Professional Education


  • Bachelor of Science, University of California Davis (2009)
  • Doctor of Philosophy, University of California Los Angeles (2014)

Stanford Advisors


All Publications


  • Anti-CD47 Treatment Stimulates Phagocytosis of Glioblastoma by M1 and M2 Polarized Macrophages and Promotes M1 Polarized Macrophages In Vivo PLOS ONE Zhang, M., Hutter, G., Kahn, S. A., Azad, T. D., Gholamin, S., Xu, C. Y., Liu, J., Achrol, A. S., Richard, C., Sommerkamp, P., Schoen, M. K., McCracken, M. N., Majeti, R., Weissman, I., Mitra, S. S., Cheshier, S. H. 2016; 11 (4)

    Abstract

    Tumor-associated macrophages (TAMs) represent an important cellular subset within the glioblastoma (WHO grade IV) microenvironment and are a potential therapeutic target. TAMs display a continuum of different polarization states between antitumorigenic M1 and protumorigenic M2 phenotypes, with a lower M1/M2 ratio correlating with worse prognosis. Here, we investigated the effect of macrophage polarization on anti-CD47 antibody-mediated phagocytosis of human glioblastoma cells in vitro, as well as the effect of anti-CD47 on the distribution of M1 versus M2 macrophages within human glioblastoma cells grown in mouse xenografts. Bone marrow-derived mouse macrophages and peripheral blood-derived human macrophages were polarized in vitro toward M1 or M2 phenotypes and verified by flow cytometry. Primary human glioblastoma cell lines were offered as targets to mouse and human M1 or M2 polarized macrophages in vitro. The addition of an anti-CD47 monoclonal antibody led to enhanced tumor-cell phagocytosis by mouse and human M1 and M2 macrophages. In both cases, the anti-CD47-induced phagocytosis by M1 was more prominent than that for M2. Dissected tumors from human glioblastoma xenografted within NOD.Cg-Prkdcscid Il2rgtm1Wjl/SzJ mice and treated with anti-CD47 showed a significant increase of M1 macrophages within the tumor. These data show that anti-CD47 treatment leads to enhanced tumor cell phagocytosis by both M1 and M2 macrophage subtypes with a higher phagocytosis rate by M1 macrophages. Furthermore, these data demonstrate that anti-CD47 treatment alone can shift the phenotype of macrophages toward the M1 subtype in vivo.

    View details for DOI 10.1371/journal.pone.0153550

    View details for Web of Science ID 000374541200027

    View details for PubMedID 27092773

  • Advances in PET Detection of the Antitumor T Cell Response. Advances in immunology McCracken, M. N., Tavaré, R., Witte, O. N., Wu, A. M. 2016; 131: 187-231

    Abstract

    Positron emission tomography (PET) is a powerful noninvasive imaging technique able to measure distinct biological processes in vivo by administration of a radiolabeled probe. Whole-body measurements track the probe accumulation providing a means to measure biological changes such as metabolism, cell location, or tumor burden. PET can also be applied to both preclinical and clinical studies providing three-dimensional information. For immunotherapies (in particular understanding T cell responses), PET can be utilized for spatial and longitudinal tracking of T lymphocytes. Although PET has been utilized clinically for over 30 years, the recent development of additional PET radiotracers have dramatically expanded the use of PET to detect endogenous or adoptively transferred T cells in vivo. Novel probes have identified changes in T cell quantity, location, and function. This has enabled investigators to track T cells outside of the circulation and in hematopoietic organs such as spleen, lymph nodes, and bone marrow, or within tumors. In this review, we cover advances in PET detection of the antitumor T cell response and areas of focus for future studies.

    View details for DOI 10.1016/bs.ai.2016.02.004

    View details for PubMedID 27235684

  • An Effective Immuno-PET Imaging Method to Monitor CD8-Dependent Responses to Immunotherapy CANCER RESEARCH Tavare, R., Escuin-Ordinas, H., Mok, S., McCracken, M. N., Zettlitz, K. A., Salazar, F. B., Witte, O. N., Ribas, A., Wu, A. M. 2016; 76 (1): 73-82

    Abstract

    The rapidly advancing field of cancer immunotherapy is currently limited by the scarcity of noninvasive and quantitative technologies capable of monitoring the presence and abundance of CD8(+) T cells and other immune cell subsets. In this study, we describe the generation of (89)Zr-desferrioxamine-labeled anti-CD8 cys-diabody ((89)Zr-malDFO-169 cDb) for noninvasive immuno-PET tracking of endogenous CD8(+) T cells. We demonstrate that anti-CD8 immuno-PET is a sensitive tool for detecting changes in systemic and tumor-infiltrating CD8 expression in preclinical syngeneic tumor immunotherapy models including antigen-specific adoptive T-cell transfer, agonistic antibody therapy (anti-CD137/4-1BB), and checkpoint blockade antibody therapy (anti-PD-L1). The ability of anti-CD8 immuno-PET to provide whole body information regarding therapy-induced alterations of this dynamic T-cell population provides new opportunities to evaluate antitumor immune responses of immunotherapies currently being evaluated in the clinic.

