Abrogation of cell surface expression of human class I transplantation antigens by an adenovirus protein in Xenopus laevis oocytes

Class I transplantation antigens form complexes with a virus protein encoded in the early region E3 of the adenovirus-2 genome. The interaction between this viral glycoprotein, E19, and nascent human class I antigens has been examined by microinjecting purified mRNA into Xenopus laevis oocytes. Both E19 and the two class I antigen subunits, the heavy chain and beta 2-microglobulin (beta 2M), were efficiently translated. The heavy chains did not become terminally glycosylated, as monitored by endoglycosidase H digestion, and were not expressed on the oocyte surface unless they were associated with beta 2M. The E19 protein did not become terminally glycosylated, and we failed to detect this viral protein on the surface of the oocytes. Co-translation of heavy chain and E19 mRNA demonstrated that the two proteins associate intracellularly. However, neither protein appeared to be transported to the trans-Golgi compartment. Similar observations were made in adenovirus-infected HeLa cells. Heavy chains bound to beta 2M became terminally glycosylated in oocytes in the presence of low concentrations of E19. At high concentrations of the viral protein, no carbohydrate modifications and no cell surface expression of class I antigens were apparent. Thus, beta 2M and E19 have opposite effects on the intracellular transport of the heavy chains. These data suggest that adenovirus-2 may impede the cell surface expression of class I antigens to escape immune surveillance.     

Cell surface expression of invariant gamma-chain of class II histocompatibility antigens in human skin

A rat monoclonal antibody (McAb 21:9) reactive with the human invariant gamma-chain of class II major histocompatibility complex (MHC)-encoded antigens was isolated and was shown to react with the carbohydrate-carrying, COOH-terminal part of the gamma-chain. The McAb 21:9 binds to a molecule that is identified as the gamma-chain for the following reasons: it has an apparent m.w. of 33,000, similar to that of the gamma-chain; it has a two-dimensional gel migration pattern identical to that of the gamma-chain; and it associates with immature, but not processed class II antigens. When used for immunohistochemical staining on sections of normal human skin, only dendritic, class II MHC antigen, and anti-Leu-6 reactive Langerhans cells are labeled in the epidermis. HLA-DR-expressing keratinocytes present in the tuberculin reaction, cutaneous T cell lymphoma, and lichen planus, however, did not react with the anti-gamma-chain antibody, nor with a HLA-DQ-reactive antibody. Cell surface expression of the gamma-chain was observed on 1 to 3% of normal viable epidermal cells in suspension. By using double indirect immunofluorescence, it was possible to demonstrate the simultaneous binding of anti-gamma-chain, anti-HLA-DR, anti-Leu-10, and anti-Leu-6 antibodies, respectively, on the same cells, thus confirming their identity as Langerhans cells. The presence of the gamma-chain on the surface of the immunocompetent Langerhans cells may indicate that the cell surface, not the cytoplasm as has been suggested, is the site of the primary function of the gamma-chain.     

Modulation of class I and class II histocompatibility antigens on human T cell lines by IFN-gamma

IFN-gamma is an immunomodulatory agent which is known to induce or enhance the expression of class II histocompatibility Ag (Ia Ag) on many lymphoid cells and cell lines of diverse origin. However, we have observed that IFN-gamma did not induce the expression of Ia Ag on Ia- human T cell lines. Neither did IFN-gamma enhance the expression of Ia Ag on Ia+ T cells. However, IFN-gamma was able to enhance the expression of class I histocompatibility Ag (HLA-A,B,C Ag) on a number of the T cell lines tested. Experiments with 125I-labeled IFN-gamma showed a relatively small degree of specific binding to these T cell lines. More extensive studies on two of the T cell lines demonstrated 1000 and 2600 IFN-gamma binding receptor sites/cell and binding affinities of 4.0 X 10(-10) M and 7.3 X 10(-10) M. Thus, although IFN-gamma can bind to human T cell lines and enhance class I histocompatibility Ag on these cells, IFN-gamma alone does not appear to regulate expression of class II histocompatibility Ag on T cell lines.     

