Mechanisms of self tolerance

Engineering of Trophoblast Extracellular Vesicle-Delivering Hydrogels for Localized Tolerance Induction in Cell Transplantation

Purpose

The need for chronic systemic immunosuppression, which presents a host of acute risks to transplantation patients, remains the primary limitation for the translation of many cell therapies, such as insulin secreting cells for the treatment of type 1 diabetes. Trophoblasts are the professional tolerogenic cells of the placenta, and they secrete a range of soluble factors to induce antigen specific tolerance toward allogeneic fetal tissue during pregnancy, including extracellular vesicles. Here we develop a trophoblast extracellular vesicle-delivering hydrogel designed for sustained, localized tolerogenic factor delivery within a transplant site to induce localized tolerance toward cell grafts.

Methods

We engineer a synthetic poly(ethylene glycol)-based hydrogel system to tether extracellular vesicles for sustained delivery, and compare this system to passive vesicle entrapment within an alginate hydrogel system. We characterize trophoblast extracellular vesicles for size and morphology, and evaluate vesicle tolerogenic protein content via proteomic analysis. We validate the retention and tethering of extracellular vesicles within the hydrogel systems via scanning electron and stimulated emission depletion microscopy, and measure vesicle release rate over time. Finally, we evaluate trophoblast extracellular vesicle influence on natural killer cell activation in vitro.

Results

We isolated trophoblast extracellular vesicles and proteomics confirmed the presence of tolerogenic factors. We confirmed the presence of extracellular vesicles within hydrogel delivery vehicles, and synthetic hydrogels extended extracellular vesicle release relative to a passive hydrogel system. Finally, extracellular vesicles reduced natural killer cell activation in vitro, confirming the tolerogenic potential of hydrogel-delivered extracellular vesicles.

Conclusions

This tolerogenic extracellular vesicle-delivering hydrogel platform designed for delivery within a transplant site could serve as an alternative to systemic immunosuppression in cell transplantation, potentially reducing the risks associated with cell therapies and widening the eligible patient population.

Apoptosis and inflammation

During the last few decades it has been recognized that cell death is not the consequence of accidental injury, but is the expression of a cell suicide programme. Kerr et al. (1972) introduced the term apoptosis. This form of cell death is under the influence of hormones, growth factors and cytokines, which depending upon the receptors present on the target cells, may activate a genetically controlled cell elimination process. During apoptosis the cell membrane remains intact and the cell breaks into apoptotic bodies, which are phagocytosed. Apoptosis, in contrast to necrosis, is not harmful to the host and does not induce any inflammatory reaction. The principal event that leads to inflammatory disease is cell damage, induced by chemical/physical injury, anoxia or starvation. Cell damage means leakage of cell contents into the adjacent tissues, resulting in the capillary transmigration of granulocytes to the injured tissue. The accumulation of neutrophils and release of enzymes and oxygen radicals enhances the inflammatory reaction. Until now there has been little research into the factors controlling the accumulation and the tissue load of granulocytes and their histotoxic products in inflammatory processes. Neutrophil apoptosis may represent an important event in the control of intlamtnation. It has been assumed that granulocytes disintegrate to apoptotic bodies before their fragments are removed by local macrophages. Removal of neutrophils from the inflammatory site without release of granule contents is of paramount importance for cessation of inflammation. In conclusion, apoptotic cell death plays an important role in inflammatory processes and in the resolution of inflammatory reactions. The facts known at present should stimulate further research into the role of neutrophil, eosinophil and macrophage apoptosis in inflammatory diseases.

IL-27 induces the differentiation of Tr1-like cells from human naive CD4+ T cells via the phosphorylation of STAT1 and STAT3

IL-27, a member of IL-6/IL-12 cytokine family, is mainly produced by activated antigen presenting cells (APC). It has been demonstrated that IL-27 has the pro- and anti-inflammatory properties during immune responses. However, the signaling pathways that contribute to the cytokine generation are still unclear in humans. In the present study, we showed that IL-27 induced IL-10 and IFN-γ, but had no effect on IL-2, TNF-α and IL-4 production from human umbilical cord blood mononuclear cells (CBMCs). For purified naive CD4(+) T cells, IL-27 elicited the differentiation of Tr1-like cells with expression of IL-10 and IFN-γ. Importantly, this induction was dependent on the signal transducer and activator of transcription (STAT) 1 and 3 factors. In addition, all of the phosphorylated STAT3 positive cells were also positive for phosphorylated STAT1, both of which could be inhibited by a JAK2/STAT inhibitor, AG490. However, there was no phosphorylation of STAT4, STAT5 and STAT6 in IL-27-stimulating conditions. Moreover, the biological function of IL-10 that was induced by IL-27 mainly enhanced the expression of CD16 without influencing CD14 expression on human monocytes. These data identify the function of IL-27 on the differentiation of human naive CD4(+) T cells and demonstrate that the STATs signaling pathways may play an important role in mediating immune responses in humans.

