Selective turnover and shedding of H-2K and H-2D antigens is controlled by the major histocompatibility complex. Implications for H-2-restricted recognition

Competition between MHC class I alleles for cell surface expression alters CTL responses to influenza A virus

Mammalian cells express up to six different MHC class I alleles, many of which differ in terms of their interaction with components of the Ag presentation pathway and level of cell surface expression. However, it is often assumed in Ag presentation studies that class I alleles function independently of each other. We have compared cell surface expression levels and function of MHC class I molecules in F(1) hybrid mice with those in the homozygous parental strains. The level of cell surface expression of certain alleles in F(1) mice differed significantly from 50% of that found on the same cell type in the corresponding parental strain, suggesting allele-specific competition for cell surface expression, and not expression solely according to gene dosage. The strongest effect was observed in H-2(b) x H-2(k) F(1) mice, in which the H-2(b) class I molecules dominated over the H-2(k) class I molecules. The magnitude of H-2(k)-restricted CTL responses to influenza A virus infection was similar in the F(1) hybrid and parental H-2(k) mice. However, in H-2(k) mice expressing a K(b) transgene, cell surface levels of the endogenous class I molecules were down-regulated to a greater degree than in F(1) hybrid mice, and H-2(k)-restricted CTL responses against influenza A virus were greatly reduced, although the CTL repertoire was apparently present. Therefore, certain MHC class I molecules compete with each other for cell surface expression, and the resulting low cell surface expression of specific alleles can lead to a severe reduction in the ability to generate a CTL response.

The multiple immune-evasion genes of murine cytomegalovirus are not redundant: m4 and m152 inhibit antigen presentation in a complementary and cooperative fashion

Both human cytomegaloviruses (HCMVs) and murine cytomegaloviruses (MCMVs) encode multiple genes that interfere with antigen presentation by major histocompatibility complex (MHC) class I, and thus protect infected targets from lysis by virus-specific cytotoxic T lymphocytes (CTLs). HCMV has been shown to encode four such genes and MCMV to encode two. MCMV m152 blocks the export of class I from a pre-Golgi compartment, and MCMV m6 directs class I to the lysosome for degradation. A third MCMV gene, m4, encodes a glycoprotein which is expressed at the cell surface in association with class I. Here we here show that m4 is a CTL-evasion gene which, unlike previously described immune-evasion genes, inhibited CTLs without blocking class I surface expression. m152 was necessary to block antigen presentation to both K(b)- and D(b)-restricted CTL clones, while m4 was necessary to block presentation only to K(b)-restricted clones. m152 caused complete retention of D(b), but only partial retention of K(b), in a pre-Golgi compartment. Thus, while m152 effectively inhibited D(b)-restricted CTLs, m4 was required to completely inhibit K(b)-restricted CTLs. We propose that cytomegaloviruses encode multiple immune-evasion genes in order to cope with the diversity of class I molecules in outbred host populations.

Soluble HLA class I antigens in chronic hepatitis C: a disease-associated manifestation or molecules modulating immunoresponsiveness?

The occurrence of high levels of soluble human leukocyte class I antigens (sHLA-I) represents an usual finding during the course of different clinical conditions, such as viral infections and autoimmune disorders. On the other hand, the well known property of sHLA-I to modulate T cell responsiveness could be taken as an advantage to improve long-term allograft acceptance. Recent data have pointed out that subjects with chronic hepatitis C virus (HCV) infection exhibit high amounts of sHLA-I, a pattern which has also been used for monitoring host responsiveness to interferon alpha (IFN-alpha) therapy. However, the lack of correlation between lymphocyte infiltration at liver site and disease biological activity suggests a potential role for sHLA-I in T cell dysfunction during chronic hepatitis C. sHLA-I antigens may, in fact, either interact with T cell receptor delivering an inhibitory signal or trigger cytotoxic T lymphocyte apoptosis by inducing CD95 ligand expression. Both events seem to favour HCV replication and liver tissue damage progression. Alltogether, these findings indicate that, besides viral variant generation and HCV core-mediated immunosuppression, sHLA-I may contribute to the imbalance of immunoresponsiveness during chronic HCV infection.

Purified truncated recombinant HLA-B7 molecules abrogate cell function in alloreactive cytotoxic T lymphocytes by apoptosis induction

BACKGROUND

Soluble MHC class I molecules are ubiquitous in human body fluids, including serum, urine, sweat, and cerebrospinal fluid. However, their biological function has remained unresolved. Membrane-derived human soluble MHC molecules (soluble human leukocyte antigen; sHLA) have been shown to induce apoptosis in alloreactive cytotoxic T lymphocytes (CTL). Here we report the efficacy of recombinant soluble HLA-B7 (rsHLA-B7) to modulate T-cell function.

METHODS

Primers of HLA-B7 were designed to allow amplification of a cDNA lacking the transmembrane and cytoplasmic domains yielding a truncated gene. rsHLA-B7 molecules were expressed in the human myeloma cell line 721.221 and purified by affinity chromatography using the BB7.7 mouse monoclonal antibody. CTL were generated from peripheral blood lymphocytes derived from healthy blood donors by stimulation with irradiated Epstein Barr virus-transformed HLA-B7-positive B cells. CTL were preincubated with rsHLA-B7, and cytotoxicity and apoptosis were tested according to standard procedure.

RESULTS

A total of 2 x 10(6) cells/ml secreted 10 microg/ml rsHLA-B7 as determined by a conformation-dependent ELISA, suggesting that rsHLA-B7 do not require the transmembrane and cytoplasmic regions for proper folding. After purification by affinity chromatography, rsHLA-B7 induced apoptosis in anti-HLA-B7 CTL, but not in anti-HLA-A2-specific, CTL. As a consequence, allorecognition of target cells by the CTL was significantly blocked.

