Expression and function of a nonglycosylated major histocompatibility class I antigen

Influence of Glycosylation Inhibition on the Binding of KIR3DL1 to HLA-B*57:01

Viral infections can affect the glycosylation pattern of glycoproteins involved in antiviral immunity. Given the importance of protein glycosylation for immune function, we investigated the effect that modulation of the highly conserved HLA class I N-glycan has on KIR:HLA interactions and NK cell function. We focused on HLA-B*57:01 and its interaction with KIR3DL1, which has been shown to play a critical role in determining the progression of a number of human diseases, including human immunodeficiency virus-1 infection. 721.221 cells stably expressing HLA-B*57:01 were treated with a panel of glycosylation enzyme inhibitors, and HLA class I expression and KIR3DL1 binding was quantified. In addition, the functional outcomes of HLA-B*57:01 N-glycan disruption/modulation on KIR3DL1ζ+ Jurkat reporter cells and primary human KIR3DL1+ NK cells was assessed. Different glycosylation enzyme inhibitors had varying effects on HLA-B*57:01 expression and KIR3DL1-Fc binding. The most remarkable effect was that of tunicamycin, an inhibitor of the first step of N-glycosylation, which resulted in significantly reduced KIR3DL1-Fc binding despite sustained expression of HLA-B*57:01 on 721.221 cells. This effect was paralleled by decreased activation of KIR3DL1ζ+ Jurkat reporter cells, as well as increased degranulation of primary human KIR3DL1+ NK cell clones when encountering HLA-B*57:01-expressing 721.221 cells that were pre-treated with tunicamycin. Overall, these results demonstrate that N-glycosylation of HLA class I is important for KIR:HLA binding and has an impact on NK cell function.

Recognition of open conformers of classical MHC by chaperones and monoclonal antibodies

There is considerable evidence that the conformation and stability of class I and class II major histocompatibility complex (MHC) proteins is dependent upon high-affinity peptide ligation, but structural data for an empty MHC protein unfortunately is lacking. However, several monoclonal antibodies (mAbs) that specifically detect open MHC conformers have been characterized, and they provide insights into the changes associated with peptide loading and unloading. Here, the structural changes make the argument that certain of these open conformer-specific mAbs recognize analogous MHC segments as the molecular chaperones tapasin and DM. MHC residues located in regions flanking the peptide-terminal anchoring pockets have been implicated in both chaperone and monoclonal antibody binding. Indeed, we propose these regions serve as peptide-binding hinges that are uniquely accessible in open MHC.

N-linked carbohydrates in tyrosinase are required for its recognition by human MHC class II-restricted CD4(+) T cells

Glycosylation of mammalian proteins is known to influence their intracellular trafficking, half life, and susceptibility to enzymatic degradation. Rare instances of natural T cell epitopes dependent upon glycosylation for recognition have been described. We report here on human CD4(+) T lymphocyte cultures and clones from two melanoma patients that recognize the melanoma-associated Ag tyrosinase in the context of HLA-DR4 and -DR8. These T cells recognize tyrosinase, normally a heavily glycosylated molecule, when expressed constitutively in melanoma cells or in COS-7 transfectants pulsed as lysates onto autologous APC. However, these T cells fail to recognize tyrosinase expressed in bacteria, nor do they react with overlapping peptides covering full-length tyrosinase, suggesting a critical role for glycosylation in the processing and / or composition of the stimulatory epitopes. The requirement for glycosylation was demonstrated by the failure of tyrosinase-specific CD4(+) T cells to recognize tyrosinase synthesized in the presence of glycosylation inhibitors, or deglycosylated enzymatically. Site-directed mutagenesis of each of seven potential N-glycosylation sites showed that four sites were required to generate forms of tyrosinase that could be recognized by individual T cell clones. These data indicate that certain carbohydrate moieties are required for processing the tyrosinase peptides recognized by CD4(+) T cells. Post-translational modifications of human tumor-associated proteins such as tyrosinase could be a critical factor for the development of antitumor immune responses.

Folding of the human immunodeficiency virus type 1 envelope glycoprotein in the endoplasmic reticulum

The lumen of the endoplasmic reticulum (ER) provides a unique folding environment that is distinct from other organelles supporting protein folding. The relatively oxidizing milieu allows the formation of disulfide bonds. N-linked oligosaccharides that are attached during synthesis play multiple roles in the folding process of glycoproteins. They stabilize folded domains and increase protein solubility, which prevents aggregation of folding intermediates. Glycans mediate the interaction of newly synthesized glycoproteins with some resident ER folding factors, such as calnexin and calreticulin. Here we present an overview of the present knowledge on the folding process of the heavily glycosylated human immunodeficiency virus type 1 (HIV-1) envelope glycoprotein in the ER.

Genetic remodeling of protein glycosylation in vivo induces autoimmune disease

Autoimmune diseases are among the most prevalent of afflictions, yet the genetic factors responsible are largely undefined. Protein glycosylation in the Golgi apparatus produces structural variation at the cell surface and contributes to immune self-recognition. Altered protein glycosylation and antibodies that recognize endogenous glycans have been associated with various autoimmune syndromes, with the possibility that such abnormalities may reflect genetic defects in glycan formation. We show that mutation of a single gene, encoding alpha-mannosidase II, which regulates the hybrid to complex branching pattern of extracellular asparagine (N)-linked oligosaccharide chains (N-glycans), results in a systemic autoimmune disease similar to human systemic lupus erythematosus. alpha-Mannosidase II-deficient autoimmune disease is due to an incomplete overlap of two conjoined pathways in complex-type N-glycan production. Lymphocyte development, abundance, and activation parameters are normal; however, serum immunoglobulins are increased and kidney function progressively falters as a disorder consistent with lupus nephritis develops. Autoantibody reactivity and circulating immune complexes are induced, and anti-nuclear antibodies exhibit reactivity toward histone, Sm antigen, and DNA. These findings reveal a genetic cause of autoimmune disease provoked by a defect in the pathway of protein N-glycosylation.

Calreticulin and Calnexin Interact with Different Protein and Glycan Determinants During the Assembly of MHC Class I

Before peptide binding, a variety of endoplasmic reticulum (ER) proteins are associated with class I including calnexin, TAP, calreticulin, and tapasin. Although the selective functions of any one of these ER proteins have been difficult to define, individually or in combination they perform two general chaperone functions for class I. They promote assembly of the class I heterotrimeric molecule (heavy (H) chain, β2m, and peptide) and they retain incompletely assembled complexes in the ER. In this study, we present evidence that calreticulin clearly differs from calnexin in how it associates with class I. Regarding the structural basis of the association, the oligosaccharide moiety in the α1 domain and the amino acid residue at position 227 in the α3 domain were both found to be critical for the interaction of class I with calreticulin. Interestingly, calreticulin displayed sensitivity to class I peptide binding even in TAP-deficient human or mouse cells. Thus, calreticulin is clearly more specific than calnexin in the structures and conformation of the class I molecule with which it can interact.

