Review Article
Chimeric Antigen Receptor Modified T Cells for B Cell Malignancies: An Advance in Cellular Therapy
Jie Wang*, Michael Klichinsky and Stephen J Schuster
Department of General Oncology, University of Pennsylvania, USA
*Corresponding author: Jie Wang, Department of General Oncology, University of Pennsylvania, USA
Published: 21 Oct, 2016
Cite this article as: Wang J, Klichinsky M, Schuster SJ.
Chimeric Antigen Receptor Modified
T Cells for B Cell Malignancies: An
Advance in Cellular Therapy. Clin
Oncol. 2016; 1: 1129.
Abstract
CD19 directed Chimeric Antigen Receptor (CAR) modified T cells have recently shown dramatic results in the treatment of relapsed and refractory B cell malignancies. CARs endow autologous T cells with antibody-like specificity and are capable of redirecting them to target tumor antigens and kill tumor cells. Investigational CD19 directed CARs have gained breakthrough therapy designation from the FDA and may represent the beginning of a paradigm shift in the field of cellular therapies. By redirecting the patient’s own T cells to tumor cells, CAR modified T cells harness the power of cellular immunity resulting in prolonged remissions in many patients with refractory disease, while avoiding the risks of allogeneic stem cell transplantation. In this review, we address CAR T cell design and function, discuss the clinical experience in CD19 directed CAR T cell therapy, and summarize the efficacy of CD19 CAR T cells for the treatment of different B cell malignancies. CAR T cell therapy addresses several areas of unmet clinical need in the treatment of B cell malignancies in the current era, and represents an important advance in the field of cellular therapies.
Introduction
Addition of the chimeric anti-CD20 monoclonal antibody, rituximab, to chemotherapy regimens
revolutionized the treatment of B cell malignancies, significantly improving progression-free
survival and, in some B cell malignancies, overall survival. Thus, chemoimmunotherapy has become
the mainstay of therapy for this group of diseases. However, despite the impact of rituximab, several
areas of unmet clinical need remain. Patients with Diffuse Large B Cell Lymphoma (DLBCL) who
relapse less than 1 year after combination rituximab, cyclophosphamide, doxorubicin, vincristine,
prednisone (R-CHOP) therapy and those who have MYC gene rearrangements have particularly
poor outcomes despite standard salvage chemotherapy and autologous stem cell transplant (auto
HSCT) [1,2]. Patients with indolent B cell lymphomas who have clinically significant disease that
is refractory to rituximab and subsequent lines of therapy, as well as follicular lymphoma patients
who have early progression of disease after initial therapy R-CHOP, also need alternative treatment
strategies [3]. Patients with Chronic Lymphocytic Leukemia (CLL) with high risk cytogenetic
features have an inferior prognosis with standard rituximab, fludarabine, cyclophosphamide
chemoimmunotherapy [4]. Those CLL patients who have progressive disease on kinase inhibitors
or who are intolerant of these drugs also require therapeutic alternatives [5].
In the 1970s, allogeneic hematopoietic stem cell transplantation (allo HSCT) was demonstrated
to be capable of maintaining complete remissions after salvage chemotherapy in patients with
relapsed and, in some cases, refractory hematologic malignancies [6,7]. The mechanism behind allo
HSCT is the transfer of T cell-mediated cellular immunity from the stem cell donor to the recipient.
In the rituximab era, allo HSCT is still an option that can result in long term survival in relapsed
B cell malignancies; however, the need to find a suitable donor, the occurrence of graft versus host
disease (GVHD), and the high mortality rate in the initial two years following transplantation, limit
allo HSCT as a feasible therapeutic option for many patients [8,9]. Given these drawbacks of allo
HSCT, there has been unrelenting interest in devising ways to redirect a patient’s own immune cells
to target his/her malignancy.
Chimeric Antigen Receptor (CAR) modified T cell therapy is a novel form of cellular therapy in
which a patient’s own T cells are engineered to target their malignancy. The unique design of CAR
modified T cells combines antibody-like antigen specificity and high affinity binding with T cell
effector function. By using autologous T cells, GVHD is completely avoided. CD19 directed CAR
T cells have already achieved remarkable success in the treatment of B cell Acute Lymphoblastic
Leukemia (ALL) and other B cell malignancies. We believe that this therapeutic approach is a “game
changer” for patients with relapsed and refractory B cell malignancies, and may replace the role
for allo HSCT and, for some patients, auto HSCT in this group of
diseases.
