Clinical Instructor, Medicine - Hematology
Honors & Awards
Trainee Research Award, American Society of Hematology (2005)
MacKenzie Foundation Award, MacKenzie Foundation (2006-2009)
Medical Student Research Training Fellowship, Howard Hughes Medical Institute (2007-2009)
First Place Award, Medical Student Research Symposium, Stanford University (2008)
Keystone Symposia Scholarship, Keystone Symposia (2008)
Medical Student Summer Fellowship, American Brain Tumor Association (2009)
Graduate Student Scholar, Genentech Scholars Program (2009)
Merck Scholar-in-Training Award, American Association for Cancer Research (2010)
M.D., Stanford University School of Medicine, Medicine (2011)
Ph.D., Stanford University, Cancer Biology (2010)
Current Research and Scholarly Interests
I am interested in the characterization and therapeutic targeting of cancer stem cells in human malignancies. Specifically I have been investigating antibody-specific approaches to therapeutically target leukemic stem cells in acute myeloid leukemia as well as targeting of cancer cells in other hematologic malignancies. This work has led to the identification and development of a monoclonal antibody targeting the anti-phagocytic signal CD47 for the treatment of human malignancies.
Prospective separation of normal and leukemic stem cells based on differential expression of TIM3, a human acute myeloid leukemia stem cell marker
PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA
2011; 108 (12): 5009-5014
Hematopoietic tissues in acute myeloid leukemia (AML) patients contain both leukemia stem cells (LSC) and residual normal hematopoietic stem cells (HSC). The ability to prospectively separate residual HSC from LSC would enable important scientific and clinical investigation including the possibility of purged autologous hematopoietic cell transplants. We report here the identification of TIM3 as an AML stem cell surface marker more highly expressed on multiple specimens of AML LSC than on normal bone marrow HSC. TIM3 expression was detected in all cytogenetic subgroups of AML, but was significantly higher in AML-associated with core binding factor translocations or mutations in CEBPA. By assessing engraftment in NOD/SCID/IL2R?-null mice, we determined that HSC function resides predominantly in the TIM3-negative fraction of normal bone marrow, whereas LSC function from multiple AML specimens resides predominantly in the TIM3-positive compartment. Significantly, differential TIM3 expression enabled the prospective separation of HSC from LSC in the majority of AML specimens with detectable residual HSC function.
View details for DOI 10.1073/pnas.1100551108
View details for Web of Science ID 000288712200061
View details for PubMedID 21383193
CD137 stimulation enhances the antilymphoma activity of anti-CD20 antibodies
2011; 117 (8): 2423-2432
Antibody-dependent cell-mediated cytotoxicity (ADCC), which is largely mediated by natural killer (NK) cells, is thought to play an important role in the efficacy of rituximab, an anti-CD20 monoclonal antibody (mAb) used to treat patients with B-cell lymphomas. CD137 is a costimulatory molecule expressed on a variety of immune cells after activation, including NK cells. In the present study, we show that an anti-CD137 agonistic mAb enhances the antilymphoma activity of rituximab by enhancing ADCC. Human NK cells up-regulate CD137 after encountering rituximab-coated tumor B cells, and subsequent stimulation of these NK cells with anti-CD137 mAb enhances rituximab-dependent cytotoxicity against the lymphoma cells. In a syngeneic murine lymphoma model and in a xenotransplanted human lymphoma model, sequential administration of anti-CD20 mAb followed by anti-CD137 mAb had potent antilymphoma activity in vivo. These results support a novel, sequential antibody approach against B-cell malignancies by targeting first the tumor and then the host immune system.
