E3 Ubiquitin Ligase Cbl-b Research Articles

CBLs downregulation foretells T cell ubiquitination leading to autoimmunity

In a study published in the July issue of Immunity, Li etal.1 demonstrate that expression of the E3 ubiquitin ligases CBL and CBL-B is downregulated in Tfh cells in SLE with Tfh cell expansion and autoimmunity. This leads to reduced ubiquitination of the Tcell costimulator ICOS which regulates proteostasis of the Tfh cell transcription factor BCL6 via chaperone-mediated autophagy.

Abstract 4042: Cbl inhibition increases the response of cytotoxic T cells to sub-optimal stimulation and drives enhanced anti-tumor responses alone or in combination with PD-1 blockade

Abstract INTRODUCTION: Herein, we describe a series of experiments to define the unique biology of dual Cbl-b/c-Cbl (Cbl) inhibition in amplifying T cell activation and anti-tumor responses. Through their E3 ubiquitin ligase activity, Cbl-b and c-Cbl have been shown to negatively regulate multiple signaling proteins downstream of the T cell receptor (TCR), suggesting that inhibition may yield enhanced anti-tumor T cell activity. Indeed, using a dual Cbl-b/c-Cbl small molecule inhibitor, we demonstrate that Cbl blockade enhances T cell activity under standard and sub-optimal stimulation conditions. Moreover, Cbl inhibition increases mouse syngeneic tumor control alone or in combination with α-PD-1, as described below. METHODS: Human CD8+ T cells were isolated from healthy human blood and activated using α-CD3 or α-CD3/CD28 stimulation. Mouse OT-I splenocytes were stimulated with SIINFEKL or lower affinity ovalbumin peptides and cytokine production was assessed. For tumor studies, mice were inoculated with syngeneic cancer cells and were treated with Cbl inhibitor (30 mg/kg, QD) +/- α-PD-1 (10 mg/kg, Q5D) starting at a tumor volume of 75 mm3. RESULTS: Human CD8+ T cells, stimulated in the presence of a Cbl inhibitor, demonstrated a concentration-dependent increase in activation hallmarks including cytokine secretion (IL-2, IFN-γ and granzyme B), proliferation, and the presence of highly polyfunctional cells. Furthermore, using serially stimulated CD8+ T cells with reduced overall activity, Cbl inhibition showed a marked enhancement of IFN-γ production and cytotoxicity compared to controls upon re-stimulation. We then used an antigen-specific T cell assay where OT-I splenocytes were stimulated by peptides with optimal or reduced TCR affinities, Cbl inhibition produced significantly higher IFN-γ levels with all peptides relative to controls demonstrating that Cbl inhibition can increase the response of T cells to sub-optimal stimulation. Using mouse CT26 and MC38 tumor models, Cbl inhibition was able to significantly enhance tumor control in combination with α-PD-1 and as a monotherapy in CT26 tumors. Notably, a greater number of tumor antigen-specific cytotoxic T cells were found in Cbl inhibitor-treated tumors. Higher expression levels of the co-stimulatory molecule CD226 and the TCR in these anti-tumor cells were also observed, demonstrating the unique biology of Cbl inhibition in enhancing anti-tumor responses. Finally, Cbl inhibition had no effect on the growth or survival of cancer cells in vitro consistent with the notion that enhanced tumor control was driven by immune activity. CONCLUSIONS: Altogether, these data demonstrate that pharmacologic Cbl inhibition robustly enhances T cell activity in vitro and in vivo and provide a mechanistic rationale for targeting Cbl-b/c-Cbl to amplify anti-tumor immune responses. Citation Format: Livia Yamashiro, Sachie Marubayashi, Rameshwari Rayaji, Anudari Battsooj, Karim Mrouj, Hema Singh, Yue Tong Lee, Ada Chen, Lunda Shen, Artur Mailyan, Kai Yu, Masha Podunavac, Julie Yu, Dongdong Liu, Clayton Hardman, Johnny Yin, Srikanth Gangam, Ken Lawson, Manmohan Leleti, Matthew Walters, Daniel DiRenzo. Cbl inhibition increases the response of cytotoxic T cells to sub-optimal stimulation and drives enhanced anti-tumor responses alone or in combination with PD-1 blockade [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 4042.

