Major histocompatibility complex and other allergy-related candidate genes associated with insect bite hypersensitivity in Icelandic horses

Insect bite hypersensitivity (IBH) is an allergic dermatitis of horses caused by bites of insects. IBH is a multifactorial disease with contribution of genetic and environmental factors. Candidate gene association analysis of IBH was performed in a group of 89 Icelandic horses all born in Iceland and imported to Europe. Horses were classified in IBH-affected and non-affected based on clinical signs and history of recurrent dermatitis, and on the results of an in vitro sulfidoleukotriene (sLT)-release assay with Culicoides nubeculosus and Simulium vittatum extract. Different genetic markers were tested for association with IBH by the Fisher's exact test. The effect of the major histocompatibility complex (MHC) gene region was studied by genotyping five microsatellites spanning the MHC region (COR112, COR113, COR114, UM011 and UMN-JH34-2), and exon 2 polymorphisms of the class II Eqca-DRA gene. Associations with Eqca-DRA and COR113 were identified (p < 0.05). In addition, a panel of 20 single nucleotide polymorphisms (SNPs) in 17 candidate allergy-related genes was tested. During the initial screen, no marker from the panel was significantly (p < 0.05) associated with IBH. Five SNPs associated with IBH at p < 0.10 were therefore used for analysis of combined genotypes. Out of them, SNPs located in the genes coding for the CD14 receptor (CD14), interleukin 23 receptor (IL23R), thymic stromal lymphopoietin (TSLP) and transforming growth factor beta 3 (TGFB3) molecules were associated with IBH as parts of complex genotypes. These results are supported by similar associations and by expression data from different horse populations and from human studies.

Antibodies against major histocompatibility complex class II antigens directly inhibit the growth of T cells infected with Theileria parva without affecting their state of activation

We have analyzed the effect of antibodies (Abs) directed against major histocompatibility complex (MHC) class II Abs on the proliferation of Theileria parva-infected (Tpi) T cells. Anti-MHC class II Abs exert a direct effect on Tpi T cells causing an acute block in their proliferation. The inhibition does not involve apoptosis and is also entirely reversible. The rapid arrest of DNA synthesis caused by anti-MHC class II Abs is not due to interference with the state of activation of the T cells since the transcriptional activator NF-kappa B remains activated in arrested cells. In addition, interleukin 2 (IL-2), IL-2R, and c-myc gene expression are also unaffected. By analyzing the cell-cycle phase distribution of inhibited cells, it could be shown that cells in all phases of the cell cycle are inhibited. The signal transduction pathway that results in inhibition was shown to be independent of protein kinase C and extracellular Ca2+. Tyrosine kinase inhibitors, however, partly reduced the level of inhibition and, conversely, phosphatase inhibitors enhanced it. The possible relevance of this phenomenon in other systems is discussed.

Exploring the MHC-peptide matrix of central tolerance in the human thymus

Ever since it was discovered that central tolerance to self is imposed on developing T cells in the thymus through their interaction with self-peptide major histocompatibility complexes on thymic antigen-presenting cells, immunologists have speculated about the nature of these peptides, particularly in humans. Here, to shed light on the so-far unknown human thymic peptide repertoire, we analyse peptides eluted from isolated thymic dendritic cells, dendritic cell-depleted antigen-presenting cells and whole thymus. Bioinformatic analysis of the 842 identified natural major histocompatibility complex I and II ligands reveals significant cross-talk between major histocompatibility complex-class I and II pathways and differences in source protein representation between individuals as well as different antigen-presenting cells. Furthermore, several autoimmune- and tumour-related peptides, from enolase and vimentin for example, are presented in the healthy thymus. 302 peptides are directly derived from negatively selecting dendritic cells, thus providing the first global view of the peptide matrix in the human thymus that imposes self-tolerance in vivo.

Narcolepsy: autoimmunity, effector T cell activation due to infection, or T cell independent, major histocompatibility complex class II induced neuronal loss?

