Expression of T cell antigen receptor heterodimers in a lipid-linked form

Preparation of soluble recombinant T cell receptor alpha chain by using a calmodulin fusion expression system

We have isolated a full length T cell receptor alpha chain (TCR alpha) cDNA derived from a bee venom phospholipase A2-specific mouse suppressor T cell hybridoma. A bacterial fusion expression system was constructed using rat calmodulin as a fusion partner for production of soluble TCR alpha. In this system, calmodulin-TCR alpha fusion protein was expressed at a high level in the soluble fraction of bacterial cell lysate, and could be purified by binding of calmodulin portion of the protein to phenyl-Sepharose. Using this system, fusion proteins containing a TCR alpha peptide corresponding to the complete extracellular region, V alpha-J alpha region or C alpha extracellular region were isolated. TCR alpha peptides were then released from the fusion proteins by digestion with thrombin which recognizes a linker sequence between calmodulin portion and TCR alpha segment. Polyclonal antibodies against constant region of TCR alpha chain (C alpha) were obtained by immunization of rabbits with the recombinant C alpha peptide. ELISA for TCR protein was established by using the polyclonal antibodies and the monoclonal antibody specific for C alpha region.

Specific elimination of IgE production using T cell lines expressing chimeric T cell receptor genes

B cells that are destined to secrete IgE express a membrane-bound form of IgE (mIgE) on their cell surface. Thus, elimination of such mIgE-positive cells should result in the suppression of IgE production, thereby alleviating the symptoms of IgE-mediated allergy. In this study, we examined, in a model system, whether IgE-specific effector T cells can be used specifically to eradicate IgE-producing B cells. To this end, we endowed T cells with anti-IgE specificity using chimeric T cell receptors (cTCR) containing the variable region domain (Fv) of the 84.1c non-anaphylactic anti-mouse IgE monoclonal antibody (mAb). Two configurations of chimeric receptor were used: in the first, we combined the heavy and light variable region chains of 84.1c with the constant (C) regions of the TCR alpha and beta chains. The second construct consisted of a chimeric single-chain receptor (scFvR), composed of a single-chain Fv region of the 84.1c antibody and the C beta domain of the TCR. Following transfection of the cTCR or the scFvR genes into the murine MD.45 cytotoxic T cell hybridoma or the Jurkat human T cell line, functional expression of IgE-specific chimeric receptors was detected on the cell surface. The transfected cells secreted interleukin-2 upon stimulation with immobilized IgE or fixed IgE-producing hybridoma cells. Moreover, cytotoxic T cell hybridomas expressing the chimeric receptor genes specifically eliminated IgE-secreting B cells in vitro, resulting in isotype-specific suppression of IgE production.

B7-1 amplifies the response to interleukin-2-secreting tumor vaccines in vivo, but fails to induce a response by naive cells in vitro

Parental and interleukin-2 (IL-2)-secreting CMS5 tumor cells were transfected with the B7-1 costimulatory molecule to amplify anti-tumor responses. CMS5 cells transfected with B7-1 grew more slowly in vivo than did parental CMS5 cells. Moreover, tumor cells secreting levels of IL-2 too low to cause rejection alone were rejected following transfection with B7-1. To determine whether the expression of B7-1 enabled the tumor cells to activate T cells directly, their ability to stimulate in vitro functional responses by T cells was examined. We found that neither B7-1+ nor IL-2-secreting, B7-1+ CMS5 cells stimulated naive spleen cells to proliferate or to become cytotoxic. In contrast, restimulation of primed T cells by B7-1+ CMS5 cells resulted in stronger cytotoxicity responses than seen following restimulation by parental CMS5 cells. Lysis was even higher if the B7-1+ tumor cells also secreted IL-2. Our results suggest that the expression of costimulatory molecules can augment responses generated by vaccinating with IL-2-secreting tumor cells. Furthermore, they are consistent with the hypothesis that the initiation of an anti-tumor response by naive T cells may depend upon initial antigen presentation by another unidentified cell and that the major action of IL-2-secreting and/or B7-1+ tumor cell vaccines might be to potentiate the response of already primed cells.

Expression of an unusual Bw4 epitope by a subtype of HLA-B8 [B*0802]

The primary structure of a variant HLA-B8 antigen has been determined by cDNA cloning and sequencing. The variant, B*0802 differs, from the common B*0801 subtype at positions 77-83 of the alpha 1 helix that determine the Bw4 and Bw6 public epitopes. Whereas B*0801 has the common Bw6 motif, B*0802 has the Bw4 motif found in B*13 and B*44 allotypes. Serological analysis of B cell lines expressing B*0802 and of a B*0802 transfectant made with the HLA-A,B negative cell line 721.221 shows that B*0802 reacts with Bw4-specific antibodies, but at a level much lower than expected for Bw4 positive HLA-B allotypes.

Human CD6 possesses a large, alternatively spliced cytoplasmic domain

Human CD6 is a monomeric 105/130-kDa T cell surface glycoprotein that is involved in T cell activation. The apparent discrepancy between the size of the cytoplasmic domain in human (44 amino acids) and mouse (243 amino acids) CD6, led us to use reverse transcriptase-polymerase chain reaction of human peripheral blood lymphocyte mRNA to isolate cDNA clones that include the carboxyl-terminal coding region of human CD6. The nucleotide sequence of the longest human cDNA clone, CD6-PB1, predicts a protein of 668 amino acids with a 244-amino acid cytoplasmic domain similar in size to and possessing 71.5% amino acid sequence identity with the cytoplasmic domain of mouse CD6. This previously unrecognized 244-amino acid cytoplasmic domain does not have significant homology to any other known protein (except mouse CD6), but does possess two proline-rich motifs containing the SH3 domain-binding consensus sequence, a serine-threonine-rich motif repeated three times, three protein kinase C phosphorylation-site motifs, and 10 casein kinase-2 phosphorylation-site motifs. These sequences are likely to play a role in the ability of CD6-specific monoclonal antibodies to stimulate T cell proliferation. Full-length CD6 cDNA containing this cytoplasmic domain sequence encodes a monomeric 105/130-kDa protein that can be immunoprecipitated from the surface of transfected cells and comigrates upon SDS-PAGE with wild-type CD6 immunoprecipitated from PBL. We also isolated two alternatively spliced forms of human CD6 cDNA lacking sequences encoding membrane-proximal regions of the cytoplasmic domain which maintain the same reading frame as CD6-PB1. The short cytoplasmic domain of the previously reported human CD6-15 cDNA clone results from a deletion of a 20-bp segment through use of an alternative 3' splice site, resulting in a frame shift and premature termination of translation relative to the clones we have isolated. These data demonstrate that human CD6 possesses a large cytoplasmic domain containing sequence motifs that are likely to be involved in signal transduction upon stimulation of T cells through CD6 ligation.

Requirement for the synergy site for cell adhesion to fibronectin depends on the activation state of integrin alpha 5 beta 1

We investigated the influence of the activation state of integrin alpha 5 beta 1 on its dependence on the PHSRN synergy site for binding to RGD in fibronectin. K562 and MV3 cells lacked alpha v beta 3 expression and adhered to fibronectin through alpha 5 beta 1. Mel57 cells adhered through alpha v beta 3 and alpha 5 beta 1. A recombinant fibronectin polypeptide, containing five type III repeats from the central cell binding domain 3Fn6-10, and a mutated polypeptide lacking the synergy site were equally effective in promoting Mel57 adhesion. For K562 and MV3, the mutated polypeptide was not or poorly active compared to the control polypeptide. Expression of alpha v beta 3 in MV3 induced strong adhesion to the mutated polypeptide. TS2/16 stimulatory beta 1-integrin antibodies or Mn2+ induced alpha 5 beta 1-mediated adhesion of K562 and MV3 to GRGDSP. In the presence of TS2/16 or Mn2+, alpha 5 beta 1-mediated MV3 adhesion to the mutated polypeptide was equally strong as adhesion to the control polypeptide. Mn2+ or TS2/16 induced weak K562 binding to the mutated polypeptide, and in the presence of a combination of phorbol 12-myristate 13-acetate, Mn2+, and TS2/16, alpha 5 beta 1-mediated K562 adhesion to the mutated and control polypeptide was equally strong. Our findings demonstrate that requirement for the PHSRN synergy site for alpha 5 beta 1-mediated adhesion to RGD in fibronectin depends on the activation state of the integrin.

