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We systematically evaluated the primary and secondary endpoints used in acute myeloid leukemia (AML) phase III randomized controlled trials (RCTs). We included 238 phase III AML RCTs in the past 15 years that reported 279 primary endpoints and 657 secondary endpoints. Overall survival (OS), progression-free survival (PFS), event-free survival (EFS), and complete remission (CR) were primary endpoints in 120 (43%), 34 (12%), 30 (11%), and 41 (15%) studies, respectively. OS (12.5%), PFS (13.2%), CR (14%), safety (11%), and EFS (9%) were commonly reported secondary endpoints. Among primary endpoints, a higher use of OS (OR 2.03, 95%CI 1.10-3.75

Acute myeloid leukemia (AML) is a complex diagnosis that puts patients at a higher risk for developing infections, particularly invasive fungal infections (IFI). Mutations in TNFRSF13B have been shown to cause dysfunction in B-cell homeostasis and differentiation, making it a risk factor for developing immunodeficiency syndromes. In this case, a male patient in his 40s presented to our emergency department (ED) with symptoms leading to a diagnosis of AML with concurrent mucormycosis of the lungs and sinuses. Targeted next generation sequencing (NGS) of the patient’s bone marrow showed, among other variants, a loss of function mutation in the TNFRSF13B gene. While most patients present with fungal infections after prolonged periods of neutropenia associated with AML treatment, this case presented with IFI at diagnosis without neutropenia suggesting an immunodeficiency syndrome. The concurrent IFI and AML diagnoses create a delicate balance between treatment of the infection and the malignancy. This case highlights the risk of infection in patients receiving chemotherapy, especially those with unrecognized immunodeficiency syndromes, and emphasizes the importance of NGS for prognosis and treatment.

We conducted a systematic review and meta-analysis to evaluate outcomes after allogeneic hematopoietic stem cell transplantation (HSCT) in TP53-mutated acute myeloid leukemia (AML). We performed a literature search on PubMed, Embase, Cochrane Library, and ClinicalTrials.gov. After screening 592 manuscripts, eight studies were included. Data were extracted following Preferred Reporting Items for Systematic Reviews and Meta-Analysis (PRISMA) guidelines. Pooled analysis was done using the meta-package by Schwarzer et al. Proportions with 95% confidence intervals (CIs) were computed. We analyzed 297 patients. The median follow-up was 45 (0.9-407.3) months. The pooled 2-year overall survival was 29.7% (95% CI 0.17-0.43

Both mesenchymal stem cells (MSCs) and their corresponding small extracellular vesicles (sEVs, commonly referred to as exosomes) share similar immunomodulatory properties that are potentially beneficial for the treatment of acute graft versus host disease (aGvHD). We report that clinical grade Wharton's Jelly‐derived MSCs (WJMSCs) secrete sEVs enriched in programmed death‐ligand 1 (PD‐L1), an essential ligand for an inhibitory immune checkpoint. A rapid increase in circulating sEV‐associated PD‐L1 was observed in patients with aGvHD and was directly associated with the infusion time of clinical grade WJMSCs. In addition, in vitro inhibitory antibody mediated blocking of sEV‐associated PD‐L1 restored T cell activation (TCA), suggesting a functional inhibitory role of sEVs‐PD‐L1. PD‐L1‐deficient sEVs isolated from WJMSCs following CRISPR‐Cas9 gene editing fail to inhibit TCA. Furthermore, we found that PD‐L1 is essential for WJMSC‐derived sEVs to modulate T cell receptors (TCRs). Our study reveals an important mechanism by which therapeutic WJMSCs modulate TCR‐mediated TCA through sEVs or sEV‐carried immune checkpoints. In addition, our clinical data suggest that sEV‐associated PD‐L1 may be not only useful in predicting the outcomes from WJMSC clinical administration, but also in developing cell‐independent therapy for aGvHD patients.

