Convection Enhanced Delivery of the Oncolytic Adenovirus Delta24-RGD in Patients with Recurrent GBM: A Phase I Clinical Trial Including Correlative Studies.

PURPOSE: Testing safety of Delta24-RGD (DNX-2401), an oncolytic adenovirus, locally delivered by convection enhanced delivery (CED) in tumor and surrounding brain of patients with recurrent glioblastoma. PATIENTS AND METHODS: Dose-escalation phase I study with 3+3 cohorts, dosing 107 to 1 × 1011 viral particles (vp) in 20 patients. Besides clinical parameters, adverse events, and radiologic findings, blood, cerebrospinal fluid (CSF), brain interstitial fluid, and excreta were sampled over time and analyzed for presence of immune response, viral replication, distribution, and shedding. RESULTS: Of 20 enrolled patients, 19 received the oncolytic adenovirus Delta24-RGD, which was found to be safe and feasible. Four patients demonstrated tumor response on MRI, one with complete regression and still alive after 8 years. Most serious adverse events were attributed to increased intracranial pressure caused by either an inflammatory reaction responding to steroid treatment or viral meningitis being transient and self-limiting. Often viral DNA concentrations in CSF increased over time, peaking after 2 to 4 weeks and remaining up to 3 months. Concomitantly Th1- and Th2-associated cytokine levels and numbers of CD3+ T and natural killer cells increased. Posttreatment tumor specimens revealed increased numbers of macrophages and CD4+ and CD8+ T cells. No evidence of viral shedding in excreta was observed. CONCLUSIONS: CED of Delta24-RGD not only in the tumor but also in surrounding brain is safe, induces a local inflammatory reaction, and shows promising clinical responses.

Blood-based kinase activity profiling: a potential predictor of response to immune checkpoint inhibition in metastatic cancer.

BACKGROUND: Many cancer patients do not obtain clinical benefit from immune checkpoint inhibition. Checkpoint blockade targets T cells, suggesting that tyrosine kinase activity profiling of baseline peripheral blood mononuclear cells may predict clinical outcome. METHODS: Here a total of 160 patients with advanced melanoma or non-small-cell lung cancer (NSCLC), treated with anti-cytotoxic T-lymphocyte-associated protein 4 (anti-CTLA-4) or anti-programmed cell death 1 (anti-PD-1), were divided into five discovery and cross-validation cohorts. The kinase activity profile was generated by analyzing phosphorylation of peripheral blood mononuclear cell lysates in a microarray comprising of 144 peptides derived from sites that are substrates for protein tyrosine kinases. Binary grouping into patients with or without clinical benefit was based on Response Evaluation Criteria in Solid Tumors V.1.1. Predictive models were trained using partial least square discriminant analysis (PLS-DA), performance of the models was evaluated by estimating the correct classification rate (CCR) using cross-validation. RESULTS: The kinase phosphorylation signatures segregated responders from non-responders by differences in canonical pathways governing T-cell migration, infiltration and co-stimulation. PLS-DA resulted in a CCR of 100% and 93% in the anti-CTLA-4 and anti-PD1 melanoma discovery cohorts, respectively. Cross-validation cohorts to estimate the accuracy of the predictive models showed CCRs of 83% for anti-CTLA-4 and 78% or 68% for anti-PD-1 in melanoma or NSCLC, respectively. CONCLUSION: Blood-based kinase activity profiling for response prediction to immune checkpoint inhibitors in melanoma and NSCLC revealed increased kinase activity in pathways associated with T-cell function and led to a classification model with a highly accurate classification rate in cross-validation groups. The predictive value of kinase activity profiling is prospectively verified in an ongoing trial.

CD45RA+CCR7- CD8 T cells lacking co-stimulatory receptors demonstrate enhanced frequency in peripheral blood of NSCLC patients responding to nivolumab.

