In Vitro Antibacterial Activity of Microbial Natural Products against Bacterial Pathogens of Veterinary and Zoonotic Relevance

Antimicrobial resistance (AMR) is considered one of the greatest threats to both human and animal health. Efforts to address AMR include implementing antimicrobial stewardship programs and introducing alternative treatment options. Nevertheless, effective treatment of infectious diseases caused by bacteria will still require the identification and development of new antimicrobial agents. Eight different natural products were tested for antimicrobial activity against seven pathogenic bacterial species (Brachyspira sp., Chlamydia sp., Clostridioides sp., Mannheimia sp., Mycobacterium sp., Mycoplasma sp., Pasteurella sp.). In a first pre-screening, most compounds (five out of eight) inhibited bacterial growth only at high concentrations, but three natural products (celastramycin A [CA], closthioamide [CT], maduranic acid [MA]) displayed activity at concentrations <2 µg/mL against Pasteurella sp. and two of them (CA and CT) also against Mannheimia sp. Those results were confirmed by testing a larger collection of isolates encompassing 64 Pasteurella and 56 Mannheimia field isolates originating from pigs or cattle, which yielded MIC90 values of 0.5, 0.5, and 2 µg/mL against Pasteurella and 0.5, 4, and >16 µg/mL against Mannheimia for CA, CT, and MA, respectively. CA, CT, and MA exhibited higher MIC50 and MIC90 values against Pasteurella isolates with a known AMR phenotype against commonly used therapeutic antimicrobial agents than against isolates with unknown AMR profiles. This study demonstrates the importance of whole-cell antibacterial screening of natural products to identify promising scaffolds with broad- or narrow-spectrum antimicrobial activity against important Gram-negative veterinary pathogens with zoonotic potential.

Interdisciplinary studies on Coxiella burnetii: From molecular to cellular, to host, to one health research

The Q-GAPS (Q fever GermAn interdisciplinary Program for reSearch) consortium was launched in 2017 as a German consortium of more than 20 scientists with exceptional expertise, competence, and substantial knowledge in the field of the Q fever pathogen Coxiella (C.) burnetii. C. burnetii exemplifies as a zoonotic pathogen the challenges of zoonotic disease control and prophylaxis in human, animal, and environmental settings in a One Health approach. An interdisciplinary approach to studying the pathogen is essential to address unresolved questions about the epidemiology, immunology, pathogenesis, surveillance, and control of C. burnetii. In more than five years, Q-GAPS has provided new insights into pathogenicity and interaction with host defense mechanisms. The consortium has also investigated vaccine efficacy and application in animal reservoirs and identified expanded phenotypic and genotypic characteristics of C. burnetii and their epidemiological significance. In addition, conceptual principles for controlling, surveilling, and preventing zoonotic Q fever infections were developed and prepared for specific target groups. All findings have been continuously integrated into a Web-based, interactive, freely accessible knowledge and information platform (www.q-gaps.de), which also contains Q fever guidelines to support public health institutions in controlling and preventing Q fever. In this review, we will summarize our results and show an example of how an interdisciplinary consortium provides knowledge and better tools to control a zoonotic pathogen at the national level.

Genome-wide association reveals host-specific genomic traits in Escherichia coli

BACKGROUND

Escherichia coli is an opportunistic pathogen which colonizes various host species. However, to what extent genetic lineages of E. coli are adapted or restricted to specific hosts and the genomic determinants of such adaptation or restriction is poorly understood.

RESULTS

We randomly sampled E. coli isolates from four countries (Germany, UK, Spain, and Vietnam), obtained from five host species (human, pig, cattle, chicken, and wild boar) over 16 years, from both healthy and diseased hosts, to construct a collection of 1198 whole-genome sequenced E. coli isolates. We identified associations between specific E. coli lineages and the host from which they were isolated. A genome-wide association study (GWAS) identified several E. coli genes that were associated with human, cattle, or chicken hosts, whereas no genes associated with the pig host could be found. In silico characterization of nine contiguous genes (collectively designated as nan-9) associated with the human host indicated that these genes are involved in the metabolism of sialic acids (Sia). In contrast, the previously described sialic acid regulon known as sialoregulon (i.e. nanRATEK-yhcH, nanXY, and nanCMS) was not associated with any host species. In vitro growth experiments with a Δnan-9 E. coli mutant strain, using the sialic acids 5-N-acetylneuraminic acid (Neu5Ac) and N-glycolylneuraminic acid (Neu5Gc) as sole carbon source, showed impaired growth behaviour compared to the wild-type.

CONCLUSIONS

This study provides an extensive analysis of genetic determinants which may contribute to host specificity in E. coli. Our findings should inform risk analysis and epidemiological monitoring of (antimicrobial resistant) E. coli.

Portable Differential Detection of CTX-M ESBL Gene Variants, blaCTX-M-1 and blaCTX-M-15, from Escherichia coli Isolates and Animal Fecal Samples Using Loop-Primer Endonuclease Cleavage Loop-Mediated Isothermal Amplification

Cefotaximase-Munich (CTX-M) extended-spectrum beta-lactamase (ESBL) enzymes produced by Enterobacteriaceae confer resistance to clinically relevant third-generation cephalosporins. CTX-M group 1 variants, CTX-M-1 and CTX-M-15, are the leading ESBL-producing Enterobacteriaceae associated with animal and human infection, respectively, and are an increasing antimicrobial resistance (AMR) global health concern. The blaCTX-M-1 and blaCTX-M-15 genes encoding these variants have an approximate nucleotide sequence similarity of 98.7%, making effective differential diagnostic monitoring difficult. Loop-primer endonuclease cleavage loop-mediated isothermal amplification (LEC-LAMP) enables rapid real-time multiplex pathogen detection with single-base specificity and portable on-site testing. We have developed an internally controlled multiplex CTX-M-1/15 LEC-LAMP assay for the differential detection of blaCTX-M-1 and blaCTX-M-15. Assay analytical specificity was established using a panel of human, animal, and environmental Escherichia coli isolates positive for blaCTX-M-1 (n = 18), blaCTX-M-15 (n = 35), and other closely related blaCTX-Ms (n = 38) from Ireland, Germany, and Portugal, with analytical sensitivity determined using probit regression analysis. Animal fecal sample testing using the CTX-M-1/15 LEC-LAMP assay in combination with a rapid DNA extraction protocol was carried out on porcine fecal samples previously confirmed to be PCR-positive for E. coli blaCTX-M. Portable instrumentation was used to further analyze each fecal sample and demonstrate the on-site testing capabilities of the LEC-LAMP assay with the rapid DNA extraction protocol. The CTX-M-1/15 LEC-LAMP assay demonstrated complete analytical specificity for the differential detection of both variants with sensitive low-level detection of 8.5 and 9.8 copies per reaction for blaCTX-M-1 and blaCTX-M-15, respectively, and E. coli blaCTX-M-1 was identified in all blaCTX-M positive porcine fecal samples tested. IMPORTANCE CTX-M ESBL-producing E. coli is an increasing AMR public health issue with the transmission between animals and humans via zoonotic pathogens now a major area of interest. Accurate and timely identification of ESBL-expressing E. coli CTX-M variants is essential for disease monitoring, targeted antibiotic treatment and infection control. This study details the first report of portable diagnostics technology for the rapid differential detection of CTX-M AMR markers blaCTX-M-1 and blaCTX-M-15, facilitating improved identification and surveillance of these closely related variants. Further application of this portable internally controlled multiplex CTX-M-1/15 LEC-LAMP assay will provide new information on the transmission and prevalence of these CTX-M ESBL alleles. Furthermore, this transferable diagnostic technology can be applied to other new and emerging relevant AMR markers of interest providing more efficient and specific portable pathogen detection for improved epidemiological surveillance.