Melanoma treatment has been revolutionized over the past decade. Long-term results with immuno-oncology (I-O) agents and targeted therapies are providing evidence of durable survival for a substantial number of patients. These results have prompted consideration of how best to define long-term benefit and cure. Now more than ever, oncologists should be aware of the long-term outcomes demonstrated with these newer agents and their relevance to treatment decision-making. As the first tumor type for which I-O agents were approved, melanoma has served as a model for other diseases. Accordingly, discussions regarding the value and impact of long-term survival data in patients with melanoma may be relevant in the future to other tumor types. Current findings indicate that, depending on the treatment, over 50% of patients with melanoma may gain durable survival benefit. The best survival outcomes are generally observed in patients with favorable prognostic factors, particularly normal baseline lactate dehydrogenase and/or a low volume of disease. Survival curves from melanoma clinical studies show a plateau at 3 to 4 years, suggesting that patients who are alive at the 3-year landmark (especially in cases in which treatment had been stopped) will likely experience prolonged cancer remission. Quality-of-life and mixture-cure modeling data, as well as metrics such as treatment-free survival, are helping to define the value of this long-term survival. In this review, we describe the current treatment landscape for melanoma and discuss the long-term survival data with immunotherapies and targeted therapies, discussing how to best evaluate the value of long-term survival. We propose that some patients might be considered functionally cured if they have responded to treatment and remained treatment-free for at least 2 years without disease progression. Finally, we consider that, while there have been major advances in the treatment of melanoma in the past decade, there remains a need to improve outcomes for the patients with melanoma who do not experience durable survival.

BackgroundTumor-infiltrating lymphocyte (TIL) therapy is a personalized cancer treatment which involves generating ex vivo cultures of tumor-reactive T cells from surgically resected tumors and administering the expanded TILs as a therapeutic infusion. Phase 1 of many TIL production protocols use aldesleukin (IL-2) alone to establish TIL cultures (termed “PreREP” (Pre-Rapid Expansion Protocol)); however, this fails to consistently produce TIL cultures from renal cell carcinoma (RCC) in a timely manner. Adding mitogenic stimulation via anti-CD3/anti-CD28 beads along with IL-2 to the fresh tumor digest (FTD) during TIL generation (termed “FTD+ beads”) increases successful TIL culture rates; however, T cells produced by this method may be suboptimal for adoptive transfer. We hypothesize that adherent cell depletion (ACD) before TIL expansion will produce a superior TIL product by removing the immunosuppressive signals originating from adherent tumor and stromal cells. Here we investigate if “panning,” a technique for ACD prior to TIL expansion, will impact the phenotype, functionality and/or clonality of ex vivo expanded RCC TILs.MethodsTumor specimens from 55 patients who underwent radical or partial nephrectomy at the University of Kansas Medical Center (KUMC) were used to develop the panning method and an additional 19 specimens were used to validate the protocol. Next-generation sequencing, immunohistochemistry/immunocytochemistry and flow cytometry were used during method development. The phenotype, functionality and clonality of autologous TILs generated in parallel by panning, PreREP, and FTD+ beads were assessed by flow cytometry, in vitro co-culture assays, and TCRB CDR3 sequencing.ResultsTIL cultures were successfully generated using the panning protocol from 15/16 clear cell, 0/1 chromophobe, and 0/2 papillary RCC samples. Significantly fewer regulatory (CD4+/CD25+/FOXP3+) (p=0.049, p=0.005), tissue-resident memory (CD8+/CD103+) (p=0.027, p=0.009), PD-1+/TIM-3+ double-positive (p=0.009, p=0.011) and TIGIT+ T cells (p=0.049, p=0.026) are generated by panning relative to PreREP and FTD+ beads respectively. Critically, a subset of TILs generated by panning were able to degranulate and/or produce interferon gamma in response to autologous tumor cells and the average tumor-reactive TIL yield was greatest when using the panning protocol.ConclusionsRemoving immunosuppressive adherent cells within an RCC digest prior to TIL expansion allow for the rapid production of tumor-reactive T cells with optimal characteristics for adoptive transfer.

