Low-dose targeted radionuclide therapy synergizes with CAR T cells and enhances tumor response

Ionizing radiation has garnered considerable attention as a combination partner for immunotherapy due to its potential immunostimulatory effects. In contrast to the more commonly used external beam radiation, we explored the feasibility of combining chimeric antigen receptor (CAR) T cell therapy with targeted radionuclide therapy (TRT), which is achieved by delivering β-emitting 177Lu-DOTATATE to tumor via tumor-infiltrating CAR T cells that express somatostatin receptor 2 (SSTR2). We hypothesized that the delivery of radiation to tumors could synergize with CAR T therapy, resulting in enhanced antitumor immunity and tumor response. To determine the optimal dosage and timing of 177Lu-DOTATATE treatment, we measured CAR T cell infiltration and expansion in tumors longitudinally through positron emission tomography (PET) using a SSTR2-specific positron-emitting radiotracer,18F-NOTA-Octreotide. In animals receiving CAR T cells and a low-dose (2.5 Gy) of TRT following the administration of 177Lu-DOTATATE, we observed a rapid regression of large subcutaneous tumors, which coincided with a dramatic increase in serum proinflammatory cytokines. Tumor burden was also reduced when a higher radiation dose (6 Gy) was delivered to the tumor. However, this higher dose led to cell death in both the tumor and CAR T cells. Our study suggests that there may exist an optimum range of TRT dosage that can enhance T cell activity and sensitize tumor cells to T cell killing, which may result in more durable tumor control compared to a higher radiation dose.

Chimeric Antigen Receptor T Cell Therapy Targeting Epithelial Cell Adhesion Molecule in Gastric Cancer: Mechanisms of Tumor Resistance

Epithelial cell adhesion molecule (EpCAM) is a tumor-associated antigen that is frequently overexpressed in various carcinomas. We have developed chimeric antigen receptor (CAR) T cells specifically targeting EpCAM for the treatment of gastric cancer. This study sought to unravel the precise mechanisms by which tumors evade immune surveillance and develop resistance to CAR T cell therapy. Through a combination of whole-body CAR T cell imaging and single-cell multiomic analyses, we uncovered intricate interactions between tumors and tumor-infiltrating lymphocytes (TILs). In a gastric cancer model, tumor-infiltrating CD8 T cells exhibited both cytotoxic and exhausted phenotypes, while CD4 T cells were mainly regulatory T cells. A T cell receptor (TCR) clonal analysis provided evidence of CAR T cell proliferation and clonal expansion within resistant tumors, which was substantiated by whole-body CAR T cell imaging. Furthermore, single-cell transcriptomics showed that tumor cells in mice with refractory or relapsing outcomes were enriched for genes involved in major histocompatibility complex (MHC) and antigen presentation pathways, interferon-γ and interferon-α responses, mitochondrial activities, and a set of genes (e.g., CD74, IDO1, IFI27) linked to tumor progression and unfavorable disease prognoses. This research highlights an approach that combines imaging and multiomic methodologies to concurrently characterize the evolution of tumors and the differentiation of CAR T cells.

Activation of the JAK/STAT Pathway Leads to BRAF Inhibitor Resistance in BRAFV600E Positive Thyroid Carcinoma

A subset of thyroid cancers, recurrent differentiated thyroid cancers and anaplastic thyroid cancer (ATC), are difficult to treat by thyroidectomy and systemic therapy. A common mutation in thyroid cancer, BRAFV600E, has targetable treatment options; however, the results have been disappointing in thyroid cancers compared with BRAFV600E melanoma, as thyroid cancers quickly become resistant to BRAFV600E inhibitor (BRAFi). Here, we studied the molecular pathway that is induced in BRAFV600E thyroid cancer cells and patient-derived tumor samples in response to BRAFi, vemurafenib, using RNA-sequencing and molecular analysis. Both inducible response to BRAFi and acquired BRAFi resistance in BRAFV600E thyroid cancer cells showed significant activation of the JAK/STAT pathway. Functional analyses revealed that the combination of BRAFi and inhibitors of JAK/STAT pathway controlled BRAFV600E thyroid cancer cell growth. The Cancer Genome Atlas data analysis demonstrated that potent activation of the JAK/STAT signaling was associated with shorter recurrence rate in patients with differentiated thyroid cancer. Analysis of tumor RNA expression in patients with poorly differentiated thyroid cancer and ATC also support that enhanced activity of JAK/STAT signaling pathway is correlated with worse prognosis. Our study demonstrates that JAK/STAT pathway is activated as BRAFV600E thyroid cancer cells develop resistance to BRAFi and that this pathway is a potential target for anticancer activity and to overcome drug resistance that commonly develops to treatment with BRAFi in thyroid cancer.

IMPLICATIONS

Dual inhibition of BRAF and JAK/STAT signaling pathway is a potential therapeutic treatment for anticancer activity and to overcome drug resistance to BRAFi in thyroid cancer.

Inducible expression of interleukin-12 augments the efficacy of affinity-tuned chimeric antigen receptors in murine solid tumor models

The limited number of targetable tumor-specific antigens and the immunosuppressive nature of the microenvironment within solid malignancies represent major barriers to the success of chimeric antigen receptor (CAR)-T cell therapies. Here, using epithelial cell adhesion molecule (EpCAM) as a model antigen, we used alanine scanning of the complementarity-determining region to fine-tune CAR affinity. This allowed us to identify CARs that could spare primary epithelial cells while still effectively targeting EpCAM^high tumors. Although affinity-tuned CARs showed suboptimal antitumor activity in vivo, we found that inducible secretion of interleukin-12 (IL-12), under the control of the NFAT promoter, can restore CAR activity to levels close to that of the parental CAR. This strategy was further validated with another affinity-tuned CAR specific for intercellular adhesion molecule-1 (ICAM-1). Only in affinity-tuned CAR-T cells was NFAT activity stringently controlled and restricted to tumors expressing the antigen of interest at high levels. Our study demonstrates the feasibility of specifically gearing CAR-T cells towards recognition of solid tumors by combining inducible IL-12 expression and affinity-tuned CAR.

Abstract 5084: Targeted delivery of low-dose radiation alleviates tumor resistance to CAR-T cell therapy

Purpose: Current understanding of resistance to CAR-T cell therapy in solid tumors implicates inadequate CAR-T cell potency in the immunosuppressive tumor microenvironment (TME). We have previously developed a platform using somatostatin receptor 2 (SSTR2) as a positron emission tomography (PET) reporter to detect CAR-T cell expansion and trafficking. The current study aimed to leverage SSTR2 for low-dose targeted radionuclide therapy (177Lu-DOTATATE, Lutathera), which under dosimetry guidance might enhance antitumor immunity by reprograming the TME and promoting T-cell reinvigoration.

Methods: Using intercellular adhesion molecule 1 (ICAM-1) as a model antigen, we evaluated the immunomodulatory effects of low-dose radiation in a gastric cancer animal model. NSG mice were inoculated subcutaneously with firefly luciferase-expressing Hs 746T cells (0.1 × 106 per mouse) and treated 5 days later with 10 × 106 SSTR2-expressing ICAM-1 CAR-T cells. CAR-T cell expansion was monitored weekly by PET scan using 18F-NOTA-Octreotide, a radiotracer targeting SSTR2. Three weeks post-T cell infusion, a cohort of mice were injected with 7.4 MBq of 177Lu-DOTATATE via the tail vein. SPECT imaging was performed for dosimetry analysis. Tumor growth was monitored by bioluminescence imaging and tumor size measurement. Serum cytokines were analyzed.

Results: Single dose 177Lu-DOTATATE treatment (delivering 1-6 Gy to tumor) improved response of established gastric cancer tumor (>1,000 mm3) that was not responsive to CAR-T cell treatment alone and significantly prolonged survival. All mice receiving CAR-T cells plus 177Lu-DOTATATE displayed rapid tumor shrinkage, with 83% of mice achieving complete remission within 3 weeks of 177Lu-DOTATATE treatment. SPECT imaging confirmed specific delivery of 177Lu-DOTATATE by tumor-infiltrating CAR-T cells, with tumor uptake of 0.43 ± 0.24 and 0.19 ± 0.08 MBq/g at 24 and 144 hours post 177Lu-DOTATATE injection, respectively. Most radioactivity reduction over time (73%) is explained by physical decay of 177Lu, indicating persistent tumor retention of 177Lu-DOTATATE. In contrast, rapid clearance of 177Lu-DOTATATE was observed in liver and kidneys. Importantly, longitudinal CAR-T cell imaging using 18F-NOTA-Octreotide revealed increased CAR-T cell expansion induced by low-dose radiation. Furthermore, we detected high levels of IFN-γ and perforin in serum after 177Lu-DOTATATE treatment, which were >10 folds higher than that in control mice receiving CAR-T cell only, indicating activation of T cells by low-dose radiation.

Conclusions: We developed a translatable radioimmunotherapy platform that incorporates a FDA-approved theranostic endoradiotherapy (Lutathera) to improve tumor response to CAR-T cell therapy. The next steps would be to explore the immunomodulatory effects of low-dose radiation systematically and elucidate the mechanism of action.

Citation Format: Yanping Yang, Yago Alcaina, Yogindra Vedvyas, Maria Cristina Riascos, Edward K. Fung, Brett Vaughn, Sarah M. Cheal, Irene M. Min, Claire Vanpouille-Box, Moonsoo M. Jin. Targeted delivery of low-dose radiation alleviates tumor resistance to CAR-T cell therapy. [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 5084.