Evidence for Sulfate Modification of H-2Dd on N-Linked Carbohydrate(s): Possible Involvement in Ly-49A Interaction

Murine class I molecules are ligands for Ly-49 molecules, a family of regulatory receptors expressed on murine NK cells. Since soluble sulfated mono- and polysaccharides interfere with the interaction of Ly-49A, a C-type lectin, and its class I ligand, Dd, it is possible that the oligosaccharides on class I molecules are sulfated and participate in Ly-49A binding. In this report, we show that H-2Dd expressed by activated T cells and various tumor cell lines is sulfated, as demonstrated by immunoprecipitation of Dd following Na235SO4 labeling. The 35SO4−2 label on Dd expressed by a representative tumor cell, NZB1.1, is removed by peptide N-glycosidase F, but is resistant to endoglycosidase H treatment, indicating that the sulfate group is located on mature N-linked oligosaccharides. Two-dimensional SDS-PAGE analysis revealed that all major mature glycosylation variants of the Dd expressed by NZB1.1 are sulfated. Sodium chlorate, a potent inhibitor of ATP-sulfurylase, which prevents the formation of the sulfate donor, 3′-phosphoadenosine 5′-phosphosulfate, inhibited metabolic sulfation of Dd. NZB1.1 binds isolated Ly-49A immobilized on solid phase through an interaction by cell surface Dd, since cell adhesion was blocked by Abs directed against Dd or Ly-49A. Treatment of the Dd-expressing NZB1.1 tumor cells with sodium chlorate reduced their ability to bind immobilized Ly-49A, particularly when Ly-49A density was limiting. These results provide evidence for sulfation of H-2Dd oligosaccharide moieties, and suggest a role for this posttranslational modification in the interaction of Dd with Ly-49A.

The quality control of glycoprotein folding in the endoplasmic reticulum, a trip from trypanosomes to mammals

The present review deals with the stages of synthesis and processing of asparagine-linked oligosaccharides occurring in the lumen of the endoplasmic reticulum and their relationship to the acquisition by glycoproteins of their proper tertiary structures. Special emphasis is placed on reactions taking place in trypanosomatid protozoa since their study has allowed the detection of the transient glucosylation of glycoproteins catalyzed by UDP-Glc:glycoprotein glucosyltransferase and glucosidase II. The former enzyme has the unique property of covalently tagging improperly folded conformations by catalyzing the formation of protein-linked Glc1Man7GlcNAc2, Glc1Man8GlcNac2 and Glc1Man9GlcNAc2 from the unglucosylated proteins. Glucosyl-transferase is a soluble protein of the endoplasmic reticulum that recognizes protein domains exposed in denatured but not in native conformations (probably hydrophobic amino acids) and the innermost N-acetylglucosamine unit that is hidden from macromolecular probes in most native glycoproteins. In vivo, the glucose units are removed by glucosidase II. The influence of oligosaccharides in glycoprotein folding is reviewed as well as the participation of endoplasmic reticulum chaperones (calnexin and calreticulin) that recognize monoglucosylated species in the same process. A model for the quality control of glycoprotein folding in the endoplasmic reticulum, i.e., the mechanism by which cells recognize the tertiary structure of glycoproteins and only allow transit to the Golgi apparatus of properly folded species, is discussed. The main elements of this control are calnexin and calreticulin as retaining components, the UDP-Glc:glycoprotein glucosyltransferase as a sensor of tertiary structures and glucosidase II as the releasing agent.

Site-directed mutagenesis of the N-linked glycosylation site in platelet-derived growth factor B-chain results in diminished intracellular retention

Pulse-chase analysis of human platelet-derived growth factor (PDGF) B-chain was conducted in stably transfected Chinese hamster ovary cells to determine precisely the kinetics of processing, intracellular trafficking and secretion. Newly synthesized 31 kDa monomers of the B-chain (p31) dimerized rapidly via disulfide bonds to a p54 species (t1/2 < 30 min). The p54 dimer was processed to a group of intracellular, cell surface (suramin-releasable) and secreted forms whose rates of appearance and disappearance from the cell were measured over a 48 h period. The newly synthesized p31 species was quantitatively converted to p27 by treatment with endoglycosidase H, consistent with efficient N-glycosylation at a site in the N-terminal propeptide region (Asn63-Met64-Thr65). Interruption of B-chain glycosylation by oligodeoxynucleotide-directed mutagenesis resulted in a significant increase in suramin-releasable forms at the cell surface (p34-38) and a concomitant decrease in accumulation of an intracellular p24 species. The glycosylation-defective mutant exhibited slight increases in receptor binding and mitogenic activity. Our results suggest that N-linked glycosylation of the B-chain is not important for formation of mitogenically active protein, but that it plays a role in early intracellular sorting and proteolytic processing events.

Functions for MHC class I carbohydrates inside and outside the cell

Major histocompatibility complex (MHC) class I heavy chain glycoproteins have an invariant N-linked glycosylation site at Asn86, which is found between two extremely variable protein domains. For human MHC class I molecules, Asn is the only site of glycosylation and the attached oligosaccharides are remarkably uniform. The carbohydrate initiates interactions with calnexin in the endoplasmic reticulum that facilitate the assembly of MHC class I molecules and their delivery to the cell surface. However, recognition of MHC class I molecules by antibodies and T cells is indifferent to the carbohydrate, although lines of circumstantial evidence implicate lectins and carbohydrates in the recognition of MHC class I glycoproteins by natural killer cells.