Background: Chimeric antigen receptor T cell (CAR-T) therapy is an adoptive T cell immunotherapy that employs the genetically modified T cell to attack the cancer cell. It is widely studied across various hematological and solid organ malignancies. Several autologous CD19 CAR-T cell therapy constructs are now approved for various B cell lymphomas, including aggressive B cell lymphomas, indolent follicular lymphoma and mantle cell lymphoma, and acute lymphoblastic leukemia (ALL). Autologous CD19 CAR-T cell therapy has unprecedented success in relapsed and refractory disease. Long time to manufacture (2-5 weeks) and manufacture failure are challenges associated with risk of interim death and deterioration of CAR-T candidates with rapidly progressive disease. T cell fitness of the autologous product in heavily pretreated patients is also potentially compromised. To overcome these shortcomings, universal "off the shelf" allogeneic CAR-T cell therapy constructs are being developed and studied. Donor sources include healthy donors and cord or induced pluripotent stem cells (iPSCs). These CAR-T constructs have additional gene modifications to mitigate the risk of rejection and graft versus host disease (GVHD). We performed a systematic review and meta-analysis to assess the safety and efficacy of allogeneic CD19 CAR-T cell therapy. Methods: Four databases (Web of Science/MEDLINE/PubMed, Embase, and Cochrane Registry of Controlled Trials) were searched for this systematic review and meta-analysis following the Preferred Reporting Items for Systematic Reviews and Meta-Analysis (PRISMA) guidelines using MeSH terms and keywords for "Receptors, Chimeric antigen" OR "Artificial-T-cell receptor" OR "immunotherapy, adoptive" OR "CD-19". Our search produced 3506 articles and after removing duplicates, 2243 records were screened. After excluding reviews and irrelevant articles, we included 8 prospective trials of allogeneic CD-19 CAR-T cell therapy enrolling two or more than two patients from Jan 2013 to Nov 2020. We also searched ASH 2020 abstracts to include any additional trials. The methodological quality of the included studies was evaluated using NIH quality assessment tool. Inter-study variance was calculated using the Der Simonian-Laird Estimator. Proportions along with 95% confidence Interval (CI) were extracted to compute pooled analysis using the 'meta' package by Schwarzer et al. in the R programming language (version 4.16-2). Results: A total of 68 patients from 8 studies were evaluated. Median age was 22.5 (4.8-64) years. (Table 1) The median follow-up time was 10 (2-18) months with median number of prior therapies of 3.2 (2-11) as reported by 5 studies. Underlying diagnosis was acute lymphocytic lymphoma (n=49, 72%), chronic lymphocytic leukemia (n=6, 9%), and non-Hodgkin lymphoma (n=13, 19%). The pooled overall response rate (ORR) was 77% (95% CI 0.63-0.89, I 2 =22%, n=68) with a complete response (CR) of 75% (95% CI 0.57-0.90, I 2 =48%, n=65). The pooled incidence of cytokine release syndrome grade I/II and grade III/IV was 53% (95% CI 0.16-0.89, I 2 =89%, n=65) and 10% (95% CI 0.01-0.25, I 2 =50%, n=65) respectively. Neurotoxicity grade I/II was 12% (95%CI 0.01-0.30, I 2 =47%, p=0.09, n=47) and GVHD grade I/II was 8% (95%CI 0.01-0.19, I 2 =0%, p=0.57 n=53). None of the clinical trials reported the duration of response. Conclusion: "Off the shelf" universal CAR-T therapy is early in development. Our available data suggest that allogeneic CD19 CAR-T constructs offer high ORR and CR rates with acceptable safety profiles. GVHD was mainly low grade (grade I-II). Given these findings, allogeneic CAR-T cell therapy is an attractive option to improve timely access compared to available autologous therapy. Extensive preclinical research to develop novel constructs and several phase I/II clinical trials are ongoing to shape the future of "off the shelf" CAR-T cell therapy. Figure 1 Figure 1. Disclosures Hoffmann: Pharmcyclics: Consultancy, Honoraria; TG Therapeutics: Consultancy, Honoraria; Novartis: Consultancy, Honoraria; Janssen: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees; celgene: Consultancy, Honoraria. Abhyankar: Incyte/Therakos: Consultancy, Research Funding, Speakers Bureau. McGuirk: Fresenius Biotech: Research Funding; Gamida Cell: Research Funding; Pluristem Therapeutics: Research Funding; EcoR1 Capital: Consultancy; Novartis: Research Funding; Astelllas Pharma: Research Funding; Kite/ Gilead: Consultancy, Honoraria, Other: travel accommodations, expense, Kite a Gilead company, Research Funding, Speakers Bureau; Juno Therapeutics: Consultancy, Honoraria, Research Funding; Magenta Therapeutics: Consultancy, Honoraria, Research Funding; Novartis: Research Funding; Bellicum Pharmaceuticals: Research Funding; Allovir: Consultancy, Honoraria, Research Funding.

