Rheumatoid-factor-reactive sites on CH3 established by overlapping 7-mer peptide epitope analysis

Polyclonal IgM rheumatoid factors (RF) from ten patients with rheumatoid arthritis and six monoclonal IgM RF were isolated from monomeric IgG affinity columns and studied for their reactivity with the entire CH3 domain of IgG synthesized as overlapping 7-mers using a pin-ELISA assay. All ten polyclonal IgM RF showed similar profiles of reactivity which included peptides with solvent accessible residues PREPQVY (residues 343-349), PQVYTLP (residues 346-352), TLPPRSE (350-356), DGSFFLY (401-407), WQQGNVF (417-423), CSVMHEG (425-430), EGLHNHY (430-436) and KSLSLSP (439-446) of the CH3 domain. Substitution of a neutral glycine or alanine for each residue within these RF-reactive epitopes indicated that tyrosine at position 349, prolines at 343, 346 and 352, glutamine 347, valine 348, threonine 350, leucine 351, arginine 354, aspartic acid 401, tyrosine 407, serine 426, histidine 429, leucine 432, tyrosine 436 and lysine 439 represented important single amino acids within CH3 for RF reactivity. Regions of CH3 primary sequence with and without the single allotype-specific amino acid substitutions of glycine for alanine 431 (Gmx) or aspartic acid for glutamic acid (356) and leucine for methionine (358) (Gma) often showed considerable differences in reactivity with individual polyclonal and monoclonal RF. However, these differences in RF reactivity did not correlate with the individual anti-Gm RF specificity. Assays using monoclonal IgM RF produced from RA synovial B cells or peripheral blood B cells frequently showed a much more restricted spectrum of reactive CH3 epitopes. 7-mer peptides representing RF-reactive sites on CH3 preincubated with polyclonal IgM RF showed strong inhibition (55-66%) of RF binding to whole IgG on the ELISA plate. These studies indicate that it is possible to define portions of the IgG CH3 domain participating in the reaction with IgM RF using reactive epitope-mapping with sequential linear peptides derived from the primary IgG CH3 sequence.