Stable expression of biologically active recombinant bovine interleukin-4 in Trypanosoma brucei

Cyclic AMP signalling and glucose metabolism mediate pH taxis by African trypanosomes

The collective movement of African trypanosomes on semi-solid surfaces, known as social motility, is presumed to be due to migration factors and repellents released by the parasites. Here we show that procyclic (insect midgut) forms acidify their environment as a consequence of glucose metabolism, generating pH gradients by diffusion. Early and late procyclic forms exhibit self-organising properties on agarose plates. While early procyclic forms are repelled by acid and migrate outwards, late procyclic forms remain at the inoculation site. Furthermore, trypanosomes respond to exogenously formed pH gradients, with both early and late procyclic forms being attracted to alkali. pH taxis is mediated by multiple cyclic AMP effectors: deletion of one copy of adenylate cyclase ACP5, or both copies of the cyclic AMP response protein CARP3, abrogates the response to acid, while deletion of phosphodiesterase PDEB1 completely abolishes pH taxis. The ability to sense pH is biologically relevant as trypanosomes experience large changes as they migrate through their tsetse host. Supporting this, a CARP3 null mutant is severely compromised in its ability to establish infections in flies. Based on these findings, we propose that the expanded family of adenylate cyclases in trypanosomes might govern other chemotactic responses in their two hosts.

African trypanosomes collectively move in a process called social motility. Here, the authors show that procyclic forms acidify their environment as a consequence of glucose metabolism, generating pH gradients by diffusion that are sensed via cyclic AMP signalling. Parasite mutants defective in cAMP signaling are inhibited in fly infection.

Semi-stable Production of Bovine IL-4 and GM-CSF in The Mammalian Episomal Expression System

Introduction

Granulocyte-macrophage colony stimulating factor (GM-CSF) and interleukin-4 (IL-4) are cytokines widely used in ex vivo monocyte differentiation experiments, vaccine formulations and disease treatment. The aim of this study was to produce recombinant bovine GM-CSF and IL-4 in an episomal expression system that conserves the postransductional modification of the native proteins and to use the products to differentiate bovine monocytes into dendritic cells.

Material and Methods

The recombinant proteins rGM-CSF and rIL-4 were expressed in PEAKrapid CRL-2828 human kidney cells, ATCC CRL-2828. The functional activity of the recombinant cytokines was monitored by registering morphological changes in bovine monocytes and assessing the expression of CD14 upon incubation with them.

Results

Both recombinant proteins were detected in the cell culture supernatant of transfected cells. Culture supernatants of transfected cells induced in bovine monocytes morphological changes that resemble macrophages or dendritic cells. In addition, bovine cells treated with rGM-CSF and rIL-4 showed reduced expression of the macrophage surface marker CD14 compared with untreated cells. This effect indicates the expected differentiation. The expression of the cytokines was stable after many successive cell passages and a freeze/thaw cycle.

Conclusions

The semi-stable mammalian episomal expression system used in this study allowed us to easily produce functional bovine rGM-CSF and rIL-4 without the need for protein purification steps.

Flagellar cAMP signaling controls trypanosome progression through host tissues

The unicellular parasite Trypanosoma brucei is transmitted between mammals by tsetse flies. Following the discovery that flagellar phosphodiesterase PDEB1 is required for trypanosomes to move in response to signals in vitro (social motility), we investigated its role in tsetse flies. Here we show that PDEB1 knockout parasites exhibit subtle changes in movement, reminiscent of bacterial chemotaxis mutants. Infecting flies with the knockout, followed by live confocal microscopy of fluorescent parasites within dual-labelled insect tissues, shows that PDEB1 is important for traversal of the peritrophic matrix, which separates the midgut lumen from the ectoperitrophic space. Without PDEB1, parasites are trapped in the lumen and cannot progress through the cycle. This demonstrates that the peritrophic matrix is a barrier that must be actively overcome and that the parasite’s flagellar cAMP signaling pathway facilitates this. Migration may depend on perception of chemotactic cues, which could stem from co-infecting parasites and/or the insect host.

Trypanosoma brucei probably relies on chemotactic signals for movement through tsetse fly tissues, but the molecular basis is unknown. Here, the authors show that flagellar cAMP signaling is required for traversal of the peritrophic matrix and that, without it, parasites are trapped in the midgut lumen.