7033 Background: The Coronavirus Disease 2019 (COVID-19) has caused over 25 million infections in the US with over 0.4 million deaths. Hematogenic stem cell transplant (HCT) or cellular therapy (CT) recipients have a high risk of mortality with COVID-19 due to profound immune dysregulation. We aimed to assess the outcomes with COVID-19 in HCT/CT recipients. Methods: A single-center prospective study was conducted, including all (n=40) adult HCT/CT patients who were diagnosed with COVID-19 at the University of Kansas from Apr 2020 to Jan 2021. Baseline and disease-related characteristics were ascertained from medical records. Data were analyzed using SPSS version 21 (SPSS Inc, Chicago, IL). Bivariate analyses, using chi-square and t-test, and logistic regression analyses were conducted. Results: The study included 40 COVID-19 patients (72.5% Oct 2020-Jan 2021), including allogeneic HCT (n=25), autologous HCT (n=13) and CAR-T CT (n=2) with median time since HCT/CT of 12.4 (1-201.9), 37.2 (0.4-118.7), and 3.8 (2.8-4.8) months. Seventy percent were Caucasians and 17.5 were Hispanics. Primary hematologic malignancy was myeloid (37.5%), lymphoid (35%) or plasma cell disorder (27.5%). Myeloablative conditioning was performed in 65% of patients. Donors were autologous (37.5%), matched sibling (17.5%), matched unrelated (22.5%) and haploidentical (22.5%). COVID-19 was mild (42.5%), moderate (42.5%) or severe (15%). Clinical findings included pneumonia (62.5%), hypoxia (25%) and ICU admission (17.5%) while therapies included remdesivir (47.5%), convalescent plasma (40%), dexamethasone (25%) and monoclonal antibodies (17.5%). Concurrent cancer treatment, other infections and active GVHD were reported in 25% (all myeloma), 20% and 32.5% of patients. After a median follow-up of 74 days (7-269), the mortality rate was 12.5% in all patients and 20% in allo-HCT patients. Significant predictors of COVID-19 severity included allogeneic HCT, concurrent immune suppression and elevated inflammatory markers. (Table). Conclusions: Hematopoietic stem cell transplant recipients have an increased risk of mortality with COVID-19. Our findings confirm the need for vaccination prioritization, close monitoring, and aggressive treatment in HCT/CT patients.[Table: see text]

e19002 Background: The presence of measurable residual disease (MRD) preceding hematopoietic stem cell transplantation (HSCT) in acute myeloid leukemia (AML) is increasingly recognized as a risk factor for leukemic relapse and decreased survival. Over many years, attempts have been looking at developing tools to detect MRD; this includes multiparametric flow cytometry, quantitative polymerase chain reaction, and most recently, next-generation sequencing (NGS). NGS offers higher sensitivity and detection rate of disease-related gene mutations, thereby potentially improving disease outcomes. Our study sought to review the scientific literature that included NGS‐detected molecular MRD in patients with AML who underwent bone marrow transplantation. Methods: We performed a systematic search using PubMed, Google Scholar, EMBASE, and SCOPUS up until October 2020. Inclusion criteria included articles that reported the association between pre-HSCT MRD detected by NGS and post HSCT outcome in patients with AML. We extracted hazard ratios for the cumulative incidence of relapse (CIR), overall survival (OS) and leukemia free survival (LFS). A random-effect model was utilized to calculate the hazard ratio (HR) with a 95% confidence interval (CI). Results: Six studies met our inclusion criteria. Our meta-analysis showed that the detection of pre-transplant MRD by NGS was associated with increased risk of cumulative incidence of relapse (hazard ratio=2.5, CI= 1.6-3.9, with p-value 2 = 53%, 52%, and 59% for CIR, OS, and LFS, respectively). Conclusions: The application of NGS to detect MRD is a strong predictor of outcome in patients with AML who are undergoing hematopoietic stem cell transplantation. NGS-detected MRD positive status prior to HSCT is indicative of a higher risk of relapse and decreased overall survival in this meta-analysis. Despite the limitations in our study, it demonstrates the value of MRD detection by NGS in HSCT recipients.

