Isolation and expression of a cDNA clone encoding HLA-Cw6: unique characteristics of HLA-C encoded gene products

N-linked glycosylation selectively regulates the generic folding of HLA-Cw1

To resolve primary (glycosylation-assisted) from secondary (glycosylation-independent) quality control steps in the biosynthesis of HLA (human leukocyte antigen) class I glycoproteins, the unique N-linked glycosylation site of the HLA-Cw1 heavy chain was deleted by site-directed mutagenesis. The non-glycosylated Cw1S88G mutant was characterized by flow cytometry, pulse-chase, co-immunoprecipitation, and in vitro assembly assays with synthetic peptide ligands upon transfection in 721.221 and 721.220 cells. The former provide a full set of primary as well as secondary chaperoning interactions, whereas the latter are unable to perform secondary quality control (e.g. proper class I assembly with peptide antigens) as a result of a functional defect of the HLA-dedicated chaperone tapasin. In both transfectants, Cw1S88G displayed a loss/weakening in its generic chaperoning interaction with calreticulin and/or ERp57 and became redistributed toward calnexin, known to bind the most unfolded class I conformers. Despite this, and quite unexpectedly, a weak interaction with the HLA-dedicated chaperone TAP was selectively retained in 721.221. In addition, the ordered, stepwise acquisition of thermal stability/peptide binding was disrupted, resulting in a heterogeneous ensemble of Cw1S88G conformers with unorthodox and unprecedented peptide assembly features. Because a lack of glycosylation and a lack of tapasin-assisted peptide loading have distinct, complementary, and additive effects, the former is separable from (and upstream of) the latter, e.g. primary quality control is suggested to supervise a crucial, generic folding step preliminary to the acquisition of peptide receptivity.

A single bottleneck in HLA-C assembly

Poor assembly of class I major histocompatibility HLA-C heavy chains results in their intracellular accumulation in two forms: free of and associated with their light chain subunit (beta(2)-microglobulin). Both intermediates are retained in the endoplasmic reticulum by promiscuous and HLA-dedicated chaperones and are poorly associated with peptide antigens. In this study, the eight serologically defined HLA-C alleles and the interlocus recombinant HLA-B46 allele (sharing the HLA-C-specific motif KYRV at residues 66-76 of the alpha1-domain alpha-helix) were compared with a large series of HLA-B and HLA-A alleles. Pulse-labeling experiments with HLA-C transfectants and HLA homozygous cell lines demonstrated that KYRV alleles accumulate as free heavy chains because of both poor assembly and post-assembly instability. Reactivity with antibodies to mapped linear epitopes, co-immunoprecipitation experiments, and molecular dynamics simulation studies additionally showed that the KYRV motif confers association to the HLA-dedicated chaperones TAP and tapasin as well as reduced plasticity and unfolding in the peptide-binding groove. Finally, in vitro assembly experiments in cell extracts of the T2 and 721.220 mutant cell lines demonstrated that HLA-Cw1 retains the ability to form a peptide-receptive interface despite a lack of TAP and functional tapasin, respectively. In the context of the available literature, these results indicate that a single locus-specific biosynthetic bottleneck renders HLA-C peptide-selective (rather than peptide-unreceptive) and a preferential natural killer cell ligand.

Impaired Assembly Results in the Accumulation of Multiple HLA-C Heavy Chain Folding Intermediates

