Transcriptional control of SV40 T-antigen expression allows a complete reversion of immortalization

Synthetic rewiring and boosting type I interferon responses for visualization and counteracting viral infections

Mammalian first line of defense against viruses is accomplished by the interferon (IFN) system. Viruses have evolved numerous mechanisms to reduce the IFN action allowing them to invade the host and/or to establish latency. We generated an IFN responsive intracellular hub by integrating the synthetic transactivator tTA into the chromosomal Mx2 locus for IFN-based activation of tTA dependent expression modules. The additional implementation of a synthetic amplifier module with positive feedback even allowed for monitoring and reacting to infections of viruses that can antagonize the IFN system. Low and transient IFN amounts are sufficient to trigger these amplifier cells. This gives rise to higher and sustained—but optionally de-activatable—expression even when the initial stimulus has faded out. Amplification of the IFN response induced by IFN suppressing viruses is sufficient to protect cells from infection. Together, this interfaced sensor/actuator system provides a toolbox for robust sensing and counteracting viral infections.

Murine Cytomegalovirus M25 Proteins Sequester the Tumor Suppressor Protein p53 in Nuclear Accumulations

To ensure productive infection, herpesviruses utilize tegument proteins and nonstructural regulatory proteins to counteract cellular defense mechanisms and to reprogram cellular pathways. The M25 proteins of mouse cytomegalovirus (MCMV) belong to the betaherpesvirus UL25 gene family that encodes viral proteins implicated with regulatory functions. Through affinity purification and mass spectrometric analysis, we discovered the tumor suppressor protein p53 as a host factor interacting with the M25 proteins. M25-p53 interaction in infected and transfected cells was confirmed by coimmunoprecipitation. Moreover, the proteins colocalized in nuclear dot-like structures upon both infection and inducible expression of the two M25 isoforms. p53 accumulated in wild-type MCMV-infected cells, while this did not occur upon infection with a mutant lacking the M25 gene. Both M25 proteins were able to mediate the effect, identifying them as the first CMV proteins responsible for p53 accumulation during infection. Interaction with M25 proteins led to substantial prolongation of the half-life of p53. In contrast to the higher abundance of the p53 protein in wild-type MCMV-infected cells, the transcript levels of the prominent p53 target genes Cdkn1a and Mdm2 were diminished compared to cells infected with the ΔM25 mutant, and this was associated with reduced binding of p53 to responsive elements within the respective promoters. Notably, the productivity of the M25 deletion mutant was partially rescued on p53-negative fibroblasts. We propose that the MCMV M25 proteins sequester p53 molecules in the nucleus of infected cells, reducing their availability for activating a subset of p53-regulated genes, thereby dampening the antiviral role of p53.IMPORTANCE Host cells use a number of factors to defend against viral infection. Viruses are, however, in an arms race with their host cells to overcome these defense mechanisms. The tumor suppressor protein p53 is an important sensor of cell stress induced by oncogenic insults or viral infections, which upon activation induces various pathways to ensure the integrity of cells. Viruses have to counteract many functions of p53, but complex DNA viruses such as cytomegaloviruses may also utilize some p53 functions for their own benefit. In this study, we discovered that the M25 proteins of mouse cytomegalovirus interact with p53 and mediate its accumulation during infection. Interaction with the M25 proteins sequesters p53 molecules in nuclear dot-like structures, limiting their availability for activation of a subset of p53-regulated target genes. Understanding the interaction between viral proteins and p53 may allow to develop new therapeutic strategies against cytomegalovirus and other viruses.

Murine alveolar epithelial cells and their lentivirus-mediated immortalisation

In this study, we describe the isolation and immortalisation of primary murine alveolar epithelial cells (mAEpC), as well as their epithelial differentiation and barrier properties when grown on Transwell® inserts. Like human alveolar epithelial cells (hAEpC), mAEpC transdifferentiate in vitro from an alveolar type II (ATII) phenotype to an ATI-like phenotype and exhibit features of the air-blood barrier, such as the establishment of a thin monolayer with functional tight junctions (TJs). This is demonstrated by the expression of TJ proteins (ZO-1 and occludin) and the development of high transepithelial electrical resistance (TEER), peaking at 1800Ω ·cm². Transport across the air-blood barrier, for general toxicity assessments or preclinical drug development, is typically studied in mice. The aim of this work was the generation of novel immortalised murine lung cell lines, to help meet Three Rs requirements in experimental testing and research. To achieve this goal, we lentivirally transduced mAEpC of two different mouse strains with a library of 33 proliferation-promoting genes. With this immortalisation approach, we obtained two murine alveolar epithelial lentivirus-immortalised (mAELVi) cell lines. Both showed similar TJ protein localisation, but exhibited less prominent barrier properties (TEERmax ~250Ω·cm²) when compared to their primary counterparts. While mAEpC demonstrated their suitability for use in the assessment of paracellular transport rates, mAELVi cells could potentially replace mice for the prediction of acute inhalation toxicity during early ADMET studies.

Lung macrophage scavenger receptor SR-A6 (MARCO) is an adenovirus type-specific virus entry receptor

Macrophages are a diverse group of phagocytic cells acting in host protection against stress, injury, and pathogens. Here, we show that the scavenger receptor SR-A6 is an entry receptor for human adenoviruses in murine alveolar macrophage-like MPI cells, and important for production of type I interferon. Scavenger receptors contribute to the clearance of endogenous proteins, lipoproteins and pathogens. Knockout of SR-A6 in MPI cells, anti-SR-A6 antibody or the soluble extracellular SR-A6 domain reduced adenovirus type-C5 (HAdV-C5) binding and transduction. Expression of murine SR-A6, and to a lower extent human SR-A6 boosted virion binding to human cells and transduction. Virion clustering by soluble SR-A6 and proximity localization with SR-A6 on MPI cells suggested direct adenovirus interaction with SR-A6. Deletion of the negatively charged hypervariable region 1 (HVR1) of hexon reduced HAdV-C5 binding and transduction, implying that the viral ligand for SR-A6 is hexon. SR-A6 facilitated macrophage entry of HAdV-B35 and HAdV-D26, two important vectors for transduction of hematopoietic cells and human vaccination. The study highlights the importance of scavenger receptors in innate immunity against human viruses.