Bovine blood derived macrophages are unable to control Coxiella burnetii replication under hypoxic conditions

Background

Coxiella burnetii is a zoonotic pathogen, infecting humans, livestock, pets, birds and ticks. Domestic ruminants such as cattle, sheep, and goats are the main reservoir and major cause of human infection. Infected ruminants are usually asymptomatic, while in humans infection can cause significant disease. Human and bovine macrophages differ in their permissiveness for C. burnetii strains from different host species and of various genotypes and their subsequent host cell response, but the underlying mechanism(s) at the cellular level are unknown.

Methods

C. burnetii infected primary human and bovine macrophages under normoxic and hypoxic conditions were analyzed for (i) bacterial replication by CFU counts and immunofluorescence; (ii) immune regulators by westernblot and qRT-PCR; cytokines by ELISA; and metabolites by gas chromatography-mass spectrometry (GC-MS).

Results

Here, we confirmed that peripheral blood-derived human macrophages prevent C. burnetii replication under oxygen-limiting conditions. In contrast, oxygen content had no influence on C. burnetii replication in bovine peripheral blood-derived macrophages. In hypoxic infected bovine macrophages, STAT3 is activated, even though HIF1α is stabilized, which otherwise prevents STAT3 activation in human macrophages. In addition, the TNFα mRNA level is higher in hypoxic than normoxic human macrophages, which correlates with increased secretion of TNFα and control of C. burnetii replication. In contrast, oxygen limitation does not impact TNFα mRNA levels in C. burnetii-infected bovine macrophages and secretion of TNFα is blocked. As TNFα is also involved in the control of C. burnetii replication in bovine macrophages, this cytokine is important for cell autonomous control and its absence is partially responsible for the ability of C. burnetii to replicate in hypoxic bovine macrophages. Further unveiling the molecular basis of macrophage-mediated control of C. burnetii replication might be the first step towards the development of host directed intervention measures to mitigate the health burden of this zoonotic agent.

Clostridioides difficile in South American Camelids in Germany: First Insights into Molecular and Genetic Characteristics and Antimicrobial Resistance

Little is known about zoonotic pathogens and their antimicrobial resistance in South American camelids (SAC) in Germany including Clostridioides (C.) difficile. The aim of this study was to investigate prevalence, molecular characteristics and antimicrobial resistance of C. difficile in SAC. Composite SAC faecal samples were collected in 43 husbandries in Central Germany and cultured for C. difficile. Toxinotyping and ribotyping was done by PCR. Whole genome sequencing was performed with Illumina^® Miseq™. The genomes were screened for antimicrobial resistance determinants. Genetic relatedness of the isolates was investigated using core genome multi locus sequence typing (cgMLST) and single nucleotide polymorphism analysis. Antimicrobial susceptibility testing was done using the Etest^® method. Eight C. difficile isolates were recovered from seven farms. The isolates belonged to different PCR ribotypes. All isolates were toxinogenic. cgMLST revealed a cluster containing isolates recovered from different farms. Seven isolates showed similar resistance gene patterns. Different phenotypic resistance patterns were found. Agreement between phenotypic and genotypic resistance was identified only in some cases. Consequently, SAC may act as a reservoir for C. difficile. Thus, SAC may pose a risk regarding zoonotic transmission of toxinogenic, potentially human-pathogenic and resistant C. difficile isolates.

Comparative Genomic Analysis of Antimicrobial-Resistant Escherichia coli from South American Camelids in Central Germany

South American camelids (SAC) are increasingly kept in Europe in close contact with humans and other livestock species and can potentially contribute to transmission chains of epizootic, zoonotic and antimicrobial-resistant (AMR) agents from and to livestock and humans. Consequently, SAC were included as livestock species in the new European Animal Health Law. However, the knowledge on bacteria exhibiting AMR in SAC is too scarce to draft appropriate monitoring and preventive programs. During a survey of SAC holdings in central Germany, 39 Escherichia coli strains were isolated from composite fecal samples by selecting for cephalosporin or fluoroquinolone resistance and were here subjected to whole-genome sequencing. The data were bioinformatically analyzed for strain phylogeny, detection of pathovars, AMR genes and plasmids. Most (33/39) strains belonged to phylogroups A and B1. Still, the isolates were highly diverse, as evidenced by 28 multi-locus sequence types. More than half of the isolates (23/39) were genotypically classified as multidrug resistant. Genes mediating resistance to trimethoprim/sulfonamides (22/39), aminoglycosides (20/39) and tetracyclines (18/39) were frequent. The most common extended-spectrum-β-lactamase gene was bla_CTX-M-1 (16/39). One strain was classified as enteropathogenic E. coli. The positive results indicate the need to include AMR bacteria in yet-to-be-established animal disease surveillance protocols for SAC.

Using unique ORFan genes as strain-specific identifiers for Escherichia coli

BACKGROUND

Bacterial identification at the strain level is a much-needed, but arduous and challenging task. This study aimed to develop a method for identifying and differentiating individual strains among multiple strains of the same bacterial species. The set used for testing the method consisted of 17 Escherichia coli strains picked from a collection of strains isolated in Germany, Spain, the United Kingdom and Vietnam from humans, cattle, swine, wild boars, and chickens. We targeted unique or rare ORFan genes to address the problem of selective and specific strain identification. These ORFan genes, exclusive to each strain, served as templates for developing strain-specific primers.