CONCLUSION

Recombinant sHLA are sufficient binding cues for T cells, which efficiently induce apoptosis and block allorecognition of target cells by CTL. Thus, recombinant sHLA molecules may become a valuable new modality for specific immunological therapeutic intervention.

Expression of allogeneic MHC class I antigens by transgenic mouse trophoblast does not interfere with the normal course of pregnancy

Mammalian embryos express paternal histocompatibility antigens which make them potential targets for maternal immune responses. Yet, the histoincompatible fetus survives and develops normally. Down regulation of classical MHC antigen expression by trophoblast cells which are in direct contact with maternal circulation has been repeatedly shown. The trophoblast cells are unable to function properly in antigen presentation and do not induce allogeneic rejection reactions. In the present study we have created transgenic mice that express an allogeneic class I transgene whose transcription is controlled by the transferrin receptor promoter. The expression patterns of the transgene product mice from a single transgenic line were studied in each of the typical placental subpopulations. The allogeneic class I antigen was expressed in the allantoic plate region of the trophoblast, and this expression was not restricted to the endothelial region but extended also to the spongiotrophoblast, as well as the major blood vessels and in the endodermal sinuses. In contrast to the normal class I expression, prominent levels of allogeneic H-2 antigens were detected in the labyrinthine trophoblast. The fetal resorption rate in females mated with these transgenic males was not higher then the normal rate, and the embryos survived and developed normally. These data imply that the unusual expression of allogeneic class I antigens in certain trophoblast subpopulations does not affect fetal development.

Soluble HLA class I molecules: biological significance and clinical implications

Soluble class I human leukocyte antigens (sHLAs) have been detected in serum, sweat, lymphatic fluid, urine and cerebrospinal fluid. Their biological function has, however, remained a puzzle. The physiological concentration of sHLA varies more than tenfold depending on the phenotype of the individual, and is significantly upregulated in various diseases and during inflammation. This suggests that sHLAs might serve as a marker of pathological changes. Recent experiments have shown that, in vitro, sHLAs can modulate T-cell reactivity and induce cell-activated apoptosis, implicating sHLAs in the induction and maintenance of peripheral tolerance. Therefore, sHLAs have the therapeutic potential to induce tolerance to transplants.

Soluble HLA class I molecules induce apoptosis in alloreactive cytotoxic T lymphocytes

Soluble HLA class I molecules (sHLAs) have been identified in the serum of patients with inflammatory diseases, allografts and autoimmune diseases and in serum of healthy individuals. The biological significance of these molecules, particularly after allogeneic organ transplantation, has been enigmatic. Here we show that primary alloreactive CD8+ T cells interact with sHLA and undergo apoptosis in the absence of a second signal. Ligation of CD28 rescued T cells from death, implying that sHLAs induce apoptosis through selective stimulation of the T-cell receptor. CD95-L was upregulated after cytotoxic T lymphocytes were incubated with sHLAs, and cell death was blocked by a neutralizing anti-CD95-L antibody, suggesting that sHLAs induce endogenous mutual killing of activated T cells. These results provide a molecular basis for the capacity of sHLAs to downregulate T-cell responses, which may be especially relevant to organ transplantation.

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.

Production of soluble HLA-class-I molecules by IFN-gamma-induced colon-adenocarcinoma cells

High levels of soluble HLA-class-I molecules (sHLA) were found to be produced in a time- and dose-dependent manner by colon-adenocarcinoma Colo205 cells in response to IFN-gamma stimulation. Among other cytokines tested, only IL-6, TNF, IFN-alpha and IFN-beta showed weak inducibility. IFN-gamma-induced production of sHLA was synergistically enhanced by IL-1 alpha, IL-6 or TNF, and combined treatment with TNF and IL-6 exhibited an additive to synergistic induction. Expression of sHLA is unlikely to result from IFN-gamma-induced enhancement of overall HLA-class-I expression, as at low concentrations IFN-alpha, IFN-beta and IFN-gamma stimulated increased expression of cell membrane HLA-class-I molecules in Colo205 cells with almost equal efficiency, whereas only IFN-gamma induced high level production of sHLA. Immunoblotting with a monoclonal antibody recognizing beta 2-microglobulin-free HLA-class-I heavy chain revealed 3 major forms of sHLA heavy chain, i.e., 45/43-, 37- and 33-kDa molecules, in the culture supernatants of IFN-gamma-induced Colo205 cells. The 45/43-kDa proteins can be partitioned into Triton X-114, representing intact HLA-class-I heavy chains shed from the cell membrane. The hydrophilic 37- and 33-kDa heavy chain, which remained almost exclusively in the aqueous phase after extraction with Triton X-114, could well be due to alternative RNA splicing, with deletion of exon 5 encoding the hydrophobic transmembrane region of membrane-anchored HLA-class-I heavy chain.

Quantitative and biochemical characterization of soluble HLA class I antigens shed by cultured human cells

sHLA has been described in human serum and other body fluids. In this study sHLA shed by cultivated human cells and their biochemical nature in solution were studied. EBV-transformed human B-lymphoblastoid cell lines (n = 4), permanent human lymphoblastoid tumor cell lines (n = 4), and PBLs from three donors were cultivated in vitro and sHLA measured in the supernatants. The Daudi cell line was used as a negative control in all experiments. Maximum expression of sHLA was measured after 8 hours, after which the concentrations gradually declined. The allospecificities A2 and B7 were also detectable in the ELISA. sHLA in the supernatants was further characterized by 1D-IEF. All bands representing the allotypes were detected, showing that cell supernatants can be used as antigen sources for biochemical tissue typing. These data show that sHLA expression is a characteristic of viable cells.

Is restricted antigen presentation the explanation for fetal allograft survival?

