A case of too much sugar: Lung DCs flummoxed by flu

Diabetes is known to increase susceptibility to respiratory infections, but the underlying basis remains elusive. In a recent study in Nature, Nobs et al. showed that hyperglycemia impinges on the histone acetylation landscape to impair the ability of lung dendritic cells to prime adaptive immunity.

THEMIS is a substrate and allosteric activator of SHP1, playing dual roles during T cell development

THEMIS plays an indispensable role in T cells, but its mechanism of action has remained highly controversial. Using the systematic proximity labeling methodology PEPSI, we identify THEMIS as an uncharacterized substrate for the phosphatase SHP1. Saturated mutagenesis assays and mass spectrometry analysis reveal that phosphorylation of THEMIS at the evolutionally conserved Tyr34 residue is oppositely regulated by SHP1 and the kinase LCK. Similar to THEMIS-/- mice, THEMISY34F/Y34F knock-in mice show a significant decrease in CD4 thymocytes and mature CD4 T cells, but display normal thymic development and peripheral homeostasis of CD8 T cells. Mechanistically, the Tyr34 motif in THEMIS, when phosphorylated upon T cell antigen receptor activation, appears to act as an allosteric regulator, binding and stabilizing SHP1 in its active conformation, thus ensuring appropriate negative regulation of T cell antigen receptor signaling. However, cytokine signaling in CD8 T cells fails to elicit THEMIS Tyr34 phosphorylation, indicating both Tyr34 phosphorylation-dependent and phosphorylation-independent roles of THEMIS in controlling T cell maturation and expansion.

Allosterically inhibited PFKL via prostaglandin E2 withholds glucose metabolism and ovarian cancer invasiveness

Metastasis is the leading cause of high ovarian-cancer-related mortality worldwide. Three major processes constitute the whole metastatic cascade: invasion, intravasation, and extravasation. Tumor cells often reprogram their metabolism to gain advantages in proliferation and survival. However, whether and how those metabolic alterations contribute to the invasiveness of tumor cells has yet to be fully understood. Here we performed a genome-wide CRISPR-Cas9 screening to identify genes participating in tumor cell dissemination and revealed that PTGES3 acts as an invasion suppressor in ovarian cancer. Mechanistically, PTGES3 binds to phosphofructokinase, liver type (PFKL) and generates a local source of prostaglandin E2 (PGE2) to allosterically inhibit the enzymatic activity of PFKL. Repressed PFKL leads to downgraded glycolysis and the subsequent TCA cycle for glucose metabolism. However, ovarian cancer suppresses the expression of PTGES3 and disrupts the PTGES3-PGE2-PFKL inhibitory axis, leading to hyperactivation of glucose oxidation, eventually facilitating ovarian cancer cell motility and invasiveness.

Untargeted metabolomics identifies succinate as a biomarker and therapeutic target in aortic aneurysm and dissection

AIMS

Aortic aneurysm and dissection (AAD) are high-risk cardiovascular diseases with no effective cure. Macrophages play an important role in the development of AAD. As succinate triggers inflammatory changes in macrophages, we investigated the significance of succinate in the pathogenesis of AAD and its clinical relevance.

METHODS AND RESULTS

We used untargeted metabolomics and mass spectrometry to determine plasma succinate concentrations in 40 and 1665 individuals of the discovery and validation cohorts, respectively. Three different murine AAD models were used to determine the role of succinate in AAD development. We further examined the role of oxoglutarate dehydrogenase (OGDH) and its transcription factor cyclic adenosine monophosphate-responsive element-binding protein 1 (CREB) in the context of macrophage-mediated inflammation and established p38αMKOApoe-/- mice. Succinate was the most upregulated metabolite in the discovery cohort; this was confirmed in the validation cohort. Plasma succinate concentrations were higher in patients with AAD compared with those in healthy controls, patients with acute myocardial infarction (AMI), and patients with pulmonary embolism (PE). Moreover, succinate administration aggravated angiotensin II-induced AAD and vascular inflammation in mice. In contrast, knockdown of OGDH reduced the expression of inflammatory factors in macrophages. The conditional deletion of p38α decreased CREB phosphorylation, OGDH expression, and succinate concentrations. Conditional deletion of p38α in macrophages reduced angiotensin II-induced AAD.