Peptides which bind to E-selectin and block neutrophil adhesion

E-selectin is an inducible cell adhesion molecule which mediates rolling of neutrophils on the endothelium, an early event in the development of an inflammatory response. Inhibition of selectin-mediated rolling is a possible means for controlling inflammation-induced diseases, and several classes of compounds have been tested for this use. We describe here the use of recombinant peptide library screening for identification and optimization of novel ligands which bind to E-selectin. Several of these peptides bind with Kd values in the low nanomolar range and block E-selectin-mediated adhesion of neutrophils in static and flow-cell assays. Administration of the peptide to mice undergoing an acute inflammatory response reduced the extent of neutrophil transmigration to the site of inflammation, demonstrating the utility of this compound as a potential therapeutic. The identification of a peptide ligand for E-selectin suggests that the complete natural ligand for this adhesion molecule may include protein as well as carbohydrate moieties.

T-cell recognition of antigen. A process controlled by transient intermolecular interactions

As recently as ten years ago, the nature of the T-cell receptor for antigen was a mystery, as was the precise role of histocompatibility molecules in antigen-presentation to T cells. Although T-cell receptors have now been cloned and crystal structures of MHC/peptide molecules exist, our understanding of the parameters that characterize this interaction and other interactions relevant to T-cell immunity are still unclear. The engineering of soluble forms of proteins that mediate T-cell recognition of antigen has allowed the first measurements of these parameters. Interestingly, many of these interactions are of a transient nature, with very rapid off-rates. These data suggest a model whereby highly reversible intermolecular interactions mediate the cell-cell association. The association of adhesion molecules is probably the first step in the stabilization of a conjugate, because they are more numerous than any antigen-specific interaction, followed later by TCR-MHC engagements. Diffusion within each lipid bilayer should allow the congregation of MHC/TCR interactions at the cell-cell interface, with peptide-specific TCR interactions outcompeting irrelevant interactions. Rapid off-rates for both the antigen-specific and nonspecific interactions may be necessary to maintain reversibility, yet allow a rapid approach to equilibrium and consequent signaling when a specific antigen is present or disengagement when it is not.

Cloning of the gene encoding Trop-2, a cell-surface glycoprotein expressed by human carcinomas

We have cloned by expression the cDNA encoding Trop-2, a cell-surface glycoprotein expressed by most human carcinomas. Formal proof of the identity of the clone is the hybridization to DNA and RNA from genomic TROP2 transfectants. TROP2 is a single-copy gene in human cells, hybridizes to a single 1.8-kb mRNA from expressing sources and encodes a 35,709 Da type-1 transmembrane protein with a single transmembrane domain. TROP2 is essentially identical to GA733-1. Thus, we have proven that GA733-1, for which a protein product had not been identified, is a functional gene. TROP2 is also homologous to TROP1/KSA/GA733-2, confirming the serological similarities between the 2 molecules. The homology between the Trop-1 and Trop-2 peptides is clustered in 2 extracytoplasmic domains and in the transmembrane/cytoplasmic region. Twelve cysteines and a potential cytoplasmic tyrosine phosphorylation site are also conserved. Trop-1 and Trop-2 are homologous to serum IGF-II-binding proteins and appear as signal transducers. Thus, they likely represent novel cell-surface receptors and may play a role in regulating the growth of carcinoma cells. On the other hand, we have found no evidence for a role of Trop-2 and Trop-1 as homophilic adhesion molecules.

Molecular characterization of human CD94: a type II membrane glycoprotein related to the C-type lectin superfamily

Natural killer (NK) cells preferentially express several genes of the C-type lectin superfamily which have been implicated in the regulation of NK cell function. We demonstrate that CD94 is a type II membrane protein encoded by a unique gene of the C-type lectin superfamily. While homology of CD94 with the NK cell-associated NKR-P1 and NKG2 C-type lectin genes is limited to the structural motifs conserved in the carbohydrate recognition domain, all of these genes are on human chromosome 12, the syntenic of mouse chromosome 6, where genes of the NK complex (NKR-P1 and Ly-49) are located. An unexpected feature of CD94 is the essential absence of a cytoplasmic domain, implying that association with other receptors may be necessary for the function of this molecule.

A splice variant of alpha 6 integrin is associated with malignant conversion in mouse skin tumorigenesis

The epithelial-specific integrin alpha 6 beta 4 is suprabasally expressed in benign skin tumors (papillomas) and is diffusely expressed in carcinomas associated with an increase in the proliferating compartment. Analysis of RNA samples by reverse transcriptase-PCR and DNA sequencing revealed that chemically or oncogenically induced papillomas (n = 8) expressed a single transcript of the alpha 6 subunit, identified as the alpha 6 A splice variant. In contrast, carcinomas (n = 13) expressed both alpha 6A and an alternatively spliced form, alpha 6B. Primary keratinocytes and a number of keratinocyte cell lines that vary in biological potential from normal skin, to benign papillomas, to well-differentiated slowly growing carcinomas exclusively expressed alpha 6A. However, I7, an oncogene-induced cell line that produces highly invasive carcinomas, expressed both alpha 6A and alpha 6B transcript and protein. The expression of alpha 6B in I7 cells was associated with increased attachment to a laminin matrix compared to cell lines exclusively expressing alpha 6A. Furthermore, introduction of an alpha 6B expression vector into a papilloma cell line expressing alpha 6A increased laminin attachment. When a papilloma cell line was converted to an invasive carcinoma by introduction of the v-fos oncogene, the malignant cells expressed both alpha 6A and alpha 6B, while the parent cell line and cells transduced with v-jun or c-myc, which retained the papilloma phenotype, expressed only alpha 6A. Comparative analysis of alpha 6B expression in cell lines and their derived tumors indicate that alpha 6B transcripts are more abundant in tumors than cell lines, and alpha 6B is expressed to a greater extent in poorly differentiated tumors. These results establish a link between malignant conversion and invasion of squamous tumor cells and the regulation of transcript processing of the alpha 6 beta 4 integrin.

Reconstitution of interactions between tyrosine kinases and the high affinity IgE receptor which are controlled by receptor clustering

High affinity IgE receptor (Fc epsilon RI) signaling after contact with antigen occurs in response to receptor clustering. This paper describes methodology, based on vaccinia virus driven protein expression, for probing signaling pathways and its application to Fc epsilon RI interactions with the lyn and syk tyrosine kinases. Reconstitution of the complete tetrameric Fc epsilon RI receptor, lyn and syk in a non-hematopoietic 'null' cell line is sufficient to reconstruct clustering-controlled receptor tyrosine phosphorylation and activation of syk, without apparent requirement for hematopoietic specific phosphatases. The src family kinase lyn phosphorylates Fc epsilon RI in response to receptor clustering, resulting in syk binding to the phosphorylated Fc epsilon RI. Lyn also participates in the tyrosine phosphorylation and activation of syk in a manner which is dependent on phosphorylated Fc epsilon RI. Using overexpression of active and dominant negative syk proteins in a mast cell line which naturally expresses Fc epsilon RI, we corroborate syk's role downstream of receptor phosphorylation, and demonstrate that syk SH2 domains protect receptor ITAMs from ongoing dephosphorylation. Based on these results, we propose that receptor clustering controls lyn-mediated Fc epsilon RI tyrosine phosphorylation by shifting a balance between phosphorylation and dephosphorylation towards accumulation of tyrosine phosphorylated Fc epsilon RI. Fc epsilon RI tyrosine phosphorylation functions to bring syk into a microenvironment where it becomes tyrosine phosphorylated and activated, thereby allowing clustering to indirectly control syk activity.

