Ligands for the murine NKG2D receptor: expression by tumor cells and activation of NK cells and macrophages

Natural killer (NK) cells attack tumor and infected cells, but the receptors and ligands that stimulate them are poorly understood. Here we report the expression cloning of two murine ligands for the lectin-like receptor NKG2D. The two ligands, H-60 and Rae1 beta, are distant relatives of major histocompatibility complex class I molecules. NKG2D ligands are not expressed by most normal cells but are up-regulated on numerous tumor cells. We show that mouse NKG2D is expressed by NK cells, activated CD8+ T cells and activated macrophages. Expression of either NKG2D ligand by target cells triggers NK cell cytotoxicity and interferon-gamma secretion by NK cells, as well as nitric oxide release and tumor necrosis factor alpha transcription by macrophages. Thus, through their interaction with NKG2D, H-60 and Rae1 beta are newly identified potent stimulators of innate immunity.

The molecular genetics of the T-cell antigen receptor and T-cell antigen recognition

The genes encoding the alpha and beta chain of the T-cell receptor and the gamma gene have been cloned, and their structure, organization, ontogeny of expression, pattern of rearrangement, and diversification are now generally understood. In most cases, the immunoglobulin paradigm applied very well to the corresponding phenomena in T cells, although as described above, some interesting and potentially important differences exist. Nevertheless, there are still many unanswered questions regarding the ontogeny and mechanism of MHC-restricted antigen recognition, and it is not clear how far the immunoglobulin model can take us in understanding these phenomena. Although the alpha/beta heterodimer looks like an antibody and the binding sites of the two molecules may be similar, the rules governing B- and T-cell activation are clearly different, and the ligand(s) bound by the receptor are still poorly characterized. In the future, T-cell receptor genes, as well as those encoding the T-cell accessory molecules, will be altered in vitro and transferred into mammalian cells in culture and into whole organisms in an attempt to understand T-cell antigen recognition. These tools will allow us to manipulate the mammalian immune response in a variety of different ways that will have a profound impact both on our understanding of immunology and on medicine in the future.

ERAAP customizes peptides for MHC class I molecules in the endoplasmic reticulum

The ability of killer T cells carrying the CD8 antigen to detect tumours or intracellular pathogens requires an extensive display of antigenic peptides by major histocompatibility complex (MHC) class I molecules on the surface of potential target cells. These peptides are derived from almost all intracellular proteins and reveal the presence of foreign pathogens and mutations. How cells produce thousands of distinct peptides cleaved to the precise lengths required for binding different MHC class I molecules remains unknown. The peptides are cleaved from endogenously synthesized proteins by the proteasome in the cytoplasm and then trimmed by an unknown aminopeptidase in the endoplasmic reticulum (ER). Here we identify ERAAP, the aminopeptidase associated with antigen processing in the ER. ERAAP has a broad substrate specificity, and its expression is strongly upregulated by interferon-gamma. Reducing the expression of ERAAP through RNA interference prevents the trimming of peptides for MHC class I molecules in the ER and greatly reduces the expression of MHC class I molecules on the cell surface. Thus, ERAAP is the missing link between the products of cytosolic processing and the final peptides presented by MHC class I molecules on the cell surface.

Detection of rare antigen-presenting cells by the lacZ T-cell activation assay suggests an expression cloning strategy for T-cell antigens

The alpha/beta T-cell receptor a complex ligand formed by the association of antigenic peptides with molecules of the major histocompatibility complex (MHC). The inherent limitations of the conventional T-cell activation assays used to detect these peptide/MHC ligands have, until now, hampered the development of expression cloning systems for T-cell antigens. To overcome these limitations, we have recently introduced a method for detecting ligand-induced activation of individual T cells. This assay, which makes use of a lacZ reporter construct, differs from conventional ligand-induced activation assays in that it allows the detection of single, activated T cells in large pools of resting cells. We applied the lacZ assay to the problem of screening expression libraries, which requires the ability to detect ligand-bearing antigen-presenting cells when they are present at very low frequency. We show here that ligand-expressing antigen-presenting cells can be detected at frequencies of 1:10(3)-10(4), a level of sensitivity compatible with the screening of cDNA libraries. Furthermore, by using as antigen-presenting cells COS-7 cells stably transfected with the murine Kb class I MHC molecule, we demonstrate that transiently expressed ovalbumin is efficiently processed and presented to an ovalbumin/Kb-specific T-cell hybridoma. lacZ expression is induced in a detectable number of cocultured T cells, even when the ovalbumin cDNA consists of only 1:10(4) of the total DNA used to transfect the COS cells. These results suggest that unknown T-cell antigens may be identified by screening cDNA libraries in MHC-expressing COS cells using lacZ-inducible T cells as indicators of peptide antigen expression.