E3 ubiquitin ligase CBL-B suppresses vascular endothelial cell pyroptosis and injury in intracranial aneurysm by facilitating NLRP3 degradation

E3 ubiquitin ligase CBL-B suppresses vascular endothelial cell pyroptosis and injury in intracranial aneurysm by facilitating NLRP3 degradation

Knockout of c-Cbl/Cbl-b slows c-Met trafficking resulting in enhanced signaling in corneal epithelial cells

In many cell types, the E3 ubiquitin ligases c-Cbl and Cbl-b induce ligand-dependent ubiquitylation of the HGF-stimulated c-Met receptor and target it for lysosomal degradation. This study determines whether c-Cbl/Cbl-b are negative regulators of c-Met in the corneal epithelium (CE), and if their inhibition can augment c-Met-mediated CE homeostasis. Immortalized human corneal epithelial cells were transfected with Cas9 only (Cas9, control cells) or with Cas9 and c-Cbl/Cbl-b guide RNAs to knockout each gene singularly (-c-Cbl or -Cbl-b cells) or both genes (DKO cells) and monitored for their responses to HGF. Cells were assessed for ligand-dependent c-Met ubiquitylation via immunoprecipitation, magnitude and duration of c-Met receptor signaling via immunoblot, and receptor trafficking by immunofluorescence. Single knockout cells displayed a modest decrease in receptor ubiquitylation and a slight increase in phosphorylation as compared to control. DKO cells did not have detectable ubiquitylation, had delayed receptor trafficking, and a 2.3-fold increase in c-Met phosphorylation. Based on the observed changes in receptor trafficking and signaling, we examined HGF-dependent in vitro wound healing via live cell timelapse microscopy in control and DKO cells. HGF-treated DKO cells healed at approximately twice the rate of untreated cells. From these data, we have generated a model in which c-Cbl/Cbl-b mediate the ubiquitylation of c-Met, which targets the receptor through the endocytic pathway toward lysosomal degradation. In the absence of ubiquitylation, the stimulated receptor stays phosphorylated longer and enhances in vitro wound healing. We propose that c-Cbl and Cbl-b are two promising pharmacologic targets for enhancing c-Met-mediated CE re-epithelialization.

Abstract LB337: Inhibition of the E3 ligase CBL-B enhances the effector function and proliferation of natural killer cells

Abstract The E3 ubiquitin ligase CBL-B is a master regulator of effector T cells through its downstream regulation of T-cell receptor and co-stimulatory stimulation, but it has also been shown to play a negative feedback role in NK cells. CBL-B depletion enhances the production of IFNγ and the cytotoxicity of activated NK cells. Similar to the effect of depleting CD8 T cells in mice, depletion of NK cells abolishes the tumor growth delay observed in Cbl-b-/- mice. With emerging data supporting the potential role of exogenous NK cells in anti-tumor immunity, inhibition of CBL-B represents a promising approach to enhance both adaptive and innate immunity for cancer immunotherapy. Using one of our small molecules, allosteric CBL-B inhibitors, HOT-A, we demonstrated that CBL-B inhibition enhanced NK activation, IFNγ production and granzyme B secretion in response to the various stimulation conditions, including cytokines (such as IL-15 and IL12 + IL-18) and stimulatory receptors (such as NKp30 and NKG2D + CD244). NK cell proliferation was also enhanced in the presence of HOT-A. Single cell analysis further suggested that HOT-A increased NK polyfunctional cytokine expression during IL-12 + IL-18 activation. In a coculture system of NK cells and K562 target cells, HOT-A dose-dependently promoted primary human NK cell activation, cytokine production and cytotoxicity in the K562 cells. In the in vivo CT-26 tumor model, granzyme B+ NK cells in the TIL were significantly increased in the HOT-A treated tumor bearing mice and NK function gene signature was also increased by Nanostring analysis, suggesting an enhanced NK function in the tumor microenvironment. Taken together, the pre-clinical data presented here demonstrate that our CBL-B inhibitor enhances NK cell function in vitro and in vivo, suggesting that its enhancement of anti-tumor immunity may be driven through both effector T cell and NK cell-driven biology. Citation Format: Yilin Qi, Jun Kuai, Yingzhi Bi, David Greco, Samira Jaeger, Ken Carson, Hadi Danaee, Timothy Reilly, Geraldine Harriman, Fang Wang. Inhibition of the E3 ligase CBL-B enhances the effector function and proliferation of natural killer cells [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 2 (Clinical Trials and Late-Breaking Research); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(8_Suppl):Abstract nr LB337.