IL-27 is a key regulator of IL-10 and IL-17 production by human CD4+ T cells

Although the physiologic pathways that control regulatory T cells (Foxp3-expressing regulatory T cells, IL-10-secreting Tr1 cells) and Th17 cells in rodents have been defined, the factors that control these differentiation pathways in humans are not well understood. In this study, we show that IL-27 promotes the differentiation of IL-10-secreting Tr1 cells while inhibiting Th17 generation and molecules associated with Th17 function. Furthermore, IL-27 inhibits IL-17-polarizing cytokines on dendritic cells, which in turn decrease IL-17 secretion from T cells. Our results demonstrate that IL-27 plays a key role in human T cells by promoting IL-10-secreting Tr1 cells and inhibiting Th17 cells and thus provides a dual regulatory mechanism to control autoimmunity and tissue inflammation.

Induction of Oral Tolerance in Experimental Sjogren's Syndrome Autoimmunity

Previous studies have showed that immunization with peptides from Ro 60 results in Sjogren's syndrome (SS)-like condition in BALB/c mice. We hypothesized that oral feeding with Ro 60 peptide or Ro 60 would prevent the disease. Four groups (each consisting of 10) of BALB/c mice were used. Group I-III were immunized with Ro 274 peptide. Group IV mice were administered adjuvant only. Group II mice were fed orally with Ro 274 peptide and Group III with Ro 60 for 5 days before immunization. There was a significant reduction in the binding of sera from both Group II and Group III mice to most of the Ro multiple antigenic peptides bound by Group I mice. In Group III mice, salivary flow was maintained above that of the Group I mice (average: 117.5 versus 58.6 microl; t = 2.7; P = 0.02). Salivary infiltrates were drastically decreased in the Ro peptide or Ro 60-fed groups, compared to non-tolerized group. Two of eight mice in Group II and 3/6 mice in Group III had no infiltrates, whereas all eight mice studied in Group I had a significant number of infiltrates. Thus, epitope spreading was prevented, lymphocytic infiltration was blocked and saliva flow was restored by means of oral feeding of either Ro 274 or Ro 60 in this animal model of SS.

Platelet-associated antibodies mediate protective autoimmune response in schizophrenia

The peripheral blood platelets of schizophrenic patients were isolated, and the level of the platelet-associated antibodies (SPAA) was correlated with the rating scores of discrete schizophrenic symptom clusters evaluated with the Brief Psychiatric Rating Scale. Irrespective of medication and gender, symptom-dependent correlations were established between the SPAA levels and the relevant psychometric scores. The results indicate a heterogeneous origin of schizophrenia and imply the involvement of an autoimmune arm as a predominantly protective immune response.

Continuous nasal administration of antigen is critical to maintain tolerance in adoptively transferred autoimmune arthritis in SCID mice

Mucosal tolerance is a natural mechanism that prevents immunological reactions to antigens by altering the activity of immune cells of pathogenic clones without modulating the entire immune system. This 'natural immune suppression' can be exploited when antigen(s) of the target organ in an autoimmune disease is used for mucosal treatment. Being inspired by the experimental results in animal models, clinical trials using type II collagen for mucosal treatment have been conducted in rheumatoid arthritis. High-density proteoglycan (aggrecan) is another major macromolecular component in articular cartilage, and may be a candidate autoantigen for provoking immune reactions in patients with rheumatoid arthritis. Indeed, like type II collagen, systemic immunization of genetically susceptible mice with proteoglycan (PG) aggrecan induces progressive autoimmune polyarthritis. Here, we investigated whether intranasally applied PG can be effective in suppressing PG-induced arthritis (PGIA) in BALB/c mice. We found that nasal administration of 100 microg PG exerted a strong suppressive effect on both the incidence and severity of the disease, most probably by reducing responsiveness towards the immunizing PG antigen. When we transferred PGIA into genetically matched but immunodeficient SCID mice, we were able to establish a tolerized state, but only if the recipient SCID mice received lymphocytes from tolerized animals and intranasal treatment with PG was continued. Without nasally administered antigen, the transferred anergic cells recovered and arthritis rapidly developed in a severe form. Intranasal PG treatment of recipient SCID mice was ineffective when cells from non-tolerized arthritic donors were transferred, in which case the regular weekly 'tolerizing' dose of PG made the disease worse. Our results suggest that mucosal treatment in an already existing disease may result in paradoxical outcomes.