e13615 Background: Transplant Survivorship Clinic at our institution serves to improve outcomes and overall health of allogeneic transplant survivors. The COVID19 pandemic allowed for growth of telemedicine in our program. We examine the patterns of use of telehealth and hypothesize that the use of telemedicine allowed continued access to care compared to the era prior to availability. We compared our transplant survivorship clinic data from July- December 2020, when telehealth was well established and compared to July-December 2019. Methods: All patients seen by the survivorship team for end of treatment visits, graft versus host disease assessments and survivorship visits annually between July-December 2019 and July-December 2020 were included. Their zip codes were used to get direct distance to survivorship clinic, average drive time, driving distance and average household income as in zip-codes.com database. Results: Total number of office visits in July-Dec 2019 was 163 visits (0% via telehealth) and in July-Dec 2020 was 228 (66.2% via telehealth). All encounters (telehealth and office visits) were lower in July and August 2020 compared to July and August 2019 but higher in months of September -December 2020 compared to 2019. Comparing all encounters during 7/2019-12/2019 to 7/2020-12/2020, there was no statistically significant difference in median age (58yr vs 60 yr), gender (males: 58% vs 59%), race (non-white: 11% vs 8.7%), median years from transplant (4yr vs 3 yrs), median income of patient neighborhood ($63,735 vs $60,465) and average drive time to center from zip code (40 min vs 51min). Comparison of patients who chose telehealth vs. office visit is summarized in table. While there was no statistically significant difference in age when comparing all encounters in 2019 and 2020, those who chose telehealth were younger (55yrs vs 60yrs, p=0.003). Conclusions: There were higher patient encounters in the 2020 period compared to 2019. Most of these 2020 encounters were via were telehealth, demonstrating the role of telehealth in increasing access. Younger patients appeared to choose telehealth, but telehealth served patients up to the age of 77 yrs. Utilization of telehealth appeared to be irrespective of demographics such as gender, neighborhood income and driving distance to the center. Comparison of telehealth vs office visit for July-December 2019 and 2020.[Table: see text]

e18525 Background: Several recurrent genetic mutations have been described in acute myeloid leukemia (AML), which have both prognostic and therapeutic implications. Recurrent mutations in the Neurofibromin 1 ( NF1) gene are reported in 1-5 % of AML patients; however, there are limited data regarding its prognostic implications. Here, we report the outcomes for patients with newly diagnosed AML with a somatic NF1 mutation at our center. Methods: A retrospective chart review included patients with newly diagnosed AML at KUMC from 01/2016 through 09/2018. All patients had targeted next-generation sequencing (NGS) at diagnosis. Baseline characteristics were compared between patients with NF1 mutations and those who were wild-type using Fisher’s exact test for categorical variables, and the Wilcoxon rank-sum test for continuous variables. The primary outcome was overall survival (OS), which was measured from the time of diagnosis to the time of death from any cause. A stepwise Cox proportional-hazard model was used to adjust for potential confounders. Results: Data on 110 patients were included. Out of the 110 patients, 15 (13.6%) had a delectable NF1 mutation, while 95 (86.4%) patients were NF1 wild-type. The baseline characteristics of the two groups are displayed below. Median OS for patients with an NF1 mutation was 7.3 months, while it was 18.4 months for patients with NF1 wild-type, Log-rank test p-value 0.02. After adjusting for potential confounders, including age, ELN risk category, induction regimen, presence of other mutations such as TP53, the hazard of death remained significantly higher for patients with NF1 mutations, HR 2.4, CI (1.05-5.6), p-value 0.04. Conclusions: In this single-center retrospective study, the presence of a NF1 mutation was associated with worse overall survival in patients with newly diagnosis AML. [Table: see text]

