362 17 Synthetic Biology in Immunotherapy and Stem Cell Therapy Engineering
In addition to the magnitude of the immune response, the precision of effector
functions is critical to the development of safer cell-based immunotherapy. For
example, there is an emerging consensus that the lack of tumor-exclusive, sur-
face-bound antigens presents a fundamental challenge to the widespread imple-
mentation of adoptive T-cell therapy [111, 112]. T cells identify target cells
via surface receptor-mediated recognition of membrane-bound biomarker.
However, tumor cells rarely express surface antigens that are completely absent
in all healthy tissues. As a result, basal antigen expression by healthy tissues fre-
quently elicits “on-target, off-tumor” toxicities in adoptive T-cell therapy.
Bispecific CAR-T cells capable of AND- or AND-NOT-gate signal computation
discussed previously represent one approach to increasing the precision of dis-
ease-cell recognition based on surface interactions [69–73]. Another recently
reported approach endows T cells with the ability to interrogate the intracellular
environment of target cells through the delivery of intracellular antigen-respon-
sive cytotoxic switches derived from the cytotoxic proteins granzyme B (GrB)
[113]. Expression of a small ubiquitin-like modifier (SUMO)–GrB fusion protein
selectively triggered cytotoxicity in cells overexpressing the intracellular tumor-
associated sentrin-specific protease 1 (SENP1) [113]. Coupled with demonstra-
tions of recombinant GrB transfer from T cells into target cells, these results
point to a potentially viable strategy for improving the therapeutic precision of
adoptive T-cell therapy by expanding the repertoire of targetable candidate anti-
gens to include a plethora of intracellular disease signatures. Although these sys-
tems remain to be validated in vivo, they highlight the versatility and malleability
of cellular therapeutics, as well as the importance of effective engineering tech-
niques in the development and optimization of cell-based immunotherapy.17.4 Challenges and Future Outlook
The ability to efficiently design, construct, and optimize synthetic biological sys-
tems that modify and/or interface with living cells is expanding new possibilities
in the development of cellular therapeutics and offering enticing views of next-
generation strategies for disease treatment. Early synthetic biological circuits
consist of input/output devices linked in various configurations to achieve
diverse purposes, including signal oscillation, memory, and cell–cell communi-
cation [114–117]. If engineered to fit the clinical context and application-specific
requirements, such functions could significantly improve the performance of
cellular therapeutics. For example, a robust, tunable oscillation pattern would
enable the regular, pulsatory delivery of drug molecules that are either synthe-
sized or carried by therapeutic cells. The ability to memorize and keep count of
events such as cell divisions would enable timed proliferation and death of engi-
neered cells, providing an additional mechanism to ensure the safety of cellular
therapies. The ability to sense extracellular molecular signals and communicate
with other cells could enable time-, position-, and community-dependent res-
ponses that serve as disease diagnostics or enhance the specificity of cellular
therapeutics toward disease targets. In addition to synthetic circuitry that con-
fers novel functions onto engineered cells, rapidly advancing genome editing