Metastatic pancreatic neuroendocrine tumors feature elevated T cell infiltration

Pancreatic neuroendocrine tumors (PNETs) are malignancies arising from the islets of Langerhans. Therapeutic options are limited for the over 50% of patients who present with metastatic disease. We aimed to identify mechanisms to remodel the PNET tumor microenvironment (TME) to ultimately enhance susceptibility to immunotherapy. The TMEs of localized and metastatic PNETs were investigated using an approach that combines RNA-Seq, cancer and T cell profiling, and pharmacologic perturbations. RNA-Seq analysis indicated that the primary tumors of metastatic PNETs showed significant activation of inflammatory and immune-related pathways. We determined that metastatic PNETs featured increased numbers of tumor-infiltrating T cells compared with localized tumors. T cells isolated from both localized and metastatic PNETs showed evidence of recruitment and antigen-dependent activation, suggestive of an immune-permissive microenvironment. A computational analysis suggested that vorinostat, a histone deacetylase inhibitor, may perturb the transcriptomic signature of metastatic PNETs. Treatment of PNET cell lines with vorinostat increased chemokine CCR5 expression by NF-κB activation. Vorinostat treatment of patient-derived metastatic PNET tissues augmented recruitment of autologous T cells, and this augmentation was substantiated in a mouse model of PNET. Pharmacologic induction of chemokine expression may represent a promising approach for enhancing the immunogenicity of metastatic PNET TMEs.

Abstract 5568: Revitalization of affinity-tuned CAR T cells via antigen-dependent release of interleukin-12

Introduction: The application of chimeric antigen receptor (CAR) T cells in solid tumors has met many challenges, arising from the paucity of effective yet safe targets and T cell dysfunction within the immunosuppressive milieu. Clinical trials over the years have reported severe on-target off-tumor toxicities associated with CAR T cell therapies targeting tumor-associated antigens (TAAs). Here, we propose a strategy to target TAAs effectively and safely: 1) improve tumor-targeting specificity by affinity-tuning; 2) armor T cells with antigen-dependent expression of interleukin-12 (IL-12) to compensate loss of CAR T cell activity caused by affinity-tuning. In this system, IL-12 expression is tightly regulated by the nuclear factor of the activated T cells promoter (NFAT) to limit IL-12 to the environ of activated affinity-tuned CAR T cells within the tumor. We demonstrate here the feasibility of this strategy using epithelial cell adhesion molecule (EpCAM) as a model TAA.

Methods: Affinity variants were generated by alanine scanning mutagenesis in the complementary-determining region 3 (CDR3) of an EpCAM antibody (UBS54). The affinity of CAR variants was determined by flow cytometry-based saturation binding assay. On-target off-tumor cytotoxicity of CAR T cells was examined against human primary normal epithelial cells. In vivo anti-tumor efficacy was evaluated in mouse models of gastric cancer with cell line-derived (SNU-638 and MKN-45) and patient-derived xenografts.

Results: Substitution of alanine into the UBS54 CDR3 led to identification of a tyrosine (6th residue in heavy chain CDR3) as a hot spot for affinity tuning. Among several more subtle amino acid substitutions for Tyr-6, we found that CAR molecule with valine substituent (Y6V) possesses 10 µM affinity toward EpCAM, rendering CAR T cells to be selective to EpCAM-high tumors while being not reactive to primary normal epithelial cells in vitro. In gastric cancer mouse models, compared to UBS54 CAR T cells that mediated rapid tumor remission, Y6V CAR T cells produced mainly partial responses. To revitalize affinity-tuned CAR T cells, we further engineered CAR T cells to release IL-12 under NFAT. We found that both the activity of the NFAT promoter and the level of IL-12 release were tightly regulated by the antigen density of targets when inducible IL-12 is combined with 10 µM affinity Y6V CAR; however, such dependence is lost when 1 µM affinity UBS54 was examined. In mouse models of gastric cancer, inducible lL-12 armored Y6V CAR T cells produced enhanced anti-tumor responses without elevating systemic exposure to IL-12, evidenced by low levels of IL-12 in mouse sera (below 100 pg/ml).

Conclusions: Combination of affinity-tuned CAR with inducible expression of IL-12 is a promising strategy to develop CAR T cell therapy against TAAs with the potential for reduced on-target off-tumor toxicity and tumor resistance.

Citation Format: Yanping Yang, Huan Yang, Yago Alcaina, Jaclyn E. McCloskey, Janusz Puc, Alyssa Birt, Yogindra Vedvyas, Juan Gonzalez-Valdivieso, Irene M. Min, Eric von Hofe, Moonsoo M. Jin. Revitalization of affinity-tuned CAR T cells via antigen-dependent release of interleukin-12 [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 5568.

Abstract 5579: First-in-human study of ICAM-1-specific affinity tuned CAR T cells against advanced thyroid cancer

Background: The use of chimeric antigen receptor (CAR) T cells for solid tumors has a number of challenges, such as lack of tumor-specific targets, CAR T cell exhaustion, and the immunosuppressive tumor microenvironment. To address these challenges, AffyImmune has developed technologies to affinity tune and track CAR T cells in patients. The targeting moiety is affinity tuned to preferentially bind to tumor cells overexpressing the target while leaving normal cells with low basal levels untouched, thereby increasing the therapeutic window and allowing for more physiological T cell killing. The CAR T cells are engineered to co-express somatostatin receptor 2 (SSTR2), which allows for the tracking of CAR T cells in vivo via PET/CT scan using FDA-approved DOTATATE.

Methods: AIC100 was generated by affinity tuning the I-domain of LFA-1, the physiological ligand to ICAM-1. Various mutants with 106-fold difference in affinity were evaluated for structure activity relationships using targets with varying antigen densities. The AIC100 with micromolar affinity was clearly the most effective in non-clinical animal models. AIC100 is currently being evaluated to assess safety, CAR T expansion, tumor localization, and preliminary activity in patients with advanced thyroid cancer (ATC) in a phase I study (NCT04420754). Our study uses a modified toxicity probability interval design with three dosage groups of 10 × 106, 100 × 106, and 500 × 106 cells.

Results: Preclinical studies demonstrated greater in vivo anti-tumor activity and safety with micromolar affinity CAR T cells. A single dose of AIC100 resulted in tumor elimination and significantly improved survival of animals bearing ATC xenografts. AIC100 activity was confirmed in other high ICAM-1 tumor models including breast cancer, gastric cancer, and multiple myeloma. In a Phase I patient given 10 × 106 CAR T cells, near synchronous imaging of FDG and DOTATATE revealed preliminary evidence of transient CAR T expansion and tumor reduction at multiple tumor lesions, with the peak of CAR T cell density coinciding with the spike in CAR T cell numbers in blood.

Conclusion: We have developed affinity tuned CAR T cells designed to selectively target ICAM-1 overexpressing tumor cells and to spatiotemporally image CAR T cells. Near-synchronous FDG and DOTATATE scans will enhance patient safety by early detection of off-tumor CAR T activity and validation of tumor response. We anticipate that our “tune and track” technology will be widely applicable to developing potent yet safe CAR T cells against hard-to-treat solid cancers.

Citation Format: Jingmei Hsu, Yen-Michael Hsu, Koen van Besien, Thomas J. Fahey, Jana Ivanidze, Janusz Puc, Karrie Du, Yanping Yang, Yogindra Vedvyas, Irene M. Min, Eric von Hofe, Moonsoo M. Jin. First-in-human study of ICAM-1-specific affinity tuned CAR T cells against advanced thyroid cancer [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 5579.

Abstract 5270: Developing a predictive model for pancreatic neuroendocrine tumor metastatic potential: A multi-institutional analysis

Pancreatic neuroendocrine tumors (PNETs) are rare neoplasms that arise from cells in the islets of Langerhans, with surgical resection presently recommended for tumors > 2cm. While many PNETs have the propensity to be indolent, some small tumors display aggressive features with early metastatic potential. We used machine learning to develop a predictive model of metastatic potential dependent upon the transcriptomic signature of primary PNET tissue. To build this model, RNA sequencing data was obtained from the primary tissue of 96 surgically-resected PNETs from various institutions. Two cohorts were generated with equally balanced metastatic PNET composition (15 (32.6%) vs. 13 (26.5%), p=0.52). A differential gene expression analysis identified 20 concordantly differentially expressed genes associated with metastatic status between the two cohorts. Unsupervised surrogate variable analysis estimated and adjusted for significant sources of variation not related to metastatic potential and mitigated unwanted noise and batch effects. A gene set enrichment analysis identified an additional 29 genes that most frequently contributed to the enriched biologic pathways extrapolated from the sequencing data. Log transformed, batch corrected TPM values for these 49 genes were combined with an additional 10 clinically-relevant genes, including ARX and PDX1, that are known to contribute to PNET signatures or oncogenesis. The datasets were subsequently randomized in a 1:1 ratio and informative features with respect to metastatic status were identified utilizing a Boruta algorithm, with a priori exclusion of highly-correlative genes and those that displayed near zero variance. Nine genes, including AURKA, ARX, CDCA8, CPB2, MYT1L, NDC80, PAPPA2, SFMBT1 and ZPLD1, were identified as sufficient to classify the localized or metastatic outcome. Distributed random forests (DRF), generalized linear models (GLM), gradient boosting machines (GBM) and extreme gradient boosting (XGBoost) models were trained utilizing these 9 genes. Training ROC ranged from 0.92 for DRF to 1 for XGboost. When applied to 47 independent validation samples, the testing sensitivity ranged from 75% for DRF to 94% for GBM; specificity ranged from 84% for DRF to 94% to XGboost and GLM; positive predictive value ranged from 72% for DRF to 86% for GLM; negative predictive value ranged from 88% for GLM to 97% to GBM. The degree of predictive agreement between models ranged from 64% to 91%. Taken together, we have developed a highly sensitive predictive model of the metastatic PNET phenotype that is based on expression of nine genes. Its application as a guide for management should be studied prospectively in patients with newly diagnosed PNETs.