Subunit interactions of class I histocompatibility antigens

The kinetics of dissociation of iodinated beta 2-microglobulin (beta 2m) from the papain-solubilized class I histocompatibility antigen HLA-B7 have been investigated. In the presence of unlabeled beta 2m, most of the HLA dissociates according to a single rate constant, whereas in the absence of unlabeled beta 2m, the system approaches an equilibrium dependent upon the initial HLA concentration. When iodinated beta 2m is incubated with unlabeled HLA-B7, the rate of incorporation of beta 2m into the complex is much less dependent on the concentration than is expected for a simple association/dissociation system; instead, the system behaves as if the "activity" (in a thermodynamic sense) of the HLA heavy-chain intermediate cannot surpass a critical concentration. The dissociation rate for each class I specificity is a function of temperature, ionic strength, pH, and the status of the heavy chain (papain solubilized vs. detergent solubilized). High temperature, high ionic strength, and extremes of pH promote dissociation. The intact molecule dissociates about 10 times more slowly than the papain-solubilized molecule. In contrast, the rate of dissociation of all papain-solubilized class I antigens tested falls within the range of about a factor of 2. The presence of the carbohydrate has no effect on the rate of dissociation. The possibility that HLA class I antigen dissociation may occur in vivo within acidic internal vesicles is discussed.     

Generation of major histocompatibility complex class I antigens

Presentation of antigenic peptides by major histocompatibility complex (MHC) class I molecules on the surface of antigen-presenting cells is an effective extracellular representation of the intracellular antigen content. The intracellular proteasome-dependent proteolytic machinery is required for generating MHC class I-presented peptides. These peptides appear to be derived mainly from newly synthesized defective ribosomal products, ensuring a rapid cytotoxic T lymphocyte-mediated immune response against infectious pathogens. Here we discuss the generation of MHC class I antigens on the basis of the currently understood molecular, biochemical and cellular mechanisms.     

Immunocytochemical localization of class I H-2 histocompatibility antigens of mouse liver

The subcellular location of class I H-2 histocompatibility antigens was determined for mouse liver using immunocytochemical techniques and correlated with information determined by cell fractionation and analysis in situ. Surface antigens first were localized by standard procedures involving surface labeling with ferritin-labeled antibody. This approach could not be used for internal membranes either in situ or in fractions since the antigens are not expressed at the cytoplasmic surface. For this purpose, thin sections of tissues embedded in Lowicryl were analyzed and quantitated. The in situ analysis confirmed the presence of H-2 antigens on internal membrane compartments as well as on the cell surface and helped rule out the possibility that distributions based on analyses by immunoprecipitation of fractions of internal membranes were influenced greatly by plasma membrane contamination. Quantitation was provided by immunoprecipitation of H-2 antigens from radioiodinated or metabolically labeled isolated and highly purified cell fractions. The findings establish the presence of class I H-2 histocompatibility antigens in endoplasmic reticulum, Golgi apparatus and plasma membrane in the approximate ratios of 1:3:7. No class I H-2 histocompatibility antigens could be detected in mitochondria, salt extracts of isolated membranes or NP-40-insoluble membrane material.     

Effect of herpes simplex virus types 1 and 2 on surface expression of class I major histocompatibility complex antigens on infected cells

Cytotoxic T lymphocytes (CTL) generated in C57BL/6 (H-2b) mice in response to infection with the serologically distinct herpes simplex virus type 1 (HSV-1) or type 2 (HSV-2) were cross-reactive against target cells infected with either serotype. However, HSV-2-infected cells were shown to be much less susceptible to CTL-mediated lysis, and analysis through the use of HSV-1 X HSV-2 intertypic recombinants mapped the reduced susceptibility to a region contained within 0.82 to 1.00 map units of the HSV-2 genome. The study reported here was undertaken to determine the possible reasons for the reduced susceptibility of HSV-2-infected cells to lysis by CTL. Competition for the specific lysis of labeled HSV-1-infected cells by either HSV-1- or HSV-2-infected, unlabeled inhibitor cells and frequency analysis of the CTL precursor able to recognize HSV-1- and HSV-2-infected cells suggested that the reduced susceptibility of HSV-2-infected cells to lysis could be explained, at least in part, by reduced levels of target cell recognition. A determination of the surface expression of the critical elements involved in target cell recognition by CTL following infection with HSV-1 or HSV-2 revealed that all the major HSV-specific glycoprotein species were expressed. Infection with both HSV-1 and HSV-2 caused a reduction in the expression of the class I H-2 antigens. However, this reduction was much greater following infection with HSV-2. This suggested that one important factor contributing to reduced lysis of HSV-2-infected cells may be the altered or reduced expression of the class I H-2 self-antigens.     