In this review, we summarize the structure and function of CAR
T cells, describe the efficacy of this new therapeutic approach for CAR
T cells directed against CD19 in B cell malignancies, discuss adverse
reactions and appropriate management, and examine the potential
future role for this breakthrough therapy.
CAR Structure and Function
A Chimeric Antigen Receptor (CAR) is a synthetic protein
constructed from elements derived from 3 or more distinct human
proteins - a single chain variable fragment (scFv) derived from a
monoclonal antibody, a hinge and transmembrane domain, and
intracellular signaling domain(s). This “chimera”, when expressed in
T cells, endows them with antibody-like specificity and affinity for a
given antigen via the scFv.
The extracellular fragment of a CAR is composed of a scFv
tethered to the transmembrane domain by a “hinge”. The scFv
comprises the variable heavy (VH)and light (VL) chains of the parent
monoclonal antibody connected to each other by a short peptide
“linker.” The “hinge” is derived from the extracellular domain of
CD8α, IgG1, IgG4, or CD28. It allows for scFv extension from the cell
surface, providing structural flexibility to facilitate antigen binding.
As the specific distance permitting productive antigen engagement
is unique to each CAR and its target, the optimal hinge domain must
be determined empirically [10,11]. The intracellular portion of a CAR
is composed of a signal transduction domain, which activates T cell
effector function upon antigen binding. CD3ζ is often chosen because
it is sufficient for T cell activation even in the absence of the CD3 γ, δ,
and ε chains normally present in the T cell receptor (TCR) signaling
complex [12].
Analogous to activation via conventional TCRs, CAR antigen
binding leads to immune synapse formation between T cell and
target cell, resulting in phosphorylation of tyrosine residues of the
immunoreceptor tyrosine activating motif (ITAM) of CD3ζ. This
initiates a polarized cell-signaling cascade that results in antigendependent
T cell activation, target cell killing, and CAR T cell
proliferation.CAR T cells lyse the engaged target cell by releasing
the contents of preformed cytotoxic granules, such as perforin and
granzyme B [13]. Antigen activated CAR T cells also secrete cytokines
that promote their own function and proliferation. These include
IL-2, IFNγ, TNF, GM-CSF, IL-12, IL-4, IL-6, IL-10, and MIP1α.
Thus, in a given patient, CAR T cell activation and proliferation
are commensurate with tumor burden. Initially, there is rampant
CAR T cell proliferation and target cell cytolysis, but as tumor cells
die, the antigenic load decreases and cytokine levels fall, leading to
contraction of the CAR T cell population [14].
Unlike conventional TCRs, CARs recognize antigen directly and
independently of the major histo compatibility complex (MHC).
MHC independence is a critical factor in the success of CAR T cell
therapy. First, aberrant MHC expression is common in neoplasms
including B cell malignancies [15,16]. Under selective pressure,
tumor cells may down-regulate MHC expression, or undergo
immune editing, leading to loss of MHC-peptide complexes [17]. The
efficacy of CAR T cell therapy is unaffected by these changes. Second,
the MHC-independence of CARs allows one CAR construct to be
used across all HLA types. Third, CARs can be designed to recognize
a variety of potential targets, including cell surface proteins, as well as
carbohydrates, glycoproteins, and lipids, which are not presented in
the context of MHC [18].
Choice of Antigenic Target for Lymphoma
Selecting an appropriate CAR target for a given disease is
fundamental to clinical outcome. An ideal antigenic target should
be uniquely and uniformly expressed on the target tissue. In the
context of B cell malignancies, potential CAR targets include the
B cell surface antigens CD19, CD20, CD22, CD37, and CD79. B
cell antigens are a suitable choice because long-term B cell aplasia
does not cause clinically unacceptable immune suppression. Initial
preclinical studies clearly established the cytolytic activity of CAR T
cells directed against B cell surface antigens upon co-culture with B
cell lymphoma cells in vitro [19-21] and established their feasibility
for use in vivo [22].
