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Antibodies that antagonize extracellular receptor–ligand interactions are used as therapeutic agents for many diseases to inhibit signalling by cell-surface receptors1. However, this approach does not directly prevent intracellular signalling, such as through tonic or sustained signalling after ligand engagement. Here we present an alternative approach for attenuating cell-surface receptor signalling, termed receptor inhibition by phosphatase recruitment (RIPR). This approach compels cis-ligation of cell-surface receptors containing ITAM, ITIM or ITSM tyrosine phosphorylation motifs to the promiscuous cell-surface phosphatase CD452,3, which results in the direct intracellular dephosphorylation of tyrosine residues on the receptor target. As an example, we found that tonic signalling by the programmed cell death-1 receptor (PD-1) results in residual suppression of T cell activation, but is not inhibited by ligand-antagonist antibodies. We engineered a PD-1 molecule, which we denote RIPR-PD1, that induces cross-linking of PD-1 to CD45 and inhibits both tonic and ligand-activated signalling. RIPR-PD1 demonstrated enhanced inhibition of checkpoint blockade compared with ligand blocking by anti-PD1 antibodies, and increased therapeutic efficacy over anti-PD1 in mouse tumour models. We also show that the RIPR strategy extends to other immune-receptor targets that contain activating or inhibitory ITIM, ITSM or ITAM motifs; for example, inhibition of the macrophage SIRPα ‘don’t eat me’ signal with a SIRPα–CD45 RIPR molecule potentiates antibody-dependent cellular phagocytosis beyond that of SIRPα blockade alone. RIPR represents a general strategy for direct attenuation of signalling by kinase-activated cell-surface receptors. A approach termed ‘receptor inhibition by phosphatase recruitment’ is described for attenuating both tonic and ligand-activated cell-surface receptor signalling.

Sensitive detection of two biological events in vivo has long been a goal in bioluminescence imaging. Antares, a fusion of the luciferase NanoLuc to the orange fluorescent protein CyOFP, has emerged as a bright bioluminescent reporter with orthogonal substrate specificity to firefly luciferase (FLuc) and its derivatives such as AkaLuc. However, the brightness of Antares in mice is limited by the poor solubility and bioavailability of the NanoLuc substrate furimazine. Here, we report a new substrate, hydrofurimazine, whose enhanced aqueous solubility allows delivery of higher doses to mice. In the liver, Antares with hydrofurimazine exhibited similar brightness to AkaLuc with its substrate AkaLumine. Further chemical exploration generated a second substrate, fluorofurimazine, with even higher brightness in vivo. We used Antares with fluorofurimazine to track tumor size and AkaLuc with AkaLumine to visualize CAR-T cells within the same mice, demonstrating the ability to perform two-population imaging with these two luciferase systems. NanoLuc substrates with improved solubility and bioavailability, hydrofurimazine and fluorofurimazine, strongly enhance bioluminescence signals in vivo and enable bright dual-color bioluminescent imaging with AkaLuc and AkaLumine.

Chimeric antigen receptor (CAR) T cell therapy for B cell malignancies has surpassed expectations, driving an ever-expanding number of clinical trials and the first US Food and Drug Administration approvals of cell therapies for the treatment of cancer. This experience has illuminated some generalizable requirements for CAR T cell efficacy as well as the interplay between disease biology and clinical outcomes. Major CAR intrinsic variables affecting T cell behavior have been defined, and mechanisms of tumor resistance are increasingly understood. Here, we review the clinical experience with CAR T cells amassed to date, including but not limited to B cell malignancies, emphasizing factors associated with efficacy, resistance and major barriers to success. We also discuss how these insights are driving next-generation clinical trials, including those in solid tumors.

T cells engineered to express chimeric antigen receptors (CARs) that are specific for tumour antigens have led to high complete response rates in patients with haematologic malignancies. Despite this early success, major challenges to the broad application of CAR-T cells as cancer therapies remain, including treatment-associated toxicities and cancer relapse with antigen-negative tumours. Targeting solid tumours with CAR-T cells poses additional obstacles because of the paucity of tumour-specific antigens and the immunosuppressive effects of the tumour microenvironment. To overcome these challenges, T cells can be programmed with genetic modules that increase their therapeutic potency and specificity. In this Review Article, we survey major advances in the engineering of next-generation CAR-T therapies for haematologic cancers and solid cancers, with particular emphasis on strategies for the control of CAR specificity and activity and on approaches for improving CAR-T-cell persistence and overcoming immunosuppression. We also lay out a roadmap for the development of off-the-shelf CAR-T cells.

