CAR-T Cell Therapy for Solid Tumors

Adoptive immunotherapy using chimeric antigen receptor (CAR)-modified T cells has been a game changer for the treatment of selected hematologic malignancies that are refractory to conventional chemotherapy [1‒5]. There is a desire to expand the remarkable potential of CAR-T cell therapy to solid cancers, which represent approximately 93% of all cancer cases [6]. However, cell products for solid cancers have repeatedly failed to confer durable responses and clear regulatory benchmarks, despite early signs of clinical efficacy [7‒9]. Ever since the first clinical results for CAR-T cells in solid cancers were published, it has become apparent that conventional CAR-T cells – while extremely potent in hematologic malignancies – may not be sufficiently well equipped to deal with the complexity of solid tumors (reviewed in detail in [10]).

While an increasing number of CAR-T cell clinical trials focus on solid cancers, clinical efficacy of CAR-T cells against most solid tumors still lacks behind or remains elusive [11]. To this day, one major bottleneck for immunotherapy is the availability of prototypic tumor antigens that are overexpressed on tumor tissue and safe to target, due to low baseline expression on non-malignant bystander cells. While this facet is true for both hematological and solid cancers, solid tumors are mainly heterogenous and – besides rare exceptions – do not show tumor-specific epitopes. Due to this absence of cell surface antigens for solid cancers, early clinical trials have focused on assessing the safety of various cancer-associated antigens. Most early clinical trials employ first- or second-generation CAR-T cell products and only few next-generation cell products have been reported for CAR-T cell trials in solid tumors [12].

Geographically, most CAR-T cell clinical trials are registered in China (n = 221; 59.9%) and the USA (n = 124; 33.6%), and only few trials have been launched in other regions including Europe and Australia (n = 24; 6.5%) (shown in Fig. 1a). Currently, there are 46 different target antigens under clinical investigation (shown in Fig. 1b), with the top ten antigens being mesothelin (MSLN), glypican-3 (GPC3), B7-H3, disialoganglioside (GD2), Claudin18.2 (CLDN18.2), human epidermal growth factor receptor 2 (HER2), CD70, prostate-specific membrane antigen (PSMA/PCMA), carcinoembryonic antigen (CEA), NKG2D and the epidermal growth factor variant III (EGFRvIII) (shown in Fig. 1c). Interestingly, most of these trials have been phase I basket trials (n = 174) focusing on various solid tumor collectives characterized by antigen positivity, rather than specific tumor entities (shown in Fig. 1d, e). Entity specific trials are rare and mostly found for indications such as brain cancer, gastrointestinal tumors and liver cancer (shown in Fig. 1f, and refs. [13, 14]). While long-term curative responses are yet to be obtained for CAR-T cells in solid cancers, promising results have been obtained in various trials, which outline essential features to keep in mind when studying the safety and efficacy of CAR-T cells for solid cancers.

Some of the most promising data for the use of CAR-T cells in solid cancers has been obtained in trials targeting GD2 in the context of neuroblastoma, achieving overall response rates (ORR) between 57 and 63% [15, 16]. This is mainly due to the fact that neuroblastoma lesions preferably accumulate in the bone marrow as a lymphoid predilection site, but GD2 is also a quite uniformly expressed antigen resulting in an improved CAR recognition of many tumor cells. Interestingly, both clinical trials employed stringent inclusion criteria with regard to antigen expression levels, homogeneity and tumor burden, and it is reasonable to assume that these factors had a positive impact on the observed antitumor responses. Additionally, CAR-T cell trials targeting EGFRvIII [17] or IL13Rα2 [18] have reported early signs of clinical efficacy for the treatment of brain cancers. To circumvent the role of the blood-brain barrier, some of these trials have chosen locoregionally (in this case intrathecal) administration [18]. A similar approach has been reported for the use of MSLN-specific CAR-T cells in the context of pleural mesothelioma, where locoregional administration was combined with checkpoint blockade, achieving a median OS of 23.9 months [19]. Another clinical trial recently reported on the use of CLDN18.2-specific CAR-T cells in 98 patients with gastrointestinal tumors, achieving disease control in 91.8% of all patients and an ORR of 38.8%. Again, it is likely that these results are a consequence of stringent eligibility criteria: In this particular clinical trial, only patients with gastrointestinal tumors, with lesions smaller than 4 cm in diameter, and high-level CLDN18.2 expression (>40% of the tumor mass, average H-score >2) were included [20]. Such an approach is desirable when considering the best possible therapeutic outcome in the treatment cohort of interest.

