RELATION BETWEEN THE HEAVY-CHAIN COMPLEMENTARITY REGION-3 CHARACTERISTICS AND RHEUMATOID-FACTOR BINDING-PROPERTIES

Among the rheumatoid factors (RFs), monospecific and polyspecific type s can be distinguished. However the molecular basis responsible for th eir different specificity is not well understood. In a previous report , we have shown that the binding of the majority of the polyspecific a ntibodies is salt-sensitive. No binding to IgG was observed under high ionic strength (0.3-0.5 M NaCl). This salt-sensitivity was only obser ved for 18% of the monospecific RFs. Here, we have analyzed 14 RFs rep resenting the 3 different groups (6 salt-insensitive monospecific, 4 s alt-sensitive monospecific and 3 salt-sensitive polyspecific RFs). By analysis of the amino acid composition and the distribution of polar a nd non-polar residues of their heavy chain complementarity-determining region 3 (H-CDR3) in relation to mono/polyspecificity, salt-sensitivi ty and reactivity against human IgG subclasses, we have identified com mon structural features responsible for their different binding proper ties. Salt-sensitive RFs (mono as well as polyspecific antibodies) wer e characterized by long H-CDR3's (15.3 +/- 2.7) that contained large n umbers of hydrophilic residues such as arginine and serine, while salt -insensitive RFs had more hydrophobic H-CDR3's of smaller length (11.3 +/- 2.4). In addition, for the monospecific RFs, remarkably similar h ydrophilicity H-CDR3 profiles were found that were correlated with the ir specificity for IgG subclasses. These observations confirm the impo rtance of the H-CDR3 for the binding of RFs to IgG. Furthermore, on th e basis of their shorter H-CDR3's and their rather unique H-CDR3 hydro philicity profiles, it is likely that the majority of the monospecific RFs should be considered as a group of RFs that is independent of the polyspecific RF repertoire.