Mammalian embryos express paternal histocompatibility antigens which make them potential targets for the maternal immune system. The trophoblast is a major histocompatibility complex (MHC) antigen-negative barrier between mother and fetus which facilitates movement of antigenic molecules but prevents traffic of antigenic cells and is itself unable to present antigen. Gary Wood suggests that the lack of antigen presentation requirements for MHC class I-restricted T-cell responses prevent generation of paternal-antigen directed cell-mediated immunity.

Different rates of HLA class I molecule assembly which are determined by amino acid sequence in the alpha 2 domain

Assembly of HLA class I molecules was studied using pulse-chase labeling of B-lymphoblastoid cell lines with 35S-methionine, immunoprecipitation with antibodies detecting free or beta 2-microglobulin-associated heavy chain and isoelectric focusing. Marked differences between the products of different class I alleles were noted. HLA-B51 assembled very inefficiently, with considerable free heavy chain still detected in an unsialated form after a four hour chase. The closely related molecule HLA-B35 was in contrast rapidly assembled, all newly synthesized heavy chain being detected in a beta 2m-associated sialated form within 30 minutes. Analysis of naturally occurring variants related to HLA-B35 and HLA-B51 localized the region determining assembly efficiency to the alpha 2 domain, in which these molecules differ at eight amino acid residues. The effect was not due to a linked dominant gene, as both patterns of assembly were observed in a single cell line.

Soluble histocompatibility antigens in synovial fluids of patients with rheumatoid arthritis

Soluble histocompatibility antigens of the class II region have been detected in synovial fluids obtained from patients with rheumatoid arthritis. A capture immunoassay involving two monoclonal antibodies was used; interference by rheumatoid factor, which is a feature of such assays, was overcome by mild pretreatment of fluids with 2-mercaptoethanol. No HLA class II antigen could be detected in matched sera from patients, even when levels were high in synovial fluids. Released HLA-class II material was of high molecular weight (greater than 1000 kD) and was linked to HLA-class I antigen. However, no significant amounts of other common cell surface antigens were detected in the complex, suggesting a preferential release of MHC antigens from cells of the inflamed synovium. Attempts to induce production of similar material from a cell line which expresses HLA class II strongly at the cell surface, by stressing the cells in various ways did not succeed, indicating that release is an active process.

H-2 class I loci determine sensitivity to MCMV in macrophages and fibroblasts

Peritoneal (PM) and bone marrow-derived (BMM) macrophages and lung fibroblasts (LF) from inbred, intra-H-2 recombinant, H-2 mutant, and hybrid mice were infected with murine cytomegalovirus (MCMV) under centrifugal enhancement. At the concentration of virus employed, peritoneal macrophages from strains carrying Kd, Kb, Dd, Ks and/or Ds, Kq and/or Dq alleles could be infected to a level of 80%-100%, as assessed by viral antigen expression or loss of Fc receptors. Cells lacking these haplotypes and carrying Kk, Kj, Dk, Dj, or Db were resistant, yielding levels of infection below 20%. The background (non-H-2) and class II genotype and the S allele did not influence the proportions of cells infected. Furthermore, sensitivity was dominant in the F1 progeny of H-2b X H-2k and H-2d X H-2k crosses, and was not compromised by the bm1, bm3, bm10, or bm14 mutations in the alpha 1 or alpha 2 regions of Kb or Db. The proportions of cells able to release infectious virus were low, but paralleled the frequencies of viral antigen expression. The class I genotype also determined susceptibility to MCMV infection in BMM and LF, although up to 35% of H-2k BMM and 46% of H-2k LF could be infected. The findings are consistent with an association between K and D antigens and a cellular receptor for MCMV on all three cell types.

Restricted blocking of cytotoxic T-cell function by anti-H-2K/D antibodies

Antibodies specific for H-2K and H-2D, the murine major histocompatibility complex (MHC)-encoded class I antigens, can block cytotoxic T (Tc)-cell function. Antibodies specific for the Tc cell and not the target cell have been used to map inhibition to the effector cell, suggesting a role for class I antigens in Tc-cell function. These antibody effects have been demonstrated for both alloreactive and MHC-restricted Tc cells, but inhibition has only been revealed by measuring cytotoxicity in a short-term assay. Using the neutral red assay for cytotoxicity, blocking effects evident after a 1.5-hr assay were lost by 2.5 hr. For some Tc-cell responses, only anti-H-2K antibodies have been found to be inhibitory, despite evidence of the expression of both H-2K and H-2D molecules on these cells. Some Tc-cell populations can be blocked by antibodies specific for both the H-2K and H-2D molecules. B10.A(4R) anti-Sendai Tc cells can be inhibited by anti-H-2Kk antibodies, but five different anti-H-2Db antibodies have been ineffective inhibitors. In contrast, B10.A(4R) anti-ectromelia Tc cells can be inhibited very effectively by each of these anti-H-2Db antibodies, as well as by anti-H-2Kk antibodies. Anti-H-2 antibodies also inhibit the function of cloned alloreactive Tc-cell lines such that the inhibitory capacity of antibodies specific for K versus D determinants appears to be consistent and specific for each Tc-cell line. A long-term Tc-cell clone, AR1, has been inhibited specifically by anti-H-2Kb and not anti-H-2Db antibodies, suggesting a clonally 'restricted' phenomenon.

Characterization of a product of histoincompatible spleen cells restoring class I-inhibited mixed-lymphocyte culture reactions

Conditions leading to the formation of a recognition product (SEPIR) capable of restoring MLC responses inhibited by treating stimulator cells with mAb to Class I antigens have been investigated. SEPIR has been found to be present in supernatants of immunological one-way interactions of histoincompatible spleen lymphocytes from naive mice following cultivation for a few hours. It is active in relatively high dilutions. Similarities with cytokines of IL-1, IL-2, or interferon character could not be revealed. Formation of the heat-labile principle is governed by immunologically specific reactions and involves recognition of Class I (but not Class II) HMC antigens by T cells of Lyt-2 phenotype. B cells and Lyt-1+ cells failed to induce formation of the product. SEPIR appears to be a complex of T-cell receptors for Class I antigens with these antigens. The data indicate that in conventionally induced fully incompatible MLC responses a similar product might be formed.