Cloning the P. falciparum gene encoding PfEMP1, a malarial variant antigen and adherence receptor on the surface of parasitized human erythrocytes

Plasmodium falciparum-infected human erythrocytes evade host immunity by expression of a cell-surface variant antigen and receptors for adherence to endothelial cells. These properties have been ascribed to P. falciparum erythrocyte membrane protein 1 (PfEMP1), an antigenically diverse malarial protein of 200-350 kDa on the surface of parasitized erythrocytes (PEs). We describe the cloning of two related PfEMP1 genes from the Malayan Camp (MC) parasite strain. Antibodies generated against recombinant protein fragments of the genes were specific for MC strain PfEMP1 protein. These antibodies reacted only with the surface of MC strain PEs and blocked adherence of these cells to CD36 but without effect on adherence to thrombospondin. Multiple forms of the PfEMP1 gene are apparent in MC parasites. The molecular basis for antigenic variation in malaria and adherence of infected erythrocytes to host cells can now be pursued.

Identification of a novel peptide derived from the melanocyte-specific gp100 antigen as the dominant epitope recognized by an HLA-A2.1-restricted anti-melanoma CTL line

Cytotoxic T lymphocytes (CTL) reactive with human melanoma tumor cells occasionally display cross-reactivity with normal melanocytes. Previously, we identified the melanocyte lineage-specific antigen gp 100 that is expressed by both melanoma cells and normal melanocytes, as a target antigen for tumor-infiltrating lymphocytes derived from a melanoma patient (TIL 1200). Here, we demonstrate that the oligoclonal HLA-A2.1-restricted TIL 1200 line is reactive with 2 distinct peptides derived from the gp 100 protein. Apart from the peptide corresponding to gp 100 amino acids 457-466, we identified the gp 100 peptide 154-162 as a second epitope recognized by TIL 1200. A 100-fold lower concentration of this novel gp 100 peptide was required for target-cell sensitization compared to peptide 457-466, indicating that the 154-162 peptide is the dominant gp 100 epitope for TIL 1200. Together with the recently described gp 100 280-288 epitope, 3 distinct CTL epitopes have now been identified in gp 100, all presented in the context of HLA-A2.1. Therefore, gp 100 is an attractive target antigen in the development of immuno-therapeutic protocols against melanoma.

Low HLA-C expression at cell surfaces correlates with increased turnover of heavy chain mRNA

In comparison with HLA-A and -B, the protein products of the HLA-C locus are poorly characterized, in part because of their low level of expression at the cell surface. Here, we examine how protein-protein interactions during assembly and regulation of the mRNA level affect cell surface expression of HLA-C. We find that intrinsic properties of the HLA-C heavy chain proteins do not correlate with low cell surface expression: HLA-C heavy chains associate and dissociate with beta 2-microglobulin (beta 2m) at rates comparable to those found for HLA-A and -B, and increased competition for beta 2m does not alter the surface expression of HLA-C. From studies of chimeric genes spliced from the HLA-B7 and -Cw3 genes, we find that chimeric proteins containing the B7 peptide-binding groove can have low cell surface expression, suggesting that inefficiency in binding peptides is not the cause of low cell surface expression for HLA-C. The surface levels of HLA-A, -B, or -C in cells transfected with cDNA can be similar, implicating noncoding regions of HLA-C heavy chain genes in the regulation of surface expression. We find that HLA-C mRNA is expressed at lower levels than HLA-B mRNA and that this difference results from faster degradation of the HLA-C message. Experiments examining chimeric B7/Cw3 and B7/Cw6 genes suggest that a region determining low expression of HLA-C is to be found between the 3' end of exon 3 and a site in the 3' untranslated region, approximately 600 bases downstream of the translation stop codon.

Regulation of T-cell antigen receptor (TCR) alpha-chain expression by TCR beta-chain transcripts

The TCR is an alpha beta heterodimer, a part of the multimeric structure through which physiological T-cell activation occurs. The expression of TCR alpha chain is greatly diminished in a beta-chain-deficient mutant Jurkat cell line (J.RT3-T3.5). The relationship between the expression of the TCR alpha and beta chains has been examined by stable transfection of a series of TCR beta-chain mutant constructs into this mutant cell line. The level of alpha-chain transcript was dramatically upregulated by the expression of the beta chain and specifically by a transcript of the beta-chain variable region alone, including a transcript in which the ATG start codon was mutated. The downregulation of the endogenous alpha-chain transcripts in mutants cells lacking complete beta-chain transcripts occurred primarily at the posttranscriptional level. This evidence for a regulatory function of the TCR beta-chain gene represents an unusual regulatory pathway in which the transcript of one gene is required for the optimal expression of another gene.

Construction of a functional disulfide-stabilized TCR Fv indicates that antibody and TCR Fv frameworks are very similar in structure

Disulfide-stabilized Fvs (dsFv) are recombinant Fv fragments of antibodies in which the inherently unstable VH-VL heterodimer is stabilized by an interchain disulfide bond engineered between structurally conserved framework positions. We now design and produce a disulfide-stabilized Fv of a T cell receptor. It is composed of V alpha and V beta variable domains of the 2B4 TCR stabilized by a disulfide bond between framework residues of the TCR Fv at a site corresponding to that used for disulfide stabilization of antibody Fvs. For ease of production and detection, the TCRdsFv was fused to a truncated form of Pseudomonas exotoxin (PE38). The TCR(dsFv) retains its native conformation and is much more stable than a TCR scFv. Moreover, it is functional in biological assays. Because successful disulfide stabilization of the TCR Fv by the positions used for antibody Fv stabilization would not occur unless the mutated residues in TCR Fv are at positions closely similar to those in antibody Fvs, most likely within less than 1.5 A, these results provide very strong experimental evidence for the structural similarity between immunoglobulin and TCR antigen-binding variable domains.

The Bw4 public epitope of HLA-B molecules confers reactivity with natural killer cell clones that express NKB1, a putative HLA receptor

Although inhibition of natural killer (NK) cell-mediated lysis by the class I HLA molecules of target cells is an established phenomenon, knowledge of the features of class I molecules which induce this effect remains rudimentary. Using class I alleles HLA-B*1502 and B*1513 which differ only at residues 77-83 which define the Bw4 and Bw6 serological epitopes, we tested the hypothesis that the presence of the Bw4 epitope on class I molecules determines recognition by NKB1+ NK cells. HLA-B*1513 possesses the Bw4 epitope, whereas B*1502 has the Bw6 epitope. Lysis by NKB1+ NK cell clones of transfected target cells expressing B*1513 as the only HLA-A, -B, or -C molecule was inhibited, whereas killing of transfectants expressing B*1502 was not. Addition of an an anti-NKB1 monoclonal antibody reconstituted lysis of the targets expressing B*1513, but did not affect killing of targets bearing B*1502. The inhibitory effect of B*1513 could be similarly prevented by the addition of an anti-class I monoclonal antibody. These results show that the presence of the Bw4 epitope influences recognition of HLA-B molecules by NK cells that express NKB1, and suggest that the NKB1 molecule may act as a receptor for Bw4+ HLA-B alleles. Sequences outside of the Bw4 region must also affect recognition by NKB1+ NK cells, because lysis of transfectants expressing HLA-A*2403 or A*2501, which possess the Bw4 epitope but are in other ways substantially different from HLA-B molecules, was not increased by addition of the anti-NKB1 antibody. Asparagine 86, the single site of N-linked glycosylation on class I molecules, is in close proximity to the Bw4/Bw6 region. The glycosylation site of the Bw4-positive molecule B*5801 was mutated, and the mutant molecules tested for inhibition of NKB1+ NK cells. Inhibition that could be reversed by addition of the anti-NKB1 monoclonal antibody was observed, showing the presence of the carbohydrate moiety is not essential for class I recognition by NKB1+ NK cell clones.

