Purification of transiently transfected cells by magnetic affinity cell sorting

Purification of T Cell Populations

This article describes a procedure for isolating T cell subpopulations using various methods including indirect panning and immunopanning by microarray. In these methods, cells are selected by their capacity to bind to antibody-coated plates (or slides) on the basis of particular cell-surface markers. Such methods can be superior to the antibody/complement lysis method (Alternate Protocol), as they can select additional cell population for analysis. © 2020 by John Wiley & Sons, Inc. Basic Protocol 1: Isolation of T cell populations by indirect panning Basic Protocol 2: Immunopanning with microarray Alternate Protocol: Isolation of T cell populations by antibody/complement-mediated cytotoxicity.

Purification of T cell subpopulations

This unit describes a procedure for isolating T cell populations or subpopulations using the method of indirect panning. In this method, cells are selected by their capacity to bind to antibody-coated plates on the basis of particular cell-surface markers. It is superior to the antibody/complement lysis method (also presented) because the nonselected cell population can be retrieved.

Medical application of functionalized magnetic nanoparticles

Since magnetic particles have unique features, the development of a variety of medical applications has been possible. The most unique feature of magnetic particles is their reaction to a magnetic force, and this feature has been utilized in applications such as drug targeting and bioseparation including cell sorting. Recently, magnetic nanoparticles have attracted attention because of their potential as contrast agents for magnetic resonance imaging (MRI) and heating mediators for cancer therapy (hyperthermia). Magnetite cationic liposomes (MCLs), one of the groups of cationic magnetic particles, can be used as carriers to introduce magnetite nanoparticles into target cells since their positively charged surface interacts with the negatively charged cell surface; furthermore, they find applications to hyperthermic treatments. Magnetite nanoparticles conjugated with antibodies (antibody-conjugated magnetoliposomes, AMLs) are also applied to hyperthermia and have enabled tumor-specific contrast enhancement in MRI via systemic administration. Since magnetic nanoparticles are attracted to a high magnetic flux density, it is possible to manipulate cells labeled with magnetic nanoparticles using magnets; this feature has been applied in tissue engineering. Magnetic force and MCLs were used to construct multilayered cell structures and a heterotypic layered 3D coculture system. Thus, the applications of these functionalized magnetic nanoparticles with their unique features will further improve medical techniques.

Use of protein biotinylation in vivo for chromatin immunoprecipitation

We describe a system designed to express biotinylated proteins in mammalian cells in vivo and its application to the study of protein-DNA interactions in vivo by chromatin immunoprecipitation (ChIP). The system is based on coexpression of the target protein fused to a short biotin acceptor domain together with the biotinylating enzyme BirA from Escherichia coli. The superior strength of the biotin-avidin interaction allows one to employ more stringent washing conditions in the ChIP protocol, resulting in a better signal/noise ratio.

Human fibroblasts transfected with cytomegalovirus immediate-early genes show increased MHC class I expression and are targets for natural killer cell-mediated cytotoxicity

Natural killer (NK) cells are an important line of defense against viral infections, such as those caused by cytomegalovirus (CMV), but in the context of solid organ transplantation NK responses to CMV-infected graft cells might be deleterious to the graft survival. To gain a better understanding of NK responses to CMV-infected human lung fibroblasts (HLF), we transfected HLF with a plasmid expressing CMV immediate-early (IE) genes under the control of the CMV major IE promoter and compared major histocompatibility complex (MHC) class I expression and NK-mediated lysis of transfected cells, CMV-infected cells, and appropriate controls. HLF transfected with CMV IE genes showed increased MHC Class I expression and triggered NK-mediated cytotoxicity at the same level as CMV-infected HLF and at significantly higher levels than mock-infected or mock-transfected controls. Transfection of CMV genes provides an experimental model for molecular studies of CMV- and allograft-specific cell-mediated immunity and modulation.