BACKGROUND: Checkpoint inhibitors have become standard care of treatment for non-small cell lung cancer (NSCLC), yet only a limited fraction of patients experiences durable clinical benefit, highlighting the need for markers to stratify patient populations. METHODS: To prospectively identify patients showing response to therapy, we have stained peripheral blood samples of NSCLC patients treated with 2nd line nivolumab (n = 71), as well as healthy controls, with multiplex flow cytometry. By doing so, we enumerated 18 immune cell subsets and assessed expression for 28 T cell markers, which was followed by dimensionality reduction as well as rationale-based analyses. RESULTS: In patients with a partial response (PR), representing best overall response (BOR) according to RECIST v1.1, the number of CD8 T cells at baseline and during treatment is similar to those of healthy controls, but 2-fold higher than in patients with progressive and stable disease (PD and SD). CD8 T cell populations in PR patients show enhanced frequencies of T effector memory re-expressing CD45RA (TEMRA) cells, as well as T cells that express markers of terminal differentiation (CD95+) and egression from tumor tissue (CD69-). In PR patients, the fraction of CD8 T cells that lacks co-stimulatory receptors (CD28, ICOS, CD40L, 4-1BB, OX40) correlates significantly with the total numbers and differentiated phenotype of CD8 T cells. CONCLUSIONS: This study demonstrates that high numbers of peripheral CD8 T cells expressing differentiation markers and lacking co-stimulatory receptors at baseline are associated with response to nivolumab in NSCLC patients.

Autologous Dendritic Cell Therapy in Mesothelioma Patients Enhances Frequencies of Peripheral CD4 T Cells Expressing HLA-DR, PD-1, or ICOS.

Introduction: Malignant pleural mesothelioma (MPM) is a malignancy with a very poor prognosis for which new treatment options are urgently needed. We have previously shown that dendritic cell (DC) immunotherapy provides a clinically feasible treatment option. In the current study, we set out to assess the immunological changes induced by DC immunotherapy in peripheral blood of MPM patients. Methods: Peripheral blood was collected from nine patients enrolled in a phase I dose escalation study, before and after treatment with DCs that were pulsed with an allogeneic tumor lysate preparation consisting of a mixture of five cultured mesothelioma cell lines. We used immune profiling by multiplex flow cytometry to characterize different populations of immune cells. In particular, we determined frequencies of T cell subsets that showed single and combinatorial expression of multiple markers that signify T cell activation, maturation and inhibition. Therapy-induced T cell reactivity was assessed in peptide/MHC multimer stainings using mesothelin as a prototypic target antigen with confirmed expression in the clinical tumor lysate preparation. T cell receptor (TCR) diversity was evaluated by TCRB gene PCR assays. Results: We observed an increase in the numbers of B cells, CD4 and CD8 T cells, but not NK cells at 6 weeks post-treatment. The increases in B and T lymphocytes were not accompanied by major changes in T cell reactivity toward mesothelin nor in TCRB diversity. Notably, we did observe enhanced proportions of CD4 T cells expressing HLA-DR, PD-1 (at 2 weeks after onset of treatment) and ICOS (6 weeks) and a CD8 T cell population expressing LAG3 (2 weeks). Discussion: DC immunotherapy using allogeneic tumor lysate resulted in enhanced frequencies of B cells and T cells in blood. We did not detect a skewed antigen-reactivity of peripheral CD8 T cells. Interestingly, frequencies of CD4 T cells expressing activation markers and PD-1 were increased. These findings indicate a systemic activation of the adaptive immune response and may guide future immune monitoring studies of DC therapies.

Graft predominance after double umbilical cord blood transplantation: a review.

Several parameters are involved in graft predominance after double umbilical cord blood transplantation (dUCBT), of which T-cell alloreactivity between the grafts is now considered to be the major denominator. We recently showed that alloreactive CD4+ T-cells originating from the predominant CBU recognize HLA-class II allele mismatches and can readily be detected in the majority of patients. In addition, it was shown that HLA-class II allele-specific CD4+ T-cells were able to recognize primary leukemic cells when the mismatched HLA-class II allele was shared between the rejected CBU and the patient. These results further underscored the role of alloreactive T-cells, notably class II specific CD4+ T-cells, in graft-versus-graft reactions and in graft-versus-leukemia after dUCBT.

T-cell Receptors for Clinical Therapy: In Vitro Assessment of Toxicity Risk.

Adoptive therapy with T-cell receptor (TCR)-engineered T cells has shown promising results in the treatment of patients with tumors, and the number of TCRs amenable for clinical testing is expanding rapidly. Notably, adoptive therapy with T cells is challenged by treatment-related side effects, which calls for cautious selection of target antigens and TCRs that goes beyond their mere ability to induce high T-cell reactivity. Here, we propose a sequence of in vitro assays to improve selection of TCRs and exemplify risk assessments of on-target as well as off-target toxicities using TCRs directed against cancer germline antigens. The proposed panel of assays covers parameters considered key to safety, such as expression of target antigen in healthy tissues, determination of a TCR's recognition motif toward its cognate peptide, and a TCR's cross-reactivity toward noncognate peptides. Clin Cancer Res; 23(20); 6012-20. ©2017 AACR.