Liu, Chunsheng [el alt.] 19 p.-14 fig., In the tumor microenvironment, TGF-β induces transdifferentiation of quiescent pericytes and related stromal cells into myofibroblasts that promote tumor growth and metastasis. The mechanisms governing myofibroblastic activation remain poorly understood, and its role in the tumor microenvironment has not been explored. Here, we demonstrate that IQ motif containing GTPase activating protein 1 (IQGAP1) binds to TGF-β receptor II (TβRII) and suppresses TβRII-mediated signaling in pericytes to prevent myofibroblastic differentiation in the tumor microenvironment. We found that TGF-β1 recruited IQGAP1 to TβRII in hepatic stellate cells (HSCs), the resident liver pericytes. Iqgap1 knockdown inhibited the targeting of the E3 ubiquitin ligase SMAD ubiquitination regulatory factor 1 (SMURF1) to the plasma membrane and TβRII ubiquitination and degradation. Thus, Iqgap1 knockdown stabilized TβRII and potentiated TGF-β1 transdifferentiation of pericytes into myofibroblasts in vitro. Iqgap1 deficiency in HSCs promoted myofibroblast activation, tumor implantation, and metastatic growth in mice via upregulation of paracrine signaling molecules. Additionally, we found that IQGAP1 expression was downregulated in myofibroblasts associated with human colorectal liver metastases. Taken together, our studies demonstrate that IQGAP1 in the tumor microenvironment suppresses TβRII and TGF-β dependent myofibroblastic differentiation to constrain tumor growth., We thank Kah Whye Peng and Yasuhiro Ikeda for providing reagents. The authors also acknowledge grant K01 CA118722, a 2009 Research Early Career Development Award (Mayo Clinic),the P/F Award and Clinical Core (P30 NIDDK 84567), and grants R01 CA160069 to N. Kang, R01 DK059615 to V.H. Shah, and; R01 NS051746 to G.S. Bloom.

Background: The curative potential of allogeneic hematopoietic stem cell transplant (SCT) for acute myeloid leukemia (AML) depends in part on the graft-versus-leukemia (GVL) cellular immune response. Mechanisms of resistance to GVL and methods to overcome post-SCT relapse are a critical focus of research. AML blasts can inhibit activation and proliferation of immune cells in culture. We hypothesized that irradiating AML blasts would diminish their immune suppressive capacity while maintaining antigen presentation, leading to higher activation of CD8+ T cells among peripheral blood mononuclear cells (PBMC) in co-culture. Furthermore, we investigated the capacity of live and irradiated AML blasts to induce expression of immune checkpoints on CD8+ T cells in co-culture, focusing on Lymphocyte-activation gene 3 (LAG3) which interacts with class II MHC. Methods: PBMC were isolated from healthy donors in compliance with an IRB-approved protocol. PBMC were co-cultured with live human AML K-1 cells (CRL-2724) and irradiated K-1 cells (40 Gy) at the following ratios: 1:1, 1:2, 1:4 and 1:8. Cells were cultured in RPMI complete media supplemented with 10% FBS and IL-2 20 IU/ml. On day 3 of co-culture, immunophenotypic characterization of T cells was performed on an Attune NxT flow cytometer using the following antibody markers: CD3, CD4, CD8, CD25, CD137, CD154, PD-1, TIM3, TIGIT, and LAG3. PBMC were fixed and permeabilized prior to intracellular staining of IFNg and FOXP3. Regulatory T cells (Tregs) were identified as CD4+ CD25+ FOXP3+. To correlate the expression of the genes encoding for IFNg, LAG3, PD1, and antigen presentation molecules in the tumor microenvironment, we analyzed RNA sequencing data from The Cancer Genome Atlas (NCI TCGA) AML database. Associations were determined by Spearman's correlation and K-means clustering. Results: PBMC co-cultured with irradiated AML K-1 showed significant higher IFNg expression (11.8% ± 3.1 v. 7% ± 3.3; n=7, P=0.012) and higher CD137 (4-1BB) expression (9.3% ± 1.21 v. 5.7% ± 3.4; n=7, P Conclusion: In our in vitro model, LAG3 upregulation correlates with decreased activation of CD8+ cells and higher Tregs when healthy donor PBMC are co-cultured with AML K-1 cells. Antibody-mediated blocking of LAG3 may potentially reverse the suppression of CD8+ T cells by AML K-1 cells and produce fewer Tregs. Bioinformatic analysis of TCGA database confirmed a positive correlation between LAG3 transcript levels and expression of both immune activation and antigen presentation genes. LAG3 transcript levels are higher in unfavorable-risk AML. Figure. Figure. Disclosures Lin: Jazz Pharmaceuticals: Honoraria.