View details for DOI 10.1182/blood-2010-08-301945
View details for Web of Science ID 000287698800023
View details for PubMedID 21193697
Therapeutic Antibody Targeting of CD47 Eliminates Human Acute Lymphoblastic Leukemia
2011; 71 (4): 1374-1384
Acute lymphoblastic leukemia (ALL) is the most common pediatric malignancy and constitutes 15% of adult leukemias. Although overall prognosis for pediatric ALL is favorable, high-risk pediatric patients and most adult patients have significantly worse outcomes. Multiagent chemotherapy is standard of care for both pediatric and adult ALL, but is associated with systemic toxicity and long-term side effects and is relatively ineffective against certain ALL subtypes. Recent efforts have focused on the development of targeted therapies for ALL including monoclonal antibodies. Here, we report the identification of CD47, a protein that inhibits phagocytosis, as an antibody target in standard and high-risk ALL. CD47 was found to be more highly expressed on a subset of human ALL patient samples compared with normal cell counterparts and to be an independent predictor of survival and disease refractoriness in several ALL patient cohorts. In addition, a blocking monoclonal antibody against CD47 enabled phagocytosis of ALL cells by macrophages in vitro and inhibited tumor engraftment in vivo. Significantly, anti-CD47 antibody eliminated ALL in the peripheral blood, bone marrow, spleen, and liver of mice engrafted with primary human ALL. These data provide preclinical support for the development of an anti-CD47 antibody therapy for treatment of human ALL.
View details for DOI 10.1158/0008-5472.CAN-10-2238
View details for Web of Science ID 000287352600020
View details for PubMedID 21177380
Calreticulin Is the Dominant Pro-Phagocytic Signal on Multiple Human Cancers and Is Counterbalanced by CD47
SCIENCE TRANSLATIONAL MEDICINE
2010; 2 (63)
Under normal physiological conditions, cellular homeostasis is partly regulated by a balance of pro- and anti-phagocytic signals. CD47, which prevents cancer cell phagocytosis by the innate immune system, is highly expressed on several human cancers including acute myeloid leukemia, non-Hodgkin's lymphoma, and bladder cancer. Blocking CD47 with a monoclonal antibody results in phagocytosis of cancer cells and leads to in vivo tumor elimination, yet normal cells remain mostly unaffected. Thus, we postulated that cancer cells must also display a potent pro-phagocytic signal. Here, we identified calreticulin as a pro-phagocytic signal that was highly expressed on the surface of several human cancers, but was minimally expressed on most normal cells. Increased CD47 expression correlated with high amounts of calreticulin on cancer cells and was necessary for protection from calreticulin-mediated phagocytosis. Blocking the interaction of target cell calreticulin with its receptor, low-density lipoprotein receptor-related protein, on phagocytic cells prevented anti-CD47 antibody-mediated phagocytosis. Furthermore, increased calreticulin expression was an adverse prognostic factor in diverse tumors including neuroblastoma, bladder cancer, and non-Hodgkin's lymphoma. These findings identify calreticulin as the dominant pro-phagocytic signal on several human cancers, provide an explanation for the selective targeting of tumor cells by anti-CD47 antibody, and highlight the balance between pro- and anti-phagocytic signals in the immune evasion of cancer.
View details for DOI 10.1126/scitranslmed.3001375
View details for Web of Science ID 000288444900003
View details for PubMedID 21178137
Anti-CD47 Antibody Synergizes with Rituximab to Promote Phagocytosis and Eradicate Non-Hodgkin Lymphoma
2010; 142 (5): 699-713
Monoclonal antibodies are standard therapeutics for several cancers including the anti-CD20 antibody rituximab for B cell non-Hodgkin lymphoma (NHL). Rituximab and other antibodies are not curative and must be combined with cytotoxic chemotherapy for clinical benefit. Here we report the eradication of human NHL solely with a monoclonal antibody therapy combining rituximab with a blocking anti-CD47 antibody. We identified increased expression of CD47 on human NHL cells and determined that higher CD47 expression independently predicted adverse clinical outcomes in multiple NHL subtypes. Blocking anti-CD47 antibodies preferentially enabled phagocytosis of NHL cells and synergized with rituximab. Treatment of human NHL-engrafted mice with anti-CD47 antibody reduced lymphoma burden and improved survival, while combination treatment with rituximab led to elimination of lymphoma and cure. These antibodies synergized through a mechanism combining Fc receptor (FcR)-dependent and FcR-independent stimulation of phagocytosis that might be applicable to many other cancers.