Macrophages are a diverse group of phagocytic cells acting in host protection against stress, injury, and pathogens. They phenotypically and functionally adapt to their local environment, for example, peritoneal macrophages are distinct from brain-resident microglia, from liver-resident Kupffer cells or lung macrophages in the lung. Airway macrophages are among the first cells to encounter human respiratory viruses, such as adenoviruses. They release pro-inflammatory cytokines, kill pathogens, present antigens, and restore tissues. Yet, interactions of viruses with lung macrophages are poorly understood, and it is unclear, how they lead to infection or virus clearance. Here we identified the murine scavenger receptor SR-A6 as a receptor for a subset of human adenoviruses on alveolar macrophage-like cells, so-called MPI cells. Scavenger receptors comprise a large family of trans-membrane proteins, and contribute to the clearance of endogenous proteins, lipoproteins and pathogens. In a series of robust experimentation, we show that adenoviruses use SR-A6 as an entry receptor for infection of MPI cells, and production of type I interferon. MPI cells are non-transformed, self-renewing macrophages derived from fetal murine liver, and closely resemble adult alveolar macrophages. The results demonstrate that SR-A6 binds virions on the surface of alveolar macrophage-like cells, and leads to infection.

Role of fibroblast growth factor receptors (FGFR) and FGFR like-1 (FGFRL1) in mesenchymal stromal cell differentiation to osteoblasts and adipocytes

Fibroblast growth factors (FGF) and their receptors (FGFRs) regulate many developmental processes including differentiation of mesenchymal stromal cells (MSC). We developed two MSC lines capable of differentiating to osteoblasts and adipocytes and studied the role of FGFRs in this process. We identified FGFR2 and fibroblast growth factor receptor like-1 (FGFRL1) as possible actors in MSC differentiation with gene microarray and qRT-PCR. FGFR2 and FGFRL1 mRNA expression strongly increased during MSC differentiation to osteoblasts. FGF2 treatment, resulting in downregulation of FGFR2, or silencing FGFR2 expression with siRNAs inhibited osteoblast differentiation. During adipocyte differentiation expression of FGFR1 and FGFRL1 increased and was down-regulated by FGF2. FGFR1 knockdown inhibited adipocyte differentiation. Silencing FGFR2 and FGFR1 in MSCs was associated with decreased FGFRL1 expression in osteoblasts and adipocytes, respectively. Our results suggest that FGFR1 and FGFR2 regulate FGFRL1 expression. FGFRL1 may mediate or modulate FGFR regulation of MSC differentiation together with FGFR2 in osteoblastic and FGFR1 in adipocytic lineage.

The M25 gene products are critical for the cytopathic effect of mouse cytomegalovirus

Cell rounding is a hallmark of the cytopathic effect induced by cytomegaloviruses. By screening a panel of deletion mutants of mouse cytomegalovirus (MCMV) a mutant was identified that did not elicit cell rounding and lacked the ability to form typical plaques. Altered cell morphology was assigned to the viral M25 gene. We detected an early 2.8 kb M25 mRNA directing the synthesis of a 105 kDa M25 protein, and confirmed that a late 3.1 kb mRNA encodes a 130 kDa M25 tegument protein. Virions lacking the M25 tegument protein were of smaller size because the tegument layer between capsid and viral envelope was reduced. The ΔM25 mutant did not provoke the rearrangement of the actin cytoskeleton observed after wild-type MCMV infection, and isolated expression of the M25 proteins led to cell size reduction, confirming that they contribute to the morphological changes. Yields of progeny virus and cell-to-cell spread of the ΔM25 mutant in vitro were diminished and replication in vivo was impaired. The identification of an MCMV gene involved in cell rounding provides the basis for investigating the role of this cytopathic effect in CMV pathogenesis.

Mesenteric lymph node stromal cell-derived extracellular vesicles contribute to peripheral de novo induction of Foxp3+ regulatory T cells

Intestinal regulatory T cells (Tregs) are fundamental in peripheral tolerance toward commensals and food‐borne antigens. Accordingly, gut‐draining mesenteric lymph nodes (mLNs) represent a site of efficient peripheral de novo Treg induction when compared to skin‐draining peripheral LNs (pLNs), and we had recently shown that LN stromal cells substantially contribute to this process. Here, we aimed to unravel the underlying molecular mechanisms and generated immortalized fibroblastic reticular cell lines (iFRCs) from mLNs and pLNs, allowing unlimited investigation of this rare stromal cell subset. In line with our previous findings, mLN‐iFRCs showed a higher Treg‐inducing capacity when compared to pLN‐iFRCs. RNA‐seq analysis focusing on secreted molecules revealed a more tolerogenic phenotype of mLN‐ as compared to pLN‐iFRCs. Remarkably, mLN‐iFRCs produced substantial numbers of microvesicles (MVs) that carried elevated levels of TGF‐β when compared to pLN‐iFRC‐derived MVs, and these novel players of intercellular communication were shown to be responsible for the tolerogenic properties of mLN‐iFRCs. Thus, stromal cells originating from mLNs contribute to peripheral tolerance by fostering de novo Treg induction using TGF‐β‐carrying MVs. This finding provides novel insights into the subcellular/molecular mechanisms of de novo Treg induction and might serve as promising tool for future therapeutic applications to treat inflammatory disorders.

Proliferation status defines functional properties of endothelial cells

Homeostasis of solid tissue is characterized by a low proliferative activity of differentiated cells while special conditions like tissue damage induce regeneration and proliferation. For some cell types it has been shown that various tissue-specific functions are missing in the proliferating state, raising the possibility that their proliferation is not compatible with a fully differentiated state. While endothelial cells are important players in regenerating tissue as well as in the vascularization of tumors, the impact of proliferation on their features remains elusive. To examine cell features in dependence of proliferation, we established human endothelial cell lines in which proliferation is tightly controlled by a doxycycline-dependent, synthetic regulatory unit. We observed that uptake of macromolecules and establishment of cell-cell contacts was more pronounced in the growth-arrested state. Tube-like structures were formed in vitro in both proliferating and non-proliferating conditions. However, functional vessel formation upon transplantation into immune-compromised mice was restricted to the proliferative state. Kaposi's sarcoma-associated herpes virus (KSHV) infection resulted in reduced expression of endothelial markers. Upon transplantation of infected cells, drastic differences were observed: proliferation arrested cells acquired a high migratory activity while the proliferating counterparts established a tumor-like phenotype, similar to Kaposi Sarcoma lesions. The study gives evidence that proliferation governs endothelial functions. This suggests that several endothelial functions are differentially expressed during angiogenesis. Moreover, since proliferation defines the functional properties of cells upon infection with KSHV, this process crucially affects the fate of virus-infected cells.

Development of a clonal equine myoblast cell line capable of terminal differentiation into mature myotubes in vitro

OBJECTIVE

To produce a clonal equine myoblast cell line that retains the ability to divide for multiple passages and differentiate into multinucleated myotubes during specific conditions.

SAMPLE

Cultured primary equine skeletal muscle-derived cells from a healthy Thoroughbred.