From these and other studies, we know that CAR-T cells for solid cancers are expected to expand similarly to CAR-T cells in hematological malignancies reaching their expansion peaks in the blood between day 7 and 14 after infusion [21]. However, in-depth analyses regarding tumor infiltration, T-cell proliferation and/or persistence, as well as the duration of functional activity within the tumor tissue may also be needed to understand clinical efficacies in more detail. In order to answer these questions, it is essential to understand and identify CAR-T cell trafficking routes after injection. While biopsies allow for high quality data sampling regarding tumor, tumor environment and immune cell infiltrate, its clinical use is largely confined to start- and endpoint analyses due to ethical and medical reasons (including associated risks, discomfort, and costs to the patients) [22]. This approach is further confounded by the detection limit of conventional imaging approaches and a selection bias in the context of multi-focal lesions. However, our understanding regarding the trafficking of CAR-T cell to solid tumor lesions remains limited. First data on T-cell trafficking obtained from clinical trials describe a similar pattern where CAR-T cells initially accumulate in the lungs before assembling in secondary lymphoid organs [23, 24]. CAR-T cell efficacy also depends on tumor-specific factors, such as tumor localization, size and antigen density. While it is commendable that basket trials enable patient access to CAR-T cell therapy in various indications simultaneously, many clinical trials employ lenient eligibility criteria – particular with regard to antigen expression levels or homogeneity. These circumstances are of particular relevance because activation of T cells through CAR-mediated signaling generally requires higher antigen expression levels than through its native T-cell receptor (reviewed in [25]).

Another major obstacle remain bulky tumors – even if well vascularized, tumor infiltration is a major challenge and the unfavorable ratio of effector to tumor cells within the tumor mass contributes to suboptimal antitumor efficacy. To facilitate antitumor reactivity in such settings, various strategies have been proposed, including locoregional administration in addition to i.v. injection [18, 19, 26], neo-adjuvant targeting with CAR-T cells to reduce bulky masses and simplify surgical procedures as well as adjuvant local delivery of CAR-Ts in a fibrin glue-based to clear residual cancer cells following incomplete surgery [27]. For these reasons, CAR-T cell therapy may also be of particular interest to treat metastatic or micro-metastatic disease. However, intra-patient tumor heterogeneity and differential expression of tumor-associated antigens may represent a major challenge in these settings [28‒30]. Another potential area of application represents glandular tumors, for which various cell surface antigens with limited expression on other tissues have been identified [31]. The use of such tissue-specific antigens for cellular therapy harbors a risk for eradication of antigen-positive healthy tissues (on-target off-tumor toxicity), which may result in eradication of the respective gland. An evaluation of the impact of such an intervention on the patient’s quality of life is beyond the scope of this review and must be assessed on an individual basis. However, gland-to-gland tumor heterogeneity and metastatic lesions with altered surface protein expression patterns also represent a major obstacle in these cancers [32].

Considering that curative responses have rarely been reported for CAR-T cell therapy in the context of solid cancers, tremendous research efforts have been made to augment the potency of such cell products or to protect them from the detrimental effects of the tumor microenvironment (TME).

It is clear CAR-T cell efficacy largely depends on the availability of adequate tumor-associated cell surface antigens. While such target structures that are available for hematological malignancies, the identification of suitable targets has been more cumbersome than anticipated [33‒36]. Furthermore, solid tumors exhibit a high level of inter-and intra-patient antigen heterogeneity (shown in Fig. 2), and recent studies suggest that these characteristics both hinder the identification of suitable targets and limit the efficacy of cell products targeting such cell surface antigens [37].

