proteins made by Streptococci cause food poisoning and shock in man and animals. These proteins share an ability to bind to human and mouse major histocompatibility complex proteins. The complex ligand so formed has specificity for a particular part of T cell receptors, V beta, and by engaging V beta can stimulate many T cells. It is likely that some or all of the pathological effects of these toxins are caused by their ability to activate quickly so many T cells. It is also possible that encounters with such toxins have caused mice, at least, to evolve mechanisms for varying their T cell V beta repertoires, such that they are less susceptible to attack by the toxins.

Identification of transcripts of the T cell antigen receptor beta chain gene and major histocompatibility complex class II genes in antigen-presenting cloned BK-BI-2.6.C6 cells

The cloned murine cell line BK-BI-2.6.C6 has previously been shown to exhibit T cell characteristics, to synthesize and express MHC class II molecules, and to present protein antigens to antigen-dependent T cell clones. As a more definitive proof of the T-cell nature of these cells, transcripts of the rearranged T cell antigen receptor (TcR) beta gene were assessed by Northern blot analysis. BK-BI-2.6.C6 cells constitutively transcribe mRNA for the light chain of TcR and express the disulphide-linked alpha, beta TcR heterodimer at the cell surface. In addition mRNA for the polymorphic MHC class II subunits A alpha and A beta as well as for the invariant gamma chain were detected. BK-BI-2.6.C6 T cells effectively stimulated bovine insulin-reactive T hybridoma cells to lymphokine production in the presence of this antigen. Since the antigen-presenting and the responding T cell populations are maintained in culture in the absence of feeder cells, contamination by conventional accessory cells is excluded. These data unequivocally demonstrate that cloned murine Ia-expressing T cells can act as antigen-presenting accessory cells.

The production of recombinant HLA-DR beta and invariant chain polypeptides by cDNA expression in E. coli

In this report we describe the production of recombinant fusion proteins of the HLA-DRw6 beta chain and the murine Ia-associated invariant chain. cDNAs encoding the human HLA-DRw6 beta chain and the murine Ia-associated invariant chain were introduced into bacterial expression plasmids. These plasmids direct the synthesis of the respective molecules as fusion proteins of the bacteriophage MS-2 polymerase by E. coli. Fusion proteins purified from crude E. coli lysates were used to raise antisera in rabbits. These antisera were able to immunoprecipitate biosynthetically labelled class II and invariant chain antigens. Additionally, two anti-DR antisera were raised against single domains of the HLA-DR beta chain thus generating reagents with a defined fine specificity. The anti-murine invariant chain serum was shown to cross-react with the human invariant chain and therefore may be useful for studying invariant chain and Ia antigen expression in different species. The method described here permitted us to produce large quantities of immunologically relevant proteins, for use in the production of polyclonal and monoclonal antibodies. Soluble fragments of the fusion proteins representing certain DR domains may also be useful in functional immunological studies.

A phenotypically dominant regulatory mechanism suppresses major histocompatibility complex class II gene expression in a murine plasmacytoma

The expression of major histocompatibility complex (MHC) class II antigens is down-regulated when B cells differentiate into plasma cells. We have studied the mechanism of down-regulation of MHC class II expression in a BALB/c strain-derived murine plasmacytoma cell line, NS1. NS1 cells express MHC class I antigens but not MHC class II antigens. We tested 20 uncloned hybrid cell lines obtained from the fusion of NS1 cells with MHC class II-expressing splenic B cells prepared from CBA, SJL or BALB/c mice. All the hybrid cell lines expressed MHC class I antigens of either or both parental haplotypes but did not express MHC class II. One NS1 X splenic B cell hybrid clone, K3, was used to further validate these results; K3 cells expressed MHC class I but not MHC class II antigens. K3 was fused to the MHC class II-expressing B lymphoma A20, and the seven resulting hybrid cell lines were again found to express MHC class I but not MHC class II antigens. Since NS1 is a subclone of the P3-X63Ag8 murine plasmacytoma, we also tested one P3-X63Ag8 x splenic B cell hybrid, Sp2/0, and two Sp2/0 x splenic B cell hybrids. All were found to express the appropriate MHC class I antigens but did not express MHC class II. Thus, our results suggest that the NS1 plasmacytoma suppresses MHC class II expression by a phenotypically dominant regulatory mechanism. We found that NS1 cells express correctly sized mRNA for the MHC class II genes A alpha, E alpha and the invariant chain. The co-expression of MHC class I protein and I-A and I-E region gene transcripts provides strong evidence that the MHC gene cluster is structurally intact, and that lack of class II expression is due to a genetic regulatory mechanism. The amounts of class II mRNA expressed by NS1 cells were at least equivalent to those found in splenic lymphocytes. Therefore, this regulation must operate post-transcriptionally.

