Testing for isotypes does not help differentiating rheumatoid arthritis from other rheumatoid factor positive diseases

Rheumatoid factors (RFs) are useful for diagnosis and classification of rheumatoid arthritis (RA). Nephelometric and turbidimetric techniques, which detect total RF but do not reveal the antibody isotype, are common diagnostic methods in clinical routine. Given the recent development of isotype-specific immunoassays, the detection of IgG, IgM, and IgA RFs represents an interesting challenge. The aim of the study was to evaluate whether specific RF tests performed as a second step after traditional nephelometry could help differentiating RA from other RF-positive diseases. We tested 117 consecutive serum samples that were RF-positive at nephelometry (BNII nephelometric analyzer, Siemens) for IgA, IgG, and IgM RF isotypes by a fluoroimmunoenzymatic assay (FEIA) on the Phadia 250 instrument (ThermoFisher). Fifty-five subjects had RA and 62 presented non-RA diagnoses. Eighteen sera (15.4%) were positive only by nephelometry, two were positive only for IgA RF, and the remaining 97 sera were all positive for IgM RF isotype (with or without IgG and IgA RF). Positive findings did not correlate with RA or non-RA diagnosis. Spearman rho correlation coefficient between nephelometric total RF and IgM isotype was moderate (0.657), and weak between total RF and IgA (0.396) and IgG (0.360) isotypes. Despite its low specificity, measurement of total RF by nephelometry still seems to be the method that performs best. As IgM, IgA, and IgG RF isotypes showed only a moderate correlation with total RF measurement, their diagnostic use as a second level test remains controversial.

Rheumatoid factor autoantibody repertoire profiling reveals distinct binding epitopes in health and autoimmunity

BACKGROUND

Rheumatoid factors (RF) are one of the hallmark autoantibodies characteristic of rheumatoid arthritis (RA), and are frequently observed in other diseases and in healthy individuals. RFs comprise multiple subtypes with different specificities towards the constant region of human IgG. Studies indicate that these patterns differ between naturally occurring RFs and RFs associated with disease. However, individual specificities characteristic of either have not been clearly defined.

METHODS

In this study, we developed an extended set of engineered IgG-fragment crystallisable (Fc) targets with preferential RF binding to specific (conformational) epitopes, which was subsequently used for profiling of RF binding patterns in a compiled exploration cohort, consisting of sera from healthy donors with measurable RF and patients with RA, primary Sjögren's syndrome (pSS) and seropositive arthralgia.

RESULTS

We identified an epitope that is strongly associated with RA, which was targeted by both IgM-RF and IgA-RF. We also identified an epitope that is preferentially targeted by healthy donor (IgM) RFs. IgM-RFs derived from healthy donors and patients with RA and pSS all target distinct regions on the IgG-Fc, whereas overall, the IgA-RF repertoire is largely restricted to pathology-associated specificities. Using monoclonal RFs with different specificities, we furthermore demonstrate that the capacity to activate complement or even inhibit IgG-mediated complement activation varies according to the epitopes to which RFs bind.

CONCLUSIONS

Our results demonstrate both the need and feasibility to redefine 'RF' into pathological and physiological autoantibody subtypes.

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.

Mapping IgG Epitopes Bound by Rheumatoid Factors from Immunized Controls Identifies Disease-Specific Rheumatoid Factors Produced by Patients with Rheumatoid Arthritis

We have mapped the specificity of 28 monoclonal IgM rheumatoid factors (RFs) produced by heterohybridomas derived from five healthy blood donors immunized with mismatched human red blood cells (HID). The HID-RFs did not differ in their binding specificity for IgG epitopes from RFs that we previously analyzed from patients with Waldenström’s macroglobulinemia. However, IgM RFs produced by HID differed in their specificity for IgG compared with RFs expressed by patients with rheumatoid arthritis (RA-RFs). Only 1 of 28 HID-RFs bound all IgG subclasses (pan binding pattern) compared with 7 of 19 RA-RFs (p = 0.006). Three HID-RFs bound IgG3 compared with 9 RA-RFs (p = 0.007). Fine specificity differences were also identified between HID- and RA-RFs. Therefore, some RA-RFs show novel specificities for IgG not found among RFs from HID or individuals with Waldenström’s macroglobulinemia who do not have joint disease. These Abs with unique specificities may represent disease-specific autoantibodies in patients with RA. Nine of the HID-RFs from the same individual were clonally related, and several contained somatic mutations. Even when the clonally related HID-RFs were considered as one RF for comparison, the reactivity of the HID-RFs differed significantly from RA-RFs in their inability to recognize all IgG subclasses (p = 0.044) and recognize IgG3 (p = 0.041). Interestingly, among the clonally related RFs, considerable differences in the specificity for IgG were also observed, with the RF containing the most somatic mutations in VH and VL showing the most distinctive specificity changes. Therefore, these studies also demonstrate a correlation between somatic mutation and binding specificity.

Serologic crossreactions among primate immunoglobulins

We have generated and characterized 50 murine monoclonal antibodies (mAb) specific for baboon IgG. We examined crossreactivity of these mAb to baboon IgM and immunoglobulin (Ig) of various other primates including human, chimpanzee, rhesus monkey, cynomolgus monkey, and African green monkey. Those mAB that crossreacted with human IgG were further examined using myeloma proteins for specificity to human Ig subclasses. One mAB crossreacted with all four human IgG subclasses and with human IgM. We further analyzed this reactivity utilizing Bence Jones proteins representative of various light (L) chain germline gene family products. This mAB reacted with Bence Jones proteins indicating the recognition of a kappa (k) L chain specificity associated with the kappa I, kappa III, and kappa IV subgroups, but not with kappa II. Based on the differences between kappa II germ line gene encoded L chains and the other kappa L chain subgroups, we ascribe this reactivity to six amino acids that define a discontinuous epitope.