Chimeric antigen receptor (CAR) T cells targeting CD19 mediate potent effects in relapsed and/or refractory pre-B cell acute lymphoblastic leukemia (B-ALL), but antigen loss is a frequent cause of resistance to CD19-targeted immunotherapy. CD22 is also expressed in most cases of B-ALL and is usually retained following CD19 loss. We report results from a phase 1 trial testing a new CD22-targeted CAR (CD22-CAR) in 21 children and adults, including 17 who were previously treated with CD19-directed immunotherapy. Dose-dependent antileukemic activity was observed, with complete remission obtained in 73% (11/15) of patients receiving ≥1 × 106 CD22-CAR T cells per kg body weight, including 5 of 5 patients with CD19dim or CD19- B-ALL. Median remission duration was 6 months. Relapses were associated with diminished CD22 site density that likely permitted CD22+ cell escape from killing by CD22-CAR T cells. These results are the first to establish the clinical activity of a CD22-CAR in B-ALL, including leukemia resistant to anti-CD19 immunotherapy, demonstrating potency against B-ALL comparable to that of CD19-CAR at biologically active doses. Our results also highlight the critical role played by antigen density in regulating CAR function.

Here, we highlight key papers published in 2018 that advance our understanding of resistance to chimeric antigen receptor (CAR) T cell immunotherapy for leukaemia and lymphoma and in so doing reveal barriers that must be addressed to increase efficacy of this novel class of therapeutics for B cell malignancies and expand their reach to solid tumours. Several clinical studies in 2018 documented the potency of therapies based on T cells with chimeric antigen receptors (CAR T cells), but also revealed mechanisms of resistance. These insights may facilitate the design of improved CAR T cell therapies for B cell malignancies and beyond.

Chimeric antigen receptors (CARs) genetically link an antigen-binding domain with cell-signaling domains to redirect immune cell specificity toward antigens expressed on the surface of cancer cells. Progress in CAR engineering over the last two decades has elucidated fundamental principles impacting CAR potency, and today CARs can be readily generated toward essentially any cell surface target on cancer cells. Efficacy thus far has been most impressive using CD19-CAR expressing T cells to treat B cell lymphoblastic leukemia, although clear activity has also been observed using CD19-CARs in patients with chronic lymphocytic leukemic and B cell lymphoma. Much less data is available regarding CAR efficacy for solid tumors. Here, we summarize current concepts of CAR design, with a focus on the relationship between structure and function, a review of the clinical results reported thus far, and the challenges to be addressed in future studies.

Chimeric antigen receptors (CARs) targeting CD19 have mediated dramatic antitumor responses in hematologic malignancies, but tumor regression has rarely occurred using CARs targeting other antigens. It remains unknown whether the impressive effects of CD19 CARs relate to greater susceptibility of hematologic malignancies to CAR therapies, or superior functionality of the CD19 CAR itself. We show that tonic CAR CD3-ζ phosphorylation, triggered by antigen-independent clustering of CAR single-chain variable fragments, can induce early exhaustion of CAR T cells that limits antitumor efficacy. Such activation is present to varying degrees in all CARs studied, except the highly effective CD19 CAR. We further determine that CD28 costimulation augments, whereas 4-1BB costimulation reduces, exhaustion induced by persistent CAR signaling. Our results provide biological explanations for the antitumor effects of CD19 CARs and for the observations that CD19 CAR T cells incorporating the 4-1BB costimulatory domain are more persistent than those incorporating CD28 in clinical trials.