Highly efficient selection of phage antibodies mediated by display of antigen as Lpp-OmpA' fusions on live bacteria

Delayed infectivity panning (DIP) is a novel approach for the in vivo isolation of interacting protein pairs. DIP combines phage display and cell surface display of polypeptides as follows: an antigen is displayed in many copies on the surface of F(+) Escherichia coli cells by fusing it to a Lpp-OmpA' hybrid. To prevent premature, non-specific infection by phage, the cells are rendered functionally F(-) by growth at 16 degrees C. The antigen-displaying cells are used to capture antibody-displaying phage by virtue of the antibody-antigen interaction. Following removal of unbound phage, infection of the cells by bound phage is initiated by raising the temperature to 37 degrees C that facilitates F pilus expression. The phage then dissociate from the antigen and infect the bacteria through the F pilus. Using specific scFv antibodies and the human ErbB2 proto-oncogene and IL2-Ralpha chain as model antibody-antigen pairs, we demonstrate enrichment of those phage that display a specific antibody over phage that display an irrelevant antibody of over 1,000,000 in a single DIP cycle. We further show the successful isolation of anti-toxin, anti-receptor, anti-enzyme and anti-peptide antibodies from several immune phage libraries, a shuffled library and a large synthetic human library. The effectiveness of DIP makes it suitable for the isolation of rare clones present in large libraries. Since DIP can be applied for most of the phage libraries already existing, it could be a powerful tool for the rapid isolation and characterization of binders in numerous protein-protein interactions.

Endothelial cell DNA transfer and expression using petri dish electroporation and the nonreplicating vaccinia virus/T7 RNA polymerase hybrid system

The nonreplicating vaccinia virus MVA/T7 RNA polymerase hybrid system was tested with Petri dish electroporation for ectopic gene expression in human umbilical vein endothelial cells (HUVECs). A range of voltages (150-450 V), pulse times (10-40 ms), DNA concentrations (0-20 microg/ml) and infection levels (0-15 multiplicities of infection) were tested for effects on T7 promoter-directed chloramphenicol acetyltransferase (CAT) activity after MVA/T7RP infection. MVA/T7RP-directed expression was transient and at least 10 000-fold in excess of nonviral, cytomegalovirus enhancer-directed expression. Use of a Petri dish electrode with the MVA/T7RP system showed increased viability compared with a cuvette electrode. Overexpression of interleukin-2 alpha subunit (IL2Ralpha) pro- tein followed by anti-IL2Ralpha-directed magnetic immunoaffinity cell sorting allowed isolation of the transfected population. The high fidelity of cellular sorting was shown by segregation of CAT activity in the IL2Ralpha-sorted population after transfection of T7 promoter-directed bicistronic IL2Ralpha/CAT DNA. Expression of a panel of proteins including the fluorophore green fluorescent protein as detected by fluorescence microscopy and p21cip1, p27kip1, pp60c-src, FGF-1, pRb, p107 and pRb2/p130 proteins was also achieved. Thus, use of the nonreplicating vaccinia virus/T7 RNA polymerase expression system with Petri dish electroporation is feasible for certain applications for the manipulation of HUVECs by gene transfer.

Identification of glucocorticoid receptor domains necessary for transcriptional activation of the mouse mammary tumor virus promoter integrated in the genome

It has previously been determined that the mouse mammary tumor virus (MMTV) promoter when integrated in the genome assumes a defined chromatin structure which is disrupted upon addition of glucocorticoids. In contrast, a transiently introduced MMTV promoter has a random nucleoprotein structure. To reveal glucocorticoid receptor (GR) domains necessary for transcriptional activation of the MMTV promoter we compared the effects of mutations of the GR on transcriptional activation of the stably integrated versus transiently introduced MMTV promoter. For this purpose we generated a GR-negative cell line which has an MMTV promoter/reporter construct integrated in the genome and studied the transcriptional activation of this construct by different GR mutants introduced into the cells. Transcriptional activation of the integrated and transiently introduced promoter was achieved by the wild-type GR or a chimeric receptor in which the MR hormone-binding domain (HBD) replaced the GR HBD. In contrast, we found that deletion of the HBD of the GR or replacement of this region with the equivalent domain of the estrogen receptor produced receptors that were unable to activate the MMTV promoter integrated in the genome although these receptors efficiently activated the transiently introduced MMTV promoter. The HBD was not the sole determinant of MMTV transcriptional activation when integrated in the genome. Chimeric receptors which harbored the MR amino terminal domain or the wild-type MR were also unable to activate the integrated MMTV promoter. Taken together, these data indicate a rigid requirement for sequences in both the GR amino and the carboxy terminal domains for transcriptional activation of a hormone response element in the defined chromatin context of the MMTV promoter.

