The alpha chain gene of H-2O has an unexpected location in the major histocompatibility complex.

A previously unknown major histocompatibility complex class II molecule consisting of the beta chain encoded by the H-2Ob gene and an unknown alpha chain was recently described. We now report that the alpha chain occurs in two allelic forms distinguished by charge difference. Using inbred recombinant mouse strains we were able to map the H-2Oa gene to a location between the A.TL and B10.MBR recombination points. Cosmids covering this region were used to isolate the gene. Sequence analysis revealed that the H-2Oa gene is the murine equivalent of the human HLA-DNA gene. These results indicate that the human HLA-DNA gene, the existence of which has long been known, is indeed coding for DO alpha, the alpha chain pairing with DO beta.

Requirements for peptide-induced T cell receptor downregulation on naive CD8+ T cells.

The requirements for inducing downregulation of alpha/beta T cell receptor (TCR) molecules on naive major histocompatibility complex class I-restricted T cells was investigated with 2C TCR transgenic mice and defined peptides as antigen. Confirming previous results, activation of 2C T cells in response to specific peptides required CD8 expression on the responder cells and was heavily dependent upon costimulation provided by either B7-1 or ICAM-1 on antigen-presenting cells (APC). These stringent requirements did not apply to TCR downregulation. Thus, TCR downregulation seemed to depend solely on TCR/peptide/interaction and did not require either CD8 or B7-1 expression; ICAM-1 potentiated TCR downregulation, but only with limiting doses of peptides. TCR downregulation was most prominent with high affinity peptides and appeared to be neither obligatory nor sufficient for T cell activation. In marked contrast to T cell activation, TCR downregulation was resistant to various metabolic inhibitors. The biological significance of TCR downregulation is unclear, but could be a device for protecting T cells against excessive signaling.

Differing roles for B7 and intercellular adhesion molecule-1 in negative selection of thymocytes.

To ensure self tolerance, immature thymocytes with high binding affinity for self peptides linked to major histocompatibility complex (MHC) molecules are eliminated in situ via apoptosis (negative selection). The roles of two costimulatory molecules, B7-1 and intercellular adhesion molecule-1 (ICAM-1), in negative selection was examined by studying apoptosis of T cell receptor transgenic CD4+8+ thymocytes cultured with specific peptides presented by MHC class I-transfected Drosophila cells. When coexpressed on these cells, B7-1 and ICAM-1 act synergistically and cause strong class 1-restricted negative selection of thymocytes. When expressed separately, however, B7-1 and ICAM-1 display opposite functions: negative selection is augmented by B7-1, but is inhibited by ICAM-1. It is notable that B7-1 is expressed selectively in the thymic medulla, whereas ICAM-1 is expressed throughout the thymus. Because of this distribution, the differing functions of B7-1 and ICAM-1 may dictate the sites of positive and negative selection. Thus, in the cortex, the presence of ICAM-1, but not B7-1, on the cortical epithelium may preclude or reduce negative selection and thereby promote positive selection. Conversely, the combined expression of B7-1 and ICAM-1 may define the medulla as the principal site of negative selection.

Nonclassical behavior of the thymus leukemia antigen: peptide transporter-independent expression of a nonclassical class I molecule.

The thymus leukemia (TL) antigen is a major histocompatibility complex-encoded nonclassical class I molecule. Here we present data demonstrating that expression of the TL antigen, unlike other class I molecules, is completely independent of the function of the transporter associated with antigen processing (TAP). The TL antigen is expressed by transfected TAP-2-deficient RMA-S cells when these cells are grown at 37 degrees C. In transfected RMA cells, the kinetics of arrival of TL antigen on the cell surface are similar to those of a classical class I molecule. The kinetics are not altered in TAP-deficient RMA-S cells, demonstrating that surface TL expression in TAP-deficient cells is not due to the stable expression of a few molecules that leak out by a TAP-independent pathway. Soluble TL molecules produced by Drosophila melanogaster cells are highly resistant to thermal denaturation, unlike peptide-free classical class I molecules synthesized by these insect cells. In addition, these soluble TL molecules are devoid of detectable bound peptides. The results demonstrate that the TL antigen is capable of reaching the surface without bound peptide, although acquisition of peptide or some other ligand through a TAP-independent pathway cannot be formally excluded. We speculate that the ability of the TL antigen to reach the cell surface, under conditions in which other class I molecules do not, may be related to a specialized function of the TL molecule in the mucosal immune system, and possibly in the stimulation of intestinal gamma delta T cells.

