CAR T (chimeric antigen receptor T cell) therapy is one of the most promising treatment for cancer. However, without proper control
after administration of CAR-T cells, severe adverse effects associated with CAR-T therapy may bring fatal risks to the patients,
especially during the clinical trial stages. Thus we designed a universal add-on for all CAR-Ts and TCR-Ts to ensure safety.
While immunosuppressive drugs like Tocilizumab and corticosteroids are commonly used for such treatments, multiple research groups
and companies have proposed different strategies to mitigate these toxic side effects, including suicide switches, inducible
expression of receptors and receptors regulated by small molecules. We conducted exhaustive literature search to find these
strategies and compared the pros and cons of our design to theirs, finding our own strengths and potential improvements in the
future.
We also designed how to integrate our safe switch into the current CAR T therapy, and other potential applications of the unique
properties of U24 protein in other aspects of immunotherapy.
A more detailed version of each method can be seen in Description(See Our Description)
To date the most common way in clinical practices is to use immunosuppressive drugs, including Tocilizumab and corticosteroids. These
drugs is effective, exemplified by reversion of cytokine release syndrome (CRS) within hours. However, the global effects may
increase the risk of opportunistic infection.
In contrast, integrating a safe switch in CAR T, by genetic modification limits the treatment effect specifically to engineered cell
population alone. However, it remains to be seen whether the inhibition of CAR T alone is enough to counteract the adverse effects,
since CRS and neurotoxicity is a collective result of several cell types and cytokines.
Suicide genes are well-characterized and has seen applications in several clinical trials of cell transfer therapies. They are
universal as they target common pathways to induce cell death. However, they irreversibly eradicate the infused cell population.
In contrast, on/off-switch strategies enable repeated on and off cycles by adding and removing of drugs. However, few strategies have
been reported, possibly due to the difficulty in engineer receptor machinery that respond to both antigens and drugs. Strategies
reported to date rely heavily on direct modification on CAR receptors, which may not be transferable from one type of CAR to another
due to their structural difference.
On and off switch differ in their action in the presence of drugs, the former activating the T cells and the latter inhibiting them.
Although they possess distinct response dynamics and could both serve as effective safe switch, On-switch has one limitation that it
requires continual drug administration for CAR T’s antitumor functions. However, this may be expensive, especially when the
controlling drugs are antibodies recognized by CARs (Rodgers et al., 2016) . In addition, the
persistence of CAR T cells in patients are believed to keep cancer cells in check (Kalos et al., 2011)
, and such benefit of “living drugs” is diminished without adding activating drugs.
To ensure safety, it is preferred that our device integrated into the same vector for T cells transduction and CAR expression. Thus, any T cell expressing CARs are likely to have a controlling machinery.
Since our device uses transcriptional-based regulation, a shortcoming is that it takes longer to respond after drug is added. In
order to maximize the potential of our device, we proposed the following methods for usage.
First, drugs can be added before or at the same time of infusion, and slowly decrease the dose of drugs. Thus, the CAR T cells
reactivity can increase steadily without causing an overreaction.
Second, drugs can be added prior to severe CRS development, in order to prevent severe CRS in the first place. Several markers have
been used to grading the severity of CRS (Lee et al., 2014) , thus it is possible to give drugs
accordingly.
Third, when CRS develops, both doxycycline and immunosuppressive drugs are given at the same time. Immunosuppressive drugs are given
to reverse the syndrome subsequently removed to prevent prolonged inhibition of the immune system. Doxycycline is continued and
adjusted according to the patients’ situation.
In addition, our method should be combined with other in order to ensure safety. For example, suicide genes are more suitable when
the CAR T cells are fundamentally flawed, e.g. express a CAR that has strong cross-reactivity towards normal antigens. And thus a
suicide gene is highly recommended for CAR T in the clinical trial stage in addition to our device.
By combination of tet-ON promoter and U24 protein, we achieved controllable T cells activity by addition or removal of doxycycline.
However, the modularity of our design enables us to switch promoters upstream of U24 to achieve distinct functions. For example, by
expressing U24 under the control of IL-6 responsive promoter, we can construct self-regulating CAR T that downregulate its own
activity when the serum cytokines level increases. For such purpose, promoters that respond to IL-2, interferon-γ, IL-1, C-reactive
protein and others may be used, for these cytokines are common players in the cytokine release syndrome
(Brentjens et al., 2013) .
Another possible function is that by switching to promoters that reacts to a tissue specific marker, e.g. a specific component in the
cerebrospinal fluid, it is possible to engineer CAR T to shut down when infiltrating the central nervous system, possibly preventing
some neurotoxicity.
While the full induction of U24 can potentially reduce all T cells activity, the sub-optimal induction may limit some functions of
CAR T while leaving others unaffected. This notion is supported by findings that high level of CARs is required for IL-2
production (Walker et al., 2017) , and different level of PD-1, an inhibitory protein, is
required to impair different T cell functions, including granule release, cytokines production, and proliferation (Wei et al., 2013) . It is also possible that reduced surface CAR level enhances discrimination of tumor cells
and normal cells baring the same antigen at different level. (see our modeling)