CD19-directed CAR T cells have had the most success in clinical
trials for relapsed and refractory B cell malignancies. There are many
versions of CD19-directed CAR T products in clinical trials today
[23]. CD19 is an ideal target because it is constitutively expressed
on all pre-B lymphocytes through their differentiation into terminal
effector cells as well as their malignant counterparts, but is not
expressed by other bone marrow cells or other non hematopoietic
tissue [24].
The ideal form of cellular therapy would not only target malignant
cells with minimal collateral damage to normal tissues, but would
also have prolonged persistence and curative potential without the
need for repeat infusions. Unlike allogeneic stem cell transplantation
where donor CD34+ stem cells, together with more mature T cells
in most cases, are transplanted into the recipient, CAR modified T
cells are created from fully differentiated T cells. Thus, the question
arises, can CAR T cells persist long term in vivo? They can and do, but
require a co-stimulatory signal.
Co-simulation and Persistence
Early versions of CARs were composed of an scFv antigen
recognition domain linked to a single T cell signaling domain [12].
Clinical trials of these “first generation CAR T cells” had disappointing
efficacy due to limited persistence. In a proof of concept clinical trial
of CD20-directed first generation CAR modified T cells in 7 patients
with refractory B cell lymphomas, the infused T cells had a limited
persistence of 5-21 days in vivo. There was only modest improvement
with administration of adjuvant IL2 [25]. In another trial of first
generation CD19- and CD20-directed CARs, the infused CAR T cells
were not detectable beyond 1 week by quantitative PCR for detection
of recombinant plasmid despite the administration of low dose IL-2
[26]. Poor CAR T cell persistence was not explained by the cell dose
[26].
Since physiologic T cell responses require one or more costimulatory
signals in addition to TCR signaling, it was hypothesized
that delivery of an activation signal via the TCR alone in the absence
of a co-stimulatory signal, was responsible for a poor CAR T cell
proliferative response and induction of energy or apoptosis [27,28].
To test this hypothesis, CAR modified EBV-specific Cytotoxic T
Lymphocytes (CTLs) that received optimal co-stimulation upon
viral engagement of their native TCR were compared with CAR
T cells without native virus specificity. Indeed, the virus specific
CTLs had greater in vivo persistence and clinical efficacy [29,30].
Likewise, “second generation CAR T cells”, engineered to possess
a single combined chimeric antigen receptor and co-stimulatory
domain, have improved activity in vivo [31] and are capable of
multiple sequential rounds of expansion and antigen specific target
cell lysis [32] (Figure 1). Unlike CAR modified virus specific CTLs,
co-stimulation in second generation CARs is dependent upon tumor
antigen alone.
By simultaneously infusing six non-Hodgkin lymphoma (NHL)
patients with both a first and second generation CD19-directed CAR
T cell product, Savoldo et al. [33] convincingly show that having a costimulatory
domain enhances CAR T cell survival and proliferation in
human subjects. Third generation CARs that incorporate two distinct
co-stimulatory domains (Figure 1) have not yet been shown to have
superior function or persistence compared to second generation
CARs.
A number of co-stimulatory domains have been studied and
preclinical studies show that they may confer different levels of
persistence and efficacy to CAR T cells [34,35]. The optimal costimulatory
domain has not yet been determined, but the two most
commonly used co-stimulatory domains in clinical trials are derived
from CD28 and 4-1BB. CD28 is expressed on T cells and engages in
native TCR co-stimulation by binding CD80 and CD86 on antigen
presenting cells. It is considered to be the classic “second signal” of
T cell activation. CD28 co-stimulation leads to intracellular signaling
via the PI3K/AKT, PKCθ, LCK, and RAS pathways, ultimately
resulting in greater T cell function through induction of enhanced
cytokine secretion, T cell proliferation, cell cycle progression, and
survival [36]. In contrast, 4-1BB (CD137) is a co-stimulatory receptor
on T cells that is normally upregulated upon collaborative signaling
by the TCR complex and CD28. Upon binding to its ligand 4-1BBL,
4-1BB enhances T cell function through a TRAF dependent signaling
cascade [37,38].