Class I MHC H chains assemble with beta2-microglobulin (beta2m) and are loaded with peptide Ags through multiple folding steps. When free of beta2m, human H chains react with Abs to linear epitopes, such as L31. Immunodepletion and coimmunoprecipitation experiments, performed in this study, detected a preferential association of L31-reactive, beta2m-free H chains with calnexin in beta2m-defective cells, and with calreticulin and TAP in beta2m-expressing cells. In beta2m-defective cells, the accumulation of calnexin-bound H chains stoichiometrically exceeded their overall accumulation, a finding that supports both chaperoning preferences and distinct sorting abilities for different class I folds. No peptide species, in a mass range compatible with that of the classical class I ligands, could be detected by mass spectrometry of acidic eluates from L31-reactive HLA-Cw1 H chains. In vitro assembly experiments in TAP-defective T2 cells, and in cells expressing an intact Ag-processing machinery, demonstrated that L31 H chains are not only free of, but also unreceptive to, peptides. L31 and HC10, which bind nearly adjacent linear epitopes of the alpha1 domain alpha helix, reciprocally immunodepleted free HLA-C H chains, indicating the existence of a local un-/mis-folding involving the N-terminal end of the alpha1 domain alpha helix and peptide-anchoring residues of the class I H chain. Thus, unlike certain murine free H chains, L31-reactive H chains are not the immediate precursors of conformed class I molecules. A model inferring their precursor-product relationships with other known class I intermediates is presented.

Genetically modified HLA class I molecules able to inhibit human NK cells without provoking alloreactive CD8+ CTLs

Human NK cells are likely to be important effectors of xenograft rejection. Expression of HLA class I molecules by transfected porcine cells can protect them from human NK cell-mediated lysis; however, this strategy has the potential to augment the anti-graft response by recipient CD8(+) T cells recognizing foreign pig peptides presented by HLA. In this study we show that the introduction of a mutation (D227K) in the alpha(3) domain of HLA-Cw3 abrogates its recognition by CD8-dependent T cells but leaves intact its ability to function as an inhibitory ligand for NK cells. Such genetically modified molecules may have potential therapeutic applications in the prevention of delayed xenograft rejection and in the facilitation of allogeneic and xenogeneic bone marrow engraftment.

Studies on binding of HIV-1 p24gag peptide to HLA-Cw3+ cells

Human major histocompatibility complex class I antigens, HLA-C, are expressed on the cell surface at approximately a tenfold lower level than HLA-A and -B. We hypothesized that the expression of HLA-C is limited by the quantity of high affinity peptides which bind to these molecules, thus allowing only a small fraction of HLA-C molecules to be transported and/or to remain stable on the cell surface. If this assumption is correct, then the addition of exogenous peptide should increase cell surface HLA-C expression. To verify the hypothesis, we pulsed lymphoblastoid cell line PAJ (HLA-Cw3+) with synthetic HIV-1 p24gag 145-152 peptide, known to be presented to T-lymphocytes by HLA-Cw3 molecule. PAJ (HLA-Cw3+) cells bound approximately two times more of the peptide than HAJ (HLA-Cw3-), and four times more than 500/C9 (HLA-Cw3-) cells. Accordingly, overnight pulsing of PAJ cells with the p24gag 145-152 peptide caused an increase in class I HLA expression detected on the cell surface by flow cytofluorimetric analysis with anti-HLA-B,C monoclonal antibodies but not by anti-HLA-A antibody. In contrast, HLA-Cw3- cells treated in the same manner did not show any increase of HLA class I expression. Our data suggest that low concentration of high affinity peptides within the cell may be one of the factors limiting cell surface expression of HLA-C molecules.

Zinc Bound to the Killer Cell-Inhibitory Receptor Modulates the Negative Signal in Human NK Cells

The lysis of target cells by human NK cells is inhibited by several kinds of receptors with varying specificities for the MHC class I molecules of target cells. The requirements for complete inhibition of NK cytotoxicity appear to be complex and not well defined. The HLA-C-specific members of the killer cell-inhibitory receptor (KIR) family, carrying two Ig domains (KIR2D), are unusual among Ig superfamily members in their ability to bind zinc. A role for the zinc-binding site in KIR-mediated inhibition was demonstrated in this study using a functional reconstitution system in NK cells. Replacement of six histidines by alanine residues in putative zinc binding sites of a KIR2D ablated zinc binding and markedly impaired its inhibitory function, but left intact its ability to bind HLA-C and to transduce a positive signal through an immunoreceptor tyrosine-based activation motif grafted onto its cytoplasmic tail. Thus, zinc modulates specifically the negative signal transmitted by this KIR molecule. Mutation of an exposed amino-terminal zinc-binding motif alone was sufficient to impair the inhibitory function of KIR. The data suggest that complete inhibition of HLA-C-specific NK cells requires a zinc-dependent protein-protein interaction via the amino-terminal end of KIR2D.

