SIGNAL CONVERTER PROTEINS - Fusion Protein Technologies for Biopharmaceuticals: Applications and Challenges - page 438

of immunoregulation. The pivotal insight, that immuno-
regulatory cells can be generated through gene or protein
transfer, prompted the subsequent development of fusion
proteins that can be used to accomplish this cellular engi-
neering feat. SCP emerged as a preferred fusion protein
option.
According to one of cellular immunology's central dog-
mas, antigen-presenting cells (APC), such as dendritic cells
(DC), drive T-cell activation through contact-dependent
intercellular signaling. Surface-associated major histo-
compatibility (MHC)

next critical advance—artificially engineering veto cells,
using gene or protein transfer to introduce into APC addi-
tional molecules that trigger the inhibition or destruction of
target T cells, in effect creating immunoregulatory APC
Trojan horses that we named artificial veto cells, or AVC
[3,4].
Given the centrality of contact-dependent signaling at the
APC-T-cell nexus, leveraging this type of juxtacrine signal-
ing for purposes of AVC engineering was a logical way to
go. A compelling paradigm emerged: the generation of
AVC, with the capacity to inhibit T cells in an antigen-
specific manner, by altering the trans (intercellular) signal-
ing properties of APC via neoexpression of T-cell inhibitory
proteins on their surfaces. Our group coined the term
“coinhibitor” to characterize such surface-associated,
trans-signaling inhibitory proteins, drawing a parallel
between them and their well-established counterparts, the
costimulators [3]. As defined, costimulators and coinhibitors
alike mediate APC-to-T-cell contact-dependent signaling,
and operate alongside and coordinately with MHC

peptide antigen complexes and
costimulators provide the classic APC-anchored first and
second T-cell-activating signals, respectively. While the
focus has traditionally been on the APC's role as a T-cell
activator, early immunology literature also hinted at another
APC functional dimension, casting it as a T-cell inhibitor.

30.2.1 Artificial Veto Cell Engineering

The view of an APC as a T-cell inhibitor arose from a
provocative observation decades ago in transplantation immu-
nology. Specifically, intravenous injection of allogeneic cells
into an immunologically incompetent recipient induces sup-
pression, rather than sensitization, of alloimmune responses.
This finding formed the basis of two-step pretransplant trans-
fusion strategies that entailed first reducing the pool of immu-
nocompetent T cells, through irradiation and drugs, and
thereafter inducing nonresponsiveness to specific alloantigens
by exposing the regenerating T-cell repertoire to alloantigenic
cells. Once alloantigen-presenting lymphoid cells were iden-
tified as the central drivers of this suppressive effect, the notion
of the veto cell emerged—a cell inhibiting a second cell that
recognizes it [1,2]. Cells capable of suppressing, or “vetoing,”
antigen-specific responders have been referred to by a variety
of other names as well, “deletional APC” and “inhibitory
APC” among them. Functionally, the veto effect has been
invoked beyond the pretransplant transfusion phenomenon, to
explain a variety of other immunological phenomena, such as
self-tolerance and resistance to graft-versus-host disease.
While the veto cell, in its original incarnation, was a
CD8 þ lymphoid cell that inhibits a class I alloreative cyto-
toxic T-cell precursor, this narrow definition gave way to a
broader view of this cell category. The repertoire of cells
capable of mediating veto grew, with the list encompassing a
melange of cell categories; some poorly defined, for exam-
ple, lymphokine-activated killer cells, activated CD4 þ T
cells, a subset of CD8 þ murine dendritic cells, and even a
nonlymphoid cell type, thymic epithelial cells. Furthermore,
approaches were developed for expanding veto cells in vitro,
for example, by cloning T cells with lymphokines, or
incubating naive lymph node cells with relatively high
concentrations of IL-2. These in vitro stimulatory methods
were directed toward eliciting and expanding naturally
occurring cells with veto capacity. This beckoned the

peptide
antigen complexes to modulate antigen-specific T cells. In
one case, there is T-cell activation and in the other inhibition.
Thus, while the “veto” term served to conceptually link this
artificially engineered cell to the natural veto cell of an
earlier immunology literature, the “coinhibitor” term, which
has since entered the immunology mainstream, emphasized
the mechanistic parallel with costimulators.
AVC engineering via neoexpression of surface coinhibi-
tors has proven to be a robust concept. A variety of cell
surface coinhibitors have emerged, but somewhat paradoxi-
cally, the lymphoid surface molecule CD8, one of the
classical cis-acting T-cell co-receptors, was the first mole-
cule to be invoked as a trans-signaling coinhibitor. Specifi-
cally, while CD8 is primarily thought of as a lymphoid co-
receptor essential for cytotoxic T-cell activation, this same
surface molecule, when artificially neoexpressed on APC,
proved capable of signaling outward, and in doing so,
inhibiting antigen-specific responder T cells. The proposed
mechanism for the latter is inhibitory reverse signaling
through T-cell-resident MHC class I, CD8's cognate
counter-receptor. This ties into a longstanding observation
that certain agonist anti-MHC class I monoclonal antibodies
can inhibit the proliferation of TCR-triggered T cells [5].
Our original identification of CD8 as a coinhibitor, along
with our broader formulation of the CD8-engineered AVC
concept [4,6-9], has been amply substantiated by other
investigators [10-12]. The artificial veto cell has even
been periodically rediscovered, sometimes recast under a
new name [13-16].
When it comes to AVC engineering, CD8 has given way
to more conventional coinhibitors. Since an apoptotic mech-
anism has been proposed for the CD8-to-MHC class
I inhibitory pathway, it was reasonable to turn to other
signaling axes more classically linked to T-cell apoptosis.