CONCLUSION

Plasma succinate concentrations allow to distinguish patients with AAD from both healthy controls and patients with AMI or PE. Succinate concentrations are regulated by the p38α-CREB-OGDH axis in macrophages.

Dynamic changes in macrophage metabolism modulate induction and suppression of Type I inflammatory responses

During microbial infection, macrophages link recognition of microbial stimuli to the induction of Type I inflammatory responses. Such inflammatory responses coordinate host defense and pathogen elimination but induce significant tissue damage if sustained, so macrophages are initially activated to induce inflammatory responses but then shift to a tolerant state to suppress inflammatory responses. Macrophage tolerance is regulated by induction of negative regulators of TLR signaling, but its metabolic basis was not known. Here, we review recent studies that indicate that macrophage metabolism changes dynamically over the course of microbial exposure to influence a shift in the inflammatory response. In particular, an initial increase in oxidative metabolism boosts the induction of inflammatory responses, but is followed by a shutdown of oxidative metabolism that contributes to suppression of inflammatory responses. We propose a unifying model for how dynamic changes to oxidative metabolism influences regulation of macrophage inflammatory responses during microbial exposure.

Mitochondrial metabolism regulates macrophage biology

Mitochondria are critical for regulation of the activation, differentiation, and survival of macrophages and other immune cells. In response to various extracellular signals, such as microbial or viral infection, changes to mitochondrial metabolism and physiology could underlie the corresponding state of macrophage activation. These changes include alterations of oxidative metabolism, mitochondrial membrane potential, and tricarboxylic acid (TCA) cycling, as well as the release of mitochondrial reactive oxygen species (mtROS) and mitochondrial DNA (mtDNA) and transformation of the mitochondrial ultrastructure. Here, we provide an updated review of how changes in mitochondrial metabolism and various metabolites such as fumarate, succinate, and itaconate coordinate to guide macrophage activation to distinct cellular states, thus clarifying the vital link between mitochondria metabolism and immunity. We also discuss how in disease settings, mitochondrial dysfunction and oxidative stress contribute to dysregulation of the inflammatory response. Therefore, mitochondria are a vital source of dynamic signals that regulate macrophage biology to fine-tune immune responses.

mTOR Is Key to T Cell Transdifferentiation

T cell transdifferentiation to functionally distinct subsets can play a key role in balancing the protective and pathogenic features of the T cell response. In a new study, Karmaus et al. (2019) showed that mTORC1 activity influences metabolic heterogeneity within a T cell population to modulate transdifferentiation and disease pathogenesis in a setting of chronic inflammation-driven autoimmunity.

Metabolism as a guiding force for immunity

Recent studies indicate that cellular metabolism plays a key role in supporting immune cell maintenance and development. Here, we review how metabolism guides immune cell activation and differentiation to distinct cellular states, and how differential regulation of metabolism allows for context-dependent support during activation and lineage commitment. We discuss emerging principles of metabolic support of immune cell function in physiology and disease, as well as their general relevance to the field of cell biology.

STING Senses Microbial Viability to Orchestrate Stress-Mediated Autophagy of the Endoplasmic Reticulum

Constitutive cell-autonomous immunity in metazoans predates interferon-inducible immunity and comprises primordial innate defense. Phagocytes mobilize interferon-inducible responses upon engagement of well-characterized signaling pathways by pathogen-associated molecular patterns (PAMPs). The signals controlling deployment of constitutive cell-autonomous responses during infection have remained elusive. Vita-PAMPs denote microbial viability, signaling the danger of cellular exploitation by intracellular pathogens. We show that cyclic-di-adenosine monophosphate in live Gram-positive bacteria is a vita-PAMP, engaging the innate sensor stimulator of interferon genes (STING) to mediate endoplasmic reticulum (ER) stress. Subsequent inactivation of the mechanistic target of rapamycin mobilizes autophagy, which sequesters stressed ER membranes, resolves ER stress, and curtails phagocyte death. This vita-PAMP-induced ER-phagy additionally orchestrates an interferon response by localizing ER-resident STING to autophagosomes. Our findings identify stress-mediated ER-phagy as a cell-autonomous response mobilized by STING-dependent sensing of a specific vita-PAMP and elucidate how innate receptors engage multilayered homeostatic mechanisms to promote immunity and survival after infection.

