Autoimmunity during thymectomy-induced lymphopenia: role of thymus ablation and initial effector T cell activation timing in nonobese diabetic mice.

Autoimmune diseases develop in selected normal mouse strains when thymectomy (Tx) is performed at 3 days of age (d3-Tx). Insufficient T cell regulation after Tx may result from a defect in regulatory T (Treg) cells or from an augmented effector T (Teff) cell number/pathogenicity. We have previously shown that Tx at 3 wk (wk3-Tx), the age of massive islet Ag release, accelerates diabetes onset. We now have determined diabetes incidence in d3-Tx nonobese diabetic mice and compared the frequency and function of their Teff and Treg cells with those of wk3-Tx mice. We found that d3-Tx had no effect on diabetes incidence, but induced gastritis. After day 3 and week 3 Tx, Treg cells were fully competent and their frequency increased. The number of diabetogenic T cells was greatly amplified after wk3-Tx and likely overcame Treg cell control, leading to an early tolerance breakdown. By contrast, in d3-Tx mice, activation concerned few cells and Teff cell amplification remained controlled. This suggests that Tx enhances autoimmunity when it coincides with the first encounter of autoreactive T cells with their cognate Ag. The relationship between Tx-induced lymphopenia, tissue remodeling, and autoimmunity is discussed.

Lymphopenia-induced spontaneous T-cell proliferation as a cofactor for autoimmune disease development.

Lymphopenia is thought to be a major cause of tolerance breakdown. In a lymphopenic environment, self-recognition events induce some T cells to expand strongly (a mechanism known as spontaneous proliferation). In this study, we show that in C57BL/6 mice, the repertoire resulting from lymphopenia-induced spontaneous CD4(+) T-cell proliferation included a proportion of regulatory T cells as large as that observed in a normal mouse, and no autoimmune disorder was observed. By contrast, in nonobese diabetic mice, differences in the ability of conventional and regulatory T cells to expand in response to lymphopenia led to an unbalance between these 2 T-cell compartments at the expense of regulatory T cells, resulting in the onset of autoimmune diseases. Notably, this accounted for the rapid transfer of diabetes with small numbers of BDC2.5 CD4(+) T cells. Thus, lymphopenia does not itself induce autoimmunity, but it should be considered as a cofactor for the development of autoimmune disorders.

Influence of a non-NK complex region of chromosome 6 on CD4+ invariant NK T cell homeostasis.

The number and function of immunoregulatory invariant NKT (iNKT) cells are genetically controlled. A defect of iNKT cell ontogeny and function has been implicated as one causal factor of NOD mouse susceptibility to type 1 diabetes. Other factors of diabetes susceptibility, such as a decrease of regulatory T cell function or an increase in TLR1 expression, are corrected in diabetes-resistant Idd6 NOD.C3H 6.VIII congenic mice. Thus, we surmised that the iNKT cell defects found in NOD mice may also be rescued in congenic mice. Unexpectedly, we found, in both the thymus and the periphery, a 50% reduction in iNKT cell number in NOD.C3H 6.VIII mice as compared with NOD mice. This reduction only affected CD4(+) iNKT cells, and left the double negative iNKT cells unchanged. In parallel, the production of IL-4 and IFN-gamma following alpha-GalCer stimulation was proportionally reduced. Using three subcongenic strains, we have narrowed down the region controlling iNKT development within Idd6 (5.8 Mb) to Idd6.2 region (2.5 Mb). Idd6 region had no effect on NK cell number and in vivo cytotoxic activity. These results indicate that the role of iNKT cells in diabetes development is equivocal and more complex than initially considered. In addition, they bring strong evidence that the regulation of CD4(+) iNKT cell production is independent from that of DN iNKT cells, and involves genes of the Idd6 locus.

The diabetes type 1 locus Idd6 modulates activity of CD4+CD25+ regulatory T-cells.

The genetic locus Idd6 confers susceptibility to the spontaneous development of type 1 diabetes in the NOD mouse. Our studies on disease resistance of the congenic mouse strain NOD.C3H 6.VIII showed that Idd6 influences T-cell activities in the peripheral immune system and suggest that a major mechanism by which the Idd6 locus modifies diabetes development is via modulation of regulatory T-cell activities. Our transfer experiments using total splenocytes and purified T-cells demonstrated that the locus specifically controls the efficiency of disease protection mediated by the regulatory CD4(+)CD25(+) T-cell subset. Our data also implicate the Idd6 locus in controlling the balance between infiltrating lymphocytes and antigen-presenting cells within the pancreatic islet.

Role of beta-cells in type 1 diabetes pathogenesis.

