Marshall Plaut, Howard B. Dickler, and Daniel Rotrosen Division of Allergy, Immunology and Transplantation, National Institute of Allergy and Infectious Diseases, National Institutes of Health, Bethesda, Maryland
Background
Vaccines are the most effective medical intervention for preventing infectious diseases. Originally, the definition of vaccine was restricted to vaccinia preparations used for immunization. Over time, the term has evolved to include all preparations used to generate protective immunity to microbial pathogens or their toxins. More recently, the definition of vaccine has been further expanded to include antigenic materials used to tolerize-or turn off- antigen-specific immune responses to prevent or treat immune-mediated diseases. The following discussion highlights the status of recent efforts to develop tolerogenic vaccines for immune-mediated diseases and identifies promising areas of further research.
Autoimmune diseases affect nearly 5 percent of adults in North America and Europe, and allergic diseases affect more than 15 percent of children and adults. These chronic relapsing disorders affect multiple organ systems and are characterized by substantial morbidity and mortality and by their high social and economic costs. Vaccines that could prevent or treat these conditions would be an important addition to existing therapies.
The properties of immune tolerance and the underlying mechanisms have not been clearly defined, but the following are important considerations: (1) tolerance refers to a selective inability to respond to antigens-this can be a learned phenomenon or due to immunological ignorance; (2) both foreign and self-antigens can be targets of tolerogenic processes; (3) although tolerance can be mediated by suppressor cells, tolerance is not the same as immune suppression, either mechanistically or clinically; (4) tolerance can be maintained by active or passive processes and can result from cell inactivation, altered function, or death; and (5) tolerance can be induced centrally (in the thymus) or peripherally.
The activation of T cells is central to virtually all autoimmune events. Tissue injury in autoimmune diseases is mediated by cytotoxic T cells, neutrophils, and macrophages, or through the actions of autoantibodies and complement. T cells play a central role in all of these event, by providing "help" for antibody production and by producing secretory products that activate and recruit phagocytic cells to sites of inflammation.
A variety of approaches are being pursued to induce T cell tolerance. These include blocking the activation of T cells by antigen presenting cells, focusing on the interactions of the T cell receptor (TCR) with peptides presented by the major histocompatibility complex (MHC). Other strategies target costimulatory pathways in T cells, or the interactions of cell surface adhesion molecules and their counter-ligands. Some of these experimental therapies are now being developed as candidate vaccines.
Mechanisms of Tolerance
Immune responses in the gastrointestinal tract are self-limited, and repeated challenge with certain antigens results in a diminished response. Oral administration of both high- and low-dose antigen results in a phenomenon termed "oral tolerance," in which the immune response to subsequent systemic administration of antigen is blocked. At least two mechanisms appear to be important. Tolerance to high-dose antigen appears to be via inactivation or clonal deletion to Th1 and Th2 cells. In contrast, tolerance to low-dose antigen is really "bystander" immune suppression mediated by stimulation of Th2- and Th3 type cytokines, with TGF-? being the major suppressive cytokine in various model systems. Other routes of mucosal tolerance have recently been explored, including immunization via the nasal and respiratory mucosa. These routes appear to be equally or more efficient in inducing immune tolerance in animal models. By eliminating enzymatic degradation in the gastrointestinal tract, nonoral routes of immunization offer the theoretical advantage that lower doses of antigen will be needed.
In human autoimmune diseases there are often reactivities to multiple autoantigens in target organs. This intra- and interantigenic spread-"epitope spreading"-is a characteristic of chronic inflammation in autoimmune disorders. Bystander suppression refers to the generation of regulatory cells (first demonstrated after oral administration of myelin basic protein) that nonspecifically suppress inflammation in the target organs where the fed antigen in present. Thus, it may be possible to design vaccines that prevent organ injury even in those diseases where all of the relevant autoantigens are not known. As such, bystander suppression solves a major conceptual problem in developing vaccine-based therapies for many autoimmune diseases.
Different mechanisms appear to be important in other model systems. For example, T cell inactivation results when antigen is presented to the T cell, by costimulatory signals, also called second signals, are blocked. T cells made tolerant in this way will not respond when re-presented with the same antigen, even in the absence of costimulatory blockade. Generation of costimulatory signals depends on cognate interactions between B7 molecules and CD40 on antigen-presenting cells and their counter-receptors, CD28 and CD40 ligand, on the surface of T cells. Numerous approaches to block these interactions are now being explored.
In certain model systems, differences in Th1 and Th2 responsiveness correlate with the development of autoimmune (Th1-predominant) and allergic (Th2 predominant) reactivity. In addition, immune deviation from a Th2 to a Th1 predominance also creates a barrier to long-term allograft acceptance. In humans there appears to be greater plasticity of T cells responses, as opposed to the more polarized Th1/Th2 demarcation seen in the mouse. Nonetheless, in addition to vaccines that induce global tolerance, vaccines that could not direct immune responses toward a Th2 predominance might be of value in autoimmune diseases.