Rapid decreases in donor-specific cytotoxic T lymphocyte precursor frequencies and graft outcome after liver and lung transplantation

BACKGROUND

A decrease in donor-specific T cell precursor frequencies as seen late, one or more years, after transplantation is assumed to reflect transplantation tolerance, a condition important for long term acceptance of the allograft. However, such late decreases also occur in recipients that developed chronic transplant dysfunction questioning its relevance in transplantation tolerance. We investigated whether early, i.e., the first 6 months, decreases in donor-specific T cell precursor frequencies reflect transplantation tolerance and predict graft outcome after liver and lung transplantation.

METHODS

Donor and third party specific cytotoxic (CTLp) and helper T lymphocyte precursor (HTLp) frequencies were analyzed in pretransplant and 1 (or 2) and 6-month blood samples taken from liver and lung recipients and were correlated with graft outcome.

RESULTS

In liver allograft recipients with good graft function (n=7), mean donor-specific CTLp frequencies decreased as early as 1 month after transplantation and remained low thereafter. In contrast, mean CTLp frequencies did not decrease in liver allograft recipients with chronic transplant dysfunction (n=6). In lung allograft recipients, donor-specific CTLp frequencies remained relatively high and frequencies were not different between recipients without (n=6) or with (n=6) chronic transplant dysfunction. Donor-specific HTLp frequencies did not change significantly after liver or lung transplantation and did not differ between recipients without or with chronic transplant dysfunction.

CONCLUSIONS

An early decrease in donor-specific CTLp correlates with good graft outcome after liver transplantation. Such rapid decreases in alloreactivity do not occur after lung transplantation illustrating the unique capacity of liver allografts to induce transplantation tolerance.

Oral tolerance: elucidation of mechanisms and application to treatment of autoimmune diseases

Oral tolerance is the phenomenon of systemic, antigen specific, immunological hyporesponsiveness that results from oral administration of a protein. The mechanism by which tolerance is generated depends on the amount of antigen administered; low doses favor induction of regulatory T cells while higher doses favor clonal deletion and anergy. The regulatory T cells induced by low doses of oral antigen are triggered by the same antigen to secrete cytokines that suppress, in an antigen nonspecific manner, inflammation in the microenvironment where the triggering antigen is located. This makes possible the targeted delivery of antiinflammatory cytokines to a specific tissue without the requirement for identifying the antigen causing the inflammation. This attribute makes active suppression an attractive mechanism for developing therapies for autoimmune diseases. Orally administered autoantigens have been shown to suppress a wide variety of experimental autoimmune diseases and have recently been applied to the treatment of human autoimmune diseases with promising early results.

Fetal type lymphocytes in insulin dependent diabetes mellitus

CD5+ B and gamma/delta T lymphocytes constituting a major population in the fetus and newborn infant, represent two small subsets of the B and T lymphocyte compartment in healthy adults. There is evidence for their potential involvement and relative expansion in autoimmune disorders. In insulin-dependent diabetes mellitus (IDDM) CD5+ B lymphocyte counts have been found to be increased or normal. The aim of our study was to determine the percentage of both "fetal type" lymphocyte subsets in peripheral blood of 22 recently diagnosed children with IDDM, in that of 13 high risk subjects (islet cell antibody (ICA) positive non-diabetic Ist degree relatives of diabetic children) and in 43 healthy controls. The percentage of gamma/delta TCR+ cells and of CD5+ B lymphocytes was found to be significantly (p < 0.0001 and p = 0.03, respectively) higher in the diabetic and prediabetic groups as compared to controls. Young children with IDDM associated antibodies carry a higher risk of developing clinical IDDM than older individuals. In our hands, the percentage of both CD5+ B and gamma/delta T lymphocytes was higher in the younger population. However, age-dependent decrease for both lymphocyte subpopulations in IDDM-prediabetic patients was less than in healthy controls. Since the above subpopulations are supposed to play a role in immune response to conserved structures, these observations would suggest a higher capacity of older individuals to 'natural autoimmunity" and would explain at least in part the increased risk of antibody positive young children to develop IDDM.