Furthermore, solid cancers are characterized by a complex TME, containing both cellular and acellular components (shown in Fig. 2), including tumor-associated macrophages (TAM), cancer-associated fibroblasts (CAF), regulatory T cells (T_reg), and myeloid-derived suppressor cells [38‒40]. These cell populations elicit their immunosuppressive effects either through direct cell-cell interactions and ligation of cell surface receptors (e.g., PD-1, TIM-3, and LAG-3) [41], secretion of soluble ligands that inhibit immune cell activation (e.g., TGF-β, IL-10, or IL-4) [42‒45], deposition of extracellular matrix (ECM) proteins to form a physical barrier [46‒49], or secretion of repulsive chemokines [50, 51]. Furthermore, tumors are characterized by a high metabolic activity and both nutrients including glucose and amino acids, as well as oxygen are scarce in this environment [52‒54]. Therefore, these features represent an additional challenge for CAR-T cells and have been shown to have a detrimental effect on their viability and persistence within the tumor environment.

Moreover, inadequate potency, functional persistence, or T-cell exhaustion have been implicated in the suboptimal antitumor reactivity of CAR-T cells in solid cancers. While T-cell exhaustion is naturally induced by persistent antigen stimulation, it has become apparent that CAR-T cell exhaustion may depend on additional factors, including the CAR construct used in a cell product (reviewed in [25]). Additionally, it has been shown that specific components of a CAR may impact characteristics of the final cell product, such as T-cell persistence and metabolism [55‒57]. Besides, T-cell fitness has been shown to be affected by the quality of the starting material or ex vivo cultivation times [58, 59].

Antigen homogeneity and expression levels have a profound effect on the efficacy of CAR-T cell products. Therefore, various strategies have been developed, to generate cell products with specificity for multiple cell surface antigens such as bispecific CAR, tandem CAR, split CAR, or adapter CAR approaches (reviewed in [25]).

To overcome inter-patient antigen expression heterogeneity, a recent study has supported the use of HER2/EphA2/IL13Rα2-trivalent CAR-T cells in the context of glioblastoma [60]. Such approaches have also been shown to mitigate antigen escape, increase antitumor activity, and improve disease-specific survival in preclinical models. Importantly, such cell products show activation dynamics comparable to those of conventional CAR-T cells, but exhibit an increased activity, and a less exhausted phenotype compared to T cells co-expressing a HER2 and IL13Rα2 CAR [61]. To allow for the targeting of heterogeneous tumors, adapter CAR-T cell platforms have been developed, where a bio-orthogonal CAR is armed by administration of an adapter molecule (reviewed in detail in [62]). Such approaches have been reported for various bio-orthogonal molecules, such as diketone- or fluorescein isothiocyanate-based adapters [63, 64]. The integration of universal CAR receptors alongside adapter molecules that confer antigen specificity represents a promising strategy to enhance both the safety and adaptability of CAR-T cells.

However, it is reasonable to assume that safety assessment will be more challenging for such multi-specific cell products, and careful selection of target panels may be required. This facet becomes particularly clear when considering reports about on-target off-tumor toxicities of CAR-T cells in solid cancers, e.g., for cell products targeting HER2 [65], or carbonic anhydrase IX (CAIX) [34, 66]. Furthermore, recent studies suggest that toxicities may also be associated with factors other than tumor or cell product specific features, including preconditioning regimens [67], or treatment-unrelated inflammatory conditions [68]. Another approach to address heterogenous antigen expression levels is the use of more sensitive receptor designs, such as optimized CARs [69, 70], human leukocyte antigen (HLA)-independent T-cell receptor (TCRs) (HIT) [71], synthetic TCR and antigen receptors (STAR) [72], or TRuCs [73].