    View details for DOI 10.1158/0008-5472.CAN-15-1707

    View details for Web of Science ID 000367553900012

    View details for PubMedID 26573799

  • Engineering high-affinity PD-1 variants for optimized immunotherapy and immuno-PET imaging PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA Maute, R. L., Gordon, S. R., Mayer, A. T., McCracken, M. N., Natarajan, A., Ring, N. G., Kimura, R., Tsai, J. M., Manglik, A., Kruse, A. C., Gambhir, S. S., Weissman, I. L., Ring, A. M. 2015; 112 (47): E6506-E6514
  • Molecular Pathways: Activating T Cells after Cancer Cell Phagocytosis from Blockade of CD47 "Don't Eat Me" Signals CLINICAL CANCER RESEARCH McCracken, M. N., Cha, A. C., Weissman, I. L. 2015; 21 (16): 3597-3601

    Abstract

    Recent advances with immunotherapy agents for the treatment of cancer have provided remarkable, and in some cases, curative results. Our laboratory has identified CD47 as an important "don't eat me" signal expressed on malignant cells. Blockade of the CD47:SIRP-α axis between tumor cells and innate immune cells (monocytes, macrophages, and dendritic cells) increases tumor cell phagocytosis in both solid tumors (including, but not limited to, bladder, breast, colon, lung, and pancreatic) and hematologic malignancies. These phagocytic innate cells are also professional antigen-presenting cells (APC), providing a link from innate to adaptive antitumor immunity. Preliminary studies have demonstrated that APCs present antigens from phagocytosed tumor cells, causing T-cell activation. Therefore, agents that block the CD47:SIRP-α engagement are attractive therapeutic targets as a monotherapy or in combination with additional immune-modulating agents for activating antitumor T cells in vivo. Clin Cancer Res; 21(16); 3597-601. ©2015 AACR.

    View details for DOI 10.1158/1078-0432.CCR-14-2520

    View details for Web of Science ID 000361909100007

  • Noninvasive detection of tumor-infiltrating T cells by PET reporter imaging. journal of clinical investigation McCracken, M. N., Vatakis, D. N., Dixit, D., McLaughlin, J., Zack, J. A., Witte, O. N. 2015; 125 (5): 1815-1826

    Abstract

    Adoptive transfer of tumor-reactive T cells can successfully reduce tumor burden; however, in rare cases, lethal on-target/off-tumor effects have been reported. A noninvasive method to track engineered cells with high sensitivity and resolution would allow observation of correct cell homing and/or identification of dangerous off-target locations in preclinical and clinical applications. Human deoxycytidine kinase triple mutant (hdCK3mut) is a nonimmunogenic PET reporter that was previously shown to be an effective tool to monitor whole-body hematopoiesis. Here, we engineered a construct in which hdCK3mut is coexpressed with the anti-melanoma T cell receptor F5, introduced this construct into human CD34 cells or PBMCs, and evaluated this approach in multiple immunotherapy models. Expression of hdCK3mut allowed engrafted cells to be visualized within recipient bone marrow, while accumulation of [18F]-L-FMAU in hdCK3mut-expressing T cells permitted detection of intratumoral homing. Animals that received T cells coexpressing hdCK3mut and the anti-melanoma T cell receptor had demonstrably higher signals in HLA-matched tumors compared with those in animals that received cells solely expressing hdCK3mut. Engineered T cells caused cytotoxicity in HLA/antigen-matched tumors and induced IFN-γ production and activation. Moreover, hdCK3mut permitted simultaneous monitoring of engraftment and tumor infiltration, without affecting T cell function. Our findings suggest that hdCK3mut reporter imaging can be applied in clinical immunotherapies for whole-body detection of engineered cell locations.

