Insulin B chain functions as an effective competitor of antigen presentation via peptide homologies present in the thymus

Cloned T cells from a recent onset IDDM patient reactive with insulin B-chain

Insulin-dependent diabetes mellitus (IDDM) results from selective autoimmune destruction of insulin producing beta-cells. T-cell reactivity and autoantibodies to several islet proteins such as insulin, GAD and IA-2 are associated with IDDM in mice and men. In NOD mice, the majority of T cells from insulitis specifically recognize the insulin B-chain peptide amino acid 9-22, in contrast to the periphery where the precursor frequency is much lower. It is important to note that these cells are diabetogenic. Surprisingly, the same insulin B-chain region contains epitopes recognized by protective T cells. In fact, autoimmune diabetes in NOD mice could be prevented by prophylactic treatment with this immunodominant T-cell epitope. In humans, however, no immunodominant regions of insulin have yet been defined. We have isolated and characterized a human insulin-specific T-cell clone that was derived from peripheral blood of a newly diagnosed IDDM patient. This patient displayed weakly positive primary T-cell responses to insulin. The peptide recognized by the clone was mapped to the insulin B chain (B:11-27). Functionally, the human insulin-specific CD4+ T cells displayed a Th1/0 like cytokine profile and were restricted by HLA-DR. The previously proposed alternative superantigen-like binding of insulin-B chain peptide outside of the peptide binding groove of HLA-DR could not be confirmed, since T-cell recognition was inhibited in competition experiments of insulin-B chain peptide with HLA-DR16 binding influenza peptide HA307-319. Our results indicate that human clonal T cells isolated from a recent onset IDDM patient recognize an epitope overlapping with the insulin B-chain region that is immunodominant and potentially therapeutic in NOD mice. This observation may be useful in studying the role of insulin-specific T cells in IDDM, and may eventually help to establish peptide-based immunotherapies in IDDM.

Naturally processed heterodimeric disulfide-linked insulin peptides bind to major histocompatibility class II molecules on thymic epithelial cells

We determined whether disulfide-linked insulin peptides that are immunogenic in vitro for CD4+ T cells bind to major histocompatibility complex class II in vivo. Radiolabeled recombinant human insulin (rHI) was injected into BALB/c mice, and processed rHI peptides bound to I-Ad molecules on different thymic antigen-presenting cells were characterized. The A6-A11/B7-B19 and A19-A21/B14-B21 disulfide-linked I-Ad-bound rHI peptides were isolated from thymic epithelial cells but not dendritic cells. While both thymic epithelial cells and dendritic cells present rHI to HI/I-Ad-specific T cells, these antigen-presenting cells do not present the reduced or nonreduced forms of the disulfide-linked rHI peptides. Thus, a naturally processed disulfide-linked peptide can bind to major histocompatibility complex class II in vivo. The potential role of these peptides in immunological tolerance is discussed.

Identification of an autologous insulin B chain peptide as a target antigen for H-2Kb-restricted cytotoxic T lymphocytes

We have examined the CD8+ peripheral T cell repertoire of C57BL/6 (H- 2b) mice for cytotoxic T lymphocyte (CTL) reactivities to insulin, using in vitro immunization with a chymotryptic digest of reduced bovine insulin. The results presented in this study demonstrate that potentially autoreactive H-2Kb-restricted cytotoxic T cells specific for an autologous insulin B chain peptide are present in the preimmune splenic T cell repertoire. The immunogenic peptide comprises residues 7- 15 from the insulin B chain and has features in common with naturally processed Kb-restricted peptides identified by others. The minimal peptide sequence recognized by these cytotoxic T cells is 10-15, which is highly conserved in mammalian species and constitutes a self-peptide in mice. The presence of class I major histocompatibility complex- restricted CTLs with potentially autoreactive specificities in preimmune animals raises the possibility of a role for such cells in autoimmune disease states. Possible mechanisms for the in vivo expansion of insulin peptide-specific CTLs are discussed.