Infiltration of M1, but not M2, macrophages is impaired after unilateral ureter obstruction in Nrf2-deficient mice

Chronic inflammation can be a major driver of the failure of a variety of organs, including chronic kidney disease (CKD). The NLR family pyrin domain-containing 3 (NLRP3) inflammasome has been shown to play a pivotal role in inflammation in a mouse kidney disease model. Nuclear factor erythroid 2-related factor 2 (Nrf2), the master transcription factor for anti-oxidant responses, has also been implicated in inflammasome activation under physiological conditions. However, the mechanism underlying inflammasome activation in CKD remains elusive. Here, we show that the loss of Nrf2 suppresses fibrosis and inflammation in a unilateral ureter obstruction (UUO) model of CKD in mice. We consistently observed decreased expression of inflammation-related genes NLRP3 and IL-1β in Nrf2-deficient kidneys after UUO. Increased infiltration of M1, but not M2, macrophages appears to mediate the suppression of UUO-induced CKD symptoms. Furthermore, we found that activation of the NLRP3 inflammasome is attenuated in Nrf2-deficient bone marrow–derived macrophages. These results demonstrate that Nrf2-related inflammasome activation can promote CKD symptoms via infiltration of M1 macrophages. Thus, we have identified the Nrf2 pathway as a promising therapeutic target for CKD.

Metabolism Supports Macrophage Activation

Macrophages are found in most tissues of the body, where they have tissue- and context-dependent roles in maintaining homeostasis as well as coordinating adaptive responses to various stresses. Their capacity for specialized functions is controlled by polarizing signals, which activate macrophages by upregulating transcriptional programs that encode distinct effector functions. An important conceptual advance in the field of macrophage biology, emerging from recent studies, is that macrophage activation is critically supported by metabolic shifts. Metabolic shifts fuel multiple aspects of macrophage activation, and preventing these shifts impairs appropriate activation. These findings raise the exciting possibility that macrophage functions in various contexts could be regulated by manipulating their metabolism. Here, we review the rapidly evolving field of macrophage metabolism, discussing how polarizing signals trigger metabolic shifts and how these shifts enable appropriate activation and sustain effector activities. We also discuss recent studies indicating that the mitochondria are central hubs in inflammatory macrophage activation.

Akt-mTORC1 signaling regulates Acly to integrate metabolic input to control of macrophage activation

Macrophage activation/polarization to distinct functional states is critically supported by metabolic shifts. How polarizing signals coordinate metabolic and functional reprogramming, and the potential implications for control of macrophage activation, remains poorly understood. Here we show that IL-4 signaling co-opts the Akt-mTORC1 pathway to regulate Acly, a key enzyme in Ac-CoA synthesis, leading to increased histone acetylation and M2 gene induction. Only a subset of M2 genes is controlled in this way, including those regulating cellular proliferation and chemokine production. Moreover, metabolic signals impinge on the Akt-mTORC1 axis for such control of M2 activation. We propose that Akt-mTORC1 signaling calibrates metabolic state to energetically demanding aspects of M2 activation, which may define a new role for metabolism in supporting macrophage activation.

Control of macrophage metabolism and activation by mTOR and Akt signaling

Macrophages are pleiotropic cells that assume a variety of functions depending on their tissue of residence and tissue state. They maintain homeostasis as well as coordinate responses to stresses such as infection and metabolic challenge. The ability of macrophages to acquire diverse, context-dependent activities requires their activation (or polarization) to distinct functional states. While macrophage activation is well understood at the level of signal transduction and transcriptional regulation, the metabolic underpinnings are poorly understood. Importantly, emerging studies indicate that metabolic shifts play a pivotal role in control of macrophage activation and acquisition of context-dependent effector activities. The signals that drive macrophage activation impinge on metabolic pathways, allowing for coordinate control of macrophage activation and metabolism. Here we discuss how mTOR and Akt, major metabolic regulators and targets of such activation signals, control macrophage metabolism and activation. Dysregulated macrophage activities contribute to many diseases, including infectious, inflammatory, and metabolic diseases and cancer, thus a better understanding of metabolic control of macrophage activation could pave the way to the development of new therapeutic strategies.

mTOR trains heightened macrophage responses

The ability of a primary challenge to protect against secondary infection (e.g., during vaccination) independent of the adaptive immune system is mediated in part by macrophage 'training'. Two new studies show that macrophage training is associated with genome-wide epigenetic changes and is regulated by the mTOR pathway and metabolic reprogramming.