Citation Format: Jacques A. Greenberg, Nikolay A. Ivanov, Yajas Shah, Scott Kulm, Jelani Williams, Catherine G. Tran, Theresa Scognamiglio, Yeon Joo Lee, Caitlin E. Egan, Irene M. Min, Rasa Zarnegar, James Howe, Xavier Keutgen, Thomas J. Fahey, Olivier Elemento, Brendan M. Finnerty. Developing a predictive model for pancreatic neuroendocrine tumor metastatic potential: A multi-institutional analysis [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 5270.

CSPG4 Is a Potential Therapeutic Target in Anaplastic Thyroid Cancer

Background: Anaplastic thyroid cancer (ATC) is a rare cancer with poor prognosis and few treatment options. The objective of this study was to investigate new immune-associated therapeutic targets by identifying ATC-derived, human leukocyte antigen (HLA) class II-presenting peptides. One protein that generated multiple peptides in ATC was chondroitin sulfate-proteoglycan-4 (CSPG4), a transmembrane proteoglycan with increased expression in multiple aggressive cancers, but not yet investigated in ATC. Methods: We applied autologous peripheral blood T cells to ATC patient-derived xenografted mice to examine whether ATC induces a tumor-specific T cell response. We then identified peptide antigens eluted from the HLA-DQ complex in ATC patient-derived cells using mass spectrometry, detecting abundant CSPG4-derived peptides specific to the ATC sample. Next, we analyzed the surface expression level of CSPG4 in thyroid cancer cell lines and primary cell culture using flow cytometry. In addition, we used immunohistochemistry to compare the expression level and localization of the CSPG4 protein in ATC, papillary thyroid cancer, and normal thyroid tissue. We then investigated the correlation between CSPG4 expression and clinicopathological features of patients with thyroid cancer. Results: We found that ATC tissue had a high level of HLA-DQ expression and that the patient's CD4⁺ T cells showed activation when exposed to ATC. By eluting the HLA-DQ complex of ATC tissue, we found that CSPG4 generated one of the most abundant and specific peptides. CSPG4 expression at the cell surface of thyroid cancer was also significantly high when determined by flow cytometry, with the majority of ATC cell lines exhibiting ∼10-fold higher mean fluorescence intensity. Furthermore, most ATC patient cases expressed CSPG4 in the cytoplasm or membrane of the tumor cells. CSPG4 expression was correlated with tumor size, extrathyroidal extension, and intercellular adhesion molecule-1 (ICAM-1) circumferential expression. CSPG4 mRNA overexpression was associated with worse overall survival in patients with ATC and poorly differentiated thyroid cancer. Conclusions: CSPG4 expression is significantly elevated in aggressive thyroid cancers, with a strong correlation with a poor prognosis. The vast number of HLA-DQ eluted CSPG4 peptides was identified in ATC, demonstrating the potential of CSPG4 as a novel immunotherapeutic target for ATC.

Bispecific CAR T Cells against EpCAM and Inducible ICAM-1 Overcome Antigen Heterogeneity and Generate Superior Antitumor Responses

Adoptive transfer of chimeric antigen receptor (CAR) T cells has demonstrated unparalleled responses in hematologic cancers, yet antigen escape and tumor relapse occur frequently. CAR T-cell therapy for patients with solid tumors faces even greater challenges due to the immunosuppressive tumor environment and antigen heterogeneity. Here, we developed a bispecific CAR to simultaneously target epithelial cell adhesion molecule (EpCAM) and intercellular adhesion molecule 1 (ICAM-1) to overcome antigen escape and to improve the durability of tumor responses. ICAM-1 is an adhesion molecule inducible by inflammatory cytokines and elevated in many types of tumors. Our study demonstrates superior efficacy of bispecific CAR T cells compared with CAR T cells targeting a single primary antigen. Bispecific CAR T achieved more durable antitumor responses in tumor models with either homogenous or heterogenous expression of EpCAM. We also showed that the activation of CAR T cells against EpCAM in tumors led to upregulation of ICAM-1, which rendered tumors more susceptible to ICAM-1 targeting by bispecific CAR T cells. Our strategy of additional targeting of ICAM-1 may have broad applications in augmenting the activity of CAR T cells against primary tumor antigens that are prone to antigen loss or downregulation.

Chimeric Antigen Receptor T Cell Therapy Targeting ICAM-1 in Gastric Cancer

Cancer therapy utilizing adoptive transfer of chimeric antigen receptor (CAR) T cells has demonstrated remarkable clinical outcomes in hematologic malignancies. However, CAR T cell application to solid tumors has had limited success, partly due to the lack of tumor-specific antigens and an immune-suppressive tumor microenvironment. From the tumor tissues of gastric cancer patients, we found that intercellular adhesion molecule 1 (ICAM-1) expression is significantly associated with advanced stage and shorter survival. In this study, we report a proof-of-concept study using ICAM-1-targeting CAR T cells against gastric cancer. The efficacy of ICAM-1 CAR T cells showed a significant correlation with the level of ICAM-1 expression in target cells in vitro. In animal models of human gastric cancer, ICAM-1-targeting CAR T cells potently eliminated tumors that developed in the lungs, while their efficacy was more limited against the tumors in the peritoneum. To augment CAR T cell activity against intraperitoneal tumors, combinations with paclitaxel or CAR activation-dependent interleukin (IL)-12 release were explored and found to significantly increase anti-tumor activity and survival benefit. Collectively, ICAM-1-targeting CAR T cells alone or in combination with chemotherapy represent a promising strategy to treat patients with ICAM-1+ advanced gastric cancer.

PD1 Blockade Enhances ICAM1-Directed CAR T Therapeutic Efficacy in Advanced Thyroid Cancer

PURPOSE

Advanced thyroid cancers, including poorly differentiated and anaplastic thyroid cancer (ATC), are lethal malignancies with limited treatment options. The majority of patients with ATC have responded poorly to programmed death 1 (PD1) blockade in early clinical trials. There is a need to explore new treatment options.

EXPERIMENTAL DESIGN

We examined the expression of PD-L1 (a ligand of PD1) and intercellular adhesion molecule 1 (ICAM1) in thyroid tumors and ATC cell lines, and investigated the PD1 expression level in peripheral T cells of patients with thyroid cancer. Next, we studied the tumor-targeting efficacy and T-cell dynamics of monotherapy and combination treatments of ICAM1-targeting chimeric antigen receptor (CAR) T cells and anti-PD1 antibody in a xenograft model of ATC.

RESULTS

Advanced thyroid cancers were associated with increased expression of both ICAM1 and PD-L1 in tumors, and elevated PD1 expression in CD8⁺ T cells of circulating blood. The expression of ICAM1 and PD-L1 in ATC lines was regulated by the IFNγ-JAK2 signaling pathway. ICAM1-targeted CAR T cells, produced from either healthy donor or patient T cells, in combination with PD1 blockade demonstrated an improved ability to eradicate ICAM1-expressing target tumor cells compared with CAR T treatment alone. PD1 blockade facilitated clearance of PD-L1 high tumor colonies and curtailed excessive CAR T expansion, resulting in rapid tumor clearance and prolonged survival in a mouse model.

CONCLUSIONS

Targeting two IFNγ-inducible, tumor-associated antigens-ICAM1 and PD-L1-in a complementary manner might be an effective treatment strategy to control advanced thyroid cancers in vivo.

Abstract 6597: Highly localized, inducible interleukin-12 release augments ICAM-1 CAR T cell activity against solid tumors

Background: Chimeric antigen receptor (CAR) T cells have demonstrated continued success in the treatment of hematological malignancies. By contrast, limited efficacy of CAR T cells has been seen in solid tumors due to multiple obstacles including impaired T cell infiltration and immune suppressive tumor environment, resulting in lack of proliferation and function. Previously, we have developed a micromolar affinity tuned CAR T cells targeting overexpressed ICAM-1 against a variety of tumors. To combat the immunosuppressive microenvironment in solid tumors, we developed a novel lentivirus vector that incorporates expression cassettes for CAR, somatostatin receptor 2 (SSTR2) for PET/CT imaging of T cells, and CAR-activation dependent release of IL-12. Inducible IL-12 (iIL-12) expression was enabled by a synthetic promoter containing both NFkB and NFAT promoter elements by T cells to leverage IL-12's ability to promote Th1 response and at the same time to minimize systemic toxicity of IL-12. The iIL-12 CAR T exhibited much more robust killing of subcutaneous and peritoneal tumors, which showed partial to limited response to conventional CAR T cells without inducible cytokines.

Methods: CAR T cells were generated by double lentiviral transduction of primary human T cells at 24 and 48 hours after activation with anti-CD3/CD28 Dynabeads. NSG mice inoculated with subcutaneous anaplastic thyroid cancer cells (8505c) and triple negative breast cancer cells (MDA-MB-468), and intraperitoneal gastric cancer cells (MKN28) were treated with with or without iIL-12 CAR T cells. Tumor growth was monitored regularly by bioluminescence imaging. PET/CT imaging was implemented to monitor T cell localization and biodistribution using 18F-NOTA-octreotide, a radiotracer targeting SSTR2.

Results: Jurkat cells expressing iIL-12 ICAM-1 CAR produced IL-12 in an antigen density dependent manner when co-incubated with ICAM-1 positive tumor cells or HEK 293T cells transduced to express human ICAM-1. In in vitro cytotoxicity assay, iIL-12 CAR T cells exhibited significantly augmented tumor-lytic activity than conventional ICAM-1 CAR T cells. In striking contrast to conventional ICAM-1 CAR T cells which did not control tumor growth, iIL-12 CAR T cells mediated complete regression of various solid tumors and prolonged mouse survival significantly. Based on PET/CT imaging, both conventional and iIL-12 CAR T cells showed high levels of T cell infiltration and localization in tumor lesions. These results suggest that conventional CAR T cells quickly exhausted and lost function after penetration, while locally released IL-12 played an important role in improving anti-tumor responses. Furthermore, serum levels of IL-12 in mice treated with iIL-12 CAR T cells were not detectable, indicating that IL-12 secretion was tightly controlled and limited to tumor sites.

