Molecular characterization of monoclonal IgM derived from human B cell lines expressing the 4C9 rheumatoid factor associated idiotype

Human and murine immunoglobulin expression vector cassettes

We describe the construction of new immunoglobulin (Ig) expression vectors and their use in the production of recombinant chimeric Ig molecules in transfected mammalian cells. The vectors contain the cDNA encoding the constant regions of human (mu, alpha1, gammal, gamma2, gamma3, gamma4, kappa) and murine (mu, gamma2a, kappa) Ig heavy and light chains. Unique restriction sites flanking the Ig variable region allow for replacement of variable regions generated by PCR. The CMV promoter allows for the transfection and expression of Ig in non-lymphoid cells. Distinct drug selection markers for heavy chain and light chain expression vectors allows for sequential or co-transfection of the vectors. We show that secretion of recombinant Ig can reach 1.2 microg/ml per million cells per day for transfected B cells. Replacement of the variable region results in the production of functional Ig retaining antigen specificity.

A rheumatoid factor specific mimotope identified by a peptide display library

We screened a 10 amino acid peptide display library in filamentous phage with B'20, a monoclonal high affinity IgM rheumatoid factor (RF) expressing the VkIIIa-dependent 4C9 idiotype. Using direct and indirect selection techniques, 12 B'20 reactive peptides were identified, 9 of which belonged to one of two motifs. Binding of B'20 to phage-bearing peptides was inhibited by both IgG and 4C9 antiidiotype. Synthetic peptides corresponding to the two motifs inhibited the Fc binding of a low avidity IgA B'20 construct. Purified IgM from 6/8 RF-positive RA patients and 8/11 monoclonal RFs with VkIII-encoded light chains bound to the phage, whereas none of the four monoclonal RFs with VkI or VkII encoded light chains bound. Phage binding appeared to be RF specific. Three 4C9 positive/RF negative cell lines from RA patients did not bind to phage nor did three B'20 mutants that had lost RF specificity, whereas two mutants that retained RF specificity also retained phage binding. We propose that there is a common epitope(s) recognized by VkIII encoded RFs that is mimicked by the structure of these peptides. Such mimotopes might be exploited to design novel agents that interfere with autoantibody binding.

Rheumatoid Factor Specificity of a VH3-Encoded Antibody Is Dependent on the Heavy Chain CDR3 Region and Is Independent of Protein A Binding

Rheumatoid factors (RF) recognize conformational determinants located within the Fc portion of IgG. By analyzing a panel of monoclonal rheumatoid arthritis (RA)-derived RFs, we previously demonstrated that the somatically generated light chain complementarity-determining region 3 (CDR3) contributes to RF specificity. We have now generated a panel of heavy chain mutants of the B′20 Ab, a high affinity RA-derived IgM RF. B′20 also binds avidly to protein A and weakly to ssDNA and tetanus toxoid. B9601, a RF negative Ab that is highly homologous to B′20 but does not bind any of the Ags tested, and RC1, a low affinity polyreactive RF, were used to generate heavy chain mutants with framework (FR) and CDR switches. The mutated heavy chains were cotransfected into a myeloma cell line with the germline counterpart of the B′20 light chain, and the expressed Ig tested for antigenic specificity. We show that both RF specificity and polyreactivity of B′20 is dependent on its unique heavy chain CDR3 region. Replacement with a B9601 CDR3 shortened to the same length as the B′20 CDR3, and with only 5 amino acid differences, did not restore Fc binding. Conversely, absence of protein A binding of B9601 is due to the presence of a serine residue at position 82a in the B9601 heavy chain FR3 region. Together, our data suggest that Ig gene recombination events can generate B cells with autoantibody specificities in the preimmune repertoire. Abnormal release, activation, expansion, or mutation of such cells might all contribute to the generation of a high titer RF response in patients with RA.