Targeting Alpha-Fetoprotein (AFP)-MHC Complex with CAR T-Cell Therapy for Liver Cancer

PURPOSE

The majority of tumor-specific antigens are intracellular and/or secreted and therefore inaccessible by conventional chimeric antigen receptor (CAR) T-cell therapy. Given that all intracellular/secreted proteins are processed into peptides and presented by class I MHC on the surface of tumor cells, we used alpha-fetoprotein (AFP), a specific liver cancer marker, as an example to determine whether peptide-MHC complexes can be targets for CAR T-cell therapy against solid tumors.

EXPERIMENTAL DESIGN

We generated a fully human chimeric antigen receptor, ET1402L1-CAR (AFP-CAR), with exquisite selectivity and specificity for the AFP_158-166 peptide complexed with human leukocyte antigen (HLA)-A*02:01.

RESULTS

We report that T cells expressing AFP-CAR selectively degranulated, released cytokines, and lysed liver cancer cells that were HLA-A*02:01⁺/AFP⁺ while sparing cells from multiple tissue types that were negative for either expressed proteins. In vivo, intratumoral injection of AFP-CAR T cells significantly regressed both Hep G2 and AFP₁₅₈-expressing SK-HEP-1 tumors in SCID-Beige mice (n = 8 for each). Moreover, intravenous administration of AFP-CAR T cells in Hep G2 tumor-bearing NSG mice lead to rapid and profound tumor growth inhibition (n = 6). Finally, in an established intraperitoneal liver cancer xenograft model, AFP-CAR T cells showed robust antitumor activity (n = 6).

CONCLUSIONS

This study demonstrates that CAR T-cell immunotherapy targeting intracellular/secreted solid tumor antigens can elicit a potent antitumor response. Our approach expands the spectrum of antigens available for redirected T-cell therapy against solid malignancies and offers a promising new avenue for liver cancer immunotherapy. Clin Cancer Res; 23(2); 478-88. ©2016 AACR.

A discontinuous hammerhead ribozyme embedded in a mammalian messenger RNA

Structured RNAs embedded in the untranslated regions (UTRs) of messenger RNAs can regulate gene expression. In bacteria, control of a metabolite gene is mediated by the self-cleaving activity of a ribozyme embedded in its 5' UTR. This discovery has raised the question of whether gene-regulating ribozymes also exist in eukaryotic mRNAs. Here we show that highly active hammerhead ribozymes are present in the 3' UTRs of rodent C-type lectin type II (Clec2) genes. Using a hammerhead RNA motif search with relaxed delimitation of the non-conserved regions, we detected ribozyme sequences in which the invariant regions, in contrast to the previously identified continuous hammerheads, occur as two fragments separated by hundreds of nucleotides. Notably, a fragment pair can assemble to form an active hammerhead ribozyme structure between the translation termination and the polyadenylation signals within the 3' UTR. We demonstrate that this hammerhead structure can self-cleave both in vitro and in vivo, and is able to reduce protein expression in mouse cells. These results indicate that an unrecognized mechanism of post-transcriptional gene regulation involving association of discontinuous ribozyme sequences within an mRNA may be modulating the expression of several CLEC2 proteins that function in bone remodelling and the immune response of several mammals.

Intersubunit movement is required for ribosomal translocation

Translocation of tRNA and mRNA during protein synthesis is believed to be coupled to structural changes in the ribosome. The "ratchet model," based on cryo-EM reconstructions of ribosome complexes, invokes relative movement of the 30S and 50S ribosomal subunits in this process; however, evidence that directly demonstrates a requirement for intersubunit movement during translocation is lacking. To address this problem, we created an intersubunit disulfide cross-link to restrict potential movement. The cross-linked ribosomes were unable to carry out polypeptide synthesis; this inhibition was completely reversed upon reduction of the disulfide bridge. In vitro assays showed that the cross-linked ribosomes were specifically blocked in elongation factor G-dependent translocation. These findings show that intersubunit movement is required for ribosomal translocation, accounting for the universal two-subunit architecture of ribosomes.