Colitis in transgenic and knockout animals as models of human inflammatory bowel disease

Spontaneous colitis in knockout (KO) and transgenic rodents provides experimental models to study the development of mucosal inflammation and inflammatory bowel disease (Crohn's disease and ulcerative colitis). Genetic and environmental factors, particularly the normal enteric flora, are important factors in the development of mucosal inflammation. The normal mucosal homeostasis is disrupted when there is either cytokine imbalance, abrogation of oral tolerance, alteration of epithelial barrier and function or loss of immunoregulatory cells. Some but not all immunodeficiencies, in the appropriate setting, lead to colitis. CD4+ T cells have been identified as the pathogenic T cells in colitis, which mediate inflammation by either the Th1 or the Th2 pathway. The Th1 pathway dominates most colitis models and in Crohn's disease. In contrast, the colitis in TCR alpha KO mice shares many features of ulcerative colitis including the dominance of Th2 pathway in colonic inflammation. A major benefit of these models is in the development of therapeutic strategies for the treatment of inflammatory bowel disease.

Donor-specific hyporeactivity after liver transplantation: prominent decreases in donor-specific cytotoxic T lymphocyte precursor frequencies independent of changes in helper T lymphocyte precursor frequencies or suppressor cell activity

BACKGROUND

The development of immunological donor-specific hyporeactivity may account for the low incidence of chronic rejection after clinical liver transplantation. We investigated whether hyporeactivity commonly develops after liver transplantation by analyzing precursor frequencies of donor-reactive cytotoxic (CTLp) and helper (HTLp) T lymphocytes and mixed lymphocyte culture (MLC) reactivity in liver allograft recipients. We further studied whether CTLp hyporeactivity correlated with changes in donor-specific HTLp frequencies or suppressor cell activity.

METHODS

CTLp and HTLp frequencies and MLC reactivity against donor and third-party spleen cells were determined in pre- and posttransplantation peripheral blood samples from 18 recipients with good graft function 2 years after transplantation. By mixing posttransplantation samples (with "putative" suppressor cell activity) with pretransplantation samples (in which normal CTL activity with no suppressor cell activity is expected), the presence of suppressor cell activity in peripheral blood was analyzed.

RESULTS

Two years after transplantation, all but one (94%) of the recipients had developed CTLp hyporeactivity as evidenced by reduced donor-specific CTLp frequencies. The development of hyporeactivity was not specific for any particular underlying disease. The occurrence of HTL hyporeactivity, however, was less frequent: 38% and 20% of recipients were HTLp and MLC hyporeactive, respectively. Decreases in CTLp frequencies did not correlate with decreased donor-specific HTL function or suppressor cell activity in peripheral blood samples.

CONCLUSIONS

Donor-specific CTLp hyporeactivity can develop in the majority of liver allograft recipients, irrespective of underlying disease. Donor-specific HTL hyporeactivity, however, occurs infrequently. A reduction in donor-specific CTLp frequencies was found to be independent of changes in donor-specific HTLp or suppressor cell activity, suggesting that other mechanisms (e.g., clonal deletion) are operative in the reduction of donor-specific CTLp after liver transplantation.

Human CD8+ TCR-alpha beta(+) and TCR-gamma delta(+) cells modulate autologous autoreactive neuroantigen-specific CD4+ T-cells by different mechanisms

To investigate the regulatory interactions among autologous T-cells during the course of multiple sclerosis (MS), proteolipid protein peptide-specific CD4+ T-cell clones (TCCs) were irradiated and used as immunogens to stimulate purified populations of autologous CD8+ TCR-alpha beta+ and TCR-gamma delta+ T-cells isolated from the peripheral blood of MS patients, patients with other non-inflammatory neurological diseases, and healthy blood donors. The resulting blasts were expanded in the presence of hIL-2 and then cloned by limiting dilution. Two different groups of CD8+ TCCs were revealed. A first group of CD8+ TCCs recognized autologous CD4+ T-cells based in their TCRV beta structures (anti-idiotypic responsiveness). A second group of CD8+ TCCs recognized Ag activated autologous CD4+ TCCs irrespective of their Ag specificity or TCRV beta expression (anti-ergotypic responsiveness). Both groups showed MHC class I restricted cytotoxicity against CD4+ T-cells and were able to secrete IFN-gamma, TNF alpha/beta and TGF-beta. TCR-gamma delta+ TCCs isolated in response to stimulation with autologous peptide-specific CD4+ TCCs showed only anti-ergotypic cytotoxicity, which was not inhibited by anti-MHC class Ia monoclonal antibodies. Moreover, they were able to secrete IFN-gamma and TNF alpha/beta, but not TGF-beta. These data demonstrate that regulatory mechanisms among human autologous T-cells can be mediated by cytolytic interactions or by the release of specific cytokines. Furthermore, they provide evidence that CD8+ TCR-alpha beta+ and TCR-gamma delta+ cells differ in their patterns of recognition and in their abilities to modulate the immune response mediated by autologous autoreactive CD4+ T-cells.