Differential ability of isolated H-2 Kb subsets to serve as TCR ligands for allo-specific CTL clones: potential role for N-linked glycosylation

It is not known whether all forms of cell surface peptide-class I complexes, when bound with relevant peptide antigen, are recognized by T cells. We demonstrate herein that two distinct subsets of the murine H-2 Kb molecule can be separately isolated from H-2b-expressing cell lines using Y3 mAb immunoaffinity chromatography. Although both isolated Kb subsets were found to be strongly reactive with Y3 mAb by ELISA, one Kb subset is S19.8 mAb reactive (Ly-m11+Kb subset) and exhibits low reactivity with the M1/42 antibody, while the other subset is negative for the Ly-m11 epitope and highly reactive with the M1/42 antibody (M1/42high Kb subset). More importantly, whereas the M1/42high Kb subset is a very effective ligand for both TCR and CD8, the Ly-m11+ Kb subset could only function as a CD8 ligand, as determined in allo-specific CD8+ CTL clone adhesion and degranulation assays. Peptides acid-eluted from both Kb subsets sensitized Kb-transfected T2 cells expressing "peptide empty" Kb for lysis to a similar extent by allo-CTL clones, indicating that relevant endogenous peptide antigens are not limiting in the Ly-m11+ Kb subset. The major distinction identified between the two Kb subsets is that they differ substantially in their degree of N-linked glycosylation, with the Ly-m11+ subset containing Kb molecules with larger and more complex carbohydrate modifications than the M1/42high subset. The differences in glycosylation may explain the functional differences observed between the two Kb subsets. It is therefore possible that some forms of glycosylation on class I molecules interfere with TCR recognition and may limit CD8+ T cell responses, perhaps under circumstances where peptide antigen is limiting.

Infection with Listeria monocytogenes impairs sialic acid addition to host cell glycoproteins

Listeria monocytogenes is a facultative intracellular bacterium that causes severe disease in neonates and immunocompromised adults. Although entry, multiplication, and locomotion of Listeria in the cytosol of infected cells are well described, the impact of such infection on the host cell is unknown. In this report, we investigate the effect of L. monocytogenes infection on MHC class I synthesis, processing, and intracellular trafficking. We show that L. monocytogenes infection interferes with normal processing of N-linked oligosaccharides on the major histocompatibility complex (MHC) class I heavy chain molecule, H-2Kd, resulting in a reduced sialic acid content. The glycosylation defect is more pronounced as the infection progresses and results from interference with the addition of sialic acid rather than its removal by a neuraminidase. The effect is found in two different cell lines and is not limited to MHC class I molecules since CD45, a surface glycoprotein, and LGP120, a lysosomal glycoprotein, are similarly affected by L. monocytogenes infection. The glycosylation defect is specific for infection by L. monocytogenes since neither Trypanosoma cruzi nor Yersinia enterocolitica, two other intracellular pathogens, reproduces the effect. The resultant hyposialylation of H-2Kd does not impair its surface expression in infected cells. Diminished sialic acid content of surface glycoproteins may enhance host-defense by increasing susceptibility to lysis and promoting clearance of Listeria-infected cells.

Beta 2-microglobulin-independent MHC class Ib molecule expressed by human intestinal epithelium

A major histocompatibility complex class Ib protein, CD1d, is expressed by human intestinal epithelial cells (IECs) and is a ligand for CD8+ T cells. CD1d was found to be expressed on the surface of human IECs as a 37-kilodalton protein that was beta 2-microglobulin (beta 2M) independent with no N-linked carbohydrate. Transfection into a beta 2M- cell line confirmed that CD1d could be expressed at the cell surface in the absence of beta 2M. These data indicate that IECs use a specialized pathway for CD1d synthesis and that a beta 2M-independent class Ib protein may be the normal ligand for some intestinal T cells.

How N-linked oligosaccharides affect glycoprotein folding in the endoplasmic reticulum

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Increased expression of MHC class I molecules on human cells after short time IFN-gamma treatment

Human cell lines and blood lymphocytes were treated for short time periods with IFN-gamma. This treatment increased the amount of the assembled MHC class I molecules on the plasma membrane after 30 min. This early increase of the membrane expression subsided in the next few hours. A second wave of elevation occurred after 8-24 hr. Analysis of cytoplasmic and membrane molecules in pulse chase experiments showed that the cytokine enhanced both the assembly of available heavy and light chains and the transport of the complex to the plasma membrane. The membrane level of the HLA-A2 molecules showed similar kinetics. Addition of an A2 specific binding peptide stabilized the IFN-gamma induced molecules on the cell surface. It seems that IFN-gamma alone or together with a binding peptide can influence MHC class I expression solely through post-transcriptional events utilizing an available pool of free heavy and light chains already after a short time, before the enhancement of the synthesis starts.

Glycosylation requirements for intracellular transport and function of the hemagglutinin of influenza virus

The contribution of each of the seven asparagine-linked oligosaccharide side chains on the hemagglutinin of the A/Aichi/68 (X31) strain of influenza virus was assessed with respect to its effect on the folding, intracellular transport, and biological activities of the molecule. Twenty mutant influenza virus hemagglutinins were constructed and expressed, each of which had one or more of the seven glycosylation sites removed. Investigations of these mutant hemagglutinins indicated that (i) no individual oligosaccharide side chain is necessary or sufficient for the folding, intracellular transport, or function of the molecule, (ii) at least five oligosaccharide side chains are required for the X31 hemagglutinin molecule to move along the exocytic pathway to the plasma membrane, and (iii) mutant hemagglutinins having less than five oligosaccharide side chains form intracellular aggregates and are retained in the endoplasmic reticulum.

Cell-specific constraints to the lateral diffusion of a membrane glycoprotein

We have previously shown that the lateral diffusion, D, of the class I Major Histocompatibility Complex (MHC) glycoprotein H-2Ld is constrained by its glycosylation, when expressed in mouse L-cells. Removal of one or more of the 3 N-linked oligosaccharides of H-2Ld glycoproteins results in an increase in D. In order to further examine the influence of glycosylation on D, we compared lateral diffusion of H-2Ld expressed in wild-type CHO cells with lateral diffusion of the same molecule expressed in mutant CHO cells with aberrant surface glycosylation. In addition, we compared lateral diffusion of wild-type and unglycosylated H-2Ld antigens in these cells. In contrast to the large effect of glycosylation state on lateral diffusion of H-2Ld in mouse L-cells, there was little effect of glycosylation on lateral diffusion of H-2Ld in any of the CHO cells. This, together with similar results on hamster class I antigens, indicates that the constraints to D of H-2Ld and other class I MHC molecules are different in CHO cells than in L-cells. Measurements of lateral diffusion after treatment of cells with cytochalasin D make it clear that interactions between MHC class I molecules and a cytoskeleton are important in reducing the mobile fraction of diffusing molecules, R, though they cannot be shown to directly affect the diffusion coefficient, D.