Whether autoimmunity results primarily from a defect of the immune system, target organ dysfunction, or both remains an open issue in most human autoimmune diseases. The highly multigenic background on which diabetes develops in the NOD mouse and in the human suggests that numerous gene variants associate in contributing to activation of autoimmunity to beta-cells. Both immune genes and islet-related genes are involved. The presence of beta-cells is required for initiation of diabetes autoimmunity to proceed. Available experiments in the NOD mouse and epidemiological evidence in the human point to proinsulin as a key autoantigen in diabetes. The functional importance of insulin, the high number of autoantigens characterized at different stages of diabetes, and their clustering within beta-cell subparticles point to the islet as a starting point in the initiation phase of the disease. Genes that direct the autoimmune reaction toward the beta-cell target, autoantigens that are recognized by autoreactive B- and T-cells along the autoimmune process, the importance of beta-cells in the activation of autoreactive lymphocytes, and the expression level of key beta-cell molecules along diabetes development are successively considered in this review.

Invariant NKT cells exacerbate type 1 diabetes induced by CD8 T cells.

Invariant NKT (iNKT) cells have been implicated in the regulation of autoimmune diseases. In several models of type 1 diabetes, increasing the number of iNKT cells prevents the development of disease. Because CD8 T cells play a crucial role in the pathogenesis of diabetes, we have investigated the influence of iNKT cells on diabetogenic CD8 T cells. In the present study, type 1 diabetes was induced by the transfer of CD8 T cells specific for the influenza virus hemagglutinin into recipient mice expressing the hemagglutinin Ag specifically in their beta pancreatic cells. In contrast to previous reports, high frequency of iNKT cells promoted severe insulitis and exacerbated diabetes. Analysis of diabetogenic CD8 T cells showed that iNKT cells enhance their activation, their expansion, and their differentiation into effector cells producing IFN-gamma. This first analysis of the influence of iNKT cells on diabetogenic CD8 T cells reveals that iNKT cells not only fail to regulate but in fact exacerbate the development of diabetes. Thus, iNKT cells can induce opposing effects dependent on the model of type 1 diabetes that is being studied. This prodiabetogenic capacity of iNKT cells should be taken into consideration when developing therapeutic approaches based on iNKT cell manipulation.

Impaired migration of NOD mouse thymocytes: a fibronectin receptor-related defect.

We previously showed intrathymic alterations in non-obese diabetic (NOD) mice, including the appearance of giant perivascular spaces, filled with mature thymocytes, intermingled with an extracellular matrix network. This raised the hypothesis of a defect in thymocyte migration with partial arrest of exiting thymocytes in the perivascular spaces. Herein, we investigated the expression of receptors for fibronectin [very late antigen (VLA)-4 and VLA-5] and laminin (VLA-6), known to play a role in thymocyte migration. When compared with two normal and one other autoimmune mouse strains, a decrease of VLA-5 expression in NOD thymocytes was noticed, being firstly observed in late CD4/CD8 double-negative cells, and more pronounced in mature CD4(+) and CD8(+) thymocytes. Functionally, thymocyte exit from the lymphoepithelial complexes, the thymic nurse cells, was reduced. Moreover, NOD thymocyte adhesion to thymic epithelial cells as well as to fibronectin was diminished, and so was the migration of NOD thymocytes through fibronectin-containing transwell chambers. In situ, intra-perivascular space thymocytes were VLA-5-negative, suggesting a correlation between the thymocyte arrest within these structures and loss of VLA-5 expression. Overall, our data reveal impairment in NOD thymocyte migration, and correspond to the first demonstration of a functional fibronectin receptor defect in the immune system.

Pancreatic lymph nodes are required for priming of beta cell reactive T cells in NOD mice.

Nonobese diabetic (NOD) mice develop spontaneous autoimmune diabetes that results from the destruction of insulin secreting beta cells by diabetogenic T cells. The time and location of the encounter of autoantigen(s) by naive autoreactive T cells in normal NOD mice are still elusive. To address these issues, we analyzed diabetes development in mice whose spleen or pancreatic lymph nodes (panLNs) had been removed. Excision of panLNs (panLNx) at 3 wk protected mice against insulin autoantibodies (IAAs), insulitis, and diabetes development almost completely, but had no effect when performed at 10 wk. The protection afforded by panLNx at weaning was not due to modifications of the immune system, the absence of autoreactive T cells, or the increase in the potency of regulatory T cells. That panLNs are dispensable during adult life was confirmed by the capacity of 10-wk-old panLNx irradiated recipients to develop diabetes upon transfer of diabetogenic T cells. In contrast, splenectomy had no effect at any age. Partial excision of mesenteric LN at 3 wk did not prevent accelerated diabetes by cyclophosphamide as panLNx did. Thus, in normal NOD mice, autoreactive T cell initial priming occurs in LNs draining the target organ of the disease from 3 wk of age.