The TSC-mTOR pathway regulates macrophage polarization

Macrophages are able to polarize to proinflammatory M1 or alternative M2 states with distinct phenotypes and physiological functions. How metabolic status regulates macrophage polarization remains not well understood, and here we examine the role of mTOR (Mechanistic Target of Rapamycin), a central metabolic pathway that couples nutrient sensing to regulation of metabolic processes. Using a mouse model in which myeloid lineage specific deletion of Tsc1 (Tsc1^Δ/Δ) leads to constitutive mTOR Complex 1 (mTORC1) activation, we find that Tsc1^Δ/Δ macrophages are refractory to IL-4 induced M2 polarization, but produce increased inflammatory responses to proinflammatory stimuli. Moreover, mTORC1-mediated downregulation of Akt signaling critically contributes to defective polarization. These findings highlight a key role for the mTOR pathway in regulating macrophage polarization, and suggest how nutrient sensing and metabolic status could be “hard-wired” to control of macrophage function, with broad implications for regulation of Type 2 immunity, inflammation, and allergy.

IL-6 strikes a balance in metabolic inflammation

Interleukin-6 (IL-6) is a pleiotropic cytokine that exerts either proinflammatory or anti-inflammatory effects and is implicated in diverse settings, including obesity, exercise, arthritis, and colitis. A new study shows that modulation of macrophage activation by IL-6 maintains glucose homeostasis in diet-induced obesity while limiting inflammation in endotoxemia (Mauer et al., 2014).

Activation of AIM2 inflammasome triggers mitochondrial damage and blocks mitophagy (INM6P.409)

The AIM2 inflammasome is critical for host defense against certain intracellular bacteria and DNA viruses. It is a cytoplasmic complex consisting of the regulatory subunit AIM2, the adaptor ASC, and the effector subunit caspase-1. Stimulus dependent assembly of the complex activates the proteolytic activity of caspase-1, which is required for the processing and production of inflammatory cytokines IL-1β and IL-18. Here we demonstrate that mitochondrial damage is a consequence of activation of the AIM2 inflammasome. We address the underlying mechanisms, thus providing insight into regulation of a key inflammasome effector activity.

Calcium signaling and mitochondrial destabilization in the triggering of the NLRP3 inflammasome

The NLRP3 inflammasome is a cytosolic complex that activates Caspase-1, leading to maturation of interleukin-1β (IL-1β) and IL-18 and induction of proinflammatory cell death in sentinel cells of the innate immune system. Diverse stimuli have been shown to activate the NLRP3 inflammasome during infection and metabolic diseases, implicating the pathway in triggering both adaptive and maladaptive inflammation in various clinically important settings. Here I discuss the emerging model that signals associated with mitochondrial destabilization may critically activate the NLRP3 inflammasome. Together with studies indicating an important role for Ca2+ signaling, these findings suggest that many stimuli engage Ca2+ signaling as an intermediate step to trigger mitochondrial destabilization, generating the mitochondrion-associated ligands that activate the NLRP3 inflammasome.

Control of inducible gene expression by signal-dependent transcriptional elongation

Most inducible transcriptional programs consist of primary and secondary response genes (PRGs and SRGs) that differ in their kinetics of expression and in their requirements for new protein synthesis and chromatin remodeling. Here we show that many PRGs, in contrast to SRGs, have preassembled RNA polymerase II (Pol II) and positive histone modifications at their promoters in the basal state. Pol II at PRGs generates low levels of full-length unspliced transcripts but fails to make mature, protein-coding transcripts in the absence of stimulation. Induction of PRGs is controlled at the level of transcriptional elongation and mRNA processing, through the signal-dependent recruitment of P-TEFb. P-TEFb is in turn recruited by the bromodomain-containing protein Brd4, which detects H4K5/8/12Ac inducibly acquired at PRG promoters. Our findings suggest that the permissive structure of PRGs both stipulates their unique regulation in the basal state by corepressor complexes and enables their rapid induction in multiple cell types.

