Interactions between the mannose receptor and thyroid autoantigens

Glycosylation in Autoimmune Diseases

Autoimmune diseases are accompanied by changes in protein glycosylation, in both the immune system and target tissues. The best-studied alteration in autoimmunity is agalactosylation of immunoglobulin G (IgG), characterized primarily in rheumatoid arthritis (RA), and then detected also in systemic lupus erythematosus (SLE), inflammatory bowel disease (IBD), and multiple sclerosis (MS). The rebuilding of IgG N-glycans in RA correlates with the relapses and remissions of the disease, is associated with physiological states such as pregnancy but also depends on applied anti-inflammatory therapy. In turn, a decreased core fucosylation of the whole pool of IgG N-glycans is a serum glycomarker in autoimmune thyroid diseases (AITD) encompassing Hashimoto's thyroiditis (HT) and Grave's disease (GD). However, fucosylation of anti-thyroglobulin IgG (an immunological marker of HT) was elevated in HT serum. Core fucosylation of IgG oligosaccharides was also lowered in MS and SLE. In AITD and IBD, chronic inflammation T lymphocytes showed the reduced expression of MGAT5 gene encoding β1,6-N-acetylglucosaminyltransferase V (GnT-V) responsible for β1,6-branching of N-glycans, which is important for T cell receptor activation. Structural changes of glycans have a profound effect on the pro-inflammatory activity of immune cells and serum immune proteins, including IgG in autoimmunity.

Molecular Insights Into the Relationship Between Autoimmune Thyroid Diseases and Breast Cancer: A Critical Perspective on Autoimmunity and ER Stress

The etiopathologies behind autoimmune thyroid diseases (AITDs) unravel misbehavior of immune components leading to the corruption of immune homeostasis where thyroid autoantigens turn foe to the self. In AITDs lymphocytic infiltration in the thyroid shows up a deranged immune system charging the follicular cells of the thyroid gland (thyrocytes) leading to the condition of either hyperthyroidism or hypothyroidism. The inflammation in AITDs consistently associate with ER function due to which disturbances in the ER protein homeostasis leads to unfolded protein response (UPR) that promotes pathogenesis of autoimmunity. The roles of ER stress in the instantaneous downregulation of MHC class I molecules on thyrocytes and the relevance of IFN γ in the pathogenesis of AITD has been well-documented. Thyroglobulin being the major target of autoantibodies in most of the AITDs is because of its unusual processing in the ER. Autoimmune disorders display a conglomeration of ER stress-induced UPR activated molecules. Several epidemiological data highlight the preponderance of AITDs in women as well as its concurrence with breast cancer. Both being an active glandular system displaying endocrine activity, thyroid as well as breast tissue show various commonalities in the expression pattern of heterogenous molecules that not only participate in the normal functioning but at the same time share the blame during disease establishment. Studies on the development and progression of breast carcinoma display a deranged and uncontrolled immune response, which is meticulously exploited during tumor metastasis. The molecular crosstalks between AITDs and breast tumor microenvironment rely on active participation of immune cells. The induction of ER stress by Tunicamycin advocates to provide a model for cancer therapy by intervening glycosylation. Therefore, this review attempts to showcase the molecules that are involved in feeding up the relationship between breast carcinoma and AITDs.

Antibody-Based Tracers for PET/SPECT Imaging of Chronic Inflammatory Diseases

Chronic inflammatory diseases are often progressive, resulting not only in physical damage to patients but also social and economic burdens, making early diagnosis of them critical. Nuclear medicine techniques can enhance the detection of inflammation by providing functional as well as anatomical information when combined with other modalities such as magnetic resonance imaging, computed tomography or ultrasonography. Although small molecules and peptides were mainly used for the treatment and imaging of chronic inflammatory diseases in the past, antibodies and their fragments have also been emerging for chronic inflammatory diseases as they show high specificity to their targets and can have various biological half-lives depending on how they are engineered. In addition, imaging with antibodies or their fragments can visualize the in vivo biodistribution of the probes or help monitor therapeutic responses, thereby providing physicians with a greater understanding of drug behavior in vivo and another means of monitoring their patients. In this review, we introduce various targets and radiolabeled antibody-based probes for the molecular imaging of chronic inflammatory diseases in preclinical and clinical studies. Targets can be classified into three different categories: 1) cell-adhesion molecules, 2) surface markers on immune cells, and 3) cytokines or enzymes. The limitations and future directions of using radiolabeled antibodies for imaging inflammatory diseases are also discussed.

