17.3 Synthetic Biology Approaches to Cellular Immunotherapy Engineering 361
constitutive IL-15 production and the inducible caspase 9 suicide system and
demonstrated superior in vivo T-cell expansion and antitumor effects compared
with T cells expressing the CAR alone [106].
Although suicide gene systems provide a powerful countermeasure to major
adverse events such as deleterious genetic mutations in engineered cells, results
from clinical trials have also highlighted situations in which measured dampen-
ing of functions rather than complete elimination of therapeutic cells is the
preferred response. In adoptive T-cell therapy for cancer, tumor regression is
strongly associated with a dramatic increase in the level of inflammatory
cytokines, a phenomenon known as cytokine storm or tumor lysis syndrome
[107, 108]. When the intensity of the tumor lysis syndrome exceeds physiological
tolerance, corticosteroids can be administered to the patient, thereby not only
quelling the immediate dangers of therapy-associated toxicity but also terminat-
ing the treatment by effectively disabling the therapeutic cell population [109].
As a potential alternative, researchers have engineered synthetic circuits that
regulate the amplitude of T-cell activation, thus enabling fine-tuning of T-cell-
mediated responses [110]. The bacterial protein OspF downregulates T-cell
activation by inactivating the extracellular signal-regulated kinase (ERK). An
“amplitude limiter” consisting of a negative feedback loop with OspF expressed
from an NFAT promoter lowers the maximum level of T-cell activation-induced
gene expression, which can be further modulated by the addition of degradation
tags to the OspF protein. Furthermore, a “pause switch” was constructed by
expressing OspF from a doxycycline-inducible promoter, such that pulses of
doxycycline addition result in temporary reductions in T-cell activation-induced
expression [110].
(small
subunit)
Caspase 9
(large
subunit)
FKBP
iCasp9
activation
T-cell
apoptosis
Homodimerization
iCasp9
AP1903
Figure 17.4 A chemically inducible caspase 9 kill switch. Inactive pro-caspase 9 monomers
are linked to the human FK506 binding protein FKBP and constitutively expressed in the
engineered cell. Upon addition of the chemical inducer of dimerization AP1903, the FKBP
domains dimerize and lead to the cross-linking and activation of caspase 9, which triggers
downstream events in the apoptosis pathway and results in cell death.