Jacques F. A. P. Miller

The CD8alpha(+) dendritic cell is responsible for inducing peripheral self-tolerance to tissue-associated antigens.

We previously described a mechanism for the maintenance of peripheral self-tolerance. This involves the cross-presentation of tissue-associated antigens by a bone marrow–derived cell type that stimulates the proliferation and ultimate deletion of self-reactive CD8 T cells. This process has been referred to as cross-tolerance. Here, we characterize the elusive cell type responsible for inducing cross-tolerance as a CD8α+ dendritic cell (DC). To achieve this aim, transgenic mice were generated expressing yellow fluorescent protein (YFP) linked to CTL epitopes for ovalbumin and glycoprotein B (gB) of herpes simplex virus under the rat insulin promoter (RIP). Although tracking of YFP was inconclusive, the use of a highly sensitive gB-specific hybridoma that produced β-galactosidase on encounter with antigen, enabled detection of antigen presentation by cells isolated from the pancreatic lymph node. This showed that a CD11c+CD8α+ cell was responsible for cross-tolerance, the same DC subset as previously implicated in cross-priming. These data indicate that CD8α+ DCs play a critical role in both tolerance and immunity to cell-associated antigens, providing a potential mechanism by which cytotoxic T lymphocyte can be immunized to viral antigens while maintaining tolerance to self.

Effect of Neonatal Thymectomy on the Immunological Responsiveness of the Mouse

The effect of thymectomy on the lymphocyte population and immune response of C3H, (Ak ×T6)F1 and C57BL mice has been investigated. Thymectomy performed in the immediate neonatal period was associated with severe depletion in the lymphocyte population and serious impairment of the immune response of the mature animal to Salmonella typhi H antigen and to allogeneic and heterospecific skin grafts. Clinically, the mice appeared healthy until about 2 to 4 months of age when two-thirds of the animals died from a syndrome characterized by progressive wasting and diarrhoea. Thymectomy in infancy was still associated with some impairment of the immune response to skin homografts particularly when donor and hosts were closely related immunogenetically. Thymectomy after 3 weeks of age was not associated with any significant impairment of homograft immunity. Neonatally thymectomized mice subsequently grafted with thymus tissue were capable of rejecting allogeneic skin grafts and showed evidence of immunity to such grafts. The lymphoid tissue of the thymus-grafted mice appeared normal and was shown to contain cells that had been derived from the thymus graft. It is concluded that, during very early life, the thymus produces the progenitors of immunologically competent cells which mature and migrate to other sites. Present evidence does not, however, exclude the production by the young thymus of a humoral factor necessary to the maturation or proliferation of lymphocytes elsewhere in the body.

Peripheral Deletion of Autoreactive CD8 T Cells by Cross Presentation of Self-Antigen Occurs by a Bcl-2–inhibitable Pathway Mediated by Bim

By transgenic expression of ovalbumin (OVA) as a model self antigen in the β cells of the pancreas, we have shown that self tolerance can be maintained by the cross-presentation of this antigen on dendritic cells in the draining lymph nodes. Such cross-presentation causes initial activation of OVA-specific CD8 T cells, which proliferate but are ultimately deleted; a process referred to as cross-tolerance. Here, we investigated the molecular basis of cross-tolerance. Deletion of CD8 T cells was prevented by overexpression of Bcl-2, indicating that cross-tolerance was mediated by a Bcl-2 inhibitable pathway. Recently, Bim, a pro-apoptotic Bcl-2 family member whose function can be inhibited by Bcl-2, was found to play a critical role in the deletion of autoreactive thymocytes, leading us to examine its role in cross-tolerance. Bim-deficient T cells were not deleted in response to cross-presented self-antigen, strongly implicating Bim as the pro-apoptotic mediator of cross-tolerance.

Clonal Deletion of Autospecific B Lymphocytes

Using mice transgenic for functional, rearranged immunoglobulin heavy and light chain genes, it can be demonstrated that B lymphocytes reactive with cell surface-bound class I MHC antigen can be controlled by clonal elimination. Even low-affinity cell-bound ligands can induce deletion. Deletion can occur in the pre-B to B cell transitional stage or after the B cells exist the bone marrow, depending on where the cells first encounter autoantigen. IgD appears to play no role in protecting cells from deletion. It is argued that defects in B-cell tolerance alone may be sufficient to lead to systemic autoimmunity.

Signalling through CD30 protects against autoimmune diabetes mediated by CD8 T cells.