Glycosylation in the Thyroid Gland: Vital Aspects of Glycoprotein Function in Thyrocyte Physiology and Thyroid Disorders

The key proteins responsible for hormone synthesis in the thyroid are glycosylated. Oligosaccharides strongly affect the function of glycosylated proteins. Both thyroid-stimulating hormone (TSH) secreted by the pituitary gland and TSH receptors on the surface of thyrocytes contain N-glycans, which are crucial to their proper activity. Thyroglobulin (Tg), the protein backbone for synthesis of thyroid hormones, is a heavily N-glycosylated protein, containing 20 putative N-glycosylated sites. N-oligosaccharides play a role in Tg transport into the follicular lumen, where thyroid hormones are produced, and into thyrocytes, where hyposialylated Tg is degraded. N-glycans of the cell membrane transporters sodium/iodide symporter and pendrin are necessary for iodide transport. Some changes in glycosylation result in abnormal activity of the thyroid and alteration of the metabolic clearance rate of hormones. Alteration of glycan structures is a pathological process related to the progression of chronic diseases such as thyroid cancers and autoimmunity. Thyroid carcinogenesis is accompanied by changes in sialylation and fucosylation, β1,6-branching of glycans, the content and structure of poly-LacNAc chains, as well as O-GlcNAcylation, while in thyroid autoimmunity the main processes affected are sialylation and fucosylation. The glycobiology of the thyroid gland is an intensively studied field of research, providing new data helpful in understanding the role of the sugar component in thyroid protein biology and disorders.

TSH Receptor Cleavage Into Subunits and Shedding of the A-Subunit; A Molecular and Clinical Perspective

The TSH receptor (TSHR) on the surface of thyrocytes is unique among the glycoprotein hormone receptors in comprising two subunits: an extracellular A-subunit, and a largely transmembrane and cytosolic B-subunit. Unlike its ligand TSH, whose subunits are encoded by two genes, the TSHR is expressed as a single polypeptide that subsequently undergoes intramolecular cleavage into disulfide-linked subunits. Cleavage is associated with removal of a C-peptide region, a mechanism similar in some respects to insulin cleavage into disulfide linked A- and B-subunits with loss of a C-peptide region. The potential pathophysiological importance of TSHR cleavage into A- and B-subunits is that some A-subunits are shed from the cell surface. Considerable experimental evidence supports the concept that A-subunit shedding in genetically susceptible individuals is a factor contributing to the induction and/or affinity maturation of pathogenic thyroid-stimulating autoantibodies, the direct cause of Graves' disease. The noncleaving gonadotropin receptors are not associated with autoantibodies that induce a "Graves' disease of the gonads." We also review herein current information on the location of the cleavage sites, the enzyme(s) responsible for cleavage, the mechanism by which A-subunits are shed, and the effects of cleavage on receptor signaling.

Withdrawn: TSH Receptor Cleavage Into Subunits and Shedding of the A-Subunit; A Molecular and Clinical Perspective

The TSH receptor (TSHR) on the surface of thyrocytes is unique among the glycoprotein hormone receptors in comprising two subunits: an extracellular A-subunit, and a largely transmembrane and cytosolic B-subunit. Unlike its ligand TSH, whose subunits are encoded by two genes, the TSHR is expressed as a single polypeptide that subsequently undergoes intramolecular cleavage into disulfide-linked subunits. Cleavage is associated with removal of a C-peptide region, a mechanism similar in some respects to insulin cleavage into disulfide linked A- and B-subunits with lossofaC-peptideregion. The potential pathophysiological importance of TSHR cleavage into A-and B-subunits is that some A-subunits are shed from the cell surface. Considerable experimental evidence supports the concept that A-subunit shedding in genetically susceptible individuals is a factor contributing to the induction and/or affinity maturation of pathogenic thyroid-stimulating autoantibodies, the direct cause of Graves' disease. The noncleaving gonadotropin receptors are not associated with autoantibodies that induce a "Graves' disease of the gonads." We also review herein current information on the location of the cleavage sites, the enzyme(s) responsible for cleavage, the mechanism by which A-subunits are shed, and the effects of cleavage on receptor signaling. (Endocrine Reviews 37: 114-134, 2016).