The role of conserved amino acid motifs within the integrin beta3 cytoplasmic domain in triggering focal adhesion kinase phosphorylation

Integrin-mediated adhesion of cells to extracellular matrix proteins triggers a variety of intracellular signaling pathways including a cascade of tyrosine phosphorylations. In many cell types, the cytoplasmic focal adhesion tyrosine kinase, FAK, appears to be the initial protein that becomes tyrosine-phosphorylated in response to adhesion; however, the molecular mechanisms regulating integrin-triggered FAK phosphorylation are not understood. Previous studies have shown that the integrin beta1, beta3, and beta5 subunit cytoplasmic domains all contain sufficient information to trigger FAK phosphorylation when expressed in single-subunit chimeric receptors connected to an extracellular reporter. In the present study, beta3 cytoplasmic domain deletion and substitution mutants were constructed to identify amino acids within the integrin beta3 cytoplasmic domain that regulate its ability to trigger FAK phosphorylation. Cells transiently expressing chimeric receptors containing these mutant cytoplasmic domains were magnetically sorted and assayed for the tyrosine phosphorylation of FAK. Analysis of these mutants indicated that structural information in both the membrane-proximal and C-terminal segments of the beta3 cytoplasmic domain is important for triggering FAK phosphorylation. In the C-terminal segment of the beta3 cytoplasmic domain, the highly conserved NPXY motif was found to be required for the beta3 cytoplasmic domain to trigger FAK phosphorylation. However, the putative FAK binding domain within the N-terminal segment of the beta3 cytoplasmic domain was found to be neither required nor sufficient for this signaling event. We also demonstrate that the serine 752 to proline mutation, known to cause a variant of Glanzmann's thrombasthenia, inhibits the ability of the beta3 cytoplasmic domain to signal FAK phosphorylation, suggesting that a single mutation in the beta3 cytoplasmic domain can inhibit both "inside-out" and "outside-in" integrin signaling.

Role of reactive oxygen intermediates in cytomegalovirus gene expression and in the response of human smooth muscle cells to viral infection

Because cytomegalovirus (CMV) may contribute to restenosis and atherosclerosis and because smooth muscle cells (SMCs) are involved in these disease processes, we examined CMV-SMC interactions. Using confocal microscopy to identify a redox-sensitive fluorescent marker, we found that CMV infection of SMCs generates intracellular reactive oxygen intermediates (ROIs). CMV also activated nuclear factor kappa B (NF kappa B), a cellular transcription factor, as demonstrated by increased NF kappa B binding to DNA (electrophoretic mobility shift assay). Antioxidants inhibited activation, suggesting a role of ROIs in CMV-induced NF kappa B activation. By using antioxidants to assess the role of ROIs in modulating virally mediated effects, we also found that CMV-induced ROIs (1) are critical to the transactivation of the viral major immediate promoter (MIEP) by its immediate-early protein IE72 (determined by cotransfection of an IE72 expression vector and a reporter gene downstream from the MIEP) and (2) are necessary for IE72 expression (determined by immunocytochemistry) and viral replication (determined by viral titer assay on indicator cells) following CMV infection of SMCs. Because ROIs, through activation of NF kappa B, can also induce expression of cellular genes involved in immune and inflammatory responses, the ROI response to CMV infection may also represent a parallel survival mechanism that has evolved in the host cell to protect against viral infection. We conclude that CMV induces intracellular ROI generation within minutes after infection of SMCs and then uses these ROIs to facilitate its own gene expression and replication. Conversely, antioxidants inhibit CMV immediate-early gene expression and viral replication.

Development of a PCR protocol for sensitive detection of Cryptosporidium oocysts in water samples

The development of a reliable method of using PCR for detection of Cryptosporidium oocysts in environmental samples with oligonucleotide primers which amplify a portion of the sequence encoding the small (18S) subunit of rRNA producing a 435-bp product was demonstrated. The PCR assay was found to provide highly genus-specific detection of Cryptosporidium spp. after release of nucleic acids from oocysts by a simple freeze-thaw procedure. The assay routinely detected 1 to 10 oocysts in purified oocyst preparations, as shown by direct microscopic counts and by an immunofluorescence assay. The sensitivity of the PCR assay in some seeded environmental water samples was up to 1,000-fold lower. However, this interference was eliminated by either flow cytometry or magnetic-antibody capture. Sensitivity was also improved 10- to 1,000-fold by probing of the PCR product on dot blots with an oligonucleotide probe detected by chemiluminescence. Confirmation of the presence of Cryptosporidium oocysts in water samples from the outbreak in Milwaukee, Wis., was obtained with this technique, and PCR was found to be as sensitive as immunofluorescence for detection of oocysts in wastewater concentrates.