Gene regulation and signal transduction in the immune system

A report of the meeting 'Gene Expression and Signaling in the Immune System', Cold Spring Harbor, USA, 22-26 April 2008.

A report of the meeting 'Gene Expression and Signaling in the Immune System', Cold Spring Harbor, USA, 22-26 April 2008.

TRAM couples endocytosis of Toll-like receptor 4 to the induction of interferon-beta

Toll-like receptor 4 (TLR4) induces two distinct signaling pathways controlled by the TIRAP-MyD88 and TRAM-TRIF pairs of adaptor proteins, which elicit the production of proinflammatory cytokines and type I interferons, respectively. How TLR4 coordinates the activation of these two pathways is unknown. Here we show that TLR4 activated these two signaling pathways sequentially in a process organized around endocytosis of the TLR4 complex. We propose that TLR4 first induces TIRAP-MyD88 signaling at the plasma membrane and is then endocytosed and activates TRAM-TRIF signaling from early endosomes. Our data emphasize a unifying theme in innate immune recognition whereby all type I interferon-inducing receptors signal from an intracellular location.

NKG2D signaling is coupled to the interleukin 15 receptor signaling pathway

The effector functions of natural killer cells are regulated by activating receptors, which recognize stress-inducible ligands expressed on target cells and signal through association with signaling adaptors. Here we developed a mouse model in which a fusion of the signaling adaptor DAP10 and ubiquitin efficiently downregulated expression of the activating receptor NKG2D on the surfaces of natural killer cells. With this system, we identified coupling of the signaling pathways triggered by NKG2D and DAP10 to those initiated by the interleukin 15 receptor. We suggest that this coupling of activating receptors to other receptor systems could function more generally to regulate cell type-specific signaling events in distinct physiological contexts.

Unanticipated antigens: translation initiation at CUG with leucine

Major histocompatibility class I molecules display tens of thousands of peptides on the cell surface for immune surveillance by T cells. The peptide repertoire represents virtually all cellular translation products, and can thus reveal a foreign presence inside the cell. These peptides are derived from not only conventional but also cryptic translational reading frames, including some without conventional AUG codons. To define the mechanism that generates these cryptic peptides, we used T cells as probes to analyze the peptides generated in transfected cells. We found that when CUG acts as an alternate initiation codon, it can be decoded as leucine rather than the expected methionine residue. The leucine start does not depend on an internal ribosome entry site–like mRNA structure, and its efficiency is enhanced by the Kozak nucleotide context. Furthermore, ribosomes scan 5′ to 3′ specifically for the CUG initiation codon in a eukaryotic translation initiation factor 2–independent manner. Because eukaryotic translation initiation factor 2 is frequently targeted to inhibit protein synthesis, this novel translation mechanism allows stressed cells to display antigenic peptides. This initiation mechanism could also be used at non-AUG initiation codons often found in viral transcripts as well as in a growing list of cellular genes.

Proteins have been identified for which a unique translational machinery makes use of unconventional start codons

The adaptor molecule TIRAP provides signalling specificity for Toll-like receptors

Mammalian Toll-like receptors (TLRs) function as sensors of infection and induce the activation of innate and adaptive immune responses. Upon recognizing conserved pathogen-associated molecular products, TLRs activate host defence responses through their intracellular signalling domain, the Toll/interleukin-1 receptor (TIR) domain, and the downstream adaptor protein MyD88 (refs 1-3). Although members of the TLR and the interleukin-1 (IL-1) receptor families all signal through MyD88, the signalling pathways induced by individual receptors differ. TIRAP, an adaptor protein in the TLR signalling pathway, has been identified and shown to function downstream of TLR4 (refs 4, 5). Here we report the generation of mice deficient in the Tirap gene. TIRAP-deficient mice respond normally to the TLR5, TLR7 and TLR9 ligands, as well as to IL-1 and IL-18, but have defects in cytokine production and in activation of the nuclear factor NF-kappaB and mitogen-activated protein kinases in response to lipopolysaccharide, a ligand for TLR4. In addition, TIRAP-deficient mice are also impaired in their responses to ligands for TLR2, TLR1 and TLR6. Thus, TIRAP is differentially involved in signalling by members of the TLR family and may account for specificity in the downstream signalling of individual TLRs.

