The major rheumatoid factor cross-reactive idiotype in rheumatic disease

Low molecular weight IgM and CD5 B lymphocytes in rheumatoid arthritis

OBJECTIVES

To evaluate the role of low molecular weight (LMW) IgM and CD5 B cells in rheumatoid arthritis (RA) and to explore the possibility that LMW IgM is derived selectively from this subset of B cells.

METHODS

LMW IgM in sera and culture supernatants was detected by a sensitive immunoblot technique with an enhanced chemiluminescence detection system. CD5 B cells were determined by FACScan cytometry. In vitro studies were established in culture plates containing pokeweed mitogen with or without 2-mercaptoethanol (2-ME). Supernatants were obtained from CD5 positive hybridomas and CD5 negative hybridomas. Other immunological indices were measured by laser nephelometry.

RESULTS

Circulating LMW IgM was detected in all rheumatoid patients with significantly higher levels being observed in sero-positive patients. LMW IgM correlated significantly with total IgM and RF. Peripheral blood mononuclear cells (PBMC) from the majority of the patients with RA secreted LMW IgM in vitro as did mononuclear cells from a synovial fluid sample. The addition of low concentrations of 2-ME to the culture medium enhanced the proportions of secreted monomeric IgM. In contrast, PBMC from healthy subjects secreted only trace quantities of LMW IgM. In RA no significant correlations were observed between CD5 B cells and LMW IgM and RF. LMW IgM could be detected in the supernatants from both CD5+ and CD5- B cell lines. Finally, CD5 B cells were not significantly elevated in RA and levels remained constant over time.

CONCLUSION

LMW IgM exists in high concentrations in RA sera and synovial fluid. Serum level correlates with RF and IgM. In vitro studies have suggested that the occurrence of LMW IgM may be due to an intrinsic defect(s) in the assembly of the IgM pentameric molecule. LMW IgM is unlikely to be derived solely from CD5 B cells.

The major rheumatoid factor cross-reactive idiotype is dominantly expressed by Staphylococcus aureus Cowan strain I-activated CD5+ control B cells

Some seropositive (RF+) and seronegative (RF-) rheumatoid arthritis (RA) patients selectively express high concentrations of the major RF cross-reactive idiotype (RCRI) in their sera and generate high frequencies of RCRI+ pokeweed mitogen (PWM)-induced plasma cells from their peripheral blood mononuclear cells (PBM). To determine if normal individuals can express RCRI in vitro, B cells from controls were activated with Staphylococcus aureus Cowan strain I (SAC) bacteria to identify RCRI and RF production. In addition, we studied the relationship of RCRI expression with the subset of B cells bearing CD5. Control CD5+ B cells are responsible for RCRI expression following SAC activation. We also observed that RCRI is dominantly expressed by control SAC-induced B cells in frequencies comparable to that expressed by some RA and juvenile rheumatoid arthritis patients' PBM activated by PWM. Therefore, the frequency of RCRI+ B cells in control and arthritis patients' PBL may be similar, or the selection and/or regulation of RCRI+ B-cell expression in vitro and in vivo may be different in arthritis patients compared to normal individuals.

Hidden rheumatoid factor and Wa idiotype expression in juvenile rheumatoid arthritis

The major rheumatoid factor cross-reactive idiotype, which was defined by prototypic monoclonal IgM rheumatoid factors from Waldenstrom's macroglobulinaemia patient Wa, is present on 60% of all monoclonal IgM RF paraproteins. One-third of patients with juvenile rheumatoid arthritis (JRA), who are seronegative for classic IgM rheumatoid factor (RF), express the Wa idiotype in high titre in their sera. To determine if the Wa idiotype is present on hidden rheumatoid factors in JRA patient sera, we studied hidden RF expression by both ELISA and haemolytic assay techniques. The majority of JRA sera with increased concentrations of the Wa idiotype did not have increased RF activity nor hidden RF activity. In some JRA patients, the Wa idiotype may be expressed on a parallel set of immunoglobulin molecules, related to RFs, but lacking the ability to bind to IgG.

Expression of the major rheumatoid factor cross-reactive idiotype in pediatric patients with systemic lupus erythematosus

Rheumatoid factor cross-reactive idiotype (RF-CRI) is expressed in high concentrations in the sera of some patients with rheumatoid arthritis (RA) and juvenile rheumatoid arthritis (JRA). To determine if RF-CRI is specifically expressed in rheumatic disease or if it is secondary to polyclonal B-cell activation, we examined sera of 23 children with SLE, 16 adolescents with infectious mononucleosis (IM), and age-matched pediatric controls for RF-CRI expression. Concentrations of RF-CRI in serum, determined by an inhibition ELISA, were 24 +/- 17 micrograms/ml (mean +/- SD) in 25 normal children, 31 +/- 17 in 16 young adults with IM, and were significantly increased, 70 +/- 80 micrograms/ml, in the 23 children with SLE (p less than 0.036). Eleven of 23 SLE patients had serum RF-CRI greater than the mean +/- 2 SD for normal children. Ten of 23 SLE sera contained IgM rheumatoid factor (RF) activity. One patient with IM had a borderline elevated RF-CRI level, and 5 IM patients had RF in their sera. The serum IgM concentrations in sera were: SLE (192 +/- 93 mg/dl) and IM (234 +/- 77 mg/dl) sera. These levels were significantly elevated compared to controls (132 +/- 44 mg/dl), p less than 0.031 for SLE and p less than 0.001 for IM, suggesting that polyclonal activation of B cells was present in SLE and IM patient groups. Increased expression of RF-CRI in the SLE patients correlated directly with high titer anti-DNA antibody values (r = 0.3965, p less than 0.05) and RF activity when human IgG (r = 0.5026, p less than 0.05) was used as the RF binding substrate and inversely with serum C3 levels (r = 0.3925, p less than 0.05). RF-CRI expression did not correlate with RF that bound rabbit (r = 0.3123, p greater than 0.05). Increased serum RF-CRI expression is not a result of polyclonal B-cell activation. RF-CRI may be selectively up-regulated in patients with SLE.

Expression of large quantities of rheumatoid factor major cross-reactive idiotype in the serum of adults with seropositive rheumatoid arthritis

We quantified rheumatoid factor cross-reactive idiotype (RF-CRI) in whole serum from RF+ rheumatoid arthritis (RA) patients, using an inhibition enzyme-linked immunosorbent assay which is not affected by the presence of IgG. Serum from 16 RF+ RA patients contained 2-252 micrograms/ml RF-CRI (geometric mean */divided by SD 20.8 */divided by 5.2), while serum from 11 normal adults contained 1-16 micrograms/ml RF-CRI (geometric mean */divided by SD 3.9 */divided by 2.3). Serum from 8 of the RF+ RA patients contained RF-CRI at concentrations more than 2 standard deviations above the geometric mean in the normal subjects (greater than 21 micrograms/ml). Our results indicate that some RF+ RA patients express high concentrations of RF-RCRI and immunoglobulin molecules that express the RF-CRI may not be RF.