HLA-C heavy chains free of beta2-microglobulin: distribution in normal tissues and neoplastic lesions of non-lymphoid origin and interferon-gamma responsiveness

Lacking monospecific antibodies to HLA-C, the expression and synthesis of these molecules have been difficult to evaluate. Using biochemical and flow cytometry approaches, the present report demonstrates that the reactivity of the murine monoclonal antibody L31 is restricted to naturally occurring HLA-C (HLA-Cw1 through -Cw8), HLA-B8 and HLA-B51 heavy chains not associated with beta2-microglobin (beta2m). This is due to two properties of HLA-C heavy chains: (a) they share the L31 epitope which distinguishes them from all the HLA-A and most HLA-B molecules; (b) they accumulate intracellularly, in a beta2m-free form, in much greater amounts than most L31-reacting HLA-B heavy chains. On the basis of this restricted reactivity, a representative panel of normal and neoplastic human tissues and cells derived from HLA-B8- B51- individuals was selected and employed to assess the tissue distribution, surface expression and IFN-gamma responsiveness of beta2m-free HLA-C heavy chains. At variance from antibody W6/32 to beta2m-associated heavy chains, L31 stains normal and neoplastic tissues with a ground-glass pattern and weakly binds to the surface of viable cells, even after treatment with interferon gamma (IFN-gamma). Thus, beta2m-free HLA-C heavy chains are, for the most part, located intracellularly. In spite of their distinct cellular localization, L31- and W6/32-reacting molecules have an overlapping tissue distribution, undergo concordant changes upon transformation and are upregulated in their synthesis by IFN-gamma to a similar extent. These observations demonstrate a coordinate regulation of HLA-C with HLA-A and -B molecules. In addition, they indicate that the assembly of HLA-C is impaired in most body districts and IFN-gamma is unable to completely reverse this impairment. The present results are consistent with a low surface expression of HLA-C and with a privileged role of these molecules in signaling class I loss to cytotoxic effectors in pathological conditions.

Affinity biotinylation: nonradioactive method for specific selection and labeling of cellular proteins

Biotinylation has become a popular alternative to radioiodination for labeling cell surface proteins, whereas labeling of the total cellular protein pool is usually achieved metabolically with [35S]methionine and [35S]cysteine. In this paper we describe a new technique in which total cellular lysate proteins that have been affinity bound to a solid phase are labeled efficiently with biotin. This labeling technique is preferable to direct biotinylation of cell lysate since the unreacted biotin can be readily removed from the sample by washing. The affinity step permits preselection of the molecules to be labeled, thereby decreasing the potential for nonspecific binding during subsequent immunoprecipitation. We applied this affinity biotinylation method to a human cellular lysate in order to preselect the total glycoprotein pool for subsequent immunoprecipitation of HLA class I. Following immunoprecipitation, SDS-PAGE, and Western blot, the biotinylated protein could be readily revealed by enhanced chemiluminescence. The results were comparable to those obtained by radiometabolic labeling and Western blot using a monoclonal antibody probe. Overall, the affinity biotinylation method is faster and more practical than conventional radiolabeling, without any loss in sensitivity.

Tyrosine phosphorylation of a human killer inhibitory receptor recruits protein tyrosine phosphatase 1C

Natural killer (NK) cells express killer inhibitory receptors that mediate negative regulation of NK cell cytotoxicity upon binding to MHC class I molecules on target cells. Unrelated inhibitory receptors on B cells have recently been shown to function through recruitment of phosphotyrosine phosphatase 1C (PTP-1C). Here, we show that a human killer inhibitory receptor specific for HLA-C also recruits PTP-1C after phosphorylation induced either by the pharmacological agent phenylarsine oxide or by conjugation with target cells. This recruitment is mediated by the binding of specific cytoplasmic phosphotyrosine-containing sequences to PTP-1C. These results implicate PTP-1C as a cytosolic component of the negative signaling pathway through NK cell inhibitory receptors.