Involvement of the E3 ubiquitin ligase Cblb in host defense and evaluation of transcriptome during Trueperella pyogenes infection

Trueperella pyogenes (T. pyogenes) is a versatile and ingenious bacterium that causes severe suppurative injuries in lots of economically important ruminants. The underlying pathogenesis of T. pyogenes infection remains poorly understood. In the current study, we performed transcriptome sequencing of mouse blood tissue infected with T. pyogenes. A total of 36.73 G clean data were collected, and 136 differentially expressed genes were obtained in the infection group compared to the control group. In addition, we found that the E3 ubiquitin ligase Cblb exhibited significant upregulation in the infection groups compared to the control group. Mechanistically, T. pyogenes infection markedly enhanced the expression of Cblb and regulated the host defense response. Inhibiting Cblb expression with Cblb siRNA impaired the inflammatory response and reduced the effect of phagocytosis in RAW264.7 murine macrophages. Intriguingly, overexpression of Cblb induced a strong inflammatory response and enhanced phagocytosis against T. pyogenes infection in macrophages. More importantly, the overexpression of Cblb significantly reduced the bacterial load and protected mice from the T. pyogenes infections. Therefore, our findings reveal that Cblb is a novel and potential regulator in response to T. pyogenes infection and shed new light on the development of promising treatments against T. pyogenes-related diseases.

Abstract 5512: CBLB, CISH and CD70 multiplexed gene knockout with CRISPR/Cas9 enhances cytotoxicity of CD70-CAR NK cells and provides greater resistance to TGF-β for cancer immunotherapy

Abstract Natural killer (NK) cells provide an attractive platform for development of effective cancer immunotherapies. NK cells are known for their ability to kill tumor cells and do not elicit graft-versus-host disease, making them a potential source of ‘off-the-shelf’ allogeneic cell therapy. NK cells are also amenable to CRISPR genomic engineering to enhance the antitumor activity of NK cells by increasing their cytotoxicity, overcoming suppression within the tumor microenvironment, or promoting their persistence and homing to tumor sites. Cytokine inducible SH2-containing protein (CISH) is a NK cell checkpoint for IL-15 mediated NK survival, proliferation, cytotoxicity, and anti-tumor immunity. The E3 ubiquitin ligase CBLB is also a negative regulator of NK cell function and has been shown to mediate TGF-β sensitivity by downregulating inhibitory SMAD7 in primary T cells. We hypothesized that knockout of both CISH and CBLB would not only improve NK cell effector function and but also render NK cells resistant to TGF-β mediated suppression. In this study, we utilized CRISPR-Cas9 ribonucleoproteins (RNPs) to disrupt CISH and CBLB genes in isolated peripheral blood NK cells from healthy donors. Given that CD70 expression is present on activated NK cells, to target CD70 on renal cell carcinoma (RCC) with CAR NK cells, CD70 was knocked out in NK cells to avoid fratricide. Western blotting, FACS and TIDE/Amplicon NGS Sequencing data confirmed all three genes were successfully disrupted. Then we expanded these edited NK cells by using IL-2 and stimulation using NKSTIM, a modified K562 stimulatory cell line expressing membrane-bound form of IL-15 (mbIL-15) and 4-1BBL. IL-12 and IL-18 were added during expansion to drive memory-like NK cell differentiation. Furthermore, we were able to transduce CRISPR/Cas9 edited NK cells to express a CD70-CAR construct and membrane bound IL-15. CAR expression was assessed by flow cytometry. In vitro cytotoxicity was measured using the IncuCyte S3 live cell analysis system. CD70/CISH/CBLB triple knockout CD70-CAR NK cells could be produced efficiently and exhibited similar persistence as CD70/CISH or CD70/CBLB double knockout CD70-CAR NK cells in culture. Cytotoxicity assays demonstrated that CD70/CISH/CBLB triple knockout CD70-CAR NK cells had greater tumor growth control after multiple rechallenges. In the presence of exogenous TGF-β, CD70/CISH/CBLB triple knockout CD70-CAR NK cells showed greater resistance to TGF-β inhibition of cytotoxicity. In summary, we show CD70/CISH/CBLB triple knockout CD70-CAR NK cells demonstrate enhanced anti-tumor activity against relevant solid tumor cell lines and provide greater resistance to tumor microenvironment inhibition. These data support the further exploration of CD70/CISH/CBLB triple gene knockout CD70 CAR NK cells for clinical application. Citation Format: Chao Guo, Yanying Fan, Alexander Aronov, Qi Zhang, Sombeet Sahu, Mary-Lee Dequéant, Changan Guo, Sushant Karnik, Glenn D. Leary, Chandirasegaran Massilamany, Ivan H. Chan, James B. Trager. CBLB, CISH and CD70 multiplexed gene knockout with CRISPR/Cas9 enhances cytotoxicity of CD70-CAR NK cells and provides greater resistance to TGF-β for cancer immunotherapy [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 5512.