Priming of memory but not effector CD8 T cells by a killed bacterial vaccine

Killed or inactivated vaccines targeting intracellular bacterial and protozoal pathogens are notoriously ineffective at generating protective immunity. For example, vaccination with heat-killed Listeria monocytogenes (HKLM) is not protective, although infection with live L. monocytogenes induces long-lived, CD8 T cell-mediated immunity. We demonstrate that HKLM immunization primes memory CD8 T lymphocyte populations that, although substantial in size, are ineffective at providing protection from subsequent L. monocytogenes infection. In contrast to live infection, which elicits large numbers of effector CD8 T cells, HKLM immunization primes T lymphocytes that do not acquire effector functions. Our studies show that it is possible to dissociate T cell-dependent protective immunity from memory T cell expansion, and that generation of effector T cells may be necessary for long-term protective immunity.

Drosophila MyD88 is an adapter in the Toll signaling pathway

Toll-like receptors comprise a family of cell surface receptors that play a crucial role in the innate immune recognition of both Drosophila and mammals. Previous studies have shown that Drosophila Toll-1 mediates the induction of antifungal peptides during fungal infection of adult flies. Through genetic studies, Tube, Pelle, Cactus, and Dif have been identified as downstream components of the Toll-1 signaling pathway. Here we report characterization of a Drosophila homologue of human MyD88, dMyD88. We show that dMyD88 is an adapter in the Toll signaling pathway that associates with both the Toll receptor and the downstream kinase Pelle. Expression of dMyD88 in S2 cells strongly induced activity of a Drosomycin reporter gene, whereas a dominant-negative version of dMyD88 potently inhibited Toll-mediated signaling. We also show that dMyD88 associates with the death domain-containing adapter Drosophila Fas-associated death domain-containing protein (dFADD), which in turn interacts with the apical caspase Dredd. This pathway links a cell surface receptor to an apical caspase in invertebrate cells and therefore suggests that the Toll-mediated pathway of caspase activation may be the evolutionary ancestor of the death receptor-mediated pathway for apoptosis induction in mammals.

TIRAP: an adapter molecule in the Toll signaling pathway

Mammalian Toll-like receptors (TLRs) recognize conserved products of microbial metabolism and activate NF-kappa B and other signaling pathways through the adapter protein MyD88. Although some cellular responses are completely abolished in MyD88-deficient mice, TLR4, but not TLR9, can activate NF-kappa B and mitogen-activated protein kinases and induce dendritic cell maturation in the absence of MyD88. These differences suggest that another adapter must exist that can mediate MyD88-independent signaling in response to TLR4 ligation. We have identified and characterized a Toll-interleukin 1 receptor (TIR) domain-containing adapter protein (TIRAP) and have shown that it controls activation of MyD88-independent signaling pathways downstream of TLR4. We have also shown that the double-stranded RNA-binding protein kinase PKR is a component of both the TIRAP- and MyD88-dependent signaling pathways.

Structural basis for signal transduction by the Toll/interleukin-1 receptor domains

Toll-like receptors (TLRs) and the interleukin-1 receptor superfamily (IL-1Rs) are integral to both innate and adaptive immunity for host defence. These receptors share a conserved cytoplasmic domain, known as the TIR domain. A single-point mutation in the TIR domain of murine TLR4 (Pro712His, the Lps(d) mutation) abolishes the host immune response to lipopolysaccharide (LPS), and mutation of the equivalent residue in TLR2, Pro681His, disrupts signal transduction in response to stimulation by yeast and gram-positive bacteria. Here we report the crystal structures of the TIR domains of human TLR1 and TLR2 and of the Pro681His mutant of TLR2. The structures have a large conserved surface patch that also contains the site of the Lps(d) mutation. Mutagenesis and functional studies confirm that residues in this surface patch are crucial for receptor signalling. The Lps(d) mutation does not disturb the structure of the TIR domain itself. Instead, structural and functional studies indicate that the conserved surface patch may mediate interactions with the down-stream MyD88 adapter molecule, and that the Lps(d) mutation may abolish receptor signalling by disrupting this recruitment.