Human CD8 transgene regulation of HLA recognition by murine T cells.

A series of human CD8 transgenic (hCD8 Tg) mice with differential expression in the thymus and periphery were produced to investigate CD8 coreceptor regulation of repertoire selection and T cell responses. Expression of hCD8 markedly enhanced responses to both HLA class I molecules and hybrid A2/Kb molecules providing functional evidence for a second interaction site, outside of the alpha 3 domain, which is essential for optimal coreceptor function. Peripheral T cell expression of hCD8 was sufficient to augment responsiveness to HLA class I, as hCD8 Tg mice which lacked thymic expression responded as well as mice expressing hCD8 in the thymus and periphery. Both murine CD8+ and CD4+ T cells expressing hCD8 transgenes exhibited markedly enhanced responses to foreign HLA class I, revealing the ability of T cell receptor repertoires selected on either murine class I or class II to recognize human class I major histocompatibility complex (MHC). In contrast to recognition of foreign class I, thymic expression of hCD8 transgenes was absolutely required to enhance recognition of antigenic peptide restricted by self-HLA class I. Thus, our studies revealed disparate requirements for CD8 coreceptor expression in the thymus for selection of a T cell repertoire responsive to foreign MHC and to antigenic peptides bound to self-MHC, providing a novel demonstration of positive selection that is dependent on human CD8.

Species specificity and augmentation of responses to class II major histocompatibility complex molecules in human CD4 transgenic mice.

Murine T cell responses to human class II major histocompatibility complex (MHC) molecules were shown to be a minimum of 20-70-fold lower than responses to allogeneic molecules. Transgenic mice expressing slightly below normal (75-95%) or very high (250-380%) cell surface levels of human CD4 were utilized to determine whether this was due to a species-specific interaction between murine CD4 and class II molecules. Human CD4 was shown to function in signal transduction events in murine T cells based on the ability of anti-human CD4 antibody to synergize with suboptimal doses of anti-murine CD3 antibody in stimulating T cell proliferation. In mice expressing lower levels of human CD4, T cell responses to human class II molecules were enhanced up to threefold, whereas allogeneic responses were unaltered. In mice expressing high levels of human CD4, responses to human class II molecules were enhanced at least 10-fold, whereas allogeneic responses were between one and three times the level of normal responses. The relatively greater enhancement of the response to human class II molecules in both lines argues for a preferential interaction between human CD4 and human class II molecules. In mice expressing lower levels of human CD4, responses to human class II molecules were blocked by antibodies to CD4 of either species, indicating participation by both molecules. In mice expressing high levels of human CD4, responses to both human and murine class II molecules were almost completely blocked with anti-human CD4 antibody, whereas anti-murine CD4 antibody had no effect. However, anti-murine CD4 continued to synergize with anti-CD3 in stimulating T cell proliferation in these mice. Thus, overexpression of human CD4 selectively impaired the ability of murine CD4 to assist in the process of antigen recognition. The ability of human CD4 to support a strong allogeneic response under these conditions indicates that this molecule can interact with murine class II molecules to a significant extent. Despite the fact that human CD4 appeared to be the only functional coreceptor in these mice, responses to human class II molecules were still much lower than those to murine class II alloantigens. This indicates that species-specific interactions between class II molecules and CD4 expressed on peripheral T cells are not sufficient to account for the low xenogeneic response and that intrinsic differences in T cell receptor structures or the need for species specificity in the interaction between CD4 and class II molecules during positive selection are also important.

MHC class I molecules form ternary complexes with calnexin and TAP and undergo peptide-regulated interaction with TAP via their extracellular domains.