Breaking tolerance to thyroid antigens: changing concepts in thyroid autoimmunity

Thyroid autoimmunity involves loss of tolerance to thyroid proteins in genetically susceptible individuals in association with environmental factors. In central tolerance, intrathymic autoantigen presentation deletes immature T cells with high affinity for autoantigen-derived peptides. Regulatory T cells provide an alternative mechanism to silence autoimmune T cells in the periphery. The TSH receptor (TSHR), thyroid peroxidase (TPO), and thyroglobulin (Tg) have unusual properties ("immunogenicity") that contribute to breaking tolerance, including size, abundance, membrane association, glycosylation, and polymorphisms. Insight into loss of tolerance to thyroid proteins comes from spontaneous and induced animal models: 1) intrathymic expression controls self-tolerance to the TSHR, not TPO or Tg; 2) regulatory T cells are not involved in TSHR self-tolerance and instead control the balance between Graves' disease and thyroiditis; 3) breaking TSHR tolerance involves contributions from major histocompatibility complex molecules (humans and induced mouse models), TSHR polymorphism(s) (humans), and alternative splicing (mice); 4) loss of tolerance to Tg before TPO indicates that greater Tg immunogenicity vs TPO dominates central tolerance expectations; 5) tolerance is induced by thyroid autoantigen administration before autoimmunity is established; 6) interferon-α therapy for hepatitis C infection enhances thyroid autoimmunity in patients with intact immunity; Graves' disease developing after T-cell depletion reflects reconstitution autoimmunity; and 7) most environmental factors (including excess iodine) "reveal," but do not induce, thyroid autoimmunity. Micro-organisms likely exert their effects via bystander stimulation. Finally, no single mechanism explains the loss of tolerance to thyroid proteins. The goal of inducing self-tolerance to prevent autoimmune thyroid disease will require accurate prediction of at-risk individuals together with an antigen-specific, not blanket, therapeutic approach.

Anti-Saccharomyces cerevisiae antibodies are elevated in Graves' disease but not in Hashimoto's thyroiditis

BACKGROUND

Anti-Saccharomyces cerevisiae antibodies (ASCA) had been known to be specific for Crohn's disease, but they had also been found in many other autoimmune diseases.

AIM

The aim of this study was to evaluate the prevalence of ASCA in patients with autoimmune thyroid disease (AITD).

PATIENTS AND METHODS

One hundred and ninety-seven patients with AITD and 160 healthy controls were included in the study. One hundred and nineteen patients had Graves' disease (GD) and 78 patients had Hashimoto's thyroiditis (HT). ASCA IgG and IgA were determined by ELISA.

RESULTS

ASCA IgG were significantly more frequent in patients with GD than in control group (11.8% vs. 3.1%, p = 0.002). In HT, the frequency of ASCA IgG was similar to that of the control group (3.8% and 3.1% respectively). The frequency of ASCA IgA was similar in GD (0.8%), HT (2.6%), and the control group (3.1%). In all GD patients, the frequency of ASCA IgG was significantly higher than that of ASCA IgA (11.8% vs. 0.8%, p = 0.001). These results were also true even in male and female groups (10.4% vs. 1.3%, p = 0.01 and 14.3% vs. 0%, p = 0.01, respectively). ASCA IgG levels were significantly higher in GD patients (6.7 ± 11.1 vs. 2.2 ± 2.8, p = 3 × 10(-6)) and in HT patients (4.2 ± 4.7 vs. 2.2 ± 2.8, p = 0.0002) than those in the control group. ASCA IgA levels were comparable among patients with GD, HT, and the control group. In GD patients, the mean titer of ASCA IgG was significantly higher than that of ASCA IgA (6.7 ± 11.1 vs. 3.6 ± 4.2, p = 0.005).

CONCLUSION

Patients with GD had a higher frequency of ASCA IgG than controls.