E3 Ubiquitin Ligase Cbl-b Inhibits Type 2 Innate Lymphoid Cell Development by Targeting ST2 for Ubiquitination

Abstract Type 2 innate lymphoid cells (ILC2) play a crucial role in driving allergic airway inflammation, but the regulation of ILC2 development is not fully understood. Previously we have shown that E3 ubiquitin ligase Cbl-b negatively regulates Th2 response, but whether Cbl-b also regulates ILC2-mediated type 2 immunity remains to be defined. In this study, we found that Rag1−/−Cblb−/− mice are hypersensitive to inhalation of A. Fumigatus, a fungus that causes allergic asthma, as revealed by heightened airway hyperresponsiveness (AHR), increased airway inflammation, and mucus production. This hypersensitivity is associated with increased frequencies of ILC2 and eosinophils, but reduced frequency of alveolar macrophages in the lungs. Consistent with this, type 2 cytokines are also increased in the lung homogenates of Rag1−/−Cblb−/− mice. At the molecular level, ST2 undergoes ubiquitination and proteasome-mediated degradation upon IL-33 stimulation, which is abrogated in cells expressing Cbl-b C373A mutant. Therefore, our data indicate that Cbl-b inhibits the development of ILC2 and ILC2-mediated airway inflammation by targeting ST2 for ubiquitination. Supported by NIH R01 AI090901, AI121196, and AI123253

Cbl-b deficiency prevents functional but not phenotypic T cell anergy

T cell anergy is an important peripheral tolerance mechanism. We studied how T cell anergy is established using an anergy model in which the Zap70 hypermorphic mutant W131A is coexpressed with the OTII TCR transgene (W131AOTII). Anergy was established in the periphery, not in the thymus. Contrary to enriched tolerance gene signatures and impaired TCR signaling in mature peripheral CD4 T cells, CD4SP thymocytes exhibited normal TCR signaling in W131AOTII mice. Importantly, the maintenance of T cell anergy in W131AOTII mice required antigen presentation via MHC-II. We investigated the functional importance of the inhibitory receptor PD-1 and the E3 ubiquitin ligases Cbl-b and Grail in this model. Deletion of each did not affect expression of phenotypic markers of anergic T cells or T reg numbers. However, deletion of Cbl-b, but not Grail or PD-1, in W131AOTII mice restored T cell responsiveness and signaling. Thus, Cbl-b plays an essential role in the establishment and/or maintenance of unresponsiveness in T cell anergy.

Proteomic investigation of Cbl and Cbl-b in neuroblastoma cell differentiation highlights roles for SHP-2 and CDK16

SummaryNeuroblastoma is a highly heterogeneous embryonal solid tumor of the sympathetic nervous system. As some tumors can be treated to undergo differentiation, investigating this process can guide differentiation-based therapies of neuroblastoma. Here, we studied the role of E3 ubiquitin ligases Cbl and Cbl-b in regulation of long-term signaling responses associated with extracellular signal-regulated kinase phosphorylation and neurite outgrowth, a morphological marker of neuroblastoma cell differentiation. Using quantitative mass spectrometry (MS)-based proteomics, we analyzed how the neuroblastoma cell line proteome, phosphoproteome, and ubiquitylome were affected by Cbl and Cbl-b depletion. To quantitatively assess neurite outgrowth, we developed a high-throughput microscopy assay that was applied in combination with inhibitor studies to pinpoint signaling underlying neurite outgrowth and to functionally validate proteins identified in the MS data sets. Using this combined approach, we identified a role for SHP-2 and CDK16 in Cbl/Cbl-b-dependent regulation of extracellular signal-regulated kinase phosphorylation and neurite outgrowth, highlighting their involvement in neuroblastoma cell differentiation.

