Isolation of Tumor Antigen-Specific Single-Chain Variable Fragments Using a Chimeric Antigen Receptor Bicistronic Retroviral Vector in a Mammalian Screening Protocol

A cell-based phenotypic library selection and screening approach for the de novo discovery of novel functional chimeric antigen receptors

Anti-tumor therapies that seek to exploit and redirect the cytotoxic killing and effector potential of autologous or syngeneic T cells have shown extraordinary promise and efficacy in certain clinical settings. Such cells, when engineered to express synthetic chimeric antigen receptors (CARs) acquire novel targeting and activation properties which are governed and orchestrated by, typically, antibody fragments specific for a tumor antigen of interest. However, it is becoming increasingly apparent that not all antibodies are equal in this regard, with a growing appreciation that ‘optimal’ CAR performance requires a consideration of multiple structural and contextual parameters. Thus, antibodies raised by classical approaches and intended for other applications often perform poorly or not at all when repurposed as CARs. With this in mind, we have explored the potential of an in vitro phenotypic CAR library discovery approach that tightly associates antibody-driven bridging of tumor and effector T cells with an informative and functionally relevant CAR activation reporter signal. Critically, we demonstrate the utility of this enrichment methodology for ‘real world’ de novo discovery by isolating several novel anti-mesothelin CAR-active scFv candidates.

A High-Throughput Method for Characterizing Novel Chimeric Antigen Receptors in Jurkat Cells

Chimeric antigen receptor (CAR) development involves extensive empirical characterization of antigen-binding domain (ABD)/CAR constructs for clinical suitability. Here, we present a cost-efficient and rapid method for evaluating CARs in human Jurkat T cells. Using a modular CAR plasmid, a highly efficient ABD cloning strategy, plasmid electroporation, short-term co-culture, and flow-cytometric detection of CD69, this assay (referred to as CAR-J) evaluates sensitivity and specificity for ABDs. Assessing 16 novel anti-CD22 single-chain variable fragments derived from mouse monoclonal antibodies, CAR-J stratified constructs by response magnitude to CD22-expressing target cells. We also characterized 5 novel anti-EGFRvIII CARs for preclinical development, identifying candidates with varying tonic and target-specific activation characteristics. When evaluated in primary human T cells, tonic/auto-activating (without target cells) EGFRvIII-CARs induced target-independent proliferation, differentiation toward an effector phenotype, elevated activity against EGFRvIII-negative cells, and progressive loss of target-specific response upon in vitro re-challenge. These EGFRvIII CAR-T cells also showed anti-tumor activity in xenografted mice. In summary, CAR-J represents a straightforward method for high-throughput assessment of CAR constructs as genuine cell-associated antigen receptors that is particularly useful for generating large specificity datasets as well as potential downstream CAR optimization.

"Therapeutic applications of the 'NPGP' family of viral 2As"

Oligopeptide "2A" and "2A-like" sequences ("2As"; 18-25aa) are found in a range of RNA virus genomes controlling protein biogenesis through "recoding" of the host-cell translational apparatus. Insertion of multiple 2As within a single open reading frame (ORF) produces multiple proteins; hence, 2As have been used in a very wide range of biotechnological and biomedical applications. During translation, these 2A peptide sequences mediate a eukaryote-specific, self-"cleaving" event, termed "ribosome skipping" with very high efficiency. A particular advantage of using 2As is the ability to simultaneously translate a number of proteins at an equal level in all eukaryotic systems although, naturally, final steady-state levels depend upon other factors-notably protein stability. By contrast, the use of internal ribosome entry site elements for co-expression results in an unbalanced expression due to the relative inefficiency of internal initiation. For example, a 1:1 ratio is of particular importance for the biosynthesis of the heavy-chain and light-chain components of antibodies: highly valuable as therapeutic proteins. Furthermore, each component of these "artificial polyprotein" systems can be independently targeted to different sub-cellular sites. The potential of this system was vividly demonstrated by concatenating multiple gene sequences, linked via 2A sequences, into a single, long, ORF-a polycistronic construct. Here, ORFs comprising the biosynthetic pathways for violacein (five gene sequences) and β-carotene (four gene sequences) were concatenated into a single cistron such that all components were co-expressed in the yeast Pichia pastoris. In this review, we provide useful information on 2As to serve as a guide for future utilities of this co-expression technology in basic research, biotechnology, and clinical applications.

Efficacy and toxicity management of CAR-T-cell immunotherapy: a matter of responsiveness control or tumour-specificity?