Immune manipulation for Graves' disease: re-exploring an unfulfilled promise with modern translational research

Although Graves' disease is the commonest autoimmune thyroid disorder, current therapeutics typically center on the eradication of the antigenic stimulus (i.e. thyroid gland) rather than radically tackling the underlying autoimmune processes. Consequently, it is not a surprising fact that Graves' disease remains essentially a chronic drug-dependent ailment afflicting untold numbers worldwide for decades despite progress in deciphering its autoimmune nature. Addressing the latter is key to a future cure as underscored by appropriate, albeit crude, proof-of-concept scenarios of clinical remissions achieved with hematopoietic stem cell transplantation, immune down-regulation during pregnancy, use of corticosteroids or immunosuppressives, and cytokine biologics in animal models. Ongoing basic and translational research to further elucidate and refine our understanding of the pathogenesis of Graves' disease holds the promise of unraveling novel immune manipulative techniques that will bring the world a step closer to the elusive cure of the underlying autoimmunity amidst skepticisms on the value of the science from the present lack of paralleled advances at the bedside. We review the updated literature and describe the forms of immune manipulation hitherto explored that will offer a route to a future cure, from thionamides, hematopoietic stem cell transplantation to the latest immunomodulatory agents.

Review and hypothesis: does Graves' disease develop in non-human great apes?

BACKGROUND

Graves' disease, caused by stimulatory thyrotropin receptor (TSHR) autoantibodies, has not been observed in animals. In contrast, Hashimoto's thyroiditis develops in chickens, rats, mice, dogs, and marmosets. Attempts to induce an immune response in mice to the luteinizing-hormone receptor suggested that autoantigen glycosylation was one parameter involved in breaking self-tolerance. Over evolution, TSHR glycosylation increased from three asparagine-linked-glycans (N-glycans) in fish to six N-glycans in humans and great apes. All other placental mammals lack one N-glycan in the shed TSHR A-subunit, the primary Graves' disease autoantigen. We hypothesized that (a) lesser TSHR A-subunit glycosylation reduces immunogenicity, accounting for the absence of Graves' disease in most placental mammals; (b) due to human-like A-subunit glycosylation, Graves' disease might arise in great apes. Here, we review and analyze the literature on this subject and report the results of a survey of veterinarians at primate centers and zoos in North America.

SUMMARY

Previous experimental data from induced TSHR antibodies in mice support a role for A-subunit glycosylation in breaking self-tolerance. An extensive search of the great-ape literature revealed five reports of noncongenital thyroid dysfunction, four with hypothyroidism and one with hyperthyroidism. The latter was a gorilla who was treated with anti-thyroid drugs but is now deceased. Neither serum nor thyroid tissue from this gorilla were available for analysis. The survey of veterinarians revealed that none of the 979 chimpanzees in primate research centers had a diagnosis of noncongenital thyroid dysfunction and among ∼1100 great apes (gorillas, orangutans, and chimpanzees) in U.S. zoos, only three were hypothyroid, and none were hyperthyroid.

CONCLUSIONS

Graves' disease appears to be either very rare or does not occur in great apes based on the literature and a survey of veterinarians. Although the available data do not advance our hypothesis, there is a paucity of information regarding thyroid function tests and thyroid autoantibodies in the great apes In addition, these primates may be protected against TSHR autoimmunity by the absence of genetic polymorphisms and putative environmental triggers. Finally, larger numbers of great apes need to be followed, and tests of thyroid function and thyroid autoantibodies be performed, to confirm that spontaneous Graves' disease is restricted to humans.

An attempt to induce "Graves' disease of the gonads" by immunizing mice with the luteinizing hormone receptor provides insight into breaking tolerance to self-antigens

BACKGROUND

Gonadotropin receptors, unlike the thyrotropin receptor (TSHR), are not cleaved into disulfide-linked A- and B-subunits, nor do they shed A-subunits. Heavily glycosylated TSHR A-subunits initiate or amplify responses leading to stimulating TSHR-autoantibodies and Graves' hyperthyroidism.

