BASIC
SCIENCE FOR THE CLINICIAN
Children’s Hospital of Philadelphia and
the University of Pennsylvania,
Philadelphia, PA, USA
Address
correspondence request to:
Dr.
Randy Q. Cron, MD, PhD
Children’s
Hospital of Philadelphia,
3615
Civic Center Blvd.
Abramson
Research Center, Rm. 1102B
Tel.:
(215) 590-1844
FAX:
(215) 590-1258
e-mail:
rqcron@mail.med.upenn.edu
Running
title: CD40 Ligand and
lupus
CD154 (CD40 ligand) is expressed as a
soluble cytokine and as a homotrimeric type II transmembrane protein on the
surface of activated CD4 T lymphocytes. The interaction of CD154 with its
receptor, CD40, on B cells is critical for B cell growth, differentiation, and
antibody isotype switching. Because of its multiple effects in the immune
system, expression of CD154 is normally very tightly regulated. However, in
autoimmune diseases, such as systemic lupus erythematosus (SLE), CD154 is
over-expressed and contributes to disease pathology. Although monoclonal
antibody therapies directed against CD154 have been successful at treating
disease in murine models of SLE, human trials have been disappointing to date.
Therefore, approaches based on correcting the dysregulation of CD154 in SLE are
being pursued. Studies of the normal regulation of CD154, and of the
dysregulation of CD154 in SLE, are still in their infancy but clues to the
pathways and proteins involved are being reported. Ultimately, the ability to
manipulate CD154 expression may prove useful for correcting the defect(s) in
autoimmune disorders, such as SLE, and it may also aid in treating a whole host
of other diseases where CD154 is thought to play a role.
CD154 in the Immune Response
CD154 (formerly known as CD40 ligand) is
a member of the tumor necrosis factor superfamily, and it is rapidly expressed
as a cell surface activation antigen and as functionally active soluble trimers
in the serum (1). CD154 is
primarily expressed by activated CD4 T lymphocytes for brief periods of time,
up to 24 hours after stimulation (2). CD40, which
is constitutively expressed by B lymphocytes, macrophages, dendritic cells, and
many other cell types, is the only known partner receptor for CD154 (3). By
interacting with CD40, CD154 triggers a pleiotropic immune response.
B cells depend on CD154 stimulation for
growth, development, and antibody isotype class switching (4). However,
recent studies in vitro suggest that
CD154 may not be absolutely required for the generation of certain
immunoglobulin isotypes (5). In addition,
the CD154-CD40 interaction is critical for germinal center formation (6). CD154
triggering of dendritic cells also stimulates interleukin-12 production and
indirectly primes CD8 T cell effector function (6). CD154 is
important for optimal T cell dependent antibody responses as well (7). It is no
wonder that CD154 has been considered by some as the, “Center of the Immune
Universe” (8).
Because CD154 drives multiple
effector functions throughout the immune system, CD154 expression on CD4 T
cells is normally very tightly regulated. Like many cytokines, expression of
CD154 is primarily regulated at the level of transcription. The human CD154
transcriptional promoter has been partially characterized and was found to be
positively regulated by the cyclosporin A (CsA)-sensitive transcription factor
family, nuclear factor of activated T cells (NFAT) (9). Some NFAT
proteins are pre-existing in the cytoplasm of T cells and rapidly transit to
the nucleus upon sustained intracellular calcium levels following T cell
activation (10).
Recently,
several other transcription factors have also been implicated in the regulation
of the CD154 promoter (11).
In addition to the CD154 transcriptional promoter, two others
have been identifies by DNase I hypersensitivity site mapping (11).
A novel GATA- and NFAT-regulated transcriptional enhancer
element has been identified just upstream (5’) of the CD154 transcriptional
promoter (12), and a newly
described NFκB-responsive enhancer has recently been reported within and
immediately downstream (3’) of the CD154 3’ untranslated region (13). The 3’
untranslated region also serves to regulate CD154 expression at the level of
mRNA stability, and a novel RNA binding protein may specifically regulate CD154
expression (14). Thus,
although the regulation of CD154 in primary human CD4 T cells under normal
circumstances is complex, progress has been made in defining this regulation (11).
Abnormal or dysregulated expression
of CD154 by CD4 T cells, and by and other cell types that do not normally
express CD154, has been implicated in the pathogenesis of a wide array of
diseases ranging from atherosclerosis to Alzheimer disease (11). Dysregulated
expression of CD154 has also been associated with a variety of autoimmune
disorders from rheumatoid arthritis to inflammatory bowel disease (Table I).
However, the autoimmune disease best characterized in terms of CD154
dysregulation is systemic lupus erythematosus (SLE).
Murine models of SLE first reported
that CD154 was over-expressed on T cells from lupus-prone strains of mice (15). Furthermore,
blockade of the CD154-CD40 interaction in mice with lupus has been shown to delay
and decrease the incidence of glomerulonephrtitis (15,
16).
Moreover, treatment with anti-CD154 monoclonal antibody prolonged the survival
of mice with established lupus nephritis (17). Taking
another approach, two independent groups of researchers recently showed that
over-expression of CD154 in mice in vivo
triggered autoantibody production (18) and lead to a
lupus-like disease (19). Thus, CD154
over-expression appears to be a major culprit in the pathogenesis of disease in
murine models of SLE.
Over-expression of CD154 on CD4 T
cells has also been demonstrated in human SLE. Two independent labs reported
that in vitro activated peripheral
blood T cells from patients with SLE expressed increased and prolonged (over 24
hours) levels of CD154 (Table II) (20,
21).
As controls, two different activation markers, CD25 and CD69, were expressed at
similar levels after mitogen-induced activation of T cells from controls and
patients with SLE (20,
21).
Furthermore, CD154 has been found to be over-expressed immediately ex vivo on T cells from patients with
SLE (20,
22).
Similar results for baseline and in vitro-activated
hyper-expression of CD154 on T cells have also been found in children with SLE (23). Importantly,
the increased expression of CD154 on SLE T cells was demonstrated to induce
higher levels of CD80 on co-cultured B lymphocytes (20) and to
produce pathogenic-variety antinuclear antibodies in vitro (21). It is
currently unclear whether the increased/prolonged expression of CD154 helps to
break tolerance of autoreactive B cells by increasing their survival and/or by
providing growth and differentiation signals.
Nevertheless, it does seem likely that over-expression of CD154 on T
cells of patients with SLE contributes to disease pathogenesis.
In addition to the increased
expression of CD154 on CD4 T cells, patients with SLE were also found to have
ectopic expression of CD154 on B cells, CD8 T cells, and CD4-,CD8-
T cells (Table II) (21,
22).
Furthermore, soluble CD154 (sCD154) has been reported to be elevated in the
serum of patients with SLE compared to controls (Table II) (24-26). The levels
of sCD154 correlated with dsDNA titers and disease activity scores in patients
with SLE (26), and the
elevated sCD154 levels were found to be functional in that they were shown to
increase expression of accessory molecules on B cells (25,
26).
Interestingly, a correlation between sCD154 levels and coronary artery
calcification was recently noted in adults with SLE (24). Elevated
levels of sC154 and surface bound CD154 have also been shown to be a marker for
unstable angina (27). Therefore,
elevated sCD154 levels may serve as an indicator to screen for coronary artery
disease in patients with SLE.
In one study, levels of sCD154 but not surface CD154 on T cells were found to correlate with CD154 mRNA levels from SLE T cells (26). This does not address, however, whether or not the increased CD154 is due to differences in CD154 transcription and/or CD154 mRNA stability. Preliminary data argues for both increased CD154 mRNA stability (28) and increased CD154 transcription