Enhancement of thyroid allograft survival following organ culture: II. Induction of recipient peripheral tolerance

Hyperbaric oxygen culture (HOC) prolongs endocrine graft survival and decreases major histocompatibility complex (MHC) class I surface expression. If graft prolongation were the result of passenger cell inactivation and decreased class I expression, then simultaneous transplantation of both a nontreated and a HOC-treated graft should result in rejection of the nontreated graft and acceptance of the HOC-treated graft. Simultaneous transplant of a nontreated and a HOC-treated thyroid allograft beneath the kidney capsule of recipient mice resulted in prolonged survival of both grafts in three strain combinations differing at class I (K(K), D(d), D(b)). In vitro analysis of the recipient splenic population revealed the presence of primed donor-specific cytotoxic T cells. These results suggest that recipient tolerance was not because of anergy or clonal deletion. Splenectomy at the time of transplant, revealed that both graft prolongation and the induction of recipient tolerance were dependent on the spleen. Finally, analysis of graft infiltrating cells reveals the presence of CD8+ cells but no CD4+ cells in tolerant recipients, whereas graft infiltrating cells from rejecting recipients contained both populations. The results suggest that active peripheral tolerance can be generated following transplantation of a HOC-treated allograft and that tolerance results from redirection of the recipients immune response.

Premature ovarian failure and ovarian autoimmunity

Premature ovarian failure (POF) is defined as a syndrome characterized by menopause before the age of 40 yr. The patients suffer from anovulation and hypoestrogenism. Approximately 1% of women will experience menopause before the age of 40 yr. POF is a heterogeneous disorder with a multicausal pathogenesis involving chromosomal, genetic, enzymatic, infectious, and iatrogenic causes. There remains, however, a group of POF patients without a known etiology, the so-called "idiopathic" form. An autoimmune etiology is hypothesized for the POF cases with a concomitant Addison's disease and/or oöphoritis. It is concluded in this review that POF in association with adrenal autoimmunity and/or Addison's disease (2-10% of the idiopathic POF patients) is indeed an autoimmune disease. The following evidence warrants this view: 1) The presence of autoantibodies to steroid-producing cells in these patients; 2) The characterization of shared autoantigens between adrenal and ovarian steroid-producing cells; 3) The histological picture of the ovaries of such cases (lymphoplasmacellular infiltrate around steroid-producing cells); 4) The existence of various autoimmune animal models for this syndrome, which underlines the autoimmune nature of the disease. There is some circumstantial evidence for an autoimmune pathogenesis in idiopathic POF patients in the absence of adrenal autoimmunity or Addison's disease. Arguments in support of this are: 1) The presence of cellular immune abnormalities in this POF patient group reminiscent of endocrine autoimmune diseases such as IDDM, Graves' disease, and Addison's disease; 2) The more than normal association with IDDM and myasthenia gravis. Data on the presence of various ovarian autoantibodies and anti-receptor antibodies in these patients are, however, inconclusive and need further evaluation. A strong argument against an autoimmune pathogenesis of POF in these patients is the nearly absent histological confirmation (the presence of an oöphoritis) in these cases (< 3%). However, in animal models using ZP immunization, similar follicular depletion and fibrosis (as in the POF women) can be detected. Accepting the concept that POF is a heterogenous disorder in which some of the idiopathic forms are based on an abnormal self-recognition by the immune system will lead to new approaches in the treatment of infertility of these patients. There are already a few reports on a successful ovulation-inducing treatment of selected POF patients (those with other autoimmune phenomena) with immunomodulating therapies, such as high dosages of corticosteroids (288-292).

