Insect cell culture and applications to research and pest management

Maintenance of primary cell cultures of immunocytes from Cacopsylla spp. psyllids: a new in vitro tool for the study of crop pest insects

Primary cell cultures of immunocytes have been developed from the three psyllid species Cacopsylla melanoneura, Cacopsylla pyri (vectors of 'Candidatus Phytoplasma mali' and 'Candidatus Phytoplasma pyri', respectively) and Cacopsylla crataegi. The medium most suitable of those evaluated was Hert-Hunter 70 (HH70) psyllid medium. In fact, good survival and proliferation of the Cacopsylla immunocytes for over 60 d were observed, with mitosis activities starting at 15-d post culture. Moreover, adhesion and phagocytosis activities were confirmed for all the psyllid cell cultures by functionality tests. Morphological examination of cultured immunocytes revealed the presence of different cell types in all the three psyllid species in accordance to published data about insect immunocytes. The in vitro maintenance of psyllid immunocytes represents a powerful tool for a wide range of applications, especially for psyllid cell biology. In particular, in-depth studies on the biology of psyllids as vector insects as well as analyses to understand the mechanisms behind the interactions with pathogens and symbionts are now possible. These cultures can be used as an in vitro model to study psyllid humoral immune responses, which also will allow in-depth investigations on the abilities of psyllids as vectors of phytoplasmas. All these applications provide new opportunities to develop more focused and specific pest control strategies.

Serumfree culture of the suspension cell line QB-Tn9-4s of the cabbage looper, Trichoplusia ni, is highly productive for virus replication and recombinant protein expression

Serumfree cultures of insect cells play an important role in the fields of protein engineering, medicine, and biology. In this paper, the suspension cell line QB-Tn9-4s of Trichoplusia ni (Hübner) (Lepidoptera: Noctuidae) was successfully adapted to serumfree Sf-900 III medium and passaged for 52 generations. The adapted QB-Tn9-4s cells grew faster. Their population doubling time shortened from 27.4 hr in serum-containing medium to 24.1 hr, and their maximal density increased by 1.83-fold, reaching 3.50 ×10(6) cells/mL in serumfree culture in T-flasks. The cells readily adapted to spinner culture, with maximum cell density of 4.40 × 10(6) cells/mL in a spinner flask. Although the infection rate of Autographa californica multiple nucleopolyhedrovirus and production of occlusion bodies (OBs) of the adapted QB-Tn9-4s cells were 91.0% and 85.4 OBs/cell, respectively, similar to those of QB-Tn9-4s cells cultured in serum-containing medium and control BTI-Tn5B1-4 cells, their budded virus titer was 4.97 ×10(7) TCID50/mL, significantly higher than those of the latter two. In addition, the expression levels of β-galactosidase at six days postinfection and secreted alkaline phosphatase at seven days postinfection in the adapted QB-Tn9-4s cells reached 2.98 ± 0.15×10(4) IU/mL and 3.34 ± 0.13 IU/mL, respectively, significantly higher than those of QB-Tn9-4s and control BTI-Tn5B1-4 cultured in serum-containing media. The above findings establish a foundation for industrial production of virus and recombinant proteins in QB-Tn9-4s serumfree culture.

Establishment and characterization of three new cell lines from the embryonic tissue of Holotrichia oblita Faldermann (Coleoptera: Scarabaeidae)

