Honors & Awards

  • Percy Cyril Garnham Award for best PhD student, London School of Hygiene and Tropical Medicine (2010)

Professional Education

  • Doctor of Philosophy, University Of London (2010)

Stanford Advisors

Research & Scholarship

Lab Affiliations


Journal Articles

  • Genetic and environmental determinants of human NK cell diversity revealed by mass cytometry. Science translational medicine Horowitz, A., Strauss-Albee, D. M., Leipold, M., Kubo, J., Nemat-Gorgani, N., Dogan, O. C., Dekker, C. L., Mackey, S., Maecker, H., Swan, G. E., Davis, M. M., Norman, P. J., Guethlein, L. A., Desai, M., Parham, P., Blish, C. A. 2013; 5 (208): 208ra145-?


    Natural killer (NK) cells play critical roles in immune defense and reproduction, yet remain the most poorly understood major lymphocyte population. Because their activation is controlled by a variety of combinatorially expressed activating and inhibitory receptors, NK cell diversity and function are closely linked. To provide an unprecedented understanding of NK cell repertoire diversity, we used mass cytometry to simultaneously analyze 37 parameters, including 28 NK cell receptors, on peripheral blood NK cells from 5 sets of monozygotic twins and 12 unrelated donors of defined human leukocyte antigen (HLA) and killer cell immunoglobulin-like receptor (KIR) genotype. This analysis revealed a remarkable degree of NK cell diversity, with an estimated 6000 to 30,000 phenotypic populations within an individual and >100,000 phenotypes in the donor panel. Genetics largely determined inhibitory receptor expression, whereas activation receptor expression was heavily environmentally influenced. Therefore, NK cells may maintain self-tolerance through strictly regulated expression of inhibitory receptors while using adaptable expression patterns of activating and costimulatory receptors to respond to pathogens and tumors. These findings further suggest the possibility that discrete NK cell subpopulations could be harnessed for immunotherapeutic strategies in the settings of infection, reproduction, and transplantation.

    View details for DOI 10.1126/scitranslmed.3006702

    View details for PubMedID 24154599

  • Activation of human NK cells by Plasmodium-infected red blood cells. Methods in molecular biology (Clifton, N.J.) Horowitz, A., Riley, E. M. 2013; 923: 447-464


    This chapter describes a protocol to assess activation of human NK cells following in vitro stimulation with malaria-infected red blood cells. Activation is assessed by flow cytometry, staining for cell surface expression of CD69 and accumulation of intracellular IFN-?. Procedures are described for in vitro propagation and purification of Plasmodium falciparum parasites, separation of peripheral blood mononuclear cells from heparinized blood by density centrifugation, in vitro culture of PBMC and for staining and analysis of PBMC by flow cytometry. Some examples of typical FACS plots are shown.

    View details for PubMedID 22990797

  • Antigen-Specific IL-2 Secretion Correlates with NK Cell Responses after Immunization of Tanzanian Children with the RTS,S/AS01 Malaria Vaccine JOURNAL OF IMMUNOLOGY Horowitz, A., Hafalla, J. C., King, E., Lusingu, J., Dekker, D., Leach, A., Moris, P., Cohen, J., Vekemans, J., Villafana, T., Corran, P. H., Bejon, P., Drakeley, C. J., von Seidlein, L., Riley, E. M. 2012; 188 (10): 5054-5062


    RTS,S/AS01, a vaccine targeting pre-erythrocytic stages of Plasmodium falciparum, is undergoing clinical trials. We report an analysis of cellular immune response to component Ags of RTS,S-hepatitis B surface Ag (HBs) and P. falciparum circumsporozoite (CS) protein-among Tanzanian children in a phase IIb RTS,S/AS01(E) trial. RTS,S/AS01 (E) vaccinees make stronger T cell IFN-?, CD69, and CD25 responses to HBs peptides than do controls, indicating that RTS,S boosts pre-existing HBs responses. T cell CD69 and CD25 responses to CS and CS-specific secreted IL-2 were augmented by RTS,S vaccination. Importantly, more than 50% of peptide-induced IFN-?(+) lymphocytes were NK cells, and the magnitude of the NK cell CD69 response to HBs peptides correlated with secreted IL-2 concentration. CD69 and CD25 expression and IL-2 secretion may represent sensitive markers of RTS,S-induced, CS-specific T cells. The potential for T cell-derived IL-2 to augment NK cell activation in RTS,S-vaccinated individuals, and the relevance of this for protection, needs to be explored further.

