A co-evolutionary arms race: trypanosomes shaping the human genome, humans shaping the trypanosome genome

<i>Trypanosoma brucei</i> is the causative agent of African sleeping sickness in humans and one of several pathogens that cause the related veterinary disease Nagana. A complex co-evolution has occurred between these parasites and primates that led to the emergence of trypanosome-specific defences and counter-measures. The first line of defence in humans and several other <i>catarrhine</i> primates is the trypanolytic protein apolipoprotein-L1 (APOL1) found within two serum protein complexes, trypanosome lytic factor 1 and 2 (TLF-1 and TLF-2). Two sub-species of <i>T. Brucei</i> have evolved specific mechanisms to overcome this innate resistance, <i>Trypanosoma brucei gambiense</i> and <i>Trypanosoma brucei rhodesiense</i>. In <i>T. b. Rhodesiense</i>, the presence of the serum resistance associated (SRA) gene, a truncated variable surface glycoprotein (VSG), is sufficient to confer resistance to lysis. The resistance mechanism of <i>T. b. Gambiense</i> is more complex, involving multiple components: reduction in binding affinity of a receptor for TLF, increased cysteine protease activity and the presence of the truncated VSG, <i>T. b. Gambiense</i>-specific glycoprotein <i>(TgsGP)</i>. In a striking example of co-evolution, evidence is emerging that primates are responding to challenge by <i>T. b. Gambiense</i> and <i>T. b. Rhodesiense</i>, with several populations of humans and primates displaying resistance to infection by these two sub-species

Haptoglobin-hemoglobin receptor independent killing of African trypanosomes by human serum and trypanosome lytic factors

The haptoglobin-hemoglobin receptor (HpHbR) of African trypanosomes plays a critical role in human innate immunity against these parasites. Localized to the flagellar pocket of the veterinary pathogen Trypanosoma brucei brucei this receptor binds Trypanosome Lytic Factor-1 (TLF-1), a subclass of human high-density lipoprotein (HDL) facilitating endocytosis, lysosomal trafficking and subsequent killing. Recently, we found that group 1 Trypanosoma brucei gambiense does not express a functional HpHbR. We now show that loss of the TbbHpHbR reduces the susceptibility of T. b. brucei to human serum and TLF-1 by 100- and 10,000-fold, respectively. The relatively high concentrations of human serum and TLF-1 needed to kill trypanosomes lacking the HpHbR indicates that high affinity TbbHpHbR binding enhances the cytotoxicity; however, in the absence of TbbHpHbR, other receptors or fluid phase endocytosis are sufficient to provide some level of susceptibility. Human serum contains a second innate immune factor, TLF-2, that has been suggested to kill trypanosomes independently of the TbbHpHbR. We found that T. b. brucei killing by TLF-2 was reduced in TbbHpHbR-deficient cells but to a lesser extent than TLF-1. This suggests that both TLF-1 and TLF-2 can be taken up via the TbbHpHbR but that alternative pathways exist for the uptake of these toxins. Together the findings reported here extend our previously published studies and suggest that group 1 T. b. gambiense has evolved multiple mechanisms to avoid killing by trypanolytic human serum factors