Clinical Opportunities and Clinical Trials
Disease prevention should be the goal when at-risk individuals can be identified before significant injury has occurred (e.g., to prevent transplant rejection or for immune-mediated diabetes mellitus before destruction of insulin-producing islets). In other settings (e.g., rheumatoid arthritis), clinical improvement might still be possible even though at-risk individuals cannot be identified before the onset of disease.
The approach that has been most widely used is to tolerize pathogenic T cells by oral or systemic administration of the target antigen. the identification of disease-related antigens is a prerequisite for such an approach. The importance of certain autoantigens is clear because the presence of antigen- specific antibodies or T cells correlates with disease activity, and the disease can be mimicked in animal models via adoptive transfer of autoantibody or autoreactive T cells.
Examples include insulin and the enzyme glutamic acid decarboxylase (GAD) in immune-mediated diabetes mellitus, myelin basic protein (MBP) in multiple sclerosis (MS), and the keratinocyte cell adhesion molecule desmoglein 3 in pemphigus vulgaris. The evidence pointing to MDP as a target antigen in MS is substantial:
1. MBP causes an MS-like illness when injected into animals;
2. MBP-specific cells are found at increased frequency among the circulating activated T cells of MS patients;
3. MBP-specific T cells have been localized to lesional sites in the brain; and
4. an MS-like disease can be induced in immunodeficient mice (which do not reject human cells) by MBP-reactive T cells from the cerebrospinal fluid of patients with MS.
Self-antigens have been administered as tolerogenic vaccines in animal models of human autoimmune diseases including rheumatoid arthritis, insulin-dependent diabetes mellitus, multiple sclerosis, and uveitis. In most of these studies, nonresponsiveness was induced by administration of the self-antigen via the oral route. These approaches have been successful for treatment and prevention in animal models. However, the observed responses have generally been less than complete, characterized largely by delays in onset of disease or reductions in disease severity.
Based on promising results in animal models, pilot open trials or blinded randomized studies have been initiated in insulin-dependent diabetes mellitus (oral administration of insulin), multiple sclerosis (oral administration of bovine myelin), rheumatoid arthritis (oral administration of chicken type II collagen), and uveitis (oral administration of bovine retinal S antigen). Other trials are in progress or are planned. Taken as a group, these trials have shown safety with relatively few side effects. Clinical improvements have been seen in some studies, but enthusiasm has also been tempered by recent failures.
In contrast, the goal of immunotherapy for atopic diseases is to drive immune responses in the opposite direction-toward a Th1 predominance. Standard allergen immunotherapy may do this but probably with limited efficacy. One recently developed approach involves vaccination with DNA. Plasmid DNA contains short immunostimulatory sequences that specifically promote Th1 responses to the recombinant proteins encoded by these vaccines. DNA vaccines encoding a variety of natural and "model" allergens have now been tested in animal models. In a murine model of allergic asthma, DNA vaccination leads to allergen-specific reductions in IgE, induction of allergen-specific IgG "blocking" antibodies, and a decrease in allergen-induced bronchial hyperreactivity. It is likely that clinical trials with DNA vaccines will ba launched in the near future.
Another very promising approach involves immunization with short linear peptides representing the major T cells epitopes of common allergens. This approach has recently demonstrated efficacy and eliminates the major complication of standard immunotherapy, i.e., the risk of life-threatening anaphylaxis triggered by preexisting IgE (on mast cells and basophils) that is directed to epitopes of "whole" allergen.
Standard allergen immunotherapy not only causes modest reductions in interleukin-4 production by allergen-specific T cells, but also leads to production of IgG "blocking" antibodies. The role of these IgG antibodies in altering allergic responses has long remained unclear. The actions of IgG blocking antibodies may be explained by the discovery of a negative signaling pathway in mast cells and basophils. This pathway is activated by immune complexes that cross-link mast cell and basophil receptors for IgG. Turning on this negative pathway blocks the activation of these cells that would otherwise occur with cross-linking of the receptors for IgE. Novel vaccines could be developed to target more effectively this important pathway.
Future Directions
Antigen-specific immune tolerance can be induced in animal models and in human autoimmune diseases. However, many important questions remain to be answered. Key areas for future investigation include (1) further identification of disease-specific autoantigen; (2) studies to optimize vaccine delivery and antigen processing for tolerance induction; (3) characterization of costimulatory pathways and identification of new approaches for their inhibition; (4) studies of the role of the cytokine milieu tin tolerance induction; and (5) development of gene transfer-based approaches for tolerance induction. With advances in these and other areas it is likely that today promising leads can be developed into effective vaccines.
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