Tumors represent a hostile environment containing physical barriers, immunosuppressive signals, and metabolic constraints that collectively hinder CAR-T cell infiltration, persistence, and efficacy. Therefore, extensive research efforts are made to develop novel strategies to shield CAR-T cells from such signals and to reshape the TME through the use of “armored” cell products.

Such armored cell products are best exemplified by T cells redirected for universal cytokine killing (TRUCK), which are engineered to release specific cytokines within the TME, upon T-cell activation. This dual functionality not only enhances direct cytotoxic activity of the CAR-T cells, but also fosters a broader antitumor response by activating and attracting other immune effector cells within the TME [74‒76]. Various interleukins, such as IL-7, IL-12, IL-15, IL-18, IL-21, and IL-23 have been evaluated in this context [77‒82]. Due to its biological role and potency, IL-12 has been studied extensively in the context of solid cancers. A recent study showed that IL-12 secreting CEA-specific CAR-T cells led to an increased tumor infiltration with M1 polarized macrophages, which augmented the potency of CAR-T cells through phagocytosis and antigen cross-presentation [83, 84]. However, it is known that systemic administration of IL-12 can be toxic [85], and significant attempts have been made to develop a more controlled application, including the use of inducible transgene cassettes, as well as the development of membrane-bound and attenuated IL-12 variants [80, 86‒88]. The relevance of such optimizations is best exemplified by a recent study using mucin-16 (MUC16)-specific IL-12 TRUCKs (NCT02498912) for the treatment of treating ovarian, primary peritoneal and fallopian tube carcinoma, in which dose-limited toxicities associated with IL-12 were observed [89].

Another interleukin that has received a lot of attention in the TRUCK context is IL-18, which has been shown to improve the infiltration of solid tumors with CD8⁺ T cells and natural killer (NK) cells, induce M1 polarization in tumor-associated macrophages, and decrease the abundance of immunosuppressive dendritic cells, M2 macrophages, and T_reg [90]. Importantly, recombinant human IL-18 has been shown to be well tolerated in both mice and monkeys [91].

The TME in solid tumors is characterized by a dense ECM that includes heparin sulfate proteoglycans, collagen, hyaluronic acid, glycoproteins, and other matrix proteins. This composition is significantly different from that of normal tissue and has been shown to affect intra-tumor signaling and metabolism, and to hinder the infiltration and effectiveness of CAR-T cell therapy [39].

To overcome this structural barrier, CAR-T cells have been engineered to express ECM-digesting enzymes like heparinase [92] or hyaluronidase [93]. Another strategy has been to redirect CAR-T cells against the cell populations responsible for ECM deposition, such as CAF by targeting fibroblast activation protein (FAP) [94]. Using these approaches, researchers have been able to make the TME more accessible for therapeutic interventions [95, 96]. Importantly, this approach can also be combined with tumor-specific cell products: Preclinical studies have shown that the sequential administration of FAP- and tumor-specific CAR-T cells creates a “hot” TME characterized by an open stroma, low frequencies of CAF, and myeloid-derived suppressor cells, allowing for an improved antitumor response [97‒99]. However, recent studies have raised safety concerns of FAP-targeted CAR-T cell therapy, due to basal expression of FAP on the surface of healthy stromal cells in bone marrow and skeletal muscle. Eradication of these cell populations has been shown to be associated which lethal bone marrow toxicity and cachexia, underscoring the need for additional safety assessments [100]. Another approach to actively recruit CAR-T cells to the TME represents the overexpression of chemokine receptors, such as CXCR2, CXCR4, and CCR2 (reviewed in detail in [101]).

Solid tumors are characterized by high-level expression of inhibitory ligands. To combat these immunosuppressive signals, various shielding strategies have been proposed, such as antibody-based immune checkpoint blockade, genome editing, overexpression of dominant negative receptors (DNR), or signaling cascade modulation using switch receptors (SR).