    View details for DOI 10.4049/jimmunol.1102710

    View details for Web of Science ID 000303634300038

    View details for PubMedID 22504653

  • A distinct subset of human NK cells expressing HLA-DR expand in response to IL-2 and can aid immune responses to BCG EUROPEAN JOURNAL OF IMMUNOLOGY Evans, J. H., Horowitz, A., Mehrabi, M., Wise, E. L., Pease, J. E., Riley, E. M., Davis, D. M. 2011; 41 (7): 1924-1933


    Subsets of NK cells can have distinct functions. Here, we report that >25% of human peripheral blood NK cells express HLA-DR after culture with IL-2. This can be driven by an expansion of a small subset of NK cells expressing HLA-DR, in contrast to previous assumptions that HLA-DR is upregulated on previously negative cells. HLA-DR-expressing NK cells showed enhanced degranulation to susceptible target cells and expressed chemokine receptor CXCR3, which facilitated their enrichment following exposure to CXCL11/I-TAC. Suggesting HLA-DR-expressing NK cells have an important role in an immune response, stimulation of PBMCs with Mycobacterium bovis BCG (BCG) triggered expansion of this subset. Importantly, the magnitude of an individual's NK cell IFN-? response triggered by BCG was associated with the initial frequency of HLA-DR-expressing NK cells in PBMCs. More directly indicating the importance of HLA-DR-expressing NK cells, enriching the frequency of this subset in PBMCs substantially augmented the IFN-? response to BCG. Thus, HLA-DR expression marks a distinct subset of NK cells, present at low frequency in circulating blood but readily expanded by IL-2, that can play an important role during immune responses to BCG.

    View details for DOI 10.1002/eji.201041180

    View details for Web of Science ID 000293131200013

    View details for PubMedID 21491418

  • Systemic effector and regulatory immune responses to chlamydial antigens in trachomatous trichiasis. Frontiers in microbiology Gall, A., Horowitz, A., Joof, H., Natividad, A., Tetteh, K., Riley, E., Bailey, R. L., Mabey, D. C., Holland, M. J. 2011; 2: 10-?


    Trachomatous trichiasis (TT) caused by repeated or chronic ocular infection with Chlamydia trachomatis is the result of a pro-fibrotic ocular immune response. At the conjunctiva, the increased expression of both inflammatory (IL1B, TNF) and regulatory cytokines (IL10) have been associated with adverse clinical outcomes. We measured in vitro immune responses of peripheral blood to a number of chlamydial antigens. Peripheral blood effector cells (CD4, CD69, IFN?, IL-10) and regulatory cells (CD4, CD25, FOXP3, CTLA4/GITR) were readily stimulated by C. trachomatis antigens but neither the magnitude (frequency or stimulation index) or the breadth and amount of cytokines produced in vitro [IL-5, IL-10, IL-12 (p70), IL-13, IFN?, and TNF?] were significantly different between TT cases and their non-diseased controls. Interestingly we observed that CD4+ T cells account for <50% of the IFN? positive cells induced following stimulation. Further investigation in individuals selected from communities where exposure to ocular infection with C. trachomatis is endemic indicated that CD3-CD56+ (classical natural killer cells) were a major early source of IFN? production in response to C. trachomatis elementary body stimulation and that the magnitude of this response increased with age. Future efforts to unravel the contribution of the adaptive immune response to conjunctival fibrosis should focus on the early events following infection and the interaction with innate immune mediated mechanisms of inflammation in the conjunctiva.

