Type 1 Diabetes: Cellular, Molecular & Clinical Immunology
Theoretical Essay C - The New Math of Diabetes: When Th1-Th2=Th0
Donald Bellgrau and Daniel P. Gold
Thymus-derived T lymphocytes are required for diabetes mellitus
to occur; elimination of antibody production does not protect against disease,
indicating that T, not B, lymphocytes are the major players in the immune-mediated
destruction of insulin-producing beta cells. How T cells contribute to diseases
is not known, but experimental evidence indicates that there may be a "regulatory"
T cell defect. A consequence of such a defect is that an "unregulated"
immune response is free to destroy insulin-producing islet beta cells.
Two important questions to be answered are (1) what is the nature of the regulatory defect and (2) what are the characteristics of the cells normally under regulation, ie, the autoreactive cells? One view is that the regulators and regulated populations are encompassed in two subsets of peripheral T cells termed Th1 and Th2. Although it is an oversimplification to ascribe homogeneous characteristics to Th1 and Th2 populations, a generic view is that Th1 T cells are involved in "cell-mediated immune responses" associated with IL-2 and interferon gamma production while Th2 cells drive the humoral response associated with IL-4 production. A popular view is that a Th1/Th2 imbalance alters the immune homeostasis of protective versus inductive lymphokines. More specifically, in autoimmune diabetes it may be that a Th2 deficit can shift the immune response heavily toward the Th1 subset.1 The inference is that it is the Th2 regulators that control the Th1 mediators of autoreactivity. Therefore at least one diabetogenic gene should influence Th2 function. Indeed, in our studies of the diabetes prone BB rat (BB-DP) we have obtained experimental evidence indicating that the BB-DP regulatory deficit occurs as a consequence of diminished numbers of the Th2 T cells and that this characteristic is linked to the inheritance of a known diabetogenic gene.2
Is a shift in the Th1/Th2 ratio enough to cause disease? Because diabetic susceptibility is polygenic the answer is almost certainly no. In fact, genetic studies in the rat, where the gene responsible for diminished Th2 T cells is fixed our other genetic backgrounds, indicate that this diabetes susceptibility gene is necessary but not sufficient for disease.3 A second candidate for a genetic influence is the effector population. As mentioned above, the logical extension of the Th1/Th2 "imbalance leading to autoimmunity" paradigm is to propose that the Th1 subset contains the factors of beta cell destruction. However, experimental evidence does not fit easily with ascribing beta cell destruction to Th1 T cells. First, injection of interleukin-2 (IL-2), a dominant cytokine of this subset, does not exacerbate diabetic disease. In fact, injecting diabetes-prone animals with IL-2 actually prevents diabetes. Second, in both human and murine models of diabetes areas there a deficit in IL-2 production associated with disease-prone individuals.4-6 Therefore, while we agree that impairing the Th2 arm of the immune response is a predisposing factor in diabetes we do not believe that the effector cells that benefit from the release from regulation are Th1 cells.
If not Th1, then what? The autoreactivity in the BB-DP is associated with a broad cytokine profile high in both IL-4 and interferon gamma, ruling against Th2, and low levels of IL-2, ruling against Th1. Cells surface phenotype analyses of the autoreactive subset indicate that the effectors are more juvenile/immature then peripheral Th1 and Th2 T cells.7 This population proliferates to self antigen in vitro and expresses a limited T cell receptor Vbeta chain family usage.8 Experiments in the mouse have ascribed these properties to a medullary thymocyte subset termed Thy0 for thymic Th0.9 Th0 are thought to be the peripheral precursors of the Th1 and Th2 subsets.10
Although no information exists on the way in which mouse Thy0 retain their autoreactivity, our data on what we believe is the Thy0 equivalent in the rat provide some clues. In the BB-DP, the effector subset appears to be unusually resistant to apoptosis,11 defined by crosslinking CD4 followed by direct stimulation of the T cell receptor. Normally this CD4 crosslinking protocol induces apoptosis/programmed cell death in greater than 95% of CD4 + peripheral T cells from normal rats. The BB-DP effector population is also resistant to anergy induction following presentation with an exogenous source of superantigen.12 Anergy in this setting is defined as an inability to proliferate in vitro to an antigen to which the T cells were previously exposed in vivo. In total we interpret the resistance to apoptosis and anergy as characteristics of T cells that escape the selection process that normally deletes autoreactive T cells. Whether other diabetes susceptibility genes impart these resistance characteristics or they are characteristics of Thy0 in general is yet to be determined. However we favor the hypothesis that the enhanced resistance to tolerizing signals is driven by a diabetogenic gene.
In summary we believe that there is a deficit in the Th2 T cell subset that is created by a diabetogenic gene. This regulatory defect uncovers an autoreactive T cell subset that is not a Th1 but is rather a Th0, most probably a thymic Th0. This autoreactive population, released from regulation by the deficit in Th2 function, is the diabetogenic effector population. Whether it is intrinsically autoreactive or is influenced by other diabetogenic genes is not known. Besides overt autoreactivity, this population may be characterized as expressing limited TCR Vbeta chain usage, possibly associated with the nature of the thymic selecting antigens, and resistance to tolerizing signs that likely permit escape from the normal negative selection process involved in the deletion of overtly autoreactive thymocytes. The resistance to tolerizing signals manifested in the resistance to apoptotic death and resistance to anergy may be imparted by a diabetogenic gene distinct from the one that controls the regulatory T cell deficit.
Reference List - links to PubMed available in Reference List.
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