While immune checkpoint blockade is commonly used to augment tumor immune control, gene editing CAR-T cells directly can boost their antitumor activity more effectively and potentially offers a superior safety profile as compared to systemic antibody administration [102]. For example, it has been shown that disruption of PD-1 in T cells induced enhanced long-term efficacy and IFNγ production [103]. Similarly, Tang et al. [104] showed that CRISPR/Cas9-mediated knockout of the TGF-β receptor II (TGFBR2) in CAR-T cells reduced regulatory T-cell conversion and prevented CAR-T cell exhaustion. TGFBR2-edited CAR-T cells exhibited improved tumor elimination in both cell line-derived and patient-derived xenograft solid tumor models. These edited cells also had a higher proportion of central and effector memory subsets among circulating CAR-T cells. Overall, knocking out TGFBR2 significantly enhanced the in vitro and in vivo functionality of CAR-T cells in TGF-β–rich TMEs [104].

Another promising approach is the use of DNRs, decoy receptors, which do not transmit signals upon ligand binding. TGF-β DNR is a prime example for this and have been shown to shield CAR-T cells from TGF-β-mediated immunosuppression. In preclinical studies, this allowed CAR-T cells to remain active and functional even in the presence of high levels of TGF-β, in various cancer models [105, 106]. In contrast, SRs not only shield cell products, but augment their potency by redirecting immunosuppressive stimuli into pro-inflammatory signaling cascades. The most prominent approaches employ signaling modules from cytokine or TCR components for this purpose. This approach has been implemented successfully for various inhibitory molecules, including TGF-β and IL-4 [107, 108]. Similarly, SRs have also been reported for cell surface molecules, such as PD-1. For example, the PD-1/CD28-SR merges the extracellular domain of PD-1 with the intracellular signaling domain of the co-stimulatory molecule CD28, effectively redirecting the inhibitory signal of PD-1 into an activating signaling cascade on the one hand, while leading to enhanced T-cell activation, proliferation, and cytokine production on the other [109, 110]. This concept has been tested in various settings, including cMet-specific CAR-T cells [111].

During the last years, it has become apparent that various other immune cell populations may contribute to tumor control. In consequence, persistent attempts have been made to expand the concept of CARs to other immune cells. In this paragraph, we focus on the potential of CAR-NK cells and CAR macrophages (CAR-M). NK cells are an essential part of the innate immune system that respond rapidly to non-self cells, and do not require antigen presentation on major histocompatibility complex molecules. In a clinical trial using HLA-mismatched CAR-NK cells derived from cord blood, none of the 11 patients experienced graft-versus-host disease [112]. This study shows that CAR-NK cell therapy does not rely on autologous cells for antitumor efficacy, and “off-the-shelf” solutions using CAR-NK derived from cell lines (e.g., NK92), healthy donor umbilical cord blood, or induced pluripotent stem cells are currently being explored [113‒118]. Furthermore, the cytokine secretion pattern of CAR-NK cells is fundamentally different from CAR-T cells, leading to a reduced risk of CRS and neurotoxicity. While CAR-T cells release various pro-inflammatory cytokines, including tumor necrosis factor alpha, IFNγ, and IL-2, the cytokine profile of NK cells is dominated by cytokines such as GM-CSF [119, 120]. Additionally, NK cells possess multiple mechanisms for targeting cancer cells, such as antibody-dependent cell-mediated cytotoxicity, as well as ligation of cell surface death receptors, including Fas and TRAIL receptors [120‒123]. While many challenges of cell therapy for solid cancers also apply for CAR-NK, their short lifespan represents an additional challenge. While this circumstance limits the time frame for side effects of CAR-NK cell therapy, repeated infusions may be necessary for prolonged remission [124]. However, it must be stated, that anti-cell product immune responses have been shown to occur upon repeated dosing. Therefore, considerable initiatives are being made to equip CAR-NK cells with memory properties for long-term survival and continuous immune surveillance and studies show that modifying CAR-NK cells to express IL-12, IL-15, and IL-18 can boost their persistence [112, 118, 125].