The induction of cytotoxic T-cell responses with H-2 antigens shed from viable lymphocytes

Supernatant derived from the incubation of normal, unstimulated spleen cells was able to stimulate a strong specific in vitro cytotoxic lymphocyte (CTL) response in allogeneic spleen cells primed with the corresponding haplotype. The supernatant antigen (SA) was as efficient in inducing secondary CTLs as equivalent numbers of irradiated, adherent cell-depleted spleen cell stimulators present during the culture period, and the CTL stimulation with both was dependent on the presence of responder splenic adherent cells. SA obtained from P815 tumour cells was unable to stimulate a similar response under the same conditions. The stimulating fraction of the SA showed the characteristics of lipid-associated major histocompatibility (MHC) antigens shed from viable lymphocytes, in that it was removed with specific H-2 antiserum, it was sedimented at 100,000 g and its activity was reduced if spleen cells were incubated at 4 degrees instead of 37 degrees. These results indicate a possible role for the shedding of MHC antigens in the general induction of a cytotoxic response.

A simple method to eliminate the antigenicity of surface class I MHC molecules from the membrane of viable cells by acid treatment at pH 3

We describe here a simple, reproducible method which specifically eliminates the antigenicity of surface class I major histocompatibility complex (MHC) molecules by acid treatment at pH 3 from the membrane of viable cells. When fresh mononuclear cells (MNC) or established cultured cell lines were treated at 4 degrees C for 2 min with citric acid buffer at pH 3 containing 1% bovine serum albumin, the antigenicity of class I MHC molecules, but not those of class II MHC and the other non-MHC antigens, was eliminated from the surface membrane without significant cell death. This method was effective for both human and murine cells with various origins. Monoclonal and polyclonal antibodies were used to identify the expression of surface antigens in conjunction with immunofluorescence tests. The eliminated antigenicity of human class I MHC antigens (i.e., HLA-A,B,C) on MNC regenerated when cells were incubated in the medium at 37 degrees C for 10 h. The pretreatment of cells with emetine (10(-4) M), a protein synthesis inhibitor, was found to be effective in inhibiting this regeneration. The acid treatment method might be useful for future studies on the functional characterization of surface class I MHC antigens.

Analysis of the expression of H-2K, D and L locus mRNAs by use of an oligonucleotide primer

By employing an oligonucleotide-dependent cDNA synthesis/HaeIII digestion procedure followed by gel electrophoretic separation of HaeIII digested cDNAs, we have developed a method for analyzing the expression of H-2 mRNAs encoding the K, D and L locus histocompatibility antigens. This method provides unambiguous data as the individual K, D or L specific cDNA bands can be isolated from the gel and partially sequenced. The method is readily applicable to analyzing the relative expression of K, D and L locus mRNAs in normal mouse tissues and their tumors.

Aberrancy in immunogenicity and cell-surface expression of H-2 antigens on erythrocytes

Immunogenicity for T cell-independent B-cell response assessed by splenic plaque-forming cell (PFC) response and cell-surface expression measured by laser flow cytometry of various class I H-2 antigens on mouse red blood cells (RBC) were compared. It was found that the order of magnitude of both immunogenicity and cell-surface expression on RBC is H-2Dd much greater than H-2Db greater than H-2Kd, H-2Kb. Furthermore, H-2d public antigens and H-2Ld antigens were neither immunogenic nor easily demonstrable on RBC. These findings contrasted with poor immunogenicity for PFC response (Nakashima et al. 1982, 1983) and proportionally strong expression of H-2 antigens on lymphoid cells. Immunogenicity and cell-surface expression of H-2Dd antigen on RBC were not shown to be controlled by the action of genes outside H-2D. It was therefore suggested that a number of H-2 antigens, including H-2Kd private, H-2Kb private, and H-2d public specificities are at least functionally defective on RBC. This is possibly due to the structural characteristics of the antigens. Since immunogenicity and cell-surface expression were in parallel, the expression of H-2 antigens on RBC must be dictated by a subset of B cells whose activity was assessed by PFC response. This finding supports the view that the H-2 molecules display a new category of activity which is different from their ability to activate T cells and depends on their expression on RBC.

Qa gene expression: biosynthesis and secretion of Qa-2 molecules in activated T cells

The biosynthesis and expression of the tissue-specific class I molecule Qa-2 have been studied in resting and activated T-cell populations. Polyclonal activation of T lymphocytes induces a 3- to 4-fold increase in the biosynthesis of Qa-2 molecules but no increase in cell-surface levels. Analysis of the biosynthetic pathway of the Qa-2 molecule in activated lymphocytes reveals that approximately equal to 70% of the newly synthesized Qa-2 molecules are secreted as soluble molecules. In resting-cell populations, Qa-2 remains entirely cell-associated. This process is unique to the Qa-2 molecule, since other class I molecules (e.g., H-2Kb and H-2Db) synthesized by activated cells remain cell-associated. The possibility that the secreted Qa-2 molecule is the product of a new Qa gene or an alternatively spliced mRNA is considered. These results indicate that the Qa-2 molecules may not just function as a cell-surface recognition structure but also may serve a role as a soluble factor synthesized by activated lymphoid cell populations.

Expression and function of a nonglycosylated major histocompatibility class I antigen

The major histocompatibility class I antigens, expressed in most somatic cells, have carbohydrate moieties. We constructed mutant mouse MHC class I genes in which codons for the N-linked glycosylation sites were replaced by those of other amino acids. L cell transformants expressing the nonglycosylated class I antigens allowed us to investigate biological roles of carbohydrates with the highest specificity possible. The nonglycosylated antigen was unchanged in its overall serological specificities, and was recognized by alloreactive cytotoxic T cells. Further, the antigen was capable of mediating cytotoxic activity of vesicular stomatitis virus-specific T cells. These studies indicate that carbohydrates are not essential for immunological function of the MHC class I antigens. Cell surface expression of the nonglycosylated antigen was markedly reduced as compared with the native antigen, which was not attributable to accelerated degradation or rapid shedding. We conclude that the primary role of carbohydrates of the class I antigens is to facilitate the intracellular transport of the nascent proteins to the plasma membrane. The possible involvement of carbohydrate-receptor interactions in this process is discussed.