View details for DOI 10.1016/j.cell.2010.07.044
View details for Web of Science ID 000281523200014
View details for PubMedID 20813259
Macrophages as mediators of tumor immunosurveillance
TRENDS IN IMMUNOLOGY
2010; 31 (6): 212-219
Tumor immunosurveillance is a well-established mechanism for regulation of tumor growth. In this regard, most studies have focused on the role of T- and NK-cells as the critical immune effector cells. However, macrophages play a major role in the recognition and clearance of foreign, aged, and damaged cells. Macrophage phagocytosis is negatively regulated via the receptor SIRPalpha upon binding to CD47, a ubiquitously expressed protein. We recently showed that CD47 is up-regulated in myeloid leukemia and migrating hematopoietic progenitors, and that the level of protein expression correlates with the ability to evade phagocytosis. These results implicate macrophages in the immunosurveillance of hematopoietic cells and leukemias. The ability of macrophages to phagocytose tumor cells might be exploited therapeutically by blocking the CD47-SIRPalpha interaction.
View details for DOI 10.1016/j.it.2010.04.001
View details for Web of Science ID 000279427000002
View details for PubMedID 20452821
Identification, molecular characterization, clinical prognosis, and therapeutic targeting of human bladder tumor-initiating cells
PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA
2009; 106 (33): 14016-14021
Major clinical issues in bladder cancer include the identification of prediction markers and novel therapeutic targets for invasive bladder cancer. In the current study, we describe the isolation and characterization of a tumor-initiating cell (T-IC) subpopulation in primary human bladder cancer, based on the expression of markers similar to that of normal bladder basal cells (Lineage-CD44(+)CK5(+)CK20(-)). The bladder T-IC subpopulation was defined functionally by its enriched ability to induce xenograft tumors in vivo that recapitulated the heterogeneity of the original tumor. Further, molecular analysis of more than 300 bladder cancer specimens revealed heterogeneity among activated oncogenic pathways in T-IC (e.g., 80% Gli1, 45% Stat3, 10% Bmi-1, and 5% beta-catenin). Despite this molecular heterogeneity, we identified a unique bladder T-IC gene signature by gene chip analysis. This T-IC gene signature, which effectively distinguishes muscle-invasive bladder cancer with worse clinical prognosis from non-muscle-invasive (superficial) cancer, has significant clinical value. It also can predict the progression of a subset of recurring non-muscle-invasive cancers. Finally, we found that CD47, a protein that provides an inhibitory signal for macrophage phagocytosis, is highly expressed in bladder T-ICs compared with the rest of the tumor. Blockade of CD47 by a mAb resulted in macrophage engulfment of bladder cancer cells in vitro. In summary, we have identified a T-IC subpopulation with potential prognostic and therapeutic value for invasive bladder cancer.
View details for DOI 10.1073/pnas.0906549106
View details for Web of Science ID 000269078700071
View details for PubMedID 19666525
CD47 Is an Adverse Prognostic Factor and Therapeutic Antibody Target on Human Acute Myeloid Leukemia Stem Cells
2009; 138 (2): 286-299
Acute myeloid leukemia (AML) is organized as a cellular hierarchy initiated and maintained by a subset of self-renewing leukemia stem cells (LSC). We hypothesized that increased CD47 expression on human AML LSC contributes to pathogenesis by inhibiting their phagocytosis through the interaction of CD47 with an inhibitory receptor on phagocytes. We found that CD47 was more highly expressed on AML LSC than their normal counterparts, and that increased CD47 expression predicted worse overall survival in three independent cohorts of adult AML patients. Furthermore, blocking monoclonal antibodies directed against CD47 preferentially enabled phagocytosis of AML LSC and inhibited their engraftment in vivo. Finally, treatment of human AML LSC-engrafted mice with anti-CD47 antibody depleted AML and targeted AML LSC. In summary, increased CD47 expression is an independent, poor prognostic factor that can be targeted on human AML stem cells with blocking monoclonal antibodies capable of enabling phagocytosis of LSC.