e19004Background: High-dose chemotherapy and autologous hematopoietic cell transplantation (Auto-HCT) is associated with neutropenia, anemia and thrombocytopenia in the post-transplant period. The ...

Introduction: Transforming growth factor beta (TGF-β) within the tumor microenvironment induces transdifferentiation of quiescent pericytes and related stromal cells into myofibroblasts that promote tumor growth. However mechanisms governing TGF-β mediated myofibroblastic activation remain poorly understood. IQ motif containing GTPase activating protein 1 (IQGAP1) exerts diverse cellular functions, but its role in the tumor microenvironment is unexplored. In this study, we tested a hypothesis that IQGAP1 may interact with TGF-β receptor II (TβRII) and regulate its signaling in mesenchymal-type cells that activate into tumor associated myofibroblasts such as hepatic stellate cells (HSCs), which are resident liver pericytes, and that IQGAP1 of HSCs regulates liver metastatic growth. Experimental procedures: Retroviral and lentiviral based vectors were used to deliver TβRII, IQGAP1 or IQGAP1 shRNA into human primary HSCs. IQGAP1/TβRII interactions were determined by immunofluorescence staining, immunoprecipitation, in vitro GST pull down and in vitro protein binding assays. TGF-β1 activation of HSCs into myofibroblasts was determined by α-SMA immunofluorescence and Western Blot analyses for α-SMA, fibronectin and P-Smad2. The role of stellate cell IQGAP1 in tumorigenesis was assessed by in vitro assays of conditioned media, a HSC/tumor cell co-implantation mouse model and experimental liver metastasis model performed on IQGAP1 knockout mice. Additionally, IQGAP1 protein levels in the myofibroblasts of 29 human colorectal liver metastases were investigated by immunofluorescence staining. Results: IQGAP1 is recruited to TβRII by TGF-β1 with the C-terminal a.a. 1503-1657 region of IQGAP1 mediating IQGAP1/TβRII binding. Through scaffolding the E3 ubiquitin ligase Smurf1 and TβRII, IQGAP1 promotes ubiquitination, lysosomal and proteasomal degradation of TβRII, thus suppressing TβRII and TGF-β1 activation of HSCs into myofibroblasts in vitro. In mice, co-implantation of IQGAP1 knockdown HSCs promotes myofibroblastic activation of HSCs, tumor implantation and growth. In an experimental liver metastasis model, IQGAP1 deficiency in the tumor microenvironment promotes myofibroblastic activation and liver metastatic growth. Indeed, IQGAP1 knockdown in HSCs upregulates paracrine signaling molecules such as stromal derived factor 1 (SDF-1/CXCL12) and hepatic growth factor (HGF) that confer a stimulatory effect on proliferation, migration and survival of tumor cells. Additionally, we found that 24 out of 29 colorectal cancer patients display varying degrees of reduction of IQGAP1 protein in the myofibroblasts of their liver metastases as compared to IQGAP1 expression levels observed in activated HSCs and portal myofibroblasts of the adjacent non-tumorous control liver. Conclusion: Our studies demonstrate that IQGAP1 in the tumor microenvironment suppresses TβRII and the TGFβ1 dependent myofibroblastic activation switch with these events constraining tumor growth and metastasis. Citation Format: Chunsheng Liu, Haitham Abdelhakim, Vijay H. Shah, Ningling Kang. IQGAP1 in the tumor microenvironment suppresses TGF-beta mediated myofibroblastic activation and ensuing tumor growth. [abstract]. In: Proceedings of the AACR Special Conference on Tumor Invasion and Metastasis; Jan 20-23, 2013; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2013;73(3 Suppl):Abstract nr B19.