Clonal variation in cell surface display of an H-2 protein lacking a cytoplasmic tail

Truncated variants of the gene encoding H-2Ld, an integral membrane protein encoded by the major histocompatibility complex, were constructed by in vitro mutagenesis to elucidate the function of charged amino acids found on the cytoplasmic side of the transmembrane (TM) region. Analysis of cloned L cells transfected with these genes shows that the seven amino acids following the TM segment, four of which are basic, enhance the cell surface expression of H-2Ld protein but are not required for it. However, some clones do not express a tailless H-2Ld protein on the cell surface but express it intracellularly where it has a long half-life. Turnover measurements on cell surface H-2Ld proteins suggest that the basic residues following the TM segment are not a "stop transfer" sequence (Blobel, G., 1980, Proc. Natl. Acad. Sci. USA., 77:1496-1500) which anchors the H-2Ld protein in the membrane. Pulse-chase and endoglycosidase H sensitivity studies show that H-2Ld proteins lacking some or all of the basic residues and H-2Ld proteins which have a full-length cytoplasmic tail are processed with different kinetics. These results suggest an involvement of the membrane-proximal region of the cytoplasmic tail in the intracellular transport of H-2Ld. We further suggest that the L cell clones which do and do not express a tailless H-2Ld protein on the cell surface differ in the ability to transport a tailless integral membrane protein to the cell surface.

Allogeneic stimulation of cytotoxic T cells by supported planar membranes

Phospholipid vesicles containing the transmembrane protein H-2Kk spontaneously fuse to form planar membranes when incubated on treated glass surfaces. Pattern photobleaching of fluorescent lipid probes indicates that these planar membranes are continuous and that the lipids are as mobile as they are in conventional fluid bilayers or monolayers. H-2Kk molecules in these planar membranes are immobile. These membranes stimulate cytotoxic T lymphocytes when cultured with immune spleen cells. The response to H-2Kk in planar membranes is greatly enhanced by the addition of supernatant from concanavalin A-stimulated spleen cells, indicating that relatively little antigen processing or presentation by accessory cells occurs. Cytotoxic T cells induced by purified alloantigen are found to be as susceptible to antibody blockade as are effectors from conventional mixed lymphocyte culture, where the antibody is directed against a T-cell surface antigen reputed to strengthen target cell adhesion through an interaction independent of major histocompatibility antigens.

Trinitrophenyl modification of H-2k and H-2b spleen cells results in enhanced serological detection of Kk-like determinants

Several anti-H-2Kk but not anti-H-2Dd monoclonal antibodies (mAb) exhibited enhanced binding to B10.A murine spleen cells after modification of the cells with trinitrobenzene sulfonate (TNBS). The number of antibody molecules bound to TNP-modified B10.A spleen cells increased by a factor of two or more. The same anti-2Kk mAb that exhibited enhanced binding to modified B10.A cells did not bind to unmodified C57BL/10 spleen cells, as expected, but did bind to TNP-modified C57BL/10 spleen cells. This TNP-dependent binding was not a result of cross-reactions with cell surface TNP groups nor with Fc receptors. TNP modification of a variant cell line that does not express class I H-2 products did not result in enhanced binding by these mAb. These findings can account for preferential recognition of TNP-Kk by B10.A and B10.BR CTL, and also for cross-reactive lysis by C57BL/10 CTL stimulated by C57BL/10-TNP against unmodified H-2Kk targets.

Immunological surveillance of tumors in the context of major histocompatibility complex restriction of T cell function

The immunological surveillance hypothesis was formulated prior to the realization of the fact that an individual's effector T cells generally only see neoantigen if it is appropriately presented in the context of self MHC glycoproteins. The biological consequence of this mechanism is that T lymphocytes are focused onto modified cell-surface rather than onto free antigen. The discovery of MHC-restricted T cell recognition, and the realization that T cell-mediated immunity is of prime importance in promoting recovery from infectious processes, has thus changed the whole emphasis of the surveillance argument. Though the immunological surveillance hypothesis generated considerable discussion and many good experiments, there is no point in continuing the debate in the intellectual context that seemed reasonable in 1970. It is now much more sensible to think of "natural surveillance" and "T cell surveillance," without excluding the probability that these two systems have elements in common. We can now see that T cell surveillance probably operates well in some situations, but is quite ineffective in many others. Part of the reason for this may be that the host response selects tumor clones that are modified so as to be no longer recognized by cytotoxic T cells. The possibility that this reflects changes in MHC phenotype has been investigated, and found to be the case, for some experimental tumors. In this regard, it is worth remembering that many "mutations" in MHC genes that completely change the spectrum of T cell recognition are serologically silent. The availability of molecular probes for investigating the status of MHC genes in tumor cells, together with the capacity to develop cloned T cell lines, monoclonal antibodies to putative tumor antigens, and cell lines transfected with genes coding for these molecules, indicates how T cell surveillance may profitably be explored further in both experimental and human situations.