Catechol pyrazolinones as trypanocidals: fragment-based design, synthesis, and pharmacological evaluation of nanomolar inhibitors of trypanosomal phosphodiesterase B1

Trypanosomal phosphodiesterases B1 and B2 (TbrPDEB1 and TbrPDEB2) play an important role in the life cycle of Trypanosoma brucei, the causative parasite of human African trypanosomiasis (HAT), also known as African sleeping sickness. We used homology modeling and docking studies to guide fragment growing into the parasite-specific P-pocket in the enzyme binding site. The resulting catechol pyrazolinones act as potent TbrPDEB1 inhibitors with IC₅₀ values down to 49 nM. The compounds also block parasite proliferation (e.g., VUF13525 (20b): T. brucei rhodesiense IC₅₀ = 60 nM, T. brucei brucei IC₅₀ = 520 nM, T. cruzi = 7.6 μM), inducing a typical multiple nuclei and kinetoplast phenotype without being generally cytotoxic. The mode of action of 20b was investigated with recombinantly engineered trypanosomes expressing a cAMP-sensitive FRET sensor, confirming a dose-response related increase of intracellular cAMP levels in trypanosomes. Our findings further validate the TbrPDEB family as antitrypanosomal target.

MAP kinase kinase 1 (MKK1) is essential for transmission of Trypanosoma brucei by Glossina morsitans

MAP kinase kinase 1 (MKK1) is encoded by a single copy gene in Trypanosoma brucei. It has been shown recently that MKK1 is not essential for bloodstream forms [14]. To investigate the requirement for MKK1 in other life-cycle stages we generated null mutants in procyclic forms of a fly-transmissible strain. These grew normally in culture and were able to establish midgut infections in tsetse at normal rates and intensities, but were incapable of colonising the salivary glands. Transformation of null mutants with an ectopic copy of MKK1 enabled parasites to complete the life cycle in tsetse and infect mice. This is the first example of a gene that is indispensable for transmission of T. brucei. It also raises the possibility that activating the MKK1 signalling cascade in vitro might trigger the differentiation and proliferation of life-cycle stages of T. brucei that are currently refractory to culture.

Targeting cattle-borne zoonoses and cattle pathogens using a novel trypanosomatid-based delivery system

Trypanosomatid parasites are notorious for the human diseases they cause throughout Africa and South America. However, non-pathogenic trypanosomatids are also found worldwide, infecting a wide range of hosts. One example is Trypanosoma (Megatrypanum) theileri, a ubiquitous protozoan commensal of bovids, which is distributed globally. Exploiting knowledge of pathogenic trypanosomatids, we have developed Trypanosoma theileri as a novel vehicle to deliver vaccine antigens and other proteins to cattle. Conditions for the growth and transfection of T. theileri have been optimised and expressed heterologous proteins targeted for secretion or specific localisation at the cell interior or surface using trafficking signals from Trypanosoma brucei. In cattle, the engineered vehicle could establish in the context of a pre-existing natural T. theileri population, was maintained long-term and generated specific immune responses to an expressed Babesia antigen at protective levels. Building on several decades of basic research into trypanosomatid pathogens, Trypanosoma theileri offers significant potential to target multiple infections, including major cattle-borne zoonoses such as Escherichia coli, Salmonella spp., Brucella abortus and Mycobacterium spp. It also has the potential to deliver therapeutics to cattle, including the lytic factor that protects humans from cattle trypanosomiasis. This could alleviate poverty by protecting indigenous African cattle from African trypanosomiasis.

Single-celled parasites of the order Kinetoplastida are responsible for devastating diseases of humans and animals, including African trypanosomiasis, Chagas' disease and leishmaniasis. However, there are also many species of trypanosomatids that do not cause disease and are distributed globally. One example is Trypanosoma (Megatrypanum) theileri, which is restricted to bovids and ubiquitous in cattle herds worldwide. This organism is maintained extracellularly in the blood and tissues long-term without any observed ill effects on host health or productivity. Using knowledge of gene expression and protein trafficking in pathogenic trypanosomatids, we have successfully developed, from first principles, Trypanosoma theileri as a delivery system for vaccine antigens and therapeutics. Procedures for the growth, transfection and heterologous gene expression of T. theileri have been developed, and the delivery of a vaccine antigen derived from Babesia divergens evaluated in vivo. Our results demonstrate the ability of T. theileri to be used as a flexible and easily manipulated protein delivery system suitable for the control of cattle pathogens and cattle-borne zoonoses. In one notable application, we propose that the system could allow the expression of serum trypanolytic factors in cattle, with the potential to alleviate poverty in Africa through the killing of pathogenic trypanosomatids in livestock.