Cell type and maturation stage-dependent polymorphism of N-linked oligosaccharides on murine lymphocytes and lymphoma cells

We analyzed the overall structures of N-linked oligosaccharides on glycoproteins of various murine lymphocytic and lymphoma cells employing a newly developed method which was performed on high-performance liquid chromatography after derivatization of oligosaccharides with 2-aminopyridine. A total of 15 types of bi, tri- and tetra-antennary N-acetyllactosamine-type oligosaccharides with or without fucose and oligomannose-type oligosaccharides were identified on these cells in variable amounts depending on the type and maturation stage of the cells. It was found that all murine lymphocytic cells carry N-acetyllactosamine-type oligosaccharides with the additional alpha-linked galactose residue on the non-reducing ends. Thymocytes had exceptionally large amounts of oligosaccharides with one or even two alpha-galactose residues per molecule. In contrast, peripheral resting T cells possessed those oligosaccharides only in a small amount, although the cells produced more the oligosaccharides after stimulation with Con A. Two thymoma lines such as BW 5147 and EL-4 and one B cell lymphoma line WEHI231 contained relatively large amount of oligosaccharides with alpha-galactose residues. Significant change of the molar ratio of component carbohydrates by cell activation was observed also in oligommanose-type oligosaccharides which were few in resting T cells but were markedly increased in Con A activated cells. Molar ratio of triantennary oligosaccharides in total N-acetyllactosamine type oligosaccharides was high in thymocytes and low in resting T cells, but was increased in T cells after Con A activation. It was also very high in WEHI 231 B cell lymphoma. Although BW 5147 and EL-4 thymoma did not contain tri-antennary oligosaccharides in high proportion, they carried larger tetra-antennary oligosaccharides with an N-acetyllactosamine repeating unit in definitive amounts. It is suggested from these results that overall structures of oligosaccharides on cell surface proteins of lymphocytes are finely controlled with link to cell differentiation, activation and transformation.

Efficiency of acetylcholine receptor subunit assembly and its regulation by cAMP

Assembly of nicotinic acetylcholine receptor (AChR) subunits was investigated using mouse fibroblast cell lines stably expressing either Torpedo (All-11) or mouse (AM-4) alpha, beta, gamma, and delta AChR subunits. Both cell lines produce fully functional cell surface AChRs. We find that two independent treatments, lower temperature and increased intracellular cAMP can increase AChR expression by increasing the efficiency of subunit assembly. Previously, we showed that the rate of degradation of individual subunits was decreased as the temperature was lowered and that Torpedo AChR expression was acutely temperature sensitive, requiring temperatures lower than 37 degrees C. We find that Torpedo AChR assembly efficiency increases 56-fold as the temperature is decreased from 37 to 20 degrees C. To determine how much of this is a temperature effect on degradation, mouse AChR assembly efficiencies were determined and found to be only approximately fourfold more efficient at 20 than at 37 degrees C. With reduced temperatures, we can achieve assembly efficiencies of Torpedo AChR in fibroblasts of 20-35%. Mouse AChR in muscle cells is also approximately 30% and we obtain approximately 30% assembly efficiency of mouse AChR in fibroblasts (with reduced temperatures, this value approaches 100%). Forskolin, an agent which increases intracellular cAMP levels, increased subunit assembly efficiencies twofold with a corresponding increase in cell surface AChR. Pulse-chase experiments and immunofluorescence microscopy indicate that oligomer assembly occurs in the ER and that AChR oligomers remain in the ER until released to the cell surface. Once released, AChRs move rapidly through the Golgi membrane to the plasma membrane. Forskolin does not alter the intracellular distribution of AChR. Our results indicate that cell surface expression of AChR can be regulated at the level of subunit assembly and suggest a mechanism for the cAMP-induced increase in AChR expression.

Molecular analysis of H-2 class I molecules expressed on the UV-induced tumour 1591

We have biochemically characterized by 2D (two-dimensional) electrophoresis three novel class I molecules called A166, A149 and A216 expressed by 1591, a UV-induced fibrosarcoma, and have compared them to class I molecules expressed by mice of the H-2q and H-2s haplotypes. A166 and A149 are very similar if not identical to Dq and Lq respectively. We have shown, using HPLC (high-pressure liquid chromatography) tryptic peptide mapping, that the expression of A166 is approximately three fold greater than A149, reminiscent of Dd compared to Ld. In addition A216 possess an identical isoelectric point to that of the Ks molecule. We demonstrate that outbred Swiss Webster mice express an analogous constellation of class I molecules and we conclude that our results can be most easily interpreted in terms of an allogeneic origin for the novel class I molecules expressed on 1591.

N-linked glycan modification on antigen-presenting cells restores an allospecific cytotoxic T cell response

The B6 anti-bm6 allospecific CTL response is strictly dependent on CD4+ cells when using LPS blasts as stimulator cells. Altering the N-linked carbohydrates on stimulator cells by use of the N-linked trimming glycosidase inhibitors 1-deoxymannojirimycin and swainsonine, or by treatment with bacterial neuraminidase, results in a restoration of the B6 anti-bm6 response in the absence of CD4+ cells. The extent of restoration is inversely correlated with the number of sialic acids present on N-linked glycans of stimulator cells.

A trans-acting major histocompatibility complex-linked gene whose alleles determine gain and loss changes in the antigenic structure of a classical class I molecule

The RT1.A locus of the rat MHC encodes the H chain of the single classical class I molecule of this species. One of the alleles of this polymorphic locus, RT1.Aa, is present in several laboratory inbred, congenic, and MHC recombinant rat strains. Studies of the RT1.Aa class I molecule from a number of these strains as a target for CTL show that its antigenicity, both as an alloantigen and a restricting element, is subject to gain and loss alterations by the action of a gene mapping in the MHC to the right of RT1.A. This locus is apparently present in two allelic forms (one possibly a null allele) corresponding to the presence or absence of a dominant transacting modifier, and has been named class I modification, or cim. The antigenic change brought about by cim is scarcely detectable serologically but highly immunogenic for CTL. Biochemical investigations show that cim affects the post-translational modification of RT1.Aa.

Differential transport requirements of HLA and H-2 class I glycoproteins

Transport of human and mouse major histocompatibility complex class I glycoproteins has been examined in a transport deficient B-lymphoblastoid cell line X T-lymphoblastoid cell line (B-LCL X T-LCL) hybrid, 174 X CEM.T2 (T2). This cell line expresses no detectable endogenous HLA-B5 and reduced levels of HLA-A2 on its surface although these molecules are synthesized. In order to study this defect further, either HLA-Bw58 or HLA-B7 genomic clones were transfected into T2. Metabolic labeling and immune precipitation demonstrated biosynthesis of the Bw58 or B7 glycoprotein. However, like the endogenous HLA-B5 molecule, neither HLA-Bw58 nor HLA-B7 was expressed at the cell surface. The cloned genes were properly expressed on the surface of C1R, a control B-LCL. To determine if mouse class I alleles had the same transport requirements as the human class I glycoproteins, either mouse H-2Dp or H-2Kb class I genes were introduced into T2. Surprisingly, the H-2 class I glycoproteins were transported to the cell surface normally. These data suggest a fundamental difference between human and mouse histocompatibility antigens in their requirements for intracellular transport.