CD4+ T-cell alloreactivity toward mismatched HLA class II alleles early after double umbilical cord blood transplantation.

Although double umbilical cord blood transplantation (dUCBT) in adult patients may be associated with less graft failure compared with single UCBT, hematopoietic recovery generally originates from a single cord blood unit (CBU). CBU predominance is still incompletely understood. We recently showed that blood CD4+ T-cell numbers rapidly increase after dUCBT, and early CD4+ T-cell chimerism predicts for graft predominance. Given the frequent HLA class II allele mismatches between CBUs in dUCBT, we hypothesized that alloreactive HLA class II-specific CD4+ T cells from the "winning" CBU may contribute to rejection of the "loser" CBU. We evaluated whether CD4+ T cells originating from the predominant (PD)-CBU would recognize HLA class II allele mismatches, expressed by the nonengrafting (NE)-CBU. Alloreactive effector CD4+ T cells toward 1 or more mismatched HLA class II alleles of the NE-CBU were detected in 11 of 11 patients, with reactivity toward 29 of 33 (88%) tested mismatches, and the strongest reactivity toward DR and DQ alleles early after dUCBT. Mismatched HLA class II allele-specific CD4+ T cells recognized primary leukemic cells when the mismatched HLA class II allele was shared between NE-CBU and patient. Our results suggest that cytotoxicity exerted by CD4+ T cells from the PD-CBU drives the rapid rejection of the NE-CBU, whose alloreactive effect might also contribute to graft-versus-leukemia.

Plasma IFN-γ and IL-6 levels correlate with peripheral T-cell numbers but not toxicity in RCC patients treated with CAR T-cells.

Autologous T-cells genetically modified to express a chimeric antigen receptor (CAR) against carboxy-anhydrase-IX (CAIX) were administered to twelve patients with CAIX-positive metastatic renal cell carcinoma. Here, we questioned whether plasma cytokine levels following treatment or in vitro cytokine production from the T-cell infusion products could serve as predictors for peripheral T-cell persistence or in vivo T-cell activity. We demonstrated that CAR surface as well as gene expression are down-regulated following T-cell infusion, and that peripheral numbers of CAR T-cells are best captured by flow cytometry and not by qPCR. Numbers of CAR T-cells in blood correlated with plasma levels of IFN-γ and IL-6, but not with any of the other cytokines tested. Plasma IFN-γ or IL-6 levels did not correlate with liver enzyme values. Thus, out of 27 cytokines tested, IFN-γ and IL-6 levels in plasma are potential surrogate markers for CAR T-cell persistence in solid tumors.

Treatment of metastatic renal cell carcinoma (mRCC) with CAIX CAR-engineered T-cells-a completed study overview.

We studied safety and proof of concept of a phase I/II trial with chimeric antigen receptor (CAR) T-cells in patients with metastatic renal cell carcinoma (mRCC). The CAR was based on the G250 mAb that recognized an epitope of carboxy-anhydrase-IX (CAIX). Twelve patients with CAIX+ mRCC were treated in three cohorts with a maximum of 10 daily infusions of 2×10(7) to 2×10(9) CAR T-cells. Circulating CAR T-cells were transiently detectable in all patients and maintained antigen-specific immune functions following their isolation post-treatment. Blood cytokine profiles mirrored CAR T-cell presence and in vivo activity. Unfortunately, patients developed anti-CAR T-cell antibodies and cellular immune responses. Moreover, CAR T-cell infusions induced liver enzyme disturbances reaching CTC grades 2-4, which necessitated cessation of treatment in four out of eight patients (cohort 1+2). Examination of liver biopsies revealed T-cell infiltration around bile ducts and CAIX expression on bile duct epithelium, adding to the notion of on-target toxicity. No such toxicities were observed in four patients that were pretreated with G250 mAb (cohort 3). The study was stopped due to the advent of competing treatments before reaching therapeutic or maximum tolerated dose in cohort 3. No clinical responses have been recorded. Despite that, from this trial numerous recommendations for future trials and their immune monitoring could be formulated, such as choice of the target antigen, format and immunogenicity of receptor and how the latter relates to peripheral T-cell persistence.