The establishment of new insect cell lines plays important roles in the researches of insect pathology, insect toxicology, insecticide screening and activity assay, etc. Using embryos of Holotrichia oblita Faldermann (Coleoptera: Scarabaeidae) as materials, this study describes the establishment of three cell lines designated as QAU-Ho-E-3 (Ho-3), QAU-Ho-E-4 (Ho-4), and QAU-Ho-E-6 (Ho-6), respectively. Currently, the three cell lines have been passaged more than 50 times in the TNM-FH insect cell medium containing 10% fetal bovine serum (FBS). All of them showed adherent growth. The majority of Ho-3 cells are spindle-shaped, with a size of 24.35 ± 5.29 × 11.56 ± 1.67 μm. The Ho-4 cells were either spindle-shaped or oblong, with a size of 38.07 ± 8.57 × 17.62 ± 2.48 μm. The Ho-6 cells were primarily round in shape with a diameter of 14.54 ± 1.96 μm. The Ho-3 and Ho-4 cell lines contained 20 chromosomes (i.e., diploid, 2n = 20) at passages 14 and 45. The Ho-6 cell line contained 20 chromosomes (i.e., diploid, 2n = 20) at passage 14 but 40 chromosomes (i.e., polyploidy, 4n = 40) at passage 45. The results of random amplified polymorphic DNA (RAPD) analysis showed that the RAPD fingerprint of the three cell lines was consistent with that of H. oblita eggs, but clearly different from that of BTI-Tn5B1-4 and Sf-9 cells, demonstrating that the three cell lines Ho-3, Ho-4, and Ho-6 are H. oblita cell lines. The results of the growth curve test showed that the population doubling times of Ho-3, Ho-4, and Ho-6 were 101.1, 105.2, and 83.6 h, respectively. The viral infection assay indicated that these H. oblita cell lines were not permissive to infection by Autographa californica multiple nucleopolyhedrovirus (AcMNPV) or Bombyx mori nucleopolyhedrovirus (BmNPV).

Protective efficacy of a human endogenous retrovirus envelope-coated, nonreplicable, baculovirus-based hemagglutin vaccine against pandemic influenza H1N1 2009

Despite the advantages of DNA vaccines, overcoming their lower efficacy relative to that of conventional vaccines remains a challenge. Here, we constructed a human endogenous retrovirus (HERV) envelope-coated, nonreplicable, baculovirus-based HA vaccine against swine influenza A/California/04/2009(H1N1) hemagglutin (HA) (AcHERV-sH1N1-HA) as an alternative to conventional vaccines and evaluated its efficacy in two strains of mice, BALB/c and C57BL/6. A commercially available, killed virus vaccine was used as a positive control. Mice were intramuscularly administered AcHERV-sH1N1-HA or the commercial vaccine and subsequently given two booster injections. Compared with the commercial vaccine, AcHERV-sH1N1-HA induced significantly higher levels of cellular immune responses in both BALB/c and C57BL/6 mice. Unlike cellular immune responses, humoral immune responses depended on the strain of mice. Following immunization with AcHERV-sH1N1-HA, C57BL/6 mice showed HA-specific IgG titers 10- to 100-fold lower than those of BALB/c mice. In line with the different levels of humoral immune responses, the survival of immunized mice after intranasal challenge with sH1N1 virus (A/California/04/2009) depended on the strain. After challenge with 10-times the median lethal dose (MLD₅₀) of sH1N1 virus, 100% of BALB/c mice immunized with the commercial vaccine or AcHERV-sH1N1-HA survived. In contrast, C57BL/6 mice immunized with AcHERV-sH1N1-HA or the commercial vaccine showed 60% and 70% survival respectively, after challenge with sH1N1 virus. In all mice, virus titers and results of histological analyses of lung tissues were consistent with the survival data. Our results indicate the importance of humoral immune response as a major defense system against influenza viral infection. Moreover, the complete survival of BALB/c mice immunized with AcHERV-sH1N1-HA after challenge with sH1N1 virus suggests the potential of baculoviral vector-based vaccines to achieve an efficacy comparable to that of killed virus vaccines.

A cell-based reporter assay for screening for EcR agonist/antagonist activity of natural ecdysteroids in Lepidoptera (Bm5) and Diptera (S2) cell cultures, followed by modeling of ecdysteroid-EcR interactions and normal mode analysis

Ecdysteroid signal transduction is a key process in insect development and therefore an important target for insecticide development. We employed an in vitro cell-based reporter bioassay for the screening of potential ecdysone receptor (EcR) agonistic and antagonistic compounds. Natural ecdysteroids were assayed with ecdysteroid-responsive cell line cultures that were transiently transfected with the reporter plasmid ERE-b.act.luc. We used the dipteran Schneider S2 cells of Drosophila melanogaster and the lepidopteran Bm5 cells of Bombyx mori, representing important pest insects in medicine and agriculture. Measurements showed an EcR agonistic activity only for cyasterone both in S2 (EC50=3.3μM) and Bm5 cells (EC50=5.3μM), which was low compared to that of the commercial dibenzoylhydrazine-based insecticide tebufenozide (EC50=0.71μM and 0.00089μM, respectively). Interestingly, a strong antagonistic activity was found for castasterone in S2 cells with an IC50 of 0.039μM; in Bm5 cells this effect only became visible at much higher concentrations (IC50=18μM). To gain more insight in the EcR interaction, three-dimensional modeling of dipteran and lepidopteran EcR-LBD was performed. In conclusion, we showed that the EcR cell-based reporter bioassay tested here is a useful and practical tool for the screening of candidate EcR agonists and antagonists. The docking experiments as well as the normal mode analysis provided evidence that the antagonist activity of castasterone may be through direct binding with the receptor with specific changes in protein flexibility. The search for new ecdysteroid-like compounds may be particularly relevant for dipterans because the activity of dibenzoylhydrazines appears to be correlated with an extension of the EcR-LBD binding pocket that is prominent in lepidopteran receptors but less so in the modeled dipteran structure.