    View details for DOI 10.3389/fmicb.2011.00010

    View details for PubMedID 21747780

  • Activation of natural killer cells during microbial infections. Frontiers in immunology Horowitz, A., Stegmann, K. A., Riley, E. M. 2011; 2: 88-?


    Natural killer (NK) cells are large granular lymphocytes that express a diverse array of germline encoded inhibitory and activating receptors for MHC Class I and Class I-like molecules, classical co-stimulatory ligands, and cytokines. The ability of NK cells to be very rapidly activated by inflammatory cytokines, to secrete effector cytokines, and to kill infected or stressed host cells, suggests that they may be among the very early responders during infection. Recent studies have also identified a small number of pathogen-derived ligands that can bind to NK cell surface receptors and directly induce their activation. Here we review recent studies that have begun to elucidate the various pathways by which viral, bacterial, and parasite pathogens activate NK cells. We also consider two emerging themes of NK cell-pathogen interactions, namely their contribution to adaptive immune responses and their potential to take on regulatory and immunomodulatory functions.

    View details for DOI 10.3389/fimmu.2011.00088

    View details for PubMedID 22566877

  • NK Cells as Effectors of Acquired Immune Responses: Effector CD4(+) T Cell-Dependent Activation of NK Cells Following Vaccination JOURNAL OF IMMUNOLOGY Horowitz, A., Behrens, R. H., Okell, L., Fooks, A. R., Riley, E. M. 2010; 185 (5): 2808-2818


    We characterized vaccine-induced cellular responses to rabies virus in naive adult volunteers. Contrary to current paradigms, we observed potent and prolonged in vitro NK cell cytokine production and degranulation responses after restimulation of PBMCs with inactivated rabies virus in vaccinated, but not in unvaccinated, individuals. This "recall" NK cell response was absolutely dependent on Ag-specific IL-2 from CD45RO(+) CD4(+) T cells as well as IL-12 and IL-18 from accessory cells. Importantly, NK cells represented over 70% of all IFN-gamma-secreting and degranulating cells in the first 12-18 h after virus rechallenge indicating they may be required for rapid control of infection after vaccination. Activation of NK cells may be a critical function of IL-2-secreting effector memory T cells. Although IL-2-dependent postvaccination NK cell activation has been reported previously, this is the first time the magnitude of this effect and its contribution to the overall vaccine-induced response has been appreciated and the mechanisms of NK activation postvaccination have been elucidated. Our data will allow standard protocols for evaluating vaccine-induced immunity to be adapted to assess NK cell effector responses.

    View details for DOI 10.4049/jimmunol.1000844

    View details for Web of Science ID 000281122600023

    View details for PubMedID 20679529

  • Cross-Talk between T Cells and NK Cells Generates Rapid Effector Responses to Plasmodium falciparum-Infected Erythrocytes JOURNAL OF IMMUNOLOGY Horowitz, A., Newman, K. C., Evans, J. H., Korbel, D. S., Davis, D. M., Riley, E. M. 2010; 184 (11): 6043-6052


    Rapid cell-mediated immune responses, characterized by production of proinflammatory cytokines, such as IFN-gamma, can inhibit intraerythrocytic replication of malaria parasites and thereby prevent onset of clinical malaria. In this study, we have characterized the kinetics and cellular sources of the very early IFN-gamma response to Plasmodium falciparum-infected RBCs among human PBMCs. We find that NK cells dominate the early (12-18 h) IFN-gamma response, that NK cells and T cells contribute equally to the response at 24 h, and that T cells increasingly dominate the response from 48 h onward. We also find that although gammadelta T cells can produce IFN-gamma in response to P. falciparum-infected RBCs, they are greatly outnumbered by alphabeta T cells, and thus, the majority of the IFN-gamma(+) T cells are alphabeta T cells and not gammadelta T cells; gammadelta T cells are, however, an important source of TNF. We have previously shown that NK cell responses to P. falciparum-infected RBCs require cytokine and contact-dependent signals from myeloid accessory cells. In this study, we demonstrate that NK cell IFN-gamma responses to P. falciparum-infected RBCs are also crucially dependent on IL-2 secreted by CD4(+) T cells in an MHC class II-dependent manner, indicating that the innate response to infection actually relies upon complex interactions between NK cells, T cells, and accessory cells. We conclude that activation of NK cells may be a critical function of IL-2-secreting CD4(+) T cells and that standard protocols for evaluation of Ag-specific immune responses need to be adapted to include assessment of NK cell activation as well as T cell-derived IL-2.