METHODS

To investigate the possibility that mice immunized with luteinizing hormone receptor (LHR) would develop functional antibodies, we constructed adenoviruses expressing the rat-LH holoreceptor (LHR-Ad) and an LHR A-subunit equivalent (LHR-289-Ad). Female BALB/c mice were immunized with high doses (10(11) particles) of LHR-Ad, LHR-289-Ad, or control (Con)-Ad. Sera were tested using LHR-expressing eukaryotic cells for antibody binding by flow cytometry and for bioactivity by measuring cyclic adenosine monophosphate (cAMP) stimulation.

RESULTS

Elevated serum binding to LHR cells in some LHR-Ad and LHR-289-Ad immunized mice was not specific for LHR-expressing cells. Moreover, sera lacked bioactivity, consistent with unchanged serum estradiol and ovary histology. The difference between rat and mouse LHR-ectodomains is relatively small (3% at the amino-acid level). In contrast, despite amino-acid identity, immunization of mice with adenovirus expressing membrane-bound mouse thyroid peroxidase (TPO), but not soluble mouse TPO ectodomain, elicited strong TPO-specific antibodies.

CONCLUSIONS

Our investigations provide insight into antibody responses to self-antigens. First, antibodies are induced to large self-antigens like mouse-TPO when membrane bound. Second, lesser amino acid homology between the immunogen and mouse protein (91% vs. 97% for the human-TSHR and rat-LHR, respectively) favors antibody induction. Finally, from previous studies demonstrating the immunogenicity of the highly glycosylated human TSHR A-subunit versus our present data for the nonimmunogenic less glycosylated rat LHR, we suggest that the extent of glycosylation contributes to breaking self-tolerance.

Anti-Saccaromyces Cerevisiae antibodies (ASCA) are elevated in autoimmune thyroid disease ASCA in autoimmune thyroid disease

Environmental factors have been implicated in the development of autoimmune thyroid disease (AITD). Anti-Saccaromyces Cerevisiae Antibodies (ASCA) were shown to be elevated in several autoimmune diseases. The aim of the study was to determine ASCA levels and their relationship with thyroid autoantibodies in patients with AITD. One-hundred and twelve patients with AITD (age 41.1±12.8 years; F/M:96/16) and 103 healthy controls (38.5±10.3 years; F/M:82/21) were included. Twenty-four patients had Graves disease (GD), and 88 had Hashimoto's thyroiditis (HT). ASCA IgA and IgG, TSH, free T4, anti-thyroglobulin, and anti-thyroid peroxidase antibody concentrations were determined. ASCA IgA positivity in patients with GD (16.6%) was similar to patients with HT (13.6%) and was higher than controls (5.8%). No significant difference was present between the frequencies of IgG positivity among GD (12.5%), HT (7.9%), and control groups (5.8%). The mean levels of ASCA IgA and IgG were comparable within the groups. No correlation of ASCA and anti-thyroglobulin and anti-thyroid peroxidase levels was observed. Increased IgA ASCA positivity is observed in patients with GD, suggesting a role of environmental stimuli in its pathogenesis. The role of ASCA in the etiology of AITD needs to be further examined.

Attenuation of induced hyperthyroidism in mice by pretreatment with thyrotropin receptor protein: deviation of thyroid-stimulating to nonfunctional antibodies

Graves'-like hyperthyroidism is induced by immunizing BALB/c mice with adenovirus expressing the thyrotropin receptor (TSHR) or its A-subunit. Nonantigen-specific immune strategies can block disease development and some reduce established hyperthyroidism, but these approaches may have unforeseen side effects. Without immune stimulation, antigens targeted to the mannose receptor induce tolerance. TSHR A-subunit protein generated in eukaryotic cells binds to the mannose receptor. We tested the hypothesis that eukaryotic A-subunit injected into BALB/c mice without immune stimulation would generate tolerance and protect against hyperthyroidism induced by subsequent immunization with A-subunit adenovirus. Indeed, one sc injection of eukaryotic, glycosylated A-subunit protein 1 wk before im A-subunit-adenovirus immunization reduced serum T(4) levels and the proportion of thyrotoxic mice decreased from 77 to 22%. Prokaryotic A-subunit and other thyroid proteins (thyroglobulin and thyroid peroxidase) were ineffective. A-subunit pretreatment reduced thyroid-stimulating and TSH-binding inhibiting antibodies, but, surprisingly, TSHR-ELISA antibodies were increased. Rather than inducing tolerance, A-subunit pretreatment likely expanded B cells that secrete nonfunctional antibodies. Follow-up studies supported this possibility and also showed that eukaryotic A-subunit administration could not reverse hyperthyroidism in mice with established disease. In conclusion, glycosylated TSHR A-subunit is a valuable immune modulator when used before immunization. It acts by deviating responses away from pathogenic toward nonfunctional antibodies, thereby attenuating induction of hyperthyroidism. However, this protein treatment does not reverse established hyperthyroidism. Our findings suggest that prophylactic TSHR A-subunit protein administration in genetically susceptible individuals may deviate the autoantibody response away from pathogenic epitopes and provide protection against future development of Graves' disease.