RESULTS

Most of the experimental strains (14 out of 17) possessed unique ORFan genes that were used to develop strain-specific primers. The remaining three strains were identified by combining a PCR for a rare gene with a selection step for isolating the experimental strains. Multiplex PCR allowed the successful identification of the strains both in vitro in spiked faecal material in addition to in vivo after experimental infections of pigs and recovery of bacteria from faecal material. In addition, primers for qPCR were also developed and quantitative readout from faecal samples after experimental infection was also possible.

CONCLUSIONS

The method described in this manuscript using strain-specific unique genes to identify single strains in a mixture of strains proved itself efficient and reliable in detecting and following individual strains both in vitro and in vivo, representing a fast and inexpensive alternative to more costly methods.

Looking Inside Non-Destructively: Label-Free, Raman-Based Visualization of Intracellular Coxiella burnetii

The life cycle of intracellular pathogens is often complex and can include different morphoforms. Treatment of intracellular infections and unperturbed studying of the pathogen inside the host cell are frequently challenging. Here, we present a Raman-based, label-free, non-invasive, and non-destructive method to localize, visualize, and even quantify intracellular bacteria in 3D within intact host cells in a Coxiella burnetii infection model. C. burnetii is a zoonotic obligate intracellular pathogen that causes infections in ruminant livestock and humans with an acute disease known as Q fever. Using statistical data analysis, no isolation is necessary to gain detailed information on the intracellular pathogen's metabolic state. High-quality false color image stacks with diffraction-limited spatial resolution enable a 3D spatially resolved single host cell analysis that shows excellent agreement with results from transmission electron microscopy. Quantitative analysis at different time points post infection allows to follow the infection cycle with the transition from the large cell variant (LCV) to the small cell variant (SCV) at around day 6 and a gradual change in the lipid composition during vacuole maturation. Spectral characteristics of intracellular LCV and SCV reveal a higher lipid content of the metabolically active LCV.

Interaction of Salmonella Gallinarum and Salmonella Enteritidis with peripheral leucocytes of hens with different laying performance

Salmonella enterica ssp. enterica serovars Enteritidis (SE) and Gallinarum (SG) cause different diseases in chickens. However, both are able to reach the blood stream where heterophils and monocytes are potentially able to phagocytose and kill the pathogens. Using an ex vivo chicken whole blood infection model, we compared the complex interactions of the differentially host-adapted SE and SG with immune cells in blood samples of two White Leghorn chicken lines showing different laying performance (WLA: high producer; R11: low producer). In order to examine the dynamic interaction between peripheral blood leucocytes and the Salmonella serovars, we performed flow cytometric analyses and survival assays measuring (i) leucocyte numbers, (ii) pathogen association with immune cells, (iii) Salmonella viability and (iv) immune gene transcription in infected whole blood over a four-hour co-culture period. Inoculation of blood from the two chicken lines with Salmonella led primarily to an interaction of the bacteria with monocytes, followed by heterophils and thrombocytes. We found higher proportions of monocytes associated with SE than with SG. In blood samples of high producing chickens, a decrease in the numbers of both heterophils and Salmonella was observed. The Salmonella challenge induced transcription of interleukin-8 (IL-8) which was more pronounced in SG- than SE-inoculated blood of R11. In conclusion, the stronger interaction of monocytes with SE than SG and the better survivability of Salmonella in blood of low-producer chickens shows that the host-pathogen interaction and the strength of the immune defence depend on both the Salmonella serovar and the chicken line.

Newly Isolated Animal Pathogen Corynebacterium silvaticum Is Cytotoxic to Human Epithelial Cells

Corynebacterium silvaticum is a newly identified animal pathogen of forest animals such as roe deer and wild boars. The species is closely related to the emerging human pathogen Corynebacterium ulcerans and the widely distributed animal pathogen Corynebacterium pseudotuberculosis. In this study, Corynebacterium silvaticum strain W25 was characterized with respect to its interaction with human cell lines. Microscopy, measurement of transepithelial electric resistance and cytotoxicity assays revealed detrimental effects of C. silvaticum to different human epithelial cell lines and to an invertebrate animal model, Galleria mellonella larvae, comparable to diphtheria toxin-secreting C. ulcerans. Furthermore, the results obtained may indicate a considerable zoonotic potential of this newly identified species.

Shiga Toxin-Producing E. coli in Animals: Detection, Characterization, and Virulence Assessment

Cattle and other ruminants are primary reservoirs for Shiga toxin-producing Escherichia coli (STEC) strains which have a highly variable, but unpredictable, pathogenic potential for humans. Domestic swine can carry and shed STEC, but only STEC strains producing the Shiga toxin (Stx) 2e variant and causing edema disease in piglets are considered pathogens of veterinary medical interest. In this chapter, we present general diagnostic workflows for sampling livestock animals to assess STEC prevalence, magnitude, and duration of host colonization. This is followed by detailed method protocols for STEC detection and typing at genetic and phenotypic levels to assess the relative virulence exerted by the strains.

The Coxiella burnetii T4SS Effector AnkF Is Important for Intracellular Replication

Coxiella burnetii is an obligate intracellular pathogen and the causative agent of the zoonotic disease Q fever. Following uptake by alveolar macrophages, the pathogen replicates in an acidic phagolysosomal vacuole, the C. burnetii-containing vacuole (CCV). Effector proteins translocated into the host cell by the type IV secretion system (T4SS) are important for the establishment of the CCV. Here we focus on the effector protein AnkF and its role in establishing the CCV. The C. burnetii AnkF knock out mutant invades host cells as efficiently as wild-type C. burnetii, but this mutant is hampered in its ability to replicate intracellularly, indicating that AnkF might be involved in the development of a replicative CCV. To unravel the underlying reason(s), we searched for AnkF interactors in host cells and identified vimentin through a yeast two-hybrid approach. While AnkF does not alter vimentin expression at the mRNA or protein levels, the presence of AnkF results in structural reorganization and vesicular co-localization with recombinant vimentin. Ectopically expressed AnkF partially accumulates around the established CCV and endogenous vimentin is recruited to the CCV in a time-dependent manner, suggesting that AnkF might attract vimentin to the CCV. However, knocking-down endogenous vimentin does not affect intracellular replication of C. burnetii. Other cytoskeletal components are recruited to the CCV and might compensate for the lack of vimentin. Taken together, AnkF is essential for the establishment of the replicative CCV, however, its mode of action is still elusive.