Spontaneous internalization of Class I major histocompatibility complex molecules in T lymphoid cells

A low proportion of T lymphocytes in normal mouse spleen contains small intracytoplasmic vesicles showing Class I MHC molecules. After stimulation in vitro in a mixed lymphocyte reaction or by addition of Con A, the proportion of T cells with such intracytoplasmic vesicles increases progressively and becomes the majority. Labeling with fluorochrome-conjugated antibodies has shown that the vesicles are formed by internalization of molecules from the plasma membrane. The process is spontaneous and does not require cross-linking by antibodies or other ligands; it is selective inasmuch as other molecules (Thy-1 and T200 antigens) are not included and it is specific since it is not performed by other cells such as B lymphoid cells or fibroblasts. On the whole the process shows similarities with the internalization and recycling of other receptors, such as the receptors for different macromolecules of metabolic or informational significance, as seen in other cells. On the other hand, the specificity of Class I MHC mobilization in T lymphoid cells suggest a role for this process which is related to the immune function of these molecules.

Regulatory mechanisms in cell-mediated immune responses. Role of I-J and I-C determinants in the activation of H-2I and H-2K/D alloantigen-specific suppressor T cells

The role of individual H-2I subregion determinants in the activation of H-2I alloantigen-primed mixed leukocyte response suppressor T cells (MLR Ts), as well as their possible expression on stimulator cells required to trigger primed H-2K- or D-specific MLR Ts, was addressed in these studies. Both genetic and serologic studies demonstrated that MLR Ts potentially primed to alloantigens encoded by the entire H-2I region were triggered to MLR Ts factor production only by stimulator cells bearing the priming I-J and/or I-C, but not I-A or I-E alloantigens. The relevant I-J and I-C determinants were demonstrated on a single antigen-presenting cell population that is used in common by independent I-J-specific and I-C-specific MLR Ts. Unexpectedly, the stimulator cell population necessary to trigger MLR Ts primed to class I H-2K or D alloantigens expressed not only the priming class I determinant, but in addition, I-C alloantigens syngeneic with the MLR Ts haplotype. Stimulator populations bearing the appropriate H-2K or D alloantigen but serologically depleted of I-C+ cells or genetically constructed to display MLR Ts-disparate I-C determinants were ineffective stimulators of class I antigen-primed MLR Ts. Thus these data suggest that as allogeneic determinants, I-J- and I-C-encoded molecules are together the major triggering elements for MLR Ts primed to disparate H-2I region determinants. In addition, self-I-C molecule recognition appears to constitute an important feature of the triggering, and by implication, priming process of H-2 class I antigen-specific Ts cells.

Expression of H-2 and viral antigens and resistance to the antitumor lysis of tunicamycin-treated MBL-2 lymphoma cells

The role of protein glycosylation in the tumor lysis mediated by effector cells derived from Moloney-sarcoma-virus(MSV)-immune mice was studied. Treatment of the Moloney-virus-induced H-2b lymphoma target cells, MBL-2, with tunicamycin (TM), an inhibitor of the protein-N-linked glycosilation, was found to cause a loss of susceptibility to lysis by MSV-immune syngeneic effectors cells, while the same target cells remained fully sensitive to the lytic action of anti-H-2b-immune lymphocytes. Examination of MBL-2 cell surface by lactoperoxidase, 125I iodination, and immunoprecipitation by antiviral protein sera revealed that env but not gag viral gene-encoded products were expressed on the surface of this lymphoma. The TM-induced alteration of cell surface expression of H-2Db, H-2Kb, and gp70 antigens was examined by a combined approach of serological and biochemical techniques. The results were concordant in indicating that (1) after 16 h of TM treatment the cells showed a decreased expression of the three glycoproteins, (2) H-2Db (the restriction element in this system) resulted more affected by the treatment than its counterpart H-2Kb (75% vs 50% reduction as compared to untreated cells), (3) an additional lighter form of H-2Kb was found on the surface of TM-treated cells. In the context of an "associative recognition' of Db and gp70 by MSV-immune effector cells, our results may explain the loss of susceptibility to antitumor effectors of TM-treated MBL-2 cells by a quantitative reduction in the expression of both molecules which interact to create the target structure of syngeneic effectors.

A factor shed by lymphoblastoid cell lines of HLA-B27 positive patients with ankylosing spondylitis, specifically modifies the cells of HLA-B27 positive normal individuals

Epstein-Barr virus transformed lymphoblastoid cell lines from HLA-B27 positive individuals with ankylosing spondylitis (B27+AS+) release, into the culture medium, a factor capable of specifically modifying the HLA-B27 positive lymphocytes of normal individuals (B27+AS-); this modification results in a phenotypic change similar to that seen on B27+AS+ lymphocytes. This lymphoblastoid cell line derived factor appears to be physically and functionally similar to a factor present in the culture filtrate of certain Klebsiella isolates. Biogel P-100 chromatography of the material released from the cell line indicated a mol.wt of 25,000-30,000, similar to that of the Klebsiella derived factor. Chromatofocusing on a PBE 94 column revealed that cell line derived factor had an isoelectric point of 5.5 (cf. pI 5.4 for the Klebsiella derived factor). Immunoadsorption experiments suggest that the factor from the B27+AS+ cell line shares antigenic determinants with a cell surface component present on certain Klebsiella isolates. These results will form the basis for future studies on the nature of the interaction between HLA-B27 and certain enteric organisms and their products. A better understanding of this association should elucidate some of the early events in the pathogenesis of the seronegative arthropathies.

Enhanced expression of H-2K and H-2D antigens on reticulocytes infected with Plasmodium yoelii

The 17XNL strain of Plasmodium yoelii induces a highly effective and permanent T-cell dependent immunity in mice of the CBA strain; the lethal variant P. yoelii 17XL and P. berghei (ANKA) fail to activate an effective immune response in the same host. These differences in immunogenicity are unexplained. We recently observed that in CBA/CaJ mice the intracellular blood stages of P. yoelii 17XNL were almost exclusively within reticulocytes whereas lethal P. yoelii 17XL and P. berghei (ANKA), at comparable stages of infection, were predominantly erythrocytic. Induction of a reticulocytosis converted the normally lethal P. yoelii 17XL infection into a nonlethal one, and reticulocytic P. yoelii was shown to be more immunogenic than the erythrocytic form. Since one of the differences between reticulocytes and erythrocytes that might have influenced the development of immunity was greater expression of MHC antigens of the former cell type we examined the expression of H-2K, H-2D and Ia on reticulocytes infected with P. yoelii 17XNL. These cells showed a very marked increase in H-2K and D antigen expression compared to normal reticulocytes or erythrocytes. No Ia was detected. Red blood cells (RBC) infected with lethal P. yoelii 17XL or P. berghei showed no increase in H-2K or H-2D antigen expression. Finally, the level of expression of H-2K on P. yoelii 17XNL parasitized red blood cells from different strains of mice correlated closely with the ability of these strains to control the infection.