View details for DOI 10.1016/j.cell.2009.05.045
View details for Web of Science ID 000268277000011
View details for PubMedID 19632179
CD47 Is Upregulated on Circulating Hematopoietic Stem Cells and Leukemia Cells to Avoid Phagocytosis
2009; 138 (2): 271-285
Macrophages clear pathogens and damaged or aged cells from the blood stream via phagocytosis. Cell-surface CD47 interacts with its receptor on macrophages, SIRPalpha, to inhibit phagocytosis of normal, healthy cells. We find that mobilizing cytokines and inflammatory stimuli cause CD47 to be transiently upregulated on mouse hematopoietic stem cells (HSCs) and progenitors just prior to and during their migratory phase, and that the level of CD47 on these cells determines the probability that they are engulfed in vivo. CD47 is also constitutively upregulated on mouse and human myeloid leukemias, and overexpression of CD47 on a myeloid leukemia line increases its pathogenicity by allowing it to evade phagocytosis. We conclude that CD47 upregulation is an important mechanism that provides protection to normal HSCs during inflammation-mediated mobilization, and that leukemic progenitors co-opt this ability in order to evade macrophage killing.
View details for DOI 10.1016/j.cell.2009.05.046
View details for Web of Science ID 000268277000010
View details for PubMedID 19632178
Establishment of a Normal Hematopoietic and Leukemia Stem Cell Hierarchy
CONTROL AND REGULATION OF STEM CELLS
2008; 73: 439-449
Many types of adult tissues, especially for high turnover tissues such as the blood and intestinal system, stand on a hierarchical tissue-specific stem cell system. Tissue-specific stem cells concurrently have self-renewal capacity and potential to give rise to all types of mature cells in their tissue. The differentiation process of the tissue-specific stem cell is successive restriction of these capacities. The first progeny of tissue-specific stem cells are multipotent progenitors (MPPs) that lose long-term self-renewal capacity yet have full lineage potential. MPPs in turn give rise to oligopotent progenitors, which then commit into lineage-restricted progenitors. This hierarchical system enables a lifelong supply of matured functional cells that generally have a short life span and a relatively high turnover rate. In this chapter, we review our findings and other key experiments that have led to the establishment of the current cellular stem and progenitor hierarchy in the blood-forming systems of mice and humans for both normal and leukemic hematopoiesis. We also review select signaling pathways intrinsic to normal hematopoietic and leukemic stem cell populations as well our recent findings elucidating the possible origin of the leukemia stem cell.
View details for Web of Science ID 000267135700050
View details for PubMedID 19022770
Distribution, levels and phosphorylation of Raf-1 in Alzheimer's disease
JOURNAL OF NEUROCHEMISTRY
2006; 99 (5): 1377-1388
Extracellular signal-regulated kinase (ERK), a member of the mitogen-activated protein kinase pathway, has been increasingly implicated in the pathogenesis of Alzheimer's disease due to its critical role in brain function. While we previously demonstrated that ERK is activated in Alzheimer's disease, the upstream cascade leading to its activation had not been fully examined. In this study, we focused on Raf-1, one of the physiological activators of the ERK pathway. Raf-1 is activated by phosphorylation at Ser338 and Tyr340/341 and inhibited by phosphorylation at Ser259. Interestingly, phosphorylation at all three sites on Raf-1 was increased as evidenced by both immunocytochemistry and immunoblot analysis in Alzheimer's disease brains compared to age-matched controls. Both phospho-Raf-1 (Ser259) and phospho-Raf-1 (Ser338) were localized to intracytoplasmic granular structures, whereas phospho-Raf-1 (Tyr340/341) was localized to neurofibrillary tangles and granules in pyramidal neurons in Alzheimer's disease hippocampus. There is extensive overlap between phospho-Raf-1 (Ser338) and phospho-Mek1/2, the downstream effector of Raf-1, suggestive of a mechanistic link. Additionally, increased levels of Raf-1 are associated with Ras and MEK1 in Alzheimer's disease as evidenced by its coimmunoprecipitation with Ras and Mek1, respectively. Based on these findings, we speculate that Raf-1 is activated to effectively mediate Ras-dependent signals in Alzheimer's disease.
