The British Journal of Rheumatology, Vol 37, 1164-1171, Copyright © 1998 by British Society for Rheumatology
KJ Lackner, U Schlosser, B Lang and G Schmitz
Autoantibodies against calpastatin have recently been described to be
highly prevalent in sera of patients with rheumatoid arthritis (RA). When
the sera of 45 patients with RA were analysed for autoantibodies against
calpastatin by a newly developed enzyme-linked immunosorbent assay (ELISA),
only four sera (8.9%) tested positive, which is not significantly different
from the frequency observed in healthy controls. Since the ELISA is based
on a synthetic peptide containing the C-terminal 27 amino acids of
calpastatin bound to the solid phase, this negative result might be the
consequence of the small antigen used. Therefore, a systematic analysis of
the epitopes for autoantibodies in calpastatin was performed using sera
from RA patients and healthy individuals. Recombinant fusion proteins
containing fragments of calpastatin or the complete protein were produced
and sera analysed by Western blots. In the N-terminal portion (amino acids
1- 369), at least two major epitopes exist, against which 65% of normal
sera as well as 76% of RA sera show reactivity in Western blot assays.
These epitopes are not useful for clinical diagnostics. Only five out of 45
(11.1%) RA sera reacted against the C-terminal portion (amino acids
363-708) of calpastatin, while four out of 52 (7.7%) control sera showed
reactivity. Three of the five RA sera and two out of four control sera had
autoantibodies against the C-terminal 27 amino acids of calpastatin. These
three patient sera had already been tested positive in the ELISA. The
fourth patient positive in the ELISA was Western blot negative. The
differences between the group of RA patients and controls are not
statistically significant. When the clinical characteristics of the four
patients with autoantibodies against the carboxyl end of calpastatin were
analysed, it became apparent that all four had significantly elevated
C-reactive protein (>50 mg/l). This observation might indicate that
calpastatin autoantibodies are found in RA patients with more active
disease. Thus, while the majority of RA patients do not have an increased
prevalence of calpastatin autoantibodies, it cannot be ruled out
definitively that a small subgroup may be characterized by autoantibodies
to the C-terminus of calpastatin.
ORIGINAL PAPERS
Autoantibodies against human calpastatin in rheumatoid arthritis: epitope mapping and analysis of patient sera
Institut fur Klinische Chemie und Laboratoriumsmedizin, Klinikum der Universitat Regensburg, Germany.
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