T Cell Maturation Stage Prior to and During GMP Processing Informs on CAR T Cell Expansion in Patients.

Autologous T cells were genetically modified to express a chimeric antigen receptor (CAR) directed toward carboxy-anhydrase-IX (CAIX) and used to treat patients with CAIX-positive metastatic renal cell carcinoma. In this study, we questioned whether the T cell maturation stage in the pre-infusion product affected CAIX CAR expression and function in vitro as well as in vivo CAR T cell numbers and expansion. During the 14 days expansion of CAR T cells prior to administration, we observed shifts from a predominant CD4 to a CD8 T cell phenotype and from a significant fraction of naïve to central effector T cells. Surface expression of the CAR was equally distributed among different T cell subsets and T cell maturation stages. During T cell culture days 14-18 (which covered patient treatment days 1-5), T cells demonstrated a decline in CAR expression level per cell irrespective of T cell maturation stage, although the proportion of CAR-positive T cells and CAR-mediated T cell effector functions remained similar for both CD4 and CD8 T cell populations. Notably, patients with a higher fraction of naïve CD8 T cells at baseline (prior to genetic modification) or central effector CD8 T cells at 2 weeks of CAR T cell culture demonstrated a higher fold expansion and absolute numbers of circulating CAR T cells at 1 month after start of therapy. We conclude that the T cell maturation stage prior to and during CAR T cell expansion culture is related to in vivo CAR T cell expansion.

Adoptive T-cell therapy: a need for standard immune monitoring.

Cancer immune therapy, in particular the use of checkpoint inhibitors and adoptive transfer of T cells has recently demonstrated significant clinical responses against several tumor types. Unfortunately, these therapies are frequently accompanied by severe toxicities, underscoring the need for markers that provide information on therapy response. Monitoring immune responses in the tumor microenvironment and peripheral blood prior to and during these therapies will provide better insight into the mechanisms underlying clinical activities, and will potentially enable the identification of such markers. In this review, we present an overview of adoptive T-cell trials conducted with a special focus on immune monitoring, and argue that accurate monitoring of T cells is pivotal to further development of immune therapies to treat cancer.

T cell receptor-engineered T cells to treat solid tumors: T cell processing toward optimal T cell fitness.

Therapy with autologous T cells that have been gene-engineered to express chimeric antigen receptors (CAR) or T cell receptors (TCR) provides a feasible and broadly applicable treatment for cancer patients. In a clinical study in advanced renal cell carcinoma (RCC) patients with CAR T cells specific for carbonic anhydrase IX (CAIX), we observed toxicities that (most likely) indicated in vivo function of CAR T cells as well as low T cell persistence and clinical response rates. The latter observations were confirmed by later clinical trials in other solid tumor types and other gene-modified T cells. To improve the efficacy of T cell therapy, we have redefined in vitro conditions to generate T cells with young phenotype, a key correlate with clinical outcome. For their impact on gene-modified T cell phenotype and function, we have tested various anti-CD3/CD28 mAb-based T cell activation and expansion conditions as well as several cytokines prior to and/or after gene transfer using two different receptors: CAIX CAR and MAGE-C2(ALK)/HLA-A2 TCR. In a total set of 16 healthy donors, we observed that T cell activation with soluble anti-CD3/CD28 mAbs in the presence of both IL15 and IL21 prior to TCR gene transfer resulted in enhanced proportions of gene-modified T cells with a preferred in vitro phenotype and better function. T cells generated according to these processing methods demonstrated enhanced binding of pMHC, and an enhanced proportion of CD8+, CD27+, CD62L+, CD45RA+T cells. These new conditions will be translated into a GMP protocol in preparation of a clinical adoptive therapy trial to treat patients with MAGE-C2-positive tumors.

Phase I study of RGB-286638, a novel, multitargeted cyclin-dependent kinase inhibitor in patients with solid tumors.