Differential phosphorylation of murine class I major histocompatibility antigens

Recent studies have shown that the H-2K and H-2D transplantation antigens are expressed differentially in different tissues of mouse. Our previous investigations also established that in thioglycolate-stimulated peritoneal macrophages the H-2D^k antigen exists in distinct cell surface and intracellular forms. These two forms are glycosylated differently. In this report, we have found that (1) H-2D^k antigen is phosphorylated whereas H-2K^k antigen is not, and (2) only the cell surface form of H-2D^k antigen is phosphorylated in thioglycolate-stimulated macrophages derived from C3H/Heha mice. This differential phosphorylation of H-2 antigens will provide a model system for further studies on the molecular mechanism and function of phosphrrylation of H-2 antigens.     

The human cytomegalovirus US8 glycoprotein binds to major histocompatibility complex class I products

Human cytomegalovirus US8 is a type I membrane protein that partially colocalizes with cellular endosomal and lysosomal proteins. Although US8 does not have discernible effects on the processing and cell surface distribution of major histocompatibility complex (MHC) class I products, we have demonstrated that US8 binds to MHC class I heavy chains in the endoplasmic reticulum.     

Differential association between two human MHC class I antigens and an adenoviral glycoprotein

The glycoprotein E19, encoded in early region 3 of adenovirus-2, forms complexes with major histocompatibility complex class I antigens. As a result of the complex formation, the intracellular transport of the class I antigens is abrogated, and adenovirus-infected cells display gradually diminishing quantities of cell surface-expressed class I molecules. To assess whether the E19 protein interacts equally well with different class I antigens, the associations between the viral protein and HLA-A2 and HLA-B7 antigens have been estimated. By infecting transfected HeLa cells expressing various amounts of HLA-A2 and HLA-B7 molecules, respectively, with various infectious doses of adenovirus-2, experimental conditions could be established that allowed quantitative estimates of the interactions to be determined. It was found that HLA-A2 molecules and the E19 protein interacts with a binding constant that is more than twice as high as that for HLA-B7 antigens and the viral protein. It is suggested that the pathogenicity of the virus may be dependent on the HLA-type of the infected individual.     

Equine herpesvirus type 4 UL56 and UL49.5 proteins downregulate cell surface major histocompatibility complex class I expression independently of each other

Major histocompatibility complex class I (MHC-I) molecules are critically important in the host defense against various pathogens through presentation of viral peptides to cytotoxic T lymphocytes (CTLs), a process resulting in the destruction of virus-infected cells. Herpesviruses interfere with CTL-mediated elimination of infected cells by various mechanisms, including inhibition of peptide transport and loading, perturbation of MHC-I trafficking, and rerouting and proteolysis of cell surface MHC-I. In this study, we show that equine herpesvirus type 4 (EHV-4) modulates MHC-I cell surface expression through two different mechanisms. First, EHV-4 can lead to a significant downregulation of MHC-I expression at the cell surface through the product of ORF1, a protein expressed with early kinetics from a gene that is homologous to herpes simplex virus 1 UL56. The EHV-4 UL56 protein reduces cell surface MHC-I as early as 4 h after infection. Second, EHV-4 can interfere with MHC-I antigen presentation, starting at 6 h after infection, by inhibition of the transporter associated with antigen processing (TAP) through its UL49.5 protein. Although pUL49.5 has no immediate effect on overall surface MHC-I levels in infected cells, it blocks the supply of antigenic peptides to the endoplasmic reticulum (ER) and transport of peptide-loaded MHC-I to the cell surface. Taken together, our results show that EHV-4 encodes at least two viral immune evasion proteins: pUL56 reduces MHC-I molecules on the cell surface at early times after infection, and pUL49.5 interferes with MHC-I antigen presentation by blocking peptide transport in the ER.     