Defective post-thymic tolerance mechanisms during the chronic progressive stage of multiple sclerosis

We have recently isolated a panel of T-cell clones from chronic progressive multiple sclerosis (MS) patients that are capable of functioning as antigen-presenting cells and of expressing the costimulatory molecules B7-1 and B7-2. In this report we show that these T-cell clones are resistant to inhibitory regulation, including the induction of anergy and sensitivity to tumor growth factor-beta (TGF-beta)-induced growth inhibition. The resistance to anergy induction was associated with expression of B7 costimulatory molecules. These data suggest that lack of responsiveness to peripheral inhibitory signals may account for the entry of autoimmune diseases into a chronic progressive phase.

Immune responses to self peptides naturally presented by murine class II major histocompatibility complex molecules

Peptides eluted from murine Major Histocompatibility Complex (MHC) class II molecules are predominantly fragments of self proteins, which include apolipoprotein E, cystatin-c, transferrin receptor, MHC class II and Ii chains. These naturally processed self peptides are expected to be presented during ontogeny. Therefore, immune responses to these peptides in syngeneic hosts may be under physiological control so as to modulate auto-reactivity. As would be expected from our current understanding, T cells reactive to such antigens should be deleted or clonally anergized. To explore this possibility, we investigated the immunogenicity of a number of these self peptides in mice that express MHC class II, from which these peptides were eluted. T cell and antibody responses were measured following immunization of mice with the appropriate peptide. Surprisingly, many of these peptides were highly immunogenic in normal mice. T cells reactive to these self peptides are restricted by syngeneic MHC class II and were blocked by alpha CD4 antibodies. T cells primed with the native protein in vivo could be challenged with the appropriate self peptide in vitro. Some of the self epitopes induce Th1 cells as indicated by IFN-gamma but not IL-4 production and others induce Th2 cells. Antipeptide antibodies were detected only at higher doses of antigen. Our results suggest that T cells specific for many of the naturally processed self peptides are not deleted but tolerance to these peptides is still maintained in vivo. Presumably the high-affinity self-reactive T cells are deleted in the thymus and the low-affinity self peptide reactive T cells remain unresponsive to antigen challenge in vitro. Upon antigen priming in vivo, many of these self-reactive T cells become activated and readily respond to antigen challenge in vitro. These results point to the physiological control of the maintenance of tolerance to naturally processed self peptides.

Induction of anergy in Th1 lymphocytes by oral tolerance. Importance of antigen dosage and frequency of feeding

Oral tolerance, a biologically relevant pathway for inducing peripheral tolerance in T lymphocytes, occurs by two distinct mechanisms. Multiple low doses of antigen induce regulatory T lymphocytes that secrete immunosuppressive cytokines, whereas feeding a single high dose of antigen induces anergy of antigen-specific Th1 lymphocytes (diminished IgG2a, IL-2, and IFNgamma) with intact Th2 responses (IgG1 and IL-4). Anergy was demonstrated by the ability to reverse tolerant state after culturing tolerant cells in rIL-2. Reversal of the tolerant state was established in vitro by increase in frequency of IL-2 secreting cells, and in vivo by specific IgG2a production in irradiated mice adoptively transferred with cells cultured in rIL-2. Inasmuch as the induction of anergy was inhibited by the presence of antibodies specific for the tolerizing antigen, it appears that the oral induction of anergy might depend on the systemic dissemination of antigen (or its fragments) absorbed from the gut. It is suggested that tolerance is insured by the fact that this absorbed antigen is presented to Th1 lymphocytes in draining lymph nodes in the absence of inflammatory and costimulatory molecules.

Severe colitis in mice with aberrant thymic selection

Tg epsilon 26 mice display an arrest very early in T cell development that has a profound effect on the architecture of thymic stromal cells. We have recently demonstrated that transplantation of wild-type bone marrow cells restores the thymic microenvironment of fetal but not adult Tg epsilon 26 mice. Here, we report that T cell-reconstituted adult Tg epsilon 26 mice develop a spontaneous wasting syndrome characterized by extensive inflammation of the colon, resembling human ulcerative colitis. Colitis in these animals was marked by substantial infiltration of the colon by activated thymus-derived CD4+ T cells. Importantly, bone marrow-transplanted Tg epsilon 26 mice previously engrafted with a fetal Tg epsilon 26 thymus did not develop colitis. These results suggest that T cells selected in an aberrant thymic microenvironment contain a population of cells able to induce severe colitis that can be prevented by T cells that have undergone normal thymic development.