Conclusions: These findings demonstrate that local release of inducible IL-12 can help overcome hostile tumor microenvironment and augment anti-tumor immune responses. Studies are underway to evaluate the feasibility of inducible IL-12 armored ICAM-1 CAR T cells as a potential therapy with favorable safety and efficacy profiles.

Citation Format: Yanping Yang, Jaclyn E. McCloskey, Yogindra Vedvyas, Irene M. Min, Eric von Hofe, Moonsoo M. Jin. Highly localized, inducible interleukin-12 release augments ICAM-1 CAR T cell activity against solid tumors [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 6597.

Abstract 1895: The role of the IFNγ pathway in the development of vemurafenib resistance in BRAFV600E mutant thyroid carcinoma

BRAFV600E is the most common driver mutation in anaplastic thyroid cancer, and BRAF inhibitors are increasingly being used in the treatment of this disease. However, the therapeutic benefit of BRAFV600E inhibitors, such as vemurafenib, has been limited in thyroid cancer due to rapid development of drug resistance in clinical practice. Here, we characterize a targetable mechanism of vemurafenib resistance by studying vemurafenib-resistant clones derived from BRAFV600E mutant anaplastic thyroid cancer cell lines (8505C and FRO), and propose a drug combination to overcome resistance. Vemurafenib resistance in these established clones was verified by persistent activation of ERK1/2 after vemurafenib treatment by western blot analysis. RNA-sequencing and subsequent ingenuity pathway analysis (IPA) was used to identify differences in pathway activation after vemurafenib treatment in parental and vemurafenib-resistant cell lines. Treatment of parental cells with vemurafenib resulted in differential gene expression correlating with upstream activation of the IFNγ-STAT1-IRF1 pathway (All FDR < 0.05). Furthermore, parental cells treated with vemurafenib demonstrated higher HLA-II and HLA-A2 expression than untreated cells when analyzed by flow cytometry, implying the activation of IFNγ signaling pathway. In the IPA of a vemurafenib-resistant clone (8505C Res1), IFNγ, STAT1 and IRF1 expression was upregulated compared to their respective untreated parental cell line (All FDR < 0.05). Increased IRF1 expression in 8505C Res1 compared to parental cells was also demonstrated by qPCR, supporting innate IFNγ activation in the resistant clone. Inhibition of the IFNγ pathway with increasing concentrations of neutralizing anti-IFNγ antibody sensitized resistant thyroid cancer cells to vemurafenib. Pharmacologically blocking the IFNγ downstream pathway in resistant clones with AZD1480 (JAK2 inhibitor) or fludarabine (STAT1 inhibitor), resulted in decreased cell viability in combination treatment groups compared to vemurafenib treatment only (cell viability: 92% vemurafenib vs 19% fludarabine combination, p < 0.05) (cell viability: 73% vemurafenib vs 37% AZD1480 combination, p < 0.05). These results demonstrate that activation of the IFNγ pathway is a potential mechanism for the development of resistance of anaplastic thyroid cancer to BRAFV600E inhibitors. Our study suggests that blockade of the IFNγ pathway may potentiate the therapeutic benefit of vemurafenib treatment in this disease.

Citation Format: Jessica Limberg, Katherine D. Gray, Mandeep Singh, Timothy M. Ullmann, Brian Wyrwas, Weibin Wang, Zachary Lowenstein, Steve El Eshaky, Heng Liang, Wei Li, Tuo Zhang, Jenny Xiang, Dessislava Stefanova, Rasa Zarnegar, Thomas J. Fahey, Irene M. Min. The role of the IFNγ pathway in the development of vemurafenib resistance in BRAFV600E mutant thyroid carcinoma [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 1895.

Abstract LB-381: Mitigating on-target off-tumor cytotoxicity of EpCAM CAR-T by affinity tuning

Chimeric antigen receptor (CAR)-T cell therapy has shown robust anti-cancer responses in hematologic malignancies. However, application of this therapeutic approach to solid tumors has been hindered by multiple challenges, one of which is on-target/off-tumor cytotoxicity to normal tissues. Tumor-specific antigens exclusively present on tumor cells are rare. Most CAR-T cells are designed to target tumor associated antigens (TAAs) expressed in high levels on tumor cells. Yet, normal tissues express these antigens as well, albeit at much lower densities.To mitigate the on-target/off-tumor cytotoxicity, we developed a strategy for fine tuning the affinity of CARs to selectively target tumor cells.

Epithelial cell adhesion molecule (EpCAM) is highly expressed in epithelial cells and overexpressed in tumor cells in a variety of epithelial carcinomas. High-affinity (nM range) EpCAM-targeting CAR-T cells kill both normal human epithelial cells and EpCAM-high tumor cells in vitro. To develop CAR-T cells specific to EpCAM-high cancers, we identified low-affinity scFv variants by rational design of amino acid substitutions. The affinities of these scFvs were measured by Surface Plasmon Resonance (SPR). CAR-T cells equipped with low-affinity scFvs showed antigen-dependent activation, proliferation and Th1-like cytokine secretion when co-cultured with target cells having varied levels of EpCAM. Importantly, low-affinity CAR-T cells still lysed EpCAM-high tumor cells but spared normal human epithelial cells in vitro. Treatment of MKN28 xenograft mice with low-affinity CAR-T cells resulted in tumor regression and prolonged survival. Moreover, transcriptomic profiling of CAR-T cells revealed significant differences in gene expression levels between high- and low- affinity CARs, particularly as they relate to the functional state of these CAR-T cells, and their resistance to exhaustion.

Our results show that rational design of scFv can aid in the identification of CARs that are minimally reactive toward normal tissues while effectively eliminating tumors. Furthermore, affinity-tuned CARs demonstrate better overall fitness and antitumor activity in vivo. This affinity fine-tuning approach shows promise as a general strategy for identifying a therapeutic window for CAR-T cells targeting novel TAAs that may have been overlooked because of basal expression levels in normal tissues.

Citation Format: Huan Yang, Janusz Puc, Yanping Yang, Jaclyn E. McCloskey, Yogindra Vedvyas, Hongtao Li, Irene M. Min, Moonsoo M. Jin, Eric v. Hofe. Mitigating on-target off-tumor cytotoxicity of EpCAM CAR-T by affinity tuning [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr LB-381.

Dual inhibition of BRAF and MEK increases expression of sodium iodide symporter in patient-derived papillary thyroid cancer cells in vitro

BACKGROUND

The majority of papillary thyroid cancers are driven by acquired mutations typically in the BRAF or RAS genes that aberrantly activate the mitogen-activated protein kinase pathway. This process leads to malignant transformation, dedifferentiation, and a decrease in the expression of the sodium-iodide symporter (NIS; SLC5A5), which results in resistance to radioactive iodine therapy. We sought to determine whether inhibition of aberrant mitogen-activated protein kinase-signaling can restore NIS expression.

METHODS

We prospectively developed cultures of papillary thyroid cancers derived from operative specimens and applied drug treatments for 24 hours. Samples were genotyped to identify BRAF and RAS mutations. We performed quantitative PCR to measure NIS expression after treatment.

RESULTS

We evaluated 24 patient papillary thyroid cancer specimens; BRAF^V600E mutations were identified in 18 out of 24 (75.0%); 1 patient tumor had an HRAS mutation, and the remaining 5 were BRAF and RAS wildtype. Dual treatment with dabrafenib and trametinib increased NIS expression (mean fold change 4.01 ± 2.04, P < .001), and single treatment with dabrafenib had no effect (mean fold change 0.98 ± 0.42, P = .84). Tumor samples that had above-median NIS expression increases came from younger patients (39 vs 63 years, P < .05).

CONCLUSION

Dual treatment with BRAF and MEK inhibitors upregulated NIS expression, suggesting that this treatment regimen may increase tumor iodine uptake. The effect was greatest in tumor cells from younger patients.

Abstract 1442: Chimeric antigen receptor T cell therapy targeting ICAM-1 in gastric cancer

Introduction:

Despite advances in treatment, gastric cancer (GC) remains among the most fatal malignancies. Intercellular adhesion molecule-1 (ICAM-1) is overexpressed and associated with various cancers, including GC. We developed a third-generation chimeric antigen receptor (CAR) targeting ICAM-1 (ICAM-1-CAR) and investigated ICAM-1 as a immunotherapeutic target for CAR T therapy in GC.

Methods:

We investigated ICAM-1 expression in tissues of curatively resected GC patients (n=134) by immunohistochemical staining to determine its prognostic value. Effector to target assays were performed with ICAM-1-CAR T cells co-cultured with 8 GC cell lines with varying levels of ICAM-1 expression to investigate specific target cell death. We created a firefly luciferase-expressing human GC model in mice to measure tumor growth and killing by whole body bioluminescence imaging. To find the best treatment route and optimal dose of T cells in GC mice model, ICAM-1-CAR was injected via tail vein and intraperitoneal administration (IP) at two different doses, i.e., low dose (1 million CAR T) and high dose (10 million live T cells).

Results:

The ICAM-1 expression was higher in advanced stages (22.2% in stage II vs. 48.8% in stage III, p=0.002) and the patients with high ICAM-1 expression showed significantly poor survival (disease free survival, hazard ration [HR], 4.55, p&lt;0.001; overall survival, HR, 3.89, p&lt;0.001). The efficacy of ICAM-1-CAR T cells in vitro showed a strong correlation with the level of ICAM-1 expression in target cells, i.e., faster killing of GC with higher ICAM-1 expression. ICAM-1-CART facilitated regression of tumor in MKN-28 xenograft IP model. Additionally, the efficacy of ICAM-1-CAR was more prominent in mice treated with high dose of T cell and IP route. Compared to no treatment, ICAM-1CAR via IP led to tumor reduction that persisted for over 80 days and significantly improved survival without toxicity (p=0.049).