Identification of an MHC class I-restricted autoantigen in type 1 diabetes by screening an organ-specific cDNA library

Type 1 diabetes is an autoimmune disease in which the insulin-producing pancreatic beta cells are destroyed at an early age by an immune process that involves both CD4 and CD8 T lymphocytes. The identification of autoantigens in diabetes is very important for the design of antigen-specific immunotherapy. By screening a pancreatic islet cDNA library, we have identified the autoantigen recognized by highly pathogenic CD8 T cells in the non-obese diabetic mouse, one of the best animal models for human diabetes. This is the first identification, to our knowledge, of a CD8 T-cell epitope in an autoimmune disease. The peptide recognized by the cells is in the same region of the insulin B chain as the epitope recognized by previously isolated pathogenic CD4 T cells. This has very important implications for the potential use of insulin in preventative therapy.

Requirement for association of p56lck with CD4 in antigen-specific signal transduction in T cells

The T cell-specific transmembrane glycoprotein CD4 interacts with class II MHC molecules via its external domain and is associated with tyrosine kinase p56lck via a cysteine motif in its cytoplasmic domain. We have assessed the ability of CD4 to synergize with the antigen-specific T cell receptor (TCR) for induction of transmembrane signals that result in lymphokine production. Mutant CD4 molecules were introduced into T cells that lacked endogenous CD4 but expressed TCRs specific for lysozyme peptides or the superantigen SEA bound to Ab or Abm12 class II MHC molecules. With either ligand, T cell activation occurred only when CD4 was associated with p56lck. These results demonstrate that residues within the cytoplasmic domain of CD4 are required for its coreceptor function in TCR-mediated signal transduction and strongly support the notion that the association of CD4 with p56lck is critical in this process.

LacZ inducible, antigen/MHC-specific T cell hybrids

The nuclear factor of activated T cells (NFAT) enhancer element of the IL-2 gene can regulate expression of the Escherichia coli lacZ reporter gene in activated T cells. Based upon this observation, we showed that this inducible NFAT--lacZ construct could be used to measure TCR mediated, ligand-specific activation in single T cells. Here we describe a general approach to obtaining lacZ inducible, T cell hybrids by generating two new fusion partners BWZ.36 and BWZ.36 CD8 alpha derived by transfecting alpha-beta-BW5147 cells with the NFAT--lacZ construct. Using these fusion partners and normal T cells from immunized mice, we obtained T cell hybrids in which lacZ activity is specifically induced in response to antigen/MHC class II or MHC class I complexes. We show that measuring ligand induced T cell activation by the non-radioactive lacZ assay is simpler, faster, and most cost-effective relative to conventional IL-2 assays. Most importantly, the unique ability to detect activation of single T cells by the lacZ assays permits detection of rare antigen presenting cells and thus provides the basis for developing expression cloning strategies for identifying unknown T cell antigens.

Class I MHC presentation of exogenous soluble antigen via macropinocytosis in bone marrow macrophages

Extracellular proteins are not generally presented on class I MHC molecules in vitro, yet many studies show that a pathway exists in vivo for the presentation of extracellular material on class I molecules to prime CD8+ T cell responses. Here, we provide morphological evidence that proteins taken up by macropinocytosis can gain access to the cytosol and therefore into the conventional class I MHC pathway. Class I presentation of soluble ovalbumin by mouse bone marrow macrophages was dramatically enhanced by MCSF or phorbol ester and blocked by amiloride, which stimulate and inhibit membrane ruffling and macropinocytosis, respectively. Brefeldin A, gelonin, and a peptide aldehyde inhibitor of proteasomal processing each blocked presentation of macropinocytosed antigen, demonstrating that unusual access to the conventional class I MHC pathway was occurring. This novel cell type-specific endocytic pathway may facilitate presentation of exogenous material on class I MHC molecules, allowing induction of CD8+ T cell responses to soluble proteins, tumor cell fragments, and some pathogens.

