In vitro immune modulation by antibodies coupled to tumour cells

Targeting mesenchymal stem cells to activated endothelial cells

Cell surface coating is a methodology wherein specific molecules are transiently anchored onto cell membrane to modulate cell behavior. Cell surface coating was tested as a method to deliver mesenchymal stem cells (MSCs) to endothelial cells via binding to intercellular cell adhesion molecule-1 (ICAM-1). MSCs coated with palmitated protein G (PPG) followed by antibodies to ICAM-1 (Ab(ICAM-1)), and incubated on ICAM-I coated coverslips showed a 40-fold increase in cell binding over PPG-only controls. Ab(ICAM-1)-coated MSCs incubated with human vascular endothelial cells (HUVECs), with and without exposure to TNFalpha (to upregulate ICAM-1 expression), showed 2.6-fold increased binding to control HUVECs over PPG-only controls, and a 16-fold increase in binding to TNFalpha-treated HUVECs. Pretreatment of HUVECs with ICAM-1 antibody promoted the attachment of PPG-only MSCs while reducing the attachment of Ab(ICAM-1)-MSCs by approximately 50%. In flow chamber studies on TNFalpha-stimulated HUVECs, PPG-only, and MSC-only lost 80-90% of their initial binding at 4 dyne/cm(2), while Ab(ICAM-1)-MSCs maintained 100% binding at 4 dyne/cm(2) and 40% binding at 25 dyne/cm(2). These results demonstrate that cell surface coating promotes the attachment of MSCs to endothelial cells, and provides a methodology for the delivery of stem cells to sites of inflammation.

Tracking antigen-driven responses by flow cytometry: monitoring proliferation by dye dilution

Cell-tracking reagents such as the green-fluorescent protein labeling dye CFSE and the red-fluorescent lipophilic membrane dye PKH26 are commonly used to monitor cell proliferation by flow cytometry in heterogeneous cell populations responding to immune stimuli. Both reagents stain cells with a bright homogeneous fluorescence, which is partitioned between daughter cells during each cell division. Because daughter cell fluorescence intensities are approximately halved after each division, the intensity of a cell relative to its intensity at the time of staining provides information about how many divisions it has undergone. Knowing how many rounds of division have occurred and the relative number of cells in each daughter generation, one can back-calculate the number of cells in the original population (i.e., cells present at the time of stimulus) that went on to respond by proliferating. Using this information, the precursor cell frequencies and extent of expansion to a specific antigen or mitogen of interest can be calculated. Concurrently, the phenotype of the cells can be determined, as well as their ability to bind antigen or synthesize cytokines, providing more detailed characterization of all cells responding to the antigen, not just effector cells. In multiparameter flow cytometric experiments to simultaneously analyze antigen-specific tetramer binding, cytokine production and T-cell proliferation, we found that only approximately half of the cells that exhibited specific binding to influenza tetramer also proliferated, as measured by dye dilution, and synthesized IFNgamma in response to antigen. We expect the advent of new cell tracking dyes emitting from the violet to the near infrared combined with the increasing number of lasers and detectors on contemporary flow cytometers to further expand the usefulness of this approach to characterization of complex antigen-driven immunological responses.

Cell tracking 2007: a proliferation of probes and applications

The articles in this thematic issue, entitled "Tracking Cell Proliferation and Function," illustrate some of the choices made by authors pushing the envelope for cell tracking applications in their areas of interest. Over the past decade there has been a proliferation in the range of commercially available probes for these studies, the capabilities of the instrumentation used to detect them, and in the biological systems being studied. This introductory to the thematic issue presents the advantages and limitations of the more commonly used probes such as CFSE and PKH26, as well as emerging probes that expand the range of fluorescence available, including quantum dots and the new CellVue dyes. Appropriate method and instrument setup controls and possible data analysis strategies are discussed with the goal of urging experienced investigators to include all critical information and controls when publishing their data and of aiding researchers new to cell tracking to make informed decisions on which cell tracking reagent(s) are best suited for their particular application. All cell tracking assays have the common goal of determining the fate of a particular cell population within a heterogeneous environment, whether in vivo or in vitro. Some of the common themes among the contributions found in this issue include how various probes are used to track (i) cell proliferation, (ii) regulatory and effector immune cell function and (iii) membrane transfer and antigen presentation. Although these represent only a small fraction of the large and growing list of applications for cell tracking, clearly illustrate the growing trend toward the use of multiple tracking reagents and multiple detection modalities to address complex biological questions.