View details for DOI 10.1111/j.1471-4159.2006.04174.x
View details for Web of Science ID 000241951300006
View details for PubMedID 17064357
The JAK2 V617F mutation occurs in hematopoietic stem cells in polycythemia vera and predisposes toward erythroid differentiation
PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA
2006; 103 (16): 6224-6229
Although a large proportion of patients with polycythemia vera (PV) harbor a valine-to-phenylalanine mutation at amino acid 617 (V617F) in the JAK2 signaling molecule, the stage of hematopoiesis at which the mutation arises is unknown. Here we isolated and characterized hematopoietic stem cells (HSC) and myeloid progenitors from 16 PV patient samples and 14 normal individuals, testing whether the JAK2 mutation could be found at the level of stem or progenitor cells and whether the JAK2 V617F-positive cells had altered differentiation potential. In all PV samples analyzed, there were increased numbers of cells with a HSC phenotype (CD34+CD38-CD90+Lin-) compared with normal samples. Hematopoietic progenitor assays demonstrated that the differentiation potential of PV was already skewed toward the erythroid lineage at the HSC level. The JAK2 V617F mutation was detectable within HSC and their progeny in PV. Moreover, the aberrant erythroid potential of PV HSC was potently inhibited with a JAK2 inhibitor, AG490.
View details for DOI 10.1073/pnas.0601462103
View details for Web of Science ID 000236999000031
View details for PubMedID 16603627
Heme oxygenase in Candida albicans is regulated by hemoglobin and is necessary for metabolism of exogenous heme and hemoglobin to alpha-biliverdin
JOURNAL OF BIOLOGICAL CHEMISTRY
2004; 279 (5): 3426-3433
Candida albicans is an opportunistic pathogen that has adapted uniquely to life in mammalian hosts. One of the host factors recognized by this yeast is hemoglobin, which binds to a specific cell surface receptor. In addition to its regulating the expression of adhesion receptors on the yeast, we have found that hemoglobin induces the expression of a C. albicans heme oxygenase (CaHmx1p). Hemoglobin transcriptionally induces the CaHMX1 gene independent of the presence of inorganic iron in the medium. A Renilla luciferase reporter driven by the CaHMX1 promoter demonstrated rapid activation of transcription by hemoglobin and (cobalt protoporphyrin IX) globin but not by apoglobin or other proteins. In contrast, iron deficiency or exogenous hemin did not activate the reporter until after 3 h, suggesting that induction of the promoter by hemoglobin is mediated by receptor signaling rather than heme or iron flux into the cell. As observed following disruption of the Saccharomyces cerevisiae ortholog, HMX1, a CaHMX1 null mutant was unable to grow under iron restriction. This suggests a role for CaHmx1p in inorganic iron acquisition. CaHMX1 encodes a functional heme oxygenase. Exogenous heme or hemoglobin is exclusively metabolized to alpha-biliverdin. CaHMX1 is required for utilization of these exogenous substrates, indicating that C. albicans heme oxygenase confers a nutritional advantage for growth in mammalian hosts.
View details for DOI 10.1074/jbc.M311550200
View details for Web of Science ID 000188379600037
View details for PubMedID 14615478
Oxidative stress and neuronal adaptation in Alzheimer disease: The role of SAPK pathways
ANTIOXIDANTS & REDOX SIGNALING
2003; 5 (5): 571-576
Recent evidence indicates that oxidative stress occurs early in the progression of Alzheimer disease, significantly before the development of the hallmark pathologies, namely neurofibrillary tangles and senile plaques. The interaction of abnormal mitochondria, redox transition metals, and oxidative stress response elements contributes to the generation of reactive oxygen species in diseased neurons. Oxidative damage to major cellular molecules is seen in a number of disease states that are either acute or chronic and it is apparent that without eliciting compensations that restore redox balance, cells will rapidly succumb to death. Indeed, although oxidative stress is a prominent feature in Alzheimer disease, few vulnerable neurons show clear signs of apoptosis, suggesting that the level of oxidative stress does not significantly exceed neuronal oxidative defenses. In light of this observation, we propose that neurons in Alzheimer disease are exposed to low, but chronic, levels of oxidative stress that lead neurons to elicit adaptive responses such as the activation of stress-activated protein kinase pathways.
View details for Web of Science ID 000186101200009
View details for PubMedID 14580312