    View details for DOI 10.4049/jimmunol.1000106

    View details for Web of Science ID 000278439600016

    View details for PubMedID 20427769

  • Activation of human NK cells by malaria-infected red blood cells. Methods in molecular biology (Clifton, N.J.) Horowitz, A., Riley, E. M. 2010; 612: 429-446


    This chapter describes a protocol to assess activation of human NK cells following in vitro stimulation with malaria-infected red blood cells. Activation is assessed by flow cytometry, staining for cell surface expression of CD69 and accumulation of intracellular IFN-gamma. Procedures are described for in vitro propagation and purification of Plasmodium falciparum parasites, separation of peripheral blood mononuclear cells from heparinised blood by density centrifugation, in vitro culture of PBMC and for staining and analysis of PBMC by flow cytometry. Some examples of typical FACS plots are shown.

    View details for DOI 10.1007/978-1-60761-362-6_29

    View details for PubMedID 20033658

  • Use of Immobilized HLA-A2:Ig Dimeric Proteins to Determine the Level of Epitope-Specific, HLA-Restricted CD8+ T-Cell Response SCANDINAVIAN JOURNAL OF IMMUNOLOGY Horowitz, A., Li, X., Poles, M. A., Tsuji, M. 2009; 70 (5): 415-422


    A novel assay to assess antigen-specific cytokine release from stimulated CD8(+) T cells derived from the mucosal and peripheral blood compartments has been developed and standardized using the influenza A virus matrix protein (MP) peptide, GILGFVFTL. This technology is based on the capacity for the human leucocyte antigen (HLA)-A2:Ig dimeric protein to stimulate CD8(+) T cells in a major histocompatibility complex (MHC) class I-restricted fashion without the necessity for antigen presenting cells (APC). This assay has been optimized utilizing a 9-amino acid residue (9mer) peptide, the optimal peptide length for presenting an epitope to CD8(+) T cells. Compared to existing assays, this more sensitive and specific methodology requires fewer cells, enabling easier and more accurate monitoring of the CD8(+) T-cell response in biological compartments, such as the mucosa during the course of viral infection and may be utilized to assess epitope-specific CD8(+) T-cell responses in vaccine trials.

    View details for DOI 10.1111/j.1365-3083.2009.02317.x

    View details for Web of Science ID 000271251500002

    View details for PubMedID 19874545

  • Killer Ig-Like Receptor (KIR) Genotype Predicts the Capacity of Human KIR-Positive CD56(dim) NK Cells to Respond to Pathogen-Associated Signals JOURNAL OF IMMUNOLOGY Korbel, D. S., Norman, P. J., Newman, K. C., Horowitz, A., Gendzekhadze, K., Parham, P., Riley, E. M. 2009; 182 (10): 6426-6434


    IFN-gamma emanating from NK cells is an important component of innate defense against infection. In this study, we demonstrate that, following in vitro stimulation of human peripheral blood NK cells with a variety of microbial ligands, CD56(dim) as well as CD56(bright) NK cells contribute to the overall NK cell IFN-gamma response with, for most cell donors, IFN-gamma(+) CD56(dim) NK cells outnumbering IFN-gamma(+) CD56(bright) NK cells. We also observe that the magnitude of the human NK IFN-gamma response to microbial ligands varies between individuals; that the antimicrobial response of CD56(bright), but not CD56(dim), NK cells is highly correlated with that of myeloid accessory cells; and that the ratio of IFN-gamma(+) CD56(dim) to IFN-gamma(+) CD56(bright) NK cells following microbial stimulation differs between individuals but remains constant for a given donor over time. Furthermore, ratios of IFN-gamma(+) CD56(dim) to IFN-gamma(+) CD56(bright) NK cells for different microbial stimuli are highly correlated and the relative response of CD56(dim) and CD56(bright) NK cells is highly significantly associated with killer Ig-like receptor (KIR) genotype. These data reveal an influence of KIR genotype, possibly mediated via NK cell education, on the ability of NK cells to respond to nonviral infections and have implications for genetic regulation of susceptibility to infection in humans.