Hyposensitization in nickel allergic contact dermatitis: clinical and immunologic monitoring

BACKGROUND

In allergic contact dermatitis (ACD) previously sensitized T cells cause skin damage. If an ubiquitous allergen such as nickel is involved, no effective treatment is available. Down-regulation of this allergic response has been described after antigen presentation in the absence of adequate costimulatory signals. UV exposure can enhance such hyposensitization.

OBJECTIVE

The aim of this study was to establish the capability of a hyposensitization procedure to induce antigen-specific tolerance.

METHODS

Twenty-one patients with nickel ACD were randomly assigned to either a hyposensitized or control group. A schedule consisting of UVB treatment and subcutaneous nickel sulfate administration (hyposensitization) or UVB only (control) was applied. During the ensuing 2 years, several clinical and immunologic features were monitored.

RESULTS

During UVB treatment we observed a significant clinical improvement in both groups that persisted in the hyposensitized group. Except for increased slope variances of specific lymphocyte proliferation in time, no clear changes were seen in the immunologic findings.

CONCLUSION

Despite significant clinical improvement induced by UVB, hyposensitization did not induce significant changes in the immunologic findings in patients with nickel ACD.

Biological implications of HLA-DR expression in tumours

HLA-DR antigens show restricted tissue distribution in comparison with the more extensive expression of HLA class I molecules. This constitutive expression is genetically controlled by well-defined mechanisms. In addition, DR antigen expression can be induced by a variety of cytokines through different molecular genetic events that convert DR-negative epithelia into positive cells. In this review we analyse the two major pathological situations in which abnormal DR expression occurs: autoimmune diseases and tumour development. We hypothesize that conversion to DR-positivity may produce two opposite effects in both clinical situations: (1) a useful one in tumours associated with a good prognosis; and (2) a harmful one in autoimmune diseases with increased tissue damage.

Human alveolar macrophages present antigen ineffectively due to defective expression of B7 costimulatory cell surface molecules

Alveolar macrophages, resident phagocytic cells in the lung that derive from peripheral blood monocytes, are paradoxically ineffective in presenting antigen to T cells. We found that antigen presentation by alveolar macrophages could be restored by the addition of anti-CD28 mAb to cultures of T cells and macrophages, indicating that costimulation by alveolar macrophages via the CD28 pathway was defective. In addition, we found that alveolar macrophages activated with IFN-gamma failed to express B7-1 or B7-2 antigens, which normally ligate CD28 on T cells and provide a costimulatory signal required for the activation of T cells. These observations are the first to demonstrate the inability of a "professional" antigen-presenting cell type to effectively express the costimulatory molecules B7-1 and B7-2. Inasmuch as immune reactions within the lung are inevitably associated with inflammatory injury to pulmonary tissue, these observations suggest that reduced expression of B7-1 and B7-2 by alveolar macrophages may be advantageous, as a critical mechanism involved in the induction of peripheral tolerance to the abundance of antigens to which mucosal tissues are continuously exposed.

Multiple ways to cellular immune tolerance

Both central and peripheral pathways leading to T-cell tolerance were discussed at a recent meeting. The mechanisms that maintain self tolerance, as well as the conditions in which self-reactive T cells launch an autoaggressive attack, were specially emphasized.

Tumor immunity as autoimmunity: tumor antigens include normal self proteins which stimulate anergic peripheral T cells

Oncogenic transformation is often accompanied by a significant overproduction of otherwise normal cellular proteins. Because of the low expression of such self components during thymic development, T cells specific for these antigens can escape deletion and progress to the periphery. In this viewpoint, Giorgio Parmiani discusses the concept that these T cells can be harnessed to provide an effective anti-tumor response.

How diverse should the immune system be?

We develop a probability-based model that suggests a novel explanation for the enormous diversity of the lymphocyte repertoire in the immune system. Taking into account the fraction of the repertoire that is expected to be rendered tolerant because of potential self reactivity, we calculated PE, the probability that a pathogen escapes recognition by the immune system. Obviously, for PE to be low the diversity should be large. Our novel result is that the major factor in this relation between diversity and PE is formed by the number of self antigens. Thus our model suggests that, rather than the number of foreign antigens the system is protecting against, the diversity reflects the number of self antigens the immune system needs to avoid reactivity with.