A macromolecular delivery vehicle for protein-based vaccines: acid-degradable protein-loaded microgels

The development of protein-based vaccines remains a major challenge in the fields of immunology and drug delivery. Although numerous protein antigens have been identified that can generate immunity to infectious pathogens, the development of vaccines based on protein antigens has had limited success because of delivery issues. In this article, an acid-sensitive microgel material is synthesized for the development of protein-based vaccines. The chemical design of these microgels is such that they degrade under the mildly acidic conditions found in the phagosomes of antigen-presenting cells (APCs). The rapid cleavage of the microgels leads to phagosomal disruption through a colloid osmotic mechanism, releasing protein antigens into the APC cytoplasm for class I antigen presentation. Ovalbumin was encapsulated in microgel particles, 200-500 nm in diameter, prepared by inverse emulsion polymerization with a synthesized acid-degradable crosslinker. Ovalbumin is released from the acid-degradable microgels in a pH-dependent manner; for example, microgels containing ovalbumin release 80% of their encapsulated proteins after 5 h at pH 5.0, but release only 10% at pH 7.4. APCs that phagocytosed the acid-degradable microgels containing ovalbumin were capable of activating ovalbumin-specific cytoxic T lymphocytes. The acid-degradable microgels developed in this article should therefore find applications as delivery vehicles for vaccines targeted against viruses and tumors, where the activation of cytoxic T lymphocytes is required for the development of immunity.

Translation from the 5' untranslated region shapes the integrated stress response

Translated regions distinct from annotated coding sequences have emerged as essential elements of the proteome. This includes upstream open reading frames (uORFs) present in mRNAs controlled by the integrated stress response (ISR) that show "privileged" translation despite inhibited eukaryotic initiation factor 2-guanosine triphosphate-initiator methionyl transfer RNA (eIF2·GTP·Met-tRNA(i )(Met)). We developed tracing translation by T cells to directly measure the translation products of uORFs during the ISR. We identified signature translation events from uORFs in the 5' untranslated region of binding immunoglobulin protein (BiP) mRNA (also called heat shock 70-kilodalton protein 5 mRNA) that were not initiated at the start codon AUG. BiP expression during the ISR required both the alternative initiation factor eIF2A and non-AUG-initiated uORFs. We propose that persistent uORF translation, for a variety of chaperones, shelters select mRNAs from the ISR, while simultaneously generating peptides that could serve as major histocompatibility complex class I ligands, marking cells for recognition by the adaptive immune system.

Producing nature's gene-chips: the generation of peptides for display by MHC class I molecules

Gene-chips contain thousands of nucleotide sequences that allow simultaneous analysis of the complex mixture of RNAs transcribed in cells. Like these gene-chips, major histocompatibility complex (MHC) class I molecules display a large array of peptides on the cell surface for probing by the CD8(+) T cell repertoire. The peptide mixture represents fragments of most, if not all, intracellular proteins. The antigen processing machinery accomplishes the daunting task of sampling these proteins and cleaving them into the precise set of peptides displayed by MHC I molecules. It has long been believed that antigenic peptides arose as by-products of normal protein turnover. Recent evidence, however, suggests that the primary source of peptides is newly synthesized proteins that arise from conventional as well as cryptic translational reading frames. It is increasingly clear that for many peptides the C-terminus is generated in the cytoplasm, and N-terminal trimming occurs in the endoplasmic reticulum in an MHC I-dependent manner. Nature's gene-chips are thus both parsimonious and elegant.

