Complete saturation of protamine sulphate by dsDNA is necessary in order to obtain a highly sensitive and specific anti-dsDNA ELISA

Antibodies to guanosine triphosphate misidentified as anti-double-stranded DNA antibodies in a patient with antinuclear antibody-negative lupus, due to buckling of insolubilized assay DNA

OBJECTIVE

To investigate why the serum of a pediatric patient with systemic lupus erythematosus was persistently (>30 months) and strongly positive for antibodies to double-stranded DNA (dsDNA) as revealed by enzyme-linked immunosorbent assay (ELISA), but yielded negative results on the antinuclear antibody test (HEp-2 immunofluorescence [IF]).

METHODS

The patient's antibodies were isolated on dsDNA and single-stranded DNA (ssDNA) supports, which were then examined by dsDNA ELISA and HEp-2 IF. Tests included the use of various inhibitors to determine the fine specificity of the antibodies. Other tests performed included immunoblotting, immunoprecipitation, Crithidia luciliae IF, and neutrophil IF.

RESULTS

The antibodies isolated from the dsDNA and ssDNA supports were similar, in that they were of the IgG type, bound well in the dsDNA ELISA, and recognized a normally hidden nucleolar RNA antigen in HEp-2 cells. With both the dsDNA ELISA and nucleolar antigens, inhibition studies revealed that the epitope recognized was guanosine 5'-triphosphate (GTP). Binding of the antibodies was better to GTP than to guanosine 5'-monophosphate or cytidylyl (3'-5') guanosine, and, in turn, was better than to guanosine, while N7-methylated GTP was unreactive. The antibodies did not bind to dsDNA present in solution or in HEp-2 or Crithidia cells, but bound transfer RNA well and recognized a cytoplasmic RNA antigen in neutrophils.

CONCLUSION

A new problem in dsDNA ELISA is revealed in the occurrence of a hitherto-unknown and unusual buckling of the insolubilized DNA molecule, which, absent in dsDNA found in solution or in whole cells, presumably creates gaps of single-strandedness in the molecule. A new antibody specific for GTP is described in this patient, which may be clinically important.

Some autoantibodies to Ro/SS-A and La/SS-B are antiidiotypes to anti-double-stranded DNA

OBJECTIVE

To determine the relationship of anti-Ro/SS-A and anti-La/SS-B antibodies to anti-double- stranded DNA (anti-dsDNA) in sera from patients with systemic lupus erythematosus.

METHODS

Sera with anti-Ro/SS-A alone (n = 5) or those with anti-Ro/SS-A and anti-La/SS-B (n = 7) were absorbed with purified Ro/SS-A and La/SS-B, respectively. The absorbed sera were then tested for reactivity with MOLT-4 extract by Western blot and dsDNA by enzyme-linked immunosorbent assay (ELISA). With selected sera, anti-dsDNA was isolated on DNA cellulose columns and anti-Ro/SS-A and anti-La/SS-B were isolated on antigen-affinity columns. Reactivity between anti-dsDNA and autologous anti-Ro/SS-A or anti-La/SS-B, as well as inhibition by cognate antigens, was studied.

RESULTS

After absorption, all sera showed reactivity with small nuclear RNP A and D bands in Western blots, and some showed reactivity with dsDNA by ELISA. Anti-dsDNA populations (n = 4) were purified on dsDNA cellulose columns. Anti-Ro/SS-A (n = 1) and anti-La/SS-B (n = 3) were affinity purified from the same sera as the anti-dsDNA. In all cases, anti-dsDNA bound autologous anti-Ro/SS-A and anti-La/SS-B much more strongly than it bound normal pooled IgG. Moreover, dsDNA, but not RNA, blocked these interactions. In addition, Ro/SS-A blocked anti-Ro/SS-A and La/SS-B blocked anti-La/SS-B in these same interactions.

CONCLUSION

In sera with anti-Ro/SS-A and anti- La/SS-B, there are subpopulations of these antibodies that bind and mask anti-dsDNA. We hypothesize that these anti-Ro/SS-A and anti-La/SS-B antibodies are antiidiotypes to idiotypes on anti-dsDNA and that they both mask and down-regulate these anti-dsDNA antibodies.