Macrophages belong to the group of professional antigen-presenting cells and form an integral part of the innate immune system. They are known for their tissue invasion capabilities and are abundantly found within the TME of solid tumors [118]. Traditionally, macrophages have classified into the pro-inflammatory M1 and immunosuppressive M2 phenotypes. In various cancer entities, M1 polarization of TAMs is correlated with a beneficial prognosis, whereas M2 macrophages are more prevalent in advanced tumor stages [126, 127]. In 2020, Klichinsky et al. [128] first reported on the development of CAR-M. One major challenge on this road was the development of adequate gene transfer methods that circumvent the plethora of nucleic acid sensor proteins in macrophages [129]. To this end, the authors employed an adenoviral vector platform (Ad5f35), which enabled successful genetic modification and persistence of the adenoviral episome and induced a favorable M1 phenotype supporting a pro-inflammatory environment upon TME infiltration. Furthermore, the authors found that CAR-M elicit potent antitumor efficacy by antigen-specific phagocytosis and presentation of cancer neoantigens to bystander immune cells [128]. Interestingly, it has been shown that macrophages – albeit part of the myeloid and not lymphoid lineage – effectively use conventional CARs. However, recent studies show that alternative signaling modules can alter the biological functions elicited by CAR-M, and the use of CD147 as a signaling domain induced matrix metalloproteinase expression and promoted tumor immune infiltration by ECM degradation [40, 130]. While CAR-M have shown promising results in preclinical studies, their short lifespan, as well as the required amounts of adenoviral vector (1 Mio-fold excess of viral vector over cells required), have been substantial roadblock. For this reason, alternative gene delivery approaches, such as lipid nanoparticles or intra-tumoral injection for transgene delivery, are currently being explored [131, 132]. Another aspect that interferes with the potency of CAR-M is the prevalence of “do not eat me” signals, such as CD47. Upon interaction between CD47 and its receptor, SIRPα, on the cell surface of myeloid cells, phagocytosis is effectively inhibited. Blocking the CD47/SIRPα axis, either by genome editing the SIRPα gene or through anti-CD47 antibodies, could potentially restore phagocytosis and increases antigen presentation [133‒135]. Currently, several strategies, including anti-CD47 antibodies, CD47-targeting peptides and epitope editing are being explored as means to interfere with the CD47/SIRPα axis [136‒138]. However, Gilead Sciences have faced major difficulties with magrolimab, a humanized monoclonal antibody targeting CD47, including setbacks in phase 3 clinical trials and potential links to patient deaths, raising concerns about its lack of survival benefits compared to standard care. Therefore, further investigations are essential to fully understand this pathway for effective clinical translation [139].

In light of the plethora of novel strategies intended to augment the potency of immune cell products for the treatment of solid cancers, it is surprising that only few technologies are currently being validated in clinical trials. One major aspect that is currently being evaluated is strategies to circumvent tumor heterogeneity. However, the most complex cell products are directed against a maximum of two target antigens (shown in Table 1). While most of these studies are ongoing, preliminary results from one trial using EGFR/IL13Rα2 bispecific CAR-T cells showed preliminary safety in a cohort of 6 patients [13].

Additionally, novel approaches lowering the antigen detection threshold of CAR immune cell products are currently being evaluated. While these approaches hold promise for various antigen low tumor cohorts, the safety of such cell products is still to be confirmed. For example, the STAR and TRuC concepts are currently under clinical investigation in phase I trials targeting MSLN⁺ tumors (STAR: NCT05344976, TRuC: NCT03907852). Moreover, activity enhanced cell products with optimized receptor designs are currently being evaluated in two clinical trials using IL-8R (NCT05353530) and IL-7RA (NCT05577091) modifications. Besides, there is one active clinical trial using CXCR5-modified CAR-T cells in EGFR⁺ non-small cell lung cancer to achieve preferential recruitment of CAR-T cells to tumor lesions (NCT05060796).