Graves' animal models of Graves' hyperthyroidism

Fifty years after the discovery of thyroid autoimmunity, several animal models of Graves' hyperthyroidism are now available. All are inducible types, and diseases are elicited by injecting living cells (professional or nonprofessional antigen-presenting cells) expressing the recombinant thyrotropin receptor (TSHR) or by DNA vaccination with TSHR cDNA in plasmid or adenovirus vectors. Thus most Graves' models are attributed to the cloning of the TSHR cDNA and involve in vivo expression of the TSHR. These breakthroughs have provided us important insights into our understanding of the pathogenesis of Graves' disease, and also indispensable means to exploring the possibility of development of novel therapeutic modalities. In particular, recent studies have begun to scrutinize the genetic factors contributing to the susceptibility to this ailment, and to delineate the roles for central and peripheral tolerance and also for fine balance between autoreactive effector T cells and regulatory T cells in the pathophysiology of anti-TSHR autoimmunity and Graves' hyperthyroidism. Moreover, preliminary, but novel, therapeutic approaches have also been started to treat experimental hyperthyroidism.

A role for exposed mannosylations in presentation of human therapeutic self-proteins to CD4+ T lymphocytes

Several therapeutic self-proteins elicit immune responses when administered to patients. Such adverse immune responses reduce drug efficacy. To induce an immune response, a protein must interact with different immune cells, including antigen-presenting cells, T cells, and B cells. Each cell type recognizes distinct immunogenic patterns on antigens. Mannose-terminating glycans have been identified as pathogen-associated molecular patterns that are essential for internalization of microbes by antigen-presenting cells, leading to presentation. Here, we have investigated the importance of exposed mannosylation on an immunogenic therapeutic self-protein, procoagulant human factor VIII (FVIII). Administration of therapeutic FVIII to hemophilia A patients induces inhibitory anti-FVIII antibodies in up to 30% of the cases. We demonstrate that entry of FVIII into human dendritic cells (DC) leading to T cell activation, is mediated by mannose-terminating glycans on FVIII. Further, we identified macrophage mannose receptor (CD206) as a candidate endocytic receptor for FVIII on DC. Saturation of mannose receptors on DC with mannan, and enzymatic removal of mannosylated glycans from FVIII lead to reduced T cell activation. The interaction between FVIII and CD206 was blocked by VWF, suggesting that, under physiological conditions, the intrinsic mannose-dependent immunogenicity of FVIII is quenched by endogenous immunochaperones. These data provide a link between the mannosylation of therapeutic self-proteins and their iatrogenic immunogenicity. Such a link would be of special relevance in the context of replacement therapy where mechanisms of central and peripheral tolerance have not been established during ontogeny because of the absence of the antigen.

Carbohydrate recognition systems in autoimmunity

The immune system is a complex functional network of diverse cells and soluble molecules orchestrating innate and adaptive immunity. Biological information, to run these intricate interactions, is not only stored in protein sequences but also in the structure of the glycan part of the glycoconjugates. The spatially accessible carbohydrate structures that contribute to the cell's glycome are decoded by versatile recognition systems in order to maintain the immune homeostasis of an organism. Microbial carbohydrate structures are recognized by pathogen associated molecular pattern (PAMP) receptors of innate immunity including C-type lectins such as MBL, the tandem-repeat-type macrophage mannose receptor, DC-SIGN or dectin-1 of dendritic cells, certain TLRS or the TCR of NKT cells. Natural autoantibodies, a long known effector branch of this network-based operation, are effective to home in on non-self and self-glycosylation also. The recirculating pool of mammalian immune cells is recruited to inflammatory sites by a reaction pathway involving the self-carbohydrate-binding selectins as initial recognition step. Galectins, further key sensors reading the high-density sugar code, exert regulatory functions on activated T cells, among other activities. Autoimmune diseases are being associated with defined changes of glycosylation. This correlation deserves to be thoroughly studied on the levels of structural mimicry and dysregulation as well as effector molecules to devise innovative anti-inflammatory strategies. This review briefly summarizes data on sensor systems for carbohydrate epitopes and implications for autoimmunity.