Cbl-b Is Upregulated and Plays a Negative Role in Activated Human NK Cells.

The E3 ubiquitin ligase Cbl-b has been characterized as an intracellular checkpoint in T cells; however, the function of Cbl-b in primary human NK cells, an innate immune anti-tumor effector cell, is not well defined. In this study, we show that the expression of Cbl-b is significantly upregulated in primary human NK cells activated by IL-15, IL-2, and the human NK cell-sensitive tumor cell line K562 that lacks MHC class I expression. Pretreatment with JAK or AKT inhibitors prior to IL-15 stimulation reversed Cbl-b upregulation. Downregulation of Cbl-b resulted in significant increases in granzyme B and perforin expression, IFN-γ production, and cytotoxic activity against tumor cells. Collectively, we demonstrate upregulation of Cbl-b and its inhibitory effects in IL-15/IL-2/K562-activated human NK cells, suggesting that Cbl-b plays a negative feedback role in human NK cells.

E3 Ubiquitin Ligase Cbl-b Restrains Priming of Pathogenic Th17 Cells Via the Inhibition of IL-6 Production by Macrophages

E3 ubiquitin ligase Cbl-b is involved in the maintenance of a balance between immunity and tolerance. Mice lacking Cbl-b are highly susceptible to experimental autoimmune encephalomyelitis (EAE), a Th17-mediated autoimmune disease. However, how Cbl-b regulates Th17 cell responses remains unclear. In this study, utilizing adoptive transfer and cell type-specific Cblb knockout strains we show that Cbl-b expression in macrophages, but not T cells or dendritic cells (DCs), restrains the generation of pathogenic Th17 cells and the development of EAE. Cbl-b inhibits the production of IL-6 by macrophages induced by the signaling derived from C-type lectin receptors, which directs T cells to generate pathogenic Th17 cells. Therefore, our data are the first to unveil a previously unappreciated function for Cbl-b in the regulation of pathogenic Th17 responses.

Inflammatory pain revisits development

Pain An increase in N -methyl-d-aspartate (NMDA) glutamate receptors (NMDARs) containing the GluN2B subunit in sensory neuronal synapses is associated with enhanced nociception and pain. In rodents, Zhang et al. found that GluN2B content in sensory neurons was progressively restricted during early development and maintained at low abundance by the E3 ubiquitin ligase Cbl-b during adulthood. Peripheral inflammation impaired the interaction of Cbl-b with GluN2B, and the increase in GluN2B abundance enhanced NMDAR activity and neuronal sensitivity to touch. Sci. Signal. 13 , eaaw1519 (2020).

E3 Ubiquitin Ligase Cbl-b Inhibits Type 2 Innate Lymphoid Cell Development by Targeting ST2 for Ubiquitination

Abstract Type 2 innate lymphoid cells (ILC2) play a crucial role in driving allergic airway inflammation, but the regulation of ILC2 development is not fully understood. Previously we have shown that E3 ubiquitin ligase Cbl-b negatively regulates Th2 response, but whether Cbl-b also regulates ILC2-mediated type 2 immunity remains to be defined. In this study, we found that Rag1−/−Cblb−/− mice are hypersensitive to inhalation of A. Fumigatus, a fungus that causes allergic asthma, as revealed by heightened airway hyperresponsiveness (AHR), increased airway inflammation, and mucus production. This hypersensitivity is associated with increased frequencies of ILC2 and eosinophils, but reduced frequency of alveolar macrophages in the lungs. Consistent with this, type 2 cytokines are also increased in the lung homogenates of Rag1−/−Cblb−/− mice. At the molecular levels, ST2 undergoes ubiquitination and proteasome-mediated degradation upon IL-33 stimulation, which is abrogated in cells expressing Cbl-b C373A mutant. Therefore, our data indicate that Cbl-b inhibits the development of ILC2 and ILC2-mediated airway inflammation by targeting ST2 for ubiquitination.