PROCEDURES

Cell cultures were transfected by electroporation with a plasmid (pNIT) that expresses the temperature-sensitive simian vacuolating virus 40 large T antigen (TAg), which can be controlled by a doxycycline-responsive promoter. Cells that stably integrated the TAg were selected and expanded to passage 25. For each passage, differentiation and fusion properties of the cells were determined and immunocytochemical analyses were performed to evaluate expression of TAg and other muscle-specific proteins. Optimum conditions that led to cell differentiation into myotubes were also determined.

RESULTS

Compared with nontransfected control cells, myogenic, desmin-positive cells expressed the TAg when incubated at 33°C and could be maintained in culture for numerous passages. Reduced expression of TAg was identified in cells incubated at 37°C or when incubated with doxycycline at 33°C. Expression of TAg was not detected when cells were incubated with doxycycline at 37°C, and when serum was withdrawn from the culture medium, those clones differentiated into a pure population of multinucleated myotubes.

CONCLUSIONS AND CLINICAL RELEVANCE

Results indicated that production of an immortalized clonal equine skeletal muscle cell line was possible. A clonal equine skeletal muscle cell line will be a valuable in vitro tool for use in equine physiology and disease research.

Immortalized Human Schwann Cell Lines Derived From Tumors of Schwannomatosis Patients

Schwannomatosis, a rare form of neurofibromatosis, is characterized predominantly by multiple, often painful, schwannomas throughout the peripheral nervous system. The current standard of care for schwannomatosis is surgical resection. A major obstacle to schwannomatosis research is the lack of robust tumor cell lines. There is a great need for mechanistic and drug discovery studies of schwannomatosis, yet appropriate tools are not currently available. Schwannomatosis tumors are difficult to grow in culture as they survive only a few passages before senescence. Our lab has extensive experience in establishing primary and immortalized human Schwann cell cultures from normal tissue that retain their phenotypes after immortalization. Therefore we took on the challenge of creating immortalized human Schwann cell lines derived from tumors from schwannomatosis patients. We have established and fully characterized 2 schwannomatosis cell lines from 2 separate patients using SV40 virus large T antigen. One patient reported pain and the other did not. The schwannomatosis cell lines were stained with S100B antibodies to confirm Schwann cell identity. The schwannomatosis cells also expressed the Schwann cell markers, p75NTR, S100B, and NGF after multiple passages. Cell morphology was retained following multiple passaging and freeze/ thaw cycles. Gene expression microarray analysis was used to compare the cell lines with their respective parent tumors. No differences in key genes were detected, with the exception that several cell cycle regulators were upregulated in the schwannomatosis cell lines when compared to their parent tumors. This upregulation was apparently a product of cell culturing, as the schwannomatosis cells exhibited the same expression pattern of cell cycle regulatory genes as normal primary human Schwann cells. Cell growth was also similar between normal primary and immortalized tumor cells in culture. Accurate cell lines derived directly from human tumors will serve as invaluable tools for advancing schwannomatosis research, including drug screening.

Conditional immortalization of primary adipocyte precursor cells

The production of new adipocytes requires the differentiation of adipocyte precursor (AP) cells residing within the adipose tissue stromal-vascular compartment. The objective was to obtain an immortalized primary adipogenic cell line derived from FACS isolated committed APs using the conditional expression of SV40 T antigen. Adipocyte precursors were isolated from white adipose tissue (WAT) using FACS to remove non-adipogenic cell populations from mice expressing a conditionally regulated SV40 T antigen. APs were maintained by continuous culture and induced to undergo adipogenic differentiation. Adipogenesis, determined by Oil Red O staining, was assessed with each passage and compared to wildtype controls. Adipogenic capability was rapidly lost with increased passage number in committed APs with concurrent reduction in cell proliferation and expression of essential late adipogenic genes, including Pparγ and C/ebpα. Thus, FACS purified committed APs have limited capability to undergo expansion and subsequent adipogenic differentiation in vitro even if they are immortalized with the SV40 T antigen.

Delineating the regulation of energy homeostasis using hypothalamic cell models

Attesting to its intimate peripheral connections, hypothalamic neurons integrate nutritional and hormonal cues to effectively manage energy homeostasis according to the overall status of the system. Extensive progress in the identification of essential transcriptional and post-translational mechanisms regulating the controlled expression and actions of hypothalamic neuropeptides has been identified through the use of animal and cell models. This review will introduce the basic techniques of hypothalamic investigation both in vivo and in vitro and will briefly highlight the key advantages and challenges of their use. Further emphasis will be place on the use of immortalized models of hypothalamic neurons for in vitro study of feeding regulation, with a particular focus on cell lines proving themselves most fruitful in deciphering fundamental basics of NPY/AgRP, Proglucagon, and POMC neuropeptide function.

Immortalized Human Hepatic Cell Lines for In Vitro Testing and Research Purposes

The ubiquitous shortage of primary human hepatocytes has urged the scientific community to search for alternative cell sources, such as immortalized hepatic cell lines. Over the years, several human hepatic cell lines have been produced, whether or not using a combination of viral oncogenes and human telomerase reverse transcriptase protein. Conditional approaches for hepatocyte immortalization have also been established and allow generation of growth-controlled cell lines. A variety of immortalized human hepatocytes have already proven useful as tools for liver-based in vitro testing and fundamental research purposes. The present chapter describes currently applied immortalization strategies and provides an overview of the actually available immortalized human hepatic cell lines and their in vitro applications.

Experimental design to evaluate directed adaptive mutation in Mammalian cells

BACKGROUND

We describe the experimental design for a methodological approach to determine whether directed adaptive mutation occurs in mammalian cells. Identification of directed adaptive mutation would have profound practical significance for a wide variety of biomedical problems, including disease development and resistance to treatment. In adaptive mutation, the genetic or epigenetic change is not random; instead, the presence and type of selection influences the frequency and character of the mutation event. Adaptive mutation can contribute to the evolution of microbial pathogenesis, cancer, and drug resistance, and may become a focus of novel therapeutic interventions.

OBJECTIVE

Our experimental approach was designed to distinguish between 3 types of mutation: (1) random mutations that are independent of selective pressure, (2) undirected adaptive mutations that arise when selective pressure induces a general increase in the mutation rate, and (3) directed adaptive mutations that arise when selective pressure induces targeted mutations that specifically influence the adaptive response. The purpose of this report is to introduce an experimental design and describe limited pilot experiment data (not to describe a complete set of experiments); hence, it is an early report.

METHODS

An experimental design based on immortalization of mouse embryonic fibroblast cells is presented that links clonal cell growth to reversal of an inactivating polyadenylation site mutation. Thus, cells exhibit growth only in the presence of both the countermutation and an inducing agent (doxycycline). The type and frequency of mutation in the presence or absence of doxycycline will be evaluated. Additional experimental approaches would determine whether the cells exhibit a generalized increase in mutation rate and/or whether the cells show altered expression of error-prone DNA polymerases or of mismatch repair proteins.