Carbohydrate-independent recognition of collagens by the macrophage mannose receptor

Mannose receptor (MR) is the best characterised member of a family of four endocytic molecules that share a common domain structure; a cysteine-rich (CR) domain, a fibronectin-type II (FNII) domain and tandemly arranged C-type lectin-like domains (CTLD, eight in the case of MR). Two distinct lectin activities have been described for MR. The CR domain recognises sulphated carbohydrates while the CTLD mediate binding to mannose, fucose or N-acetylglucosamine. FNII domains are known to be important for collagen binding and this has been studied in the context of two members of the MR family, Endo180 and the phospholipase A2 receptor. Here, we have investigated whether the broad and effective lectin activity mediated by the CR domain and CTLD of MR is favoured to the detriment of FNII-mediated interaction(s). We show that MR is able to bind and internalise collagen in a carbohydrate-independent manner and that MR deficient macrophages have a marked defect in collagen IV and gelatin internalisation. These data have major implications at the molecular level as there are now three distinct ligand-binding sites described for MR. Furthermore our findings extend the range of endogenous ligands recognised by MR, a molecule firmly placed at the interface between homeostasis and immunity.

Targeted expression of the human thyrotropin receptor A-subunit to the mouse thyroid: insight into overcoming the lack of response to A-subunit adenovirus immunization

The thyrotropin receptor (TSHR), the major autoantigen in Graves' disease, is posttranslationally modified by intramolecular cleavage to form disulfide-linked A- and B-subunits. Because Graves' hyperthyroidism is preferentially induced in BALB/c mice using adenovirus encoding the free A-subunit rather than full-length human TSHR, the shed A-subunit appears to drive the disease-associated autoimmune response. To further investigate this phenomenon, we generated transgenic mice with the human A-subunit targeted to the thyroid. Founder transgenic mice had normal thyroid function and were backcrossed to BALB/c. The A-subunit mRNA expression was confirmed in thyroid tissue. Unlike wild-type littermates, transgenic mice immunized with low-dose A-subunit adenovirus failed to develop TSHR Abs, hyperthyroidism, or splenocyte responses to TSHR Ag. Conventional immunization with A-subunit protein and adjuvants induced TSHR Abs lacking the characteristics of human autoantibodies. Unresponsiveness was partially overcome using high-dose, full-length human TSHR adenovirus. Although of low titer, these induced Abs recognized the N terminus of the A-subunit, and splenocytes responded to A-subunit peptides. Therefore, "non-self" regions in the B-subunit did not contribute to inducing responses. Indeed, transgenic mice immunized with high-dose A-subunit adenovirus developed TSHR Abs with thyrotropin-binding inhibitory activity, although at lower titers than wild-type littermates, suggesting down-regulation in the transgenic mice. In conclusion, in mice expressing a human A-subunit transgene in the thyroid, non-self human B-subunit epitopes are not necessary to induce responses to the A-subunit. Our findings raise the possibility that autoimmunity to the TSHR in humans may not involve epitopes on a cross-reacting protein, but rather, strong adjuvant signals provided in bystander immune responses.