Newly assembled heavy chain-beta 2m heterodimers of class I histocompatibility molecules associate with the endoplasmic reticulum (ER) peptide transporter, TAP, and subsequently dissociate from TAP in parallel with their transport from the ER to the Golgi apparatus. It appears that TAP-associated class I molecules are waiting to bind appropriate peptides before they dissociate from TAP and leave the ER since binding of high affinity peptides to class I molecules in vitro leads to dissociation of TAP-class I complexes. In further support of this notion, we report that limiting peptide supply through inhibition of proteasome activities prolongs the association of mouse class I molecules with TAP and concomitantly slows their transport to the Golgi apparatus. By using a series of deletion mutants and hybrid class I molecules we demonstrate that the extracellular domains of class I molecules are sufficient for their peptide-regulated interaction with TAP. Furthermore, based on the inability of an alpha 3 domain-specific mAb to recognize TAP-class I complexes and the fact that a point mutant of the Dd molecule at residue 222 is unable to bind to TAP, it is likely that a major site of interaction with TAP resides in the membrane-proximal region of the heavy chain alpha 3 domain. Finally, we examined the relationship between the interaction of mouse heavy chain-beta 2m heterodimers with TAP and with the resident ER chaperone, calnexin. Most heterodimers that bound to TAP were found to associate simultaneously with calnexin. Upon delivery of peptide to class I molecules in permeabilized cells, dissociation from TAP was observed but the interaction with calnexin was largely maintained. Therefore, both TAP and calnexin may participate in the ER retention of peptide-deficient class I molecules. However, since release from calnexin occurs after dissociation from TAP, it appears that calnexin ultimately determines if a class I molecule is to be exported from the ER.

T cells can use either T cell receptor or CD28 receptors to absorb and internalize cell surface molecules derived from antigen-presenting cells.

At the site of contact between T cells and antigen-presenting cells (APCs), T cell receptor (TCR)-peptide-major histocompatibility complex (MHC) interaction is intensified by interactions between other molecules, notably by CD28 and lymphocyte function-associated antigen 1 (LFA-1) on T cells interacting with B7 (B7-1 and B7-2), and intracellular adhesion molecule 1 (ICAM-1), respectively, on APCs. Here, we show that during T cell-APC interaction, T cells rapidly absorb various molecules from APCs onto the cell membrane and then internalize these molecules. This process is dictated by at least two receptors on T cells, namely CD28 and TCR molecules. The biological significance of T cell uptake of molecules from APCs is unclear. One possibility is that this process may allow activated T cells to move freely from one APC to another and eventually gain entry into the circulation.

Defective interleukin (IL)-18-mediated natural killer and T helper cell type 1 responses in IL-1 receptor-associated kinase (IRAK)-deficient mice.

Interleukin (IL)-18 is functionally similar to IL-12 in mediating T helper cell type 1 (Th1) response and natural killer (NK) cell activity but is related to IL-1 in protein structure and signaling, including recruitment of IL-1 receptor-associated kinase (IRAK) to the receptor and activation of c-Jun NH2-terminal kinase (JNK) and nuclear factor (NF)-kappaB. The role of IRAK in IL-18-induced responses was studied in IRAK-deficient mice. Significant defects in JNK induction and partial impairment in NF-kappaB activation were found in IRAK-deficient Th1 cells, resulting in a dramatic decrease in interferon (IFN)-gamma mRNA expression. In vivo Th1 response to Propionibacterium acnes and lipopolysaccharide in IFN-gamma production and induction of NK cytotoxicity by IL-18 were severely impaired in IRAK-deficient mice. IFN-gamma production by activated NK cells in an acute murine cytomegalovirus infection was significantly reduced despite normal induction of NK cytotoxicity. These results demonstrate that IRAK plays an important role in IL-18-induced signaling and function.

Interleukin (IL)-1 receptor-associated kinase (IRAK) requirement for optimal induction of multiple IL-1 signaling pathways and IL-6 production.

Interleukin (IL)-1 is a proinflammatory cytokine with pleiotropic effects in inflammation. IL-1 binding to its receptor triggers a cascade of signaling events, including activation of the stress-activated mitogen-activated protein (MAP) kinases, c-Jun NH2-terminal kinase (JNK) and p38 MAP kinase, as well as transcription factor nuclear factor kappaB (NF-kappaB). IL-1 signaling results in cellular responses through induction of inflammatory gene products such as IL-6. One of the earliest events in IL-1 signaling is the rapid interaction of IL-1 receptor-associated kinases, IRAK and IRAK-2, with the receptor complex. The relative roles of IRAK and IRAK-2 in IL-1 signaling pathways and subsequent cellular responses have not been previously determined. To evaluate the importance of IRAK in IL-1 signaling, IRAK-deficient mouse fibroblast cells were prepared and studied. Here we report that IL-1-mediated activation of JNK, p38, and NF-kappaB were all reduced in embryonic fibroblasts deficient in IRAK expression. In addition, IL-6 production in response to IL-1 was also dramatically reduced in IRAK-deficient embryonic fibroblasts and in skin fibroblasts prepared from IRAK-deficient mice. Our results demonstrate that IRAK plays an essential proximal role in coordinating multiple IL-1 signaling pathways for optimal induction of cellular responses.