Different class I antigen oligosaccharides on a murine tumor and a lectin-resistant variant are not responsible for the differential recognition of the tumors by CTL

Previous studies have shown that whereas a highly malignant mouse cell line termed MDAY-D2 (d haplotype) does not elicit a detectable response by cytotoxic T lymphocytes (CTL) in DBA/2 mice, strong anti-tumor CTL are generated against a wheat-germ-agglutinin-resistant variant, designated MDW3. Additional evidence suggests these anti-MDW3 CTL may not be a consequence of a unique antigenic determinant on the variant cells. Because MDW3 cells are expected to differ from MDAY-D2 cells in their surface carbohydrate structures (due to their lectin resistance) and Class I major histocompatibility molecules play a crucial role in CTL-mediated responses, we speculated that the Asn-linked oligosaccharides present on Class I molecules of MDAY-D2 and MDW3 might be different and could potentially influence recognition analyses and Con A-Sepharose affinity chromatography clearly demonstrated that the oligosaccharides isolated from the H-2Dd molecule of MDAY-D2 cells are larger and more highly branched than those of the MDW3 variant. Taken together with the finding that anti-MDW3 CTL are restricted by H-2Dd, these results suggested that the larger H-2Dd oligosaccharides on MDAY-D2 cells could potentially mask or perturb determinants required for recognition by these CTL. To test this postulate, the surface Class I oligosaccharides of both MDAY-D2 and MDW3 cells were converted to simpler hybrid structures by treatment with the oligosaccharide processing inhibitor, swainsonine. However, no effect was observed on the lysis or binding of either MDAY-D2 or MDW3 cells by anti-MDW3 CTL. Thus, the results do not support the possibility that the larger H-2Dd oligosaccharides on MDAY-D2 cells are, in themselves, responsible for the poor recognition of the parent tumor by anti-MDW3 CTL. Our data do indicate, however, that CTL target binding and effector functions are not dependent on the fine structure of complex Asn-linked carbohydrates present on Class I molecules and possibly on other, accessory molecules at the target cell surface, since MDW3 cells maintained their sensitivity to lysis by CTL following swainsonine treatment.

Addition of carbohydrate side chains at novel sites on influenza virus hemagglutinin can modulate the folding, transport, and activity of the molecule

We have constructed and expressed a series of mutant influenza virus hemagglutinins, each containing a new consensus site for glycosylation in addition to the seven sites found on the wild-type protein. Oligosaccharide side chains were added with high efficiency at four of the five novel sites, located on areas of the protein's surface that are not normally shielded by carbohydrate. Investigations of the structure, intracellular transport, and biological activities of the mutant hemagglutinin molecules indicated that (a) supernumerary carbohydrate side chains can be used to shield or disrupt functional epitopes on the surface of hemagglutinin, and (b) the presence of an additional oligosaccharide may cause temperature-dependent defects in the transport of the glycoprotein. We discuss the addition of supernumerary oligosaccharides as a general tool for shielding chosen areas of the surface of proteins that enter or traverse the secretory pathway.

Processing requirements for the recognition of insulin fragments by murine T cells

In this study we investigated aspects of antigen processing using insulin and insulin A chain-derived fragments as model antigens in Ab alpha Ak beta-restricted T-cell stimulation. Similarly to other proteins, the immunodominant region of insulin recognized by these T cells is limited in size. It is located on the insulin A chain and encompasses a portion of the molecule that is represented faithfully by peptide A1-14(SSO3-)3. Efficient presentation of intact insulin and its entire A chain is dependent on uptake and processing by APC. Whereas peptides stemming from various globular proteins are known to be presented to T cells by APC without requiring processing, this is not the case with A-chain fragment A1-14 (SSO3-)3. This observation suggested that, in addition to proteolytic degradation, other mechanisms might play a role in the processing of these antigens. Three cys-residues are located in close proximity to those amino acid residues of the insulin A chain that are inferred to participate in the specific interaction with MHC class II molecules and the TcR. In A-chain derivatives that are stimulatory for the T cells or in intact insulin these cys residues are engaged in disulfide bonds or are S-sulfonated. Both linkages can be reversibly modified by reaction with thiols. Functional data indicate that from intact insulin and from structurally distinct A-chain derivatives a closely similar or identical peptide is formed and bound to class II molecules for recognition by the T cells. The question arises as to whether, in this processed peptide, the cys residues are present in reduced form, engaged in disulfide bonds, or are modified in some other way. Taken together, these findings suggest that modification of cys residues or isomerization of disulfide bonds may play a role in insulin processing. It can be expected that other proteins carrying cys residues in their immunodominant peptides may show similar processing requirements. The inhibition of N-glycosylation of proteins by tunicamycin in APC blocked the processing and presentation of insulin and OvA whereas, under the same conditions, the presentation of a processing-independent peptide was not affected. Furthermore, an autoreactive T-cell clone was capable of recognizing tunicamycin-treated APC. Since the expression of class II molecules was found to be unaltered as demonstrated by cytofluorometric analysis the deficient N-glycosylation appears to have little influence on class II antigen-mediated T-cell recognition but interferes with uptake of antigen and/or its processing by APC.