Differences that matter: major cytotoxic T cell-stimulating minor histocompatibility antigens

Despite thousands of genetic polymorphisms among MHC matched mouse strains, a few unknown histocompatibility antigens are targeted by the cytotoxic T cells specific for tissue grafts. We isolated the cDNA of a novel BALB.B antigen gene that defines the polymorphic H28 locus on chromosome 3 and yields the naturally processed ILENFPRL (IFL8) peptide for presentation by Kb MHC to C57BI/6 CTL. The CTL specific for the IFL8/Kb and our previously identified H60/Kb complexes represent a major fraction of the B6 anti-BALB.B immune response. The immunodominance of these antigens can be explained by their differential transcription in the donor versus the host strains and their expression in professional donor antigen-presenting cells.

Presentation of out-of-frame peptide/MHC class I complexes by a novel translation initiation mechanism

Immune surveillance by CD8 T cells requires that peptides derived from intracellular proteins be presented by MHC class I molecules on the target cell surface. Interestingly, MHC molecules can also present peptides encoded in alternate translational reading frames, some even without conventional AUG initiation codons. Using T cells to measure expression of MHC bound peptides, we identified the non-AUG translation initiation codons and established that their activity was at the level of translational rather than DNA replication or transcription mechanisms. This translation mechanism decoded the CUG initiation codon not as the canonical methionine but as the leucine residue, and its activity was independent of upstream translation initiation events. Naturally processed peptide/MHC complexes can thus arise from "noncoding" mRNAs via a novel translation initiation mechanism.

The Molecular and Functional Characterization of a Dominant Minor H Antigen, H60

Minor histocompatibility (H) Ags elicit T cell responses and thereby cause chronic graft rejection and graft-vs-host disease among MHC identical individuals. Although numerous independent H loci exist in mice of a given MHC haplotype, certain H Ags dominate the immune response and are thus of considerable conceptual and therapeutic importance. To identify these H Ags and their genes, lacZ-inducible CD8+ T cell hybrids were generated by immunizing C57BL/6 (B6) mice with MHC identical BALB.B spleen cells. The cDNA clones encoding the precursor for the antigenic peptide/Kb MHC class I complex were isolated by expression cloning using the BCZ39.84 T cell as a probe. The cDNAs defined a new H locus (termed H60), located on mouse chromosome 10, and encoded a novel protein that contains the naturally processed octapeptide LTFNYRNL (LYL8) presented by the Kb MHC molecule. Southern blot analysis revealed that the H60 locus was polymorphic among the BALB and the B6 strains. However, none of the H60 transcripts expressed in the donor BALB spleen were detected in the host B6 strain. The expression and immunogenicity of the LYL8/Kb complex in BALB.B and CXB recombinant inbred strains strongly suggested that the H60 locus may account for one of the previously described antigenic activity among these strains. The results establish the source of an immunodominant autosomal minor H Ag that, by its differential transcription in the donor vs the host strains, provides a novel peptide/MHC target for host CD8+ T cells.

The molecular and functional characterization of a dominant minor H antigen, H60

Minor histocompatibility (H) Ags elicit T cell responses and thereby cause chronic graft rejection and graft-vs-host disease among MHC identical individuals. Although numerous independent H loci exist in mice of a given MHC haplotype, certain H Ags dominate the immune response and are thus of considerable conceptual and therapeutic importance. To identify these H Ags and their genes, lacZ-inducible CD8+ T cell hybrids were generated by immunizing C57BL/6 (B6) mice with MHC identical BALB.B spleen cells. The cDNA clones encoding the precursor for the antigenic peptide/Kb MHC class I complex were isolated by expression cloning using the BCZ39.84 T cell as a probe. The cDNAs defined a new H locus (termed H60), located on mouse chromosome 10, and encoded a novel protein that contains the naturally processed octapeptide LTFNYRNL (LYL8) presented by the Kb MHC molecule. Southern blot analysis revealed that the H60 locus was polymorphic among the BALB and the B6 strains. However, none of the H60 transcripts expressed in the donor BALB spleen were detected in the host B6 strain. The expression and immunogenicity of the LYL8/Kb complex in BALB.B and CXB recombinant inbred strains strongly suggested that the H60 locus may account for one of the previously described antigenic activity among these strains. The results establish the source of an immunodominant autosomal minor H Ag that, by its differential transcription in the donor vs the host strains, provides a novel peptide/MHC target for host CD8+ T cells.