Intracellular calcium dependence of gene expression in single T lymphocytes

In T lymphocytes, intracellular Ca2+ concentration ([Ca2+]i) rises within seconds of T-cell antigen-receptor stimulation and initiates the synthesis and secretion of interleukin 2, a cytokine essential for T-cell proliferation and the immune response. Using video-imaging techniques, we tracked [Ca2+]i signals in individual T cells and measured subsequent expression of a beta-galactosidase reporter gene (lacZ) controlled by the NF-AT element of the interleukin 2 enhancer. [Ca2+]i spikes elicited by monoclonal antibody binding to the CD3 epsilon subunit of the T-cell receptor were positively correlated with gene expression, but varied widely between individual cells and were therefore difficult to relate quantitatively to lacZ expression. The [Ca2+]i dependence of NF-AT-regulated gene expression was determined by elevating [Ca2+]i with either thapsigargin or ionomycin and then "clamping" [Ca2+]i to various, stable levels by altering either extracellular [Ca2+] or extracellular [K+]. Raising [Ca2+]i from resting levels of 70 nM to between 200 nM and 1.6 microM increased the fraction of cells expressing lacZ, with Kd approximately 1 microM. Activation of protein kinase C enhanced the [Ca2+]i sensitivity of gene expression (Kd = 210 nM), whereas stimulation of protein kinase A inhibited [Ca2+]i-dependent gene expression. The experiments described here provide single-cell measurements linking a second messenger to gene expression in individual cells.

Immunodominant, protective response to the parasite Toxoplasma gondii requires antigen processing in the endoplasmic reticulum

The parasite Toxoplasma gondii replicates in a specialized intracellular vacuole and causes disease in many species. Protection from toxoplasmosis is mediated by CD8(+) T cells, but the T. gondii antigens and host genes required for eliciting protective immunity are poorly defined. Here we identified GRA6, a polymorphic protein secreted in the parasitophorous vacuole, as the source of the immunodominant and protective decapeptide HF10 presented by the H-2L(d) major histocompatibility complex class I molecule. Presentation of the HF10-H-2L(d) ligand required proteolysis by ERAAP, the endoplasmic reticulum aminopeptidase associated with antigen processing. Consequently, expansion of protective CD8(+) T cell populations was impaired in T. gondii-infected ERAAP-deficient mice, which were more susceptible to toxoplasmosis. Thus, endoplasmic reticulum proteolysis is critical for eliciting protective immunity to a vacuolar parasite.

The aminopeptidase ERAAP shapes the peptide repertoire displayed by major histocompatibility complex class I molecules

Major histocompatibility complex (MHC) class I molecules present thousands of peptides to allow CD8(+) T cells to detect abnormal intracellular proteins. The antigen-processing pathway for generating peptides begins in the cytoplasm, and the MHC molecules are loaded in the endoplasmic reticulum. However, the nature of peptide pool in the endoplasmic reticulum and the proteolytic events that occur in this compartment are unclear. We addressed these issues by generating mice lacking the endoplasmic reticulum aminopeptidase associated with antigen processing (ERAAP). We found that loss of ERAAP disrupted the generation of naturally processed peptides in the endoplasmic reticulum, decreased the stability of peptide-MHC class I complexes and diminished CD8(+) T cell responses. Thus, trimming of antigenic peptides by ERAAP in the endoplasmic reticulum is essential for the generation of the normal repertoire of processed peptides.

Leucine-tRNA initiates at CUG start codons for protein synthesis and presentation by MHC class I

Effective immune surveillance by cytotoxic T cells requires newly synthesized polypeptides for presentation by major histocompatibility complex (MHC) class I molecules. These polypeptides are produced not only from conventional AUG-initiated, but also from cryptic non-AUG-initiated, reading frames by distinct translational mechanisms. Biochemical analysis of ribosomal initiation complexes at CUG versus AUG initiation codons revealed that cells use an elongator leucine-bound transfer RNA (Leu-tRNA) to initiate translation at cryptic CUG start codons. CUG/Leu-tRNA initiation was independent of the canonical initiator tRNA (AUG/Met-tRNA(i)(Met)) pathway but required expression of eukaryotic initiation factor 2A. Thus, a tRNA-based translation initiation mechanism allows non-AUG-initiated protein synthesis and supplies peptides for presentation by MHC class I molecules.

In the absence of aminopeptidase ERAAP, MHC class I molecules present many unstable and highly immunogenic peptides

Immunosurveillance by cytotoxic T cells requires that cells generate a diverse spectrum of peptides for presentation by major histocompatibility complex (MHC) class I molecules. Those peptides are generated by proteolysis, which begins in the cytoplasm and continues in the endoplasmic reticulum by the unique aminopeptidase ERAAP. The overall extent to which trimming by ERAAP modifies the peptide pool and the immunological consequences of ERAAP deficiency are unknown. Here we show that the peptide-MHC repertoire of ERAAP-deficient mice was missing many peptides. Furthermore, ERAAP-deficient cells presented many unstable and structurally unique peptide-MHC complexes, which elicited potent CD8+ T cell and B cell responses. Thus, ERAAP is a 'quintessential editor' of the peptide-MHC repertoire and, paradoxically, its absence enhances immunogenicity.