A novel antibody-TCR (AbTCR) platform combines Fab-based antigen recognition with gamma/delta-TCR signaling to facilitate T-cell cytotoxicity with low cytokine release

The clinical use of genetically modified T-cell therapies has led to unprecedented response rates in leukemia and lymphoma patients treated with anti-CD19 chimeric antigen receptor (CAR)-T. Despite this clinical success, FDA-approved T-cell therapies are currently limited to B-cell malignancies, and challenges remain with managing cytokine-related toxicities. We have designed a novel antibody-T-cell receptor (AbTCR) platform where we combined the Fab domain of an antibody with the γ and δ chains of the TCR as the effector domain. We demonstrate the ability of anti-CD19-AbTCR-T cells to trigger antigen-specific cytokine production, degranulation, and killing of CD19-positive cancer cells in vitro and in xenograft mouse models. By using the same anti-CD19 binding moiety on an AbTCR compared to a CAR platform, we demonstrate that AbTCR activates cytotoxic T-cell responses with a similar dose-response as CD28/CD3ζ CAR, yet does so with less cytokine release and results in T cells with a less exhausted phenotype. Moreover, in comparative studies with the clinically validated CD137 (4-1BB)-based CAR, CTL019, our anti-CD19-AbTCR shows less cytokine release and comparable tumor inhibition in a patient-derived xenograft leukemia model.

The hammerhead ribozyme: structure, catalysis, and gene regulation

The hammerhead ribozyme has long been considered a prototype for understanding RNA catalysis, but discrepancies between the earlier crystal structures of a minimal hammerhead self-cleaving motif and various biochemical investigations frustrated attempt to understand hammerhead ribozyme catalysis in terms of structure. With the discovery that a tertiary contact distal from the ribozyme's active site greatly enhances its catalytic prowess, and the emergence of new corresponding crystal structures of full-length hammerhead ribozymes, a unified understanding of catalysis in terms of the structure is now possible. A mechanism in which the invariant residue G12 functions as a general base, and the 2'-OH moiety of the invariant G8, itself forming a tertiary base pair with the invariant C3, is the general acid, appears consistent with both the crystal structure and biochemical experimental results. Originally discovered in the context of plant satellite RNA viruses, the hammerhead more recently has been found embedded in the 3'-untranslated region of mature mammalian mRNAs, suggesting additional biological roles in genetic regulation.

The sarcin-ricin loop of 23S rRNA is essential for assembly of the functional core of the 50S ribosomal subunit

The sarcin-ricin loop (SRL) of 23S rRNA in the large ribosomal subunit is a factor-binding site that is essential for GTP-catalyzed steps in translation, but its precise functional role is thus far unknown. Here, we replaced the 15-nucleotide SRL with a GAAA tetraloop and affinity purified the mutant 50S subunits for functional and structural analysis in vitro. The SRL deletion caused defects in elongation-factor-dependent steps of translation and, unexpectedly, loss of EF-Tu-independent A-site tRNA binding. Detailed chemical probing analysis showed disruption of a network of rRNA tertiary interactions that hold together the 23S rRNA elements of the functional core of the 50S subunit, accompanied by loss of ribosomal protein L16. Our results reveal an influence of the SRL on the higher-order structure of the 50S subunit, with implications for its role in translation.

Abstract 1537: Anti-CD19 ARTEMIS™ T cells prevent excessive inflammatory cytokine release, including IL-6, in a co-culture model of CRS

We designed the ARTEMISTM T-cell signaling platform to be a safer ACT therapy by relying on endogenous T-cell signaling factors. We show evidence that the ARTEMISTM receptor disentangles efficacy from CRS and holds the potential to be a clinically safer therapy by preventing ACT-triggered CRS. Despite the remarkable curative potential of anti-CD19-CAR T cells the therapy continues to trigger a potentially life threatening cytokine release syndrome (CRS) that is characterized by excessive concentrations of inflammatory cytokines. We reasoned that by redesigning the T-cell activating antigen receptor to require signaling through the endogenous CD3 complex the T-cell could better regulate its cellular responses and thus reduce the risk of CRS when used as an adoptive T-cell transfer (ACT) therapy. Through protein engineering, we developed the ARTEMIS™ signaling platform which when coupled with Eureka's human anti-CD19 antibody, ET190L1, and expressed on primary T-cells, results in a significant reduction of cytokine release during antigen specific T-cell activation. When CAR vs. ARTEMIS™ comparisons were tested in vitro, ET190L1-ARTEMISTM T-cells and ET190L1-CD28z-CAR T-cells specifically lysed multiple CD19+ leukemia and lymphoma cell lines with similar potencies. However, during the 16 hour in vitro killing assays, ARTEMIS™ T-cells secreted demonstrably less IL-2, IFN-γ, GM-CSF, and TNFα as compared to CAR T-cells. ARTEMISTM T-cells also accumulated less PD-1, LAG3, and TIM3 on their surface during culturing and following in vitro killing, indicating a diminished propensity for exhaustion compared to corresponding CAR T-cells. The ability of the anti-IL6 antibody tocilizumab to mitigate toxicity in patients that develop severe CRS in the clinic suggests that limiting IL-6 expression could improve the safety of adoptive T-cell therapies. However, T-cells are not efficient expressors of IL-6 and the vast majority of elevated IL-6 is produced by monocytes in response to activated CAR T-cells. To assay the effects of T-cell cytotoxicity on upregulation of cytokine expression within monocytes as a model for CRS, we co-cultured T-cells with CD19-positive ALL tumor cells on one side of a transwell and monocytes on the other side. After tumor cell lysis, RNA expression levels were measured from both T-cells and monocytes. Using ET190L1-ARTEMIS™ T-cells to kill NALM-6 tumor cells resulted in dramatically reduced levels of IL-6 and IL-5 mRNA upregulation within monocytes compared to that observed with CAR transduced T-cells. When tested in vivo against CD19+ xenografts mouse models, intravenous administration of ET190L1-ARTEMISTM T-cells caused efficient tumor regression. In agreement with the in vitro data, mice treated with ARTEMISTM T-cells released significantly lower levels of cytokines in their blood at 24 hours post dosing than mice treated with CAR T-cells.