Quantitative export of FGF-2 occurs through an alternative, energy-dependent, non-ER/Golgi pathway

Although basic fibroblast growth factor (bFGF/FGF-2) is found outside cells, it lacks a conventional signal peptide sequence; the mechanism underlying its export from cells is therefore unknown. Using a transient COS-1 cell expression system, we have identified a novel membrane-associated transport pathway that mediates export of FGF-2. This export pathway is specific for the 18-kD isoform of FGF-2, is resistant to the anti-Golgi effects of Brefeldin A, and is energy-dependent. In FGF-2-transfected COS-1 cells, this ER/Golgi-independent pathway appears to be constitutively active and functions to quantitatively export metabolically-labeled 18-kD FGF-2. Co-transfection and co-immunoprecipitation experiments, using a vector encoding the cytoplasmic protein neomycin phosphotransferase, further demonstrated the selectivity of this export pathway for FGF-2. When neomycin phosphotransferase was appended to the COOH-terminus of 18-kD FGF-2, the chimera was exported. However, the transmembrane anchor sequence of the integral membrane glycoprotein (G protein) of vesicular stomatitis virus (VSV) blocked export. The chimeric protein localized to the plasma membrane with its FGF-2 domain extracellular and remained cell-associated following alkaline carbonate extraction. Taken together, the data suggest that FGF-2 is "exported" from cells via a unique cellular pathway, which is clearly distinct from classical signal peptide-mediated secretion. This model system provides a basis for the development and testing of therapeutic agents which may block FGF-2 export. Such an intervention may be of considerable use for the treatment of angiogenesis-dependent diseases involving FGF-2.

Different roles for the Fc epsilon RI gamma chain as a function of the receptor context

The high affinity immunoglobulin E receptor (Fc epsilon RI) and the B and T cell antigen receptors (TCR) are multimeric complexes containing subunits with cytoplasmic antigen recognition activation motifs (ARAMs). The presence of multiple motifs may be a way to amplify a single signal or provide independent activation modules. Here we have compared the signaling capacity of the same Fc epsilon RI gamma motif in the context of two different receptors, Fc epsilon RI and TCR/CD3, simultaneously reconstituted on the surface of the same zeta-deficient T cell line. Both reconstituted receptors mediate early (phosphorylation) and late (interleukin [IL]-2 release) signals. Mutation of the two tyrosine residues of ARAM gamma alters early signaling by both receptors, but the set of substrates phosphorylated via ARAM gamma is different for each receptor and is thus dependent on the receptor context. Furthermore, the mutations prevent Fc epsilon RI- but not TCR/CD3-mediated IL-2 release. These data demonstrate that ARAM gamma is necessary for allowing both receptors to phosphorylate the complete set of substrates, and that the CD3 complex, unlike the Fc epsilon RI beta chain, contains activation modules capable of compensating for the absence of a functional ARAM gamma in generating late signals such as IL-2 release.

Kinetics of T-cell receptor binding to peptide/I-Ek complexes: correlation of the dissociation rate with T-cell responsiveness

Recognition by T-cell antigen receptors (TCRs) of processed peptides bound to major histocompatibility complex (MHC) molecules is required for the initiation of most T-lymphocyte responses. Despite the availability of soluble forms of TCRs and MHC heterodimers, this interaction has proven difficult to study directly due to the very low affinity. We report here on the kinetics of TCR binding to peptide/MHC complexes in a cell-free system using surface plasmon resonance. The apparent association rates for the interactions of related peptide/MHC complexes to one such TCR are relatively slow (900-3000 M-1.s-1) and dissociation rates are very fast (0.3-0.06 s-1) with t1/2 of 2-12 s at 25 degrees C. The calculated affinity of the engineered soluble molecules compares well with previously reported competition data for native TCRs or competition data reported here for native peptide/MHC complexes, indicating that these soluble heterodimers bind in the same manner as the original molecules expressed on cells. We also find that the peptide variants which give weaker T-cell stimulatory responses have similar affinities but distinctly faster dissociation rates compared with the original peptide (when loaded onto the MHC molecule) and that this later property may be responsible for their lower activity. This has implications for both downstream signaling events and models of TCR-peptide antagonists.

Functional three-domain single-chain T-cell receptors

T-cell receptors (TCRs) are membrane anchored heterodimers structurally related to antibody molecules. Single-chain antibodies can be engineered by linking the two variable domains, which fold properly by themselves. However, proper assembly of the variable domains of a human TCR (V alpha and V beta) that recognize the HLA-DR2b/myelin basic protein-(85-99) peptide complex was critically dependent on the addition of a third domain, the constant region of the TCR beta chain (C beta), to the single-chain construct. Single-chain molecules with the three-domain design, but not those with the two-domain design, expressed in a eukaryotic cell as chimeric molecules linked either to glycosyl phosphatidylinositol or to the transmembrane/cytoplasmic domains of the CD3 zeta chain were recognized by a conformation-sensitive monoclonal antibody. The chimeric three-domain single-chain TCR linked to CD3 zeta chain signaled in response to both the specific HLA-DR/peptide and the HLA-DR/superantigen staphylococcal enterotoxin B complexes. Thus, by using this three-domain design, functional single-chain TCR molecules were expressed with high efficiency. The lipid-linked single-chain TCR was solubilized by enzymatic cleavage and purified by affinity chromatography. The apparent requirement of the constant domain for cooperative folding of the two TCR variable domains may reflect significant structural differences between TCR and antibody molecules.

Activation-induced proteolysis of cytoplasmic domain of zeta in T cell receptors and Fc receptors

The CD3-T cell receptor (TCR) complex on T cells and the Fc gamma receptor type III (Fc gamma RIII)-zeta-gamma complex on natural killer cells are functionally analogous activation receptors that associate with a family of disulfide-linked dimers composed of the related subunits zeta and gamma. Immunochemical analysis of receptor complexes separated on two-dimensional diagonal gels allowed the identification of a previously uncharacterized zeta-p14 heterodimer. zeta-p14 is a component of both CD3-TCR and Fc gamma RIII-zeta-gamma. Peptide mapping analysis shows that p14 is structurally related to zeta, suggesting that it is either: (i) derived from zeta proteolytically or (ii) the product of an alternatively spliced mRNA. The observation that COS cells transformed with a cDNA encoding zeta express zeta-p14 supports the former possibility. The expression of CD3-TCR complexes including zeta-p14 increases following activation with phorbol 12-myristate 13-acetate or concanavalin A, suggesting that proteolysis of zeta may contribute to receptor modulation or desensitization.

A general method for facilitating heterodimeric pairing between two proteins: application to expression of alpha and beta T-cell receptor extracellular segments

Generation of soluble T-cell receptor (TCR) molecules by a variety of genetic engineering methods has been hampered by inefficient pairing of alpha and beta subunits in the absence of their respective transmembrane regions and associated CD3 components. To overcome this obstacle, we have added 30-amino acid-long segments to the carboxyl termini of alpha and beta extracellular domains via a cleavable flexible linker. These peptide segments (BASE-p1 for alpha and ACID-p1 for beta) have been previously shown to selectively associate to form a stable heterodimeric coiled coil termed a leucine zipper. Homodimeric structures are not permitted due to electrostatic repulsion among amino acid side chains. Expression of a representative TCR-leucine zipper fusion protein in a baculovirus expression system results in production of alpha beta TCR heterodimer at 0.6-1.4 mg/liter. This yield is 5- to 10-fold greater than that of the TCR expressed in the absence of the synthetic leucine zipper sequence. The structure of the TCR component of the fusion heterodimer was judged to be native when probed with a panel of 17 mAbs specific for alpha and beta constant and variable domains. A mAb specific for the isolated BASE-p1/ACID-p1 coiled coil was also generated and shown to react with the TCR fusion protein. The above technology should be broadly useful in the efficient production and purification of TCRs as well as other heterodimeric proteins.