Conclusion:

ICAM-1 specific CAR T cells demonstrated significant therapeutic efficacy in vitro and in vivo against preclinical GC models. ICAM-1-CAR T cells may be developed into a promising treatment strategy for patients with ICAM-1 positive GC cancers.

Citation Format: Minkyu Jung, Marjan Zaman, Ygindra Vedvyas, Xianglan Zhang, Jaclyn E. McCloskey, Yanping Yang, Irene M. Min, Raza Zamegar, Yoon Young Choi, Jae-Ho Cheong, Sung Hoon Noh, Sun Young Rha, Hyun Cheol Chung, Moonsoo M. Jin. Chimeric antigen receptor T cell therapy targeting ICAM-1 in gastric cancer [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 1442.

Abstract 2329: Clinical manufacturing of CAR T cells targeting ICAM-1 for a phase I study against advanced thyroid cancer therapy

Background: Patients with anaplastic or poorly differentiated recurrent thyroid carcinomas have a poor prognosis, with a median survival of &lt; 1 year. Previously, we have shown a significant correlation between ICAM-1 overexpression and malignancy in thyroid cancer and have pioneered the use of ICAM-1 targeted CAR T cells as a novel treatment modality. For clinical translation, we designed CAR T cells possessing micromolar affinity to ICAM-1 to avoid cytotoxicity in normal cells with basal levels of ICAM-1 expression. Here, we report the automated process of CAR T cell manufacturing with CliniMACS Prodigy (Miltenyi Biotec) using cryopreserved peripheral blood leukopak cells as starting material.

Methods: We have chosen to use CliniMACS Prodigy system for a closed, automated, and robust CAR T cell manufacturing process. Using a cryopreserved leukopak, TCT v2.0 protocol began with rapidly thawing the leukopak while directly welded to the Prodigy. T cell lentivirus transduction was performed 2x, 24 and 48 hr post activation. CAR T cells were released after ~10 days of culture and cryopreserved in ready-to-infuse formulation (AIC100). Products were then subject to in vitro cytotoxicity, in vivo efficacy and safety in xenografted mice with ATC cells (8505C), and PET/CT imaging using 18F-NOTA-octreotide to detect T cell dynamics.

Results: Prodigy manufactured CAR T cells were subjected to qualification and functional assays (n=7): cell viability, cell number, CAR expression, virus copy number (VCN), T cell subset and phenotype, and E to T assays. Consistent with reported values for Prodigy, our manufacturing protocol produced 55±9% transduction rate, 96±1.8% viability, and 2.9±0.7x10^9 final cell number. The VCN was in the range of 1.0-2.1, below the criteria of &lt; 5 copies per cell. In NSG mice xenografted with 8505C ATC cells and treated with CAR T cells (1X= 1 million live CAR T maximum tolerable dose (MTD)= 10 million live CAR T) or non-transduced (NT) T cells, tumor burden was quantitatively evaluated by whole body luminescence imaging. Compared to cohorts with no treatment (NoT) or NT, the cohorts of AIC100 exhibited complete or near-complete tumor elimination lasting variable times before tumor relapse was seen in some subjects. Median survival time for NoT and NT groups were similar (38.5 vs. 42 days), while it was significantly longer for AIC100 1X and MTD groups (72 vs. &gt;100 days).

Conclusions: Selective anti-tumor activity in the absence of toxicity provides proof-of-concept that micromolar affinity tuned CAR T cells can be used to target tumors expressing high levels of antigen while avoiding normal tissues expressing basal levels of the same antigen. These studies support the initiation of a phase I study to examine the safety and potential efficacy of micromolar affinity tuned CAR T cells against newly diagnosed anaplastic and refractory or recurrent thyroid cancers.

Citation Format: Yogindra Vedvyas, Jaclyn E. McCloskey, Yanping Yang, Irene M. Min, Thomas J. Fahey, Yen-Michael S. Hsu, Jing-Mei Hsu, Koen V. Besien, Ian Gaudet, Ping Law, Joon Kim, Eric V. Hofe, Moonsoo M. Jin. Clinical manufacturing of CAR T cells targeting ICAM-1 for a phase I study against advanced thyroid cancer therapy [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 2329.

Abstract 2322: ICAM-1 targeting CAR T cell therapy for triple negative breast cancer

Background: Triple negative breast cancer (TNBC) is an aggressive disease with a poor prognosis. TNBC patients do not respond to hormone receptor or HER2-targeted therapies owing to low expression of these cell surface biomarkers. The lacking of targeted therapy leaves conventional combination of surgery, chemotherapy and radiation therapy as the standard-of-care treatment options for TNBC. But even this combination usually fails to prevent disease recurrence. Intercellular adhesion molecule-1 (ICAM-1) was recently discovered to be upregulated in TNBC and could serve as an attractive molecular target (Guo et al., PNAS 2014). Chimeric antigen receptor (CAR) T cell therapy has shown remarkable success against hematological malignancies, however there has been little success in the treatment of solid tumors. Here, we developed an ICAM-1 targeting CAR T cell-based immunotherapeutic strategy to redirect T cell to kill solid TNBC.

Methods: Patient-derived xenograft (PDX) models of TNBC were utilized to determine ICAM-1 expression by immunohistochemistry. ICAM-1 surface protein expression in TNBC cell line MDA-MB-231 was measured by flow cytometry. Primary T cells (n = 4 donors) were isolated and transduced with ICAM-1 CAR lentivirus twice at 24 and 48 hours after activation with anti-CD3/CD28 Dynabeads. In vitro killing ability was determined by co-incubation of GFP-Firefly Luciferase (GFP/Fluc) transduced target cells (HeLa, MDA-MB-231, and 293T) with transduced and non-transduced T cells. NSG mice bearing MDA-MB-231 xenografts were treated with CAR T cells to test the in vivo efficacy. Bioluminescence was used to quantify cell lysis in vitro and to track tumor growth in vivo.

Results: 4 out of 8 TNBC PDX models (JAX, tumor markers: TER−/PR−/HER2−) showed strong IHC ICAM-1 staining. We validated by flow cytometry that ICAM-1 is highly expressed on TNBC cell line MDA-MB-231. These results provide further evidence supporting ICAM-1 as a molecular target for TNBC by CAR T cell therapy. Primary T cells were transduced to express ICAM-1 targeting CAR at approximately 50%. Co-incubation of CAR T cells or non-transduced T cells with ICAM-1 positive cell lines (HeLa and MDA-MB-231) or negative control cell line (293T) showed that CAR T cell-mediated killing was ICAM-1 expression dependent. After 48 hours, 70% of HeLa and 85% of MDA-MB-231 cells were specifically lysed at effector to target ratio of 5:1, while no obvious killing of 293T cells was observed. Additionally, non-transduced T cells produced little killing with less than 20% of target cell lysis. Ongoing in vivo studies will determine the efficacy of this ICAM-1 targeting CAR T cell against TNBC.

Conclusions: We developed an ICAM-1 targeting CAR T cell with the ability to induce potent and specific killing of TNBC cells. Preclinical studies are being conducted to evaluate the feasibility of ICAM-1 specific CAR T cells as a potential therapy for patients with ICAM-1 positive TNBC.

Citation Format: Yanping Yang, Yogindra Vedvyas, Jaclyn E. McCloskey, Irene M. Min, Moonsoo M. Jin. ICAM-1 targeting CAR T cell therapy for triple negative breast cancer [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 2322.

UCHL1 loss alters the cell-cycle in metastatic pancreatic neuroendocrine tumors

Loss of ubiquitin carboxyl-terminal hydrolase L1 (UCHL1) expression by CpG promoter hypermethylation is associated with metastasis in gastroenteropancreatic neuroendocrine tumors; however, the mechanism of how UCHL1 loss contributes to metastatic potential remains unclear. In this study, we first confirmed that loss of UCHL1 expression on immunohistochemistry was significantly associated with metastatic tumors in a translational pancreatic neuroendocrine tumor (PNET) cohort, with a sensitivity and specificity of 78% and 89%, respectively. To study the mechanism driving this aggressive phenotype, BON and QGP-1 metastatic PNET cell lines, which do not produce UCHL1, were stably transfected to re-express UCHL1. In vitro assays, RNA-sequencing, and reverse-phase protein array (RPPA) analyses were performed comparing empty-vector negative controls and UCHL1-expressing cell lines. UCHL1 re-expression is associated with lower anchorage-independent colony growth in BON cells, lower colony formation in QGP cells, and a higher percentage of cells in the G0/G1 cell-cycle phase in BON and QGP cells. On RPPA proteomic analysis, there was an upregulation of cell-cycle regulatory proteins CHK2 (1.2 fold change, p=0.004) and P21 (1.2 fold change, p=0.023) in BON cells expressing UCHL1; western blot confirmed upregulation of phosphorylated CHK2 and P21. There were no transcriptomic differences detected on RNA-Sequencing between empty-vector negative controls and UCHL1-expressing cell lines. In conclusion, UCHL1 loss correlates with metastatic potential in PNETs and its re-expression induces a less aggressive phenotype in vitro, in part by inducing cell-cycle arrest through post-translational regulation of phosphorylated CHK2. UCHL1 re-expression should be considered as a functional biomarker in detecting PNETs capable of metastasis.