RESULTS

We performed the initial stages of characterizing our system and have limited preliminary data from several pilot experiments. Cell growth and DNA sequence data indicate that we have identified a cell clone that exhibits several suitable characteristics, although further study is required to identify a more optimal cell clone.

CONCLUSIONS

The experimental approach is based on a quantum biological model of basis-dependent selection describing a novel mechanism of adaptive mutation. This project is currently inactive due to lack of funding. However, consistent with the objective of early reports, we describe a proposed study that has not produced publishable results, but is worthy of report because of the hypothesis, experimental design, and protocols. We outline the project's rationale and experimental design, with its strengths and weaknesses, to stimulate discussion and analysis, and lay the foundation for future studies in this field.

Strategies for immortalization of primary hepatocytes

The liver has the unique capacity to regenerate in response to a damaging event. Liver regeneration is hereby largely driven by hepatocyte proliferation, which in turn relies on cell cycling. The hepatocyte cell cycle is a complex process that is tightly regulated by several well-established mechanisms. In vitro, isolated hepatocytes do not longer retain this proliferative capacity. However, in vitro cell growth can be boosted by immortalization of hepatocytes. Well-defined immortalization genes can be artificially overexpressed in hepatocytes or the cells can be conditionally immortalized leading to controlled cell proliferation. This paper discusses the current immortalization techniques and provides a state-of-the-art overview of the actually available immortalized hepatocyte-derived cell lines and their applications.

Mouse cytomegalovirus infection overrules T regulatory cell suppression on natural killer cells

Background

Cytomegalovirus establishes lifelong persistency in the host and leads to life threatening situations in immunocompromised patients. FoxP3⁺ T regulatory cells (Tregs) critically control and suppress innate and adaptive immune responses. However, their specific role during MCMV infection, especially pertaining to their interaction with NK cells, remains incompletely defined.

Methods

To understand the contribution of Tregs on NK cell function during acute MCMV infection, we infected Treg depleted and undepleted DEREG mice with WT MCMV and examined Treg and NK cell frequency, number, activation and effector function in vivo.

Results

Our results reveal an increased frequency of activated Tregs within the CD4⁺ T cell population shortly after MCMV infection. Specific depletion of Tregs in DEREG mice under homeostatic conditions leads to an increase in NK cell number as well as to a higher activation status of these cells as compared with non-depleted controls. Interestingly, upon infection this effect on NK cells is completely neutralized in terms of cell frequency, CD69 expression and functionality with respect to IFN-γ production. Furthermore, composition of the NK cell population with regard to Ly49H expression remains unchanged. In contrast, absence of Tregs still boosts the general T cell response upon infection to a level comparable to the enhanced activation seen in uninfected mice. CD4⁺ T cells especially benefit from Treg depletion exhibiting a two-fold increase of CD69⁺ cells 40 h and IFN-γ⁺ cells 7 days p.i. while, MCMV infection per se induces robust CD8⁺ T cell activation which is also further augmented in Treg-depleted mice. Nevertheless, the viral burden in the liver and spleen remain unaltered upon Treg ablation during the course of infection.

Conclusions

Thus, MCMV infection abolishes Treg suppressing effects on NK cells whereas T cells benefit from their absence during acute infection. This study provides novel information in understanding the collaborative interaction between NK cells and Tregs during a viral infection and provides further knowledge that could be adopted in therapeutic setups to improve current treatment of organ transplant patients where modulation of Tregs is envisioned as a strategy to overcome transplant rejection.

Electronic supplementary material

The online version of this article (doi:10.1186/1743-422X-11-145) contains supplementary material, which is available to authorized users.

Hepatitis C virus replication in mouse cells is restricted by IFN-dependent and -independent mechanisms

BACKGROUND & AIMS

Current treatment strategies for hepatitis C virus (HCV) infection include pegylated interferon (IFN)-alfa and ribavirin. Approximately 50% of patients control HCV infection after treatment, but the broad range of patients' outcomes and responses to treatment, among all genotypes, indicates a role for host factors. Although the IFN system is important in limiting HCV replication, the virus has evolved mechanisms to circumvent the IFN response. However, direct, IFN-independent antiviral processes also might help control HCV replication. We examined the role of IFN-independent responses against HCV replication.

METHODS

We analyzed replication of the subgenomic JFH1 replicon in embryonic fibroblasts and primary hepatocytes from mice with disruptions in genes encoding factors in the IFN-dependent and alternative antiviral pathways (signal transducers and activators of transcription 1 [STAT1], protein kinase R, interferon regulatory factors (IRF) IRF-1, IRF-3, IRF-5, IRF-7, mitochondrial antiviral signaling molecule [MAVS], and IFN receptor [IFNAR]). We also assessed the effects of expression of these factors by mouse primary hepatocytes on HCV replication.

RESULTS

In addition to IRF-3- and IFN-mediated antiviral responses, IFN-independent, but IRF-1- and IRF-5-dependent mechanisms, restrict HCV replication in mouse embryonic fibroblasts. In primary hepatocytes these IFN-independent require MAVS and IRF-1.

CONCLUSIONS

HCV replication is limited by interferon-mediated pathways as well pathways that are independent of type I IFNs. IRF1 and IRF5 control IFN-independent signaling events that lead to antiviral responses. We observed antiviral roles of IRF1 and IRF5 that were IFN-independent and cell-type specific. These mechanisms are important in controlling viruses that interfere with the IFN signaling because cells retain the ability to induce functional but local antiviral states through expression of interferon-stimulated genes.

Eternity and functionality – rational access to physiologically relevant cell lines

In the first 50 years of cell culture, the development of new cell lines was mainly based on trial and error. Due to the understanding of the molecular networks of aging, senescence, proliferation, and adaption by mutation, the generation of new cell lines with physiologic properties has become more systematic. This endeavor has been supported by the availability of new technological achievements and increasing knowledge about the biology of cell differentiation and cell-cell communication. Here, we review some promising developments that are contributing toward this goal. These include molecular tools frequently used for the immortalization process. In addition to these broadly acting immortalization regimens, we focus on the developments of cell type-specific immortalization and on the methodologies of how to control the growth of newly established cell lines.

Nanoporous silica coatings on implant surfaces: characterization, stability, biocompatibility and drug release properties

Nanoporous silica coatings for drug release purposes were prepared on medical implants. As substrate, we chose Bioverit® II, which is a commercial available glass-mica ceramic implant material. The coating was prepared by a dip-coating technique in which long-chain organic molecules act as placeholders for the pores. Characterization of the coatings by scanning transmission electron microscopy and X-ray diffraction showed a disordered nanoporous system with a layer thickness of 30–150 nm. The nanoporous structure was stable for about 12 h in a typical cell culture medium and rearranged to a packing of silica nanoparticles. The coating allowed cell attachment and showed excellent biocompatibility in cell culture tests independently of the particular cell type examined. In vivo, implant-tissue interactions were examined in the middle ear in a novel mouse model. Whole genome expression profiling showed no persisting inflammatory response in the presence of the implants. Release profiles of the antibiotic ciprofloxacin demonstrated that the coating is suitable for a local drug delivery. The drug loading capacity could be drastically increased after sulfonic acid modification of the Bioverit® II surface.