B70/B7-2 is identical to CD86 and is the major functional ligand for CD28 expressed on human dendritic cells

Dendritic cells comprise a system of highly efficient antigen-presenting cells involved in the initiation of T cell responses. Herein, we investigated the role of the CD28 pathway during alloreactive T cell proliferation induced by dendritic-Langerhans cells (D-Lc) generated by culturing human cord blood CD34+ progenitor cells with granulocyte/macrophage colony-stimulating factor and tumor necrosis factor alpha. In addition to expressing CD80 (B7/BB1), a subset of D-Lc expressed B70/B7-2. Binding of the CTLA4-Ig fusion protein was completely inhibited by a combination of monoclonal antibodies (mAbs) against CD80 and B70/B7-2, indicating the absence of expression of a third ligand for CD28/CTLA-4. It is interesting to note that mAbs against CD86 completely prevented the binding of CTLA4-Ig in the presence of mAbs against CD80 and bound to a B70/B7-2-transfected fibroblast cell line, demonstrating that the B70/B7-2 antigen is identical to CD86. CD28 triggering was essential during D-Lc-induced alloreaction as it was inhibited by mAbs against CD28 (9 out of 11 tested). However, none of six anti-CD80 mAbs demonstrated any activity on the D-Lc-induced alloreaction, though some were previously described as inhibitory in assays using CD80-transfected cell lines. In contrast, a mAb against CD86 (IT-2) was found to suppress the D-Lc-dependent alloreaction by 70%. This inhibitory effect was enhanced to > or = 90% when a combination of anti-CD80 and anti-CD86 mAbs was used. The present results demonstrate that D-Lc express, in addition to CD80, the other ligand for CTLA-4, CD86 (B70/B7-2), which plays a primordial role during D-Lc-induced alloreaction.

Protein transfer of preformed MHC-peptide complexes sensitizes target cells to T cell cytolysis

Recombinant GPI-anchored HLA-A2.1 (HLA-A2.1-GPI/beta 2m) was used as a protein transfer vehicle to deliver a hepatitis B virus antigenic peptide to the surfaces of cytotoxic T cell targets. Empty HLA-A2.1-GPI/beta 2m was first produced in D. melanogaster cotransfectants and immunoaffinity purified. Cell coating with HLA-A2.1-GPI/beta 2m was shown to occur rapidly, and to be protein concentration dependent. Protein-transferred HLA-A2.1-GPI/beta 2m effectively presented a hepatitis B virus peptide to peptide-specific HLA-A2.1-restricted T cell clones in cytotoxicity assays. Protein transfer of functional GPI-modified class I MHC-antigenic peptide complexes represents a novel strategy for delivering functional antigenic complexes to cell surfaces that bypasses limitations of gene transfer and permits control of antigenic peptide densities at cell surfaces.

The receptor with high affinity for IgE on rat mast cells is a functional receptor for rat IgG2a

Rat mast cells express high-affinity receptors for IgE (Fc epsilon RI) and low-affinity receptors for IgG (Fc gamma R). In this study, the capacity of IgG to activate the rat basophilic leukemia (RBL-2H3) and rat peritoneal mast cells was investigated. Immune complexes formed with purified rat IgG and antigen as well as chemically cross-linked rat IgG induced histamine release from RBL-2H3 cells. This stimulation was inhibited by pre-incubation of the cells with saturating concentrations of monomeric IgE. With chemically cross-linked rat IgG of each subclass, only IgG2a stimulated histamine release from RBL-2H3 cells and this release was also inhibited by prior saturation of the Fc epsilon RI with monomeric IgE. Identical results were obtained with rat peritoneal mast cells. In binding experiments, IgE and cross-linked rat IgG2a bound to rat Fc epsilon RI transfected into CHO cells. Monomeric rat IgG2a, cross-linked rat IgG1, IgG2b, IgG2c and rabbit IgG did not bind to Fc epsilon RI. Stimulation of RBL-2H3 cells with aggregated IgG2a induced phosphorylation of tyrosines in the beta and gamma subunits of the Fc epsilon RI. Thus, although RBL-2H3 and rat peritoneal mast cells have Fc gamma R, the IgG-mediated stimulation of these cells for histamine release was by the Fc epsilon RI. Altogether, these data demonstrate that the rat Fc epsilon RI is a functional receptor with low affinity for rat IgG2a.

Binding of soluble natural ligands to a soluble human T-cell receptor fragment produced in Escherichia coli

An Escherichia coli expression system has been developed to produce milligram quantities of the variable domains of a human T-cell receptor from a cytotoxic T cell that recognizes the HLA-A2-influenza matrix peptide complex as a single polypeptide chain. The recombinant protein was purified by metal-chelate chromatography and then refolded in a redox buffer system. The refolded protein was shown to directly bind both Staphylococcus aureus enterotoxin B and the major histocompatibility complex protein-peptide complex using a BIAcore biosensor. Thus this preparation of a single-chain, variable-domain, T-cell receptor fragment can bind both of its natural ligands and some of it is therefore a functional fragment of the receptor molecule.

Alloantigenic recognition of artificial glycosyl phosphatidylinositol-anchored HLA-A2.1

Alloantigen presentation by GPI-reanchored variants of the human class I MHC molecule HLA-A2.1 was studied in human cellular systems. To this end, we generated chimeric coding sequences for two GPI-modified HLA-A2.1 heavy chain derivatives. In these chimeras, the coding sequence for the HLA-A2.1 heavy chain was fused in-frame to alternative overlapping sequences from the 3'-end of human DAF containing the GPI-modification signal sequence. The encoded polypeptides HLA-A2.1:DAF-S and HLA-A2.1:DAF-L differed by 53 amino acids of additional DAF sequence in the latter. Both were detected on stably transfected C1R cell surfaces by HLA-A2.1-specific mAb, and their GPI-modification was confirmed by PI-PLC enzymatic cleavage. Immunoprecipitation analysis of surface-biotinylated C1R transfectants revealed heterodimeric association for both HLA-A2.1:DAF-L and HLA-A2.1:DAF-S heavy chains with beta 2m. Alloantigenic stimulation by, and cytotoxic recognition of, both HLA-A2.1:DAF-S/C1R and HLA-A2.1/CIR cells was observed; however, HLA-A2.1:DAF-L/C1R cells could not serve as allostimulators or allotargets. These findings establish that polymorphic human class I MHC molecules can function, when artificially GPI-reanchored, as alloantigenic targets. Moreover, the data suggest that the sequence bridging the HLA-A2 extracellular domain and the membrane can influence alloantigenic presentation.

Immunochemical and molecular analysis of antigen binding to lipid anchored and soluble forms of an MHC independent human alpha/beta T cell receptor

We have constructed antigen-specific chimeric human T cell receptor (TCR) molecules deleted of the transmembrane domain and containing the signal sequence for the biosynthesis of the phosphatidyl inositol glycan (GPI) linkage. These membrane-anchored forms of the TCR alpha and beta chains have been expressed in non-T cells, and they are recognized by alpha or beta TCR specific monoclonal antibodies. We have utilized both immunochemical methods and flow cytometry to prove that the enzyme phosphatidylinositol phospholipase C (PI/PLC) is able to cleave the GPI anchored TCR as a heterodimer from the CHO cell surface. We have demonstrated that the alpha/beta TCR heterodimer on the surface of CHO cells will recognize and bind polymers containing fluorescein (FL-polymer), and the binding activity is completely eliminated by the enzyme, PI/PLC. Moreover, soluble forms of the alpha/beta heterodimer will bind tightly to FL substituted sepharose, which demonstrates the retention of biological activity by the TCR after solubilization. Molecular modelling of the putative antigen binding site of the alpha FL beta FL TCR was derived from the known atomic coordinates of eight different hapten or peptide specific antibodies. Mutagenesis of several residues predicted from the model to be important in FL binding gave results consistent with involvement of Ig equivalent CDR2 and CDR3 domains in the antigen binding pocket. Therefore, using a model hapten system in studying recognition of the TCR independent of MHC interactions, we conclude that amino acid residues located in similar positions within CDR domains as compared to the case of MHC restricted TCR recognition are used in the binding of either hapten or peptide antigens.

