NIAID Plan for Research on Immune Tolerance
PART 2: Background

Tolerance is defined here as a selective block in the immune response to particular antigens. Methods of tolerance induction include: 1) the deletion or specific inactivation (anergy) of antigen-reactive lymphocytes; 2) altering profiles of cytokine secretion to prevent inflammation and injury; and 3) preferential induction of regulatory T cells or cytokines that inhibit destructive T cell functions. In animal models, these approaches have resulted in long-lasting, antigen-specific nonresponsiveness.

A solid experimental foundation already exists and many unique reagents are now available to support translational research and pursue promising clinical applications. The potential impact for human health is great, encompassing all allergic and immune-mediated diseases, allograft rejection, graft-versus-host disease, and responses to "neoantigens" introduced via gene therapy.

Major Scientific Approaches to Immune Tolerance

1) Costimulator-Deficient Antigen Recognition

A major area of interest is the induction of tolerance by preventing costimulatory second signals. It is now well established that lymphocyte activation requires two signaling events. Signal-one is triggered by antigen presentation through the T cell receptor (TCR) and signal-two involves cognate interactions between costimulatory molecules on antigen presenting cells and lymphocytes. Propagation of signal-one in the absence of signal-two inactivates or tolerizes the responding lymphocyte. Second-signal blockade ultimately results in either death or anergy of the responding cell.

For T cells, blockade can be accomplished by antibodies or engineered soluble receptors that bind to costimulatory ligands to prevent normal T cell activation and divert the response to tolerance. The major costimulatory signal studied thus far is initiated by CD28:B7 interactions, which can be prevented by antibodies to B7 or by a soluble form of the CTLA4 receptor protein (CTLA4-Ig) that binds B7 with high affinity. Other costimulatory targets include a T cell molecule called CD40-ligand (CD40-L), which normally induces B7 expression by binding CD40 on antigen-presenting cells. Both CTLA4-Ig and anti-CD40-L antibodies were found to induce long-term tolerance when given during kidney transplantation in mice and in rhesus monkeys, and to induce tolerance in some murine models of autoimmune disease. The great advantage of this approach is that CTLA4-Ig or anti-CD40-L antibody treatment is required only during the initial exposure to antigen. Thereafter, grafts are retained or autoimmune disease is prevented in most cases without subsequent treatment of any kind. In those cases where transplant tolerance ultimately breaks down, a "rejection-free" state has been restored by a second course of CTLA4-Ig or anti-CD40-L antibody.

Additional agents should also be tested, such as those that target the costimulatory molecules, CD2, CD30 or 4-1BB, target adhesion molecules such as ICAM or VLA, or interrupt intracellular signaling pathways necessary for T cell activation, such as ZAP-70. Combinations of these reagents may also be useful to induce more robust and durable tolerance.

2) Deletion of Responding Lymphocytes

Further development is needed for approaches that induce lymphocyte death upon antigen recognition. Although much has been learned about activation-induced cell death, tolerance due to cell death has been verified in only a few systems in vivo. Given the recent explosion of information on the molecules and signaling pathways responsible for the induction or prevention of apoptosis, it is likely that methods can be developed to specifically target disease-associated lymphocytes for death via antigen recognition. Examples of this approach include expression of recombinant Fas-ligand on tissue to kill infiltrating T cells and the use of anti-IL-2 receptor antibodies to eliminate activated T cells.

3) Cytokine Modulation

A third major focus is cytokine modulation to prevent destructive immune responses. In general, cytokines that promote inflammatory conditions, such as IFNß, TNFß and LT, will promote graft rejection and T cell-mediated autoimmune attacks, whereas those that are anti-inflammatory, such as IL-10 and TGFß, will promote autoantibody production, but may alleviate graft rejection and suppress inflammatory autoimmune attack. The mechanisms responsible for the destructive immune response must be determined before cytokine modulation is applicable, and the feasibility, safety and efficacy of cytokine modulation in man must be assessed. Important issues include systemic versus local cytokine delivery, synergistic or antagonistic effects, and the possibility of replacing one type of immune response with another type that is still destructive.

4) Other Approaches

Additional approaches include a focus on the migration of activated lymphocytes which might be blocked to prevent tissue damage, on anti-CD3 antibody or other reagents that are coupled with toxins to selectively destroy activated T or B cells, and on molecularly engineered tissues that would delete or inactivate tissue-infiltrating lymphocytes. Peptide-based therapies might be effective when the specific antigens are known, since altered peptide analogues might induce tolerance. Furthermore, peptides derived from T cell antigen receptors (TCR) have been shown in some systems to induce regulatory T cells that recognize and inhibit the TCR+ disease-mediating effector cells.

Ethical Issues in the Design, Conduct and Monitoring of Clinical Trials of Immune Tolerance

Transplantation is now routine therapy for end-stage renal disease, with one-year graft survival approaching (85% using current immunosuppressive therapy. However, long-term graft survival has not improved appreciably since the early 1980s, with approximately 45% of cadaveric kidneys surviving at 10 years post-transplant. For other organs (e.g., liver, lung, and pancreas), graft survival does not approach this level of success. While new immunosuppressive drugs are reducing acute rejection in the first year post-transplant, it is increasingly clear that these therapeutic improvements will not significantly alter long-term clinical outcomes. Therefore, much recent attention has focused on the potential for the induction of donor-specific immune tolerance to achieve long-term graft survival without the need for non-specific immunosuppressive therapy.

A major ethical dilemma in moving forward with clinical trials for the induction of transplant tolerance results from the growing body of knowledge that standard immunosuppressive therapy blocks intracellular signals necessary for at least some types of tolerance induction. Therefore, evaluating the safety and efficacy of tolerogenic approaches will require withholding standard immunosuppressive therapy. Although certain promising tolerogenic molecules are being tested in humans for the treatment of some autoimmune diseases, and have been shown to induce donor-specific tolerance in rodent and non-human primate transplant models, these approaches have not been evaluated in transplantation clinical trials.

In April 1998, the NIAID convened an expert panel to begin developing guidelines for the design, conduct and monitoring of scientifically and ethically acceptable clinical trials of the safety and efficacy of new tolerance induction approaches in transplant recipients. A group of experts in bioethics, law and basic and clinical research in transplantation joined NIH staff and representatives of the Food and Drug Administration and the NIH Office of Protection from Research Risks. The recommendations of this expert panel will be incorporated into NIAID-sponsored clinical trials of tolerance induction in the transplant setting.