Human serum resistance in Trypanosoma brucei

Trypanosoma brucei is the causative agent of both sleeping sickness in humans and the related veterinary disease, Nagana. Both diseases have a wide distribution across sub-Saharan Africa and affect some of the poorest areas of the world. T. brucei can be segregated into three morphologically identical sub-species based on host, geography and pathology. T. b. brucei is limited to domestic and wild animals throughout sub-Saharan Africa and is non-infective to humans due to trypanosome lytic factors found in human serum. T. b. gambiense and T. b. rhodesiense are human infective sub-species, named due to their relative geographic locations. T. b. gambiense is the dominant form of the disease, causing over 90% of reported cases. Study of T. b. gambiense is complicated in that there are two distinct groups. Group 1 is invariably resistant to lysis and by far the more prevalent group. Group 2 T. b. gambiense exhibit a variable resistance phenotype and are only found at a small number of Côte d’Ivoire disease foci. There are two trypanosome lytic factors in human serum (TLF-1 & 2), both containing the proteins Apolipoprotein L1 (ApoL1) and Haptogoblin-related protein (Hpr). It has been conclusively demonstrated that the lytic component of TLF is ApoL1, although Hpr is required for maximal lysis by facilitating uptake of TLF particles via the HpHbR cell surface receptor. This thesis has exposed several features of the human infectivity phenotype in both groups of T. b. gambiense, an area of research for which data has been lacking due to the difficulty of working with the organism. Fluorescence microscopy indicated that group 1 T. b. gambiense exhibit avoidance of TLF-1 particles by down-regulating HpHbR receptor expression and function. However, they are also able to resist the effects of recombinant ApoL1, suggesting an additional neutralisation or compensatory mechanism. Due to group 1 T. b. gambiense avoidance of TLF-1, TLF-2 is the more important lytic particle for this sub-species group and future research must take this into consideration. Unlike group 1, group 2 T. b. gambiense displays a variable human serum resistance phenotype that involves a neutralisation or compensatory mechanism for ApoL1, with no significant avoidance of lytic particles. Despite the high variability of the phenotype of group 2 T. b. gambiense, Quantitative Trait Analysis (QTL) using twenty-five F1 progeny from a T. b. brucei / group 2 T. b. gambiense cross indicated a strong heritable component to human serum resistance largely determined by a 30 gene locus on chromosome 8. Finally, a six multi-locus genotype population analysis of a Côte d’Ivoire T. b. gambiense focus was conducted, revealing little relationship between the two groups of T. b. gambiense in the field. The differences in the human serum resistance phenotypes and population genetics of both groups of T. b. gambiense revealed both prior and during this study make it appear likely that the two groups have evolved distinct human serum resistance strategies

Planar laser-induced fluorescence imaging of Si and SiO during pulsed laser ablation of Si

Planar laser induced fluorescence has been used to acquire time sequence images of ground-state, neutral Si and SiO during laser ablation of an Si target in vacuum and in the presence of a background gas at a fluence of 3-4 J/cm^2. The SiO images, taken in air, strongly suggest that the observed SiO is created through reaction of silicon with oxygen at the contact front as the plume expands

Fatal Clostridium sordellii-mediated hemorrhagic and necrotizing gastroenteropathy in a dog: case report

Background: Canine hemorrhagic gastroenteritis (also canine gastrointestinal hemorrhagic syndrome) is commonly associated with Clostridium perfringens, although in some cases the etiology remains unclear. This report describes a fatal acute hemorrhagic and necrotizing gastroenteropathy in a dog associated with Clostridium sordellii, a bacterial species never before identified as the etiological agent of hemorrhagic and necrotizing gastroenteropathy in dogs. Case presentation: A fully vaccinated, eight-year-old, female neutered Labrador presented with a history of vomiting without diarrhea. Clinical examination revealed pink mucous membranes, adequate hydration, normothermia, and normocardia. The dog was discovered deceased the following day. Post-mortem examination showed moderate amounts of dark red, non-clotted fluid within the stomach that extended into the jejunum. Discoloration was noted in the gastric mucosa, liver, lungs, and kidneys, with small petechial hemorrhages present in the endocardium over the right heart base and thymic remnants. Histological analysis demonstrated that the gastric fundic mucosa, the pyloric region, small intestine, and large intestine exhibited superficial coagulative necrosis and were lined with a layer of short Gram-positive rods. Anaerobic culture of the gastric content revealed C. sordellii as the dominant bacterial species and neither Salmonella spp., Campylobacter spp., C. perfringens, nor C. difficile were isolated. Unexpectedly, whole genome sequencing of the C. sordellii isolate showed that it lacked the main plasmid-encoded virulence factors typical of the species, indicating that the genetic determinants of pathogenicity of this strain must be chromosomally encoded. Further phylogenetic analysis revealed it to be genetically similar to C. sordellii isolates associated with gastroenteric disease in livestock, indicating that the infection may have been acquired from the environment. Conclusions: This case demonstrates that C. sordellii can associate with a canine hemorrhagic and necrotizing gastroenteropathy in the absence of C. perfringens and illustrates the benefits of using bacterial whole genome sequencing to support pathological investigations in veterinary diagnostics. These data also update the molecular phylogeny of C. sordellii, indicating a possible pathogenic clade in the environment that is distinct from currently identified clades