A comparison of monoclonal antibody productivity in different hollow fiber bioreactors

Cell culture in hollow fiber bioreactors has been used as a method for large-scale production of monoclonal antibodies, viruses, cell-associated proteins and cancer antigens. We have examined an important variable in culturing cells in hollow fiber bioreactors: bioreactor composition. Eight different bioreactor designs which varied in nominal molecular weight cutoff, surface area, fiber material and ultra filtration rate were compared in large-scale hollow fiber cultures. A standard protocol utilizing the hybridoma 3C11 (ATCC HB 8511) or African green monkey kidney cells (Vero cells, ATCC CRL 1587) was designed so that the only variable examined was the hollow fiber bioreactor in use. The results suggest that surface area has little effect on antibody productivity, while fiber composition and ultrafiltration rate may play an important role. Vero cell growth was affected by both fiber composition and ultrafiltration rate.

Investigation of the interaction between the class I MHC-related Fc receptor and its immunoglobulin G ligand

The neonatal Fc receptor (FcRn) is structurally similar to class I major histocompatibility molecules. FcRn transports maternal immunoglobulin G (IgG) from ingested milk into the blood. IgG is bound at the pH of milk (pH 6.0-6.5) in the gut and released at the pH of blood (pH 7.5). We find that alteration of a histidine pair within the alpha 3 domain of FcRn and of a nearby loop (the FcRn counterpart of the class I CD8-binding loop) affects the affinity for IgG. Inhibition studies suggest the involvement of the FcRn B2-microglobulin domain in IgG binding. Fragment B of protein A inhibits FcRn binding to IgG, localizing the binding site on Fc for FcRn to the CH2-CH3 domain interface. Three histidines present at the CH2-CH3 domain interface of Fc could be partially responsible for the pH-dependent interaction between FcRn and IgG.

The cytoplasmic domain of CD28 is both necessary and sufficient for costimulation of interleukin-2 secretion and association with phosphatidylinositol 3'-kinase

T-cell activation requires two signaling events. One is provided by the engagement of the T-cell antigen receptor, and the second represents a costimulatory signal provided by antigen-presenting cells. CD28 mediates a costimulatory signal by binding its ligands, B7-1 and B7-2, on antigen-presenting cells, but the signaling pathway activated by CD28 has not been identified. A homologous molecule, CTLA-4, expressed on activated T cells, also binds to B7-1 and B7-2, but whether it has a signaling function is not known. We performed a structure-function analysis of CD28 to identify the functional domain which activates signal transduction. Truncation of the 40-amino-acid CD28 cytoplasmic tail abrogated costimulatory signaling. Chimeric constructs containing the extracellular and transmembrane regions of CD8 linked to the cytoplasmic region of CD28 had a costimulatory signaling function. Similar chimeras containing the cytoplasmic tail of CTLA-4 did not signal. Thus, the cytoplasmic region of CD28, but not CTLA-4, is sufficient to mediate costimulatory signaling. In addition, after CD28 stimulation, the p85 subunit of phosphatidylinositol 3'-kinase and phosphatidylinositol 3'-kinase activity were found in CD28 immunoprecipitates. The CD8-CD28 chimera, which has a costimulatory signaling function, associates with p85, while the nonfunctioning CD8-CTLA-4 chimera and a CD8-zeta chimera do not associate with p85. These results suggest that phosphatidylinositol 3'-kinase is specifically activated by CD28 and may mediate proximal events in the costimulatory signaling pathway regulated by CD28.

The cytoplasmic domain of CD28 is both necessary and sufficient for costimulation of interleukin-2 secretion and association with phosphatidylinositol 3'-kinase

T-cell activation requires two signaling events. One is provided by the engagement of the T-cell antigen receptor, and the second represents a costimulatory signal provided by antigen-presenting cells. CD28 mediates a costimulatory signal by binding its ligands, B7-1 and B7-2, on antigen-presenting cells, but the signaling pathway activated by CD28 has not been identified. A homologous molecule, CTLA-4, expressed on activated T cells, also binds to B7-1 and B7-2, but whether it has a signaling function is not known. We performed a structure-function analysis of CD28 to identify the functional domain which activates signal transduction. Truncation of the 40-amino-acid CD28 cytoplasmic tail abrogated costimulatory signaling. Chimeric constructs containing the extracellular and transmembrane regions of CD8 linked to the cytoplasmic region of CD28 had a costimulatory signaling function. Similar chimeras containing the cytoplasmic tail of CTLA-4 did not signal. Thus, the cytoplasmic region of CD28, but not CTLA-4, is sufficient to mediate costimulatory signaling. In addition, after CD28 stimulation, the p85 subunit of phosphatidylinositol 3'-kinase and phosphatidylinositol 3'-kinase activity were found in CD28 immunoprecipitates. The CD8-CD28 chimera, which has a costimulatory signaling function, associates with p85, while the nonfunctioning CD8-CTLA-4 chimera and a CD8-zeta chimera do not associate with p85. These results suggest that phosphatidylinositol 3'-kinase is specifically activated by CD28 and may mediate proximal events in the costimulatory signaling pathway regulated by CD28.

Blimp-1, a novel zinc finger-containing protein that can drive the maturation of B lymphocytes into immunoglobulin-secreting cells

We describe a novel gene, Blimp-1 (for B lymphocyte-induced maturation protein), transcripts of which are rapidly induced during the differentiation of B lymphocytes into immunoglobulin secretory cells and whose expression is characteristic of late B and plasma cell lines. The 856 amino acid open reading frame contains five Krüppel-type zinc finger motifs and proline-rich and acidic regions similar to those of known transcription factors. Serological studies show an approximately 100 kd protein that localizes to the nucleus. Stable or transient transfection of Blimp-1 into B cell lymphoma lines leads to the expression of many of the phenotypic changes associated with B cell differentiation into an early plasma cell stage, including induction of J chain message and immunoglobulin secretion, up-regulation of Syndecan-1, and increased cell size and granularity. Thus, Blimp-1 appears to be a pleiotropic regulatory factor capable of at least partially driving the terminal differentiation of B cells.

IDX-1: a new homeodomain transcription factor expressed in rat pancreatic islets and duodenum that transactivates the somatostatin gene

We describe the cloning from a rat islet somatostatin-producing cell line of a 1.4 kb cDNA encoding a new homeoprotein, IDX-1 (islet/duodenum homeobox-1), with close sequence similarity to the Drosophila melanogaster homeobox protein Antennapedia (Antp) and the Xenopus laevis endoderm-specific homeoprotein XlHbox8. Analyses of IDX-1 mRNA and protein in rat tissues show that IDX-1 is expressed in pancreatic islets and ducts and in the duodenum. In electrophoretic mobility shift assays IDX-1 binds to three sites in the 5' flanking region of the rat somatostatin gene. In co-transfection experiments IDX-1 transactivates reporter constructs containing somatostatin promoter sequences, and mutation of the IDX-1 binding sites attenuates transactivation. Reverse transcription-polymerase chain reaction of islet RNA using degenerate amplimers for mRNAs encoding homeoproteins indicates that IDX-1 is the most abundant of 12 different Antp-like homeodomain mRNAs expressed in adult rat islets. The pattern of expression, relative abundance and transcriptional regulatory activity suggests that IDX-1 may be involved in the regulation of islet hormone genes and in cellular differentiation in the endocrine pancreas and the duodenum.

Melanocyte lineage-specific antigen gp100 is recognized by melanoma-derived tumor-infiltrating lymphocytes

We recently isolated a cDNA clone that encodes the melanocyte lineage-specific antigen glycoprotein (gp)100. Antibodies directed against gp100 are an important tool in the diagnosis of human melanoma. Since the gp100 antigen is highly expressed in melanocytic cells, we investigated whether this antigen might serve as a target for antimelanoma cytotoxic T lymphocytes (CTL). Here, we demonstrate that cytotoxic tumor-infiltrating lymphocytes (TIL) derived from a melanoma patient (TIL 1200) are directed against gp100. HLA-A2.1+ melanoma cells are lysed by TIL from this patient. In addition, murine double transfectants, expressing both HLA-A2.1 and gp100, are lysed by TIL 1200, whereas transfectants expressing only HLA-A2.1 are not susceptible to lysis. Furthermore, the HLA-A2.1+ melanoma cell line BLM, which lacks gp100 expression and is resistant to lysis, becomes susceptible after transfection of gp100 cDNA. Finally, HLA-A2.1+ normal melanocytes are lysed by TIL 1200. These data demonstrate that the melanocyte differentiation antigen gp100 can be recognized in the context of HLA-A2.1 by CTL from a melanoma patient. Gp100 may therefore constitute a useful target for specific immunotherapy against melanoma, provided that no unacceptable cytotoxicity towards normal tissue is observed.