Allogeneic transplantation for myelodysplastic syndrome (MDS)

Haematopoietic stem cell transplantation (HSCT) remains the only curative option for patients with myelodysplastic syndrome (MDS). Developing conditioning regimens with low toxicity, at the same time as preserving an effective graft versus tumour response, is pivotal to expanding the scope for allogeneic transplantation in older patients with MDS. With the introduction of reduced intensity conditioned regimens, transplant centres worldwide are able to offer allogeneic HSCT to a much larger cohort of patients. Graft versus host disease (GvHD) remains a significant cause of morbidity and mortality, however with the use of T-cell depletion, centres have been able to utilise volunteer unrelated donors with an increasing degree of HLA disparity. The graft versus dysplasia effect resulting from allogeneic HSCT and the infusion of donor leukocytes has led to a greater understanding of the immunological mechanisms that govern outcome following transplantation in MDS.

Immunological weapons against acute myeloid leukaemia

A better understanding of the biology of malignant cells and of the host immune system together with dramatic advances in technology have led to the design of innovative immune-mediated approaches to control neoplastic clones, including various haematological malignancies. One of the major problems with conventional cancer therapies is their inability to eradicate residual cancer cells that are resistant to therapy, hence immune intervention might improve the clinical outcome of patients. This mini-review will focus mainly on immunological approaches to the therapy of acute myeloid leukaemia (AML), a subset of a much larger family of leukaemias. Immune-mediated approaches ranging from allogeneic lymphocyte transplants to cytokine therapy, immune-gene therapy and vaccination by dendritic-cell-based vaccines will be discussed.

Streptavidin paramagnetic particles provide a choice of three affinity-based capture and magnetic concentration strategies for retroviral vectors

Three strategies have been designed to concentrate infectious retroviral vectors from the supernatants of human- (HT1080) and murine- (NIH 3T3) based packaging cells. Streptavidin-conjugated paramagnetic particles in conjunction with (i) antibodies directed against murine fibronectin, (ii) biotinylated lectins, or (iii) biotin-modified packaging cell-surface proteins allow affinity-mediated magnetic concentration of retroviral vectors. Retroviral titers (assayed by colony formation of human myeloid K562 cells) are increased by 1-4 x 10(3)-fold after volume reductions of only 125-fold. Using these procedures, preparations of 5 x 10(8) cfu/ml are routinely made from relatively low-titer (2-5 x 10(5) cfu/ml) starting material. High-titer (paramagnetic) retroviral vector preparations can be used for magnetic field-dependent retroviral infection in vitro. Magnetic field-dependent localization such as this may enable the in vivo administration of formulations that concentrate retroviral infection to the required target tissues and organs.

Targeted transfer of polyethylenimine-avidin-DNA bioconjugates to hematopoietic cells using biotinylated monoclonal antibodies

Here we examine whether attachment of biotinylated antibodies to proteins on the cell surface increases the transfection efficiency of polyethylenimine-avidin-DNA bioconjugate gene transfer. Preliminary experiments were performed to compare avidin endocytosis into cells incubated with biotinylated antibodies. Antibody biotinylation resulted in the endocytosis of avidin-FITC into nearly 100% of cells compared with no detectable binding or entry into unbiotinylated cells. Gene transfer was accomplished with avidin conjugated to polyethylenimine (PEI) at a molar ratio of 4:1 (PA4). Plasmid DNA encoding the green fluorescent protein (GFP) gene was condensed on the PA4, and transfection efficiencies were measured by flow cytometry as the percentage of cells that fluoresced at levels greater than two standard deviations above the negative control. Gene transfer efficiencies were compared among K562, HEL, and Jurkat leukemia cell lines. Control transfections with DNA alone or untargeted PEI-DNA resulted in </=2% GFP positive cells. Targeting PEI-avidin-DNA to antibody biotinylated cells increased transfection efficiency several fold over untargeted PEI. For each cell type, the increase in transfection efficiency was not significantly different among four biotinylated antibodies tested (antiCD55, antiCD59, antiCD71, and antiCD98). These data suggest biotinylated antibodies may be useful for targeting polyethylenimine-avidin mediated gene transfer.