Establishment of a versatile cell line for juvenile hormone signaling analysis in Tribolium castaneum

The red flour beetle, Tribolium castaneum, has been widely used as a laboratory model for analyzing gene function. In this study, we established a novel cell line (Tc81) from T. castaneum embryos and validated the utility of this cell line by analyzing the juvenile hormone (JH) signaling pathway. In Tc81 cells, the Krüppel homolog 1 gene (Kr-h1), which is a JH-dependent repressor of insect metamorphosis, was rapidly induced by subnanomolar levels of JHs. Bioinformatics analysis and reporter assays identified 2 JH response elements (kJHREs) located in the region upstream of the transcription start site and in the first intron of Kr-h1. Furthermore, methoprene tolerant (Met) and steroid receptor co-activator (SRC) RNAi reduced JH-dependent induction of Kr-h1 transcripts and kJHRE-reporter activities. Thus, this novel Tc81 cell line is useful for the elucidation of JH signaling and is a promising tool for the functional analysis of genes by RNAi and reporter assays.

Molluscan cells in culture: primary cell cultures and cell lines

In vitro cell culture systems from molluscs have significantly contributed to our basic understanding of complex physiological processes occurring within or between tissue-specific cells, yielding information unattainable using intact animal models. In vitro cultures of neuronal cells from gastropods show how simplified cell models can inform our understanding of complex networks in intact organisms. Primary cell cultures from marine and freshwater bivalve and gastropod species are used as biomonitors for environmental contaminants, as models for gene transfer technologies, and for studies of innate immunity and neoplastic disease. Despite efforts to isolate proliferative cell lines from molluscs, the snail Biomphalaria glabrata Say, 1818 embryonic (Bge) cell line is the only existing cell line originating from any molluscan species. Taking an organ systems approach, this review summarizes efforts to establish molluscan cell cultures and describes the varied applications of primary cell cultures in research. Because of the unique status of the Bge cell line, an account is presented of the establishment of this cell line, and of how these cells have contributed to our understanding of snail host-parasite interactions. Finally, we detail the difficulties commonly encountered in efforts to establish cell lines from molluscs and discuss how these difficulties might be overcome.

High entomotoxicity and mechanism of the fungal GalNAc/Gal-specific Rhizoctonia solani lectin in pest insects

Whole insect assays where Rhizoctonia solani agglutinin (RSA) was fed to larval stages of the cotton leaf-worm Spodoptera littoralis and the pea aphid Acyrthosiphon pisum demonstrated a high concentration-dependent entomotoxicity, suggesting that this GalNAc/Gal-specific fungal lectin might be a good control agent for different pest insects. RSA at 10 mg/g in the solid diet of 2nd-instar caterpillars caused 84% weight reduction after 8 days with none of the caterpillars reaching the 4th-instar stage. In sucking aphids, 50% mortality was achieved after 3 days with 9 μM of RSA in the liquid diet. Feeding of FITC-labeled RSA to both insect pest species revealed strong lectin binding at the apical/luminal side of the midgut epithelium with the brush border zone, suggesting the insect midgut as a primary insecticide target tissue for RSA. This was also confirmed with cell cultures in vitro, where there was high fluorescence binding at the microvillar zone with primary cultures of larval midgut columnar cells of S. littoralis, and also at the surface with the insect midgut CF-203 cell line without lectin uptake in the midgut cells. In vitro assays using insect midgut CF-203 cells, revealed that RSA was highly toxic with an EC50 of 0.3 μM. Preincubation with GalNAc and saponin indicated that this action of RSA was carbohydrate-binding dependent and happened at the surface of the cells. Intoxicated CF-203 cells showed symptoms of apoptosis as nuclear condensation and DNA fragmentation, and this concurred with an increase of caspase-3/7, -8 and -9 activities. Finally, RSA affinity chromatography of membrane extracts of CF-203 cells followed by LC-MS/MS allowed the identification of 5747 unique peptides, among which four putatively glycosylated membrane proteins that are associated with apoptosis induction, namely Fas-associated factor, Apoptosis-linked gene-2, Neuroglian and CG2076, as potential binding targets for RSA. These data are discussed in relation to the physiological effects of RSA.