A new reporter mouse cytomegalovirus reveals maintained immediate-early gene expression but poor virus replication in cycling liver sinusoidal endothelial cells

Background

The MCMV major immediate early promoter/enhancer (MIEP) is a bidirectional promoter that drives the expression of the three immediate early viral genes, namely ie1, ie2 and ie3. The regulation of their expression is intensively studied, but still incompletely understood.

Methods

We constructed a reporter MCMV, (MCMV-MIEP^r) expressing YFP and tdTomato under the control of the MIEP as proxies of ie1 and ie2, respectively. Moreover, we generated a liver sinusoidal endothelial cell line (LSEC-uniLT) where cycling is dependent on doxycycline. We used these novel tools to study the kinetics of MIEP-driven gene expression in the context of infection and at the single cell level by flow cytometry and by live imaging of proliferating and G₀-arrested cells.

Results

MCMV replicated to higher titers in G₀-arrested LSEC, and cycling cells showed less cytopathic effect or YFP and tdTomato expression at 5 days post infection. In the first 24 h post infection, however, there was no difference in MIEP activity in cycling or G₀-arrested cells, although we could observe different profiles of MIEP gene expression in different cell types, like LSECs, fibroblasts or macrophages. We monitored infected LSEC-uniLT in G₀ by time lapse microscopy over five days and noticed that most cells survived infection for at least 96 h, arguing that quick lysis of infected cells could not account for the spread of the virus. Interestingly, we noticed a strong correlation between the ratio of median YFP and tdTomato expression and length of survival of infected cells.

Conclusion

By means of our newly developed genetic tools, we showed that the expression pattern of MCMV IE1 and IE2 genes differs between macrophages, endothelial cells and fibroblasts. Substantial and cell-cycle independent differences in the ie1 and ie2 transcription could also be observed within individual cells of the same population, and marked ie2 gene expression was associated with longer survival of the infected cells.

Measurement of mouse cytomegalovirus-induced interferon-β with immortalized luciferase reporter cells

The production of cytokines is a crucial element of the host response to viral and bacterial infections. To follow these events in vivo, transgenic mice have become a valuable tool to study cytokine production through induction of reporter genes. We describe here the generation and immortalization of cells derived from transgenic reporter mice for development of a high-throughput assay system for virus- or bacteria-induced cytokine induction. As an example we describe mouse cytomegalovirus (MCMV) infection of immortalized fibroblasts derived from mice expressing the firefly luciferase reporter downstream of the IFN-β promoter. Common methods to determine IFN-β production, including ELISA, quantitative real-time PCR (qPCR), and transient reporter assays using plasmid-based reporter constructs, have disadvantages and limitations. Transient transfections influence type I IFN responses in most cell types, and IFN-β ELISA as well as qPCR are both laborious and expensive. The method presented here is highly sensitive as well as cost-effective, and allows monitoring of a robust and dose-dependent induction of IFN-β upon virus infection in cell lysates as well as living cells.

Genetic aspects of cell line development from a synthetic biology perspective

Animal cells can be regarded as factories for the production of relevant proteins. The advances described in this chapter towards the development of cell lines with higher productivity capacities, certain metabolic and proliferation properties, reduced apoptosis and other features must be regarded in an integrative perspective. The systematic application of systems biology approaches in combination with a synthetic arsenal for targeted modification of endogenous networks are proposed to lead towards the achievement of a predictable and technologically advanced cell system with high biotechnological impact.

RNA-binding protein HuR autoregulates its expression by promoting alternative polyadenylation site usage

RNA-binding protein HuR modulates the stability and translational efficiency of messenger RNAs (mRNAs) encoding essential components of the cellular proliferation, growth and survival pathways. Consistent with these functions, HuR levels are often elevated in cancer cells and reduced in senescent and quiescent cells. However, the molecular mechanisms that control HuR expression are poorly understood. Here we show that HuR protein autoregulates its abundance through a negative feedback loop that involves interaction of the nuclear HuR protein with a GU-rich element (GRE) overlapping with the HuR major polyadenylation signal (PAS2). An increase in the cellular HuR protein levels stimulates the expression of long HuR mRNA species containing an AU-rich element (ARE) that destabilizes the mRNAs and thus reduces the protein production output. The PAS2 read-through occurs due to a reduced recruitment of the CstF-64 subunit of the pre-mRNA cleavage stimulation factor in the presence of the GRE-bound HuR. We propose that this mechanism maintains HuR homeostasis in proliferating cells. Since only the nuclear HuR is expected to contribute to the auto-regulation, our model may explain the longstanding observation that the increase in the total HuR expression in cancer cells often correlates with the accumulation of its substantial fraction in the cytoplasm.

Lentivirus production is influenced by SV40 large T-antigen and chromosomal integration of the vector in HEK293 cells

Currently, lentiviral vectors for research and gene therapy are produced from 293-T cells that are transiently transfected with plasmids encoding the vector and helper functions. However, transiently transfected vectors as well as the presence of SV40 virus large T-antigen (T-Ag) cause serious technical and safety considerations. We aimed to exploit single copy integration sites in the HEK293 genome supporting lentiviral vector production. We found that lentiviral vectors result in minimal infectious particle production from single copy integrants in HEK293. Moreover, once this cell line harbors single copy integrations of lentiviral vectors, its ability to transiently produce lentiviral vectors becomes strongly impaired. T-Ag has a dramatic effect on virus production. Low levels of constitutive T-Ag expression can overcome the production restriction imposed by integrated lentiviral vectors copies. Interestingly, T-Ag does not exert its role at the level of transcriptional activity of the vector; rather, it seems to impose an indirect effect on the cell thereby enabling lentiviral vector production. Altogether, our study highlights the restrictions for integrated lentiviral vectors that are relevant for the establishment of stable and safe producer cell lines.

