Reduced IgG anti-small nuclear ribonucleoprotein autoantibody production in systemic lupus erythematosus patients with positive IgM anti-cytomegalovirus antibodies

Abstract

Introduction: Systemic lupus erythematosus is characterized by production of autoantibodies to RNA or DNA-protein complexes such as small nuclear ribonucleoproteins (snRNPs). A role of Epstein-Barr virus in the pathogenesis has been suggested. Similar to Epstein-Barr virus, cytomegalovirus (CMV) infects the majority of individuals at a young age and establishes latency with a potential for reactivation. Homology of CMV glycoprotein B (UL55) with the U1snRNP-70 kDa protein (U1-70 k) has been described; however, the role of CMV infection in production of anti-snRNPs is controversial. We investigated the association of CMV serology and autoantibodies in systemic lupus erythematosus.

Methods: Sixty-one Mexican patients with systemic lupus erythematosus were tested for CMV and Epstein-Barr virus serology (viral capsid antigen, IgG, IgM) and autoantibodies by immunoprecipitation and ELISA (IgG and IgM class, U1RNP/Sm, U1-70 k, P peptide, rheumatoid factor, dsDNA, beta2-glycoprotein I).

Results: IgG anti-CMV and IgM anti-CMV were positive in 95% (58/61) and 33% (20/61), respectively, and two cases were negative for both. Clinical manifestation and autoantibodies in the IgM anti-CMV+ group (n = 20) versus the IgM anti-CMV(-)IgG+ (n = 39) group were compared. Most (19/20) of the IgM anti-CMV+ cases were IgG anti-CMV+, consistent with reactivation or reinfection. IgM anti-CMV was unrelated to rheumatoid factor or IgM class autoantibodies and none was positive for IgM anti-Epstein-Barr virus-viral capsid antigen, indicating that this is not simply due to false positive results caused by rheumatoid factor or nonspecific binding by certain IgM. The IgM anti-CMV+ group has significantly lower levels of IgG anti-U1RNP/Sm and IgG anti-U1-70 k (P = 0.0004 and P = 0.0046, respectively). This finding was also confirmed by immunoprecipitation. Among the IgM anti-CMV(-) subset, anti-Su was associated with anti-U1RNP and anti-Ro (P < 0.05). High levels of IgG anti-CMV were associated with production of lupus-related autoantibodies to RNA or DNA-protein complex (P = 0.0077).

Conclusions: Our findings suggest a potential role of CMV in regulation of autoantibodies to snRNPs and may provide a unique insight to understand the pathogenesis.

Figure 1

Serum rheumatoid factor and other IgM class antibodies in IgM anti-CMV(+) versus anti-CMV(-) patients. IgM anti-cytomegalovirus (CMV) (microparticle enzyme immunoassay), rheumatoid factor (RF) (laser nephelometry), IgM or IgG antibodies to mouse IgG, IgM anti-U1snRNP-70 kDa protein (anti-U1–70 k) and anti-β₂-glycoprotein I (anti-β2GPI) (ELISA) were performed as described in Materials and methods. Serum dilution for ELISA was 1:100 for IgM class and 1:500 for IgG anti-mouse IgG.

Figure 2

IgG autoantibodies by ELISA in IgM anti-CMV(+) versus IgM anti-CMV(-) patients. (a) IgG anti-U1RNP/Sm and IgG anti-U1snRNP-70 kDa protein (anti-U1–70 k) by ELISA. Anti-small nuclear ribonucleoprotein (Anti-snRNP) immunoprecipitation-positive sera (1:2,500 dilution, n = 7 for the IgM anti-cytomegalovirus (CMV)(+) group and n = 12 for the anti-CMV IgM(-)IgG(+) group) were tested for anti-U1RNP/Sm and U1–70 k antibodies by ELISA. (b) IgG anti-U1–70 k, P peptide, dsDNA, and anti-β₂-glycoprotein I (anti-β2GPI) by ELISA. ELISAs were performed as described in Materials and methods using 1:500 diluted sera. For anti-U1RNP/Sm and U1–70 k, data from 1:2,500 diluted sera are shown.

Figure 3

Autoantibodies to small nuclear ribonucleoproteins by immunoprecipitation. Immunoprecipitation by anti-small nuclear ribonucleoprotein immunoprecipitation-positive sera (lanes 1 to 7, all seven sera from the anti-cytomegalovirus (CMV) IgM(+) group; lanes 8 to 13, six randomly selected sera from the 12 in the anti-CMV IgM(-)IgG(+) group) and a normal human serum (NHS) are shown. Positions of UsnRNP components A, B'/B, C, D1/D2/D3, E/F, G, U5–200 k doublet, and 60 k Ro, and molecular weights (kDa) are indicated. White arrowheads, U5–200 k.