PURPOSE: RGB-286638 is a multitargeted inhibitor with targets comprising the family of cyclin-dependent kinases (CDK) and a range of other cancer-relevant tyrosine and serine/threonine kinases. The objectives of this first in human trial of RGB-286638, given i.v. on days 1 to 5 every 28 days, were to determine the maximum tolerated dose (MTD) and to evaluate the pharmacokinetic (PK) and pharmacodynamic (PD) profiles of this new drug. EXPERIMENTAL DESIGN: Sequential cohorts of 3 to 6 patients were treated per dose level. Blood, urine samples, and skin biopsies for full PK and/or PD analyses were collected. RESULTS: Twenty-six patients were enrolled in 6-dose levels from 10 to 160 mg/d. Four dose-limiting toxicities were observed in 2 of the 6 patients enrolled at the highest dose level. These toxicities were AST/ALT elevations in 1 patient, paroxysmal supraventricular tachycardias (SVTs), hypotension, and an increase in troponin T in another patient. The plasma PK of RGB-286638 was shown to be linear over the studied doses. The interpatient variability in clearance was moderate (variation coefficient 7%-36%). The PD analyses in peripheral blood mononuclear cells, serum (apoptosis induction) and skin biopsies (Rb, p-Rb, Ki-67, and p27(KIP1) expression) did not demonstrate a consistent modulation of mechanism-related biomarkers with the exception of lowered Ki-67 levels at the MTD level. The recommended MTD for phase II studies is 120 mg/d. CONCLUSIONS: RGB-286638 is tolerated when administered at 120 mg/d for 5 days every 28 days. Prolonged disease stabilization (range, 2-14 months) was seen across different dose levels.

Genetically modified T lymphocytes: more than just direct effectors.

Evaluation of: Russo V, Pilla L, Lunghi F et al. Clinical and immunologic responses in melanoma patients vaccinated with MAGE-A3-genetically modified lymphocytes. Int. J. Cancer 132(11), 2557-2566 (2012). When one mentions T lymphocytes, one easily recognizes the effective and antigen-specific manner by which T lymphocytes execute cellular immune responses towards pathogen-infected or cancerous cells. Russo and coworkers recognized the other side of the coin and exploited the potency of T cells to act as a cellular vaccine, to which end they used T cells transduced with the cancer-testis antigen MAGE-A3. Twenty-three patients with MAGE-A3-expressing melanoma were treated and six patients developed MAGE-A3-specific immune responses and showed clinical benefit, whereas patients without a MAGE-A3-specific immune response did not show clinical benefit. This report includes and extends on results from a pilot study including ten patients, of which three developed MAGE-A3-specific immune responses. The present study further explores a potential beneficial application of the observed immunogenicity of genetically modified T cells.

Long-term stability of T-cell activation and transduction components critical to the processing of clinical batches of gene-engineered T cells.

BACKGROUND AIMS: The generation of gene-modified T cells for clinical adoptive T-cell therapy is challenged by the potential instability and concomitant high financial costs of critical T-cell activation and transduction components. As part of a clinical trial to treat patients with metastatic renal cell cancer with autologous T cells engineered with a chimeric antigen receptor (CAR) recognizing carboxy-anhydrase-IX (CAIX), we evaluated functional stability of the retroviral vector, T-cell activation agent Orthoclone OKT3 (Janssen-Cilag, Beerse, Belgium) monoclonal antibody (mAb) and the transduction promoting agent RetroNectin (Takara, Otsu, Japan). METHODS: Carboxy-anhydrase-IX chimeric antigen receptor retrovirus-containing culture supernatants (RTVsups) were generated from two packaging cell lines, Phoenix-Ampho (BioReliance, Sterling, UK) and PG13, and stored at -80°C over 10 years and 14 years. For Orthoclone OKT3 and RetroNectin, aliquots for single use were prepared and stored at -80°C. Transduction efficiencies of both batches of RTVsups were analyzed using the same lots of cryopreserved donor peripheral blood mononuclear cells, Orthoclone OKT3 and RetroNectin over time. RESULTS: We revisit here an earlier report on the long-term functional stability of the RTVsup, observed to be 9 years, and demonstrate that this stability is at least 14 years. Also, we now demonstrate that Orthoclone OKT3 and RetroNectin are functionally stable for periods of at least 6 years and 10 years. CONCLUSIONS: High-cost critical components for adoptive T-cell therapy can be preserved for ≥10 years when prepared in aliquots for single use and stored at -80°C. These findings may significantly facilitate, and decrease the financial risks of, clinical application of gene-modified T cells in multicenter studies.

Inflammation and fatigue dimensions in advanced cancer patients and cancer survivors: an explorative study.

