|Ahead of print publication
CAR T-cells to drive away autoimmunity in lupus
Department of Clinical Immunology and Rheumatology, Kalinga Institute of Medical Sciences, KIIT University, Bhubaneswar, Odisha, India
Department of Clinical Immunology and Rheumatology, Kalinga Institute of Medical Sciences, KIIT University, Bhubaneswar - 751 024, Odisha
Source of Support: None, Conflict of Interest: None
The world possibly first heard of chimeric antigen receptor (CAR) T-cells in the context of refractory acute lymphocytic leukemia (ALL). Refractory ALL had carried very poor prognosis and the first Phase 1 trial of CAR-T cells is being considered a turning point of cancer immunology., Although success of use of CAR-T cells had been reported previously,, this trial had proved the real potential of this therapy to the world. By August 2017, the American Food and Drug Administration had approved CAR-T cell therapy for relapsed or refractory ALL in patients below 25 years.
Normally, T-cell receptors (TCRs) recognize antigens presented in the context of a histocompatibility molecule (MHC) molecule (“signal 1”). If this is accompanied by a co-stimulatory signal (“signal 2”), there is T-cell activation and proliferation. This reaction is even stronger in the presence of certain cytokines (“signal 3”). The concept of an engineered TCR that can promulgate the activation of a T cell–independent of the MHC is not new. The credit for designing the first chimeric TCR goes back to 1989.
The initial chimeric TCRs or CARs could not cause sustained T-cell activation due to the lack of “signal 2.” The second-generation CARs included an intracellular domain associated with the CAR that could provide the signal 2 (cluster of differentiation; [CD] 28 signal) itself. These were much more successful. The third generation CAR-T cells have chimeric receptors that are capable of providing all the three signals needed for sustained T-cell activation and proliferation. Fourth-generation or “TRUCK” T-cells also have a transcription activator along with these three signals [Figure 1].
|Figure 1: Generations of CAR-T cells. CD: Cluster of differentiation; NFAT: Nuclear factor activator of T-cells; IL: interleukin, CAR-T cells: Chimeric antigen receptor T-cells|
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The success of CAR-T cells is paralleled only by the development of immune-checkpoint inhibitors (that won the last year's Nobel Prize for Physiology and Medicine). It was only a matter of time that such successful therapy would percolate into other branches of medicine. The initial methods of production of CAR-T cells depended on the use of complex viral vectors. Nowadays, vectors used are lentiviruses and retroviruses that are much easier to manipulate. Plus the availability of transposon/transposase systems and CRISPR editing have opened up newer possibilities in designing CARs.
Pemphigus vulgaris is an autoimmune disease where the antigens have been well characterized along with immune repertoire profiling. Thus, it had provided the first opportunity to develop a specific CAR-T cell-driven therapy. Since it targeted autoimmune B-cells recognizing a specific autoantigen, it was called chimeric autoantigen receptor (CAAR) T-cell. A proof-of-concept study using CAAR targeting the pemphigus autoantigen desmoglein (Dsg)-3 was used in a mouse model. It was shown that the Dsg-3 CAAR-T cells expanded in vitro and specifically eliminated Dsg 3–specific B-cells in vivo.
The problem with diseases such as lupus is that they cannot be defined by a single (or a small number of) autoantigen(s) asin the case of pemphigus. Thus, development of CAAR-T cells for lupus is technically difficult. However, this did not stop Kansal et al. from using CAR-T cells to abrogate murine lupus in mouse models. They used a simple and time tested method of B-cell depletion using CAR-T cells as a proof of concept. Although B-cell depletion using anti-CD20 monoclonal antibody seems to work in lupus, two major trials did not reach their primary end points., Similarly, most mice experiments failed to show benefit of B-cell depletion in murine lupus.,
The experiments from Kansal et al. have not only shown the proof of concept of utility of CAR-T cells but have also established the role of B-cell depletion in the management of lupus. They have used lupus prone mixed New Zealand mice and MRL/MpJ-Fas (lpr) mice. Both these species have been shown to be poorly responsive to B-cell depletion using monoclonal antibodies., Moreover, the CAR-T cell-induced B-cell depletion has shown clear mortality benefit in lupus mice. Since CAR-T cells have already been used in humans, these experiments will certainly pave the way toward trials in refractory lupus in humans.
One question naturally arises: Why is B-cell depletion with CAR-T cell working better than with a monoclonal antibody? One major difference is that monoclonals usually target CD20 while CAR-T cells target CD19. CD19 is arguably a better marker for B cells, and it has a functional role in the activation of B-cells. Another argument is that an antiCD20 antibody coats the Bcells, and the B cell is killed only via complement activation or phagocytosis. However, in the case of CAR-T cells, the single event of contact is sufficient to kill the B-cell as these are CD8-positive effector T-cells. There is also hypothesis that anti-CD20 has different mechanisms beyond B-cell depletion. Moreover, CAR-T cells are too new to autoimmunity even to have too many hypotheses about them!
CAR-T cells have been shown to assume a long-term memory phenotype. The clinical data in the last decade have shown that CAR-T cells have persisted in the tissue of patients without major adverse effects. In patients with ALL, there is a risk of cytokine release syndrome. Nevertheless, this occurs in ALL even with chemotherapy and can be managed with drugs such as interleukin-6 blockers. Thus, safety of CAR-T cells is not a concern. Obviously, the risk–benefit ratio seems better for cancers than for autoimmune disease, but often a pediatric lupus nephritis can be more life-threatening than a pediatric ALL!
B-cell depletion is not the only utility of CAR-T cells! They can be used in a plethora of other ways. As proven in pemphigus, if the autoantigen is known, we can highly selectively delete B-cells reacting to that antigen. Then, CAR regulatory Tcells can be manufactured that can be triggered by specific tissue antigens induce an immune tolerant environment, e.g., in the setting of glomerulonephritis. In disease like hemophilia, CAR-B cells can be used to reduce neutralizing antibodies or “inhibitors.” As experience increases, CAR-T cells can be manufactured to specifically delete or induce certain dendritic cells or follicular helper T-cells, thus orchestrating the entire immune response!
Although long-term effects of CAR-T cells have not shown to be harmful, there are a number of strategies that can be used to limit the lifespan or function of these cells. First, a inducible suicide gene (say caspase 9) can be introduced: In the presence of the specific inducers, the CAR-T cells will undergo apoptosis. Second, a reverse suicide gene can be constitutively active but unable to work in the presence of an inhibitor: The CAR-T cells will be depleted once the inhibitor is removed. In the third strategy, the surface expression of the CAR will be under a molecular switch: Once turned “off,” the T-cell will no longer express the chimeric receptor! The advent of CRISPR technology has made such manipulation feasible.
Thus, as CAR-T cells are making inroads into rheumatology, we will have to keep a watch out for them. They have the potential to overtake conventional immunosuppressants! Proof of concept has been demonstrated in murine models of pemphigus and now of lupus. The entire gamut of autoimmune diseases may be susceptible to different CAR-T cell-based strategies!
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