Phenotypic and molecular characterization of proliferating and differentiated GnRH-expressing GnV-3 cells

GnRH neurons provide the primary driving force upon the neuroendocrine reproductive axis. Here we used GnV-3 cells, a model of conditionally immortalized GnRH-expressing neurons, to perform an analysis of cell cycle and compare the gene expression profile of proliferating cells with differentiated cells. In the proliferation medium, 45 ± 1.5% of GnV-3 cells are in S-phase by FACS analysis. In the differentiation medium, only 9 ± 0.9% of them are in S-phase, and they acquire the characteristic bipolar shape displayed by preoptic GnRH neurons in vivo. In addition, GnV-3 cells in the differentiated state exhibit electrophysiological properties characteristic of neurons. Transcriptomic analysis identified up-regulation of 1931 genes and down-regulation of 1270 genes in cells grown in the differentiation medium compared to cells in the proliferation medium. Subsequent gene ontology study indicated that genes over-expressed in proliferating GnV-3 cells were mainly involved in cell cycle regulations, whereas genes over-expressed in differentiated cells were mainly involved in processes of differentiation, neurogenesis and neuronal morphogenesis. Taken together, these data demonstrate the occurrence of morphological and physiological changes in GnV-3 cells between the proliferating and the differentiated state. Moreover, the genes differentially regulated between these two different states are providing novel pathways potentially important for a better understanding of the physiology of mature GnRH neurons.

Strict control of transgene expression in a mouse model for sensitive biological applications based on RMCE compatible ES cells

Recombinant mouse strains that harbor tightly controlled transgene expression proved to be indispensible tools to elucidate gene function. Different strategies have been employed to achieve controlled induction of the transgene. However, many models are accompanied by a considerable level of basal expression in the non-induced state. Thereby, applications that request tight control of transgene expression, such as the expression of toxic genes and the investigation of immune response to neo antigens are excluded. We developed a new Cre/loxP-based strategy to achieve strict control of transgene expression. This strategy was combined with RMCE (recombinase mediated cassette exchange) that facilitates the targeting of genes into a tagged site in ES cells. The tightness of regulation was confirmed using luciferase as a reporter. The transgene was induced upon breeding these mice to effector animals harboring either the ubiquitous (ROSA26) or liver-specific (Albumin) expression of CreER^T2, and subsequent feeding with Tamoxifen. Making use of RMCE, luciferase was replaced by Ovalbumin antigen. Mice generated from these ES cells were mated with mice expressing liver-specific CreER^T2. The transgenic mice were examined for the establishment of an immune response. They were fully competent to establish an immune response upon hepatocyte specific OVA antigen expression as indicated by a massive liver damage upon Tamoxifen treatment and did not show OVA tolerance. Together, this proves that this strategy supports strict control of transgenes that is even compatible with highly sensitive biological readouts.

Synthetic gene regulation circuits for control of cell expansion

A major drawback in the analysis of primary cells and in regenerative sciences concerns the limited number and homogeneity of cells. This limitation could be overcome by in vitro cell expansion that retains the properties of the cell types of interest. However, for most primary differentiated cells the proliferation capacity is finite and/or proliferation is associated with dedifferentiation of cells. We have developed a flexible cell expansion strategy that allows strict and reliable control of cell proliferation. This system relies on synthetic gene modules that employ positive feedback loops based on Tetracycline control. These gene modules were constructed and transduced by lentiviral vectors. We succeeded in the generation of murine and importantly also of human endothelial cell lines. The key feature of the established cell lines is that their proliferation status can be strictly controlled while the expression of relevant markers is maintained. This strict control of proliferation was observed in cell clones and in cell pools and was even maintained when two independent immortalizing genes were simultaneously employed. Thus, this strategy is flexible, easy to handle, and reliable. Most importantly, it allows expansion of human cells with a primary-like phenotype.

Precise conditional immortalization of mouse cells using tetracycline-regulated SV40 large T-antigen

Cellular immortalization provides a way for expansion and subsequent molecular characterization of rare cell types. Ideally, immortalization can be achieved by the reversible expression of immortalizing proteins. Here, we describe the use of conditional immortalization based on a modified tetracycline-regulated system for the expression of SV40 large T-antigen in embryonic stem (ES) cells and mice. The modified system relies on a codon improved reverse tetracycline transactivator (irtTA) fused to the ligand-binding domain (LBD) of the androgen receptor (irtTA-ABD) or of a mutated glucocorticoid receptor (irtTA-GBD*). Induction of T-antigen is conferred only after addition of two ligands, one to activate the LBD (mibolerone for irtTA-ABD or dexamethasone for irtTA-GBD*) and one to activate the tetracycline transactivator (doxycycline). In ES cells, changes in gene expression upon large T induction were limited and reversible upon deinduction. Similarly, expression of T-antigen was very tightly regulated in mice. We have isolated and expanded bone marrow mesenchymal stem cells that could be genetically manipulated and maintained their differentiation properties after several passages of expansion under conditions that induce the expression of large T-antigen.

Common signatures for gene expression in postnatal patients with patent arterial ducts and stented arteries

The detailed molecular processes associated with postnatal remodelling of blood vessels are presently not understood. To characterize the response of the patients undergoing stenting of the patent arterial duct, we harvested samples of vascular tissue during surgical repair. Histological analysis of explanted ducts confirmed the patency of the ducts immediately after birth. As expected, a previously unstented duct that was examined 7 months after birth had become closed and ligamentous. Whole genome expression profiling of these samples showed that a large fraction, over 10%, of the gene sequences examined were expressed differentially between the samples taken from patients with open as opposed to the ligamentous duct. Interestingly, in 2 patients in whom closure was prevented by insertion of stents, one showed an expression profile that was similar to that of the patient initially having an unstented open duct, whereas the other was more closely related to the profile of the patient with a duct that had become ligamentous. Moreover, in 2 specimens obtained from patients with stented pulmonary arteries, a large fraction of the genes that were differentially expressed were identical to the pattern seen in the samples from the patients with open ducts. The gene regulation appeared to be independent of the nature of the respective malformations, and the site of implantation of the stents. These findings suggest that a set of differentially expressed genes are indicative for a transcriptional programme in neonatal remodelling of the arterial duct, which may also take place in patients in whom ductal closure is prevented by stents, or in those with stented pulmonary arteries. The differentially expressed genes included a significant number of extracellular matrix synthetic genes, and could therefore be predictive for vascular remodelling and neointimal formation.

Alternatives to animal experimentation for hormonal compounds research

Alternatives to animal testing and the identification of reliable methods that may decrease the need for animals are currently the subject of intense investigation worldwide. Alternative testing procedures are particularly important for synthetic and natural chemicals that exert their biological actions through binding nuclear receptors, called nuclear receptors-interacting compounds (NR-ICs), for which research is increasingly emphasizing the limits of several models in the accurate estimation of the physiological consequences of exposure to these compounds. In particular, estrogen receptor interacting compounds (ER-ICs) have a great impact on human health from the therapeutic, nutritional, and toxicological point of view due to the highly permissive nature of the estrogen receptors towards a large number of natural and synthetic compounds. Similar to in vitro systems, recently generated animal models (e.g., animal models generated for the study of estrogen receptor ligands) may fulfill the 3R principles: refine, reduce, and replace. If used correctly, NR-regulated models, such as reporter mice, xenopus, or zebrafish, and models obtained by somatic gene transfer in reporter systems, combined with imaging technologies, may contribute to strongly decreasing the overall number of animals required for NR-IC testing and research. With these models, flexible and highly standardized parameters and reporter marker quantification can be obtained. Here, we highlight the need for the substitution of currently used testing models with more appropriate ones that can reproduce the features and reactivity of specific mammalian target tissue/organs. We consider the promotion of this advancement a research priority bearing scientific, economic, social, and ethical relevance.

