Regulation of anti-Sm autoantibodies by the immunoglobulin heavy chain locus.

Anti-Sm antibodies are specific markers for systemic lupus erythematosus in MRL mice and in humans. The prevalence of anti-Sm positivity in inbred MRL/Mp-lpr/lpr(MRL/lpr) mice is consistently about 25% at 5 mo of age, when the disease is at its peak. The control of the development of anti-Sm in individual MRL/lpr mice has been shown to be the result of stochastic factors, and previous research has indicated that the immunoglobulin heavy chain (IgH) b allotype may be more amenable to the production of anti-Sm. We have now further investigated the influence of the IgH genetic locus on the production of anti-Sm and other autoantibodies in an allotype congenic MRL strain, the MRL/Mp-Ipr/Ipr-IgHb (MRL/lpr-IgHb). Strikingly, 78% of MRL/lpr-IgHb mice produced anti-Sm, compared with 27% of contemporaneous MRL/lpr (IgHj) mice. Of those mice that were positive for anti-Sm, the MRL/lpr-IgHb strain produced significantly higher levels of anti-Sm than did the anti-Sm positive MRL/lpr mice. No differences were observed between the conventional MRL/lpr and the MRL/lpr-IgHb levels of antichromatin, anti-ssDNA, antiribosomal P, or anti-Su. In addition, kidney function, which was assessed by measuring serum urea nitrogen levels, was similar in the two strains. These results support the notion that the control of anti-Sm production in MRL/lpr mice operates through the IgH locus.

Regulation of the anti-Sm autoantibody response in systemic lupus erythematosus mice by monoclonal anti-Sm antibodies.

The administration of certain monoclonal anti-Sm antibodies (2G7, 7.13) induced most MRL/lpr mice to become anti-Sm positive by 5 mo of age, although other anti-Sm monoclonals (Y2, Y12) suppressed the spontaneous response. Positive anti-Sm antibody enhancement occurred efficiently only in MRL/lpr mice and not in other systemic lupus erythematosus mice that have little spontaneous anti-Sm production. The enhancement by anti-Sm antibodies was specific for the anti-Sm response. The mechanism of the passive antibody enhancement was apparently not isotype- or idiotype-related. The fine specificity of the anti-Sm monoclonal antibody may be essential to its enhancing or suppressing effects, since both enhancing monoclonals recognized only the D Sm polypeptide, whereas both suppressing monoclonals saw the D and the B polypeptides. Furthermore, analysis of serial bleeds from unmanipulated MRL mice that developed anti-Sm positivity showed that the D specificity almost always appeared first. We hypothesize, therefore, that those animals in which an anti-Sm response is initiated by D-specific B-cell clones can become serologically positive with the aid of a positive feedback loop. In contrast, animals in which the initial specificity is for both B and D peptides would be prevented from developing a full anti-Sm response.

Isotype analysis of the anti-CENP-B anticentromere autoantibody: evidence for restricted clonality.

Utilizing the centromere B fusion protein (CENP-B) and specific, matched monoclonal antiisotype reagents in an enzyme-linked immunosorbent assay, we found that anti-CENP-B autoantibodies were skewed to the IgG1 isotype. The overall kappa:lambda light chain ratio was 2:1, although several individual sera showed a strong predominance of one of the light chains. Isoelectric focusing of light chain-skewed sera showed polyclonal patterns. Our findings are consistent with the anti-CENP-B autoantibody response being a chronic, antigen-driven response.

The genetics of autoantibody production in MRL/lpr lupus mice.

We have investigated the influence of background genes in the MRL strain, as compared to C57BL/6, on the induction of autoimmunity in homozygous lpr/lpr mice. We have concentrated on two autoantibodies, anti-Sm and anti-chromatin. The propensity to make anti-Sm is controlled by dominant genes from the MRL background. However, the prevalence of this response is under the control of additional recessive genes. The anti-chromatin response is found in both MRL/Mp-lpr/lpr and in C57BL/6-lpr/lpr mice, but it appears earlier and in higher titers in the former strain. This high responder effect is controlled by dominant genes. In F1 mice between these two strains, both anti-Sm and anti-chromatin antibodies preferentially use the b Igh allotype from the low responder B6/lpr parent. In addition, in the progeny of backcross of the F1 to the MRL/lpr strain, the production of both anti-Sm and anti-chromatin is linked to the b allotype. These results demonstrate the contribution of dominant genes from the MRL background on the induction of severe autoimmunity. They also suggest that the B6 background expresses an Igh allotype particularly amenable to autoantibody production, in spite of the relatively mild SLE-like syndrome in this strain.