Distinctive features of the alpha 1-domain alpha helix of HLA-C heavy chains free of beta 2-microglobulin

Only a few monoclonal antibodies are available with a restricted specificity to HLA-C products. In the present report, we demonstrate that antibody L31, previously shown to react with beta 2m-less (free) class I MHC heavy chains, binds to an epitope (residues 66-68 of the alpha 1 domain alpha helix) present on all the HLA-C alleles corresponding to the accepted (CW1 through CW8) serologic specificities, and on a few HLA-B heavy chains sharing with HLA-C an aromatic residue at position 67. Extensive IEF blot testing of HLA homozygous, EBV-transformed B-lymphoid cells indicates that HLA-C molecules are present at significantly lower levels than HLA-B polypeptides not only at cell surface, as previously demonstrated, but also in total cellular extracts. Testing of metabolically labeled HLA-CW1, -CW5, and -CW6 transfectants and HLA homozygous lymphoid cells, particularly HLA-CW1-expressing cells, demonstrates that the L31 epitope is present on a subpopulation of naturally occurring HLA-C molecules distinct from that identified by antibody W6/32 to beta 2m-associated heavy chains. Pulse-chase experiments demonstrate that this epitope is transiently made available to antibody binding at early biosynthetic stages, but becomes hidden upon assembly with beta 2m. Thus, free HLA-C and other Y/F67+ heavy chains are characterized by distinctive antibody binding features in a region (residues 66-68) included in a previously identified HLA-C restricted motif, which has been suggested to be the primary cause of distinctive features of the antigen-binding groove, low affinity for endogenous peptide antigens and beta 2m, and preferential uptake of exogenous peptides, possibly of viral origin. We also show that HLA-CW1 heavy chains, both free and beta 2m associated, acquire sialilation. Free HLA-CW1 heavy chains are expressed at the cell surface even when unsialilated, albeit at low levels.

Cytokine regulation of HLA-G expression in human trophoblast cell lines

HLA class I genes are differentially expressed among subpopulations of cells in first trimester human placentas. In this study, HLA class I protein was detected in extravillous cytotrophoblast cells by immunohistochemistry using the monoclonal antibody W6/32. In the same trophoblast subpopulation, class Ib proteins were identified with two monoclonal antibodies, 87G (anti-HLA-G) and 131 (anti-HLA-A/G) and class Ia protein was detected with the monoclonal antibody, 4E (anti-HLA-B/C). All of the antibodies also identified antigens on the human trophoblast-derived choriocarcinoma cell line, JEG-3. Therefore, the JEG-3 cells were used as a model system to study cytokine regulation of HLA-G in trophoblast cells. Northern blot hybridization studies showed that interferons (IFN-alpha, IFN-beta, IFN-gamma) and tumor necrosis factor-alpha (TNF-alpha) modestly enhanced steady state levels of HLA-G mRNA. Yet analysis of HLA-G protein by immunocytochemistry and flow cytometry failed to identify any changes in intracellular or membrane expression of HLA-G protein following cytokine treatment. Resistance to upregulation of HLA class I antigens was not a general feature of JEG-3 cells; IFNs enhanced expression of HLA-B/C as well as HLA class I light chain, beta 2-microglobulin. HLA null Jar choriocarcinoma cells did not contain HLA-G mRNA or antigen and exposure to cytokines had no effect on HLA-G. The results of this study are consistent with the postulate that trophoblast cell expression of HLA-G is stringently regulated and is controlled in part by post-transcriptional mechanisms.

