The epitopes for natural polyreactive antibodies are rich in proline

"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.