The anti-apoptotic Coxiella burnetii effector protein AnkG is a strain specific virulence factor

The ability to inhibit host cell apoptosis is important for the intracellular replication of the obligate intracellular pathogen Coxiella burnetii, as it allows the completion of the lengthy bacterial replication cycle. Effector proteins injected into the host cell by the C. burnetii type IVB secretion system (T4BSS) are required for the inhibition of host cell apoptosis. AnkG is one of these anti-apoptotic effector proteins. The inhibitory effect of AnkG requires its nuclear localization, which depends on p32-dependent intracellular trafficking and importin-α1-mediated nuclear entry of AnkG. Here, we compared the sequences of ankG from 37 C. burnetii isolates and classified them in three groups based on the predicted protein size. The comparison of the three different groups allowed us to identify the first 28 amino acids as essential and sufficient for the anti-apoptotic activity of AnkG. Importantly, only the full-length protein from the first group is a bona fide effector protein injected into host cells during infection and has anti-apoptotic activity. Finally, using the Galleria mellonella infection model, we observed that AnkG from the first group has the ability to attenuate pathology during in vivo infection, as it allows survival of the larvae despite bacterial replication.

Metabolic Traits of Bovine Shiga Toxin-Producing Escherichia Coli (STEC) Strains with Different Colonization Properties

Cattle harbor Shiga toxin-producing Escherichia coli (STEC) in their intestinal tract, thereby providing these microorganisms with an ecological niche, but without this colonization leading to any clinical signs. In a preceding study, genotypic characterization of bovine STEC isolates unveiled that their ability to colonize cattle persistently (STEC^per) or only sporadically (STEC^spo) is more closely associated with the overall composition of the accessory rather than the core genome. However, the colonization pattern could not be unequivocally linked to the possession of classical virulence genes. This study aimed at assessing, therefore, if the presence of certain phenotypic traits in the strains determines their colonization pattern and if these can be traced back to distinctive genetic features. STEC^spo strains produced significantly more biofilm than STEC^per when incubated at lower temperatures. Key substrates, the metabolism of which showed a significant association with colonization type, were glyoxylic acid and L-rhamnose, which were utilized by STEC^spo, but not or only by some STEC^per. Genomic sequences of the respective glc and rha operons contained mutations and frameshifts in uptake and/or regulatory genes, particularly in STEC^per. These findings suggest that STEC^spo conserved features leveraging survival in the environment, whereas the acquisition of a persistent colonization phenotype in the cattle reservoir was accompanied by the loss of metabolic properties and genomic mutations in the underlying genetic pathways.

Dynamic Interplay of Host and Pathogens in an Avian Whole-Blood Model

Microbial survival in blood is an essential step toward the development of disseminated diseases and blood stream infections. For poultry, however, little is known about the interactions of host cells and pathogens in blood. We established an ex vivo chicken whole-blood infection assay as a tool to analyze interactions between host cells and three model pathogens, Escherichia coli, Staphylococcus aureus, and Candida albicans. Following a systems biology approach, we complemented the experimental measurements with functional and quantitative immune characteristics by virtual infection modeling. All three pathogens were killed in whole blood, but each to a different extent and with different kinetics. Monocytes, and to a lesser extent heterophils, associated with pathogens. Both association with host cells and transcriptional activation of genes encoding immune-associated functions differed depending on both the pathogen and the genetic background of the chickens. Our results provide first insights into quantitative interactions of three model pathogens with different immune cell populations in avian blood, demonstrating a broad spectrum of different characteristics during the immune response that depends on the pathogen and the chicken line.

Defying Death - How Coxiella burnetii Copes with Intentional Host Cell Suicide

The obligate intracellular pathogen Coxiella burnetii is the causative agent of the worldwide zoonotic disease Q fever. This Gram-negative bacterium infects macrophages where it establishes a replicative niche in an acidic and phagolysosome-like vacuole. Establishing and maintaining the niche requires a functional type IV secretion system (T4SS) which translocates multiple effector proteins into the host cell. These effector proteins act by manipulating diverse cellular processes allowing the bacterium to establish an infection and complete its complex biphasic developmental cycle. The lengthy nature of this life cycle suggests that C. burnetii has to successfully deal with cellular defense processes. Cell death is one mechanism infected cells frequently utilize to control or to at least minimize the impact of an infection. To date, four effector proteins have been identified in C. burnetii, which interfere with the induction of cell death. Three, AnkG, CaeA, and CaeB, affect intrinsic apoptosis, CaeA additionally extrinsic apoptosis. The proteins target different steps of the apoptotic pathway and are not conserved among isolates suggesting redundancy as an important feature of cell death inhibition. The fourth effector protein, IcaA, interferes with the non-canonical pathway of pyroptosis, an important inflammatory cell death pathway for controlling infectious disease. Autophagy is relevant for the C. burnetii life-cycle, but to which extent autophagic cell death is a factor in bacterial survival and proliferation is still not clear. To convincingly understand how bacterial manipulation of autophagy affects cell death either directly or indirectly will require further experiments. Collectively, C. burnetii modulates the extrinsic and intrinsic apoptotic pathways and non-canonical pyroptosis to inhibit host cell death, thereby providing a stable, intracellular niche for the course of the pathogen's infectious cycle.

Modulation of host cell metabolism by T4SS-encoding intracellular pathogens

Intracellular bacterial pathogens intimately interact with the infected host cell to prevent elimination and to ensure survival. One group of intracellular pathogens, including Coxiella burnetii, Legionella pneumophila, Brucella spp., Anaplasma phagocytophilum, and Ehrlichia chaffeensis, utilizes a type IV secretion system (T4SS) that injects effectors to modulate host cell signalling, vesicular trafficking, autophagy, cell death and transcription to ensure survival [1]. So far, little emphasis has been directed towards understanding how these bacteria manipulate host cell metabolism. This manipulation is not only important for gaining access to nutrients, but also for regulating specific virulence programs [2,3]. Here, we will summarize recent progress made in characterizing the manipulation of host cell metabolism by C. burnetii and other intracellular pathogens utilizing a T4SS.

Genetic instability associated with loop or stem-loop structures within transcription units can be independent of nucleotide excision repair

Simple sequence repeats (SSRs) are found throughout the genome, and under some conditions can change in length over time. Germline and somatic expansions of trinucleotide repeats are associated with a series of severely disabling illnesses, including Huntington's disease. The underlying mechanisms that effect SSR expansions and contractions have been experimentally elusive, but models suggesting a role for DNA repair have been proposed, in particular the involvement of transcription-coupled nucleotide excision repair (TCNER) that removes transcription-blocking DNA damage from the transcribed strand of actively expressed genes. If the formation of secondary DNA structures that are associated with SSRs were to block RNA polymerase progression, TCNER could be activated, resulting in the removal of the aberrant structure and a concomitant change in the region's length. To test this, TCNER activity in primary human fibroblasts was assessed on defined DNA substrates containing extrahelical DNA loops that lack discernible internal base pairs or DNA stem-loops that contain base pairs within the stem. The results show that both structures impede transcription elongation, but there is no corresponding evidence that nucleotide excision repair (NER) or TCNER operates to remove them.