Hierarchical costimulator thresholds for distinct immune responses: application of a novel two-step Fc fusion protein transfer method

Activation of T cells is dependent upon coordinate engagement of Ag and costimulator receptors on their surfaces. In the case of the Ag receptors (TCRs), activation thresholds have been defined, with the number of TCRs that must be triggered to stimulate cytokine secretion by individual activated T cells differing for the various cytokines. In the present study, we have determined whether comparable activation thresholds exist for the costimulator receptors on T cells. To facilitate this type of quantitative costimulator analysis, we developed a novel two-step protein transfer approach that permits delivery of graded amounts of proteins to APC surfaces. By adding a human B7-1. Fcgamma1 (Fc domain of human IgG1) fusion protein to cells precoated with palmitated protein A, fine titration of the B7-1 extracellular domain was achieved. The B7-1. Fcgamma1 reincorporated into cell membranes by this method retained costimulator function, as measured by an in vitro proliferation assay. The degree of proliferation was dependent on the surface density of B7-1. Fcgamma1. Significantly, the threshold B7-1. Fcgamma1 density required for cytokine production differed between IFN-gamma and IL-2 and mirrored the hierarchy (IFN-gamma < IL-2) described previously for the TCR activation threshold. Hence, this study invokes a novel protein transfer strategy to establish that the levels of surface costimulator on APCs can dictate both the magnitude and the quality of evoked T cell responses. The notion of costimulator receptor activation thresholds emerges.

Effect of Costimulation and the Microenvironment on Antigen Presentation by Leukemic Cells

Costimulatory signals supplied by genetically modified tumor cells can enable T-cell recognition of tumor-associated antigens that were previously silent when presented by unmodified tumor cells. Although the mechanism of the CD80/CD28 costimulation has been studied extensively in the normal T-cell/antigen-presenting cell (APC) interactions, it is unclear how expression of CD80 by tumor cells mediates its effect. We demonstrate here that optimal CD80 expression on a leukemic cell enhances T-cell recognition of alloantigen primarily by lowering the level of T-cell receptor (TCR) stimulation required for activation. CD80 expression by leukemic cells leads to increased survival of activated T cells by inducing upregulation of the antiapoptotic protein BCL-2, but not BCL-XL. The cytokine microenvironment in which T cells are activated is crucial in determining their differentiation and consequently the nature of the immune response generated. Many tumor cells produce immunosuppressive cytokines that may not favor the induction of cell-mediated immunity. In this study, the presence of CD80 on leukemic cells increased T-cell activation in vitro, but this did not result in the production of Th1 cytokines. We show that this is due to a leukemia-derived soluble factor that inhibits the production of Th1 cytokines. Optimal expression of a costimulatory molecule, therefore, enhances the ability of leukemic cells to present antigen by amplifying TCR signals, but the microenvironment generated by leukemic cells may suppress the immune response required for their eradication. Thus, strategies aimed at inducing antileukemic immunity by providing leukemic cells with costimulatory functions must ensure the presence of an appropriate microenvironment.

The Scope of Viral Vectors for the Transduction of Haemopoietic Cells

Over the last five years significant progress has been made towards the transfer of foreign genetic material to eukaryotic cells. The eventual aim to devise novel therapeutic strategies to treat human diseases, in particular solid tumours and monogenic disorders associated with various enzyme deficiency states. The easy accessibility and the ability of haemopoetic stem cells to self replicate and repopulate makes them desirable targets for gene transfer. In theory the introduction of a small number of gene modifed haemopoetic progenitor cells can allow therapy of an individual for life without any further intervention. This approach has been used for the treatment of single gene defects such as ADA deficiency. Furthermore, gene transfer technology has increasingly been exploited for bone marrow and peripheral blood stem cell marking studies, modification of cell sensitivity to cytotoxic drugs and the genetic modification of leukemic cells with the intention of inducing a leukemia specific cytotoxic T cell response. Vector development is of crucial importance for the successful delivery of genes in haemopoetic stem cells and leukemia cells. The objective of this review is to discuss in detail the properties of current vector technology that are pertinent to haemopoietic cell gene transduction.