Nonimmune thyroid destruction results from transgenic overexpression of an allogeneic major histocompatibility complex class I protein

HLA-B35 upregulates endothelin-1 and downregulates endothelial nitric oxide synthase via endoplasmic reticulum stress response in endothelial cells

The presence of the HLA-B35 allele has emerged as an important risk factor for the development of isolated pulmonary hypertension in patients with scleroderma, however the mechanisms underlying this association have not been fully elucidated. The goal of our study was to determine the molecular mechanisms that mediate the biological effects of HLA-B35 in endothelial cells (ECs). Our data demonstrate that HLA-B35 expression at physiological levels via adenoviral vector resulted in significantly increased endothelin-1 (ET-1) and a significantly decreased endothelial NO synthase (eNOS), mRNA, and protein levels. Furthermore, HLA-B35 greatly upregulated expression of chaperones, including heat shock proteins (HSPs) HSP70 (HSPA1A and HSPA1B) and HSP40 (DNAJB1 and DNAJB9), suggesting that HLA-B35 induces the endoplasmic reticulum (ER) stress and unfolded protein response in ECs. Examination of selected mediators of the unfolded protein response, including H chain binding protein (BiP; GRP78), C/Ebp homologous protein (CHOP; GADD153), endoplasmic reticulum oxidase, and protein disulfide isomerase has revealed a consistent increase of BiP expression levels. Accordingly, thapsigargin, a known ER stress inducer, stimulated ET-1 mRNA and protein levels in ECs. This study suggests that HLA-B35 could contribute to EC dysfunction via ER stress-mediated induction of ET-1 in patients with pulmonary hypertension.

Defective MHC class I antigen surface expression promotes cellular survival through elevated ER stress and modulation of p53 function

Defects in Major Histocompatibility class I cell surface expression is thought to allow escape of tumor cells from immune surveillance. Hitherto, it is unclear whether this deficiency confers immune-independent survival advantage. We show here that class I cell surface expression deficiency due to defects in beta2 microglobulin or the transporter-associated with antigen processing (TAP) results in resistance to apoptosis in response to various cytotoxic signals. Reduced apoptosis correlated with altered p53 activation, which was due to compromised nuclear translocation of p53. Binding of p53 to glycogen synthase kinase-3beta (GSK3beta), which is known to phosphorylate and lead to cytoplasmic sequestration of p53, was enhanced in these cells. Consistently, endoplasmic reticulum (ER) stress, which promotes binding of p53 to GSK3beta was constitutively elevated in the absence of class I cell surface expression. Taken together, the results suggest a non-immunological causal role for defective class I cell surface expression in regulating cellular survival in a p53-dependent manner, through the upregulation of ER stress, which could be another mechanism leading to carcinogenesis.

HLA-B35 influences the apoptosis rate in human peripheral blood mononucleated cells and HLA-transfected cells

Human leukocyte antigen (HLA) class I antigens can act as signal-transducing molecules that influence individual reactivity to external stimuli and the existence of haplotype-specific cell signal regulation has been suggested. In this article, we provide definite experimental evidence for the existence of a HLA-B35 haplotype-specific regulation of cell apoptosis in different experimental models. First, we demonstrated that HLA-B35, but not other HLA-class I antigens, was associated with an increased cell susceptibility to apoptosis in human peripheral mononuclear cells (PBMCs) exposed in vitro to thapsigargin. Second, we confirmed this association in human ECV 304 cells transfected with HLA-B35 or with HLA-B8, an antigen that did not appear to influence the apoptosis rate in the thapsigargin-treated PBMCs. Third, we confirmed the specific influence of HLA-B35 on cell apoptosis in non human cells (i.e., HLA-B35-transfected NIH3T3 murine fibroblasts). Our data show the existence of HLA-B35 haplotype-specific regulation of cell apoptosis and open new perspectives on the role of HLA class I genes in cell activation and disease susceptibility.

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.

Role of major histocompatibility complex class I molecules in autoimmune myositis

PURPOSE OF REVIEW

Recent work has continued to clarify the role of major histocompatibility complex class I in the pathogenesis of autoimmune myositis. In the past year, several new observations have been made in this area. This review describes these findings and discusses their relevance to the pathogenesis of autoimmune myositis.

RECENT FINDINGS

Recent studies have confirmed earlier observations of the up-regulation of major histocompatibility complex class I antigens in myositis. In particular, a recent study has strengthened the conclusion that major histocompatibility complex class I expression is highly specific to inflammatory myopathies and may be of diagnostic value. Two new studies have indicated that endoplasmic reticulum stress response pathway (the endoplasmic reticulum overload [NF-kB] and unfolded protein response [GRP78]) are highly activated in patients with myositis. One study using transgenic mice has further indicated that abnormal accumulation of major histocompatibility complex class I in the endoplasmic reticulum of muscle may be responsible for the initiation of this endoplasmic reticulum stress response. Furthermore, studies of normal muscle cells have shown that endoplasmic reticulum stress also plays an important role in skeletal muscle development. Investigations of autoantigen expression in myositis biopsies have revealed that regenerating muscle cells express high levels of autoantigens and major histocompatibility complex class I, indicating that these cells are the targets of cytotoxic T-cell attack and may participate in the initiation of a myositis-specific autoimmune response.