The role of ROS toxicity in spontaneous aneuploidy in cultured cells

It is well known that the karyotype of animal cells cultured in vitro tends to become aneuploid as the culture ages. Aneuploidy can cause genetic instability, alter the biological properties of cells, and affect their application in genetic studies and cell engineering. Understanding the causes and mechanisms of aneuploidy is primary to control its occurrence in cultured cells, and is also helpful to understand the mechanisms of tumorigenesis because aneuploidy is a hallmark of tumor cells. This review underscores the potential role of reactive oxygen species (ROS) toxicity in spontaneous aneuploidy of cultured cells. The underlying mechanisms and possible sources of ROS are also discussed.

Establishment of shrimp cell lines: perception and orientation

Development of continuous shrimp cell lines for effective investigation on shrimp viruses remains elusive with an arduous history of over 25 years. Despite presenting challenges to researchers in developing a cell line, the billion dollar aquaculture industry is under viral threat. Advances in molecular biology and various gene transfer technologies for immortalization of cells have resulted in the development of hundreds of cell lines from insects and mammals, but yet not a single cell line has been developed from shrimp and other marine invertebrates. Though improved growth and longevity of shrimp cells in vitro could be achieved by using modified growth media this did not make any leap to spontaneous transformation; probably due to the fact that shrimp cells inhibited neoplastic transformations. Oncogenic induction and immortalization are considered as the possible ways, and an exclusive medium for shrimp cell culture and an appropriate mode of transformation are crucial. In this review status of shrimp cell line development and its future orientation are discussed.

A cell line derived from the red flour beetle Tribolium castaneum (Coleoptera: Tenebrionidae)

The red flour beetle, Tribolium castaneum, is a model organism for agricultural and medical research and its complete genome is sequenced. We established a continuously replicating T. castaneum cell line to complement existing physiological, genetic, and genomic research tools. We set up trial cell cultures from egg, pupa, and adult stages as tissue sources and incubated them in six separate cell culture media to determine the optimal combination of tissue source and medium for cell replication. Our most promising culture was generated by co-culturing adult (∼75 %) and pupal tissues in EX-CELL 420 medium containing 9 % FBS. Our new cell culture is designated BCIRL-TcA-CLG1 (TcA) and it has been subcultured more than 90 times. Amplification of genomic DNA with species-specific primers yielded DNA fragments of the expected sizes and with sequences identical to those from the published Tribolium genome. Additionally, we characterized this line using DNA fingerprinting (DAF-PCR) and compared it with three other coleopteran cell lines and its conspecific pupae to confirm identity. Its doubling time is 155.2 hr. Early passages consisted of attached cells and vesicles in suspension, whereas later passages consisted primarily of attached, spherical cells. Similar to other established cell lines, the ploidy of TcA cells was variable, ranging from 20 chromosomes/cell (diploid) to above 30 chromosomes/cell. TcA cells withstood incubation at 40°C for 1 h with no decrease in viability. We recorded increased levels of one heat shock protein (43 kDa) and of the hsp68a transcript following exposure to 40°C. Taken together, this represents the first report of a continuously replicating T. castaneum cell line. We expect the BCIRL-TcA-CLG1 line will become a useful tool in Tribolium research.

