SIGNAL CONVERTER PROTEINS - Fusion Protein Technologies for Biopharmaceuticals: Applications and Challenges - page 441

well, with evidence for antigen-induced cell death of allor-
eactive cells [51]. The demonstration of CTLA-4 FasL's
in vivo efficacy has also been extended to animal models of
autoimmune and alloimmune pathogenesis, including auto-
immune diabetes [55] and cardiac and corneal transplanta-
tion [56,57].

permeability of the neurovascular unit [63,64], and its
endogenous expression in the central nervous system is
elevated during autoimmune encephalomyelitis and acute
cerebral ischemia. Moreover, TWEAK has proangiogenic
activity [65], which is of interest given the association
between angiogenesis and autoimmune pathogenesis [66].
TWEAK exacerbates neurodegeneration and disease sever-
ity in experimental autoimmune encephalitis (EAE), a
murine model for multiple sclerosis [67,68]. The induction
of inhibitory anti-TWEAK or anti-Fn14 antibodies, via
vaccination with the extracellular domain of either TWEAK
or Fn14, ameliorates autoimmune encephalomyelitis mani-
festations in rodent models [69].
TRAIL (TNF-related apoptosis-inducing ligand;
CD253) binds to a number of different cognate receptors
of the TNF receptor superfamily, some activating and others
decoy [70,71]. The activating receptors in humans are
TRAIL-R1 (CD261), TRAIL-R2 (CD262), and osteoprote-
grin, and in mice the sole activating receptor is DR5.
Virtually all cells of the immune system (T/B/NK cells,
DC, monocytes, granulocytes) upregulate surface TRAIL
in response to IFN and other activation signals. TRAIL
inhibits autoimmune manifestations in animal models of
EAE [72-75] and autoimmune diabetes [76,77]. Evidence
for TRAIL's capacity to inhibit EAE has come from
experiments invoking TRAIL / knockout mice, soluble
TRAIL receptor or neutralizing anti-TRAIL antibodies
capable of blocking TRAIL function, and embryonic
stem cell-derived dendritic cells co-expressing TRAIL
and auto-antigen [73-75,78]. Interestingly, in multiple
sclerosis patients, soluble TRAIL has emerged as a
response marker for IFN- b therapy [79], with those most
likely to respond to treatment showing early and sustained
soluble TRAIL induction after therapy.
On the basis of this constellation of activities associated
with the TWEAK:Fn14 and TRAIL:TRAIL-R signaling axes,
we chose to bridge them within a chimeric Fn14

30.4 EXPANDING TRANS SIGNAL CONVERSION
OPTIONS: REDIRECTING SIGNALS

As defined, TSCP convert intercellular signals between two
interacting cells. In looking to new functional SCP catego-
ries, we envisioned one that would bring third-party cells
into the fold. Along these lines, we conceived of trans signal
redirecting proteins (TSRP), which would redirect con-
verted signals to third-party cells, outside of the original
cellular dyad. While for TSCP, the cell being modulated
bears receptors for both the blocked and converted signals,
for TSRP, these two receptors reside on different cells. In
effect, one is using a fusion protein to elicit “bystander
veto.” This phenomenon is exemplified by the composite
grafting of allogeneic islets of Langerhans with syngeneic
myoblasts expressing FasL (CD95L) to induce apoptosis in
infiltrating mononuclear cells and protect the islet graft from
immune rejection [58].
We designed Fn14 TRAIL as a paradigmatic immune-

inhibitory TSRP. This fusion protein taps into two well-
studied TNF superfamily signaling axes, TWEAK:Fn14
(CD266) and TRAIL (CD253):TRAIL-R (TRAIL-R1,
CD261; TRAIL-R2, CD262; TRAIL-R3, CD263; TRAIL-
R4, CD264), which are linked to inflammation and immu-
noregulation, respectively [59,60]. TWEAK (TNF-related
weak inducer of apoptosis), a type II transmembrane protein,
is expressed on a range of immune and nonimmune cell
types [61], and is upregulated on activated monocy-
te/macrophages. TWEAK's counter-receptor, Fn14 (fibro-
blast growth factor-inducible 14 kDa protein), is also widely
expressed on many cell types, with the notable exceptions of
T and B cells. Fn14 on other immune cells (monocyte/
macrophages, NK cells, DC) is upregulated by interferon
(IFN)- g , and though generally low in normal tissues, is
overexpressed in the context of tissue injury.
Functionally, the TWEAK/Fn14 axis contributes to tissue
repair and remodeling [62]. TWEAK induces several proin-
flammatory cytokines and adhesion molecules, yet sup-
presses production of cytokines (IFN- g , IL-12) linked to
the transition from innate to adaptive Th1 immunity.
TWEAK promotes the proliferation of some cell types
(astrocytes, endothelial cells, and certain human tumor
cell lines) [59]. However, TWEAK:Fn14 signaling effects
go beyond cytokine production and cell proliferation per se;
encompassing a broader set of functions that contribute to
autoimmune inflammatory disorders. TWEAK increases the

TRAIL

protein [80]. The Fn14 component of Fn14 TRAIL binds

to surface TWEAK at inflammatory sites, while the TRAIL
ligand component triggers the inhibitory, pro-apoptotic
TRAIL-R receptors on third-party (Fn14-negative) activated
T cells in these locations. Fn14

TRAIL has a number of

interesting features as follows:

As a blocker of TWEAK, this fusion protein interferes
with a variety of TWEAK proinflammatory activities,
including triggering of cells of the innate immune
system, blocking angiogenesis, and preventing
TWEAK-mediated opening of the blood-brain barrier
(in the context of autoimmune pathogenesis in the
central nervous system). Presumably, key targets are
TWEAK þ professional APC (macrophages, DC) and
other cells capable of antigen-presenting function
(such as endothelial cells).