Modulation of the expression of histocompatibility antigens class I (HLA-A,B,C) in the human pancreas

In this study the expression of HLA class I molecules was analysed on caudal portions of ten pancreata from cadaver donors by means of indirect immunoperoxidase and immunophosphatase techniques. In 7 out of ten pancreata the results showed that islets tissue was almost negative for the expression of HLA Class I antigens as opposed to exocrine tissue that appeared positive. Within exocrine tissue and large sized islets strongly positive interstitial cells were also detected. Double stainings showed that the strongly positive interstitial cells expressed also Leu M1 antigens. Preliminary studies on the remaining three pancreata demonstrate an increase of hematic interstitial cells together with a parallel increase of HLA class I antigens by endocrine parenchyma. The above data suggest that an increase of interstitial cells within pancreas may influence islets antigenicity.     

Role of beta 2-microglobulin in the intracellular transport and surface expression of murine class I histocompatibility molecules

We have examined the requirement for beta 2-microglobulin (beta 2m) in the intracellular transport of murine class I histocompatibility molecules to the cell surface. R1E cells that are defective in the synthesis of beta 2m were transfected with either the class I H-2Kb or H-2Db genes alone, or together with the beta 2m gene. Kb or Db heavy chains synthesized in the presence of beta 2m were transported rapidly through the cell and expressed efficiently at the cell surface. In the absence of beta 2m, no "free" Kb heavy chains were detectable at the cell surface and their intracellular transport was blocked at an early stage. In contrast, a significant quantity of "free" Db heavy chains could be detected at the cell surface as we have reported previously. However, we have shown here that defects in intracellular transport were apparent in that the majority (approximately 70%) of newly synthesized Db heavy chains accumulated intracellularly and were degraded. Therefore, although Kb and Db heavy chains differ in their abilities to be expressed at the cell surface in the absence of beta 2m they both require association with beta 2m for efficient intracellular transport. In addition, R1E cells transfected with a deletion construct of the Kb gene expressed a truncated molecule lacking the alpha 3 extracellular domain (Kb-3) at the cell surface, but, like free Db, most newly synthesized Kb-3 molecules accumulated intracellularly. The free Kb, Kb-3, and Db heavy chains were not recognized by most mAb specific for Kb and Db, respectively. Therefore, even the transported forms of free Db and Kb-3 were not native in conformation, which is surprising given the current view that correct folding is essential for intracellular transport. Interestingly, the free Db and Kb-3 heavy chains that reached the cell surface differed in their detergent binding properties from those retained within the cell. This suggests that the transported heavy chains may have folded differently thus allowing their export to the cell surface.     

Construction of novel class I histocompatibility antigens by interspecies exon shuffling

Human and mouse class I histocompatibility antigens share considerable structural homology at both the protein and DNA sequence level. This homology has allowed the production of hybrid class I molecules by the reciprocal exchange of DNA sequences corresponding to equivalent domains of HLA-B7 and either H-2Ld or H-2Dd. It is shown that these genes give rise to protein products that are stably expressed on the surface of murine L cells after DNA-mediated gene transfer. These proteins express only those monoclonal antibody-defined H-2 determinants that are expected based on their genetic construction. The molecules have allowed the localization of a number of polymorphic and monomorphic HLA-specific epitopes. In all but one case, expression of an epitope on a domain does not appear to be influenced by the replacement of adjacent human domains with their murine equivalents, suggesting a considerable degree of structural independence of the domains. Cells expressing the hybrid molecules have also been tested as targets for a panel of HLA-B7-specific cytotoxic T cell clones. The results show that the polymorphic determinants recognized by these clones map to the alpha 1 and alpha 2 domains of the HLA-B7 molecule. No evidence for an influence of species-related amino acid sequence differences in the third extracellular domain on T cell recognition was seen. The results are discussed in light of the proposed domain structure of the class I proteins and the potential use of such molecules for further functional studies.