Peptide length and sequence specificity of the mouse TAP1/TAP2 translocator.

The transporter associated with antigen processing (TAP) delivers peptides to the lumen of the endoplasmic reticulum in an adenosine triphosphate (ATP) dependent fashion for presentation by major histocompatibility complex class I molecules. We show that the mouse TAP translocator (H-2b haplotype) selects peptides based on a minimal size of nine residues, and on the presence of a hydrophobic COOH-terminal amino acid. The preponderance of COOH-terminal hydrophobic amino acids in peptides capable of binding to mouse class I molecules thus fits remarkably well with the specificity of the TAP translocator. In addition to transport in the lumenal direction, efflux of peptide in the cytosolic direction is observed in an ATP- and temperature-dependent manner. By maintaining a low peptide concentration at the site of class I assembly, this efflux mechanism may ensure that class I molecules are loaded preferentially with high affinity peptides.

Invariant chain--a regulator of antigen presentation.

MHC class II molecules present internalized antigens to the immune system. They have long been known to associate with a polypeptide called the invariant chain. Recent findings have revealed that this polypeptide performs two functions. First, it prevents class II molecules from binding antigenic peptides at the site of synthesis of class II molecules in the endoplasmic reticulum.Second, it targets class II molecules to their destination in the endocytic pathway, where they pick up antigenic peptides derived from endocytosed antigens. Short sequences in the cytoplasmic portion of the invariant chain serve as subcellular address labels. The functions of the invariant chain help to explain how the immune system divides its defence against foreign pathogens between cytotoxic T cells and antibodies.

Beta 2-microglobulin induces intracellular transport of human class I transplantation antigen heavy chains in Xenopus laevis oocytes.

Human class I transplantation antigens are cell-surface-expressed molecules composed of one glycosylated, membrane-integrated heavy chain and one nonglycosylated, water-soluble subunit, beta 2-microglobulin (beta 2m). We have examined the intracellular transport of the two subunits by microinjecting mRNA into Xenopus laevis oocytes. Beta 2m, translated in oocytes, was transported and secreted into the medium in the absence of heavy chains whereas heavy chains were retained in the endoplasmic reticulum if not cotranslated with beta 2m. In the presence of beta 2m, heavy chains resisted digestion by endoglycosidase H (Endo H), suggesting that beta 2m promotes the transport of heavy chains from endoplasmic reticulum to the Golgi compartment. Pulse-chase experiments confirmed this notion. The possibility that heavy chains aggregate irreversibly when synthesized in the absence of beta 2m was ruled out and it is demonstrated that performed heavy chains will become transported once beta 2m is available. It is suggested that intracellular transport is controlled by structural features that are part of the transported polypeptide. If so, beta 2m but not heavy chains may possess such features.

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.

Retrieval of transmembrane proteins to the endoplasmic reticulum.

A COOH-terminal double lysine motif maintains type I transmembrane proteins in the ER. Proteins tagged with this motif, eg., CD8/E19 and CD4/E19, rapidly receive post-translational modifications characteristic of the intermediate compartment and partially colocalized to this organelle. These proteins also received modifications characteristic of the Golgi but much more slowly. Lectin staining localized these Golgi modified proteins to ER indicating that this motif is a retrieval signal. Differences in the subcellular distribution and rate of post-translational modification of CD8 maintained in the ER by sequences derived from a variety of ER resident proteins suggested that the efficiency of retrieval was dependent on the sequence context of the double lysine motif and that retrieval may be initiated from multiple positions along the exocytotic pathway.

Ligand-dependent regulation of intracellular protein transport: effect of vitamin a on the secretion of the retinol-binding protein.

As a model of ligand-dependent protein secretion the biosynthesis, intracellular transport, and release of the retinol-binding protein (RBP) were studied in primary cultures of rat hepatocytes pulse-labeled with [35S]methionine. After various periods of chase RBP was isolated by immunoprecipitation and identified by SDS PAGE. Both normal and vitamin A-deficient hepatocytes synthesized RBP. The normal cells secreted the pulse-labeled RBP within 2 h. RBP synthesized by deficient cells was not secreted, and intracellular degradation of the protein appeared to be slow. Deficient cells could be induced to secrete RBP on the addition of retinol to the culture medium. This occurred also after protein synthesis had been blocked by cycloheximide. Since retinol induces the secretion of RBP, accumulated in the endoplasmic reticulum (ER), it seems reasonable to conclude that the transport of RBP from the ER to the Golgi complex is regulated by retinol.