BACKGROUND: Inflammation may underlie cancer-related fatigue; however, there are no studies that assess the relation between fatigue and cytokines in patients with advanced disease versus patients without disease activity. Furthermore, the relation between cytokines and the separate dimensions of fatigue is unknown. Here, association of plasma levels of inflammatory markers with physical fatigue and mental fatigue was explored in advanced cancer patients and cancer survivors. METHODS: A total of 45 advanced cancer patients and 47 cancer survivors completed the subscales Physical Fatigue and Mental Fatigue of the Multidimensional Fatigue Inventory. Plasma concentrations of C-reactive protein (CRP), interleukin-1 receptor antagonist (IL-1-ra), interleukin-6 (IL-6), interleukin-8 (IL-8), and neopterin were measured. Nonparametric tests were used to assess differences in fatigue intensity and levels of inflammatory markers and to determine correlation coefficients between the fatigue dimensions and inflammatory markers. RESULTS: Compared with cancer survivors, patients with advanced cancer had higher levels of physical fatigue (median 16 vs 9, P < .001) and mental fatigue (median 11 vs 6, P = .01). They also had higher levels of all cytokines (P < .01). In advanced cancer, CRP (r = 0.49, P = .001), IL-6 (r = 0.43, P = .003), IL-1-ra (r = 0.32, P = .03), and neopterin (r = 0.25, P = .10) were correlated with physical but not with mental fatigue. In cancer survivors, only IL-1-ra was related to both physical fatigue (r = 0.24, P = .10) and mental fatigue (r = 0.35, P = .02). CONCLUSIONS: In advanced cancer, inflammation seems to be associated with physical fatigue, but not to mental fatigue. In cancer survivors, there was no convincing evidence that inflammation plays a major role in fatigue.

Three sensitive assays do not provide evidence for circulating HuD-specific T cells in the blood of patients with paraneoplastic neurological syndromes with anti-Hu antibodies.

Anti-Hu antibody-associated paraneoplastic neurological syndromes (Hu-PNSs) are severe and often precede the detection of a malignancy, usually small-cell lung cancer. In Hu-PNS, it is hypothesized that neuronal cells are destroyed by T cells targeted against HuD, a protein expressed by small-cell lung cancer cells and neurons. There is only limited evidence for the existence of HuD-specific T cells. To detect these T cells in the blood of Hu-PNS patients, we employed 3 highly sensitive assays that included T cell stimulation with dendritic cells (DCs) to specifically expand the number of any HuD-specific T cells. A total of 17 Hu-PNS patients were tested with 1 or more of the following 3 assays: (1) tetramer staining after stimulation of T cells with conventionally generated DCs (n = 9), (2) interleukin (IL)-13 enzyme-linked immunosorbent spot (ELISpot; n = 3), IL-4 and IL-5 and interferon (IFN)-γ multiplex cytokine bead array (n = 2) to assay cytokine production by T cells after stimulation with conventionally generated DCs, and (iii) IFN-γ ELISpot and tetramer staining after T cell stimulation with accelerated co-cultured DCs (n = 11). No circulating HuD-specific T cells were found. We suggest that either autoaggressive T cells in Hu-PNS are not targeted against HuD or that their numbers in the blood are too low for detection by highly sensitive techniques.

Immune responses to transgene and retroviral vector in patients treated with ex vivo-engineered T cells.

Adoptive transfer of immune effector cells that are gene modified by retroviral transduction to express tumor-specific receptors constitutes an attractive approach to treat cancer. In patients with metastatic renal cell carcinoma, we performed a study with autologous T cells genetically retargeted with a chimeric antibody receptor (CAR) directed toward carbonic anhydrase IX (CAIX), an antigen highly expressed in renal cell carcinoma. In the majority of patients, we observed distinct humoral and/or cellular anti-CAIX-CAR T-cell immune responses in combination with a limited peripheral persistence of transferred CAIX-CAR T cells in the majority of patients. Humoral immune responses were anti-idiotypic in nature and neutralized CAIX-CAR-mediated T-cell function. Cellular anti-CAIX-CAR immune responses were directed to the complementarity-determining and framework regions of the CAR variable domains. In addition, 2 patients developed immunity directed against presumed retroviral vector epitopes. Here, we document the novel feature that therapeutic cells, which were ex vivo engineered by means of transduction with a minimal γ-retroviral vector, do express immunogenic vector-encoded epitopes, which might compromise persistence of these cells. These observations may constitute a critical concern for clinical ex vivo γ-retroviral gene transduction in general and CAR-retargeted T-cell therapy in particular, and underscore the need to attenuate the immunogenicity of both transgene and vector.