Human narcolepsy with cataplexy is a neurological disorder, which develops due to a deficiency in hypocretin producing neurons in the hypothalamus. There is a strong association with human leucocyte antigens HLA-DR2 and HLA-DQB1*0602. The disease typically starts in adolescence. Recent developments in narcolepsy research support the hypothesis of narcolepsy being an immune-mediated disease. Narcolepsy is associated with polymorphisms of the genes encoding T cell receptor alpha chain, tumour necrosis factor alpha and tumour necrosis factor receptor II. Moreover the rate of streptococcal infection is increased at onset of narcolepsy. The hallmarks of anti-self reactions in the tissue--namely upregulation of major histocompatibility antigens and lymphocyte infiltrates--are missing in the hypothalamus. These findings are questionable because they were obtained by analyses performed many years after onset of disease. In some patients with narcolepsy autoantibodies to Tribbles homolog 2, which is expressed by hypocretin neurons, have been detected recently. Immune-mediated destruction of hypocretin producing neurons may be mediated by microglia/macrophages that become activated either by autoantigen specific CD4(+) T cells or superantigen stimulated CD8(+) T cells, or independent of T cells by activation of DQB1*0602 signalling. Activation of microglia and macrophages may lead to the release of neurotoxic molecules such as quinolinic acid, which has been shown to cause selective destruction of hypocretin neurons in the hypothalamus.

Selection and recombination drive the evolution of MHC class II DRB diversity in ungulates

Major histocompatibility complex (MHC) antigen-presenting genes are the most variable loci in vertebrate genomes. Host-parasite co-evolution is assumed to maintain the excessive polymorphism in the MHC loci. However, the molecular mechanisms underlying the striking diversity in the MHC remain contentious. The extent to which recombination contributes to the diversity at MHC loci in natural populations is still controversial, and there have been only few comparative studies that make quantitative estimates of recombination rates. In this study, we performed a comparative analysis for 15 different ungulates species to estimate the population recombination rate, and to quantify levels of selection. As expected for all species, we observed signatures of strong positive selection, and identified individual residues experiencing selection that were congruent with those constituting the peptide-binding region of the human DRB gene. However, in addition for each species, we also observed recombination rates that were significantly different from zero on the basis of likelihood-permutation tests, and in other non-quantitative analyses. Patterns of synonymous and non-synonymous sequence diversity were consistent with differing demographic histories between species, but recent simulation studies by other authors suggest inference of selection and recombination is likely to be robust to such deviations from standard models. If high rates of recombination are common in MHC genes of other taxa, re-evaluation of many inference-based phylogenetic analyses of MHC loci, such as estimates of the divergence time of alleles and trans-specific polymorphism, may be required.

Pharmacological interaction of drugs with immune receptors: the p-i concept

Drug-induced hypersensitivity reactions have been explained by the hapten concept, according to which a small chemical compound is too small to be recognized by the immune system. Only after covalently binding to an endogenous protein the immune system reacts to this so called hapten-carrier complex, as the larger molecule (protein) is modified, and thus immunogenic for B and T cells. Consequently, a B and T cell immune response might develop to the drug with very heterogeneous clinical manifestations. In recent years, however, evidence has become stronger that not all drugs need to bind covalently to the MHC-peptide complex in order to trigger an immune response. Rather, some drugs may bind directly and reversibly to immune receptors like the major histocompatibility complex (MHC) or the T cell receptor (TCR), thereby stimulating the cells similar to a pharmacological activation of other receptors. This concept has been termed pharmacological interaction with immune receptors the (p-i) concept. While the exact mechanism is still a matter of debate, non-covalent drug presentation clearly leads to the activation of drug-specific T cells as documented for various drugs (lidocaine, sulfamethoxazole (SMX), lamotrigine, carbamazepine, p-phenylendiamine, etc.). In some patients with drug hypersensitivity, such a response may occur within hours even upon the first exposure to the drug. Thus, the reaction to the drug may not be due to a classical, primary response, but rather be mediated by stimulating existing, pre-activated, peptide-specific T cells that are cross specific for the drug. In this way, certain drugs may circumvent the checkpoints for immune activation imposed by the classical antigen processing and presentation mechanisms, which may help to explain the peculiar nature of many drug hypersensitivity reactions.