Expression of programmed death ligand 1 and 2 in adrenocortical cancer tissues: An exploratory study

BACKGROUND

Inhibition of the interaction of programmed death 1 with programmed death ligand 1 and 2 has been used successfully for treatment of multiple advanced cancers, but expression has not been studied in adrenocortical carcinoma. In this study, we investigated programmed death ligand 1 and 2 expression in adrenocortical carcinoma to determine the potential usefulness of checkpoint inhibitors in these malignant neoplasms.

METHODS

A total of 56 tissue samples from patients with adrenocortical carcinoma (34) and benign adrenal tissues (22) were identified. Immunohistochemistry was performed for programmed death ligand 1, programmed death ligand 2, and CD8 and scored for membranous staining on adrenal and stromal tissue according to the immunoreactive score and absolute percentage, respectively. Descriptive statistics, a Mann-Whitney U test, and Fisher exact tests were calculated.

RESULTS

In total, 15 adrenocortical carcinoma (44%) stained positive for programmed death ligand 2 and 1 adrenocortical carcinoma for programmed death ligand 1 (P = .03). Adrenocortical carcinoma samples were more likely to express programmed death ligand 2 on tumor cells or in stromal tissues than benign samples (OR = 2.3, P = .03). There was no relationship between programmed death ligand 2 and CD8 expression (P = .08). There were also no relationships between programmed death ligand 2 or CD8 expression and tumor characteristics.

CONCLUSION

Programmed death ligand 2, but not programmed death ligand 1, is expressed commonly in adrenocortical carcinoma samples. The utility of certain checkpoint inhibitors should, therefore, be evaluated in further studies.

CHL1 expression differentiates Hürthle cell carcinoma from benign Hürthle cell nodules

BACKGROUND AND OBJECTIVES

Hürthle cell carcinoma (HCC) is an unusual and relatively rare type of differentiated thyroid cancer. Currently, cytologic analysis of fine-needle aspiration biopsy is limited in distinguishing benign Hürthle cell neoplasms from malignant ones. The aim of this study was to determine whether differences in the expression of specific genes could differentiate HCC from benign Hürthle cell nodules by evaluating differential gene expression in Hürthle cell disease.

METHODS

Eighteen benign Hürthle cell nodules and seven HCC samples were analyzed by whole-transcriptome sequencing. Bioinformatics analysis was carried out to identify candidate differentially expressed genes. Expression of these candidate genes was re-examined by quantitative real-time polymerase chain reaction (qRT-PCR). Protein expression was quantified by immunohistochemistry.

RESULTS

Close homolog of L1 (CHL1) was identified as overexpressed in HCC. CHL1 was found to have greater than 15-fold higher expression in fragments per kilobase million in HCC compared with benign Hurthle cell tumors. This was confirmed by qRT-PCR. Moreover, the immunoreactivity score of the CHL1 protein was significantly higher in HCC compared with benign Hürthle cell nodules.

CONCLUSIONS

CHL1 expression may represent a novel and useful prognostic biomarker to distinguish HCC from benign Hürthle cell disease.

Abstract 3592: Pharmacological intervention to temporally stimulate or inhibit ICAM-1 targeting CAR-T cells

Gene-engineered adoptive T cell therapies have recently been approved by the FDA. We have previously published data utilizing this technology to target intercellular adhesion molecule-1 (ICAM-1), a broad tumor biomarker. Using an affinity modified version of the physiological ligand of ICAM-1,leukocyte function-associated antigen-1 (LFA-1) I-domain, we have shown efficient clearance of a human metastatic anaplastic thyroid cancer (ATC). We were able to visualize and quantify the bio-distribution of CAR-T cells with a PET gene reporter, somatostatin receptor 2 (SSTR2) (Park et. al, Scientific Reports 7, Article number: 14366, 2017). To expand on this platform, we now want to control the activity of our CAR-T cells by exploiting these features using two FDA approved drugs. First, a somatostain analogue, Lanreotide, has been approved for treating Acromegaly and we have recently observed that Lanreotide can activate SSTR2-transduced T cells via calcium release. Second, Lovastatin, a drug clinically used for lowering cholesterol levels, was found to bind the LFA-1 I-domain and inhibit LFA-1 binding to ICAM-1. We hypothesized that in addition to our I-domain CAR-T cell, systemic use of Lanreotide will stimulate CAR-mediated tumor killing while systemic use of Lovastatin will inhibit CAR-mediated tumor killing.

In-vitro testing involved effector to target (E:T) assays performed in the presence and absence of Lovastatin or Lanreotide. ICAM-1(+) ATC target cells or ICAM-1(-) 293T off-target cells were utilized, both of which were transduced with GFP-Firefly Luciferase (GFP/Fluc) for quantification. Effector T-cells derived from healthy donor PBMCs were either transduced with our I-domain CAR-T cell construct that contains SSTR2, or were left nontransduced. Similar assays were translated in-vivo using a previously established solid tumor model with GFP/Fluc ATC ICAM-1(+) cells. Luminescence was used to track tumor progression and regression throughout both in-vivo and in-vitro experiments. [Ga68] DOTA-TOC was administered via IV injections to visualize the bio-distribution of the CAR-T cells using SSTR2 via PET/CT.

Both in-vitro and in-vivo experiments showed Lovastatins ability to compromise I-domain CAR-mediated tumor killing while Lanreotide expedited the rate of I-domain CAR-mediated killing. We report here that Lovastatin inhibits the interaction of our I-domain CAR with ICAM-1 that is present on our target tumor cells and compromises tumor killing. In addition, we have shown a unique phenomena where we can activate SSTR2-transduced CAR-T cells using Lanreotide. The ability to control CAR-T cell activity is an essential tool when addressing adverse effects and/or less than robust treatment outcomes. This will be crucial when translating our adoptive T cell therapy to the clinic.

Citation Format: Yogindra Vedvyas, Jaclyn McCloskey, Yanping Yang, Irene M. Min, Moonsoo M. Jin. Pharmacological intervention to temporally stimulate or inhibit ICAM-1 targeting CAR-T cells [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 3592.

STMN1 is Overexpressed in Adrenocortical Carcinoma and Promotes a More Aggressive Phenotype In Vitro

BACKGROUND

Adrenocortical carcinoma (ACC) is a rare endocrine malignancy with a poor prognosis and few therapeutic options. Stathmin1 (STMN1) is a cytosolic protein involved in microtubule dynamics through inhibition of tubulin polymerization and promotion of microtubule depolymerization, which has been implicated in carcinogenesis and aggressive behavior in multiple epithelial malignancies. We aimed to evaluate expression of STMN1 in ACC and to elucidate how this may contribute to its malignant phenotype.

METHODS

STMN1 was identified by RNA sequencing as a highly differentially expressed gene in human ACC samples compared with benign adrenal tumors. Expression was confirmed by quantitative reverse transcription-polymerase chain reaction (qRT-PCR), Western blot, and immunohistochemical (IHC) staining of a tissue microarray (TMA) from two independent cohorts. The biologic relevance of STMN1 was investigated in NCI-H295R cells by lentivirus-mediated silencing.

RESULTS

Differential gene expression demonstrated an eightfold increase in STMN1 messenger RNA (mRNA) in malignant compared with benign adrenal tissue. IHC showed significantly higher expression of STMN1 protein in ACC compared with normal and benign tissues. STMN1 knockdown in an ACC cell line resulted in decreased cell viability, cell-cycle arrest at G₀/G₁, and increased apoptosis in serum-starved conditions compared with scramble short hairpin RNA (shRNA) controls. STMN1 knockdown also decreased migration, invasion, and anchorage-independent growth compared with controls.

CONCLUSIONS

STMN1 is overexpressed in human ACC samples, and knockdown of this target in vitro resulted in a less aggressive phenotype of ACC, particularly under serum-starved conditions. Further study is needed to investigate the feasibility of interfering with STMN1 as a potential therapeutic target.

CAR T Therapy Targeting ICAM-1 Eliminates Advanced Human Thyroid Tumors

Purpose: Poorly differentiated thyroid cancer and anaplastic thyroid cancer (ATC) are rare yet lethal malignancies with limited treatment options. Many malignant tumors, including papillary thyroid cancer (PTC) and ATC, are associated with increased expression of ICAM-1, providing a rationale for utilizing ICAM-1-targeting agents for the treatment of aggressive cancer. We developed a third-generation chimeric antigen receptor (CAR) targeting ICAM-1 to leverage adoptive T-cell therapy as a new treatment modality.Experimental Design: ICAM-1 CAR T cells were applied to multiple malignant and nonmalignant target cells to investigate specific target cell death and "off-tumor" toxicity in vitroIn vivo therapeutic efficacy of ICAM-1 CAR T cells was examined in ATC mouse models established from a cell line and patient-derived tumors that rapidly develop systemic metastases.Results: ICAM-1 CAR T cells demonstrated robust and specific killing of PTC and ATC cell lines in vitro Interestingly, although certain ATC cell lines showed heterogeneous levels of ICAM-1 expression, addition of cytotoxic CAR T cells induced increased ICAM-1 expression such that all cell lines became targetable. In mice with systemic ATC, a single administration of ICAM-1 CAR T cells mediated profound tumor killing that resulted in long-term remission and significantly improved survival. Patient-derived ATC cells overexpressed ICAM-1 and were largely eliminated by autologous ICAM-1 CAR T cells in vitro and in animal models.Conclusions: Our findings are the first demonstration of CAR T therapy against both a metastatic, thyroid cancer cell line and advanced ATC patient-derived tumors that exhibit dramatic therapeutic efficacy and survival benefit in animal studies. Clin Cancer Res; 23(24); 7569-83. ©2017 AACR.