Autoantigens found on pancreatic islets can move to draining lymph nodes, where they are able to cause the activation and consequent deletion of autoreactive T cells by a mechanism termed cross-tolerance,. This deletion depends on signalling through CD95 (also known as Fas), a member of the superfamily of tumour-necrosis-factor receptors. Here we describe a new mechanism that protects against autoimmunity: this mechanism involves another member of this superfamily, CD30, whose function was largely unknown. CD30-deficient islet-specific CD8-positive T cells are roughly 6,000-fold more autoaggressive than wild-type cells, with the transfer of as few as 160 CD30-deficient T cells leading to the complete destruction of pancreatic islets and the rapid onset of diabetes. We show that, in the absence of CD30 signalling, cells activated but not yet deleted by the CD95-dependent cross-tolerance mechanism gain the ability to proliferate extensively upon secondary encounter with antigen on parenchymal tissues, such as the pancreatic islets. Thus, CD30 signalling limits the proliferative potential of autoreactive CD8 effector T cells and protects the body against autoimmunity.

Beta cells cannot directly prime diabetogenic CD8 T cells in nonobese diabetic mice.

Type 1 diabetes (T1D) is caused by the destruction of insulin-producing islet β cells. CD8 T cells are prevalent in the islets of T1D patients and are the major effectors of β cell destruction in nonobese diabetic (NOD) mice. In addition to their critical involvement in the late stages of diabetes, CD8 T cells are implicated in the initiation of disease. NOD mice, in which the β2-microglobulin gene has been inactivated by gene targeting (NOD.β2M−/−), have a deficiency in CD8 T cells and do not develop insulitis, which suggests that CD8 T cells are required for the initiation of T1D. However, neither in humans nor in NOD mice have the immunological requirements for diabetogenic CD8 T cells been precisely defined. In particular, it is not known in which cell type MHC class I expression is required for recruitment and activation of CD8 T cells. Here we have generated transgenic NOD mice, which lack MHC class I on mature professional antigen-presenting cells (pAPCs). These “class I APC-bald” mice developed periislet insulitis but not invasive intraislet insulitis, and they never became diabetic. Recruitment to the islet milieu does not therefore require cognate interaction between CD8 T cells and MHC class I on mature pAPCs. Conversely, such an interaction is critically essential to allow the crucial shift from periislet insulitis to invasive insulitis. Importantly, our findings demonstrate unequivocally that CD8 T cells cannot be primed to become diabetogenic by islet β cells alone.

Ligand density determines the efficiency of negative selection in the thymus.

To study the influence of antigen density on the efficiency of negative selection in the thymus, MHC class I (H-2Kb, Kb) transgenic mice were generated, which expressed a Kb transgene under the control of its natural promoter at 33% (Kb-lo) or 150% (Kb-hi) the surface density of Kb in C57BL/6 (B6, H-2b) mice. These mice were crossed to anti-Kb T-cell receptor (Des-TCR) transgenic mice. In Des-TCRxKb-hi double transgenic mice, Des-TCR bearing T cells were completely eliminated during thymocyte maturation. In contrast, in Des-TCRxKb-lo double transgenic mice, two populations of Des-TCR T cells were evident, which either expressed the Des-TCR at intermediate density in the absence of CD8 (Des-TCRintCD8-) or expressed both the Des-TCR and CD8 at low density (Des-TCRloCD8lo). In the thymus of both types of double transgenic mice, no Des-TCR+CD4+CD8+ thymocytes were detected, suggesting that deletion of Des-TCR cells occurred before the CD4+CD8+ stage. Because only very few Des-TCR+ thymocytes were found in Des-TCRxKb-hi transgenic mice, deletion of these T cells apparently occurred upon expression of the Des-TCR. By contrast, Des-TCRxKb-lo transgenic mice showed distinct populations of Des-TCRintCD4-8- and Des-TCRloCD8lo thymocytes, suggesting that expression of the CD8 coreceptor was required to allow negative selection to proceed. Functional analyses showed that sublethally irradiated Des-TCRxKb-lo double transgenic mice were protected from lethal graft-versus-host disease by injected Des-TCR lymph node cells.

correction: Signalling through CD30 protects against autoimmune diabetesmediated by CD8 T cells

This corrects the article DOI: 18692

Self-Tolerance in Thet Cell Repertoire

Publisher Summary A thorough understanding of the various mechanisms leading to self-tolerance is thus required to enable strategies to be designed to prevent or reverse autoimmune reactions. Among these, may be cited various immunosuppressive regimes, such as, for example, some that could target monoclonal antibodies reacting with MHC molecules or with the TCR of specific T cells involved in the disease process. Other measures could be worked out to deliver peptides tailor-made to have a high affinity for susceptible MHC alleles, so as to compete effectively with the peptides eliciting the antiself response. There is a need to determine the mechanisms, which induce post-thymic tolerance so as to exploit this knowledge clinically to ensure long-term survival of foreign grafts, and to limit autoimmune and allergic reactions. On the other hand, it is desirable to learn how to break such tolerance to a particular self component, as then, one might use measures to persuade the body to mount an active immune response, which could limit the growth of those neoplasms that may express their own unique oncogenes to produce antigenic molecules on tumor cell surfaces.