Citation Format: Hong Liu, Lucas H. Horan, Stephan A. Grupp, David M. Barrett, Cheng Liu. Anti-CD19 ARTEMIS™ T cells prevent excessive inflammatory cytokine release, including IL-6, in a co-culture model of CRS [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 1537.

Abstract 2299: ET1402L1-CART, a T cell therapy targeting the intracellular tumor antigen AFP, demonstrates potent antitumor activity in hepatocellular carcinoma models

Hepatocellular carcinoma (HCC) is the 5th most prevalent and 3rd most lethal cancer worldwide, due to limited treatment options. The lack of tumor-specific membrane targets is one of the key challenges currently facing immunotherapy for solid tumors, where on-target/off-tumor activity can lead to severe adverse responses. Most HCCs overexpress Alpha-Fetoprotein (AFP), a secreted fetal glycoprotein rarely expressed in adult tissues, making AFP an ideal target for immunotherapy. However, since AFP is only expressed intracellularly and secreted, it cannot be targeted with traditional antibodies against cell-surface antigens. To overcome this limitation, we exploited the fact that intracellular antigens, including AFP, are processed into peptides and presented by class I major histocompatibility complexes (MHCs) on the surface of tumor cells. Using an antibody discovery platform optimized for identifying candidates that bind peptide/MHC complexes, we developed ET1402L1, a fully-human scFv specifically targeting an immunogenic AFP peptide complexed with human leukocyte antigen (HLA)-A*02:01. ET1402L1 binds HLA-A*02:01/AFP with exquisite specificity and does not cross-react with naked HLA-A*02:01 molecules or with HLA-A*02:01 complexed with a panel of control peptides from endogenous human proteins. T cells engineered to express a second generation chimeric antigen receptor (CAR) constructed with ET1402L1 responded to HLA-A*02:01/AFP-expressing cells by degranulating and releasing IFN-γ, IL-2, IL-6, IL-8, IL-10, GM-CSF, and TNFα in an antigen-selective manner. In a cell killing assay, ET1402L1-CART selectively lysed HCC cells that expressed both HLA-A*02:01 and AFP, while sparing cells from multiple tissue types that were negative for either. In vivo, ET1402L1-CART demonstrated potent anti-tumor activity in multiple HCC xenograft models. While intravenous administration of ET1402L1-CART significantly inhibited the growth of established subcutaneous (s.c.) Hep G2 tumors (an HCC cell line positive for HLA-A*02:01 and AFP), intratumoral delivery resulted in rapid and prolonged tumor regression with complete regression observed in 75% of animals. These effects were recapitulated in a s.c. tumor model using another liver-derived HLA-A*02:01-positive cell line (SK-HEP-1) engineered to express the AFP peptide. Lastly, intraperitoneal injection of ET1402L1-CART led to regression of tumors in a disseminated abdominal HCC xenograft model. Our data demonstrates that CAR-T cell therapy targeting intracellular antigens via peptide/MHC complexes can effectively eradicate solid tumors in vivo. This approach expands the spectrum of antigens available for redirected T cell therapy against solid malignancies and provides a promising future therapy for HCC. A phase I clinical trial testing ET1402L1-CART in HCC patients is expected to begin in 2016.

Citation Format: Hong Liu, Yiyang Xu, Jingyi Xiang, Li Long, Shon Green, Zhiyuan Yang, Jingwei Lu, Neal Cheng, Lucas H. Horan, Bin Liu, Su Yan, Pei Wang, Juan Diaz, Lian-xing Liu, Vivien Chan, Cheng Liu. ET1402L1-CART, a T cell therapy targeting the intracellular tumor antigen AFP, demonstrates potent antitumor activity in hepatocellular carcinoma models. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 2299.