Preclinical studies suggest that the 4-1BB co-stimulatory domain
confers increased CAR T cell persistence and reduced susceptibility
to PD-1 inhibition and T cell exhaustion compared to CD28 [39-
41]. The underlying mechanisms for these differences relate to
the ability of 4-1BB containing CARs to take on the phenotype of
central memory T cells, and the propensity for CD28 containing
CARs to induce immune checkpoint receptor upregulation, e.g. PD1,
TIM3, and LAG3 [40,42]. In addition, Kawalekar et al. [40] recently
demonstrated that the difference in persistence between CD28 and
4-1BB CAR T cells is due in part to metabolic reprogramming–CD28
enhances glycolytic metabolism while 4-1BB enhances oxidative
metabolism. CD28 and 4-1BB co-stimulatory domains likely confer
different pharmacokinetics to the CAR T cell product, with CD28
containing CARs having an enhanced acute response and 4-1BB
CARs having greater persistence and long-term activity [32]. The
differences between CD28 and 4-1BB as co-stimulatory domains
are summarized in Figure 2. There have been no clinical trials that
directly compare CD28 and 4-1BB containing CAR T cells, but longer
persistence has been reported in the 4-1BB CARs in human subjects
compared to CD28 containing CARs [14,43-47]. Nevertheless, both
CD28 and 4-1BB containing CAR T cells have shown capacity for
remarkable clinical efficacy.
Figure 1
Figure 2
Figure 2
Venn diagram summarizing the functional differences between
CD28 and 4-1BB co-stimulated CAR T cells. Both CD28 and 4-1BB
co-stimulated CARs demonstrate enhanced functionality relative to 1st
generation CD3ζ-only CAR T cells
Clinical Trials
Use of 2nd and 3rd generation CARs in B cell malignancies
have led to dramatic and durable remissions in precursor B cell
ALL, CLL, and B cell NHLs. The majority of trials in the US using
second generation CAR constructs were conducted at four research
institutions: Memorial Sloan Kettering Cancer Center (MSKCC),
Baylor College of Medicine (BCM), National Cancer Institute (NCI)
and the University of Pennsylvania (UPenn)-Children’s Hospital of
Philadelphia (CHOP) group. With the exception of trials conducted at
UPenn-CHOP, most reported trials used CD28 as the co-stimulatory
domain. These trials show that successful engraftment, expansion,
and persistence of CAR T cells are critical to clinical success. Figure
3 summarizes the cumulative clinical response rates in precursor B
cell ALL, CLL, and B cell NHLs to CD19-directed CAR T cells in the
published literature. Multicenter trials are currently in progress.
Precursor B cell acute lymphoblastic leukemia
CD19-directed CAR T cells can achieve clinical Complete
Remission (CR) in up to 90% of patients with relapsed and refractory
B cell ALL [43,45,48,49]. These trials demonstrated that robust T cell
expansion and persistence is critical to clinical efficacy. CAR T cells
can also induce deep molecular remissions [14,43] and can traffic
to the CNS [48]. While some institutions have used CAR T cells to
achieve CR in order to bridge patients to allo HSCT, long term follow
up at UPenn-CHOP shows that durable remissions are possible with
CD19-41BB CARs, without bridging to allo HSCT [45,50].
CD19-directed CAR T cells for relapsed or refractory B cell ALL
show remarkable clinical efficacy and can rapidly induce molecular
remissions. Five patients with relapsed ALL treated at MSK with
CD19-28ζ CAR T cells all achieved tumor eradication and MRD
negativity [43]. Four eligible patients went on to undergo allogeneic
stem cell transplantation per protocol and durable remissions were
achieved. Subsequently, an expanded cohort of 16 patients were
treated and all but 2 patients achieved a CR, with 12 patients achieving
MRD negative CR [14]. Treatment failures were correlated with poor
in vivo expansion of the T cells. Seven eligible patients underwent allo
HSCT and 2 patients electively declined further therapy. One patient
died from allo HSCT related complications. The others had durable
remissions with follow up as long as 24 months. The success of CD19-
28ζ CAR T cells as a bridge to allo HSCT was also demonstrated by
the NCI in a cohort of children and young adults [48]. Fourteen of
21 patients achieved CR with 12 of these patients achieving MRD
negativity prior to proceeding to allogeneic stem cell transplant. At a
median follow up of 10 months, all patients achieving CR remained
disease-free.