Cell cultures from marine invertebrates: new insights for capturing endless stemness

Despite several decades of extensive research efforts, there is yet no single permanent cell line available from marine invertebrates as these cells stop dividing in vitro within 24-72 h after their isolation, starting cellular quiescence. This ubiquitous quiescent state should be modified in a way that at least some of the quiescent cells will become pluripotent, so they will have the ability to divide and become immortal. Following the above need, this essay introduces the rationale that the discipline of marine invertebrates' cell culture should gain from applying of two research routes, relevant to mammalian systems but less explored in the marine arena. The first is the use of adult stem cells (ASC) from marine organisms. Many marine invertebrate taxa maintain large pools of ASC in adulthood. Ample evidence attests that these cells from sponges, cnidarians, flatworms, crustaceans, mollusks, echinoderms, and ascidians play important roles in maintenance, regeneration, and asexual cloning, actively proliferating in vivo, resembling the vertebrates' cancer stem cells features. The second route is to target resting somatic cell constituents, manipulating them in the same way as has recently been performed on mammalian induced pluripotent stem (iPS) cells. While "iPS cells" are the outcome of an experimental manipulation, ASC are natural and rather frequent in a number of marine invertebrates. Above two cell categories reveal that there are more than a few types of seeds (cells) waiting to be sowed in the right soil (in vitro environmental conditions) for acquiring stemness and immortality. This rationale carries the potential to revolutionize the discipline of marine invertebrate cell cultures. When cultured "correctly," ASC and "iPS cells" from marine invertebrates may stay in their primitive stage and proliferate without differentiating into cells lineages, harnessing the stem cell's inherent abilities of self-replication versus differentiated progenies, toward the development of immortal cell lines.

Antihypertensive effect of insect cells: in vitro and in vivo evaluation

In this study, we investigated the in vitro ACE inhibitory and in vivo antihypertensive effect of insect cell extracts. The IC(50) of three insect cell lines from different type and insect species origin: S2 (embryo, Drosophila melanogaster), Sf21 (ovary, Spodoptera frugiperda) and Bm5 (ovary, Bombyx mori), were evaluated. Most interesting results were that the IC(50) values ranged between 0.4 and 0.9 mg/ml, and that an extra hydrolysis with gastrointestinal enzymes did not increase the ACE inhibitory activity conspicuously. Finally, a single oral administration with a gavage of 150 mg cell extract/kg BW to spontaneous hypertensive rats (SHR) significantly decreased (p<0.05) their systolic blood pressure (SBP) with 5-6% (9-12 mmHg) compared to the controls at 6 h post-administration. Here the undigested and digested insect S2 cell extracts were equal in activity to lower the SBP. To the best of our knowledge, this is the first report of in vivo antihypertensive activity of insect cell extracts and this without an extra digestion requirement.

Ecdysone signaling and transcript signature in Drosophila cells resistant against methoxyfenozide

Methoxyfenozide (RH-2485) is a non-steroidal ecdysteroid agonist with a dibenzoylhydrazine structure, representing a group used as novel biorational insecticides in the control of insect pests. Here we report on the selection of Drosophila melanogaster S2 cells for resistance to inhibition of cell proliferation by methoxyfenozide by ∼ 1000-fold over 4 months. Cells were exposed to gradually increasing concentrations of methoxyfenozide and selected out based on the ecdysteroid-sensitive response for cell proliferation. In the resistant cells, the ecdysteroid receptor (EcR/USP) complex was no longer active in the presence of methoxyfenozide. But when resistant cells were relaxed from pressure in methoxyfenozide-free medium, induction of the reporter construct was observed. In parallel, EcR/USP functionality was also restored when resistant cells were rescued by a Drosophila EcR plasmid. However, it was striking that in the resistant cells the ecdysteroid-sensitive response for cell proliferation was not restored upon methoxyfenozide withdrawal, indicating permanent changes in the physiology of the cells during selection. To investigate changes in gene expression caused by inactivation of the EcR/USP complex in resistant cells, Drosophila oligo 14kv1 microarrays were used and probed with cDNAs from resistant cells in the presence and absence of ecdysone agonist on one hand and from unselected sensitive cells on the other hand. A selection of 324 differentially expressed genes was assigned covering diverse functions as transport, enzyme activity, cytoskeleton organization, cell cycle machinery, transcription/translation and ecdysteroid signaling. Besides the identification of (primary and secondary) target genes of the EcR/USP signaling pathway, this analysis also allows to gain insights into the mechanism of resistance and on the crosstalk between ecdysteroid signaling and cell proliferation-linked processes.