E3 Ubiquitin Ligase Cbl-b Restrains Priming of Pathogenic Th17 Cells via Inhibiting IL-6 Production by Macrophages

Abstract E3 ubiquitin ligase Cbl-b is involved in the maintenance of a balance between immunity and tolerance. Mice lacking Cbl-b are highly susceptible to experimental autoimmune encephalomyelitis (EAE), a Th17-mediated autoimmune disease. However, how Cbl-b regulates Th17 cell response remains unclear. In this study, utilizing adoptive transfer and cell type-specific Cblb knockout strains we show that Cbl-b expression in macrophages but not T cells or dendritic cells (DCs) restrains the generation of pathogenic Th17 cells and the development of EAE. Cbl-b inhibits the production of IL-6 by macrophages induced by the signaling derived from C-type lectin receptors, which directs T cells to generate pathogenic Th17 cells. Therefore, our data are the first to unveil a previously-unappreciated function for Cbl-b in the regulation of pathogenic Th17 response.

Sequential ubiquitination of NLRP3 by RNF125 and Cbl-b limits inflammasome activation and endotoxemia.

Aberrant NLRP3 inflammasome activation contributes to the development of endotoxemia. The importance of negative regulation of NLRP3 inflammasomes remains poorly understood. Here, we show that the E3 ubiquitin ligase Cbl-b is essential for preventing endotoxemia induced by a sub-lethal dose of LPS via a caspase-11/NLRP3-dependent manner. Further studies show that NLRP3 undergoes both K63- and K48-linked polyubiquitination. Cbl-b binds to the K63-ubiquitin chains attached to the NLRP3 leucine-rich repeat domain (LRR) via its ubiquitin-associated region (UBA) and then targets NLRP3 at K496 for K48-linked ubiquitination and proteasome-mediated degradation. We also identify RNF125 as an additional E3 ubiquitin ligase that initiates K63-linked ubiquitination of the NLRP3 LRR domain. Therefore, NLRP3 is sequentially ubiquitinated by K63- and K48-linked ubiquitination, thus keeping the NLRP3 inflammasomes in check and restraining endotoxemia.

Abstract C048: Novel master checkpoint Cbl-b siRNA-based adoptive cellular therapy: Superior antitumor efficacy in a syngeneic murine hepatocellular carcinoma Hepa1-6 model following APN401 monotherapy and synergistic effects with anti-PD1

Abstract Introduction: The E3 ubiquitin ligase Cbl-b is an intracellular master checkpoint which negatively regulates both adaptive and innate anti-tumor immune responses. Selective targeting of Cbl-b not only induces anti-tumor activity in vivo but also overrides immune regulation by the PD-L1/PD-1 pathway in vitro (2017 Fujiwara et al). Human APN401 is an autologous adoptive cellular therapy of ex vivo Cbl-b-silenced human PBMCs currently in a clinical Phase 1b multiple dose study demonstrating early clinical safety and tolerability in patients with advanced solid tumors. Herein the preclinical Proof of Concept efficacy of murine APN401 immunotherapy is established in the syngeneic mouse hepatocellular carcinoma Hepa1-6 model. Experimental Procedure: Hepa1-6-C57/BL6-tumor bearing mice were treated with murine APN401 ex vivo silenced immune cells with Cbl-b specific siRNA as monotherapy or in combination with anti-PD1 (clone RMP1-14) versus control siRNA. Mice were treated for 14 d starting on D7 after inoculation of Hepa1-6 cells and tumor volume was measured daily. Results: Murine APN401 treatment alone significantly suppressed Hepa1-6 tumor growth by 56% (p<0.0001) versus control siRNA-treated mice. Combination therapy of APN401 with anti-PD1 induced profound synergistic anti-tumor efficacy with tumor growth inhibition of 76% (p<0.0001) versus anti-PD1 alone with control siRNA. APN401 was well tolerated and APN401-treated mice were unremarkable with optimal body conditions. Conclusion: APN401 monotherapy demonstrates striking anti-tumor efficacy in suppressing tumor growth in a murine hepatocellular carcinoma Hepa1-6 model. The preclinical synergistic effects of APN401 with anti-PD1 support its therapeutic utility as a combination therapy with immune checkpoint anti-PD1 treatment. These findings highlight the potential promise of selective Cbl-b silencing by APN401 as a novel immunotherapy for cancer. Citation Format: Anderson Gaweco, Kathrin Thell, Maria Urban, Julia Harrauer, Isabella Haslinger, Alexander Dohnal, Guenther Lametschwandtner, Karin Drobits, Romana Gugenberger, Hannes Muehleisen, Josef Penninger, Carlos Becerra, Vincent Chung, Anthony El-Khoueiry. Novel master checkpoint Cbl-b siRNA-based adoptive cellular therapy: Superior antitumor efficacy in a syngeneic murine hepatocellular carcinoma Hepa1-6 model following APN401 monotherapy and synergistic effects with anti-PD1 [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference on Molecular Targets and Cancer Therapeutics; 2019 Oct 26-30; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2019;18(12 Suppl):Abstract nr C048. doi:10.1158/1535-7163.TARG-19-C048