Novel African trypanocidal agents: membrane rigidifying peptides

The bloodstream developmental forms of pathogenic African trypanosomes are uniquely susceptible to killing by small hydrophobic peptides. Trypanocidal activity is conferred by peptide hydrophobicity and charge distribution and results from increased rigidity of the plasma membrane. Structural analysis of lipid-associated peptide suggests a mechanism of phospholipid clamping in which an internal hydrophobic bulge anchors the peptide in the membrane and positively charged moieties at the termini coordinate phosphates of the polar lipid headgroups. This mechanism reveals a necessary phenotype in bloodstream form African trypanosomes, high membrane fluidity, and we suggest that targeting the plasma membrane lipid bilayer as a whole may be a novel strategy for the development of new pharmaceutical agents. Additionally, the peptides we have described may be valuable tools for probing the biosynthetic machinery responsible for the unique composition and characteristics of African trypanosome plasma membranes

Should action take priority over further research on public health?

We have evidence on which to act, and inaction costs lives, argue Simon Capewell and Paul Cairney. But Aileen Clarke says our understanding of the human behaviour that leads to unhealthy choices is still lackin

Sheep as host species for zoonotic Babesia venatorum, United Kingdom

Babesia venatorum is an increasingly prominent zoonotic parasite that predominantly infects wild deer. Our molecular examination of Babesia infecting mammals in the United Kingdom identified 18S sequences in domestic sheep isolates identical to zoonotic B. venatorum. Identification of this parasite in livestock raises concerns for public health and farming policy in Europe

To the skin and beyond: the immune response to African trypanosomes as they enter and exit the vertebrate host

African trypanosomes are single-celled extracellular protozoan parasites transmitted by tsetse fly vectors across sub-Saharan Africa, causing serious disease in both humans and animals. Mammalian infections begin when the tsetse fly penetrates the skin in order to take a blood meal, depositing trypanosomes into the dermal layer. Similarly, onward transmission occurs when differentiated and insect pre-adapted forms are ingested by the fly during a blood meal. Between these transmission steps, trypanosomes access the systemic circulation of the vertebrate host via the skin-draining lymph nodes, disseminating into multiple tissues and organs, and establishing chronic, and long-lasting infections. However, most studies of the immunobiology of African trypanosomes have been conducted under experimental conditions that bypass the skin as a route for systemic dissemination (typically via intraperitoneal or intravenous routes). Therefore, the importance of these initial interactions between trypanosomes and the skin at the site of initial infection, and the implications for these processes in infection establishment, have largely been overlooked. Recent studies have also demonstrated active and complex interactions between the mammalian host and trypanosomes in the skin during initial infection and revealed the skin as an overlooked anatomical reservoir for transmission. This highlights the importance of this organ when investigating the biology of trypanosome infections and the associated immune responses at the initial site of infection. Here, we review the mechanisms involved in establishing African trypanosome infections and potential of the skin as a reservoir, the role of innate immune cells in the skin during initial infection, and the subsequent immune interactions as the parasites migrate from the skin. We suggest that a thorough identification of the mechanisms involved in establishing African trypanosome infections in the skin and their progression through the host is essential for the development of novel approaches to interrupt disease transmission and control these important diseases

NMD3 regulates both mRNA and rRNA nuclear export in African trypanosomes via an XPOI-linked pathway

Trypanosomes mostly regulate gene expression through post-transcriptional mechanisms, particularly mRNA stability. However, much mRNA degradation is cytoplasmic such that mRNA nuclear export must represent an important level of regulation. Ribosomal RNAs must also be exported from the nucleus and the trypanosome orthologue of NMD3 has been confirmed to be involved in rRNA processing and export, matching its function in other organisms. Surprisingly, we found that TbNMD3 depletion also generates mRNA accumulation of procyclin-associated genes (PAGs), these being co-transcribed by RNA polymerase I with the procyclin surface antigen genes expressed on trypanosome insect forms. By whole transcriptome RNA-seq analysis of TbNMD3-depleted cells we confirm the regulation of the PAG transcripts by TbNMD3 and using reporter constructs reveal that PAG1 regulation is mediated by its 5′UTR. Dissection of the mechanism of regulation demonstrates that it is not dependent upon translational inhibition mediated by TbNMD3 depletion nor enhanced transcription. However, depletion of the nuclear export factors XPO1 or MEX67 recapitulates the effects of TbNMD3 depletion on PAG mRNAs and mRNAs accumulated in the nucleus of TbNMD3-depleted cells. These results invoke a novel RNA regulatory mechanism involving the NMD3-dependent nuclear export of mRNA cargos, suggesting a shared platform for mRNA and rRNA expor