In vivo targeting of dendritic cells for activation of cellular immunity using vaccine carriers based on pH-responsive microparticles

Activating the immune system to trigger a specific response is a major challenge in vaccine development. In particular, activating sufficient cytotoxic T lymphocyte-mediated cellular immunity, which is crucial for the treatment of many diseases including cancer and AIDS, has proven to be especially challenging. In this study, antigens were encapsulated in acid-degradable polymeric particle carriers to cascade cytotoxic T lymphocyte activation. To target dendritic cells, the most potent antigen-presenting cells, the particle carriers, were further conjugated with monoclonal antibodies. A series of ex vivo and in vivo studies have shown increased receptor-mediated uptake of antibody-conjugated particles by dendritic cells as well as migration of particle-carrying dendritic cells to lymph nodes and stimulation of naïve T cells leading to enhanced cellular immune response as confirmed by specific cell lysis and IFN-gamma secretion.

Measurement of ligand-induced activation in single viable T cells using the lacZ reporter gene

We have used the bacterial beta-galactosidase gene (lacZ) as a reporter gene for the rapid measurement of T-cell antigen receptor (TCR)-mediated activation of individual T cells. The reporter construct contained the lacZ gene under the control of the nuclear factor of activated T cells (NF-AT) element of the human interleukin 2 enhancer [Fiering, S., Northrop, J. P., Nolan, G. P., Matilla, P., Crabtree, G. R. & Herzenberg, L. A. (1990) Genes Dev. 4, 1823-1834]. The activity of the intracellular lacZ enzyme was analyzed by flow cytometric measurement of fluorescein accumulation in cells loaded with the fluorogenic beta-galactosidase substrate fluorescein di-beta-D-galactopyranoside. As a model system, the T-cell hybridoma BO4H9.1, which is specific for the lysozyme peptide (amino acids 74-88)/Ab complex, was transfected with the NF-AT-lacZ construct. lacZ activity was induced in 50-100% of the transfectant cells following exposure to pharmacological agents, to the physiological peptide/major histocompatibility complex ligand, or to other TCR-specific stimuli. Interestingly, increasing concentrations of the stimulus increased the fraction of lacZ+ cells, but not the level of lacZ activity per cell. Even under widely varying levels of stimulus, the level of lacZ activity in individual lacZ+ cells remained within a remarkably narrow range. These results demonstrate that TCR-mediated activation can be readily measured in single T cells and strongly suggest that, once committed to activation, the level of NF-AT transcriptional activity in individual T cells is independent of the form or concentration of stimulus. This assay is likely to prove useful for the study of early activation events in individual T cells and of TCR ligands.

ER aminopeptidases generate a unique pool of peptides for MHC class I molecules

We define here the specificity and significance of proteases in the endoplasmic reticulum (ER) that generate peptides for presentation by major histocompatibility complex (MHC) class I molecules. We show that aminopeptidases efficiently trimmed all residues except proline that flank the NH2-termini of antigenic precursors in the ER and caused an accumulation of X-P-Xn peptides. An aminopeptidase inhibitor blocked peptide trimming in the ER and, consequently, the generation of peptide-loaded MHC molecules. Peptide trimming in the ER is therefore a key step in the MHC class I antigen-processing pathway and also explains the paradox of why many MHC class I molecules display peptides with the X-P-Xn motif despite the inability of the transporter associated with antigen processing to transport such peptides from the cytoplasm.