1231P - Master checkpoint Cbl-b inhibition: Anti-tumour efficacy in a murine colorectal cancer model following siRNA-based cell therapy

BackgroundThe E3 ubiquitin ligase Cbl-b is a key master immune checkpoint regulator that limits immune activation critical for anti-tumor immunity. We first reported that Cbl-b deficiency in mice confers spontaneous in vivo rejection of tumor cells. This checkpoint role in anti-tumor immunity was confirmed in multiple mouse and human studies. Further discovery efforts led to the advancement of APN401, an ex vivo human Cbl-b siRNA-based autologous cellular therapy, currently in clinical development in patients with advanced solid tumors. APN401 has been target validated and its preclinical efficacy was established in various mouse syngeneic tumor models. We present here the potent anti-tumor efficacy of APN401 immunotherapy of Cbl-b silenced murine T cells in a syngeneic MC38 colorectal tumor model. MethodsT cells isolated from MC38 tumor-bearing donor C57Bl/6 mice were silenced ex vivo with APN401 murine Cbl-b specific siRNA vs control siRNA, adoptively transferred into MC38 tumor-bearing recipient mice, and tumor growth was monitored using an in vivo imaging system. ResultsMurine APN401 treatment resulted in a significant MC38 tumor growth inhibition of 63% (p=0.008) vs controls after just a single dose application. Profound anti-tumor efficacy induced by Cbl-b-silenced T cells strongly correlated with enhanced production of TH1 cytokines IL-2 and IFN-γ. Furthermore, in vivo tracking of fluorescently labeled and silenced T cells revealed migration to relevant lymphoid organs and local tumor sites. Murine APN401 treatment was safe and well-tolerated similar to recent human APN401 clinical phase Ia study results in patients with advanced solid tumors. ConclusionsIn a model of murine colon cancer, Cbl-b-silencing induced vigorous in vivo anti-tumorigenic immune responses. The treatment modality was safe and well tolerated. These data provide direct preclinical proof of concept that siRNA-based silencing of Cbl-b provides a novel, effective and tunable approach as cellular immunotherapy. Targeting Cbl-b through human APN401 cell therapy is a promising novel therapeutic approach for the treatment of solid cancers, also highlighted by our fast-tracked global APN401 clinical development. Legal entity responsible for the studyThe authors. FundingHas not received any funding. DisclosureAll authors have declared no conflicts of interest.

