Synthetic Biology Parts, Devices and Applications

17.3 Synthetic Biology Approaches to Cellular Immunotherapy Engineering 359

molecules such as tumor growth factor-beta (TGF-β) or PD-1 (Figure 17.3b)
[86–90]. These genetically modified T cells can resist immunosuppression and
retain greater in vivo antitumor activity and are currently under clinical evalua-
tion (NCT00889954). The rapid progression of clustered regularly interspaced
short palindromic repeat (CRISPR)/Cas9 and mammalian genome editing tech-
nologies has also enabled the corollary approach of ablating immunosuppressive
signaling by knocking out inhibitory receptor expression (Figure 17.3b). Indeed,
the first FDA-approved clinical trial involving CRISPR/Cas9-edited cells will
examine the use of T cells that had been genetically modified to knockout PD-1
expression [91, 92], and a similar trial has already begun accruing patients abroad
(NCT02793856).
To further combat the effect of tumor-mediated immunosuppression,
researchers have sought to actively invert suppressive cues to promote T-cell
activation. One example of signal inversion was accomplished by fusing the
extracellular ligand-binding domain of the inhibitory IL-4 receptor to the intra-
cellular signaling domain of the immunostimulatory IL-7 receptor [93, 94]
(Figure 17.3c). Expression of the resulting IL-4/IL-7 inverted cytokine receptor
(ICR) reversed the suppression of PSCA-CAR T-cell activity in culture condi-
tions mimicking the tumor milieu of pancreatic cancers [94]. Similarly, another
study demonstrated that a chimeric receptor that converts PD-L1 binding to
CD28 costimulation could elevate the effector functions of low-avidity T cells
to levels observed in high-avidity T cells, suggesting a method to boost the ther-
apeutic efficacy of T cells previously deemed unsuitable for adoptive T-cell
therapy [95].

17.3.3 Generating Safer T-Cell Therapeutics with Synthetic Biology

Although extensive research has focused on improving the efficacy of cellular
immunotherapy, safety remains the paramount priority in therapeutics develop-
ment. Even precisely engineered cells retain the possibility of mutation after
prolonged periods of expansion inside the host organism. Similarly, sustained

(c)

ICR-expressing

T cell

Tumor cell

Inverted cytokine

receptor

Immunosuppressive

cytokine

(e.g., IL-4)

Inverted signaling

ICR

Proliferation

Persistence

Effector function

Figure 17.3 (Continued )