Evidence for Contemporary Switching of the O-Antigen Gene Cluster between Shiga Toxin-Producing Escherichia coli Strains Colonizing Cattle

Shiga toxin-producing Escherichia coli (STEC) comprise a group of zoonotic enteric pathogens with ruminants, especially cattle, as the main reservoir. O-antigens are instrumental for host colonization and bacterial niche adaptation. They are highly immunogenic and, therefore, targeted by the adaptive immune system. The O-antigen is one of the most diverse bacterial cell constituents and variation not only exists between different bacterial species, but also between individual isolates/strains within a single species. We recently identified STEC persistently infecting cattle and belonging to the different serotypes O156:H25 (n = 21) and O182:H25 (n = 15) that were of the MLST sequence types ST300 or ST688. These STs differ by a single nucleotide in purA only. Fitness-, virulence-associated genome regions, and CRISPR/CAS (clustered regularly interspaced short palindromic repeats/CRISPR associated sequence) arrays of these STEC O156:H25 and O182:H25 isolates were highly similar, and identical genomic integration sites for the stx converting bacteriophages and the core LEE, identical Shiga toxin converting bacteriophage genes for stx1a, identical complete LEE loci, and identical sets of chemotaxis and flagellar genes were identified. In contrast to this genomic similarity, the nucleotide sequences of the O-antigen gene cluster (O-AGC) regions between galF and gnd and very few flanking genes differed fundamentally and were specific for the respective serotype. Sporadic aEPEC O156:H8 isolates (n = 5) were isolated in temporal and spatial proximity. While the O-AGC and the corresponding 5' and 3' flanking regions of these aEPEC isolates were identical to the respective region in the STEC O156:H25 isolates, the core genome, the virulence associated genome regions and the CRISPR/CAS elements differed profoundly. Our cumulative epidemiological and molecular data suggests a recent switch of the O-AGC between isolates with O156:H8 strains having served as DNA donors. Such O-antigen switches can affect the evaluation of a strain's pathogenic and virulence potential, suggesting that NGS methods might lead to a more reliable risk assessment.

The Novel Aminomethylcycline Omadacycline Has High Specificity for the Primary Tetracycline-Binding Site on the Bacterial Ribosome

Omadacycline is an aminomethylcycline antibiotic with potent activity against many Gram-positive and Gram-negative pathogens, including strains carrying the major efflux and ribosome protection resistance determinants. This makes it a promising candidate for therapy of severe infectious diseases. Omadacycline inhibits bacterial protein biosynthesis and competes with tetracycline for binding to the ribosome. Its interactions with the 70S ribosome were, therefore, analyzed in great detail and compared with tigecycline and tetracycline. All three antibiotics are inhibited by mutations in the 16S rRNA that mediate resistance to tetracycline in Brachyspira hyodysenteriae, Helicobacter pylori, Mycoplasma hominis, and Propionibacterium acnes. Chemical probing with dimethyl sulfate and Fenton cleavage with iron(II)-complexes of the tetracycline derivatives revealed that each antibiotic interacts in an idiosyncratic manner with the ribosome. X-ray crystallography had previously revealed one primary binding site for tetracycline on the ribosome and up to five secondary sites. All tetracyclines analyzed here interact with the primary site and tetracycline also with two secondary sites. In addition, each derivative displays a unique set of non-specific interactions with the 16S rRNA.

A blast without power - cell death induced by the tuberculosis-necrotizing toxin fails to elicit adequate immune responses

In this study, we deploy a doxycycline-dependent suicide switch integrated in a tumor challenge model. With this experimental setup, we characterized the immunological consequences of cells dying by four distinct cell death stimuli in vivo. We observed that apoptotic cell death induced by expression of the truncated form of BH3 interacting-domain death agonist (tBid) and a constitutively active form of caspase 3 (revC3), respectively, showed higher immunogenicity than cell death induced by expression of the tuberculosis-necrotizing toxin (TNT). Our data indicate that the early release of ATP induces the silent clearance of dying cells, whereas the simultaneous presence of 'find me' signals and danger-associated molecular patterns (DAMPs) promotes inflammatory reactions and increased immunogenicity. This proposed model is supported by findings showing that the production and release of high concentrations of IL-27 by bone-marrow-derived macrophages (BMDM) is limited to BMDM exposed to those forms of death that simultaneously released ATP and the DAMPs heat-shock protein 90 (HSP90) and high-mobility group box-1 protein (HMGB1). These results demonstrate that the tissue microenvironment generated by dying cells may determine the subsequent immune response.

Nucleotide Excision Repair and Transcription-coupled DNA Repair Abrogate the Impact of DNA Damage on Transcription

DNA adducts derived from carcinogenic polycyclic aromatic hydrocarbons like benzo[a]pyrene (B[a]P) and benzo[c]phenanthrene (B[c]Ph) impede replication and transcription, resulting in aberrant cell division and gene expression. Global nucleotide excision repair (NER) and transcription-coupled DNA repair (TCR) are among the DNA repair pathways that evolved to maintain genome integrity by removing DNA damage. The interplay between global NER and TCR in repairing the polycyclic aromatic hydrocarbon-derived DNA adducts (+)-trans-anti-B[a]P-N(6)-dA, which is subject to NER and blocks transcription in vitro, and (+)-trans-anti-B[c]Ph-N(6)-dA, which is a poor substrate for NER but also blocks transcription in vitro, was tested. The results show that both adducts inhibit transcription in human cells that lack both NER and TCR. The (+)-trans-anti-B[a]P-N(6)-dA lesion exhibited no detectable effect on transcription in cells proficient in NER but lacking TCR, indicating that NER can remove the lesion in the absence of TCR, which is consistent with in vitro data. In primary human cells lacking NER, (+)-trans-anti-B[a]P-N(6)-dA exhibited a deleterious effect on transcription that was less severe than in cells lacking both pathways, suggesting that TCR can repair the adduct but not as effectively as global NER. In contrast, (+)-trans-anti-B[c]Ph-N(6)-dA dramatically reduces transcript production in cells proficient in global NER but lacking TCR, indicating that TCR is necessary for the removal of this adduct, which is consistent with in vitro data showing that it is a poor substrate for NER. Hence, both global NER and TCR enhance the recovery of gene expression following DNA damage, and TCR plays an important role in removing DNA damage that is refractory to NER.