Osteoclast-specific monoclonal antibodies coupled to magnetic beads provide a rapid and efficient method of purifying avian osteoclasts

Osteoclasts are the major cell type responsible for normal and pathologic bone resorption. Obtaining highly purified populations of these multinucleated cells has been problematic, although such populations would greatly facilitate investigations of osteoclast regulation and activity. A new immunomagnetic protocol has been devised to surmount these difficulties, employing avian osteoclast-directed monoclonal antibodies (designated 121F, 35L, and 75B) surface coupled to uniformly small, magnetic polystyrene beads covalently conjugated with sheep antimouse IgG. Presentation of these antiosteoclast antibody-coated beads to mixed cell preparations derived from marrow-depleted, collagenase- and/or trypsin-treated chick tibiae and wing bones, followed by magnetic separation and washing, results in efficient and selective binding of osteoclasts to the immunomagnetic beads within minutes. The specific nature of this bead-cell interaction is further demonstrated by the progressive decline in antiosteoclast antibody-coated bead binding to osteoclasts by uncoated beads or beads coated with an irrelevant antibody. Under optimal conditions, these isolations typically yield more than a 100-fold enrichment and greater than a 90% purification of osteoclasts from subpopulations of either predominantly nonviable or viable osteoclasts. Although scanning electron microscopy reveals that immunomagnetically purified and cultured osteoclasts internalize large numbers of the antibody-coated beads, such cells appear unimpaired in their ability to attach to tissue culture plastic or devitalized cortical bone slices and to produce resorption pits characteristic for osteoclasts. Additional studies to ascertain the most effective method for removal (desorption) of antibody-coated beads from magnetically isolated osteoclasts demonstrate that moderate physical agitation is at present the most effective protocol to dislodge antibody-coated beads from the cell surface while maintaining osteoclast viability and function. This immunomagnetic technique therefore provides a gentle method for the isolation of highly purified populations of osteoclasts from heterogeneous bone cell populations in a rapid, efficient, and selective manner.

Quiescent T lymphocytes as an inducible virus reservoir in HIV-1 infection

To better understand the basis for human immunodeficiency virus type 1 (HIV-1) persistence and latency, the form in which viral DNA exists in the peripheral T lymphocyte reservoir of infected individuals was investigated. In asymptomatic individuals, HIV-1 was harbored predominantly as full-length, unintegrated complementary DNA. These extrachromosomal DNA forms retained the ability to integrate upon T cell activation in vitro. In patients with acquired immunodeficiency syndrome (AIDS), there was an increase in integrated relative to extrachromosomal DNA forms. By analysis of DNA from patient lymphocyte subpopulations depleted of human lymphocyte antigen-Dr receptor-positive cells, quiescent T cells were identified as the source of extrachromosomal HIV-1 DNA. Thus quiescent T lymphocytes may be a major and inducible HIV-1 reservoir in infected individuals.

Sequence-specific toxicity of transfected retroviral DNA

Experimental gene transfer and viral infections can result in the accumulation of unintegrated DNA in target cells. The effects of such accumulation on target cell metabolism have not been directly studied. The experiments reported in this paper show that transfection of cloned retroviral long-terminal-repeat (LTR) DNA, or of a variety of eukaryotic promoters, into proliferating HeLa cells results in rapid, sequence-specific, and dose-dependent cell death. Plasmids containing the Rous sarcoma virus LTR or the human immunodeficiency virus LTR cloned in pUC-related plasmids are 5 to 10 times more toxic than pUC19. The demonstrated sensitivity of eukaryotic cells to exogenously introduced DNA has important implications for the interpretation of gene transfer experiments and may be relevant to the pathogenic mechanisms in the course of retroviral infections such as AIDS.

Isolation of a population of transiently transfected quiescent and senescent cells by magnetic affinity cell sorting

Rous sarcoma virus (RSV) and cytomegalovirus (CMV) promoters were tested for activity in proliferating and nonproliferating (quiescent or senescent) human embryo fibroblasts. These promoters were cloned upstream of the coding sequence for the Tac subunit of the interleukin 2 receptor, and activity was calculated from the fraction of Tac antigen positive cells detected in a coupled transient transfection/magnetic affinity cell sorting assay. Differences in promoter activities are substantial in quiescent cells: the efficiency of the RSV promoter is no greater than background whereas the CMV promoter is equally active in serum concentrations ranging from 0.5 to 20%. While both promoters are functional in growing cells (WI-38 and HeLa), the CMV promoter exhibits twofold greater activity. Surprisingly, in senescent cells both promoters exhibit the same degree of activity.