While achieving CR allowed patients to undergo allo HSCT, this
approach did not permit evaluation of CAR T cell therapy as definitive
therapy. Given the treatment related toxicities of allo HSCT and that
many patients may be either ineligible for or decline allo HSCT, it
is important to evaluate the curative potential of CAR T cells as a
definitive therapy. At UPenn-CHOP, infusion of CD19-41BB CAR T
cells resulted in long term remissions without subsequent allogeneic
stem cell transplant, proving the durability of this treatment
approach. After two children with refractory B ALL achieved rapid
MRD negative CRs, an expanded cohort of 30 children and adults
were treated. Ninety percent (27/30) achieved complete morphologic
remission at 1 month after infusion, 22 of whom were MRD negative
[49]. Importantly, even in patients with a high burden of disease with
greater than 50% bone marrow involvement, the CR rate was 82%.
Maude et al. [49], reported that 15 out of 19 patients had sustained
remissions of up to two years. These patients did not undergo
allogeneic stem cell transplantation due to patient choice, lack of
suitable donor, or history of prior allo HSCT, thus allowing for longer
follow up of the efficacy of the CAR T cell therapy. Remarkably, the
infused T cells were detectable in the peripheral blood for up to 11
months by flow cytometry and 2 years by qPCR.
Chronic lymphocytic leukemia
CD19-directed CAR T cells can result in durable and deep
remissions in CLL, regardless of high risk cytogenetic features.
Three patients with chemotherapy refractory CLL were treated with
CD19-4-1BBζ CAR T cells at the University of Pennsylvania. Two
patients, including one with del (17p), achieved complete remission;
the third patient had a partial response. Remarkably, the patient with
del (17p) had an ongoing remission at 10 months and FISH became
negative for deletion TP53 in 198/200 cells [47]. The CAR T cells
demonstrated in vivo persistence of greater than 6 months and CAR
T cells of central memory phenotype were established. In an updated
study, 8 of 14 patients with relapsed and refractory CLL responded
to CD19-4-1BBζ CAR T cells (CTL019). Molecular remissions were
achieved, as determined by deep sequencing of the immunoglobulin
heavy chain (IGH) locus, and response was not affected by del (17p).
In the first two patients achieving CR, B cell aplasia was sustained for
over four years suggesting long term CAR T cell persistence. To date,
all but one patient achieving CR have remained in CR.
CD19-28ζ CAR T cells can also induce long lasting remissions in
heavily pretreated CLL patients with a high disease burden [51,52].
A patient with progressive CLL after 3 prior therapies achieved a
CR lasting more than 15 months following CD19-28ζ CAR T cell
infusion. Pre-treatment, 96% of his peripheral blood B cells were
CLL cells and 50-60% of his bone marrow was involved by CLL. In
a subsequent study, 3 out of 4 patients with CLL achieved CRs with
CD19-28ζ CAR T cell infusion lasting 14-23+ months with follow up
ongoing at time of publication [52].
Non-Hodgkin lymphoma
CD19-directed CAR modified T cells have also been effective
in the treatment of relapsed and refractory B cell non-Hodgkin
lymphomas. Kochenderfer et al. [52] at the NCI first reported the
case of a patient with progressive stage IVB follicular lymphoma after
multiple lines of therapy who achieved a partial response lasting 32
weeks following infusion of CD19-28ζ CAR T cells. A subsequent
clinical trial conducted at the NCI included four patients with NHL
- 3 patients with follicular lymphoma and one with splenic marginal
zone lymphoma [51]. This time, a course of IL2 was given following
infusion of CD19-CD28ζ CAR T cells. Aside from one patient who
died due to influenza, partial response was achieved in the patients
with follicular lymphoma, lasting upwards of 8 to 18 months with
follow up ongoing at time of publication. The patient with splenic
marginal zone lymphoma had a partial response lasting 12 months,
and was enrolled on a subsequent CAR T cell study, was reinfused
with CAR T cells, and achieved a second partial response lasting
greater than 22 months [52].
Remarkably, lasting remissions can also be achieved in
chemotherapy refractory diffuse large B cell lymphoma (DLBCL)
patients. Out of 7 evaluable patients with relapsed DLBCL, three
patients had primary mediastinal B cell lymphoma, one patient had
DLBCL transformed from CLL, and the remainder had DLBCL NOS.
At the time of the report, four patients had complete remissions
lasting upwards of 6 - 22 months, 2 patients had partial responses (1
month - 6+ months), and one had stable disease at one month follow
up [52]. This study demonstrated that anti-CD19 CAR T cells can be
efficacious in the treatment of refractory DLBCL.