RNA aptamers as genetic control devices: the potential of riboswitches as synthetic elements for regulating gene expression

RNA utilizes many different mechanisms to control gene expression. Among the regulatory elements that respond to external stimuli, riboswitches are a prominent and elegant example. They consist solely of RNA and couple binding of a small molecule ligand to the so-called "aptamer domain" with a conformational change in the downstream "expression platform" which then determines system output. The modular organization of riboswitches and the relative ease with which ligand-binding RNA aptamers can be selected in vitro against almost any molecule have led to the rapid and widespread adoption of engineered riboswitches as artificial genetic control devices in biotechnology and synthetic biology over the past decade. This review highlights proof-of-principle applications to demonstrate the versatility and robustness of engineered riboswitches in regulating gene expression in pro- and eukaryotes. It then focuses on strategies and parameters to identify aptamers that can be integrated into synthetic riboswitches that are functional in vivo, before finishing with a reflection on how to improve the regulatory properties of engineered riboswitches, so that we can not only further expand riboswitch applicability, but also finally fully exploit their potential as control elements in regulating gene expression.

Applying an Inducible Expression System to Study Interference of Bacterial Virulence Factors with Intracellular Signaling

The technique presented here allows one to analyze at which step a target protein, or alternatively a small molecule, interacts with the components of a signaling pathway. The method is based, on the one hand, on the inducible expression of a specific protein to initiate a signaling event at a defined and predetermined step in the selected signaling cascade. Concomitant expression, on the other hand, of the gene of interest then allows the investigator to evaluate if the activity of the expressed target protein is located upstream or downstream of the initiated signaling event, depending on the readout of the signaling pathway that is obtained. Here, the apoptotic cascade was selected as a defined signaling pathway to demonstrate protocol functionality. Pathogenic bacteria, such as Coxiella burnetii, translocate effector proteins that interfere with host cell death induction in the host cell to ensure bacterial survival in the cell and to promote their dissemination in the organism. The C. burnetii effector protein CaeB effectively inhibits host cell death after induction of apoptosis with UV-light or with staurosporine. To narrow down at which step CaeB interferes with the propagation of the apoptotic signal, selected proteins with well-characterized pro-apoptotic activity were expressed transiently in a doxycycline-inducible manner. If CaeB acts upstream of these proteins, apoptosis will proceed unhindered. If CaeB acts downstream, cell death will be inhibited. The test proteins selected were Bax, which acts at the level of the mitochondria, and caspase 3, which is the major executioner protease. CaeB interferes with cell death induced by Bax expression, but not by caspase 3 expression. CaeB, thus, interacts with the apoptotic cascade between these two proteins.

Dying autologous cells as instructors of the immune system

In an organism, cell death occurs at many different sites and in many different forms. It is frequently part of normal development or serves to maintain cell homeostasis. In other cases, cell death not only occurs due to injury, disease or infection, but also as a consequence of various therapeutic interventions. However, in all of these scenarios, the immune system has to react to the dying and dead cells and decide whether to mount an immune response, to remain quiet or to initiate healing and repopulation. This is essential for the organism, testified by many diseases that are associated with malfunctioning in the cell death process, the corpse removal, or the ensuing immune responsiveness. Therefore, dying cells generally have to be considered as instructors of the immune system. How this happens and which signals and pathways contribute to modulate or shape the immune response is still elusive in many conditions. The articles presented in this Special Issue address such open questions. They highlight that the context in which cell death occurs will not only influence the cell death process itself, but also affect the surrounding cellular milieu, how the generation and presence of 'eat me' signals can have an impact on cell clearance, and that the exact nature of the residual 'debris' and how it is processed are fundamental to determining the immunological consequences. Hopefully, these articles initiate new approaches and new experiments to complete our understanding of how cell death and the immune system interact with each other.

Tet-Trap, a genetic approach to the identification of bacterial RNA thermometers: application to Pseudomonas aeruginosa

Modulation of mRNA translatability either by trans-acting factors (proteins or sRNAs) or by in cis-acting riboregulators is widespread in bacteria and controls relevant phenotypic traits. Unfortunately, global identification of post-transcriptionally regulated genes is complicated by poor structural and functional conservation of regulatory elements and by the limitations of proteomic approaches in protein quantification. We devised a genetic system for the identification of post-transcriptionally regulated genes and we applied this system to search for Pseudomonas aeruginosa RNA thermometers, a class of regulatory RNA that modulates gene translation in response to temperature changes. As P. aeruginosa is able to thrive in a broad range of environmental conditions, genes differentially expressed at 37 °C versus lower temperatures may be involved in infection and survival in the human host. We prepared a plasmid vector library with translational fusions of P. aeruginosa DNA fragments (PaDNA) inserted upstream of TIP2, a short peptide able to inactivate the Tet repressor (TetR) upon expression. The library was assayed in a streptomycin-resistant merodiploid rpsL(+)/rpsL31 Escherichia coli strain in which the dominant rpsL(+) allele, which confers streptomycin sensitivity, was repressed by TetR. PaDNA fragments conferring thermosensitive streptomycin resistance (i.e., expressing PaDNA-TIP2 fusions at 37°C, but not at 28°C) were sequenced. We identified four new putative thermosensors. Two of them were validated with conventional reporter systems in E. coli and P. aeruginosa. Interestingly, one regulates the expression of ptxS, a gene implicated in P. aeruginosa pathogenesis.

The progression of cell death affects the rejection of allogeneic tumors in immune-competent mice - implications for cancer therapy

Large amounts of dead and dying cells are produced during cancer therapy and allograft rejection. Depending on the death pathway and stimuli involved, dying cells exhibit diverse features, resulting in defined physiological consequences for the host. It is not fully understood how dying and dead cells modulate the immune response of the host. To address this problem, different death stimuli were studied in B16F10 melanoma cells by regulated inducible transgene expression of the pro-apoptotic active forms of caspase-3 (revCasp-3), Bid (tBid), and the Mycobacterium tuberculosis-necrosis inducing toxin (CpnT_CTD). The immune outcome elicited for each death stimulus was assessed by evaluating the allograft rejection of melanoma tumors implanted subcutaneously in BALB/c mice immunized with dying cells. Expression of all proteins efficiently killed cells in vitro (>90%) and displayed distinctive morphological and physiological features as assessed by multiparametric flow cytometry analysis. BALB/c mice immunized with allogeneic dying melanoma cells expressing revCasp-3 or CpnT_CTD showed strong rejection of the allogeneic challenge. In contrast, mice immunized with cells dying either after expression of tBid or irradiation with UVB did not, suggesting an immunologically silent cell death. Surprisingly, immunogenic cell death induced by expression of revCasp-3 or CpnT_CTD correlated with elevated intracellular reactive oxygen species (ROS) levels at the time point of immunization. Conversely, early mitochondrial dysfunction induced by tBid expression or UVB irradiation accounted for the absence of intracellular ROS accumulation at the time point of immunization. Although ROS inhibition in vitro was not sufficient to abrogate the immunogenicity in our allo-immunization model, we suggest that the point of ROS generation and its intracellular accumulation may be an important factor for its role as damage associated molecular pattern in the development of allogeneic responses.