SUMMARY

Defining the role of major histocompatibility complex class I in autoimmune myositis may be useful not only for diagnosis of this group of diseases but also for therapeutic opportunities for these difficult disorders.

Self-reactive B cells are not eliminated or inactivated by autoantigen expressed on thyroid epithelial cells

Graves' Disease results from the production of autoantibodies against receptors for thyroid stimulating hormone (TSH) on thyroid epithelial cells, and represents the prototype for numerous autoimmune diseases caused by autoantibodies that bind to organ-specific cell membrane antigens. To study how humoral tolerance is normally maintained to organ-specific membrane antigens, transgenic mice were generated selectively expressing membrane-bound hen egg lysozyme (mHEL) on the thyroid epithelium. In contrast to the deletion of autoreactive B cells triggered by systemic mHEL (Hartley, S.B., J. Crosbie, R. Brink, A.B. Kantor, A. Basten, and C.C. Goodnow. 1991. Nature. 353:765-769), selective expression of mHEL autoantigen on thyroid cells did not trigger elimination or inactivation of circulating HEL-reactive B cells. These results provide evidence that tolerance is not actively acquired to organ-specific antigens in the preimmune B cell repertoire, underscoring the importance of maintaining tolerance to such antigens by other mechanisms. The role of an intact endothelial barrier in sequestering organ-specific antigens from circulating preimmune B cells is discussed.

Self and non-self antigen in diabetic autoimmunity: molecules and mechanisms

In this article, we have summarized current facts, models and views of the autoimmunity that leads to destruction of insulin-producing beta-cells and consequent Type 1 (insulin-dependent) diabetes mellitus. The presence of strong susceptibility and resistance gene loci distinguishes this condition from other autoimmune disorders, but environmental disease factors must conspire to produce disease. The mapping of most of the genetic risk (or disease resistance) to specific alleles in the major histocompatibility locus (MHC class II) has direct functional implications for our understanding of autoimmunity in diabetes and directly implies that presentation of a likely narrow set of peptides is critical to the development of diabetic autoimmunity. While many core scientific questions remain to be answered, current insight into the disease process is beginning to have direct clinical impact with concerted efforts towards disease prevention or intervention by immunological means. In this process, identification of the critical antigenic epitopes recognized by diabetes-associated T cells has achieved highest priority.

MHC genes in autoimmunity

In the last year progress has been made towards elucidating the roles of the MHC gene products in autoimmunity. A major advance has been the recent determination of the crystallographic structure of the human MHC class II molecule, which will be invaluable in delineating the minimum structural requirements for peptides that induce autoimmune disease. In addition, the use of animal models and transgenic mouse technology is continuing to increase our understanding of the involvement of the MHC gene products in immunopathogenesis.

Transforming growth factor beta 1 (TGF-beta 1) controls expression of major histocompatibility genes in the postnatal mouse: aberrant histocompatibility antigen expression in the pathogenesis of the TGF-beta 1 null mouse phenotype

The phenotype of the transforming growth factor beta 1 (TGF-beta 1) null mouse has been previously described and is characterized by inflammatory infiltrates in multiple organs leading to a wasting syndrome and death as early as 3 weeks after birth. Since this phenotype occurs in the absence of any detectable pathogen, potential autoimmune disease mechanisms were investigated. We examined major histocompatibility complex (MHC) mRNA expression in tissues of the TGF-beta 1 null mouse and found levels of both the class I and class II MHC mRNA elevated compared to normal or TGF-beta 1 heterozygous littermates. This elevated expression was seen prior to any evidence of inflammatory infiltrates, suggesting a causal relationship between increased MHC expression and activation of immune cell populations. Cell surface expression of MHC molecules was detected by immunohistochemistry and correlated well with mRNA levels. Expression of mRNA for interferon gamma and its receptor was unchanged at the ages when increased MHC expression became apparent. Down-regulation of class I MHC expression by TGF-beta 1 was also demonstrated in vitro in fibroblasts isolated from TGF-beta 1 null mice. These findings suggest that one natural function of TGF-beta 1 is to control expression of both MHC classes. Altered regulation of MHC expression may be a critical step leading to the multifocal inflammation and wasting syndrome seen in the TGF-beta 1 null mouse. These results suggest potential applications for TGF-beta in the management of autoimmune disease, allograft rejection, and other problems associated with altered MHC expression.