Figure 1. The circle of tolerance. Tolerance denotes the absence of a detectable, functional immune response in the absence of immunosuppression. It is the loss of immunological tolerance that leads to autoimmunity,^[61] specifically the autoimmune destruction of β-cells in type 1 diabetes. If we interfere with T-cell function after the loss of tolerance (striped arrow), as we can with immunosuppressive drugs like cyclosporine, we can stop the disease. Better than that would be stopping the loss of tolerance (dotted arrow), thereby preventing the disease. And what of the people who already have diabetes? For them we need a separate but related strategy—tolerance induction.^[10] If we can induce islet transplantation tolerance, we may be able to cure the disease.

Figure 2. The balance hypothesis of autoimmune diabetes developed in studies of BBDP and BBDR rats. The lower half of the figure depicts the imbalance between regulatory (R) and autoreactive (A) cell populations associated with insulitis and the destruction of pancreatic β-cells. The upper half depicts these forces in a state of equilibrium. Both BBDP and BBDR rats share a genetic susceptibility to autoimmunity based on the presence of the RT1^u MHC haplotype and the Iddm4 gene(s).^[62] In the case of the BBDP rat, on the left, the presence of an additional mutation in the Ian4L1 gene causes lymphopenia and an imbalance between autoreactive and regulatory cell populations, leading to diabetes. Restorative transfusions of MHC-compatible lymphocytes restore immunological balance and prevent diabetes. In the case of the BBDR rat, on the right, autoreactive cells are present but do not lead to the expression of diabetes unless the balance is perturbed. Perturbants that lead to diabetes include regulatory cell depletion with anti-ART2.1 mAb; infection with Kilham rat virus, which can decrease regulatory cell numbers; and injection of the TLR3 ligand poly I:C, which may alter the immunological balance by activating APCs.^[13]

Figure 3. The basic principles of T-cell activation in response to the presentation of alloantigen and tolerance induction by costimulation blockade. A: The process of alloimmune response occurs in three steps. The first is alloantigen recognition: the engagement of the T-cell receptor (TCR) by the MHC-antigen complex on an APC. The antigenic peptide is represented by the black object between the MHC and TCR. Engagement of the TCR then induces CD154 expression on the responding T-cell. This second step, the induction of CD154, permits engagement of CD40 on the APC. Engagement of CD40, in turn, upregulates the costimulatory molecules B7-1 and B7-2 (step 3). As a consequence of increased expression of B7-1/2, the APC reciprocally triggers the T-cell via CD28, a process termed costimulation.^[63] The activated allospecific T-cell now becomes an effector cell capable of mediating allograft rejection. B: Tolerance induction by costimulation blockade.^[10] Donor origin cells are used as nonprofessional APCs to engage the TcR of alloantigen-specific T-cells. Anti-CD154 mAb prevents engagement of CD40 by CD154 on APCs, thereby preventing the upregulation of B7 costimulatory molecules. This hypothesis predicts allospecific tolerance induction, and permanent islet graft survival should be achieved by the coadministration of a DST and anti-CD154 mAb.

Figure 4.  A: Experimental protocol for inducing islet allograft tolerance.^[64] Recipients are C57BL/6 mice (H2^b) treated with streptozotocin (STZ) to induce diabetes. BALB/c mice (H2^d) provide spleen cells for a DST (DST) (10 × 10⁶ spleen cells) on day -7 and an islet graft on day 0. Injections of the hamster anti-mouse MR1 anti-CD154 mAb (0.25 mg per dose) are given immediately before the DST and then on days -3, 0, and +4. No other therapy is given. B: Life table showing duration of BALB/c islet allograft survival in C57BL/6 mice treated with DST and anti-CD154 mAb (solid line) or no treatment (dotted lines). Data represent a composite of several studies performed in our laboratory.

Figure 5. Stages of transplantation tolerance induction using DST and anti-CD154 mAb.^[10] Induction with DST and anti-CD154 mAb leads to the deletion of alloreactive T-cells. The process depends on the presence of CD4-positive T-cells, cells that express the costimulatory molecule CTLA4, and the cytokine interferon-γ.^[55] After transplantation, the graft goes through a transition during which the alloreactive (A) and regulatory (R) forces come into balance. Permanence of grafts, we hypothesize, depends on an equilibrium that stably favors regulation over alloreactivity.