An optimized conditional suicide switch using doxycycline-dependent expression of human tBid

We have examined in a HeLa model system whether tight doxycycline-dependent expression of human tBid can be used as a fast and efficient suicide switch. A stably transfected cell line expressing human tBid in a strictly dox-dependent manner showed fast and efficient killing with up to 98 % specific cell death 24 h after induction. The survival rate was only 0.6 % after eleven days of dox treatment. Very low amounts of tBid were sufficient to induce apoptosis indicating that stringent control of the dox-dependent suicide gene is essential for cell survival in the absence of inducer. Using human tBid, an endogenous effector protein with low immunogenic potential, and doxycycline, an inducer with low cytotoxicity and a long record of safe use in humans, this conditional suicide switch provides a tool for various applications, e.g., adoptive immune transfer.

Riboswitch engineering - making the all-important second and third steps

Synthetic biology uses our understanding of biological systems to develop innovative solutions for challenges in fields as diverse as genetic control and logic devices, bioremediation, materials production or diagnostics and therapy in medicine by designing new biological components. RNA-based elements are key components of these engineered systems. Their structural and functional diversity is ideal for generating regulatory riboswitches that react with many different types of output to molecular and environmental signals. Recent advances have added new sensor and output domains to the existing toolbox, and demonstrated the portability of riboswitches to many different organisms. Improvements in riboswitch design and screens for selecting in vivo active switches provide the means to isolate riboswitches with regulatory properties more like their natural counterparts.

tLivin displays flexibility by promoting alternative cell death mechanisms

Livin is a member of the Inhibitor of Apoptosis (IAP) protein family that inhibits apoptosis triggered by a variety of stimuli. We previously demonstrated that while Livin inhibits caspase activity, caspases can cleave Livin to produce a truncated protein, tLivin and that this newly formed tLivin paradoxically induces cell death. However to date, the mechanism of tLivin-induced cell death is not fully understood. In this study, we set out to characterize the form of cell death mediated by tLivin. Here we demonstrate that, unlike most death-promoting proteins, tLivin is a flexible inducer of cell death capable of promoting necrosis or apoptosis in different cell lines. The unusual flexibility of tLivin is displayed by its ability to activate an alternative form of cell death when apoptosis is inhibited. Thus, tLivin can promote more than one form of cell death in the same cell type. Interestingly, in cells where tLivin induces necrosis, deletion of the caspase binding BIR domain results in tLivin-induced apoptosis, suggesting the BIR domain can potentially hamper the ability of tLivin to induce apoptosis. We further elucidate that tLivin activates the JNK pathway and both tLivin-induced apoptosis and necrosis are partially mediated by JNK activity. Acquired resistance to apoptosis, common in many tumors, impinges on the efficiency of conventional anti-cancer agents that function primarily by inducing apoptosis. The ability of tLivin to induce death of apoptosis-compromised cells makes it an attractive candidate for targeted cancer therapy.

A novel TetR-regulating peptide turns off rtTA-mediated activation of gene expression

Conditional regulation of gene expression is a powerful and indispensable method for analyzing gene function. The “Tet-On” system is a tool widely used for that purpose. Here, the transregulator rtTA mediates expression of a gene of interest after addition of the small molecule effector doxycycline. Although very effective in rapidly turning on gene expression, the system is hampered by the long half-life of doxycycline which makes shutting down gene expression rapidly very difficult to achieve. We isolated an rtTA-binding peptide by in vivo selection that acts as a doxycycline antagonist and leads to rapid and efficient shut down of rtTA-mediated reporter gene expression in a human cell line. This peptide represents the basis for novel effector molecules which complement the “Tet-system” by enabling the investigator to rapidly turn gene expression not just on at will, but now also off.

AP4 is a mediator of epithelial-mesenchymal transition and metastasis in colorectal cancer

The basic helix-loop-helix transcription factor AP4/TFAP4/AP-4 is encoded by a c-MYC target gene and displays up-regulation concomitantly with c-MYC in colorectal cancer (CRC) and numerous other tumor types. Here a genome-wide characterization of AP4 DNA binding and mRNA expression was performed using a combination of microarray, genome-wide chromatin immunoprecipitation, next-generation sequencing, and bioinformatic analyses. Thereby, hundreds of induced and repressed AP4 target genes were identified. Besides many genes involved in the control of proliferation, the AP4 target genes included markers of stemness (LGR5 and CD44) and epithelial-mesenchymal transition (EMT) such as SNAIL, E-cadherin/CDH1, OCLN, VIM, FN1, and the Claudins 1, 4, and 7. Accordingly, activation of AP4 induced EMT and enhanced migration and invasion of CRC cells. Conversely, down-regulation of AP4 resulted in mesenchymal-epithelial transition and inhibited migration and invasion. In addition, AP4 induction was required for EMT, migration, and invasion caused by ectopic expression of c-MYC. Inhibition of AP4 in CRC cells resulted in decreased lung metastasis in mice. Elevated AP4 expression in primary CRC significantly correlated with liver metastasis and poor patient survival. These findings imply AP4 as a new regulator of EMT that contributes to metastatic processes in CRC and presumably other carcinomas.

Recognition of operator DNA by Tet repressor

The recognition of operator DNA by Tet repressor was analyzed by fluorescence stopped flow measurements. The main part of the fluorescence change observed for the reaction of the repressor with operator DNA reflects a second-order binding reaction including the expected concentration dependence. Global fitting of transients measured at different concentrations reveal at least one intramolecular step in addition to the bimolecular step. The rate constant for the bimolecular step is strongly salt dependent approaching the limit of diffusion control 2×10(8) M(-1) s(-1) at 50 mM NaCl and decreasing to 5×10(4) M(-1) s(-1) at 600 mM NaCl. These data are consistent with initial formation of a pre-equilibrium complex; electrostatic steering resulting from the high dipole moment of the repressor may contribute to the strong salt dependence. The rate constants of the intramolecular step are in the range of ~0.1 s(-1). Fluorescence quenching is salt dependent; the overall binding constant to operator O1 at 150 mM NaCl is 5×10(8) M(-1); binding constants at different salt concentrations indicate ~5 ion contacts for the specific complex of the Tet repressor dimer. The binding constant to operator O2 is higher than to O1 by a factor of ~2 at 400 mM NaCl.