Abstract 1707: Longitudinal and quantitative imaging of the localization, expansion, and contraction of tumor targeted adoptively transferred T cells

Introduction:

Current methodologies for monitoring adoptive cell transfer (ACT) rely on soluble markers and T cell quantity in serum reflecting mixed responses of T cell efficacy or toxicity. Quantitative, longitudinal T cell visualization can directly probe T cell distribution, expansion, and off-tumor localization, and provide rational to predict therapy successes or failures. To enable real-time PET imaging of adoptively transferred T cells in a manner directly translatable to clinics, we utilized a genetic reporter, somatostatin receptor 2 (SSTR2) and a clinically approved radiotracer to quantitatively and longitudinally visualize whole body T cell distribution and anti-tumor dynamics. SSTR2 imaging was also applied to chimeric antigen receptor (CAR) T cells in a setting of solid cancer.

Methods:

We developed subcutaneous tumors with a mixture of SSTR2 positive and negative Jurkat T cells, ranging from 0 to 100%. From this model and PET/CT imaging of SSTR2 radiotracer uptake, diagnostic performance of our imaging technique was rigorously defined. To apply SSTR2 imaging to CAR T cells, a new lentivirus vector was designed to allow dual expression of SSTR2 and CAR. Longitudinal PET/CT and luminescence imaging was performed to concurrently measure the rate of

tumor growth/killing and T cell expansion and contraction. Blood was also collected to correlate serum cytokines with T cell distribution at on- and off-tumor sites. Histology was performed to confirm the validity of SSTR2 imaging of CAR T cells and relating it to T cell activity.

Results:

Our T cell imaging technique was found to provide 1% limit of detection (one T cell in one hundred neighboring cells or ~4x106 cell/cm3 in solid tissues) with 95% specificity and 87% sensitivity. When applied to CAR T cells in solid cancer, a biphasic T cell expansion and contraction was observed in survivors where a temporal change of T cell density closely followed the change in tumor burden with some time delay. In contrast, nonsurvivors displayed unrelenting increases in tumor and T cell burden, indicating that tumor growth was outpacing T cell killing. Cytokine release syndrome indicated by weight loss and elevated proinflammatory cytokines was also apparent in a cohort of nonsurvivors.

Conclusion:

Our study is the first high quality and quantitative, longitudinal imaging of T cells, correlating T cell dynamism with therapy and toxicity responses in subjects. Correlative changes in cytokines and T cell density can be further developed for early prediction of the onset of cytokine release syndrome as well as for screening adjuvant therapies to augment CAR T cell efficacy against solid cancer. Owing to the use of a human gene and FDA-approved radiotracer, our imaging technique can be directly applicable to CAR T cells and other T cell based therapy. We are currently preparing for a phase I clinical study for real-time imaging CD19 CAR T cells in patients.

Citation Format: Yogindra Vedvyas, Enda Shevlin, Marjan Zaman, Irene M. Min, Moonsoo M. Jin. Longitudinal and quantitative imaging of the localization, expansion, and contraction of tumor targeted adoptively transferred T cells [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 1707. doi:10.1158/1538-7445.AM2017-1707

Abstract 3750: Micromolar affinity CAR T cells to ICAM-1 achieves rapid tumor elimination while avoiding systemic toxicity

Introduction: Adoptive immune therapy has achieved great success in eradicating blood-borne cancers, prominently, the CD19 CAR T cells in B cell leukemia and lymphomas. However, CAR T cell therapy in solid tumors has

been limited due to the scarcity of tumor antigens that are deemed safe for targeting. One strategy to overcome scarcity of tumor antigen is by tuning the affinity of CAR to limit T cell reaction with cells overexpressing target antigen while sparing cells with basal level expression. To rigorously test the idea of “affinity tuning”, we built variants of CARs possessing one million-fold difference in affinity spanning low nanomolar to high micromolar affinity to a target antigen, and examined the influence of CAR affinity on the rate of tumor killing and systemic toxicity.

Methods: Antigen-binding domain of CAR was built from the inserted or I domain belonging to integrin LFA-1. Affinity of CAR expressed in T cells was confirmed by ICAM-1 binding by flow cytometry. For in vivo study, mice with systemic growth of ICAM-1 positive thyroid tumor were used, where tumors grew in lungs, liver, and bones. Tumor growth and killing were monitored by whole body luminescence imaging. Sera were collected for cytokine analysis. Body weight, cytokine profile, and overall behavior were used to assess the severity of systemic toxicity.

Results: CAR T cells with a step-wise, one million-fold variation in affinity to ICAM-1 resulted in a rate of target killing in proportion to the increase in affinity and in ICAM-1 density. Owing to cross-reaction of human LFA-1 with murine ICAM-1, the influence of CAR affinity on efficacy and on-target, off-tumor toxicity was tested in mice bearing ICAM-1 positive human tumors. In vivo tumor elimination by CAR T cells was in contrast to in vitro affinity-dependent rate of target killing, demonstrating that micromolar affinity CAR T cells was superior to nanomolar affinity T cells in both tumor killing and safety aspects. Highest affinity CAR T cells led to uniform death of the host, caused by on-target, off-tumor toxicity, and high level cytokine release.

Conclusion: Our study is the first comprehensive report examining the effect of CAR affinity on the rate of tumor killing, efficacy, and toxicity. In contrast to in vitro tumor killing effect, the increase in affinity of CAR beyond certain threshold was deleterious to T cell persistence and associated with more frequent tumor relapse. Our study highlights that CAR T cells approximating natural T cell receptor affinity is more efficacious in eliminating tumors with overexpressed antigens, and is safer by avoiding potential reaction with normal cells with basal expression of the same antigen.

Citation Format: Enda Shevlin, Spencer Park, Yogindra Vedvyas, Marjan Zaman, Susan Park, Irene M. Min, Moonsoo M. Jin. Micromolar affinity CAR T cells to ICAM-1 achieves rapid tumor elimination while avoiding systemic toxicity [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 3750. doi:10.1158/1538-7445.AM2017-3750

Abstract 3624: CAR T cells targeting ICAM-1 trigger strong antitumor effects against advanced human thyroid tumors

Introduction:

Poorly differentiated thyroid cancer and anaplastic thyroid cancer (ATC) are rare yet inherently lethal malignancies with limited treatment options. Many malignant tumors, including papillary thyroid cancer (PTC) and ATC, are associated with increased expression of intercellular adhesion molecule-1 (ICAM-1), providing a rationale for utilizing ICAM-1-targeting agents for the treatment of aggressive types of thyroid cancer. Therefore, we developed a third-generation chimeric antigen receptor (CAR) targeting ICAM-1 to leverage adoptive T cell therapy as a new treatment modality against advanced thyroid cancer.

Methods:

We created a firefly luciferase-expressing human ATC model in mice that develops systemic metastases very rapidly. ATC engrafted mice were treated with human peripheral blood T cells modified with a lentivirus encoding an ICAM-1 specific CAR (ICAM-1-CAR) to investigate their therapeutic efficacy. Tumor burden was longitudinally measured by whole body bioluminescence imaging of luciferase-positive tumor cells. Effector:target assays consisting of ICAM-1-CAR T cells co-cultured with multiple malignant and non-malignant target cells were used to investigate specific target cell death and ‘off-tumor’ toxicity in vitro using luminescence and flow cytometry.

Results:

ICAM-1-CAR T cells demonstrated robust and specific killing of PTC and ATC cell lines in vitro. Strikingly, despite heterogeneous expression of ICAM-1 in ATC cell lines, addition of cytotoxic CAR T cells induced increased ICAM-1 expression by T cell-derived interferon gamma such that all cell lines became targetable by ICAM-1-CAR T cells. Patient-derived, poorly differentiated PTC cells overexpressed ICAM-1 and were also mostly eliminated by autologous ICAM-1 CAR T cells in vitro. In mice with systemic ATC, a single administration of ICAM-1-CAR-T cells at a clinical dose mediated significant tumor killing with a 100-fold reduction in primary tumor burden compared to pre-treatment. Reductions in tumor burden persisted for over 80 days and treated mice demonstrated significantly improved survival without toxicity.

Conclusion:

Our findings are the first demonstration of the potential for CAR-T cell therapy for metastatic, advanced thyroid cancers. ICAM-1-CAR T cells demonstrated significant therapeutic efficacy in vitro and in vivo and extended survival benefits in animal models.

Citation Format: Irene M. Min, Yogindra Vedvyas, Enda Shevlin, Marjan Zaman, Brian Wyrwas, Weibin Wang, Susan Park, Maureen Moore, Theresa Scognamiglio, Rasa Zarnegar, Thomas J. Fahey, Moonsoo M. Jin. CAR T cells targeting ICAM-1 trigger strong antitumor effects against advanced human thyroid tumors [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 3624. doi:10.1158/1538-7445.AM2017-3624

Longitudinal PET imaging demonstrates biphasic CAR T cell responses in survivors

Clinical monitoring of adoptive T cell transfer (ACT) utilizes serial blood analyses to discern T cell activity. While useful, these data are 1-dimensional and lack spatiotemporal information related to treatment efficacy or toxicity. We utilized a human genetic reporter, somatostatin receptor 2 (SSTR2), and PET, to quantitatively and longitudinally visualize whole-body T cell distribution and antitumor dynamics using a clinically approved radiotracer. Initial evaluations determined that SSTR2-expressing T cells were detectable at low densities with high sensitivity and specificity. SSTR2-based PET was applied to ACT of chimeric antigen receptor (CAR) T cells targeting intercellular adhesion molecule-1, which is overexpressed in anaplastic thyroid tumors. Timely CAR T cell infusions resulted in survival of tumor-bearing mice, while later infusions led to uniform death. Real-time PET imaging revealed biphasic T cell expansion and contraction at tumor sites among survivors, with peak tumor burden preceding peak T cell burden by several days. In contrast, nonsurvivors displayed unrelenting increases in tumor and T cell burden, indicating that tumor growth was outpacing T cell killing. Thus, longitudinal PET imaging of SSTR2-positive ACT dynamics enables prognostic, spatiotemporal monitoring with unprecedented clarity and detail to facilitate comprehensive therapy evaluation with potential for clinical translation.