    View details for DOI 10.4049/jimmunol.0804224

    View details for Web of Science ID 000265899800061

    View details for PubMedID 19414796

  • Function of NKT cells, potential anti-HIV effector cells, are improved by beginning HAART during acute HIV-1 infection INTERNATIONAL IMMUNOLOGY Vasan, S., Poles, M. A., Horowitz, A., Siladji, E. E., Markowitz, M., Tsuji, M. 2007; 19 (8): 943-951


    NKT cells are a subset of lymphocytes that share features of T cells and NK cells and bridge the innate and adaptive immune responses. They are able to be infected by HIV, but their function in HIV-infected individuals is not known. NKT cell percentage and function was measured in individuals with acute HIV infection before and 1 year into highly active anti-retroviral therapy (HAART). This study demonstrates that percentages of both CD161+ NKT cells and CD161+, CD4+ NKT cells decline within the first few months after HIV-1 infection, but initiating therapy during the acute infection period can prevent a further decline in these NKT cell subsets during the first year. NKT cell function is also impaired during early HIV infection, but significantly improved by effective treatment with HAART. Finally, preservation of NKT cell function may be important in HIV-infected individuals, as NKT cells display an anti-HIV-1 activity in vitro, mediated by IFN-gamma secretion.

    View details for DOI 10.1093/intimm/dxm055

    View details for Web of Science ID 000250681500005

    View details for PubMedID 17702988

  • Synthesis and human NKT cell stimulating properties of 3-O-sulfo-alpha/beta-galactosylceramides BIOORGANIC & MEDICINAL CHEMISTRY Xing, G. W., Wu, D., Poles, M. A., Horowitz, A., Tsuji, M., Ho, D. D., Wong, C. H. 2005; 13 (8): 2907-2916


    Two novel hybrid molecules 3-O-sulfo-alpha/beta-galactosylceramide 3 and 4, which are derived from an immunostimulatory agent alpha-GalCer 1 and self-glycolipid ligand sulfatide 2, were designed and synthesized. Compound 3 was shown to efficiently stimulate human NKT cells to secret IL-4 and IFN-gamma, with activities similar to 1, suggesting that modification of the 3''-OH position of the galactose moiety with sulfate has no significant effect on NKT cell stimulation. As a comparison, the beta-isomer 4 has no affinity to NKT cells, which demonstrates that the alpha-glycosidic bond of galactosylceramide is crucial to the NKT cells activation.

    View details for DOI 10.1016/j.bmc.2005.02.018

    View details for Web of Science ID 000228254000019

    View details for PubMedID 15781400

  • Infection with multidrug resistant, dual-tropic HIV-1 and rapid progression to AIDS: a case report LANCET Markowitz, M., Mohri, H., Mehandru, S., Shet, A., Berry, L., Kalyanaraman, R., Kim, A., Chung, C., Jean-Pierre, P., Horowitz, A., La Mar, M., Wrin, T., Parkin, N., Poles, M., Petropoulos, C., Mullen, M., Boden, D., Ho, D. D. 2005; 365 (9464): 1031-1038


    Rapid progression to AIDS after acute HIV-1 infection, though uncommon, has been noted, as has the transmission of multidrug resistant viruses. Here, we describe a patient in whom these two factors arose concomitantly and assess the effects.We did a case study of a patient with HIV-1 seroconversion. We genotyped the virus and host genetic markers by PCR and nucleotide sequencing. To ascertain the drug susceptibility of our patient's HIV-1 we did phenotypic studies with the PhenoSense assay. We assessed viral coreceptor use via syncytium formation in vitro and with a modified PhenoSense assay.Our patient seems to have been recently infected by a viral variant of HIV-1 resistant to multiple classes of antiretroviral drugs. Furthermore, his virus population is dual tropic for cells that express CCR5 or CXCR4 coreceptor. The infection has resulted in progression to symptomatic AIDS in 4-20 months.The intersection of multidrug resistance and rapid development of AIDS in this patient is of concern, especially in view of his case history, which includes high-risk sexual contacts and use of metamfetamine. The public health ramifications of such a case are great.