Evidence that the phagocytosis mediated by the peanut agglutinin-like activity of IgG(Fc) receptors of human monocytes is selectively modulated by estradiol and natural estrogens

The percentage of human monocytes (MCs) that are able to form rosettes with, and to phagocytose, IgG-coated sheep red blood cells (IgG-SRBCs) has been first determined in vitro by a classical rosette assay in 12 postmenopausal (PM) women. Half of them never received any suppletive estrogen (E) therapy at the time of testing, whereas the other six were chronically treated with E. Three different preparations of the same anti-SRBC IgG antibody batch were coated to SRBCs: the first one was the starting antibody preparation [IgG(total] and the other two were purified by affinity chromatography either on Sepharose-concanavalin A (Con A) or on agarose-peanut agglutinin (PNA) columns specifically recognizing terminal, and/or accessible, alpha-mannosyl [IgG(Con A)] or beta-galactosyl [IgG(PNA)] residues of the Fc domain, respectively. The three IgG preparations exhibited similar hemagglutinating antibody titers (1/100). All experiments were conducted using a coating range of 5000 to 6000 IgG antibody molecules per SRBC. In PM women with E, the rosetting capacity of autologous MCs (percentage of MCs rosetting at least three IgG-SRBCs), their phagocytosing capacity (percentage of MCs ingesting at least three IgG-SRBCs), and the phagocytosis index (number of SRBCs ingested/100 MCs) were similar for each IgG-SRBC preparation considered. In contrast, in PM women without E, the capacity of MCs to phagocytose IgG(PNA)-SRBCs, as well as the phagocytosis index measured with those SRBCs, was strongly reduced (P less than 0.01 at least), when compared to the same parameters determined using IgG(total)-SRBCs and IgG(Con A)-SRBCs.(ABSTRACT TRUNCATED AT 250 WORDS)

Constraint of the translational diffusion of a membrane glycoprotein by its external domains

The translational diffusion of wild-type and underglycosylated molecules of a membrane-integral glycoprotein the Ld class I major histocompatibility complex (MHC) antigen has been measured. The Ld mutant molecules, which lack one or more glycosylation sites, had larger translational diffusion coefficients, D, than did wild-type Ld molecules glycosylated at three sites. The increase in D is linear with loss of glycosylation. The highest value of D approaches that for translational diffusion of molecules constrained only by viscosity of the membrane lipid bilayer. These results indicate that the external portions of cell surface glycoproteins interact significantly with other nearby molecules.

Cytotoxic T lymphocytes from HLA-A2 transgenic mice specific for HLA-A2 expressed on human cells

CTL clones were derived from HLA-A2.1 transgenic mice by immunization with a human cell expressing HLA-A2.1. None of these clones lysed murine transfectants, and only 3 of 23 lysed monkey transfectants expressing HLA-A2. In contrast, all of these clones lysed a wide variety of human cells expressing HLA-A2.1. These results demonstrate the existence of species-specific epitopes on the HLA-A2.1 molecule, and suggest that these epitopes are formed by the association of class I MHC products with one or more endogenous species-specific molecules. These results provide an explanation for the frequently observed failure of HLA class I-specific CTL to recognize these antigens on murine transfectants. These results also suggest that such endogenous proteins may also contribute to the formation of epitopes recognized by allospecific CTL.

Allele and locus-specific differences in cell surface expression and the association of HLA class I heavy chain with beta 2-microglobulin: differential effects of inhibition of glycosylation on class I subunit association

The assembly of HLA class I antigens, and the contribution of the single N-linked glycan to this process were examined. We observed a requirement for N-linked glycosylation in the proper assembly and surface expression of HLA-B locus products in particular, although considerable variation was seen within the allelic series of the HLA-A and B loci. We conclude that the single N-linked glycan can contribute in a major way to that conformation of the heavy (H) chain which is competent to associate with the light chain beta 2-microglobulin, and that the presence, rather than the type, of carbohydrate chain is important in this respect. The association of human class I H chains with beta 2-microglobulin shows biphasic kinetics, where an initially rapid phase is followed by a prolonged period during which no further association can be measured. It appears that HLA-C H chains are initially synthesized in amounts similar to HLA-A and B H chains, but associate inefficiently with beta 2-microglobulin, resulting in low expression of HLA-C at the cell surface. The individual stages of assembly and maturation of class I antigens including the transfer from Golgi to cell surface were found to display characteristic allelic variation.

Use of carbohydrate probes in conjunction with fluorescence-activated cell sorting to select mutant cell lines of Chlamydomonas with defects in cell surface glycoproteins

Two carbohydrate-binding probes (the lectin concanavalin A and the anti-carbohydrate monoclonal antibody FMG-1) have been utilized in conjunction with fluorescence-activated cell sorting to select cell lines of Chlamydomonas reinhardtii that contain defects in cell surface-exposed glycoproteins. Two very different selection strategies (sorting cells with the lowest binding for the FMG-1 monoclonal antibody or the highest binding of concanavalin A) yield a class of mutant cells that exhibit a total lack of binding of the monoclonal antibody to cell wall and plasma membrane glycoproteins along with an increased affinity for concanavalin A. Detailed characterization of one such mutant cell line, designated L-23, is provided. The subtle glycosylation defect exhibited by this cell line does not alter the ability of the affected glycoproteins to be targeted to the flagellar membrane and does not affect the expression of flagellar surface motility, a phenomenon that appears to involve the major concanavalin A-binding glycoprotein of the flagellar membrane. This approach has general applicability for dissecting the role of carbohydrate epitopes in the targeting and function of any cell surface glycoprotein for which suitable carbohydrate probes are available.

Site-directed mutagenesis of class I HLA genes. Role of glycosylation in surface expression and functional recognition

We have investigated the role of the carbohydrate moiety on the HLA-B7 molecule in mAb and CTL recognition using oligonucleotide-directed mutagenesis and gene transfer techniques. A conservative substitution of asparagine to glutamine at amino acid 86 in HLA-B7 was created to abolish the unique glycosylation site present on all HLA molecules. A second mutant B7 molecule was made by substituting asparagine-aspartic acid-threonine for the resident lysine-aspartic acid/lysine tripeptide at amino acids 176-178, thus creating an N-linked glycan at amino acid 176, which is additionally present on all known murine H-2 class I antigens. Upon gene transfer into mouse and human cell recipients, the HLA-B7M176+ mutant and normal HLA-B7 expressed identical levels of surface protein. However, the binding of two mAbs (MB40.2 and MB40.3) thought to recognize different epitopes of the HLA-B7 molecule was completely eliminated. In contrast, the HLA-B7M86- mutant displayed no surface expression (mouse L cells) or minimal surface expression (human RD cells or mouse L cells coexpressing human beta 2 microglobulin [beta 2m]) after indirect immunofluorescence (IIF) and flow cytometric analysis with a panel of 12 HLA-B7 mAb reactive with monomorphic and polymorphic determinants. Immunoprecipitation analysis demonstrated that intracellular denatured mutant protein was present. Tunicamycin treatment did not rescue the expression of HLA-B7M86- antigens to the cell surface; while interferon did induce higher levels of surface expression. Tunicamycin treatment also did not allow binding of the mAbs MB40.2 or MB40.3 to HLA-B7M176+ mutant antigens, suggesting that the carbohydrate moiety itself was not directly involved in the recognition or conformation of these mAb epitopes. Further mutation of the B7M86- molecule to create a glycan moiety at amino acid position 176 (B7M86-/176+) did not rescue normal levels of surface expression. Finally, neither mutation was seen to affect recognition by a panel of 12 allospecific CTL clones. The low expression of HLA-B7M86- on the surface of human cell transfectants was sufficient to achieve lysis, albeit at a reduced efficiency, and lysis could be increased by interferon induction of higher levels of expression. Thus, the carbohydrate moiety on HLA antigens plays a minimal or nonexistent role in recognition by available mAb and allospecific CTL clones.

