Objective. To examine humoral immune responses to the native Ku antige
n and to evaluate the role of autoantibodies in stabilizing intermolec
ular contacts between the p70 and p80 Ku subunits. Methods. Recombinan
t free human p70 and p80 Ku subunits and p70/p80 heterodimers were exp
ressed in Sf9 (insect) cells using baculovirus vectors. Affinity-purif
ied recombinant human p70, p80, and p70/p80 dimer were studied by enzy
me-linked immunosorbent assay (ELISA) and immunoprecipitation to evalu
ate autoantibody specificities in sera from 58 patients with systemic
autoimmune disease. Results. Anti-Ku antibodies were detected by ELISA
or immunoprecipitation using K562 cell Ku antigen. All of the sera we
re reactive with the native recombinant p70, p80, or p70/p80 antigens:
47% were anti-p70+,anti-p80+ and 32% were anti-p70-,anti-p80 +, but o
nly 3% were anti-p70+,anti-p80-. Unexpectedly, 18% of the sera recogni
zed the p70/p80 dimer but did not recognize native p70 or p80 alone. A
subset of sera containing autoantibodies that prevent the dissociatio
n of p70 from p80 by high salt and detergent treatment was identified;
monoclonal antibody (MAb) 162, a murine anti-Ku MAb, displays the sam
e property. Autoantibodies that stabilize the p70-p80 interaction were
found most frequently in sera containing both anti-p70 and anti-p80 a
ntibodies. Conclusion. Autoantibodies to the native p80 subunit of Ku
are more common than are anti-p70 antibodies. When anti-p70 antibodies
were detected, they generally were found together with anti-p80. A no
vel type of autoantibody capable of stabilizing the p70/p80 heterodime
r was identified in human sera for the first time. These ''stabilizing
'' autoantibodies are found in sera containing both anti-p70 and anti-
p80 antibodies, and also are produced by mice immunized with human Ku
antigen. Autoimmunity to Ku may be initiated with an immune response t
o p80, followed by spreading to p70. We hypothesize that stabilizing a
ntibodies could facilitate the spreading of autoimmunity from one subu
nit of Ku to another by altering the processing of p70 or p80 by antig
en-presenting cells.