Transient analysis for antiproliferative gene activity

A subset of tumor suppressor genes presumably functions by the inhibition of cellular proliferation; however, antiproliferative activity after transfection with putative suppressor genes has been difficult to demonstrate and often requires lengthy selection either in nude mice or in vitro. A rapid alternative is presented here that utilizes a gene encoding a surface marker protein to identify transfectants in a transient expression assay. In this assay the labeling index, rate of DNA synthesis, cell-cycle distribution, and surface receptor display are measured by flow cytometry. Human beta-interferon, a gene with proven antiproliferative activity, was studied using the transient analysis system. The beta-interferon gene was introduced into human tumor cells along with the marker gene encoding the 55-kDa subunit of the human interleukin 2 receptor. Within a few days of transfection, analysis of transfectants by flow cytometry revealed a decrease in the fraction of cells in G2/M and an increase in the fraction of cells in G1/G0 and S phases. The distortion of the cell cycle was accompanied by as much as a 69% reduction in the rate of DNA synthesis and, in some experiments, an unanticipated increase in the labeling index. Therefore, cells accumulating in S phase were not blocked but continued to synthesize DNA although at a reduced rate. These studies on DNA synthetic rates revealed the caveat that screening for antiproliferative candidate genes with a labeling index alone could, in certain circumstances, exclude potentially interesting sequences from further consideration. Although this transient analysis system was developed for studies on cellular proliferation, it may prove suitable for phenotypic assays on other genes as well.

Expression cloning of a receptor for human granulocyte-macrophage colony-stimulating factor

Two cDNA clones encoding a receptor for human granulocyte-macrophage colony-stimulating factor (hGM-CSF-R) were isolated by expression screening of a library made from human placental mRNA. Pools of recombinant plasmid DNA were electroporated into COS cells which were then screened for their capacity to bind radioiodinated hGM-CSF using a sensitive microscopic autoradiographic approach. The cloned GM-CSF-R precursor is a 400 amino acid polypeptide (Mr 45,000) with a single transmembrane domain, a glycosylated extracellular domain and a short (54 amino acids) intracytoplasmic tail. It does not contain a tyrosine kinase domain nor show homology with members of the immunoglobulin super gene family, but does show some significant sequence homologies with receptors for several other haemopoietic growth factors, including those for interleukin-6, erythropoietin and interleukin-2 (beta-chain) and also to the prolactin receptor. When transfected into COS cells the cloned cDNA directed the expression of a GM-CSF-R showing a single class of affinity (KD = 2(-8) nM) and specificity for human GM-CSF but not interleukin-3. Messenger RNA coding for this receptor was detected in a variety of haemopoietic cells known to display hGM-CSF binding, and cross-linking experiments revealed a similar size for the glycosylated receptors in transfected COS and haemopoietic cells.

Enhanced transfection efficiency and improved cell survival after electroporation of G2/M-synchronized cells and treatment with sodium butyrate

To achieve high transfection efficiency in human fibroblasts with good preservation of proliferative capacity we developed an electroporation procedure that combines two distinct modalities: use of recipient cells synchronized in the late G2/mitotic phase of the cell cycle and treatment of cells post-electroporation with 5 mM butyrate. This combination enabled reduction of plasmid DNA concentration and electroporation voltage, both associated with cytotoxicity, while greatly enhancing transfection efficiencies. Although the method was primarily developed for transient expression it was also found to improve stable expression. This procedure should have wide applicability, particularly in studies seeking to identify DNA sequences that lead to inhibition of DNA synthesis and proliferation in human fibroblasts and other cells refractory to transfection.

Studies on the molecular-genetic basis of replicative senescence in Werner syndrome and normal fibroblasts

Based on evidence that human diploid fibroblasts (HDF) from the Werner syndrome (WS) of premature aging might overexpress an inhibitor of DNA synthesis (IDS), we prepared a eukaryotic cDNA expression library from WS mRNA and tested it for IDS activity in a transient assay. Two of six WS cDNA pools tested gave IDS activity, then on plus/minus screening revealed several differentially expressed cDNA clones. By slot blot and Northern analysis, one cDNA clone was found to be overexpressed in WS and normal senescent HDF, but not in quiescent normal HDF, indicating that it is senescence-specific. Further studies are needed to clarify: a) whether this cDNA truly acts as an IDS; b) if so, whether it acts alone or in concert with other cDNAs; and c) whether it is involved in the degenerative and malignant sequelae of WS and normal aging.