    View details for DOI 10.1172/JCI77326

    View details for PubMedID 25822024

  • ImmunoPET of murine T cell reconstitution post-adoptive stem cell transplant using anti-CD4 and anti-CD8 cys-diabodies. Journal of nuclear medicine : official publication, Society of Nuclear Medicine Tavare, R., McCracken, M. N., Zettlitz, K. A., Salazar, F. B., Olafsen, T., Witte, O. N., Wu, A. M. 2015

    Abstract

    The proliferation and trafficking of T lymphocytes in immune responses are crucial events in determining inflammatory responses. To study whole body T lymphocyte dynamics non-invasively in vivo, we have generated anti-CD4 and -CD8 cys-diabodies (cDbs) derived from the parental antibody hybridomas GK1.5 and 2.43, respectively, for (89)Zr-immunoPET detection of helper and cytotoxic T cell populations.Anti-CD4 and -CD8 cys-diabodies were engineered, produced via mammalian expression, purified using immobilized metal affinity chromatography, and characterized for T cell binding. The cys-diabodies were site-specifically conjugated to maleimide-desferrioxamine for (89)Zr radiolabeling and subsequent microPET/CT acquisition and ex vivo biodistribution in both wild type mice and a model of hematopoietic stem cell (HSC) transplantation.ImmunoPET and biodistribution studies demonstrate targeting and visualization of CD4 and CD8 T cell populations in vivo in the spleen and lymph nodes of wild type mice, with specificity confirmed through in vivo blocking and depletion studies. Subsequently, a murine model of HSC transplantation demonstrated successful in vivo detection of T cell repopulation at 2, 4, and 8 weeks post-HSC transplant using the (89)Zr-radiolabeled anti-CD4 and -CD8 cDbs.These newly developed anti-CD4 and -CD8 immunoPET reagents represent a powerful resource to monitor T cell expansion, localization and novel engraftment protocols. Future potential applications of T cell targeted immunoPET include monitoring immune cell subsets in response to immunotherapy, autoimmunity, and lymphoproliferative disorders, contributing overall to preclinical immune cell monitoring.

    View details for DOI 10.2967/jnumed.114.153338

    View details for PubMedID 25952734

  • Targeted noninvasive imaging of the innate immune response. Proceedings of the National Academy of Sciences of the United States of America McCracken, M. N., Radu, C. G. 2015; 112 (19): 5868-9

    View details for DOI 10.1073/pnas.1505899112

    View details for PubMedID 25934920

  • Deoxycytidine Kinase Augments ATM-Mediated DNA Repair and Contributes to Radiation Resistance PLOS ONE Bunimovich, Y. L., Nair-Gill, E., Riedinger, M., McCracken, M. N., Cheng, D., McLaughlin, J., Radu, C. G., Witte, O. N. 2014; 9 (8)

    Abstract

    Efficient and adequate generation of deoxyribonucleotides is critical to successful DNA repair. We show that ataxia telangiectasia mutated (ATM) integrates the DNA damage response with DNA metabolism by regulating the salvage of deoxyribonucleosides. Specifically, ATM phosphorylates and activates deoxycytidine kinase (dCK) at serine 74 in response to ionizing radiation (IR). Activation of dCK shifts its substrate specificity toward deoxycytidine, increases intracellular dCTP pools post IR, and enhances the rate of DNA repair. Mutation of a single serine 74 residue has profound effects on murine T and B lymphocyte development, suggesting that post-translational regulation of dCK may be important in maintaining genomic stability during hematopoiesis. Using [(18)F]-FAC, a dCK-specific positron emission tomography (PET) probe, we visualized and quantified dCK activation in tumor xenografts after IR, indicating that dCK activation could serve as a biomarker for ATM function and DNA damage response in vivo. In addition, dCK-deficient leukemia cell lines and murine embryonic fibroblasts exhibited increased sensitivity to IR, indicating that pharmacologic inhibition of dCK may be an effective radiosensitization strategy.

    View details for DOI 10.1371/journal.pone.0104125

    View details for Web of Science ID 000339993900042

    View details for PubMedID 25101980

  • Positron emission tomography probe demonstrates a striking concentration of ribose salvage in the liver PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA Clark, P. M., Flores, G., Evdokimov, N. M., McCracken, M. N., Chai, T., Nair-Gill, E., O'Mahony, F., Beaven, S. W., Faull, K. F., Phelps, M. E., Jung, M. E., Witte, O. N. 2014; 111 (28): E2866-E2874

    Abstract

    PET is a powerful technique for quantifying and visualizing biochemical pathways in vivo. Here, we develop and validate a novel PET probe, [(18)F]-2-deoxy-2-fluoroarabinose ([(18)F]DFA), for in vivo imaging of ribose salvage. DFA mimics ribose in vivo and accumulates in cells following phosphorylation by ribokinase and further metabolism by transketolase. We use [(18)F]DFA to show that ribose preferentially accumulates in the liver, suggesting a striking tissue specificity for ribose metabolism. We demonstrate that solute carrier family 2, member 2 (also known as GLUT2), a glucose transporter expressed in the liver, is one ribose transporter, but we do not know if others exist. [(18)F]DFA accumulation is attenuated in several mouse models of metabolic syndrome, suggesting an association between ribose salvage and glucose and lipid metabolism. These results describe a tool for studying ribose salvage and suggest that plasma ribose is preferentially metabolized in the liver.