Reconstitution of an operational MHC class II compartment in nonantigen-presenting cells.

Professional antigen-presenting cells (APCs) have a distinct compartment in which class II molecules are proposed to acquire antigenic peptides. Genetic evidence suggests that human leukocyte antigen (HLA)-DM, an unusual class II molecule, participates in this process. Peptide acquisition was reconstituted in nonprofessional APCs by transfection of class II, invariant chain (li), and H-2M, the murine equivalent of DM. The H-2M heterodimer appeared in an endosomal compartment, not at the cell surface, and the localization was independent of li. The data presented show that H-2M, class II, and li are the minimally required components for efficient formation of stable class II-peptide complexes, and thus for a functional class II compartment.

Crystal structures of two viral peptides in complex with murine MHC class I H-2Kb.

The x-ray structures of a murine MHC class I molecule (H-2Kb) were determined in complex with two different viral peptides, derived from the vesicular stomatitis virus nucleoprotein (52-59), VSV-8, and the Sendai virus nucleoprotein (324-332), SEV-9. The H-2Kb complexes were refined at 2.3 A for VSV-8 and 2.5 A for SEV-9. The structure of H-2Kb exhibits a high degree of similarity with human HLA class I, although the individual domains can have slightly altered dispositions. Both peptides bind in extended conformations with most of their surfaces buried in the H-2Kb binding groove. The nonamer peptide maintains the same amino- and carboxyl-terminal interactions as the octamer primarily by the insertion of a bulge in the center of an otherwise beta conformation. Most of the specific interactions are between side-chain atoms of H-2Kb and main-chain atoms of peptide. This binding scheme accounts in large part for the enormous diversity of peptide sequences that bind with high affinity to class I molecules. Small but significant conformational changes in H-2Kb are associated with peptide binding, and these synergistic movements may be an integral part of the T cell receptor recognition process.

Emerging principles for the recognition of peptide antigens by MHC class I molecules.

Class I major histocompatibility complex (MHC) molecules interact with self and foreign peptides of diverse amino acid sequences yet exhibit distinct allele-specific selectivity for peptide binding. The structures of the peptide-binding specificity pockets (subsites) in the groove of murine H-2Kb as well as human histocompatibility antigen class I molecules have been analyzed. Deep but highly conserved pockets at each end of the groove bind the amino and carboxyl termini of peptide through extensive hydrogen bonding and, hence, dictate the orientation of peptide binding. A deep polymorphic pocket in the middle of the groove provides the chemical and structural complementarity for one of the peptide's anchor residues, thereby playing a major role in allele-specific peptide binding. Although one or two shallow pockets in the groove may also interact with specific peptide side chains, their role in the selection of peptide is minor. Thus, usage of a limited number of both deep and shallow pockets in multiple combinations appears to allow the binding of a broad range of peptides. This binding occurs with high affinity, primarily because of extensive interactions with the peptide backbone and the conserved hydrogen bonding network at both termini of the peptide. Interactions between the anchor residue (or residues) and the corresponding allele-specific pocket provide sufficient extra binding affinity not only to enhance specificity but also to endure the presentation of the peptide at the cell surface for recognition by T cells.

Regulation of MHC class I transport by the molecular chaperone, calnexin (p88, IP90).

Assembled class I histocompatibility molecules, consisting of heavy chain, beta 2-microglobulin, and peptide ligand, are transported rapidly to the cell surface. In contrast, the intracellular transport of free heavy chains or peptide-deficient heavy chain-beta 2-microglobulin heterodimers is impaired. A 90-kilodalton membrane-bound chaperone of the endoplasmic reticulum (ER), termed calnexin, associates quantitatively with newly synthesized class I heavy chains, but the functions of calnexin in this interaction are unknown. Class I subunits were expressed alone or in combination with calnexin in Drosophila melanogaster cells. Calnexin retarded the intracellular transport of both peptide-deficient heavy chain-beta 2-microglobulin heterodimers and free heavy chains. Calnexin also impeded the rapid intracellular degradation of free heavy chains. The ability of calnexin to protect and retain class I assembly intermediates is likely to contribute to the efficient intracellular formation of class I-peptide complexes.