Rapid degradation of the heavy chain of class I major histocompatibility complex antigens in the endoplasmic reticulum of human cytomegalovirus-infected cells

Human cytomegalovirus (HCMV) infection results in a marked reduction in the surface expression of class I major histocompatibility complex antigens on the host cells, which is thought to be one of the means for HCMV to evade the host immune system. To clarify the precise mechanism(s) of this phenomenon, we investigated the fate of the heavy chain of class I major histocompatibility complex antigens in HCMV-infected human embryonic lung fibroblasts (HEL) by pulse-chase analysis and immunocytochemical techniques. In HCMV-infected HEL, the heavy chain was synthesized at an increasing rate. However, instead of being transported to the cell surface through the Golgi apparatus, it was retained in the endoplasmic reticulum (ER) without acquisition of a complex-type N-linked oligosaccharide. In addition, it was rapidly degraded, with a half time of 20 min, and the amount of the heavy chain remaining at the end of 3 h of chase was 10% (or less) of that initially synthesized. ER degradation of host glycoproteins in HCMV-infected HEL was selective for the heavy chain, since the posttranslational processing of the transferrin receptor in these cells was not affected. The heavy chain degradation in infected cells was resistant to inhibitors of a lysosomal proteolytic pathway and to metabolic poisons. These observations suggest the presence of an energy-dependent nonlysosomal proteolytic pathway in the ER.

HLA-C is the inhibitory ligand that determines dominant resistance to lysis by NK1- and NK2-specific natural killer cells

Natural killer (NK) cells recognize alloantigens on normal cells. One of these alloantigens correlates with homozygosity for a dimorphism of HLA-C at positions 77-80, which is shared by a number of HLA-C alleles. A second allelic alloantigen correlates with homozygosity for the alternative HLA-C dimorphism, which is shared by the remaining HLA-C alleles. Moreover, NK1- and NK2-specific NK cell lines can be generated by mixed leukocyte cultures in which donor and stimulator are homozygous for the alternative dimorphisms at positions 77-80 of HLA-C. In the present work, the role of HLA-C in NK cell-mediated allorecognition was directly investigated by analyzing the effects produced by transfection of several HLA-C alleles on NK sensitivity of class I-deleted mutant cell lines. Transfection of cells with HLA-C alleles encoding Asn-77-Lys-80 (including HLA-Cw4, -Cw5, and -Cw6) inhibited the lysis of the targets by NK1-specific NK cells, whereas HLA-C alleles encoding Ser-77-Asn-80 (including HLA-Cw1, -Cw7, and -Cw13) protected the targets from NK2-specific NK cells. Thus, HLA-C alleles are the dominant inhibitory ligands that protect targets from lysis by these allospecific NK cells.

Molecular cloning and polymerase chain reaction-sequence-specific oligonucleotide detection of the allele encoding the novel allospecificity HLA-Cw6.2 (Cw*1502) in Spanish gypsies

A proposed novel allospecificity, HLA-Cw6.2, has been reported to be commonly found (approximately 25%) in Spanish Gypsies. Full-length cDNAs of the allele (Cw*1502) coding for this antigen have been cloned in this study. A simple PCR-SSO method for its detection has been standardized and a correlation with the serologic Cw6.2 phenotype has been established. This specificity has been also detected in the homozygous typing cell RML. Although the primary structures of Cw*1502 and Cw*0601 are not closely related, they share specific motifs that may account for their serologic cross-reactivity. Two novel HLA-C alleles (Cw*12022 and Cw*0602) are also reported.