High lateral mobility of endogenous and transfected alkaline phosphatase: a phosphatidylinositol-anchored membrane protein

The lateral mobility of alkaline phosphatase (AP) in the plasma membrane of osteoblastic and nonosteoblastic cells was estimated by fluorescence redistribution after photobleaching in embryonic and in tumor cells, in cells that express AP naturally, and in cells transfected with an expression vector containing AP cDNA. The diffusion coefficient (D) and the mobile fraction, estimated from the percent recovery (%R), were found to be cell-type dependent ranging from (0.58 +/- 0.16) X 10(-9) cm2s-1 and 73.3 +/- 10.5 in rat osteosarcoma cells ROS 17/2.8 to (1.77 +/- 0.51) X 10(-9) cm2s-1 and 82.8 +/- 2.5 in rat osteosarcoma cells UMR106. Similar values of D greater than or equal to 10(-9) cm2s-1 with approximately 80% recovery were also found in fetal rat calvaria cells, transfected skin fibroblasts, and transfected AP-negative osteosarcoma cells ROS 25/1. These values of D are many times greater than "typical" values for membrane proteins, coming close to those of membrane lipid in fetal rat calvaria and ROS 17/2.8 cells (D = [4(-5)] X 10(-9) cm2s-1 with 75-80% recovery), estimated with the hexadecanoyl aminofluorescein probe. In all cell types, phosphatidylinositol (PI)-specific phospholipase C released 60-90% of native and transfection-expressed AP, demonstrating that, as in other tissue types, AP in these cells is anchored in the membrane via a linkage to PI. These results indicate that the transfected cells used in this study possess the machinery for AP insertion into the membrane and its binding to PI. The fast AP mobility appears to be an intrinsic property of the way the protein is anchored in the membrane, a conclusion with general implications for the understanding of the slow diffusion of other membrane proteins.

A single amino acid substitution in the alpha 3 domain of an H-2 class I molecule abrogates reactivity with CTL

We previously described a somatic cell expressing a variant H-2Dd molecule that did not serve as a target for alloreactive anti-Dd CTL. The mutant cell line had been isolated by its failure to express a serological epitope present on the H-2Dd alpha 3 domain. In the present study the alpha 3 domain of the Dd molecule of this somatic cell variant was sequenced and a single nucleotide change resulting in a glutamic acid to lysine substitution at residue 227 was identified. This change was reproduced in the cloned H-2Dd gene by oligonucleotide-directed mutagenesis. Cells transfected with this mutant gene were not killed by anti-H-2Dd CTL. Because previous studies using hybrid H-2 class I molecules had established that the alpha 3 domain does not express allele-specific determinants recognized by CTL, our results raise the possibility that residues in the alpha 3 domain of H-2 class I molecules are critical for CTL recognition and constitute a conserved (or monomorphic) determinant recognized by CTL.

Introduction of H-2Dd determinants into the H-2Ld antigen by site-directed mutagenesis

We used site-directed mutagenesis to localize serologically defined (s) and CTL (c)-defined alloantigenic determinants to discrete amino acid sequences of a murine MHC class I antigen. Based on the prediction that amino acid position 63-73 of the H-2Dd antigen forms s-allodeterminants, the H-2Ld gene was mutated in a sequential fashion to replace codons for amino acid positions 63, 65, 66, 70, and 73 with those of the H-2Dd amino acids. Epitopes of the mutant antigens expressed in L-cells were examined by the binding of a series of mAbs specific for the H-2Dd antigen. The mutant antigen M66 had substitutions at residues 63, 65, and 66, and resulted in the acquisition of a number of H-2Dd-specific s-epitopes. Mutant M70 had an additional substitution at residue 70, which led to the gain of multiple additional H-2Dd s-epitopes. Together, more than half of all the relevant H-2Dd s-epitopes were mapped into amino acid position 63-70 of the H-2Dd molecule, which was expressed in the mutant H-2Ld gene. The final mutation at residue 73 (M73) caused no new epitope gains, rather, a few Dd s-epitopes acquired by the preceding mutations were lost. All of the H-2Ld-specific s-determinants were retained in the mutant molecules, as were H-2Dd s-determinants specific for the alpha-2 or alpha-3 domains. Changes of these residues affected c-determinants defined by CTL. Anti-H-2Dd CTL cultures and an anti-H-2Dd CTL clone recognized the mutant H-2Ld molecules, M66 and M70. Some CTL clones generated against the Q10d molecule, which has an identical sequence to H-2Dd between residues 61 and 73, failed to recognize native H-2Dd or Ld but did crossreact with mutant Ld. While bulk-cultured anti-H-2Ld CTL cultures reacted strongly against M73, bulk-cultured H-2Ld restricted anti-vesicular stomatitis virus CTL did not. Finally, at the clonal level two of three anti-H-2Ld CTL clones lost reactivity with some or all of these mutant molecules. From these results we conclude that a stretch of amino acids from position 63 to 70 of the alpha-1 domain controls major s- and c-antigenic sites on the H-2Dd antigen and c-sites on H-2Ld antigen.

Changes in HLA A, B, C expression during "spontaneous" transformation of human urothelial cells in vivo

The immortalized but non-tumourigenic and non-invasive human urothelial cell line, Hu 609, known to express the appropriate HLA A,B antigens (A2,-; B5,-) has previously been demonstrated to undergo "spontaneous" in vitro transformation into an invasive and tumourigenic subline, Hu 609T/MV. This subline does not express the polymorphic HLA epitopes. In the present investigation we have followed two additional "spontaneous" transformations of the Hu 609 cell line into malignant sublines. Evidence is presented that the development of morphological changes and tumourigenicity were accompanied by a gradual loss of the expression of polymorphic HLA A,B epitopes and a reduction in the expression of monomorphic HLA A,B,C antigens. Antigens could be detected again after neuraminidase treatment. We conclude that the urothelial Hu 609 cell line after "spontaneous" transformation still possesses the HLA A,B epitopes. The observed quantitative differences in HLA expression between Hu 609 and its malignant sublines may be due to masking of the HLA antigens by sialic acid containing tumour-associated highly branched glycoproteins.