Bidirectional silencing of RNA polymerase I transcription by a strand switch region in Trypanosoma brucei

The procyclin genes in Trypanosoma brucei are transcribed by RNA polymerase I as part of 5–10 kb long polycistronic transcription units on chromosomes VI and X. Each procyclin locus begins with two procyclin genes followed by at least one procyclin-associated gene (PAG). In procyclic (insect midgut) form trypanosomes, PAG mRNA levels are about 100-fold lower than those of procyclins. We show that deletion of PAG1, PAG2 or PAG3 results in increased mRNA levels from downstream genes in the same transcription unit. Nascent RNA analysis revealed that most of the effects are due to increased transcription elongation in the knockouts. Furthermore, transient and stable transfections showed that sequence elements on both strands of PAG1 can inhibit Pol I transcription. Finally, by database mining we identified 30 additional PAG-related sequences that are located almost exclusively at strand switch regions and/or at sites where a change of RNA polymerase type is likely to occur.

Major surface glycoproteins of insect forms of Trypanosoma brucei are not essential for cyclical transmission by tsetse

Procyclic forms of Trypanosoma brucei reside in the midgut of tsetse flies where they are covered by several million copies of glycosylphosphatidylinositol-anchored proteins known as procyclins. It has been proposed that procyclins protect parasites against proteases and/or participate in tropism, directing them from the midgut to the salivary glands. There are four different procyclin genes, each subject to elaborate levels of regulation. To determine if procyclins are essential for survival and transmission of T. brucei, all four genes were deleted and parasite fitness was compared in vitro and in vivo. When co-cultured in vitro, the null mutant and wild type trypanosomes (tagged with cyan fluorescent protein) maintained a near-constant equilibrium. In contrast, when flies were infected with the same mixture, the null mutant was rapidly overgrown in the midgut, reflecting a reduction in fitness in vivo. Although the null mutant is patently defective in competition with procyclin-positive parasites, on its own it can complete the life cycle and generate infectious metacyclic forms. The procyclic form of T. brucei thus differs strikingly from the bloodstream form, which does not tolerate any perturbation of its variant surface glycoprotein coat, and from other parasites such as Plasmodium berghei, which requires the circumsporozoite protein for successful transmission to a new host.

A family of stage-specific alanine-rich proteins on the surface of epimastigote forms of Trypanosoma brucei

A 'two coat' model of the life cycle of Trypanosoma brucei has prevailed for more than 15 years. Metacyclic forms transmitted by infected tsetse flies and mammalian bloodstream forms are covered by variant surface glycoproteins. All other life cycle stages were believed to have a procyclin coat, until it was shown recently that epimastigote forms in tsetse salivary glands express procyclin mRNAs without translating them. As epimastigote forms cannot be cultured, a procedure was devised to compare the transcriptomes of parasites in different fly tissues. Transcripts encoding a family of glycosylphosphatidyl inositol-anchored proteins, BARPs (previously called bloodstream alanine-rich proteins), were 20-fold more abundant in salivary gland than midgut (procyclic) trypanosomes. Anti-BARP antisera reacted strongly and exclusively with salivary gland parasites and a BARP 3' flanking region directed epimastigote-specific expression of reporter genes in the fly, but inhibited expression in bloodstream and procyclic forms. In contrast to an earlier report, we could not detect BARPs in bloodstream forms. We propose that BARPs form a stage-specific coat for epimastigote forms and suggest renaming them brucei alanine-rich proteins.

Single-cell analysis divides bovine monocyte-derived dendritic cells into subsets expressing either high or low levels of inducible nitric oxide synthase

Dendritic cells (DC) are important cells at the interface between innate and adaptive immunity. DC have a key role in antigen processing and presentation to T cells. Effector functions of DC related to innate immunity have not been explored extensively. We show that bovine monocyte-derived DC (mDC) express inducible nitric oxide synthase (iNOS) mRNA and protein and produce NO upon triggering with interferon-gamma (IFN-gamma) and heat-killed Listeria monocytogenes (HKLM). An immunocytochemical analysis revealed that a sizeable subset (20-60%) copiously expresses iNOS (iNOShi) upon IFN-gamma/HKLM triggering, whereas the other subset expressed low levels of iNOS (iNOSlo). Monocyte-derived macrophages (mMphi) are more homogeneous with regard to iNOS expression. The number of cells within the iNOSlo mDC subset is considerably larger than the number of dead cells or cells unresponsive to IFN-gamma/HKLM. The large majority of cells translocated p65 to the nucleus upon triggering by IFN-gamma/HKLM. A contamination of mDC with iNOS-expressing mMphi was excluded as follows. (i) Cell surface marker analysis suggested that mDC were relatively homogeneous, and no evidence for a contaminating subset expressing macrophage markers (e.g. high levels of CD14) was obtained. (ii) iNOS expression was stronger in iNOShi mDC than in mMphi. The use of maturation-promoting stimuli revealed only subtle phenotypic differences between immature and mature DC in cattle. Nevertheless, these stimuli promoted development of considerably fewer iNOShi mDC upon triggering with IFN-gamma/HKLM. Immunocytochemical results showed that although a significant proportion of cells expressed iNOS only or TNF only upon triggering with IFN-gamma/HKLM, a significant number of cells expressed both iNOS and TNF, suggesting that TNF and iNOS producing (TIP) DC are present within bovine mDC populations obtained in vitro.