HLA class I non-coding nucleotide sequences, 1992

We present a compilation of the nucleotide sequences of the non-coding regions of the human HLA class I genes which complements previously published information on exon sequences. The listing includes the 5' and 3' untranslated (UT) regions, and introns 1-7. The HLA class I loci and their alleles from which non-coding sequences were derived are listed in Table 1, together with source references. Where possible, locus and allele designations follow the Nomenclature for factors of the HLA system 1991 (Bodmer et al., 1992). In aligning sequences, nucleotides which are conserved between all class I genes are specified only by the consensus sequence, and are indicated by a hyphen (-). To maintain the alignment between different alleles, an asterisk (*) is inserted where there is a gap in the sequence. An unavailable sequence is indicated by a period (.). Regions of sequence too diverse to be accurately compared are represented by an exclamation mark (!). Sequence motifs previously classified as having an important role in HLA class I regulation or processing, such as enhancer sequences, are identified at the bottom of the sequence comparison. It is not our intention in this paper to present an analysis of the many features revealed by this compilation. However, we hope that the information will provide important reference material for studies of HLA class I mRNA processing (Cianetti et al., 1989), promoter regulation (David-Watine et al., 1990) and in the design of allele, locus or region specific PCR primers (Summers et al., 1991). We hope to update this compilation in due course, and we would welcome sequence information not included in this publication, as well as comments and corrections that help to maintain the accuracy of the information.

Distinctive polymorphism at the HLA-C locus: implications for the expression of HLA-C

The HLA-C locus remains an enigma. The serological polymorphism is poorly defined, HLA-C molecules are expressed at the cell surface at about 10% the levels of HLA-A and -B, and their importance for antigen presentation to either CD8-bearing T cells or natural killer cells is unclear. Our understanding of HLA-C polymorphism has also lagged behind that of HLA-A and -B. We have applied the polymerase chain reaction to the characterization of cDNA encoding HLA-C antigens. Combining the recent results with previously characterized HLA-C alleles gives a data base of 26 sequences, which was used to analyze the nature of HLA-C polymorphism and compare it to the variation seen in HLA-A and -B. The sequences form 10 families of alleles that correlate well with the patterns of serological crossreactivity, including the C blanks, and all major HLA-C allelic families appear to have been sampled. The families further divide into two groups of HLA-C alleles defined on the basis of linked substitutions in the 3' exons. In comparison with HLA-A and -B, HLA-C alleles are more closely related to each other, there being less variation in residues of the antigen recognition site and more variation at other positions. In particular, the helix of the alpha 1 domain of HLA-C molecules is unusually conserved. Despite the reduced diversity in the antigen recognition site, it is evident that HLA-C genes have been the target of past selection for polymorphism. Within the antigen recognition site, it is the alpha 1 domain that is most diagnostic of HLA-C, whereas the alpha 2 domain is similar to that of HLA-B, the locus to which HLA-C is most closely related. In particular, conserved motifs in the alpha 1 helix and the conserved glycine at the base of the B pocket (position 45) provide a combination of features that is uniquely found in HLA-C molecules. We hypothesize that these features restrict the peptides bound by HLA-C molecules and in this manner reduce the efficiency of HLA-C assembly and expression at the cell surface. The overall picture HLA-C polymorphism obtained from this sampling of HLA-C alleles is unlikely to change as further alleles are characterized.

Polymerase chain reaction analysis of transcriptional patterns of expression of class I HLA genes

In order to study transcriptional patterns of expression of individual class I HLA genes we have constructed a series of cDNA libraries from human cell lines including normal lymphoblastoid cell lines MANN and HOM2, two colorectal carcinoma cell lines, WiDr and SW480, and a fetal lung fibroblast cell line, MRC-5. Between 0.5 and 1 x 10(6) independent clones were screened in each library using a class I HLA-specific DNA probe and the frequency of class I HLA cDNA clones was found to vary between 0.23% (WiDr) and 0.76% (HOM2). Polymerase chain reaction (PCR)-based analyses of possible alternative splicing events showed that each of 161 class I HLA cDNA clones which had insert sizes exceeding 0.6 kb exhibited normal splicing patterns for exons 5 and 6. Similar PCR-based analyses in clones with appropriately large inserts revealed no exceptions to the normal splicing patterns for each of exons 2, 3, 4, and 7. Sixty of the class I HLA cDNA clones selected from the WiDr, MRC-5, and MANN cDNA libraries were assigned to individual loci following identification of locus-specific DNA sequences by PCR sequencing across exon 5. The sequences obtained from the 60 clones were each interpreted to correspond to one of the classical loci, HLA-A, HLA-B, and HLA-C. While representatives of the HLA-A locus predominated in the MANN library, HLA-B-specific clones were the most abundant in the WiDr and MRC-5 libraries.

