Gene therapy for diabetes

Type 1 diabetes mellitus (T1DM) occurs as a result of T-cell-mediated destruction of the insulin-producing β-cells in the pancreas. Non-obese diabetic (NOD) mice, which spontaneously develop diabetes, are a useful model system of this disease. In this study, the authors show that expression of diabetes-resistant MHC class II alleles by NOD mice — using retroviral transduction of autologous bone-marrow cells — is sufficient to prevent the development of diabetes.

In humans, the development of T1DM is associated with the inheritance of particular MHC class II alleles that lack a charged amino acid at position 57 of the β-chain. In NOD mice, the single MHC class II allele present is I–A^g7, which also lacks a charged residue at position 57. It is thought that lack of a charged residue at this position prevents the formation of a salt bridge between the α- and β-chains of the MHC class II molecule, which could affect the ability of these molecules to mediate negative selection of autoreactive T cells.

Previous studies using transgenic mice and allogeneic bone-marrow chimeras have shown that it is possible to prevent diabetes in NOD mice, but it has not been possible to determine the preventative mechanism.

Retroviral constructs with green fluorescent protein fused to the cytoplasmic tails of diabetes-resistant I–A β-chains (which have a charged residue at position 57) were shown to be expressed at the cell surface of MHC-class-II-positive cells and could pair with endogenous α-chains. Young NOD mice were lethally irradiated and transplanted with bone marrow retrovirally transduced with the I–Aβ constructs, and their blood-glucose levels were monitored each week. All of the mice were protected from diabetes, compared with four of six mice that received bone marrow transfected with a control construct. Even when diabetes was aggressively induced using cyclophosphamide, the mice that received I–Aβ-transduced bone marrow were resistant to the development of diabetes for up to 46 weeks after transfer, and the level of insulitis was reduced.

To address the mechanism of resistance, the authors used enzyme-linked immunosorbent spot (ELISPOT) assays to look at the frequency of T cells that produced cytokines in response to peptide 206–220 from glutamic-acid decarboxylase, an immunodominant self-antigen peptide in NOD mice. No cytokine production was detectable in NOD mice that received I-Aβ-transduced bone marrow, indicating that self-reactive T cells had been functionally inactivated or eliminated. To determine whether this was owing to deletion of self-reactive T cells in the thymus or to peripheral tolerance, the authors used I–A^g7 tetramers loaded with a peptide known to stimulate pancreatic-islet-reactive T-cell clones. The frequency of CD4⁺ T cells labelled with the tetramer was significantly reduced among thymocytes from NOD mice reconstituted with I–Aβ-transduced cells compared with NOD mice that received cells transduced with the control construct, supporting the idea that I–Aβ mediated the removal of self-reactive T cells by negative selection.

This study offers the prospect that T1DM could be prevented by providing susceptible individuals with protective MHC class II alleles, using autologous bone marrow. This approach would be preferable to the use of allogeneic cells, because graft-versus-host disease would be avoided and it might be possible to use milder conditioning regimens before transplantation.