Cell death:
a trigger of autoimmunity?
R.J.T. Rodenburg, J.M.H. Raats, G.J.M. Pruijn,
and W.J. van Venrooij*
Summary
Systemic autoimmune diseases are characterized by the
production of antibodies against a broad range of self-
antigens. Recent evidence indicates that the majority of
these autoantigens are modified in various ways during
cell death. This has led to the hypothesis that the
primary immune response in the development of auto-
immunity is directed to components of the dying cell. In
this article, we summarize data on the modification of
autoantigens during cell death and the possible con-
sequences of this for autoimmunity. BioEssays
22:627±636, 2000. ß 2000 John Wiley & Sons, Inc.
Introduction
One of the most important questions in immunology is how
the immune system can distinguish between ``self'' and ``non-
self''. In a normal, healthy individual, the immune system is
able to specifically eliminate unwanted, non-self and poten-
tially dangerous organisms without attacking its own tissues
or cellular components. In some cases, however, this fine
tuning is disturbed, leading to autoimmunity: the activation
and proliferation of autoreactive lymphocytes, or even to an
autoimmune disease. In the case of organ-specific autoim-
munity, the antibodies produced by the activated B-lympho-
cytes are directed to self-components expressed only in a
specific tissue or type of cells. In the case of systemic auto-
immunity, the autoantibodies are directed to various auto-
antigens,
(1)
which are usually expressed in a wide variety of
tissues and, at the cellular level, can be present in the
nucleus, in the cytoplasm or at the cell surface. Although
many autoantigens are ubiquitously expressed, the auto-
immune diseases in which they are autoantigenic are often
limited. For example, Jo-1 is an aminoacyl-tRNA synthetase
and is autoantigenic in autoimmune disorders involving
primarily muscle tissue (myositis).
(2)
The factors that are
known to be involved in the etiology of systemic autoimmune
disorders can be divided into at least three groups: hormonal,
genetic and environmental. The fact that women are
approximately four times more likely than men to develop
an autoimmune disorder is a clear indication that sex
hormones play an important role. In addition, several genetic
factors have been shown to be involved in the development of
autoimmunity, including major histocompatibility complex
(MHC) class-II. Environmental factors that may affect the
development of systemic autoimmunity include UV light,
mercury intoxication and several immunomodulatory drugs.
Although all of the above-mentioned factors can be involved
in the development of autoimmunity, the primary cause of
these diseases remains unclear. In the past, several
hypotheses have been proposed. For example, the molecular
mimicry model implies that immune responses directed to
non-self-proteins (e.g. of viral origin) may spread to autoanti-
gens that resemble these non-self-proteins, thus triggering an
autoimmune response. An alternative model hypothesizes
that anti-idiotypic antibodies may be involved in the genera-
tion of autoimmunity; for example, antibodies directed against
a viral protein that interacts with a host receptor might induce
anti-idiotypic antibodies that also recognize the host receptor,
thus initiating an autoimmune response.
(3)
Another model
implies that uncommon modifications of self-proteins could
trigger autoimmunity. The last model has received increased
attention in the past few years, owing to a number of
discoveries suggesting that the origin of such modified self-
antigens might be the dying cell. In this review, we will discuss
the latest evidence supporting the idea that a defect in
apoptosis might be the triggering event leading to the
production of autoantibodies.
The first clear indication for a link between autoimmunity
and cell death came from research on the MRL/lpr mouse
strain. These mice lack a functional CD95 (Fas) gene,
(4)
which encodes the receptor for CD95L (or Fas ligand). The
interaction of the CD95 receptor protein with CD95L induces
apoptosis in the CD95-bearing cell.
(5)
The CD95-system is
involved in the depletion of self-reactive T and B lymphocytes,
and thus plays an important role in the maintenance of
peripheral tolerance.
(6)
The MRL/lpr mice show defective
clearance of autoreactive T cells, which probably leads to the
BioEssays 22:627±636, ß 2000 John Wiley & Sons, Inc. BioEssays 22.7 627
Department of Biochemistry, University of Nijmegen, Nijmegen, The
Netherlands.
Funding agencies: The Netherlands Organization for Scientific
Research (NWO), Het Nationaal Reumafonds of the Netherlands.
The work of Dr. J. Raats has been made possible by a fellowship of
the Royal Netherlands Academy of Arts and Sciences.
*Correspondence to: Walther J. van Venrooij, University of Nijmegen,
Department of Biochemistry, PO Box 9101, NL-6500 HB Nijmegen,
The Netherlands. E-mail: W.vanVenrooij@bioch.kun.nl
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