Elsevier

Journal of Autoimmunity

Volume 50, May 2014, Pages 38-41
Journal of Autoimmunity

Short communication
Chromogranin A is a T cell antigen in human type 1 diabetes

https://doi.org/10.1016/j.jaut.2013.10.003Get rights and content

Highlights

  • New onset type 1 diabetes patients have a significantly elevated T cell response to the peptide WE14.

  • Some patients show an improved T cell response to the transglutaminase-treated peptide WE14.

  • Posttranslationally modified antigens may play a critical role in the development of type 1 diabetes.

Abstract

Chromogranin A (ChgA) is a beta cell secretory granule protein and a peptide of ChgA, WE14, was recently identified as a ligand for diabetogenic CD4 T cell clones derived from the NOD mouse. In this study we compared responses of human CD4 T cells from recent onset type 1 diabetic (T1D) and control subjects to WE14 and to an enzymatically modified version of this peptide. T cell responders to antigens were detected in PBMCs from study subjects by an indirect CD4 ELISPOT assay for IFN-γ. T1D patients (n = 27) were recent onset patients within one year of diagnosis, typed for HLA-DQ8. Controls (n = 31) were either 1st degree relatives with no antibodies or from the HLA-matched general population cohort of DAISY/TEDDY. A second cohort of patients (n = 11) and control subjects (n = 11) was tested at lower peptide concentrations. We found that WE14 is recognized by T cells from diabetic subjects vs. controls in a dose dependent manner. Treatment of WE14 with transglutaminase increased reactivity to the peptide in some patients. This work suggests that ChgA is an important target antigen in human T1D subjects and that post-translational modification may play a role in its reactivity and relationship to disease.

Introduction

Autoreactive T cells directed toward β-cell antigens play a central role in the destruction of β-cells in human type 1 diabetes (T1D) and in the NOD mouse model of T1D. The presence of autoreactive T cells in individuals may indicate an ongoing attack on β-cells and can serve as an important biomarker for pre-diabetic status. This time-window in disease development, before overt hyperglycemia, is of key interest for therapeutic intervention strategies, such as antigen-specific tolerance induction, to prevent disease onset [1], [2].

Using a proteomic strategy, we identified Chromogranin A (ChgA) as the target antigen for diabetogenic CD4 T cell clones, including the widely used BDC-2.5 T cell, derived from NOD mice [3]. The peptide WE14, a naturally occurring cleavage product of ChgA, is a weak antigen for these T cell clones in vitro, but the antigenic activity of the peptide is dramatically increased upon treatment with the enzyme tissue transglutaminase (TGase), which is known to covalently crosslink proteins through the formation of isopeptide bonds as well as catalyze glutamine deamidation reactions, such as those that occur in the posttranslational modification of gliadin in celiac disease. In the case of ChgA-reactive T cell clones from non-obese diabetic (NOD) mice, crosslinking rather than deamidation is involved in rendering WE14 more antigenic upon TGase treatment [4]. Since the amino acid sequences of human and mouse WE14 are identical, with the exception of one amino acid [5], we tested T cells from newly diagnosed T1D patients and controls for responses to human WE14 or a TGase-converted form of this peptide. We employed an indirect ELISPOT assay to compare responses to WE14 and TGase-modified WE14 in CD4 T cells from diabetic and control subjects.

Section snippets

Study participants

Participants of both genders, aged 9–44 years, and typed for HLA DQ8, were recruited from patients, relatives, and volunteers attending the Barbara Davis Center for Diabetes, in accordance with protocols approved by the Colorado Multiple Institutional Review Board. Subjects with clinical disease of over one year duration at the time of blood draw were excluded from the study, as were control subjects who tested positive for one or more diabetes autoantibodies (insulin autoantibody [IAA],

Population characteristics

Recent onset patients averaged 15.5 years of age with a range of 9–39 vs. control subjects who averaged 25.5 years (14–44). The majority of both subject populations were HLA-DQ8, the proposed restriction element for reactivity to WE14. Most of the T1D patients were positive for at least one autoantibody (IAA, GADA, ICA512, and ZnT8).

T cell responses of first cohort to WE14

Patient and control PBMC were analyzed for T cell responses to WE14 and other antigens by ELISPOT. Results for the first cohort are illustrated in Table 2 and show

Discussion

Results of this study of cellular autoimmunity indicate that the ChgA-derived peptide WE14 is a target for autoreactive cells in new onset T1D patients. Additionally, modification of WE14 by the enzyme TGase improves the antigenicity of this peptide in some patients. ChgA is an attractive target of the immune response since it is localized in the beta cell granule and so joins insulin, IA-2 and ZnT8 in that respect [10]. Post-translational modification of proteins may strengthen weak immune

Conflicts of interest disclosure

The authors declare no conflict of interest related to this study.

Acknowledgments

We thank the DAISY and TEDDY studies for providing control subjects for these studies.

Author contributions: P.G., T.D., R.B., L.F., R.W., and K.H. designed and/or performed experimental work; G.C., M.R., and A.M. provided patients and reagents utilized in these studies. T.D., P.G., R.B., and K.H. wrote the manuscript.

Peter Gottlieb and Kathryn Haskins are the guarantors of this work, had full access to all the data, and take full responsibility for the integrity of data and the accuracy of data

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