At the University of Pennsylvania, Schuster et al. [53] are
conducting a phase IIa clinical trial to evaluate the safety and efficacy
of CAR T cells directed against CD19 (CTL019) in patients with
relapsed or refractory CD19+ NHL including DLBCL, follicular
lymphoma, and mantle cell lymphoma. Preliminary results were
presented at the American Society of Hematology Annual Meeting
in December 2015 (Abstract # 0183). At that time of the report, 22
patients were evaluable for response, including 13 patients with
DLBCL, 7 patients with follicular lymphoma, and 2 patients with
mantle cell lymphoma. Fifty-four percent (7/13) of patients with
DLBCL had an objective response, whereas all patients with follicular
lymphoma (7/7) had an objective response. Two patients with mantle
cell lymphoma have been treated so far with one CR. At the median
follow up of 11.7 months, PFS was 43% for DLBCL and 100% for
follicular lymphoma.
Figure 3
Figure 3
Cumulative rates of Complete Responses (CR), Non-Responses
(NR), and Stable Disease (SD)/Partial Responses (PR) in the published
literature, combining anti-CD19 CAR clinical trial results for B-ALL [14,43-
45,48,49,60-62], CLL [44,46,47,52,58,60,62,63], large B cell lymphomas
[33,48,52,62], and follicular/indolent NHL [33,52,53,63].
Toxicities
Cytokine release syndrome
Cytokine Release Syndrome (CRS) is a potentially life threatening
immunologic response that has been observed after CAR T cell
infusion. Our understanding of CRS in the context of CAR T cell
therapies for B cell malignancies largely derives from the experience
in ALL and CLL. CRS is associated with large scale immune activation
and release of pro-inflammatory cytokines, including IL6, IL2, IFNγ,
and TNF. The clinical manifestations can range from mild to severe,
and symptoms include fever, tachycardia, hypoxia, and hypotension.
CRS can manifest as Macrophage Activation Syndrome (MAS) [14,55]
and may clinically mimic sepsis. Davila et al. [14] proposed criteria
for the diagnosis of severe CRS (sCRS) secondary to CAR T cells.
These include fevers for at least three consecutive days, two cytokine
maximum fold changes of at least 75 or one cytokine maximum fold
change of at least 250, hypotension requiring a vasoactive pressor,
hypoxia, or neurologic changes. Some critical complications of CRS
that have occurred in ALL and CLL patients include respiratory
failure requiring mechanical ventilation, cardiac arrest, macrophage
activation syndrome, acute renal failure, and death [45,48,56].
Fortunately, not all patients exhibit sCRS. It is therefore important
to identify patients at high risk of sCRS in order to provide timely and
appropriate care. Predicting onset before overt clinical manifestations
is challenging because cytokine measurements are not routinely
available in most hospital laboratories. Using CRP levels to identify
patients with sCRS has been proposed [14] but this awaits prospective
validation. High dose corticosteroids can reverse the symptoms of
CRS, but may result in the decreased expansion and efficacy of CAR
modified T cells [14]. The IL-6 receptor blocker tocilizumab and
TNF inhibitor etanercept are generally preferred because they do not
compromise T cell proliferation [14,45] and are effective treatments.
In ALL, the severity of cytokine mediated toxicities is correlated
with the presence of greater tumor bulk at the time of CAR T cell
infusion [14,43,48,57]. In a clinical trial of CD19-CD28ζCAR T cells
for refractory ALL patients, the two patients with highest tumor
burden had the greatest CAR T cell expansion and experienced the
highest cytokine elevations, whereas patients with only minimal
residual disease at time of CAR T cell infusion demonstrated
relatively modest cytokine elevations [43]. The correlation between
severity and tumor bulk is less well established in CLL and NHL and
NHL patients with primarily extra medullary disease may be less
susceptible to severe CRS. In CLL, severity of illness has also been
correlated with cytokine elevation [48,51] and the occurrence of CRS
has been associated with clinical response [58].
The onset of CRS is concurrent with T cell proliferation and
typically occurs within a week of CAR T cell infusion [48,51,58], but
delayed onset sometimes occurs. Elevations are seen in IL2, sIL2R,
IFNγ, TNF, and IL6, among other cytokines, and timing of cytokine
elevation is correlated with clinical symptoms of CRS [45,49,51].