Abstract 2696: Genetic and pharmacologic evaluation of the ubiquitin ligase CBL-B as a small-molecule, tumor immunotherapy target

Abstract E3 ubiquitin ligases play critical roles in directing cellular protein fate by controlling the specificity of ubiquitin conjugation to substrate proteins and targeting them for cellular relocalization or degradation by the ubiquitin proteasome system. The E3 ubiquitin ligase CBL-B is expressed in immune cell lineages and negatively regulates activity of the T-cell receptor (TCR) by imposing a requirement for a costimulatory signal to mount a productive immune response upon TCR engagement. Mice deficient in Cbl-b, and more specifically in the RING Zn-finger ligase domain of Cbl-b, demonstrate a tumor rejection phenotype mediated by CD8+ T cells (Paolino et al., JI, 2011). We have reproduced these results and demonstrate that Cbl-b deficient mice show enhanced anti-tumor activity. In addition, we show that CD4+ and CD8+ T cells from mice deficient in Cbl-b have 5 to 10-fold enhanced secretion of IL-2 and IFN γ when stimulated ex vivo. These data provide a genetic rationale for the development of a small molecule inhibitor of CBL-B ligase activity for use in patients with tumor-mediated immune suppression of effector T cells. We have identified a series of small molecule inhibitors of CBL-B activity with biochemical potency at low nanomolar concentrations. CBL-B inhibitors increased cytokine secretion in vitro at low nanomolar concentrations, as measured by IL-2 and IFN γ secretion, in primary human and mouse T cells stimulated with CD3/CD28 or CD3 alone. The compounds also stimulated proliferation and elevated levels of the T cell surface activation markers CD25 and CD69. CBL-B inhibitors enhanced an antigen recall response in human PBMCs ex vivo, as measured by approximately 5-fold higher secretion of GM-CSF, TNF-α and RANTES, and demonstrated effects in an ex vivo model of exhausted T cell function. Oral dosing of an optimized CBL-B inhibitor enhanced anti-CD3 stimulated T cell activation in mouse CD4+ and CD8+ T cells, demonstrating a dose proportional pharmacodynamic effect. Oral administration over 28 days in the syngeneic CT-26 tumor model was well tolerated and resulted in single agent tumor growth inhibition. These data support the continued advancement of small molecule oral CBL-B inhibitors for future development in immuno-oncology. Citation Format: Jennifa Gosling, Ryan Rountree, Asad Taherbhoy, Chenbo Wang, Thomas Cummins, Frederick Cohen, Hiroko Tanaka, Dahlia Weiss, Mario Cardozo, Christopher Karim, May Tan, Joseph Juan, Austin Tenn-McClellan, Szerenke Kiss von Soly, Julie Sheung, Kathleen Boyle, Ketki Dhamnaskar, Katherine Kurylo, Jilliane Bruffey, Jennifer McKinnell, Dane Karr, Andria Christianson, Anne-Renee Van Der Vuurst de Vries, Pallavur Sivakumar, Mark Gallop, Paul A. Barsanti, Anjanabha Saha, Neil F. Bence, Christoph W. Zapf. Genetic and pharmacologic evaluation of the ubiquitin ligase CBL-B as a small-molecule, tumor immunotherapy target [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 2696.

RAS P21 Protein Activator 3 (RASA3) specifically promotes pathogenic T helper 17 cell generation by repressing T helper 2-biased programs

Abstract Th17 cells that produce the signature cytokine interleukin 17 (IL-17) can be both pathogenic and non-pathogenic. Pathogenic-Th17 (pTh17) cells drive inflammation and immune-pathology, but whether pTh17 cells are a Th17-subset whose generation is under specific molecular control remains unaddressed. We found that Ras p21 protein activator 3 (RASA3), a GTPase activating protein, was highly expressed by IL-1β/IL-6/IL-23 induced-pTh17 cells relative to TGFβ1/IL-6 induced-non-pTh17 cells and was required specifically for pTh17 generation in vitro and in vivo. Mice conditionally deficient for Rasa3 in T cells showed less pathology during experimental autoimmune encephalomyelitis. By RNA-seq analysis, Rasa3-deficient T cells were found to acquire a Th2-biased program that dominantly trans-suppressed pTh17 cell generation via interleukin 4 production. The Th2-bias of Rasa3-deficient T cells was due to aberrantly elevated transcription factor IRF4 expression. Furthermore, RASA3 promoted proteasome-mediated IRF4 protein degradation by facilitating interaction of IRF4 with E3-ubiquitin ligase Cbl-b. Therefore, a RASA3-IRF4-Cbl-b pathway specifically directs pTh17 cell generation by balancing reciprocal Th17-Th2 programs. These findings indicate that a distinct molecular program directs pTh17 generation and reveals targets for treating pTh17-related pathology and diseases.