The epitopes for natural polyreactive antibodies are rich in proline.
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Journal: 1997/July - Proceedings of the National Academy of Sciences of the United States of America
ISSN: 0027-8424
PUBMED: 9177218
Abstract:
"Natural" polyreactive antibodies, which bind in a nonspecific manner to a range of biological molecules both of self- and nonself- origin, are normal constituents of serum and are a significant part of the immune repertoire in many species, including humans. Autoantibodies to sTNF-R (the 55-kDa extracellular domain of the human receptor to tumor necrosis factor alpha) were affinity purified from normal human sera using immobilized sTNF-R. The isolated anti-sTNF-R IgG bound both native and denatured forms of the receptor with low affinity. These antibodies also bound to different proteins and therefore are considered to be polyreactive. We used the anti-sTNF-R antibodies and purified polyreactive antibodies to mannose-specific lectin from garlic (Allium sativum) for screening a peptide library displayed on filamentous M13 phage. After the biopanning procedure, we failed to find epitopes with a consensus sequence; however, we found that proline is the most frequent amino acid in the selected phagotopes. Proline is commonly present at solvent-exposed sites in proteins, such as loops, turns, N-terminal first turn of helix, and random coils. Thus, structures containing proline can serve as conformation-dependent common "public" epitopes for polyreactive natural antibodies. Our findings may be important for understanding polyreactivity in general and for the significance of polyreactive natural antibodies in immunological homeostasis.
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Proc Natl Acad Sci U S A 94(12): 6335-6339
PMID: 9177218

The epitopes for natural polyreactive antibodies are rich in proline

Department of Membrane Research and Biophysics, The Weizmann Institute of Science, Rehovot 76100, Israel
Leo Sachs, The Weizmann Institute of Science, Rehovot, Israel
Received 1997 Feb 12; Accepted 1997 Apr 3.
Copyright © 1997, The National Academy of Sciences of the USA

Abstract

“Natural” polyreactive antibodies, which bind in a nonspecific manner to a range of biological molecules both of self- and nonself- origin, are normal constituents of serum and are a significant part of the immune repertoire in many species, including humans. Autoantibodies to sTNF-R (the 55-kDa extracellular domain of the human receptor to tumor necrosis factor α) were affinity purified from normal human sera using immobilized sTNF-R. The isolated anti-sTNF-R IgG bound both native and denatured forms of the receptor with low affinity. These antibodies also bound to different proteins and therefore are considered to be polyreactive. We used the anti-sTNF-R antibodies and purified polyreactive antibodies to mannose-specific lectin from garlic (Allium sativum) for screening a peptide library displayed on filamentous M13 phage. After the biopanning procedure, we failed to find epitopes with a consensus sequence; however, we found that proline is the most frequent amino acid in the selected phagotopes. Proline is commonly present at solvent-exposed sites in proteins, such as loops, turns, N-terminal first turn of helix, and random coils. Thus, structures containing proline can serve as conformation-dependent common “public” epitopes for polyreactive natural antibodies. Our findings may be important for understanding polyreactivity in general and for the significance of polyreactive natural antibodies in immunological homeostasis.

Keywords: human serum, natural autoantibodies, peptide display library, polyreactivity/soluble tumor necrosis factor receptor
Abstract

Natural autoantibodies, directed against a wide range of self-antigens, are present in the sera of healthy humans (1). The role of these autoantibodies is unknown. Autoimmune diseases are characterized by the presence of autoantibodies to self-proteins (2). Evidence has accumulated that indicates that self-proteins are processed and presented similarly to foreign antigens by class I and class II major histocompatibility complex (MHC) molecules (3). Self-antigenic determinants (epitopes) are presented by the class II MHC. In this context, it was shown that most (about 90%) of the class II ligands found so far originate from cellular proteins of the animal itself, mostly from plasma membrane proteins (4). Among the ligands that have been found on MHC class II are peptides derived from the transferrin receptor, Na–K ATPase and from MHC class I molecules (5).

Recently, we described natural antibodies to dietary proteins in normal human serum that can be divided into two categories—specific or polyreactive (6, 7). The specific antibodies reacted with a single protein, whereas polyreactive antibodies crossreacted with a series of unrelated proteins including autoantigens (8).

One of the autoantigens that crossreacted with polyreactive natural antibodies was sTNF-R, the soluble type I (p55) truncated form of the receptor of tumor necrosis factor α (TNFα). sTNF-R exists in two different forms, type I (p55) and type II (p75), isolated from either urine or sera, from both healthy donors and cancer patients (9). These proteins function as inhibitors of TNFα activity (10).

Because sTNF-R is the cell surface portion of the membrane-bound TNF-R and circulates normally in serum, we determined whether healthy individuals have autoantibodies to sTNF-R and succeeded to purify such autoantibodies. These antibodies were shown to be polyreactive, and, using a phage display library, we determined some of the epitopes (11). For comparison, we also determined the epitopes of human natural antibodies against a lectin (ASA) from garlic, which also proved to be polyreactive (6).

We found that most of the peptides that interact with the two natural antibodies to self- and nonself-proteins contain proline. We concluded that the low affinity polyreactive antibodies are against the exposed epitopes of proteins, namely, against loops and turns. Most of the protein antigens contain proline in these sites, and this property may contribute to the polyreactive character of natural antibodies.

The values represent the number of phages sequences. The corresponding phage epitopes (single-letter amino acid code) were deduced from the DNA sequence.

Acknowledgments

We thank Prof. D. Wallach for fruitful discussions and Dr. M. Boldin and A. Dibman for help during our study. We also thank Dr. E. A. Bayer for critical reading of the manuscript. This work was supported by research grants from the Lynne and William Frankel Fund for the Diagnosis and Treatment of Ovarian and Breast Cancer and MINERVA, Deutsch-Israelische Wissenschftliche Zusammenarbeit, Germany.

Acknowledgments

ABBREVIATIONS

ASAAllium sativum agglutinin
TNFαtumor necrosis factor α
sTNF-R55-kDa extracellular domain of the human receptor for TNFα
MHCmajor histocompatibility complex
ABBREVIATIONS

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