Gene expression profiles differ markedly in mouse strains that are (or are not) susceptible to hyperthyroidism induced using thyrotropin receptor-expressing adenovirus

BALB/c mice are susceptible and C57BL/6 mice are resistant to Graves' hyperthyroidism induced by immunization with adenovirus encoding the thyrotropin receptor (TSHR) A-subunit. Both strains develop comparable levels of TSHR antibodies, but potent TSH blocking antibody activity in C57BL/6 mice likely blocks development of hyperthyroidism. We used microarrays to compare gene expression in spleens of mice immunized with A-subunit adenovirus (TSHR-Ad) or control adenovirus (Con-Ad). To preclude the effects of variable thyroxine (T(4)) levels, mice were studied when euthyroid as follows: BALB/c mice immunized three times with TSHR-Ad or Con-Ad and C57BL/6 mice immunized three times with TSHR-Ad or Con-Ad. Among the 14,000 expressed probe sets, there were no statistically significant differences in gene expression in BALB/c mice immunized with TSHR-Ad versus Con-Ad. In contrast, expression of 57 transcripts (representing 40 genes) changed in response to TSHR-Ad in C57BL/6 mice. Diverse genes were identified, including proteins involved in immune responses, inflammation, and cell cycling as well as heat-shock proteins and proteases. Down-regulation of chitinase 3- and-4 gene expression likely reflects cytokines produced by T-helper 2 (Th2) type cells. Indeed, the immunoglobulin (IgG) subclass for TSHR antibodies reflects a deviation away from Th2 cytokines and toward Th1 in C57BL/6 mice. In conclusion, TSHR-Ad immunization altered gene expression profiles in C57BL/6, but not in BALB/c, mice. This response primarily involved reduced gene expression. In C57BL/6 mice, decreased expression of genes such as cathelicidin, calgranulins, and lipocalin following TSHR A-subunit adenovirus immunization suggests the importance of innate immunity in this response.

Insight into Graves' hyperthyroidism from animal models

Graves' hyperthyroidism can be induced in mice or hamsters by novel approaches, namely injecting cells expressing the TSH receptor (TSHR) or vaccination with TSHR-DNA in plasmid or adenoviral vectors. These models provide unique insight into several aspects of Graves' disease: 1) manipulating immunity toward Th1 or Th2 cytokines enhances or suppresses hyperthyroidism in different models, perhaps reflecting human disease heterogeneity; 2) the role of TSHR cleavage and A subunit shedding in immunity leading to thyroid-stimulating antibodies (TSAbs); and 3) epitope spreading away from TSAbs and toward TSH-blocking antibodies in association with increased TSHR antibody titers (as in rare hypothyroid patients). Major developments from the models include the isolation of high-affinity monoclonal TSAbs and analysis of antigen presentation, T cells, and immune tolerance to the TSHR. Studies of inbred mouse strains emphasize the contribution of non-MHC vs. MHC genes, as in humans, supporting the relevance of the models to human disease. Moreover, other findings suggest that the development of Graves' disease is affected by environmental factors, including infectious pathogens, regardless of modifications in the Th1/Th2 balance. Finally, developing immunospecific forms of therapy for Graves' disease will require painstaking dissection of immune recognition and responses to the TSHR.

Animal models of Graves' hyperthyroidism

Graves' disease is a common organ-specific autoimmune disease characterized by overstimulation of the thyroid gland with agonistic anti-thyrotropin (TSH) receptor autoantibodies, which leads to hyperthyroidism and diffuse hyperplasia of the thyroid gland. Several groups including us have recently established several animal models of Graves' hyperthyroidism using novel immunization approaches, such as in vivo expression of the TSH receptor by injecting syngeneic living cells co-expressing the TSH receptor, the major histocompatibility complex (MHC) class II antigen and a costimulatory molecule, or genetic immunization using plasmid or adenovirus vectors coding the TSH receptor. This breakthrough has made it possible for us to study the pathogenesis of Graves' disease in more detail and has provided important insights into our understanding of disease pathogenesis. The important new findings that have emerged include: (i) the shed A subunit being the major autoantigen for TSAb, (ii) the significant role played by dendritic cells (DCs) as professional antigen-presenting cells in initiating disease development, (iii) contribution of MHC and particularly non-MHC genetic backgrounds in disease susceptibility, and (iv) influence of some particular infectious pathogens on disease development. However, the data regarding Th1/Th2 balance of TSH receptor-specific immune response or the association of Graves' hyperthyroidism with intrathyroidal lymphocytic infiltration are rather inconsistent. Future studies with these models will hopefully lead to better understanding of disease pathogenesis and help develop novel strategies for treatment and ultimately prevention of Graves' disease in humans.