Human interferon-gamma enhances the expression of class I and class II major histocompatibility complex products in neoplastic cells more effectively than interferon-alpha and interferon-beta

Human interferon-gamma was more effective than interferon-beta or -alpha in stimulating production of immunoassociated antigens; HLA-A, -B, and -C; and beta(2)-microglobulin in human M14 and Namalva cells. The comparison was made on the basis of antiviral units, and the stimulation could be abolished by treatment of the interferon-gamma preparation with pH 2 or anti-interferon-gamma serum.

Monoclonal antibody reveals H-2-linked quantitative and qualitative variation in the expression of a Qa-2 region determinant

We have produced a monoclonal antibody, Y-7, that reacts with a Qa-2 region-controlled determinant. Cellular and strain distribution analyses, coupled with quantitative variation in the amount of Y-7 antigen expressed among strains, provide overwhelming evidence that Y-7 reacts with the Qa-2a determinant. The determinant detected by Y-7 is differentially expressed in T and B lymphocytes in a strain specific manner. Y-7 reacts with the majority of T lymphocytes (greater than 95%) and approximately one-half of B lymphocytes in certain strains (+ + strains), and with the majority of T lymphocytes (greater than 95%) and no B lymphocytes in other strains (+ strains). T lymphocytes in + strains express approximately three fold less of the Y-7 determinant than T lymphocytes from + + strains. In addition, we show that the Y-7 determinant is expressed in approximately one-third to one-half of Lyb-3-, 5- B lymphocytes. Possible mechanisms determining quantitative and qualitative variation in the expression of the Y-7 determinant in T and B lymphocytes are discussed.

Differential endocytosis of T and B lymphocyte surface molecules evaluated with antibody-bearing fluorescent liposomes containing methotrexate

Antibody-bearing fluorescent liposomes containing methotrexate became bound to cells expressing determinants recognized by the antibody. The number of bound liposomes could be evaluated by fluorometry, and the internalization of liposomes was evaluated by the methotrexate-mediated inhibition of radio-labeled deoxyuridine incorporation. The effect of methotrexate transferred from the liposomes into the cells was a function not of the number of liposomes bound but of the nature of the cells and of the target molecules. Liposomes bearing antibodies with specificity for the H-2K or Mr 94,000 and 180,000 molecules were much more effective at drug delivery into T than B cells, even though these determinants were expressed by both cell types. B cells were more sensitive to the effect of methotrexate in anti-H-2 I-A and I-E liposomes than in anti-H-2K liposomes. Inhibition of the methotrexate effect by NH4Cl suggested that methotrexate entered the cell by endocytosis of the liposomes. The results are consistent with differential internalization of H-2K, I-A, I-E, and Mr 94,000 and 180,000 cell surface molecules by mitogen-stimulated T and B cells.

Functional and phenotypic properties of subpopulations of murine thymocytes. I. The bulk of peanut agglutinin-positive Lyt-1,2,3 thymocytes lacks precursors of cytotoxic T lymphocytes responsive to interleukin 2 (T cell growth factor)

Thymocytes were separate according to their surface phenotype and tested for cytotoxic T lymphocyte (CTL) precursor function by stimulation with allogeneic or hapten-coupled cells, with or without addition of T cell growth factor (interleukin 2). The data show that only a minor subpopulation of thymocytes agglutinated by peanut agglutinin, expressing relatively high amounts of H-2K antigen, contained CTL precursors. The remaining population, approximately 80%, could not be induced to generate CTL, even in the presence of interleukin 2.

Phenotypic and functional properties of murine thymocytes. I. Precursors of cytolytic T lymphocytes and interleukin 2-producing cells are all contained within a subpopulation of "mature" thymocytes as analyzed by monoclonal antibodies and flow microfluorometry

The correlation between surface phenotype and function in subpopulations of murine thymocytes has been investigated using flow microfluorometry (FMF). C57BL/6 thymocytes stained with monoclonal antibodies directed against Lyt-2, H-2K(b), and Thy-l.2 and passed on an FACS II flow cytometer could be resolved into at least four distinct subpopulations on the basis of fluorescence and forward light scatter (FLS) measurements. (a) Medium-sized Lyt-2(+) cells that stained strongly with H-2K(b) and weakly with Thy-l.2 (5 percent of total cells); (b) medium-sized Lyt-2(-) cells with other properties as in (a) (10 percent); (c) small Lyt-2(+) cells that stained weakly with H-2K(b) and strongly with Thy-l.2 (60 percent); and (d) large Lyt-2(+) cells that stained weakly with H-2K(b) and very strongly with Thy- 1.2 (23 percent). Cortisone-resistant thymocytes (CRT) were found to correspond phenotypically to populations (a) and (b). The distribution of cytolytic T lymphocyte precursors (CTL-P) directed against H-2(d) alloantigens in subpopulations of C57BL/6 thymocytes that had been sorted according to the phenotypic criteria described above was then investigated. CTL-P in sorted and control populations were quantitated by limiting dilution analysis of mixed leukocyte microcultures established in an excess of interleukin 2 (IL-2). These studies established that all thymus CTL-P could be quantitatively recovered in a subpopulation of cells that was cortisone-resistant, medium-sized, Lyt-2(+), H-2K(b+), and weakly stained with Thy-l.2. In parallel studies, the production of IL-2 by subpopulations of C57BL/6 thymocytes was quantitatively assessed using a recently developed sensitive microassay system. Graded numbers of sorted or control thymocytes were stimulated with irradiated T cell-depleted allogeneic cells and assayed for their ability to support the growth of an IL-2-dependent cytolytic T lymphocyte clone. Using this method, IL-2 production was found to reside entirely in a subpopulation of cortisone-resistant, medium-sized Lyt-2(-) thymocytes. Further phenotypic analysis of this subpopulation of cells indicated that it was homogeneously H-2K(b+) and weakly staining with Thy- 1.2. Taken together with the CTL-P results, these data directly demonstrate that a subpopulation of thymocytes with a mature phenotype (i.e., cortisone- resistant, medium-sized, H-2K(b+), and weakly staining with Thy-l.2) accounts for all the functional activity in the thymus. Reasons for the apparent discrepancy between these results and other recent studies will be discussed.