Navigation to the graveyard-induction of various pathways of necrosis and their classification by flow cytometry

Apoptosis and necrosis reflect the program of cell death employed by a dying cell and the final stage of death, respectively. Whereas apoptosis is defined as a physiological, highly organized cell death process, necrosis is commonly considered to be accidental and uncontrolled. Physiological and weak pathological death stimuli preferentially induce apoptosis, while harsh non-physiological insults often immediately instigate (primary) necrosis. If an apoptosing cell transits into a phase of plasma membrane disintegration, this stage of death is referred to as secondary or post-apoptotic necrosis.Here, we present several conditions that stimulate primary and/or secondary necrosis and show that necrosis displays considerably different time courses. For subclassification of necrotic phenotypes we employed a flow cytometric single-tube 4-color staining technique including annexin A5-FITC, propidium iodide, DiIC1(5), and Hoechst 33342.

Guiding Differentiation of Stem Cells in Vivo by Tetracycline-Controlled Expression of Key Transcription Factors

Transplantation of stem or progenitor cells is an attractive strategy for cell replacement therapy. However, poor long-term survival and insufficiently reproducible differentiation to functionally appropriate cells in vivo still present major obstacles for translation of this methodology to clinical applications. Numerous experimental studies have revealed that the expression of just a few transcription factors can be sufficient to drive stem cell differentiation toward a specific cell type, to transdifferentiate cells from one fate to another, or to dedifferentiate mature cells to pluripotent stem/progenitor cells (iPSCs). We thus propose here to apply the strategy of expressing the relevant key transcription factors to guide the differentiation of transplanted cells to the desired cell fate in vivo. To achieve this requires tools allowing us to control the expression of these genes in the transplant. Here, we describe drug-inducible systems that allow us to sequentially and timely activate gene expression from the outside, with a particular emphasis on the Tet system, which has been widely and successfully used in stem cells. These regulatory systems offer a tool for strictly limiting gene expression to the respective optimal stage after transplantation. This approach will direct the differentiation of the immature stem/progenitor cells in vivo to the desired cell type.

The Coxiella burnetii type IV secretion system substrate CaeB inhibits intrinsic apoptosis at the mitochondrial level

Manipulation of host cell apoptosis is a virulence property shared by many intracellular pathogens to ensure productive replication. For the obligate intracellular pathogen Coxiella burnetii anti-apoptotic activity, which depends on a functional type IV secretion system (T4SS), has been demonstrated. Accordingly, the C. burnetii T4SS effector protein AnkG was identified to inhibit pathogen-induced apoptosis, possibly by binding to the host cell mitochondrial protein p32 (gC1qR). However, it was unknown whether AnkG alone is sufficient for apoptosis inhibition or if additional effector proteins are required. Here, we identified two T4SS effector proteins CaeA and CaeB (C. burnetii anti-apoptotic effector) that inhibit the intrinsic apoptotic pathway. CaeB blocks apoptosis very efficiently, while the anti-apoptotic activity of CaeA is weaker. Our data suggest that CaeB inhibits apoptosis at the mitochondrial level, but does not bind to p32. Taken together, our results demonstrate that C. burnetii harbours several anti-apoptotic effector proteins and suggest that these effector proteins use different mechanism(s) to inhibit apoptosis.

Promoter strength driving TetR determines the regulatory properties of Tet-controlled expression systems

Bacteria frequently rely on transcription repressors and activators to alter gene expression patterns in response to changes in the surrounding environment. Tet repressor (TetR) is a paradigm transcription factor that senses the environmental state by binding small molecule effectors, the tetracyclines. However, recently isolated peptides that act as inducers of TetR after having been fused to the C-terminus of a carrier protein, suggest that TetR can also regulate gene expression in a signal-transduction pathway. For this shift in regulatory mechanism to be successful, induction of TetR must be sensitive enough to respond to an inducing protein expressed at its endogenous level. To determine this regulatory parameter, a synthetic Tet-regulated system was introduced into the human pathogen Salmonella enterica serovar Typhimurium and tested for inducibility by a peptide. Reporter gene expression was detected if the peptide-containing carrier protein Thioredoxin 1 was strongly overproduced, but not if it was expressed at a level similar to the physiological level of Thioredoxin 1. This was attributed to high steady-state amounts of TetR which was expressed by the promoter of the chloramphenicol acetyl transferase gene (P(cat)). Reducing P(cat) strength either by directed or by random mutagenesis of its -10 element concomitantly reduced the intracellular amounts of TetR. Sensitive and quantitative induction of TetR by an inducing peptide, when it was fused to Thioredoxin 1 at its native locus in the genome, was only obtained with weak P(cat) promoter variants containing GC-rich -10 elements. A second important observation was that reducing the TetR steady-state level did not impair repression. This permits flexible adjustment of an inducible system's sensitivity simply by altering the expression level of the transcription factor. These two new layers of expression control will improve the quality and, thus, the applicability of the Tet and other regulatory systems.

A recombinant triplebody with specificity for CD19 and HLA-DR mediates preferential binding to antigen double-positive cells by dual-targeting

To test the hypothesis that dual-targeting confers the novel ability of selective binding to antigen double-positive over antigen single-positive cells, a single-chain triplebody (sctb), HLA-ds16-hu19, was produced and characterized. The molecule carries three single-chain Fv (scFv) antibody fragments in a single polypeptide chain, the two distal ones specific for the human histocompatibility protein HLA-DR and the B-lymphoid cell surface protein CD19, the central one for CD16, the human low affinity Fc-receptor FcγRIII. For comparison, the bispecific scFvs (bsscFv) hu19-ds16 and HLA-ds16 were also produced. All CD16 binding modules are disulfide-stabilized (ds). The sctb bound simultaneously to both CD19 and HLA-DR on the same cancer cell and, thus, showed functional dual-targeting. In a mixing-experiment with HLA-DR single-positive HUT-78 cells and (HLA-DR plus CD19) double-positive SEM cells, the triplebody showed preferential binding to the double-positive cells, even when the single-positive cells were present in a numerical excess of up to 20-fold. In antibody-dependent cellular cytotoxicity experiments with mononuclear cells as effector cells, the sctb promoted equal lysis of Raji cells, an antigen double-positive cell line, at 130-fold lower concentrations than the bsscFv hu19-ds16, indicating that both distal scFvs of the sctb contributed to tumor cell lysis. A panel of stably-transfected HEK293 cell lines was generated that included CD19- and HLA-DR single-positive and (HLA-DR plus CD19) double-positive lines with antigen-surface densities varying over a broad range. Using a pair of cell lines with matching densities, the sctb eliminated double-positive target cells preferentially single-positive cells. This ability of preferential or selective targeting of antigen double-positive over single-positive cells opens attractive new perspectives for the use of dual-targeting sctbs in cancer therapy.