Background to hematopoietic cell transplantation, including post transplant immune recovery

Dendritic cells (DCs) serve as central regulators of adaptive immunity by presenting antigens and providing necessary co-signals. Environmental information received by the DCs determines the co-signals delivered to the responding adaptive cells and, ultimately, the outcome of the interaction. DCs loaded with relevant antigens have been used as therapeutic cellular vaccines, but the optimal antigen loading method has not been determined. We compared different methods to load class I and class II epitopes from the male antigenic complex, HY, onto DCs for the potency of the immune response induced in vivo. Co-incubation of female DCs with HY peptides, RNA or cell lysate from HY expressing tumor induced immune responses equivalent to male DCs. In contrast, female DCs incubated with irradiated, apoptotic HY expressing tumor cells (or male B cells) generated a stronger immune response than male DCs or female DCs loaded using any of the other methods. DC loading with apoptotic tumor resulted in complete protection against high dose HY-expressing tumor challenge whereas 100% lethality was observed in groups receiving DCs that were loaded with peptides, RNA, or lysate. We conclude that signals provided to the DCs by apoptotic cells substantially augment the potency of DC vaccines.

CD4(+)CD25(+) regulatory T (T(reg)) cells have a crucial role in maintaining immune tolerance. Mice and humans born lacking T(reg) cells develop severe autoimmune disease, and depletion of T(reg) cells in lymphopenic mice induces autoimmunity. Interleukin (IL)-2 signaling is required for thymic development, peripheral expansion and suppressive activity of T(reg) cells. Animals lacking IL-2 die of autoimmunity, which is prevented by administration of IL-2-responsive T(reg) cells. In light of the emerging evidence that one of the primary physiologic roles of IL-2 is to generate and maintain T(reg) cells, the question arises as to the effects of IL-2 therapy on them. We monitored T(reg) cells during immune reconstitution in individuals with cancer who did or did not receive IL-2 therapy. CD4(+)CD25(hi) cells underwent homeostatic peripheral expansion during immune reconstitution, and in lymphopenic individuals receiving IL-2, the T(reg) cell compartment was markedly increased. Mouse studies showed that IL-2 therapy induced expansion of existent T(reg) cells in normal hosts, and IL-2-induced T(reg) cell expansion was further augmented by lymphopenia. On a per-cell basis, T(reg) cells generated by IL-2 therapy expressed similar levels of FOXP3 and had similar potency for suppression compared to T(reg) cells present in normal hosts. These studies suggest that IL-2 and lymphopenia are primary modulators of CD4(+)CD25(+) T(reg) cell homeostasis.

Vaccine based immunotherapy directed toward recently-defined tumor antigens represents a new approach for the treatment of neoplastic disease. Because minimal tumor burdens are most susceptible to such therapy, it is important to define how this approach could be used successfully in hosts immediately following debulking chemotherapy. Previously, we studied immune reconstitution in patients aged 1-24 years after intensive T cell depleting chemotherapy and found that thymic involution occurred in all patients studied immediately post-chemotherapy. For patients over 10 years of age, thymic inactivity persisted for at least 6 months after completion of chemotherapy. Hence, even relatively young patients appear dependent upon thymic-independent pathways of T cell regeneration for a prolonged period post-chemotherapy. Using thymectomized mice, we now have studied thymic-independent pathways of T cell regeneration after in vivo T cell depletion. We report that thymic-independent T cell regeneration primarily involves expansion of peripheral T cell populations and requires T cell-MHC interactions since CD8 expansion did not occur in β2m-/- mice (which lack MHC Class I) and CD4 expansion was diminished in mice treated with MHC Class II-blocking antibodies. When CD4 and CD8 T cells bearing transgenic T cell receptors (TCRs) were used as T cell inocula, expansion occurred only when antigen specific for the transgenic TCR was supplied. These results provide evidence that thymic-independent T cell regeneration occurs primarily via antigen-driven expansion of peripheral T cell populations. Such antigen-driven expansion of T cells bearing transgenic TCRs resulted in a marked skewing of the regenerated T cell repertoire toward the particular target antigens with approximately 40% of the regenerated CD4 or CD8 T cells expressing an antigen-specific TCR. Thus, when a T cell inoculum containing appropriate TCR specificities is supplied in combination with the target antigen during a period of T cell regeneration in thymic-deficient hosts, significant skewing of the T cell repertoire toward a particular target antigen can occur. We are using such an approach as the basis for a clinical trial of vaccine-driven expansion of antigen-specific T cells directed toward tumor-specific proteins expressed by Ewing's sarcoma and alveolar rhabdomyosarcoma which will follow a debulking phase of intensive cytotoxic chemotherapy.