Early degenerate selection of thymocytes by class I major histocompatibility complex

Ontogeny of T cells is accomplished in the thymus by a process of positive selection, in which interaction of the T cell receptor (TcR) expressed on CD4+8+ thymocytes with self major histocompatibility complex (MHC), expressed on cortical epithelial cells, determines the progress along the maturation pathway and confers self restriction to T cells. Conversely, cells behaving as self reactive by interaction with bone marrow-derived antigen-presenting cells are negatively selected by apoptosis. We show here that the presence of a class I-restricted soluble TcR (sTcR) in the fetal thymic microenvironment, early in T cell ontogeny, determines an enhanced negative selection of a sizeable number of CD4+8+ thymocytes, which have been previously subjected to a positive-selection event. We hypothesize that the generation of the mature thymic T cell repertoire stems from an interaction of TcR, under a critical affinity threshold, with a self peptide-MHC complex which is common to a great number of TcR specificities using the same restriction element. A shift in this affinity threshold, caused by sTcR, results in the generation of cells acting in a self-reactive manner, which are then deleted. In extended fetal thymus organ culture in the presence of sTcR, we have also observed the appearance of mature CD8+ T cells, which once adoptively transferred to syngeneic nude mice are expanded in the periphery, consistent with an enhanced avidity of these cells for self MHC.

The nature of major histocompatibility complex recognition by gamma delta T cells

Despite intensive efforts, the general rules for gamma delta T cell recognition remain undefined. Here, we take advantage of the detailed knowledge of the molecular structure and biosynthetic pathways of major histocompatibility complex (MHC) molecules to analyze the recognition properties of the gamma delta T cell clones LBK5 (specific for the class II MHC, IEk) and G8 (specific for the nonclassical class I MHC, TL10b). We find that the activation of these clones requires neither class I nor class II antigen-processing and that peptides do not confer specificity. Epitope mapping also shows that the topology of gamma delta T cell receptor interaction with the MHC is distinct from that of alpha beta T cells. These results suggest that the molecular nature of gamma delta T cell recognition is fundamentally different than that of alpha beta T cells.

The T cell antigen receptor alpha and beta chains interact via distinct regions with CD3 chains

Selective pairwise interactions between CD3 chains and the clonotypic T cell antigen receptor (TCR)-alpha, -beta chains has recently been established. In this study, the region of interaction between clonotypic and CD3 chains involved with assembly was examined. To determine the site of protein interaction a variety of genetically altered TCR chains were constructed. These included: truncated proteins, lacking transmembrane and or cytosolic domains; chimeric proteins, in which extracellular, transmembrane or cytosolic domains were replaced with similar domains derived from either the Tac antigen or CD4; and point mutagenized TCR chains. COS-1 cells were transfected with cDNA, metabolically labeled, and immunoprecipitates analyzed using non-equilibrium pH gel electrophoresis (NEPHGE)-SDS/PAGE. The results demonstrated that assembly between TCR-alpha and TCR-beta chains occurred at the extracellular level. Assembly of the TCR-alpha chain with CD3-delta, and CD3-epsilon was localized to an eight-amino acid motif within the transmembrane domain of TCR-alpha. Site-specific mutations of the TCR-alpha charged residues within this motif (arginine, lysine) to leucine and similar point mutations of the transmembrane CD3-epsilon and CD3-delta charge groups resulted in the abrogation of assembly. In contrast, TCR-beta and CD3-epsilon binary complexes interacted via their extracellular domain. Analogous to TCR-alpha, the site of TCR-beta and CD3-delta assembly was at the transmembrane region. Despite multiple genetic manipulations on CD3-gamma and zeta these proteins failed to assemble with TCR-alpha. Similarly, there was no interaction between TCR-beta and zeta. These findings when coupled with the information on pairwise interactions and formation of higher order subcomplexes extend our model for the structure of the TCR complex.

The anion-binding exosite is critical for the high affinity binding of thrombin to the human thrombin receptor

The thrombin receptor has been shown to be a novel member of the family of G-protein coupled receptors (Vu, T.-K. H., Hung, D.T., Wheaton, V.I., and Coughlin, S.R. (1991) Cell 64, 1057-1068). This receptor appears to be activated through a thrombin-mediated proteolytic mechanism which exposes a "tethered ligand" responsible for receptor activation. In order to investigate the initial interactions of thrombin with this receptor, we have constructed cell lines which express high levels of the human thrombin receptor and studied the binding of various forms of thrombin to the cell surface. Analysis of transfected cells with thrombin receptor monoclonal antibodies identified a particular cell line (clone #5-18) which displayed > 150,000 thrombin receptors per cell. Clone #5-18 appeared to express functional receptors since treatment with thrombin resulted in both a 15-20 fold increase of cytoplasmic phosphoinositide levels and a comparable shift in the EC50 of thrombin-mediated calcium mobilization when compared to non-transfected CHO cells. Binding of 125I-alpha-thrombin to clone #5-18 did not reach equilibrium at 37 degrees C. However, direct binding studies of 125I-alpha-, 125I-diisopropylphospho (DIP)-alpha-, and 125I-beta-thrombin to clone #5-18 demonstrated that binding at 4 degrees C was saturable and reversible for each ligand. Analysis of the binding data revealed Kd's of 0.8 nM, 0.7 nM and 9.7 nM for 125I-alpha-, 125I-DIP-alpha- and 125I-beta-thrombin respectively. Association of 125I-alpha-, DIP-alpha, and beta-thrombin could be competed by unlabelled alpha- and DIP-alpha-thrombin. Unlabelled beta-thrombin, which has a modified anion-binding exosite, was a poor competitor for 125I-alpha- and 125I-DIP-alpha-thrombin, but did compete for 125I-beta-thrombin. In addition, the hirudin54-65 peptide competed at submicromolar concentrations for the binding of alpha- and DIP-alpha-thrombin, but not for beta-thrombin. This peptide binds specifically at the anion-binding exosite of alpha-thrombin and has been shown to have a lower affinity for beta-thrombin. These results demonstrate directly a high affinity interaction between thrombin and its receptor, and suggest that an important component is the high affinity association of the thrombin receptor with the anion-binding exosite of thrombin.

Characterization of stable Chinese hamster ovary cells expressing wild-type, secreted, and glycosylphosphatidylinositol-anchored human immunodeficiency virus type 1 envelope glycoprotein

We generated Chinese hamster ovary cell lines that stably express wild-type, secreted, and glycosylphosphatidylinositol (GPI)-anchored envelope glycoprotein of human immunodeficiency virus type 1 (HIV-1). The cells expressing wild-type Env (WT cells) express both the precursor gp160 and the mature gp120/gp41 and readily form large syncytia when cocultivated with CD4+ human cells. The cells expressing secreted Env (SEC cells) release 140-kDa precursor and mature 120-kDa envelope glycoproteins into the supernatants. The cells expressing GPI-anchored Env (PI cells) express both 140-kDa precursor and mature gp120/gp41 envelope glycoproteins, which can be released from the cell surface by treatment with phosphatidylinositol-specific phospholipase C (PI-PLC). Both the secreted and PI-PLC-released envelope glycoproteins form oligomers that can be detected on nonreducing sodium dodecyl sulfate-polyacrylamide gels. In contrast to the WT cells, the SEC and PI cells do not form syncytia when cocultivated with CD4+ human cells. The availability of cells producing water-soluble oligomers of HIV-1 Env should facilitate studies of envelope glycoprotein structure and function. The WT cells, which readily induce syncytia with CD4+ cells, provide a convenient system for assessing potential fusion inhibitors and for studying the fusion mechanism of the HIV Env glycoprotein.