Brain microvessel cross-presentation is a hallmark of experimental cerebral malaria

Cerebral malaria is a devastating complication of Plasmodium falciparum infection. Its pathogenesis is complex, involving both parasite- and immune-mediated events. CD8⁺ T cells play an effector role in murine experimental cerebral malaria (ECM) induced by Plasmodium berghei ANKA (PbA) infection. We have identified a highly immunogenic CD8 epitope in glideosome-associated protein 50 that is conserved across rodent malaria species. Epitope-specific CD8⁺ T cells are induced during PbA infection, migrating to the brain just before neurological signs manifest. They are functional, cytotoxic and can damage the blood–brain barrier in vivo. Such CD8⁺ T cells are also found in the brain during infection with parasite strains/species that do not induce neuropathology. We demonstrate here that PbA infection causes brain microvessels to cross-present parasite antigen, while non-ECM-causing parasites do not. Further, treatment with fast-acting anti-malarial drugs before the onset of ECM reduces parasite load and thus antigen presentation in the brain, preventing ECM death. Thus our data suggest that combined therapies targeting both the parasite and host antigen-presenting cells may improve the outcome of CM patients.

Tyrosyl phosphorylation and activation of MAP kinases by p56lck

T cell signaling via the CD4 surface antigen is mediated by the associated tyrosyl protein kinase p56lck. The 42-kilodalton mitogen-activated protein (MAP) kinase (p42mapk) was tyrosyl-phosphorylated and activated after treatment of the murine T lymphoma cell line 171CD4+, which expresses CD4, with antibody to CD3. Treatment of the CD4-deficient cell line 171 with the same antibody did not result in phosphorylation or activation of p42mapk. Purified p56lck both tyrosyl-phosphorylated and stimulated the seryl-threonyl phosphotransferase activity of purified p44mpk, a MAP kinase isoform from sea star oocytes. A synthetic peptide modeled after the putative regulatory phosphorylation site in murine p42mapk (Tyr185) was phosphorylated by p56lck with a similar Vmax, but a fivefold lower Michaelis constant (Km) than a peptide containing the Tyr394 autophosphorylation site from p56lck. MAP kinases may participate in protein kinase cascades that link Src family protein-tyrosyl kinases to seryl-threonyl kinases such as those encoded by rsk and raf, which are putative substrates of MAP kinases.

Discrete proteolytic intermediates in the MHC class I antigen processing pathway and MHC I-dependent peptide trimming in the ER

The antigen processing pathway generates the peptides displayed by MHC I molecules on the cell surface. Whether these peptides are generated in the cytosol or from longer intermediates transported into the ER is unclear, because peptides other than those bound to MHC I have been difficult to find. Using a novel assay, we show that N-terminally extended antigenic analogs were associated with high-molecular weight material in the cytosol and were transported by TAP. In the ER, a nonapeptide was predominant that was converted to the final octapeptide only in presence of the appropriate MHC I molecule. The existence of extended peptides and their MHC I-dependent trimming suggest a mechanism for efficiently satisfying the distinct sequence preferences of polymorphic MHC I molecules.

Rearranged beta T cell receptor genes in a helper T cell clone specific for lysozyme: no correlation between V beta and MHC restriction

The helper T cell clone 3H.25 is specific for hen egg white lysozyme and the class II MHC molecule I-Ab. This TH cell has three rearrangements in the beta-chain gene family-a V beta-D beta-J beta 1 and a D beta 2-J beta 2 rearrangement on one homolog and a D beta 1-J beta 2 rearrangement on the other. These observations demonstrate that this functional T lymphocyte expresses only a single V beta gene segment and, accordingly, exhibits allelic exclusion of beta-chain gene expression. The rearranged 3H.25 V beta gene segment is the same as that expressed in a T helper cell specific for cytochrome c and an I-Ek MHC molecule. Thus, there is no simple correlation between the V beta gene segment and antigen specificity or MHC restriction.

ERAAP synergizes with MHC class I molecules to make the final cut in the antigenic peptide precursors in the endoplasmic reticulum

The major histocompatibility complex class I molecules display peptides (pMHC I) on the cell surface for immune surveillance by CD8(+) T cells. These peptides are generated by proteolysis of intracellular polypeptides by the proteasome in the cytoplasm and then in the endoplasmic reticulum (ER) by the ER aminopeptidase associated with antigen processing (ERAAP). To define the unknown mechanism of ERAAP function in vivo, we analyzed naturally processed peptides in cells with or without appropriate MHC I and ERAAP. In the absence of MHC I, ERAAP degraded the antigenic precursors in the ER. However, MHC I molecules could bind proteolytic intermediates and were essential for generation of the final peptide by ERAAP. Thus, ERAAP synergizes with MHC I to generate the final pMHC I repertoire.