Noncovalent interactions of drugs with immune receptors may mediate drug-induced hypersensitivity reactions

Drug-induced hypersensitivity reactions are instructive examples of immune reactions against low molecular weight compounds. Classically, such reactions have been explained by the hapten concept, according to which the small antigen covalently modifies an endogenous protein; recent studies show strong associations of several HLA molecules with hypersensitivity. In recent years, however, evidence has become stronger that not all drugs need to bind covalently to the major histocompatibility complex (MHC)-peptide complex in order to trigger an immune response. Rather, some drugs may bind reversibly to the MHC or possibly to the T-cell receptor (TCR), eliciting immune reactions akin to the pharmacological activation of other receptors. While the exact mechanism is still a matter of debate, noncovalent drug presentation clearly leads to the activation of drug-specific T cells. In some patients with hypersensitivity, such a response may occur within hours of even the first exposure to the drug. Thus, the reaction to the drug may not be the result of a classical, primary response but rather be mediated by existing, preactivated T cells that display cross-reactivity for the drug and have additional (peptide) specificity as well. In this way, certain drugs may circumvent the checkpoints for immune activation imposed by the classical antigen processing and presentation mechanisms, which may help to explain the idiosyncratic nature of many drug hypersensitivity reactions.

Predicting and diagnosing abacavir and nevirapine drug hypersensitivity: from bedside to bench and back again

There is a growing discussion surrounding the issue of personalized approaches to drug prescription based on an individual's genetic makeup. This field of investigation has focused primarily on identifying genetic factors that influence drug metabolism and cellular disposition, thereby contributing to dose-dependent toxicities and/or variable drug efficacy. However, pharmacogenetic approaches have also proved valuable in predicting drug hypersensitivity reactions in selected patient populations, including HIV-infected patients receiving long-term antiretroviral therapy. In this instance, susceptibility has been strongly linked to genetic loci involved in antigen recognition and presentation to the immune system--most notably within the major histocompatibility complex (MHC) region--consistent with the notion that hypersensitivity reactions represent drug-specific immune responses that are largely dose independent. Here the authors describe their experiences with the development of pharmacogenetic approaches to hypersensitivity reactions associated with abacavir and nevirapine, two commonly prescribed antiretroviral drugs. It is demonstrated that prospective screening tests to identify and exclude individuals with a certain genetic makeup may be largely successful in decreasing or eliminating incidence of these adverse drug reactions in certain populations. This review also explores the broader implications of these findings.

Cathepsin G is differentially expressed in primary human antigen-presenting cells

Cathepsins are required for the processing of antigens in order to make them suitable for loading on major histocompatibility complex (MHC) class II molecules, for subsequent presentation to CD4(+) T cells. It was shown that antigen processing in monocyte-derived dendritic cells (DC), a commonly used DC model, is different from that of primary human DC. Here, we report that the two subsets of human myeloid DC (mDC) and plasmacytoid DC (pDC) differ in their cathepsin distribution. The serine protease cathepsin G (CatG) was detected in mDC1, mDC2, pDC, cortical thymic epithelial cells (cTEC) and high levels of CatG were determined in pDC. To address the role of CatG in the processing and presentation of a Multiple Sclerosis-associated autoantigen myelin basic protein (MBP), we used a non-CatG expressing fibroblast cell line and fibroblasts, which were preloaded with purified CatG. We find that preloading fibroblasts with CatG results in a decrease of MBP84-98-specific T cell proliferation, when compared to control cells. Our data suggest a different processing signature in primary human antigen-presenting cells and CatG may be of functional importance.