Specific nucleotide sequence of HLA-C is strongly associated with psoriasis vulgaris

The association of specific HLA-C nucleotide sequences with psoriasis vulgaris was investigated in 75 Japanese patients by the polymerase chain reaction method, followed by slot-blot hybridization using two specific oligonucleotide probes. The synthesized nucleotide primers were C180P, 5'-GACCGGGAGACACAGAAGTACAAG-3' (coding for amino acid residues 61 to 68 of the alpha 1 domain of the HLA-C molecule) and C243PR, 5'-GCTCTGGTTGTAGTAGCCGCG-3' (residues 82 to 88), respectively. The amplified sequence detected with the probe C208A (5'-AGGCACAGGCTGACCGA-3'), including the coding region for alanine at position 73, was significantly increased in frequency in the patients compared with the healthy individuals (81% versus 48%, relative risk = 4.7, chi 2 = 15.3, p less than 0.0001). This specific nucleotide sequence is common to Cw6 and Cw7, but some other HLA-C alleles including Cw4 and C blank (Cx52) also proved to have this sequence. It is suggested that alanine at position 73 of HLA-C molecules can be a good marker for psoriasis vulgaris and that this residue may play an important role in determining susceptibility to this disease.

Application of polymerase chain reaction (PCR) to HLA-C locus typing

Polymerase chain reaction/oligonucleotide typing was used to identify HLA-Cw*0601 (Cw6) in patients with psoriasis and psoriatic arthritis. The assignment of HLA-Cw*0601 was established by the concordant presence of codons for alanine (position 73), lysine (position 80) and tryptophan (position 97). The frequencies of all three codons were increased in the patient groups.

Nucleotide sequence of the HLA-A26 class I gene: identification of specific residues and molecular mapping of public HLA class I epitopes

A cosmid clone bearing an HLA class I gene has been isolated from a human genomic library by hybridization to a class I-specific probe. This clone encodes the HLA-A26 molecule characterized by immunologic reagents on murine transfected L cells. Nucleotide sequencing of the A26 allele has been performed, and the deduced amino acid sequence was compared with previously published HLA class I sequences. Amino acid sequence homologies between HLA-A26 molecules and members of the HLA-AW19 cross-reactive group were observed and allowed us to demonstrate that residue Q144 is the only critical residue involved in the binding of the 4E monoclonal antibody defining an epitope common to all HLA-B, -C, and -Aw19 alleles. This study also permitted designation of a V residue at position 189 in the third domain as possibly involved in the binding of the B1-23-2 monoclonal antibody. Furthermore, we located clusters of variability in reference to the three-dimensional structure of the HLA-A molecules, i.e., the ninth residue of the first beta-strand domain, the upper surface of the first helical region, and both beta and alpha structures of the alpha 2 domain.

HLA-C "blank" alleles express class I gene products. Biochemical analysis of four different HLA-C "blank" polypeptides

Monoclonal antibodies (mAbs) W6/32, HC10, and 4E were used to precipitate class I antigens from 21 selected individuals with at least one HLA-C "blank" allele. In 19 of these individuals, characteristic HLA-C banding patterns which could be precipitated by all three HLA class I mAbs were observed on one-dimensional isoelectric focusing gels--obviously the gene products of HLA-C "blank". At least four allelic HLA-C "blank" gene products with different isoelectric points could be discerned. All of them segregated with HLA-C "blank" haplotypes in informative families; two of them were associated with HLA-B51, one with HLA-B38, and one with HLA-B18. Reactivity of the HLA-C "blank" heavy chains with mAb W6/32 indicates that they are able to associate with beta-2 microglobulin, and hence are most probably expressed at the cell surface.