Some observations on immunologically mediated inhibited Teladorsagia circumcincta and their subsequent resumption of development in sheep

Integrating immune mechanisms to model nematode worm burden: an example in sheep

Gastrointestinal nematodes represent important sources of economic losses in farmed ruminants, and the increasing frequency of anthelmintic resistance requires an increased ability to explore alternative strategies. Theoretical approaches at the crossroads of immunology and epidemiology are valuable tools in that context. In the case of Teladorsagia circumcincta in sheep, the immunological mechanisms important for resistance are increasingly well-characterized. However, despite the existence of a wide range of theoretical models, there is no framework integrating the characteristic features of this immune response into a tractable phenomenological model. Here, we propose to bridge that gap by developing a flexible modelling framework that allows for variability in nematode larval intake which can be used to track the variations in worm burdens. We parameterize this model using data from trickle infection of sheep and show that using simple immunological assumptions, our model can capture the dynamics of both adult worm burdens and nematode fecal egg counts. In addition, our analysis reveals interesting dose-dependent effects on the immune response. Finally, we discuss potential developments of this model and highlight how an improved cross-talk between empiricists and theoreticians would facilitate important advances in the study of infectious diseases.

Immune modulation by helminth parasites of ruminants: implications for vaccine development and host immune competence

Parasitic helminths reside in immunologically-exposed extracellular locations within their hosts, yet they are capable of surviving for extended periods. To enable this survival, these parasites have developed complex and multifaceted mechanisms to subvert or suppress host immunity. This review summarises current knowledge of immune modulation by helminth parasites of ruminants and the parasite-derived molecules involved in driving this modulation. Such immunomodulatory molecules have considerable promise as vaccine targets, as neutralisation of their function is predicted to enhance anti-parasite immunity and, as such, current knowledge in this area is presented herein. Furthermore, we summarise current evidence that, as well as affecting parasite-specific immunity, immune modulation by these parasites may also affect the ability of ruminant hosts to control concurrent diseases or mount effective responses to vaccination.

Suppression of ovine lymphocyte activation by Teladorsagia circumcincta larval excretory-secretory products

Teladorsagia circumcincta is an important pathogenic nematode of sheep. It has been demonstrated previously that stimulation of murine T lymphocytes with excretory-secretory (ES) products derived from fourth stage larvae of T. circumcincta (Tci-L4-ES) results in de novo expression of Foxp3, a transcription factor intimately involved in regulatory T cell function. In the current study, Foxp3⁺ T cell responses in the abomasum and the effects of Tci-L4-ES on ovine peripheral blood mononuclear cells (PBMC) following T. circumcincta infection were investigated. T. circumcincta infection resulted in a significant increase in numbers of abomasal Foxp3⁺ T cells, but not an increase in the proportion of T cells expressing Foxp3. Unlike in mice, Tci-L4-ES was incapable of inducing T cell Foxp3 expression but instead suppressed mitogen-induced and antigen-specific activation and proliferation of ovine PBMC in vitro. This effect was heat labile, suggesting that it is mediated by protein(s). Suppression was associated with up-regulation of interleukin-10 (IL-10) mRNA, and specific monoclonal antibody neutralisation of IL-10 resulted in a 50% reduction in suppression, indicating involvement of the IL-10 signaling pathway. Suppression was significantly reduced in PBMC isolated from T. circumcincta infected vs. helminth-naïve lambs, and this reduction in suppression was associated with an increase in Tci-L4-ES antigen-specific T cells within the PBMC. In conclusion, we have identified a mechanism by which T. circumcincta may modulate the host adaptive immune response, potentially assisting survival of the parasite within the host. However, the impact of Tci-L4-ES-mediated lymphocyte suppression during T. circumcincta infection remains to be determined.

Merino ewes bred for parasite resistance reduce larval contamination onto pasture during the peri-parturient period

The peri-parturient period is crucial for controlling worms as the acquired immunity of ewes is disrupted, resulting in an increase in faecal worm egg counts. Two hypotheses were tested in this experiment - that ewes bred for worm resistance would have lower faecal worm egg counts than unselected control ewes, during late pregnancy and lactation, under similar but separate grazing areas; and also that numbers of infective nematode larvae would be lower on pastures grazed by resistant ewes than pastures grazed by unselected control ewes. Faecal samples were collected from resistant and unselected ewes in late pregnancy and early lactation, during the winter rainfall season, and analysed for numbers of Trichostrongylus colubriformis and Teladorsagia circumcincta. Pasture samples were taken 1 week before and 7 weeks after lambing started and analysed for infective larvae. In all sheep, worm egg counts rose 2 weeks prior to lambing and continued into lactation. Worm egg counts were significantly lower in the resistant ewes from 1 week before lambing to 2 weeks after lambing. There were no differences in egg counts between single- and twin-bearing ewes in the resistant line. However, twin-bearing control ewes had significantly higher egg counts than single-bearing control ewes. Following lactation, plots grazed by resistant ewes had substantially less contamination with T. colubriformis larvae, but there were no differences in numbers of T. circumcincta larvae. Our results demonstrate that sheep bred for worm resistance has lower worm burdens during the peri-parturient phase and that lambs born to resistant ewes face a lower larval challenge during their introduction to grazing. In our environment, selection for low worm egg counts has produced sheep highly resistant to T. colubriformis, but has had less impact on resistance towards T. circumcincta.

