Autoimmune disease development based on possible mislocalization of intracellular and extracellular proteins
Introduction
Zinkernagel and Doherty first reported the viewpoint that cytosolic proteins bind to major histocompatibility complex class I (MHC I) receptors and are subsequently presented to CD8+ T lymphocytes, whereas T helper cells (CD4+ T cells) recognize peptides bound to MHC II receptors through T cell receptors (TCRs) and CD4 protein (Zinkernagel and Doherty, 1974a, Zinkernagel and Doherty, 1974b). These two pathways are used to recognize the peptides produced by the body and to identify foreign peptides on antigen-presenting cells (Zinkernagel and Doherty, 1974a).
Although these two basic theorems, collectively known as MHC restriction, are widely accepted, the medical consequences of this biochemical and cellular differentiation of the entire “antigen repertoire” or “antigen spectrum” into two parts have not been fully considered. As reported by Zinkernagel and Doherty, 1974a, Zinkernagel and Doherty, 1974b, antigen-presenting cells bind and internalize foreign extracellular proteins and display peptides formed from the digestion of these proteins on class II MHC proteins, whereas peptides derived from cytosolic proteins are bound to class I MHC proteins.
Cytotoxic T cells recognize foreign peptides presented by class I MHC proteins with the aid of the coreceptor CD8 (Fig. 1A). Peptides derived from cytosolic proteins are bound to MHC I molecules and then presented and recognized by CD8+ T lymphocytes (Fig. 1B). CD4+ T cell receptors interact with class II MHC molecules in a manner analogous to the way CD8+ T cell receptors interact with class I MHC molecules (Fig. 1A). However, helper T cells and cytotoxic T cells are distinguished by other proteins that are expressed on their surfaces.
This separation of CD4+ and CD8+ TCRs corresponds to the MHC restriction identified by Zinkernagel and Doherty at the T cell level. In particular, helper T cells express CD4 protein instead of CD8. CD4 consists of four immunoglobulin domains extending from the T cell surface and a small cytoplasmic region. This study focused on the consequences of MHC restriction on T cells and subsequently on the antigenicity of the body’s own peptides and proteins.
It is thought that helper T cells express CD4 protein to recognize foreign (extracellular) peptides on antigen-presenting cells. Consequently, the antigen repertoire, or antigen spectrum, is separated into two parts, each associated with a different T cell population. The antigen repertoire includes TCRs, which specialize in recognition of cytosolic peptide structures. Therefore, cytosolic self-peptides are presented on MHC I and are recognized by TCRs with the help of the CD8 protein by CD8+ T cells. In addition, the TCR repertoire has an extracellular component. Specifically, extracellular peptides are presented on MHC II receptors and are recognized by TCRs on CD4+ T cells using CD4 protein. The research described herein led to the hypothesis that the cytosolic peptide structures produced by the body (or, more precisely, their complementary binding structures) should be “missing” from the TCR spectra of CD8+ T cells and therefore not recognizable, whereas physiological extracellular peptide structures should be present and recognizable. Furthermore, the present study proposed that physiological extracellular proteins should be missing from the TCR spectra of CD4+ T cells, whereas physiological cytosolic peptide structures should be present and recognizable.
The logical consequence of these hypotheses is that physiological intracellular proteins are recognized by TCRs on CD4+ T cells, whereas their recognition-specific patterns are absent from TCRs on CD8+ T cells. Conversely, physiological extracellular proteins are recognized by TCRs on CD8+ T cells, whereas their recognition-specific patterns are absent from TCRs on CD4+ T cells.
This theory provides a completely new viewpoint that has significant effects on understanding how autoimmune disease develops. The best approach to test this theory would be through clinical studies of such disorders. The theory predicts that an innate intracellular protein will be recognized as a foreign body if it arises or accumulates extracellularly. Through similar reasoning, an extracellular protein will be recognized as a foreign body if it arises or accumulates intracellularly. In other words, the localization of a specific protein plays a substantial role in antigenic determination in the context of whether an autoimmune reaction develops toward the misplaced protein.
Changing the localization of a specific protein is therefore critical. This theory predicts that an autoimmune disease will develop if changes in protein localization occur continuously within the body, either rapidly or slowly. This theory has been presented to the scientific community at several congresses, including the Frontiers in Immunology Research Network (FIRN) congresses in Hawaii and Athens and the German Society of Immunology (DGfI) congress in Leipzig, and has also been reported in several publications (Arneth, 2003, Arneth, 2004, Arneth and Birklein, 2009, Arneth, 2009). As a result, the validity of the role of protein mislocalization in the development of autoimmunity is becoming more widely considered.
Section snippets
Materials and methods
To test the above-described hypotheses in detail with various human proteins, human albumin and human insulin were added to human whole-blood samples from healthy subjects (N = 12) as extracellular proteins, and glyceraldehyde 3-phosphate dehydrogenase (GAPDH) and ß-actin were added as intracellular proteins. All participants provided informed consent, and the study was approved by the review board of the University of Mainz, Germany.
Extracellular fluids were obtained by centrifuging EDTA-treated
Results
In this study, extracellular proteins were represented by plasma generally and albumin and insulin specifically. Intracellular proteins were represented by intracellular fluid generally and ß-actin, GAPDH and hemoglobin specifically.
Fig. 2, Fig. 3 show the experimental results. The whole-blood aliquots incubated with intracellular proteins (GAPDH 6.5 ± 1.3% and ß-actin 5.8 ± 1.2%) showed significantly greater incorporation of BrdU and significantly more cells in S-phase than did the aliquots
Discussion
In the current study, we investigated the mislocalization of extra- and intracellular proteins in the context of autoimmune disease. Our data suggest that a mislocalized protein can be recognized as foreign and can lead to activation of CD4+ and CD8+ T lymphocytes.
The new concept of antigenicity and identification of foreign molecules by the immune system provided here leads to certain inferences, including that defects in the immune system are not only related to the emergence of autoimmune
Conclusions
The development of autoimmune disease can be explained not only by defects in the immune system but also by polymorphisms or mutations in autoantigens that affect their targeting. Thus, cellular defects in protein trafficking likely contribute to the development of autoimmune disease.
Competing interests
The author declares no competing interests.
Acknowledgement
There are no acknowledgements.
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