PCR-based simultaneous analysis of the interferon-alpha family reveals distinct kinetics for early interferons

Here we describe a PCR-based analysis system that allows the simple simultaneous assessment of murine interferons (IFN)-alpha and IFN-beta induction in a single reaction. In this analysis, the so-called early IFN-alpha4 can be distinguished from the so-called late IFN-nonalpha4 by employing a primer mixture that amplifies a part of the IFN-alpha genes in which IFN-alpha4 exhibits a deletion of 15 nucleotides compared to IFN-nonalpha4. By including a final denaturation and a slow cooling step at the end of the PCR procedure, hybrid formation was avoided that regularly occurred when standard protocols were used. Separation of the amplification products on 4.5% agarose gels allowed the comparative assessment of the classical type I IFNs. Using this analysis system, we could show that in immortalized adult fibroblasts, IFN-beta is induced first and the two types of IFN-alpha are induced later and simultaneously. When similar fibroblasts derived from mice that lack IFN-beta were tested, the IFN response was delayed. However, now IFN-alpha4 appeared first and apparently induced the cascade of IFN-nonalpha4. This confirms the role of IFN-beta as master regulator of the normal IFN response.

Development of an immortalised, post-pubertal gonadotrophin-releasing hormone neuronal cell line

Gonadotrophin-releasing hormone (GnRH) is important in reproduction, although some of the mechanisms for its synthesis and release remain elusive. Progress in understanding the GnRH neurone has been hampered by the limited number and diffuse distribution of the neurone in the mammalian brain. Several stable GnRH-expressing cell lines have been developed using in vivo expression of the simian virus 40 T Antigen (TAg), and they have been helpful for the study of gene expression and neuronal function. However, expression of an immortalising gene may interfere with normal cellular function. We developed a novel GnRH-secreting cell line transgenic mouse model suitable for targeted transformation in post-pubertal mice using a tetracycline-regulated TAg transgene. This clonal cell line, GRT, expresses neuronal markers and GnRH. GRT cells grown in medium containing tetracycline-free serum express increasing mRNA levels of GnRH associated with declining levels of TAg expression. The novelty and ultimately the usefulness of this cell line is that TAg expression, which could affect the GnRH neuronal phenotype, can be regulated by tetracycline.

Inhibition of prostate carcinogenesis by combined active immunoprophylaxis

The aim of this study is to investigate whether an active immunoprophylactic approach combining specific antigens and adjuvant stimuli would be able to inhibit prostate carcinogenesis in transgenic TRAMP mice. A vaccine consisting of allogeneic large T antigen (TAg)-positive SV40-transformed cells combined with systemic recombinant IL-12 was administered to TRAMP mice, starting from when they were still tumor-free at 5-6 weeks of age. The combined vaccine significantly inhibited prostate carcinogenesis, giving a more than doubled median latency time of prostatic tumors (53 weeks in comparison to 26 weeks in control mice). Vaccination with cells alone or IL-12 treatment alone was poorly effective (median latency of 30 and 39 weeks, respectively). The combined vaccine induced a very high CD4 response biased toward the Th1 pathway, with the induction of a humoral response that included TAg-specific antibodies. Therefore, such active immunoprophylactic approach based on the combination of allogeneic SV40 TAg-positive cells and systemic administration of recombinant IL-12 significantly delayed autochthonous urogenital carcinogenesis driven by SV40 TAg in TRAMP mice.

Reversible transfection of human melanocytes mediated by Cre/loxP site-specific recombination system and SV40 large T antigen

OBJECTIVE

To study the reversible transfection of human melanocytes mediated by simian virus 40 large T antigen (SV40LTAg) and Cre/loxP site-specific recombination system.

METHODS

The reconstructed SV40LTAg-EGFP-neo-loxP vector was transfected into primary cultured human melanocytes with Sofast(TM) transfection reagent and the positive cells were selected using G418. After expanding culture of these positive cell clones, the expression of SV40LTAg was detected by polymerase chain reaction (PCR), reverse transcription-polymerase chain reaction (RT-PCR) and immunofluorescent method. After that, these positive cells were infected by virus supernatant of Cre-ER(T2) retrovirus vector and Cre recombinase was induced to act by tamoxifen. On the 6th and 10th day after Cre recombinase acting, the expression of SV40LTAg was detected using the same methods as above, and cell tumorigenicity was studied using soft agar assay, athymic mouse study and karyotype analysis. On 10th day after tamoxifen treatment, cell biological characters were identified with immunofluorescent staining and transmission electron microscopy. Then these cells were transplanted into vitiligo animal model to observe their melanogenesis ability in vivo.

RESULTS

The genome DNA and total RNA were isolated from the positive cells transfected by SV40LTAg (designated as MCT) and specific 288 bp fragment was amplificated using PCR and RT-PCR methods. The results of immunofluorescence confirmed the expression of SV40LTAg in cell nucleus. On the 6th day after tamoxifen treatment in infected cells by Cre-ER(T2) retrovirus vector (designated as MCT-Cre), there could be detected SV40LTAg expression, but on 10th day, there could not be detected SV40LTAg expression in cells. These results showed that the excised efficiency of Cre recombinase increased along with time prolongation, and would obtain complete recombination efficiency. The identification of MCT-Cre cell biological characters showed that these cells had normal parent-cell-like cell phenotype and no tumorigenicity in vitro. The pigmentation started in 4 weeks and formed black macula in 3 months after grafting. The pathological results showed that there had been significant melanocytes and melanin accumulation in epidermis and some hair follicle in transplanted area, which confirmed that MCT-Cre had melanogenesis function in vivo.

CONCLUSION

Human melanocytes could be mediated by reversible transfection by SV40LTAg and Cre/loxP site-specific recombination system, which had stable parent-cell-like phenotypic characters and no tumorigenicity in vitro; moreover, these cells still had melanogenesis function in vivo.