Sol-gel SELEX circumventing chemical conjugation of low molecular weight metabolites discovers aptamers selective to xanthine

Sensitive detection of the metabolites indicative of a particular disease contributes to improved therapy outcomes. Developing binding reagents for detection of low molecular weight metabolites is hampered by the difficulty with immobilization of targets through appropriate covalent chemical linkage while ensuring that selected reagents retain specificity to unmodified metabolites. To circumvent chemical modification of targets, we employed sol-gel droplets deposited onto a porous silicon chip to entrap a purine metabolite, xanthine, which was found at lower levels in urine samples from patients with non-Hodgkin lymphoma. By sol-gel SELEX (systematic evolution of ligands by exponential enrichment) against xanthine, specific aptamers (KD ∼ 10 μM) with sensitivity of detection at as low as 1 μM were isolated, which bound to other purine metabolites at more than 100-fold lower affinity. In contrast, we failed to isolate xanthine-specific aptamers when SELEX was performed against xanthine covalently linked to polymer resin. This study demonstrates that the sol-gel platform for entrapping low molecular weight metabolites without chemical modifications can be utilized for SELEX to discover aptamers against clinical metabolite markers for diagnosis application.

Development of temperature-sensitive mutants of the Drosophila melanogaster P-TEFb (Cyclin T/CDK9) heterodimer using yeast two-hybrid screening

P-TEFb complex, a heterodimer of the kinase CDK9 and Cyclin T, is a critical factor that stimulates the process of transcription elongation. Here, we explored a fast and large-scale screening method to induce a temperature-dependent conditional disruption of the CDK9/Cyclin T interaction and developed an assay to validate their mutant phenotypes in a biological context. First, we used the yeast two-hybrid system to screen Drosophila melanogaster Cyclin T mutants at a large scale for temperature or cold sensitive (TS or CS) CDK9 interaction phenotypes. The isolated P-TEFb TS mutants were then expressed in Drosophila cells and were investigated for their effects on Drosophila hsp70 transcriptional activity. Our results showed that these P-TEFb TS mutants had a reduced level of hsp70 transcription at restrictive temperatures. A model structure of the Cyclin T and CDK9 complex suggested that the key TS mutations were found within the α2- and α3-helices at the interface of the complex, which may disrupt the binding of Cyclin T to CDK9 directly or indirectly by affecting the conformation of Cyclin T. The yeast two-hybrid-based screening strategy described here for isolating TS or CS interaction phenotypes can be directly applicable to other complexes in higher organisms. The use of TS or CS mutants will enable a 'real-time and reversible perturbation' restricted to specific protein-protein interactions, providing a mechanistic insight into the biological process mediated by a target complex.

Regulating RNA polymerase pausing and transcription elongation in embryonic stem cells

Transitions between pluripotent stem cells and differentiated cells are executed by key transcription regulators. Comparative measurements of RNA polymerase distribution over the genome's primary transcription units in different cell states can identify the genes and steps in the transcription cycle that are regulated during such transitions. To identify the complete transcriptional profiles of RNA polymerases with high sensitivity and resolution, as well as the critical regulated steps upon which regulatory factors act, we used genome-wide nuclear run-on (GRO-seq) to map the density and orientation of transcriptionally engaged RNA polymerases in mouse embryonic stem cells (ESCs) and mouse embryonic fibroblasts (MEFs). In both cell types, progression of a promoter-proximal, paused RNA polymerase II (Pol II) into productive elongation is a rate-limiting step in transcription of ∼40% of mRNA-encoding genes. Importantly, quantitative comparisons between cell types reveal that transcription is controlled frequently at paused Pol II's entry into elongation. Furthermore, "bivalent" ESC genes (exhibiting both active and repressive histone modifications) bound by Polycomb group complexes PRC1 (Polycomb-repressive complex 1) and PRC2 show dramatically reduced levels of paused Pol II at promoters relative to an average gene. In contrast, bivalent promoters bound by only PRC2 allow Pol II pausing, but it is confined to extremely 5' proximal regions. Altogether, these findings identify rate-limiting targets for transcription regulation during cell differentiation.

Drosophila heat shock system as a general model to investigate transcriptional regulation

Whereas the regulation of a gene is uniquely tailored to respond to specific biological needs, general transcriptional mechanisms are used by diversely regulated genes within and across species. The primary mode of regulation is achieved by modulating specific steps in the transcription cycle of RNA polymerase II (Pol II). Pol II "pausing" has recently been identified as a prevalent rate-limiting and regulated step in the transcription cycle. Many sequence-specific transcription factors (TFs) modulate the duration of the pause by directly or indirectly recruiting positive transcription elongation factor b (P-TEFb) kinase, which promotes escape of Pol II from the pause into productive elongation. These specialized TFs find their target-binding sites by discriminating between DNA sequence elements based on the chromatin context in which these elements reside and can result in productive changes in gene expression or nonfunctional "promiscuous" binding. The binding of a TF can precipitate drastic changes in chromatin architecture that can be both dependent and independent of active Pol II transcription. Here, we highlight heat-shock-mediated gene transcription as a model system in which to study common mechanistic features of gene regulation.

The transcription factor EGR1 controls both the proliferation and localization of hematopoietic stem cells

EGR1 is a member of the immediate early response transcription factor family and functions in cell growth, development, and stress responses in many tissues. Here we report an additional role for EGR1 in regulating homeostasis of hematopoietic stem cells (HSCs). HSCs normally express Egr1 at high levels, but dramatically downregulate its expression when induced to divide and migrate. Consistent with this finding, mice lacking Egr1 exhibit significant increases in steady-state levels of dividing HSCs in the bone marrow (BM), and a striking spontaneous mobilization of HSCs into the peripheral blood. These data identify EGR1 as a transcriptional regulator of stem cell migration that normally functions to promote HSC quiescence and retention in the niche. The ability of this single factor to regulate both proliferation and mobilization of HSCs suggests that EGR1 commands a genetic program that coordinates stem cell division and migration to maintain appropriate HSC number and function.

Ovulated oocytes in adult mice derive from non-circulating germ cells

Decades of research in reproductive biology have led to the generally accepted belief that in female mammals, all surviving germ cells enter meiosis at the end of fetal development and as a result, the postnatal ovary harbours a limited supply of oocytes that cannot be replenished or regenerated if lost to injury or disease. However, recent reports have challenged this view, suggesting instead that oocyte production is maintained through continual seeding of the ovary by circulating, bone-marrow-derived germ cells. To test directly the physiological relevance of circulating cells for female fertility, we established transplantation and parabiotic mouse models to assess the capacity of circulating bone marrow cells to generate ovulated oocytes, both in the steady state and after induced damage. Our studies showed no evidence that bone marrow cells, or any other normally circulating cells, contribute to the formation of mature, ovulated oocytes. Instead, cells that travelled to the ovary through the bloodstream exhibited properties characteristic of committed blood leukocytes.

Shifts in targeting of class switch recombination sites in mice that lack mu switch region tandem repeats or Msh2

The mechanisms that target class switch recombination (CSR) to antibody gene switch (S) regions are unknown. Analyses of switch site locations in wild-type mice and in mice that lack the Sμ tandem repeats show shifts indicating that a 4–5-kb DNA domain (bounded upstream by the Iμ promoter) is accessible for switching independent of Sμ sequences. This CSR-accessible domain is reminiscent of the promoter-defined domains that target somatic hypermutation. Within the 4–5-kb CSR domain, the targeting of S site locations also depends on the Msh2 mismatch repair protein because Msh2-deficient mice show an increased focus of sites to the Sμ tandem repeat region. We propose that Msh2 affects S site location because sequences with few activation-induced cytidine deaminase targets generate mostly switch DNA cleavages that require Msh2-directed processing to allow CSR joining.

Antibody class switch recombination: roles for switch sequences and mismatch repair proteins

Mechanisms and targeting of antibody class switch DNA recombination are reviewed. Particular emphasis is on the roles for the DNA sequences comprising switch (S) regions, including the S-region tandem repeats, and on the roles of proteins that are involved in both DNA mismatch repair and in class switch recombination.

The Smu tandem repeat region is critical for Ig isotype switching in the absence of Msh2

Deficiencies of the Msh2 protein or the Smu tandem repeat (SmuTR) sequences each reduce isotype switching in mice by about 2- to 3-fold. We find that switching in mice deficient for both Msh2 and SmuTR is nearly ablated. We propose that the SmuTR provides closely spaced cleavage sites that can undergo switch recombination independent of Msh2, whereas cleavages in sequences flanking the SmuTR require Msh2 processing to allow recombinational joining. We also find that changes in Smu sequences alter the focus of switch junctions within Sgamma sequences, indicating that sequences of switch regions act together in the choice of switch recombination junctions. These findings help to explain the conservation of tandemly repeated switch regions associated with heavy chain constant genes in species capable of switching.

Activation of interferon regulatory factor 3 in response to DNA-damaging agents

Genotoxic stress triggers signal transduction pathways that mediate either the protection or apoptosis of affected cells. The interferon regulatory factors (IRFs) are involved in a wide range of host defense mechanisms against environmental stresses. Treatment with DNA-damaging agents, including doxorubicin and UV radiation, caused phosphorylation of the IRF3 transcription factor. Phosphorylation of IRF3 induced its interaction with the transcriptional co-activator cAMP-response element binding protein-binding protein. Furthermore, genotoxic stress-induced phosphorylation of IRF3 resulted in its movement from the cytoplasm to the nucleus, where it activated transcription from its binding site. These observations suggest that IRF3 plays a role in the defensive responses induced by genotoxic stress.