examination under fluorescent illumination. The
pattern of fluorescence staining can give some
indication of the specificity of the autoantibodies but
with the exception of the anti-centromere pattern,
which implies anti-centromere autoantibodies, such
inferences are not highly reliable. Instead, the
specificity of autoantibodies is evaluated by im-
munoassay with purified antigens.
Unlike the organ-localized autoimmune diseases,
autoantibodies of the same specificity develop in
many of the systemic autoimmune diseases (Keren
and Warren 1993). Table 9.3 lists those autoanti-
bodies that are highly specific markers of particular
systemic autoimmune diseases. Detection of these
autoantibodies is useful in the diagnostic differentia-
tion of the systemic diseases. Regrettably, the utility
of most of these markers is limited by their low
sensitivity. Only anti-centromere antibodies and
anti-neutrophil proteinase III autoantibodies (also
called anti-neutrophil cytoplasmic antibody, ANCA)
combine high specificity with high sensitivity.
Laboratory methods for autoantibodies.
The
laboratory measurement of autoantibodies requires
the use of methods that can detect antibodies of a
defined specificity. The obvious analytical approach
for achieving the separation of the autoantibodies
from all the other antibodies present in a test sample
is to utilize the inherent specificity of the autoanti-
body. That means having the autoantibodies bind to
their cognate antigen which is present in the test
system as a reagent (Miles
et al.
1998).
If the reagent antigen is immobilized, autoanti-
bodies will become immobilized when they bind to
the antigen. All the uninteresting antibodies, which
are unbound, can then be washed away and the
autoantibodies measured using a signal generating
reagent that reacts with immunoglobulins. This is
the methodology that underlies autoantibody detec-
tion by indirect immunofluorescence microscopy in
which histologic tissue sections or tissue culture sub-
strates constitute the immobilized reagent antigen
and fluorochrome conjugated anti-human immuno-
globulins are the signal generating reagent. It is also
the methodology employed in immunoassays, the
most popular form of which is enzyme-linked
immunosorbent assay (ELISA), in which reagent
antigen is bound to the walls of the reaction tube and
chromogenic enzyme conjugated anti-human im-
munoglobulins are the signal generating reagent.
An alternative method for detecting the binding
of autoantibodies to the reagent antigen is by particle
agglutination. Here the reagent antigen coats small,
inert particles. The binding of autoantibodies, which
are bivalent in the case of IgG and decavalent in the
case of IgM, bridge the particles producing a
gel-like lattice. When allowed to settle thoroughly,
unbound particles clump together into a button while
autoantibody-bound particles remain in a diffuse gel,
thereby signaling the presence of autoantibodies. In
the investigation of autoantibodies to red cells,
reagent red cells serve as both the source of antigen
and the agglutinating particle. In contrast to inert
particle tests, in red cell tests, clumping indicates the
presence of autoantibodies. This is because the
outer red cell membrane is negatively charged
producing a repulsive force between red cells that
keeps them apart. Autoantibody bridging overcomes
this repulsion producing red cell clumps.
Tissue transplantation
Transplantation of body tissue from another
individual, other than an identical twin, elicits an
immune response in the recipient that, if not
suppressed, leads to rejection of the allograft. This
response, which consists of cellular and humoral
components, is primarily to the histocompatibility
molecules (HLA antigens) expressed by the donor
tissue (Sykes 1996, Vartdal and Thorsby 1999). The
HLA antigens, which are the products of a set of
linked loci on chromosome 6, show an extreme
degree of polymorphism, meaning that there are
very many alleles for each of the loci. The intensity
of the immune response is roughly proportional to
the number of differences between the HLA antigen
Tissue Injury
9-8
Table 9.3
Autoantibodies of High Specificity in Systemic Autoim-
mune Diseases (McCarty-Farid 1994, Miles
et al.
1998,
Moder 1998)
Autoantibody
Disease
Sensitivity
anti-dsDNA
systemic lupus
low
erythematosus
anti-Sm proteins
systemic lupus
low
erythematosus
anti-topoisomerase I
diffuse forms
low
of scleroderma
anti-centromere
CREST variant of
high
scleroderma
anti-neutrophil
Wegener’s
high
proteinase III
granulomatosis
anti-aminoacyl-tRNA polymyositis
low
synthetase
dermatomyosistis