Generation of a water-soluble oligomeric ectodomain of the Rous sarcoma virus envelope glycoprotein

Sequences encoding the transmembrane domain of the Rous sarcoma virus envelope (Env) glycoprotein were deleted and replaced with sequences that signal addition of a glycosyl phosphatidylinositol (GPI) membrane anchor. Stable NIH 3T3 cell lines expressing either the wild-type transmembrane-anchored Env or the Env chimera with a GPI tail were established. The GPI-anchored envelope glycoprotein is expressed, oligomerized, and transported to the cell surface in a manner identical to that of its wild-type transmembrane-anchored counterpart. The GPI-linked protein is quantitatively removed from the cell surface by treatment with phosphatidylinositol phospholipase C. The phosphatidylinositol phospholipase C-released, water-soluble Env glycoprotein ectodomain retains the wild-type oligomeric structure and provides a useful tool for studying the subgroup-specific binding and fusion activities of a prototypic retroviral Env glycoprotein.

Specificity of HLA class I antigen recognition by human NK clones: evidence for clonal heterogeneity, protection by self and non-self alleles, and influence of the target cell type

Prior studies using polyclonal populations of natural killer (NK) cells have revealed that expression of certain major histocompatibility complex (MHC) class I molecules on the membrane of normal and transformed hematopoietic target cells can prevent NK cell-mediated cytotoxicity. However, the extent of clonal heterogeneity within the NK cell population and the effect of self versus non-self MHC alleles has not been clearly established. In the present study, we have generated more than 200 independently derived human NK cell clones from four individuals of known human histocompatibility leukocyte antigens (HLA) type. NK clones were analyzed for cytolytic activity against MHC class I-deficient Epstein Barr virus (EBV) transformed B lymphoblastoid cell lines (B-LCL) stably transfected with several HLA-A, -B, or -C genes representing either self or non-self alleles. All NK clones killed the prototypic HLA-negative erythroleukemia K562 and most lysed the MHC class I-deficient C1R and 721.221 B-LCL. Analysis of the panel of HLA-A, -B, and -C transfectants supported the following general conclusions. (a) Whereas recent studies have suggested that HLA-C antigens may be preferentially recognized by NK cells, our findings indicate that 70% or more of all NK clones are able to recognize certain HLA-B alleles and many also recognize HLA-A alleles. Moreover, a single NK clone has the potential to recognize multiple alleles of HLA-A, HLA-B, and HLA-C antigens. Thus, HLA-C is not unique in conferring protection against NK lysis. (b) No simple patterns of HLA specificity emerged. Examination of a large number of NK clones from a single donor revealed overlapping, yet distinct, patterns of reactivity when a sufficiently broad panel of HLA transfectants was examined. (c) Both autologous and allogeneic HLA antigens were recognized by NK clones. There was neither evidence for deletion of NK clones reactive with self alleles nor any indication for an increased frequency of NK clones recognizing self alleles. (d) With only a few exceptions, protection conferred by transfection of HLA alleles into B-LCL was usually not absolute. Rather a continuum from essentially no protection for certain alleles (HLA-A*0201) to very striking protection for other alleles (HLA-B*5801), with a wide range of intermediate effects, was observed. (e) Whereas most NK clones retained a relatively stable HLA specificity, some NK clones demonstrated variable and heterogeneous activity over time. (f) NK cell recognition and specificity cannot be explained entirely by the presence or absence of HLA class I antigens on the target cell.(ABSTRACT TRUNCATED AT 400 WORDS)

Characterization of monoclonal antibodies specific for the V beta 3 family of the human T cell receptor generated using soluble TCR beta-chain

A soluble, recombinant form of the human T cell receptor (TCR) beta-chain containing the V beta 3.1 sequence has been constructed, expressed in Chinese hamster ovary cells, amplified by dihydrofolate reductase selection, and purified in quantities appropriate for the generation of monoclonal antibodies (mAb). The V beta 3 sequence was chosen because of its reported elevated usage in the synovial T cells of rheumatoid arthritis patients but the approach described should be applicable to other known human V beta gene sequences. By this method, two mAb were prepared which reacted with up to 10% of normal, live peripheral blood T cells but with reactivity varying greatly among individual donors. Both mAb specifically bound to a murine T cell line transfected with a human TCR V beta 3.1 and immunoprecipitated a protein of the expected molecular weight for the TCR beta-chain. Both antibodies were mitogenic for T cells and analysis of peripheral blood lymphocyte cultures stimulated with the mAb suggested that both were specific for the V beta 3.1 subfamily and not D beta or J beta. Clones expressing V beta 3, which were derived from mAb-stimulated peripheral blood lymphocytes of a single individual, preferentially (8/13), but not exclusively, utilized the J beta 2.7 gene segment. The V beta 3.1 usage showed no preference for the CD8+ or CD4+ subpopulations of normal peripheral blood T cells.

GPI- and transmembrane-anchored influenza hemagglutinin differ in structure and receptor binding activity

We investigated the influence of a glycosylphosphatidylinositol (GPI) anchor on the ectodomain of the influenza hemagglutinin (HA) by replacing the wild type (wt) transmembrane and cytoplasmic domains with a GPI lipid anchor. GPI-anchored HA (GPI-HA) was transported to the cell surface with equal efficiency and at the same rate as wt-HA. Like wt-HA, cell surface GPI-HA, and its ectodomain released with the enzyme PI-phospholipase C (PI-PLC), were 9S trimers. Compared to wt-HA, the GPI-HA ectodomain underwent additional terminal oligosaccharide modifications; some of these occurred near the receptor binding pocket and completely inhibited the ability of GPI-HA to bind erythrocytes. Growth of GPI-HA-expressing cells in the presence of the mannosidase I inhibitor deoxymannojirimycin (dMM) abrogated the differences in carbohydrate modification and restored the ability of GPI-HA to bind erythrocytes. The ectodomain of GPI-HA produced from cells grown in the presence or absence of dMM underwent characteristic low pH-induced conformational changes (it released its fusion peptides and became hydrophobic and proteinase sensitive) but at 0.2 and 0.4 pH units higher than wt-HA, respectively. These results demonstrate that although GPI-HA forms a stable trimer with characteristics of the wt, its structure is altered such that its receptor binding activity is abolished. Our results show that transmembrane and GPI-anchored forms of the same ectodomain can exhibit functionally important differences in structure at a great distance from the bilayer.

Distinct superinfection interference properties yet similar receptor utilization by cytopathic and noncytopathic feline leukemia viruses

Cell killing by cytopathic retroviruses is often associated with a delay or failure in the establishment of superinfection interference. Superinfection has been observed during T-cell killing and fatal immunodeficiency disease induction by the feline leukemia virus (FeLV) chimera FeLV-FAIDS-EECC, containing the surface envelope glycoprotein (SU) of FeLV-FAIDS clone 61C. We demonstrate here that 61C SU has a defect that results in a nearly complete failure to establish superinfection interference against homologous virus challenge. This failure was evident only in feline T (FeT) cell clones expressing envelope protein, not in the rare cells that have survived cytopathic infection to become chronically infected. The regions of SU responsible for this defect were the same as those previously identified as responsible for T-cell killing. The superinfection interference properties of a noncytophatic molecular clone, FeLV-FAIDS-61E, were different in that 61E established interference to homologous virus challenge, both in SU-expressing cell clones and in chronically infected cells. Neither 61E nor EECC established interference against heterologous virus challenge. Viruses expressing chimeric SU proteins displayed varied and intermediate interference properties. Purified 61E and 61C SU competed for binding sites on FeT cell surfaces, and purified 61E SU blocked infection of virus bearing 61E or 61C SU. In addition, purified 61E and 61C SU each coprecipitated 70-kDa FeT cell surface proteins. Our data are consistent with the hypothesis that there are multiple cellular components necessary for 61E and 61C attachment to and penetration of FeT cells, a primary receptor that is utilized by both 61E and 61C, and secondary receptors that are likely to be virus specific.