    View details for DOI 10.1073/pnas.1410326111

    View details for Web of Science ID 000338985700009

    View details for PubMedID 24982199

  • Engineered antibody fragments for immuno-PET imaging of endogenous CD8(+) T cells in vivo PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA Tavare, R., McCracken, M. N., Zettlitz, K. A., Knowles, S. M., Salazar, F. B., Olafsen, T., Witte, O. N., Wu, A. M. 2014; 111 (3): 1108-1113

    Abstract

    The noninvasive detection and quantification of CD8(+) T cells in vivo are important for both the detection and staging of CD8(+) lymphomas and for the monitoring of successful cancer immunotherapies, such as adoptive cell transfer and antibody-based immunotherapeutics. Here, antibody fragments are constructed to target murine CD8 to obtain rapid, high-contrast immuno-positron emission tomography (immuno-PET) images for the detection of CD8 expression in vivo. The variable regions of two anti-murine CD8-depleting antibodies (clones 2.43 and YTS169.4.2.1) were sequenced and reformatted into minibody (Mb) fragments (scFv-CH3). After production and purification, the Mbs retained their antigen specificity and bound primary CD8(+) T cells from the thymus, spleen, lymph nodes, and peripheral blood. Importantly, engineering of the parental antibodies into Mbs abolished the ability to deplete CD8(+) T cells in vivo. The Mbs were subsequently conjugated to S-2-(4-isothiocyanatobenzyl)-1,4,7-triazacyclononane-1,4,7-triacetic acid for (64)Cu radiolabeling. The radiotracers were injected i.v. into antigen-positive, antigen-negative, immunodeficient, antigen-blocked, and antigen-depleted mice to evaluate specificity of uptake in lymphoid tissues by immuno-PET imaging and ex vivo biodistribution. Both (64)Cu-radiolabeled Mbs produced high-contrast immuno-PET images 4 h postinjection and showed specific uptake in the spleen and lymph nodes of antigen-positive mice.

    View details for DOI 10.1073/pnas.1316922111

    View details for Web of Science ID 000329928400058

    View details for PubMedID 24390540

  • HSV-sr39TK positron emission tomography and suicide gene elimination of human hematopoietic stem cells and their progeny in humanized mice. Cancer research Gschweng, E. H., McCracken, M. N., Kaufman, M. L., Ho, M., Hollis, R. P., Wang, X., Saini, N., Koya, R. C., Chodon, T., Ribas, A., Witte, O. N., Kohn, D. B. 2014; 74 (18): 5173-83

    Abstract

    Engineering immunity against cancer by the adoptive transfer of hematopoietic stem cells (HSC) modified to express antigen-specific T-cell receptors (TCR) or chimeric antigen receptors generates a continual supply of effector T cells, potentially providing superior anticancer efficacy compared with the infusion of terminally differentiated T cells. Here, we demonstrate the in vivo generation of functional effector T cells from CD34-enriched human peripheral blood stem cells modified with a lentiviral vector designed for clinical use encoding a TCR recognizing the cancer/testes antigen NY-ESO-1, coexpressing the PET/suicide gene sr39TK. Ex vivo analysis of T cells showed antigen- and HLA-restricted effector function against melanoma. Robust engraftment of gene-modified human cells was demonstrated with PET reporter imaging in hematopoietic niches such as femurs, humeri, vertebrae, and the thymus. Safety was demonstrated by the in vivo ablation of PET signal, NY-ESO-1-TCR-bearing cells, and integrated lentiviral vector genomes upon treatment with ganciclovir, but not with vehicle control. Our study provides support for the efficacy and safety of gene-modified HSCs as a therapeutic modality for engineered cancer immunotherapy. Cancer Res; 74(18); 5173-83. ©2014 AACR.