Synthesis and expression of MHC class II molecules in the absence of attached invariant chains by recombinant-interferon-gamma-activated bone-marrow-derived macrophages

Pure populations of in vitro propagated bone marrow-derived macrophages are constitutively Ia negative. Co-culturing of these cells with recombinant interferon-gamma (rIFN-gamma) resulted in the appearance of high amounts of Ia antigens at the cell surface of essentially all cells. The continuous presence of the stimulus was a prerequisite for sustained Ia expression because removal of the stimulus resulted in rapid decline of surface Ia. Two-dimensional (2D) gel analysis (1D isoelectric focusing, 2D sodium dodecyl sulfate-polyacrylamide gel electrophoresis) of class II molecules synthesized by rIFN-gamma-stimulated bone marrow macrophages (BMM phi) revealed that, in contrast to class II complexes hitherto described, BMM phi-derived I-A and I-E subregion-encoded subunits are synthesized without invariant chains. The invariant chain-deficient alpha,beta heterodimers are expressed at the cell surface in high proportions demonstrating that their correct assembly and transport to the cell surface is accomplished in the absence of invariant chains. The lack of invariant chains appears not to be due to a failure of rIFN-gamma to induce transcription of the gamma-chain gene because rIFN-gamma-induced, in contrast to uninduced, BMM phi accumulate high levels of invariant chain-specific transcripts as evidenced by Northern blot analysis. These findings suggest that translation of gamma-chain-specific mRNA is blocked in BMM phi for as yet unknown reasons. Alternatively, newly synthesized gamma chains might have escaped their regular intracellular maturation pathway as a result of unidentified modifications mediated by altered post-translational processing mechanisms.

Molecular structure of human lymphocyte receptor for immunoglobulin E

We have isolated and sequenced a cDNA clone encoding the human lymphocyte receptor for IgE (Fc epsilon R). The deduced protein sequence reveals that Fc epsilon R consists of 321 amino acids, without any signal sequence, and is oriented with its N-terminus on the cytoplasmic side and its C-terminus on the outside of the cell. This molecule shows striking sequence homology with chicken asialoglycoprotein receptor (hepatic lectin), suggesting a possible role for Fc epsilon R in endocytosis. Fc epsilon R mRNA is expressed in B cells, B cell lines, and macrophage cell lines. It is not expressed in T cells or T cell lines, with the exception of an HTLV-transformed T cell line. mRNAs expressed in a macrophage line and in the latter T cell line differ in size from mRNA expressed in B cells. Human BSF-1 (or IL-4) induces the expression of Fc epsilon R mRNA in B cells, but not in T cells.

Expression of Ia antigens in a murine T-lymphoma variant

ESb, a cellular high metastatic variant derived from the murine T-cell lymphoma L5178Y (Eb), was found to synthesize Ia antigens. Ia-specific antibodies reacted with the ESb cells and precipitated Ia-like molecules from them. Two-dimensional gel electrophoretic analysis of immunoprecipitates of metabolically labeled ESb cells indicated that the Ia molecules on ESb were indistinguishable from those on murine B-cells. No Ia antigens were detectable on the parental tumor line Eb. Treatment with recombinant interferon-gamma (IFN-gamma) caused enhancement of class I histocompatibility antigen expression on Eb and ESb tumor lines. In ESb cells the expression of Ia and of Ia-associated invariant chain (Ii) was also increased upon IFN-gamma treatment. No induction of either Ia and Ii antigens was observed upon IFN-gamma treatment of the Eb line. These studies demonstrate a substantial difference between the Eb and ESb tumor lines with respect to: (i) constitutive expression of class II major histocompatibility antigens, and (ii) response to IFN-gamma treatment.

Ia-associated invariant chain is fatty acylated before addition of sialic acid

The murine invariant chain (Ii) was found to incorporate radioactive palmitic acid. This binding of fatty acid inhibits the formation of interchain S-S bonds, probably because the cysteine residue in the transmembrane region of the Ii chain is palmitylated. The inhibition of fatty acylation by cerulenin blocks further posttranslational maturation of the invariant chain as shown by two-dimensional gel electrophoresis of Ii immunoprecipitates. In particular, the addition of sialic acid residues is blocked. Thus, it appears that fatty acylation is essential for carbohydrate processing of the Ii chain.

An Ia-positive mouse T-cell clone is functional in presenting antigen to other T cells

In this report we present data demonstrating the endogenous expression of I-region associated (Ia) antigens on a cloned line of mouse T cells, CTLL, as well as transcription of the invariant chain gene in these cells. We demonstrate further that this Ia-bearing T-cell clone, CTLL, can utilize the expressed Ia molecules to present antigen to Ia-restricted antigen-specific T cells.

The gene encoding the Ia-associated invariant chain is located on chromosome 18 in the mouse

The chromosomal assignment of the gene encoding the invariant (Ii) chain associated with the mouse immune response antigens (Ia) was determined by Southern blot analysis of DNA from a panel of mouse X Chinese hamster somatic cell hybrids cleaved with Hind III or Eco RI. Using a mouse li cDNA as a hybridization probe, we localized the gene coding for the invariant chain to mouse chromosome 18.

Primary structure of human transferrin receptor deduced from the mRNA sequence

In vertebrates all iron is taken up via the carrier protein transferrin. The carrier first binds its receptor and the receptor-ligand complex is then internalized via coated pits. The transferrin receptor is a transmembrane glycoprotein (apparent molecular weight (MW) 180,000) composed of two disulphide-bonded sub-units (each of apparent MW 90,000) It contains three N-linked glycan units and is post-translationally modified with both phosphate and fatty acyl groups. Here we have determined the nucleotide sequence of the coding region of the human transferrin receptor mRNA and from this deduced the amino acid sequence of the protein. The receptor does not contain an N-terminal signal peptide but there is a membrane-spanning segment 62 amino acids from the N-terminus. It therefore has a somewhat unusual configuration with a small N-terminal cytoplasmic domain and a C-terminal extracellular domain of 672 amino acids.