    View details for Web of Science ID 000227731800027

    View details for PubMedID 15781098

  • Bacterial glycolipids and analogs as antigens for CD1d-restricted NKT cells PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA Wu, D., Xing, G. W., Poles, M. A., Horowitz, A., Kinjo, Y., Sullivan, B., Bodmer-Narkevitch, V., Plettenburg, O., Kronenberg, M., Tsuji, M., Ho, D. D., Wong, C. H. 2005; 102 (5): 1351-1356


    The CD1 family of proteins binds self and foreign glycolipids for presentation to CD1-restricted T cells. To identify previously uncharacterized active CD1 ligands, especially those of microbial origin, numerous glycolipids were synthesized and tested for their ability to stimulate mouse and human natural killer T (NKT) cells. They included analogs of the well known NKT cell agonist alpha-galactosyl ceramide (alpha-GalCer), bacterial glycolipids, and variations of the self-glycolipid, sulfatide. Bacterial glycolipids, alpha-galacturonosyl-ceramides from Sphingomonas wittichii, although structurally similar to alpha-GalCer, have significant differences in the sugar head group as well as the ceramide portion. The Sphingomonas glycosphingolipids (GSLs) and sulfatide variants were shown to activate human NKT cells as measured by IL-4 and IFN-gamma secretion. Moreover, CD1d-dimer staining revealed human NKT cell reactivity toward these GSLs and to the sulfatides in a fashion comparable with alpha-GalCer. Because alpha-GalCer is a marine-sponge-derived ligand, our study here shows that bacterium-derived antigens are also able to stimulate mouse and human NKT cells.

    View details for DOI 10.1073/pnas.0408696102

    View details for Web of Science ID 000226877300020

    View details for PubMedID 15665086

  • Primary HIV-1 infection is associated with preferential depletion of CD4(+) T lymphocytes from effector sites in the gastrointestinal tract JOURNAL OF EXPERIMENTAL MEDICINE Mehandru, S., Poles, M. A., Tenner-Racz, K., Horowitz, A., Hurley, A., Hogan, C., Boden, D., Racz, P., Markowitz, M. 2004; 200 (6): 761-770


    Given its population of CCR5-expressing, immunologically activated CD4(+) T cells, the gastrointestinal (GI) mucosa is uniquely susceptible to human immunodeficiency virus (HIV)-1 infection. We undertook this study to assess whether a preferential depletion of mucosal CD4(+) T cells would be observed in HIV-1-infected subjects during the primary infection period, to examine the anatomic subcompartment from which these cells are depleted, and to examine whether suppressive highly active antiretroviral therapy could result in complete immune reconstitution in the mucosal compartment. Our results demonstrate that a significant and preferential depletion of mucosal CD4(+) T cells compared with peripheral blood CD4(+) T cells is seen during primary HIV-1 infection. CD4(+) T cell loss predominated in the effector subcompartment of the GI mucosa, in distinction to the inductive compartment, where HIV-1 RNA was present. Cross-sectional analysis of a cohort of primary HIV-1 infection subjects showed that although chronic suppression of HIV-1 permits near-complete immune recovery of the peripheral blood CD4(+) T cell population, a significantly greater CD4(+) T cell loss remains in the GI mucosa, despite up to 5 yr of fully suppressive therapy. Given the importance of the mucosal compartment in HIV-1 pathogenesis, further study to elucidate the significance of the changes observed here is critical.

    View details for DOI 10.1084/jem.20041196

    View details for Web of Science ID 000224105500007

    View details for PubMedID 15365095

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