    View details for DOI 10.1158/0008-5472.CAN-14-0376

    View details for PubMedID 25038231

  • Application of a Rapid, Simple, and Accurate Adenovirus-Based Method to Compare PET Reporter Gene/PET Reporter Probe Systems MOLECULAR IMAGING AND BIOLOGY Gil, J. S., Machado, H. B., Campbell, D. O., McCracken, M., Radu, C., Witte, O. N., Herschman, H. R. 2013; 15 (3): 273-281

    Abstract

    This study aims to use a simple, quantitative method to compare the HSV1sr39TK/(18) F-FHBG PET reporter gene/PET reporter probe (PRG/PRP) system with PRGs derived from human nucleoside kinases.The same adenovirus vector is used to express alternative PRGs. Equal numbers of vectors are injected intravenously into mice. After PRP imaging, quantitative hepatic PET signals are normalized for transduction by measuring hepatic viral genomes.The same adenovirus vector was used to express equivalent amounts of HSV1sr39TK, mutant human thymidine kinase 2 (TK2-DM), and mutant human deoxycytidine kinase (dCK-A100VTM) in mouse liver. HSV1sr39TK expression was measured with (18) F-FHBG, TK2-DM and dCK-A100VTM with (18) F-L-FMAU. TK2-DM/(18) F-L-FMAU and HSV1sr39TK/(18) F-FHBG had equivalent sensitivities; dCK-A100VTM/(18) F-L-FMAU was twice as sensitive as HSV1sr39TK/(18) F-FHBG.The human PRG/PRP sensitivities are comparable and/or better than HSV1sr39TK/(18) F-FHBG. However, for clinical use, identification of the best PRP substrate for each enzyme, characterization of probe distribution, and consequences of overexpressing nucleoside kinases must be evaluated.

    View details for DOI 10.1007/s11307-012-0596-5

    View details for Web of Science ID 000318794800005

    View details for PubMedID 23054556

  • Long-term in vivo monitoring of mouse and human hematopoietic stem cell engraftment with a human positron emission tomography reporter gene PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA McCracken, M. N., Gschweng, E. H., Nair-Gill, E., McLaughlin, J., Cooper, A. R., Riedinger, M., Cheng, D., Nosala, C., Kohn, D. B., Witte, O. N. 2013; 110 (5): 1857-1862

    Abstract

    Positron emission tomography (PET) reporter genes allow noninvasive whole-body imaging of transplanted cells by detection with radiolabeled probes. We used a human deoxycytidine kinase containing three amino acid substitutions within the active site (hdCK3mut) as a reporter gene in combination with the PET probe [(18)F]-L-FMAU (1-(2-deoxy-2-(18)fluoro-β-L-arabinofuranosyl)-5-methyluracil) to monitor models of mouse and human hematopoietic stem cell (HSC) transplantation. These mutations in hdCK3mut expanded the substrate capacity allowing for reporter-specific detection with a thymidine analog probe. Measurements of long-term engrafted cells (up to 32 wk) demonstrated that hdCK3mut expression is maintained in vivo with no counter selection against reporter-labeled cells. Reporter cells retained equivalent engraftment and differentiation capacity being detected in all major hematopoietic lineages and tissues. This reporter gene and probe should be applicable to noninvasively monitor therapeutic cell transplants in multiple tissues.

    View details for DOI 10.1073/pnas.1221840110

    View details for Web of Science ID 000314558100058

    View details for PubMedID 23319634

  • Oxidation of Free L-histidine by tert-Butylhydroperoxide PHARMACEUTICAL RESEARCH Mason, B. D., McCracken, M., Bures, E. J., Kerwin, B. A. 2010; 27 (3): 447-456

    Abstract

    L-histidine, a commonly used buffer for protein formulations, has the potential to oxidize and form multiple byproducts. Previous studies were performed using metal catalyzed oxidation with Fe(2+) or Cu(2+). We re-examined the oxidation of L-histidine under conditions more appropriate to protein formulations.Solutions of free L-histidine, protected from light, were initially reacted with tert-butylhydroperoxide and the products analyzed by UV absorption spectroscopy, reversed phase HPLC and mass spectrometric analysis and NMR. Experimental parameters investigated were oxidizing agent, pH, temperature, metal ion and metal chelator content.The initial reaction produced a number of known products, along with an unknown product that was identified as 4(5)-imidazolecarboxaldehyde. The reaction was highly pH and oxidizing-agent specific. The product was not observed at pH 5.0 or below, while there was a dramatic increase for reactions carried out at pH 6.0 or above. Addition of FeSO(4) to the reaction dramatically increased the amount of 4(5)-imidazolecarboxaldehyde produced, while addition of the metal chelators EDTA or DTPA completely inhibited the reaction.The presence of oxidants and trace concentrations of metal ions in high purity L-histidine solutions results in the formation of 4(5)-imidazolecarboxaldehyde which has the potential to covalently modify proteins.

    View details for DOI 10.1007/s11095-009-0032-y

    View details for Web of Science ID 000275123600007

    View details for PubMedID 20127149