Peptide competition at the level of MHC-binding sites using T cell clones from a rheumatoid arthritis patient

The inhibition of antigen presentation in rheumatoid arthritis by blocking peptide binding to MHC at the antigen presenting cell (APC) level was investigated using various synthetic peptides derived from the 65 kDa mycobacterial protein. Human T cell clones from tuberculosis and rheumatoid arthritis patients were stimulated with peptides in the presence of irradiated APCs (autologous or DR homozygous EBV-B cell lines). Two peptides (residues 65-85 and 412-426) were found to be able to bind to the HLA-DR1 protein. Cross-competition was observed between these peptides when APCs were cultured with a suboptimal concentration of stimulator peptide in the presence of various concentrations of competitor peptides and T cell clones from tuberculosis patients as responder cells. These T cell clones responded not only to the peptides but also to the native protein. In other experiments, we used T cell clones from a rheumatoid arthritis patient to demonstrate the blocking of MHC-binding sites by adding the p412-426 in the recognition of DR1 restricted T cell clone specific to p65-85; MHC binding was not observed using a control peptide (residues 198-217). This approach has permitted the identification of MHC-specific blockers. Further experimentation is required to determine the particular amino acids involved in MHC binding. Our data support the idea that modulation of antigen presentation by peptide competition could be a useful tool for immunotherapy in autoimmune diseases.

Influence of antigen processing on thymic T-cell selection

The design of a specific blocking peptide for the immunosuppressive therapy of an autoimmune disease requires the identification of peptides of an autoantigen that are physiologically processed in vivo and bind to MHC-encoded membrane glycoproteins. However, knowledge of how an antigen is physiologically processed by antigen-presenting cells (APC) in vivo, particularly in the thymus, is lacking. It is also unknown whether the processing of an antigen by different APC in the thymus can influence thymic T-cell selection. This is an important consideration for attempts to delete or inactivate autoreactive T cells that elicit autoimmune disease. To address these issues, we investigated the processing of biosynthetically labelled recombinant human insulin (rHI), a model autoantigen, injected into mice and characterized the insulin peptides associated with MHC class II molecules on thymic epithelial cells and dendritic cells. These APC were found to differ in the way they process insulin. The detection of MHC-class-II-bound insulin peptides on the surface of the epithelial cells but not the dendritic cells correlated with their capacity to either present or not present insulin to T cells, respectively. Thus, antigen processing may control the appearance of different peptide-MHC class II complexes on thymic APC that mediate positive and negative selection, and thereby influence the development of the T-cell repertoire. Our findings could have important bearing on the future design of synthetic blocking peptides that reduce or eliminate the onset of autoimmune disease.

Characterization of agretopes and epitopes involved in the presentation of beef insulin to T cells

Beef insulin-specific I-Ad-restricted T cell hybridomas were derived from the fusion of antigen-primed (BALB/c X B6)F1 T cells with BW5147 thymoma. Specificity analysis revealed that the A-chain loop region is involved in antigen recognition. Hybridoma A20.2.15 is specific for beef insulin and cross-reacted with sheep insulin, but not with pork insulin. Using synthetic peptides we showed that the A-chain loop containing peptide A1-A14 jointed to the B7-B15 peptide by a disulfide bond can activate this hybridoma. Fragments generated by enzyme digest further suggest that the peptide recognized on beef insulin appears to involve A-chain loop residues A5-A12 and B-chain residues B7-B13 that are linked by the A7-B7 disulfide bridge. We found that beef insulin needs to be processed prior to T cell activation. Glutaraldehyde fixation and chloroquine treatment of presenting cells abolished their capacity to present insulin. Beef insulin denatured by pH changes cannot activate, thus suggesting that simple denaturation is not sufficient for presentation by antigen presenting cells. Finally, the agretope on beef insulin is comprised of two functional regions B7-B13 on the B chain and the A-chain loop in the A-chain, while residues A8 and A10 are probably involved in interaction with the T cell receptor.

Peptide competition for antigen presentation

Since each major histocompatibility complex (MHC) molecule can bind many different peptides, it might be predicted that competition for the same MHC-binding site takes place between peptides with unrelated sequences. As Luciano Adorini and Zoltan Nagy report here, this does indeed occur, both in vitro and in vivo. In-vivo competition between peptides for antigen presentation to T lymphocytes is an important influence on the immunodominance of T-cell determinants. In addition, it is possible to modulate T-cell activation by interfering with the binding of antigenic peptides to MHC class II molecules. This could represent a suitable approach to a rational treatment of autoimmune diseases and, possibly, of allograft rejection.