Carbohydrate moieties of rat MHC class I antigens

The rat major histocompatibility complex class I antigens RT1.Au and RT1.Eu from the u haplotype and RT1. An from the n haplotype were labeled with 14C-asparagine or with 3H-fucose, mannose, galactose, and N-acetylglucosamine. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis showed complete removal of radioactivity from the sugar-labeled antigen heavy chains by digestion with glycopeptidase F, an enzyme that removes N-linked glycans completely. High performance liquid chromatography analysis of the tryptic digests of the mixed sugar-labeled and asparagine-labeled antigens demonstrated that all the sugar-labeled peptides were coincident with asparagine-labeled peptides. The An antigen showed three glycopeptides, each of which had different amounts of sugar radioactivity. The antigens Au and Eu showed two glycopeptides with different amounts of radioactivity but at identical positions in the two antigens. Antigen Eu had an additional glycopeptide with a lower amount of radioactivity. The positions of the glycopeptides from the Au and Eu antigens were different from those of the An antigen. The peptide profiles of the 14C-asparagine-labeled Au and Eu antigens demonstrated distinct differences between the molecules. The results of this study show that: (a) all the glycans on rat class I antigens are N-linked, as they are on H-2 and HLA class I antigens; (b) there are compositional differences among the glycans in each of the three antigens; (c) the glycosylation pattern of the rat class I antigens is similar to that of the mouse class I antigens, which contain two or three glycans, in contrast to that of the human class I antigens, which contain only one glycan; and (d) the antigens Au and Eu from the same haplotype are more closely related to each other than they are to the An antigen.

Amino acid sequences in the alpha 1 domain and not glycosylation are important in HLA-A2/beta 2-microglobulin association and cell surface expression

The role of the single carbohydrate moiety present on the HLA-A2 molecule was studied by introducing several amino acid substitutions (by site-directed mutagenesis of the HLA-A2 gene) in the consensus glycosylation sequence Asn-X-Ser. Two different amino acid substitutions of the asparagine residue at position 86 (glutamine and aspartic acid) resulted in the synthesis of ca. 39,000-molecular-weight nonglycosylated heavy chains that were detected in the cytoplasm but not on the surface of mouse L-cell transfectants. However, a low level of surface expression was detected following transfection of human (rhabdomyosarcoma) cells or mouse L cells containing human beta 2-microglobulin. The defect in surface expression was not due to the absence of the glycan moiety, since the substitution of a glycine for a serine at amino acid 88 did not have the same drastic effect in the presence of human beta 2-microglobulin. These and other data suggest that the asparagine residue may play a critical role in the conformation of the HLA heavy chain and its interaction with beta 2-microglobulin. Immunofluorescence microscopy following permeabilization of the transfectants demonstrated that the unglycosylated HLA heavy chains are sequestered in an unidentified cellular compartment that is different from the Golgi structure.

Amino acid sequences in the alpha 1 domain and not glycosylation are important in HLA-A2/beta 2-microglobulin association and cell surface expression

The role of the single carbohydrate moiety present on the HLA-A2 molecule was studied by introducing several amino acid substitutions (by site-directed mutagenesis of the HLA-A2 gene) in the consensus glycosylation sequence Asn-X-Ser. Two different amino acid substitutions of the asparagine residue at position 86 (glutamine and aspartic acid) resulted in the synthesis of ca. 39,000-molecular-weight nonglycosylated heavy chains that were detected in the cytoplasm but not on the surface of mouse L-cell transfectants. However, a low level of surface expression was detected following transfection of human (rhabdomyosarcoma) cells or mouse L cells containing human beta 2-microglobulin. The defect in surface expression was not due to the absence of the glycan moiety, since the substitution of a glycine for a serine at amino acid 88 did not have the same drastic effect in the presence of human beta 2-microglobulin. These and other data suggest that the asparagine residue may play a critical role in the conformation of the HLA heavy chain and its interaction with beta 2-microglobulin. Immunofluorescence microscopy following permeabilization of the transfectants demonstrated that the unglycosylated HLA heavy chains are sequestered in an unidentified cellular compartment that is different from the Golgi structure.

Cell-surface expression of H-2Db requires N-linked glycans

The question of whether beta-2 microglobulin (B2m)-independent expression of the mouse major histocompatibility complex (MHC) antigen H-2Db results from the atypical glycosylation pattern associated with this MHC antigen (i.e., three glycans instead of two) has been addressed. Cell-surface expression of transfected H-2Db in the B2m deficient cell line R1E was completely abolished by the drug tunicamycin (Tm). Introduction of a functional B2m gene by transfection did not re-establish cell-surface expression of Db in the presence of Tm. Tm had no effect, however, on the expression of a truncated Db molecule lacking the alpha 1 and alpha 2 domains which is glycosylated at amino acid position 256, suggesting that the Db molecule, unlike other class I antigens, possesses an unstable conformation in the alpha 1 and/or alpha 2 domains which requires the attachment of glycans before it is transported to the cell surface. Once attached, however, glycans may confer a stable alpha 1/alpha 2 conformation apparently peculiar to Db which allows cell-surface expression in the absence of B2m.

Structure and function of three novel MHC class I antigens derived from a C3H ultraviolet-induced fibrosarcoma

The UV-induced, C3H fibrosarcoma, 1591, expresses at least three unique MHC class I antigens not found on normal C3H tissue. Here we report the complete DNA sequence of the three novel class I genes encoding these molecules, and describe in detail the recognition of the individual products by tumor-reactive and allospecific CTL. Remarkably, although C3H does not appear to express H-2L locus information, this C3H tumor expresses two distinct antigens, termed A149 and A166, which are extremely homologous to each other and to the H-2Ld antigen from BALB/c. The gene encoding the third novel class I antigen from 1591, A216, is quite homologous to H-2Kk) throughout its 3' end. Since all three of these genes account for polymorphic restriction fragments not found in C3H, it is likely that they were derived by recombination from the endogenous class I genes of C3H. The DNA sequence homology of A149, A166, and H-2Ld is especially significant given the functional conservation observed between the products of these genes. Limited sequence substitutions appear to correlate with some of the discrete serological differences observed between these molecules. In addition, both A149 and A166 crossreact, but to differing extents, with H-2Ld at the level of T cell recognition. Our results are consistent with the view that CTL recognize complex conformational determinants on class I molecules, but extend previous observations by comparing a set of antigens with discrete and overlapping structural and functional differences.