Quantitative variations in the expression of H-2 antigens on murine leukemia virus-induced tumor cells can affect the H-2 restriction patterns of tumor-specific cytolytic T lymphocytes

Comparative quantitative experiments were designed to study the expression of H-2Kd and H-2Dd antigens on three different leukemia cell lines induced by Gross murine leukemia virus (MuLV)in BALB/c (H-2d) mice. The H-2 restriction patterns of syngeneic cytolytic T lymphocytes (CTL) directed against Gross MuLV-induced tumors were correlated with these quantitations of H-2Kd and H-2Dd antigens, Our results obtained by quantitative absorption of monospecific antisera indicated that the three BALB/c tumor cell lines expressed different amounts of H-2Kd and H-2Dd antigens, with H-2Dd antigen showing the greatest variability in expression because it ranged from barely detectable levels to one-eighth the amount of H-2Dd antigen expressed on normal BALB/c spleen cells. The H-2 restriction patterns of Gross MuLV-specific CTL were directly affected by these quantitative modulations in the expression of H-2Kd and H-2Dd antigens, as revealed by three independent approaches: (a) inhibition of CTL activity by monospecific anti-H-2 sera in the absence of complement; (b)competitive inhibition of CTL-mediated cytotoxicity by the addition of excess tumor cells into the reaction mixture; and (c) analysis of CTL specificities using cloned CTL populations. Our results thus indicate that H-2 restriction of tumor-specific CTL activity can be directed at the target cell level by variations in the expression of H-2 antigens.

Regulation of murine B lymphocyte plasma membrane protein turnover and shedding

Lactoperoxidase-catalyzed cell surface radioiodination was employed to radiolabel murine splenic B-cell membrane immunoglobulins (IgM and IgD) and alloantigens encoded by the Major Histocompatibility Complex (I-Ak, I-Ek, H-2Kk, H-2Dk). The fate of the radiolabeled proteins was monitored by in vitro culture of labeled cells and isolation of labeled antigens from detergent lysates of the cells or culture fluids obtained at different times during culture. The effects of temperature, antimetabolites, colchicine, and cytochalasins on membrane protein catabolism demonstrated heterogeneity in rate, energy dependence, and cytoskeletal control of turnover suggesting that functional domains of turnover control exist in the B lymphocyte membrane.

Antigen-inducible, H-2-restricted, interleukin-2-producing T cell hybridomas. Lack of independent antigen and H-2 recognition

We developed a method for production of antigen-specific, H-2-restricted T cell hybrids. The tumor cell partner in the fusions was itself a T cell hybrid, FS6-14.13.AG2 (or its derivatives), which could be induced to produce the growth factor, interleukin-2 (IL-2), in response to a challenge with concanavalin A, but had no known antigen specificity. The normal T cell partner in the fusions was a population of lymph node T cell blasts that had been highly enriched in antigen-specific, H-2-restricted T cells by in vivo immunization, followed by in vitro challenge with antigen and clonal expansion in IL-2-containing medium. These fusions produced hybrids that grew constitutively in culture. A sizable proportion of the hybrids demonstrated the ability to produce IL-2 in response to a challenge with specific antigen presented by irradiated spleen cells of the appropriate H-2 type. Four cloned antigen/H-2-specific hybrid lines were produced. AO-40.10 responded to chicken ovalbumin (OVA) when presented by I-A(k)-bearing cells. DC1.18.3 responded to the apo form of beef cytochrome c when presented with I-A(d). AODK-10.4 responded to keyhole limpet hemocyanin (KLH) presented with I-A (d). AODK-1.16 also responded to KLH presented by a product of the I region of H-2(d), but the data were consistent with either a product of the I-J-I-E(d) region or a combinatorial molecule with elements from both I-A(d) and I-E(d)/I-C(d). Coincidentally, AO-40.10 was shown to have an unexpected alloreactivity with a product of H-2(b) mapping to the K-I-A region. These hybrids should prove invaluable as sources of monoclonal material for the study of the receptor(s) on T cells with H-2-restricted antigen specificities. We also generated T cell hybrids with two antigen/H-2 specificities by fusing an azaguanine-resistant clone of AO-40.10 to normal T cells with a different antigen/H-2 specificity. Many of the hybrids retained reactivity to OVA plus H-2(a) and to the second antigen/H-2 combination. None reacted to either OVA plus the second H-2 type or to the second antigen plus H-2(a). One of these hybrids was successfully cloned to produce the line AOFK- 11.11.1. It retained the ability to recognize OVA plus I-A(k) inherited from one parent, and KLH plus IA(f) inherited from the other. It did not recognize OVA plus IA(f) or KLH plus I-A(k). These results have some bearing on models describing the nature of T cell receptors for antigen recognized in association with H-2 products. They do not support models in which antigen and H-2 are recognized separately by two independent T cell receptors.