Active immunosurveillance in the tumor microenvironment of colorectal cancer is associated with low frequency tumor budding and improved outcome.

Tumor budding (single tumor cells or small tumor cell clusters) at the invasion front of colorectal cancer (CRC) is an adverse prognostic indicator linked to epithelial-mesenchymal transition. This study characterized the immunogenicity of tumor buds by analyzing the expression of the major histocompatibility complex (MHC) class I in the invasive tumor cell compartment. We hypothesized that maintenance of a functional MHC-I antigen presentation pathway, activation of CD8+ T-cells, and release of antitumoral effector molecules such as cytotoxic granule-associated RNA binding protein (TIA1) in the tumor microenvironment can counter tumor budding and favor prolonged patient outcome. Therefore, a well-characterized multipunch tissue microarray of 220 CRCs was profiled for MHC-I, CD8, and TIA1 by immunohistochemistry. Topographic expression analysis of MHC-I was performed using whole tissue sections (n = 100). Kirsten rat sarcoma viral oncogene homolog (KRAS) and B-Raf proto-oncogene, serine/threonine kinase (BRAF) mutations, mismatch repair (MMR) protein expression, and CpG-island methylator phenotype (CIMP) were investigated. Our results demonstrated that membranous MHC-I expression is frequently down-regulated in the process of invasion. Maintained MHC-I at the invasion front strongly predicted low-grade tumor budding (P = 0.0004). Triple-positive MHC-I/CD8/TIA1 in the tumor microenvironment predicted early T-stage (P = 0.0031), absence of lymph node metastasis (P = 0.0348), lymphatic (P = 0.0119) and venous invasion (P = 0.006), and highly favorable 5-year survival (90.9% vs 39.3% in triple-negative patients; P = 0.0032). MHC-I loss was frequent in KRAS-mutated, CD8+ CRC (P = 0.0228). No relationship was observed with CIMP, MMR, or BRAF mutation. In conclusion, tumor buds may evade immune recognition through downregulation of membranous MHC-I. A combined profile of MHC-I/CD8/TIA1 improves the prognostic value of antitumoral effector cells and should be preferred to a single marker approach.

Intracellular survival of apicomplexan parasites and host cell modification

The intracellular stages of apicomplexan parasites are known to extensively modify their host cells to ensure their own survival. Recently, considerable progress has been made in understanding the molecular details of these parasite-dependent effects for Plasmodium-, Toxoplasma- and Theileria-infected cells. We have begun to understand how Plasmodium liver stage parasites protect their host hepatocytes from apoptosis during parasite development and how they induce an ordered cell death at the end of the liver stage. Toxoplasma parasites are also known to regulate host cell survival pathways and it has been convincingly demonstrated that they block host cell major histocompatibility complex (MHC)-dependent antigen presentation of parasite epitopes to avoid cell-mediated immune responses. Theileria parasites are the masters of host cell modulation because their presence immortalises the infected cell. It is now accepted that multiple pathways are activated to induce Theileria-dependent host cell transformation. Although it is now known that similar host cell pathways are affected by the different parasites, the outcome for the infected cell varies considerably. Improved imaging techniques and new methods to control expression of parasite and host cell proteins will help us to analyse the molecular details of parasite-dependent host cell modifications.

Immunization with a circumsporozoite epitope fused to Bordetella pertussis adenylate cyclase in conjunction with cytotoxic T-lymphocyte-associated antigen 4 blockade confers protection against Plasmodium berghei liver-stage malaria