Chimaeric antigen receptor (CAR)-expressing T-cells have demonstrated potent clinical efficacy in patients with haematological malignancies. However, the use of CAR-T-cells targeting solid tumour-associated antigens (TAAs) has been limited by organ toxicities related to activation of T-cell effector functions through the CAR. Most existing CARs recognize TAAs, which are also found in normal tissues. CAR-T-cell-mediated destruction of normal tissues constitutes a major roadblock to CAR-T-cell therapy, and must be avoided or mitigated. There is a broad range of strategies for modulating antigen responsiveness of CAR-T-cells, with varying degrees of complexity. Some of them might ameliorate the acute and chronic toxicities associated with current CAR constructs. However, further embellishments to CAR therapy may complicate clinical implementation and possibly create new immunogenicity issues. In contrast, the development of CARs targeting truly tumour-specific antigens might circumvent on-target/off-tumour toxicities without adding additional complexity to CAR-T-cell therapies, but these antigens have been elusive and may require novel selection strategies for their discovery.

CAR T-cell therapy: toxicity and the relevance of preclinical models

Chimeric antigen receptor (CAR) T cells form part of a broad wave of immunotherapies that are showing promise in early phase cancer clinical trials. This clinical delivery has been based upon preclinical efficacy testing that confirmed the proof of principle of the therapy. However, CAR T-cell therapy does not exist alone as T cells are generally given in combination with patient preconditioning, most commonly in the form of chemotherapy, and may also include systemic cytokine support, both of which are associated with toxicity. Consequently, complete CAR T-cell therapy includes elements where the toxicity profile is well known, but also includes the CAR T cell itself, for which toxicity profiles are largely unknown. With recent reports of adverse events associated with CAR T-cell therapy, there is now concern that current preclinical models may not be fit for purpose with respect to CAR T-cell toxicity profiling. Here, we explore the preclinical models used to validate CAR T-cell function and examine their potential to predict CAR T-cell driven toxicities for the future.

Receptor-targeted lentiviral vectors are exceptionally sensitive toward the biophysical properties of the displayed single-chain Fv

An increasing number of applications require the expression of single-chain variable fragments (scFv) fusion proteins in mammalian cells at the cell surface membrane. Here we assessed the CD30-specific scFv HRS3, which is used in immunotherapy, for its ability to retarget lentiviral vectors (LVs) to CD30 and to mediate selective gene transfer into CD30-positive cells. Fused to the C-terminus of the type-II transmembrane protein hemagglutinin (H) of measles virus and expressed in LV packaging cells, gene transfer mediated by the released LV particles was inefficient. A series of point mutations in the scFv framework regions addressing its biophysical properties, which substantially improved production and increased the melting temperature without impairing its kinetic binding behavior to CD30, also improved the performance of LV particles. Gene transfer into CD30-positive cells increased ∼100-fold due to improved transport of the H-scFv protein to the plasma membrane. Concomitantly, LV particle aggregation and syncytia formation in packaging cells were substantially reduced. The data suggest that syncytia formation can be triggered by trans-cellular dimerization of H-scFv proteins displayed on adjacent cells. Taken together, we show that the biophysical properties of the targeting ligand have a decisive role for the gene transfer efficiency of receptor-targeted LVs.

Chimeric antigen receptor-redirected CD45RA-negative T cells have potent antileukemia and pathogen memory response without graft-versus-host activity

Chimeric antigen receptor (CAR)-redirected cellular therapy is an attractive modality for cancer treatment. We hypothesized that allogeneic CAR-engineered CD45RA-negative T cells can control cancer and infection without the risk of graft-versus-host disease (GVHD). We used CD19(+) MLL-rearranged leukemia as prototype because it is an aggressive and generally drug-resistant malignancy. CD45RA(-) cells that were transduced with anti-CD19 CAR containing 4-1BB and CD3ζ signaling domains effectively lysed MLL-rearranged leukemia cell lines and primary blasts in vitro. In a disseminated leukemia mouse model, CAR(+)CD45RA(-) cells significantly reduced leukemia burdens and prolonged overall survival without GVHD. CAR(+) cells were sustainable in blood, and all the treated mice remained leukemia-free even after they were re-challenged with leukemia cells. Despite the transduction process, CD45RA(-) cells retained recall activity both in vitro and in vivo against human pathogens commonly found in cancer patients. In comparison with CD45RA(+) cells, CD45RA(-) cells showed less allogeneic activity in mixed leukocyte reactions and in mouse models. Thus, the use of CAR(+)CD45RA(-) cells can separate GVHD from graft-versus-malignancy effect and infection control. These cells should also be useful in nontransplant settings and may be administered as off-the-shelf third-party cells.