The use of diverse CAR constructs bearing different co-stimulatory
domains may affect the kinetics of the pro-inflammatory cytokine
response as well as the cytokines released. CD28-containing CAR
modified T cells, such as used at the NCI and MSKCC, elicit earlier
cytokine responses compared to the 4-1BB containing construct used
by the UPenn-CHOP group [14]. This observation is most likely
due to differential kinetics of expansion of the two CAR constructs,
as CD28-containing CAR T cells expand more rapidly than those
bearing the 4-1BB co-stimulatory domains [14,43,46]. It has also
been suggested that 4-1BB containing CARs are less likely to trigger
IL2 and TNF secretion compared to CD28 CARs [39]. Clinical trial
experience in the treatment of CLL patients supports this notion
[46,47]. The two different CAR constructs, however, have not been
shown to result in different clinical toxicity profiles. Further research
into different co-stimulatory domains may lead to elucidation of a
CAR with the greatest therapeutic index.
Neurologic toxicities
CAR T cells can penetrate the Central Nervous System (CNS)
and can eradicate leukemic cells in the cerebrospinal fluid (CSF).
However, CNS trafficking may also contribute to neurologic toxicities.
Symptoms are usually self-limited and include delirium, aphasia,
confusion, hallucinations, and seizures [14,48,57]. The mechanism(s)
underlying neurotoxicity are not fully understood; however, the
potential for rare irreversible neurotoxicity is recognized. In one
study, higher concentrations of CAR T cells in the CSF were correlated
with the occurrence of neurologic symptoms [48]. Interestingly, only
one patient who developed CNS toxicity had leukemia cells in the
CSF by cytology. Since the CNS lacks CD19 expression [52,59,60],
this finding suggests that CNS penetration may not be antigen driven.
In another study, some patients who had neurologic toxicity did not
have CAR T cells identified in the CSF [14]. This suggests that CAR
T cell penetration of the CNS may not be the only mechanism for
neurologic toxicity. Tocilizumab has been reported to be ineffective
in the management of CAR T cell related CNS toxicity [52]; optimal
management of severe CNS toxicity has not been determined and
warrants further investigation. Fortunately, most cases of neurologic
toxicity are self-limited and do not require specific intervention.
Conclusions
CAR T cells directed against CD19 have earned breakthrough
therapy status by the FDA. Having shown efficacy in the treatment of
relapsed and refractory precursor B cell ALL, DLBCL, CLL, and other
B cell lymphomas, this treatment approach has already resulted in
many long term remissions in patients who otherwise would not have
had viable treatment alternatives.
CD19-directed CAR T cells have a real potential to address several
areas of unmet clinical need in the rituximab era. Namely these
are patients with 1) DLBCL who relapse within 1 year of R-CHOP
chemotherapy, 2) DLBCL with MYC rearrangement and so called
“double hit” DLBCL, 3) CLL with poor risk cytogenetic features
and refractory to or intolerant of kinase inhibitors, and 4) multiply
relapsed and progressive indolent B cell lymphomas. Patients with
these conditions are unlikely to respond to further chemotherapy
with or with stem cell transplantation, but may achieve durable
remissions following CD19-directed cellular therapy.
As a cellular therapy, CAR T cells provide an alternative to allo
HSCT in B cell malignancies. Like allo HSCT, CAR T cells exhibit
prolonged in vivo persistence and can induce long lasting remissions
with a single infusion. For example, some patients in the ALL trials
achieved long lasting remissions without subsequently undergoing
allo HSCT. While allo HSCT is associated with significant morbidity
and high treatment related mortality in the first 2 years, therapy with
CAR T cells is generally well tolerated and treatment related toxicities
are usually reversible. Since CAR T cells can be manufactured from
the patient’s own T cells, this treatment approach does not cause
GVHD. CAR T cells offer an attractive option for patients who are
medically unfit for allo HSCT, for those who do not have a suitable
stem cell donor, and for those unwilling to accept the risks of allo
HSCT. It is anticipated that clinical trials comparing CAR modified T
cells to allo HSCT or high-dose chemotherapy with auto HSCT will
be forthcoming.
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