However, most clinical trials focus on approaches to augment T-cell fitness and to maintain long-term functional persistence. Promising data for this approach have recently been reported for ROR1-specific CAR-T cells with constitutive c-Jun overexpression, which showed an ORR of 37.5% (6/16 patients) (NCT05274451). There are various CAR-T cell products under clinical investigation that employ genome editing to shield cell products from the TME, such as PD-1, CTLA-4 knockout CAR-T cells (NCT05732948, NCT04842812, NCT05812326), or CAR-T cells with a TGF-βR-knockout (NCT03089203, NCT04976218, NCT06084884). Furthermore, multiple armored cell products are currently being investigated, such as MSLN-specific CAR-T cells secreting PD-1 and CTLA-4 nanobodies (NCT05373147, NCT05089266, NCT06248697), IL-15- or IL-21-armored CAR-T cells (NCT06198296, NCT04715191), cell products secreting T-cell engaging antibody molecules (NCT05660369), as well as CAR-T cells expressing a constitutively active IL-7 receptor (NCT04099797).

Although, CAR-T cell therapy has been struggling to prove its applicability for solid cancer treatment, recent studies have shown early signs of clinical efficacy while detailed studies of solid tumor and T-cell biology have yielded a plethora of strategies to augment CAR-T cell therapy. While the availability of tumor-associated antigens remains a major challenge, the rise of novel diagnostic methods will contribute to the development of novel cell therapies. During the next years, clinical data of first-in-class next-generation cell products will become available and deepen our understanding of the intricate interplay between tumor, tumor environment and immune cells. Safety and efficacy data from such clinical trials are expected to allow for the iterative improvement of cellular therapy, in order to advance toward the common goal in the field: the development of a curative cell therapy approach for the treatment of solid tumors.

Figure 2 was created with BioRender.com.

J.W. is an inventor of patent applications and granted patents related to CAR technology, speaker honoraria: BMS. M.H. is an inventor of patent applications and granted patents related to CAR technology, licensed in part to industry, a co-founder and equity owner of T-CURX GmbH, and declares speaker honoraria from BMS, Janssen, Kite/Gilead, and Novartis. All other co-authors have nothing to disclose.

The authors have been supported by the patient advocacy group “Hilfe im Kampf gegen den Krebs e.V.,” Würzburg, Germany and “Forschung hilft” – Stiftung zur Förderung der Krebsforschung an der Universität Würzburg. The authors were also supported by the Federal Ministry for Education and Research (BMBF, Bundesministerium für Bildung und Forschung) – Project ROR2-CAR-T, grant No. 01 EN2306A, as well as the German Cancer Aid (Stiftung Deutsche Krebshilfe), Project AvantCAR.de, grant No. 70114707. The authors were supported by the German Research Foundation (Deutsche Forschungsgemeinschaft), grant 338/1 2021-452881907, project A02 (M.H.) [LETSimmun], the German Research Foundation (Deutsche Forschungsgemeinschaft), grant SFB/TRR221, project A03 (M.H.) [GvH/GvL], the Bavarian Center for Cancer Research (Bayerisches Zentrum fur Krebsforschung, Leuchtturm Immuntherapien; M.H.), the German Cancer Aid (Stiftung Deutsche Krebshilfe), grant 70114707 (TransOnc Avant-CAR.de; M.H.) and grant 70115200 (CAR FACTORY; M.H.), the Federal Ministry for Education and Research (Bundesministerium für Bildung und Forschung), grant 13N15987 (IMAGINE; M.H.), grant 01EN2306A (ROR2 CAR-T; M.H.), and grant 03ZU1111BA (SaxoCell), Innovative Medicine Initiative 2 Joint Undertaking (JU), grant 853988 (imSAVAR) and grant 945393(T2EVOLVE). The JU receives support from the European Union’s Horizon 2020 research and innovation program and EFPIA and JDRF International.

L.T., A.S., and M.S.: conceptualization, visualization, and writing. J.W. and M.H.: conceptualization, writing, supervision, and funding acquisition.