The procyclin-associated genes of Trypanosoma brucei are not essential for cyclical transmission by tsetse

EP and GPEET procyclins are the major surface glycoproteins of Trypanosoma brucei in the midgut of tsetse flies (Glossina spp.). The procyclin genes are located at the beginning of polycistronic transcription units and are followed by at least one procyclin-associated gene (PAG). The EP/PAG1 locus on one copy of chromosome X begins with the three genes EP1, EP2 and PAG1; the end of this unit has not been characterized previously. The EP/PAG2 locus on the other copy of chromosome X contains the same procyclin genes followed by PAG2 and PAG4. Here we show that the EP/PAG1 locus in AnTat1.1 has to be extended by three more PAGs, which we named PAG5, PAG2* and PAG4. The EP/PAG2 locus most likely evolved from the EP/PAG1 locus by deletion of a fragment from within PAG1 to PAG2*. The procyclin loci on the two copies of chromosome VI are indistinguishable, and contain the genes GPEET, EP3, PAG3 and GRESAG2.1. The mRNA levels of PAG1, PAG2 and PAG3 are transiently increased during differentiation of bloodstream forms to procyclic forms. Unexpectedly, procyclic forms of a PAG knockout clone lacking all eight PAGs in the procyclin loci were transmissible by Glossina morsitans. Furthermore, the deletion mutant could still establish midgut infections when competing with a tagged clone with the full complement of PAGs. Cyclical transmission was also possible when tsetse flies were infected with bloodstream forms of the deletion mutant, demonstrating that the PAGs are not essential for the differentiation of bloodstream to procyclic forms in vivo.

Development of an inducible protein expression system based on the protozoan host Leishmania tarentolae

Production of functional eukaryotic proteins in recombinant form is a bottle-neck in various post-genomic applications and in life science in general. At least partially this is due to the problems associated with the use of endogenous RNA polymerase II for high-level transcription of heterologous genes in eukaryotic expression systems. To circumvent these problems we developed a new inducible protein expression system based on the protozoan host Leishmania tarentolae (Trypanosomatidae). We have created a strain of L. tarentolae constitutively co-expressing T7 RNA polymerase and tetracycline repressor. This strain could be stably transformed with the heterologous target gene under control of the T7 promoter/TET operator assembly, which can initiate transcription upon addition of tetracycline to the culture medium. Using this system, we demonstrated that enhanced green fluorescent protein (EGFP) could be overexpressed to a level of ca. 1% of total cellular protein. The developed system was tested for its ability to inducibly co-express multiple genes. Using two copies of the egfp gene integrated at two different genomic sites, we could obtain expression levels reaching 4% of total cellular protein. Further possible improvements and applications of the developed system are discussed.

The effect of recombinant swine interleukin-4 on swine immune cells and on pro-inflammatory cytokine productions in pigs

The in vitro effect and the in vivo influence of recombinant swine IL-4 (rSwIL-4) were characterized in various swine cells and in nursery pigs on LPS-induced endotoxic shock and pro-inflammatory cytokine productions. In in vitro experiment, the rSwIL-4 induced a proliferation of CD4 positive T cells in mitogen-prestimulated peripheral blood mononuclear cell (PBMC). In addition, the rSwIL-4, which was produced from insect cells, promoted the differentiation of monocytes into immature dendritic cells in combination with granulocyte macrophage-colony stimulating factor (GM-CSF). Furthermore, the rSwIL-4 successfully suppressed the LPS-induced secretion of TNF-alpha, IL-1alpha, IL-6, IL-8, and IL-18 from swine alveolar macrophages when rSwIL-4 was treated at the same time with LPS. In in vivo experiment in nursery pigs, subcutaneous pretreatment of rSwIL-4, which was produced from baculovirus expression system, enhanced the severity of respiratory failure with endotoxic shock, and increased the production of TNF-alpha and IL-18 in response to inoculation with LPS. These results indicate that the rSwIL-4 is biologically active in both in vitro and in vivo treatments. Depending on the administration time, pro-inflammatory cytokine productions by IL-4 can cause either inhibitory or stimulatory regulation.