The adenylate cyclase toxoid (ACT) of Bordetella pertussis is capable of delivering its N-terminal catalytic domain into the cytosol of CD11b-expressing professional antigen-presenting cells such as myeloid dendritic cells. This allows delivery of CD8+ T-cell epitopes to the major histocompatibility complex (MHC) class I presentation pathway. Recombinant detoxified ACT containing an epitope of the Plasmodium berghei circumsporozoite protein (CSP), indeed, induced a specific CD8+ T-cell response in immunized mice after a single application, as detected by MHC multimer staining and gamma interferon (IFN-gamma) ELISPOT assay. This CSP-specific response could be significantly enhanced by prime-boost immunization with recombinant ACT in combination with anti-CTLA-4 during the boost immunization. This increased response was accompanied by complete protection in a number of mice after a challenge with P. berghei sporozoites. Transient blockade of CTLA-4 may overcome negative regulation and hence provide a strategy to enhance the efficacy of a vaccine by amplifying the number of responding T cells.

Drug hypersensitivity reactions involving skin

Immune reactions to drugs can cause a variety of diseases involving the skin, liver, kidney, lungs, and other organs. Beside immediate, IgE-mediated reactions of varying degrees (urticaria to anaphylactic shock), many drug hypersensitivity reactions appear delayed, namely hours to days after starting drug treatment, showing a variety of clinical manifestations from solely skin involvement to fulminant systemic diseases which may be fatal. Immunohistochemical and functional studies of drug-specific T cells in patients with delayed reactions confirmed a predominant role for T cells in the onset and maintenance of immune-mediated delayed drug hypersensitivity reactions (type IV reactions). In these reactions, drug-specific CD4+ and CD8+ T cells are stimulated by drugs through their T cell receptors (TCR). Drugs can stimulate T cells in two ways: they can act as haptens and bind covalently to larger protein structures (hapten-carrier model), inducing a specific immune response. In addition, they may accidentally bind in a labile, noncovalent way to a particular TCR of the whole TCR repertoire and possibly also major histocompatibility complex (MHC)-molecules - similar to their pharmacologic action. This seems to be sufficient to reactivate certain, probably in vivo preactivated T cells, if an additional interaction of the drug-stimulated TCR with MHC molecules occurs. The mechanism was named pharmacological interaction of a drug with (immune) receptor and thus termed the p-i concept. This new concept may explain the frequent skin symptoms in drug hypersensitivity to oral or parenteral drugs. Furthermore, the various clinical manifestations of T cell-mediated drug hypersensitivity may be explained by distinct T cell functions leading to different clinical phenotypes. These data allowed a subclassification of the delayed hypersensitivity reactions (type IV) into T cell reactions which, by releasing certain cytokines and chemokines, preferentially activate and recruit monocytes (type IVa), eosinophils (type IVb), or neutrophils (type IVd).

Cytotoxic T cells induce proliferation of chronic myeloid leukemia stem cells by secreting interferon-γ

Chronic myeloid leukemia (CML) is a clonal myeloproliferative neoplasia arising from the oncogenic break point cluster region/Abelson murine leukemia viral oncogene homolog 1 translocation in hematopoietic stem cells (HSCs), resulting in a leukemia stem cell (LSC). Curing CML depends on the eradication of LSCs. Unfortunately, LSCs are resistant to current treatment strategies. The host’s immune system is thought to contribute to disease control, and several immunotherapy strategies are under investigation. However, the interaction of the immune system with LSCs is poorly defined. In the present study, we use a murine CML model to show that LSCs express major histocompatibility complex (MHC) and co-stimulatory molecules and are recognized and killed by leukemia-specific CD8+ effector CTLs in vitro. In contrast, therapeutic infusions of effector CTLs into CML mice in vivo failed to eradicate LSCs but, paradoxically, increased LSC numbers. LSC proliferation and differentiation was induced by CTL-secreted IFN-γ. Effector CTLs were only able to eliminate LSCs in a situation with minimal leukemia load where CTL-secreted IFN-γ levels were low. In addition, IFN-γ increased proliferation and colony formation of CD34+ stem/progenitor cells from CML patients in vitro. Our study reveals a novel mechanism by which the immune system contributes to leukemia progression and may be important to improve T cell–based immunotherapy against leukemia.