Identification of key genes responsible for cytokine-induced erythroid and myeloid differentiation and switching of hematopoietic stem cells by RAGE

We utilized a unique culture system to analyze the expression patterns of gene, protein, and cell surface antigen, and the biological process of the related genes in erythroid and myeloid differentiation and switching of hematopoietic stem cells (HSCs) in response to cytokine alterations. Gene-specific fragments (266) identified from five populations of cytokine-stimulated HSCs were categorized into three groups: (1) expressed specifically in a single cell population; (2) expressed in two cell populations, and (3) expressed in three or more populations. Of 145 defined cDNAs, three (2%) were novel genes. Protein two-dimensional gel electrophoresis and flow cytometry analyses showed overlapped and distinguished protein expression profiles in the cell populations studied. Biological process mapping of mRNAs expressed in erythroid and myeloid lineages indicated that mRNAs shared by both lineages attended 'core processes,' whereas genes specifically expressed in either lineage alone were related to specific processes or cellular maturation. Data from this study support the hypothesis that committed HSCs (E14 or G14) cells can still be redirected to develop into myeloid or erythroid cells when erythropoietin (EPO) is replaced with granulocyte-colony stimulating factor (G-CSF) under erythroid-cultured condition or G-CSF with EPO in myeloid-cultured environment, respectively. Our results suggest that genes or proteins co-expressed in erythroid and myeloid lineages may be essential for the lineage maintenance and switching in hematopoiesis.

Current status of transcriptional regulation systems

Many attempts have been undertaken to control transgene activity in mammalian cells. This is of importance for both applied biotechnology and basic research activities. State of the art regulatory systems use elements for transgene regulation which are unrelated to host regulatory networks and thus do not interfere with endogenous activities. Most of these regulation systems consist of transregulators and transregulator responding promoter elements that are derived from non mammalian origin. Apart from the tetracycline (Tet) regulated system which is most widely used for conditional gene expression at the moment, a number of new systems were created. These systems have been significantly refined and their performance makes them suitable for regulating transgenes not only in cellular systems but also in transgenic animals and for human therapeutic use.

Epigenetic silencing and tissue independent expression of a novel tetracycline inducible system in double‐transgenic pigs

The applicability of tightly regulated transgenesis in domesticated animals is severely hampered by the present lack of knowledge of regulatory mechanisms and the long generation intervals. To capitalize on the tightly controlled expression of mammalian genes made possible by using prokaryotic control elements, we have used a single-step transduction to introduce an autoregulative tetracycline-responsive bicistronic expression cassette (NTA) into transgenic pigs. Transgenic pigs carrying one NTA cassette showed a mosaic transgene expression restricted to single muscle fibers. In contrast, crossbred animals carrying two NTA cassettes with different transgenes, revealed a broad tissue-independent and tightly regulated expression of one cassette, but not of the other one. The expression pattern correlated inversely with the methylation status of the NTA transcription start sites indicating epigenetic silencing of one NTA cassette. This first approach on tetracycline regulated transgene expression in farm animals will be valuable for developing precisely controlled expression systems for transgenes in large animals relevant for biomedical and agricultural biotechnology.

Application of a reversible immortalization system for the generation of proliferation-controlled cell lines

To employ physiological mechanisms to control cell growth primary cells were reversibly immortalized using the SV40 TAg. The cells showed a fibroblast-like morphology. When the expression of the TAg was turned off, the cells arrested in the G0/G1 cell cycle phase. The cell culture could be kept for over 1 week in the proliferation-controlled state while the growth arrest remained fully reversible. The regulation was highly efficacious in that the arrested cell population did not spontaneously resume growth, suggesting that in the absence of the immortalizing gene expression endogenous growth-control mechanisms can keep these cells in a viable state for a prolonged time. Recombinant protein expression increased in growth-controlled cells when compared to conventionally cultured cells. Analysis of a secreted pharmaceutical protein revealed high product integrity without any signs of degradation. Therefore, it is feasible to apply genetic regulation of cell immortalization to obtain proliferation-controlled cell lines and this technique may be of interest to generate novel biotechnological producer cells.

Transformation of mouse fibroblasts alters the induction pattern of type I IFNs after virus infection

Type I interferons (IFNs) have been shown to be involved in many immune defence and inflammatory responses. We here show that IFN-beta plays an absolute essential role in the efficient induction of all type I IFNs after infection of primary embryonic as well as primary adult fibroblasts with Sendai virus. In contrast, after immortalization of such fibroblasts with SV40 large T antigen, IFN-alpha4 can be induced independently of IFN-beta. However, efficient secretion of type I IFNs even in immortalized fibroblasts is only found when the complete signalling loop is induced by IFN-beta.

Establishment of murine cell lines by constitutive and conditional immortalization

Mouse cell lines were immortalized by introduction of specific immortalizing genes. Embryonic and adult animals and an embryonal stem cell line were used as a source of primary cells. The immortalizing genes were either introduced by DNA transfection or by ecotropic retrovirus transduction. Fibroblasts were obtained by expression of SV40 virus large T antigen (TAg). The properties of the resulting fibroblast cell lines were reproducible, independent of the donor mouse strains employed and the cells showed no transformed properties in vitro and did not form tumors in vivo. Endothelial cell lines were generated by Polyoma virus middle T antigen expression in primary embryonal cells. These cell lines consistently expressed relevant endothelial cell surface markers. Since the expression of the immortalizing genes was expected to strongly influence the cellular characteristics fibroblastoid cells were reversibly immortalized by using a vector that allows conditional expression of the TAg. Under inducing conditions, these cells exhibited properties that were highly similar to the properties of constitutively immortalized cells. In the absence of TAg expression, cell proliferation stops. Cell growth is resumed when TAg expression is restored. Gene expression profiling indicates that TAg influences the expression levels of more than 1000 genes that are involved in diverse cellular processes. The data show that conditionally immortalized cell lines have several advantageous properties over constitutively immortalized cells.

Transcriptionally regulated immortalization overcomes side effects of temperature-sensitive SV40 large T antigen

The temperature-sensitive mutant of the SV40 virus large T antigen (TAg) tsA58 is frequently employed for the conditional immortalization of primary cells. By increasing the temperature to 39 degrees C, the activity of the mutant TAg is reduced and the status of such cells may then resemble more closely that of primary cells. As an alternative, we used a novel immortalization vector with a tetracycline-regulated expression of the wild-type TAg. This enabled us to investigate the effects of the immortalizing gene expression and of temperature shifts independently of each other. Even for wild-type TAg-derived cell lines the elevated temperatures led to various clone-dependent phenotypes. This suggests that in freshly established cell lines temperature-sensitive growth phenotypes can arise spontaneously and independently of a temperature-sensitive immortalizing gene. Similar effects were observed with spontaneously immortalized cells. On the other hand, not all of the ts-TAg-derived cell lines were proliferation arrested at the non-permissive temperature. Therefore, the assumption that temperature-sensitive growth is solely due to the ts-TAg must be verified for each ts-TAg-derived cell line individually. This complexity could be avoided by using the autoregulatory immortalization vector expressing the wild-type TAg.