352 17 Synthetic Biology in Immunotherapy and Stem Cell Therapy Engineering
characteristic and significant advantage of cellular therapies. Viewing cells as
chasses, synthetic biologists have demonstrated that biological functions can be
rationally designed, systematically optimized, and translated across organisms
[39]. A core competency of synthetic biology is the rapid construction, integra-
tion, and characterization of biological systems, leading to well-defined, ration-
ally engineered cell products. This approach to cell engineering has generated
early examples with potential therapeutic functions and converges with work
that has been well established in the field of cellular therapeutics [40–43].
Immune system engineering has played a dominant role in cellular therapies.
The application of cell-based immunotherapy can be broadly divided into two
categories: immunosuppressive and immunostimulatory. Immunosuppressive
therapies aim to dampen aberrant immune responses that characterize inflam-
matory and autoimmune diseases such as multiple sclerosis, inflammatory bowel
diseases, and organ transplant rejection [44, 45]. For example, regulatory T cells
and myeloid-derived suppressor cells are naturally immunosuppressive cell types
under intensive investigation as treatment options for conditions ranging from
ocular inflammation to stroke-induced cerebral ischemia [46, 47]. In contrast,
immunostimulatory therapies aim to boost immune responses against infectious
agents and tumor growths. Prominent examples in this category include the use
of natural killer (NK) cells and cytotoxic T cells that directly kill diseased cells, as
well as dendritic cells that stimulate immune responses by presenting disease-
associated antigen peptides to effector cells including T cells and B cells [48–50].
The engineering of immune cells provides ample opportunity for synthetic biol-
ogy to make a real impact on the improvement of health and medicine.17.3.1 CAR Engineering for Adoptive T-Cell Therapy
Adoptive T-cell therapy is an emerging treatment paradigm in which T cells
expressing either TCRs or CARs that target specific disease markers are expanded
ex vivo prior to infusion into a patient (Figure 17.1). These systemically adminis-
tered T cells have the ability to seek and destroy target cells that display the cog-
nate antigen, thereby serving as a living drug against otherwise intractable
diseases such as refractory cancers and posttransplantation viral infections
[51, 52]. In particular, the adoptive transfer of T cells that express anti-CD19
CARs has shown remarkable curative potential against advanced B-cell malig-
nancies, achieving up to 90% complete remission rate in the treatment of acute
lymphoblastic leukemia [27, 28, 53].
The development of CARs offers an example of a synthetic biological approach to
efficient cell therapy engineering. CARs are synthetic receptors that redirect T-cell
specificity toward diseased targets, such as virally infected or cancerous cells, that
do not naturally provoke robust immune responses from endogenous T cells. CARs
are fusion proteins in which antibody-derived single-chain variable fragments
(scFvs) serve as extracellular sensing domains and are fused (via extracellular spacer
sequences and transmembrane domains) to cytoplasmic CD3ζ signaling domains
derived from the natural TCR [54] (Figure 17.2a). When the CAR is expressed by
conventional T cells, ligation of the scFv domain to cognate antigens triggers signal-
ing through the CD3ζ chain, leading to T-cell activation and unleashing cytotoxic