Exposure of insect midgut cells to Sambucus nigra L. agglutinins I and II causes cell death via caspase-dependent apoptosis

Sambucus nigra agglutinins I and II, further referred to as SNA-I and SNA-II, are two ricin-related lectins from elderberry. SNA-I is a chimeric lectin composed of an A-chain with enzymatic activity and a B-chain with carbohydrate-binding activity, and therefore belongs to the group of type 2 ribosome-inactivating proteins. In contrast, SNA-II consists only of carbohydrate-binding B-chains. The physiological effect of SNA-I was tested on different insect cell lines (midgut, ovary, fat body, embryo). In sensitive midgut CF-203 cells, SNA-I induced cell death with typical characteristics such as cell shrinkage, plasma membrane blebbing, nuclear condensation and DNA fragmentation. The effect was dose-dependent with 50% death of 4-day-exposed cells at 3nM. SNA-I exposure induced caspase-3 like activities, suggesting that SNA-I can induce the apoptotic pathway. Interestingly, the hololectin SNA-II also induced apoptosis in CF-203 cells at similar doses with the same physiological events. SNA-I and SNA-II both induced caspase-dependent apoptosis at low concentrations (nM order), leading to typical symptoms of cell death in sensitive cells. This effect seems independent from the catalytic activity of the A-chain, but depends on the carbohydrate-binding B-chain.

Ao38, a new cell line from eggs of the black witch moth, Ascalapha odorata (Lepidoptera: Noctuidae), is permissive for AcMNPV infection and produces high levels of recombinant proteins

BACKGROUND

The insect cell line is a critical component in the production of recombinant proteins in the baculovirus expression system and new cell lines hold the promise of increasing both quantity and quality of protein production.

RESULTS

Seventy cell lines were established by single-cell cloning from a primary culture of cells derived from eggs of the black witch moth (Ascalapha odorata; Lepidoptera, Noctuidae). Among 8 rapidly growing lines, cell line 38 (Ao38) was selected for further analysis, based on susceptibility to AcMNPV infection and production of secreted alkaline phosphatase (SEAP) from a baculovirus expression vector. In comparisons with low-passage High Five (BTI-Tn-5B1-4) cells, infected Ao38 cells produced beta-galactosidase and SEAP at levels higher (153% and 150%, respectively) than those measured from High Five cells. Analysis of N-glycans of SEAP produced in Ao38 cells revealed two N-glycosylation sites and glycosylation patterns similar to those reported for High Five and Sf9 cells. Glycopeptide isoforms consisted of pauci- or oligomannose, with and without fucose on N-acetylglucosamine(s) linked to asparagine residues. Estimates of Ao38 cell volume suggest that Ao38 cells are approximately 2.5x larger than Sf9 cells but only approximately 74% of the size of High Five cells. Ao38 cells were highly susceptible to AcMNPV infection, similar to infectivity of Sf9 cells. Production of infectious AcMNPV budded virions from Ao38 cells peaked at approximately 4.5 x 10(7) IU/ml, exceeding that from High Five cells while lower than that from Sf9 cells. Ao38 cells grew rapidly in stationary culture with a population doubling time of 20.2 hr, and Ao38 cells were readily adapted to serum-free medium (Sf-900III) and to a suspension culture system. Analysis of Ao38 and a parental Ascalapha odorata cell line indicated that these lines were free of the alphanodavirus that was recently identified as an adventitious agent in High Five cell lines.

CONCLUSIONS

Ao38 cells represent a highly productive new insect cell line that will be useful for heterologous protein expression and other applications in biotechnology.

Enhancing the multiplication of nucleopolyhedrovirus in vitro by manipulation of the pH

Insect nucleopolyhedroviruses (NPVs) are studied widely as agents for biological control, as expression vectors for the production of heterologous proteins, and as transduction vectors for gene therapy applications. Most of these applications rely on the existence of cell lines that allow in vitro multiplication of the virus. The influence of pH in the medium culture on the multiplication of SeMNPV, HearSNPV and AcMNPV in different cell culture lines was investigated. The study showed a strong influence of the medium pH on the virus multiplication with the best results at pH 6.5, about half pH unit above the pH of insect culture media used most commonly. Additional experiments using a recombinant AcMNPV, expressing the green fluorescent protein, suggested that the enhanced virus multiplication at pH 6.5 is due mainly to a facilitated entry of the budded virions into the cells.