Expression and purification of recombinant swine interleukin-4

The swine interleukin-4 (SwIL-4) cDNA was cloned by RT-PCR. It was expressed using an expression vector pQE30 in E. coli, a baculovirus AcNPV vector pVL1392 in insect cells, and a pCAGGS vector in mammalian cells. The rSwIL-4 proteins expressed from bacteria and insect cells were purified using a chelating affinity column and a mAb-coupled immunoaffinity column. The amount of the products and their bioactivities were compared. All recombinant cytokines were efficiently reacted with the specific antibodies and the molecular weight of rSwIL-4 was approximately 16 kDa in E. coli, 15 and 18 kDa in insect cells, and 15 and 20 kDa in mammalian cells. Variations of molecular weight observed in insect and mammalian cells were probably due to different modification ways of glycosylation. All these recombinant proteins retained their antigenicity and were biologically active in inducing human TF-1 cell proliferation in vitro. The simple purification method will make it possible to evaluate the in vitro and in vivo effects of IL-4 in pigs.

Differential production of cytokines, reactive oxygen and nitrogen by bovine macrophages and dendritic cells stimulated with Toll-like receptor agonists

Toll-like receptors (TLR) have been described as partially sharing signalling pathways but showing unique ligand specificity and tissue distribution. Here, the response of bovine macrophages (Mphi) and dendritic cells (DC), both derived from monocytes, was compared by exposing them to the TLR-specific ligands lipopolysaccharide, poly(I:C)-double-stranded RNA, and CpG-DNA, as well as inactivated Gram-negative and Gram-positive bacteria, shown to bind to TLR. The production of NO, superoxide anion, interleukin-10 (IL-10), IL-12 and tumour necrosis factor (TNF) was determined. Compared to monocytes, Mphi expressed more TLR2 and similar levels of TLR4 mRNA transcripts, as analysed by quantitative polymerase chain reaction, whereas DC expressed reduced amounts. Although both DC and Mphi recognized the TLR ligands, dramatic differences were seen in their reaction pattern to them. Both cell types responded with the production of TNF, but DC produced more IL-12, whereas Mphi produced more IL-10, regardless of the TLR agonist used. Co-stimulation with interferon-gamma influenced the amount of cytokine production, but did not alter the cell type-specific response pattern. Compared to Mphi, DC produced > 10 times less NO upon triggering with TLR ligands. In addition, DC produced superoxide anion to opsonized and non-opsonized zymosan, but not to phorbol 12-myristate 13-acetate, a response pattern confirmed for human Mphi and DC, respectively. Different protein kinase C isoforms and extracellular signal-regulated kinase patterns were detected in cell lysates of resting and stimulated Mphi and DC. Collectively, our results point to profound differences in pathogen-derived signal-response coupling occurring commensurate with distinct functions carried out by Mphi or DC.

Bovine monocytes induce immunoglobulin production in peripheral blood B lymphocytes

Although a role for monocytes and monocyte-derived dendritic cells (DC) in the activation of T cells is well established, it is less clear to what extent DC and their precursors, monocytes, regulate B cell immune responses. Here we show that regulatory mechanisms similar to those in humans are in place in the bovine immune system. In vitro culture of bovine monocytes with bovine B cells activated by the anti-CD3 triggered CD4+ T cells or through immunoglobulin (Ig) receptor crosslinking induces B cell Ig secretion. Unlike bovine monocyte-derived DC, monocytes do not promote Ig class switching to IgG and IgA in activated peripheral blood B cells. These results suggest that bovine monocytes are capable of directly inducing Ig secretion in activated bovine peripheral blood B cells, but do not provide the signals for B cell Ig class switching.

Bovine dendritic cells generated from monocytes and bone marrow progenitors regulate immunoglobulin production in peripheral blood B cells

We examined whether bovine monocyte-derived and bone marrow (BM) dendritic cells (DCs) regulate antibody production in activated peripheral blood B cells. DCs were generated from monocytes and BM progenitors in the presence of bovine recombinant granulocyte/macrophage colony-stimulating factor (GM-CSF) and interleukin 4 (IL-4). Monocyte-derived DCs promoted B cells activated by the anti-CD3 triggered CD4(+) T cells or through immunoglobulin M (IgM) receptor to increase the level of IgG secretion. Furthermore, the addition of DCs triggered B cells activated through IgM receptors to produce IgG2 and IgA, thus inducing an isotype switch. BM-derived DCs increased the production of IgG in B cells activated by the anti-CD3 triggered CD4(+) T cells, but unlike monocyte-derived DCs did not have any effect on B cells activated through surface IgM. These data suggest that the regulation of humoral immune responses in cattle depends on the origin of DCs and the mode of B cell activation.