Stochastic control of anti-Sm autoantibodies in MRL/Mp-lpr/lpr mice.

MRL/Mp-lpr/lpr autoimmune mice consistently show an approximately 25% incidence of the systemic lupus erythematosus marker autoantibody anti-Sm. In the present report, we show that the failure to find anti-Sm antibodies in three-quarters of 5-mo-old MRL/lpr mice was not an artifact of an insensitive assay, but rather that the mice fell into two populations as regards their anti-Sm positivity. Based on an extensive analysis of the incidence of anti-Sm positivity in 5-mo-old mice according to their cage of residence, we found no evidence for genetic, environmental, or parental influences on the propensity of an individual animal to become anti-Sm positive. Also, the gender of the mouse, its Sm antigen level, or its length of survival were not related to anti-Sm antibody, nor was the anti-Sm antibody status of either parent. Some animals became anti-Sm positive after 5 mo of age, but this was less likely than becoming positive before 5 mo of age. Finally, a survey of 205 autoimmune C57BL/6-lpr/lpr mice confirmed the uniqueness of the MRL background for this autoantibody response. These results together indicate that the possibility of making anti-Sm antibodies is under genetic control, but that the expression of this capability in an individual animal is governed by stochastic events. We hypothesize further that such random processes may involve the expression of particular immunoglobulin variable-region genes combined with mechanisms of extensive somatic mutation or positive feedback amplification, which would transmute an initial monoclonal response into an eventual polyclonal one.

Isotype progression and clonality of anti-Sm autoantibodies in MRL/Mp-lpr/lpr mice.

Antibodies to the nuclear ribonucleoprotein Sm are found in 25% of MRL/Mp-lpr/lpr mice, which develop a syndrome similar to human systemic lupus erythematosus. We have previously described that these autoantibodies are relatively restricted to the IgG2a isotype. In the current study, we analyze the isotype distribution of anti-Sm antibodies in these mice over time. The most common pattern observed was an initial response of the IgG2a isotype, which progressed such that this isotype was the major one at the time of peak response. No IgM to IgG class switch was observed. Additional studies directed at the clonality of the anti-Sm response indicated that both kappa- and lambda-light chains could be involved, and that the isoelectric focusing pattern of the anti-Sm antibodies was in general characteristic of multiple spectrotypes. These results also support a special role for the IgG2a isotype in the anti-Sm response in MRL/Mp-lpr/lpr mice. Despite this heavy chain isotype restriction, the response usually evidences substantial diversity, which suggests either multiple B cell clones or somatic mutation of antibody variable region genes.

Subclass restriction and polyclonality of the systemic lupus erythematosus marker antibody anti-Sm.

Anti-Sm antibodies are highly specific markers for the diagnosis of systemic lupus erythematosus (SLE). This specificity suggests that the immunoregulation of these autoantibodies would reflect fundamental immune abnormalities in this disorder. As a clue to this immunoregulation, we have investigated the isotype distribution of anti-Sm antibodies by enzyme-linked immunosorbent assays. We have found that the anti-Sm response is markedly restricted to the IgG1 heavy chain isotype. On the other hand, the light chain distribution reflects that in normal serum, while isoelectric focusing analysis fails to show an oligoclonal pattern. The related specificity, anti-ribonucleoprotein, is also restricted to IgG1, while the SLE-specific antibody anti-double-stranded DNA is mostly IgG1 with a lesser contribution by IgG3. These results suggest that antinuclear antibodies that are strongly associated with SLE are produced by a T cell-dependent response, probably driven by antigen. The immunoregulation of the response to several autoantigens may be quite similar.