Transcriptional changes in Teladorsagia circumcincta upon encountering host tissue of differing immune status

The aim of this study was to elucidate transcriptional changes in the parasitic nematode Teladorsagia circumcincta upon encountering either naïve or immune ovine hosts. Pools of 100 000 exsheathed 3rd- stage T. circumcincta larvae were exposed in vitro to either an immune or naïve ovine abomasal environment, RNA was extracted from the larvae and sequenced using the Roche 454 platform. Each sample produced approximately 82 000 reads that assembled to give approximately 5500 Isotigs (contigs). The two sequence datasets were clustered together to give a total of 6969 clusters of which 18 were differentially expressed (P<0·001) between the two groups. Clusters with a predominance of reads in larvae exposed to the immune abomasal environment encoded homologues of peptidyl-glycine alpha-amidating monooxygenase, heat shock-protein 16-2 and IDA-1, a tyrosine phosphatase-like receptor protein. Clusters with a predominance of reads in the naïve environment encoded homologues of cytochrome b, EGg Laying defective family member 21 and NADH dehydrogenase subunit 5. Gene ontology analyses indicated that larvae exposed to the immune environment showed an increase in expression of genes involved in 'carbon utilization', 'response to stimulus' and 'developmental process'. These data suggest that T. circumcincta modulates gene expression in response to the immune status of the host.

GTP-cyclohydrolase and development in Teladorsagia circumcincta and Dictyocaulus viviparus (Nematoda: Strongylida)

GTP-Cyclohydrolase (GTP-CH) is necessary for the production of tetrahydrobiopterin, a required cofactor for the three aromatic amino acid hydroxylases and nitric oxide synthases. The gene encoding GTP-CH is transcribed at high levels in infective third larval stages of a number of parasitic trichostrongylid nematodes. We explore the potential role of GTP-CH within the processes of nematode development and environmentally-induced hypobiosis. For two species of parasitic nematode that are of major economic and welfare importance to livestock in temperate regions, Teladorsagia circumcincta and Dictyocaulus viviparus, we have demonstrated that each of the pre-parasitic larval stages transcribe high mean levels of cat-4 (the gene encoding GTP-CH). Using quantitative real-time polymerase chain reaction analysis and two different isolates of D. viviparus, only one of which is capable of entering hypobiosis, we have shown that there were only minor differences between these isolates in mean cat-4 transcript levels, both during the parasitic stages and during the earlier environmental life cycle stages (L(1)-L(3)). Taken together, these data indicate that, although both species of nematode produce high levels of cat-4 transcript in pre-parasitic larval stages, GTP-CH levels are unlikely to be involved in the induction of parasite hypobiosis. Alternative roles for GTP-CH in larval development are discussed.

The dynamic influence of the DRB1*1101 allele on the resistance of sheep to experimental Teladorsagia circumcincta infection

Suffolk sheep carrying the DRB1*1101 (previously referred to as-DRB1*0203 or G2) allele have been reported to show increased resistance to natural Teladorsagia circumcincta infection compared to non-carriers. The objective of this study was to compare the biochemical and physiological responses of DRB1*1101 carrier and non-carrier twin lambs to an experimental infection with 3 × 10⁴ L3 Teladorsagia circumcincta. The variables studied included worm burden, faecal egg count, abomasal mast cells, IgA, IgE, IgG1 plus IgG2 and haematological parameters at 0, 3, 7, 21 and 35 days post infection (dpi), and duodenal smooth muscle contractility at 0 and 35 dpi. DRB1*1101 carrier lambs had significantly lower worm burden, higher mast cell and plasma platelet counts than the DRB1*1101 non-carriers (P < 0.05). Before infection, the non-carrier lambs exhibited significantly higher mucosal levels of all antibody isotypes measured compared to the carriers; these levels remained relatively stable over the course of infection in the non-carriers while there was a slow build up of these antibodies in the carriers up to day 21 post infection (pi). The DRB1*1101 non-carrier lambs had a significantly higher plasma lymphocyte count, and produced greater duodenal contractile force relative to the carrier lambs (P < 0.05). There was no significant difference between genotypes in the level of plasma eosinophils, monocytes, neutrophils or FEC. This evidence suggests that resistance conferred by DRB1*1101 is acquired rather than innate, depends on worm expulsion rather than fecundity and is dependent on mucosal mast cell proliferation, platelet activation, and IgA and IgE antibody responses.

Teladorsagia circumcincta in the sheep abomasum: defining the role of dendritic cells in T cell regulation and protective immunity

Parasitic nematodes of the small-ruminant gastrointestinal tract pose a problem worldwide. The impact of these pathogens is worsened by the emergence of anthelmintic resistance to all three available classes of drugs. In addition to causing considerable economic loss, these parasites are detrimental to the health and welfare of sheep and goats. Vaccination offers an alternative approach to drug-based control and a great deal of investment has gone into the investigation of protective antigens for some of these nematode species. However, attempts at vaccination are hindered by a lack of understanding of how best to promote protective immunity to nematode species, such as Teladorsagia circumcincta, which inhabits the abomasum of sheep. This situation contrasts with that in murine models of gastrointestinal nematode infection, where the basis of protective immunity is increasingly well understood. In this review, we discuss the current knowledge of the immune effector mechanisms elicited by T. circumcincta and consider the probable role of dendritic cells in the initiation of both effector and regulatory responses in the abomasum.

The experimental establishment of ruminant nematodes in European hares (Lepus europaeus)

The factors that control the demography of European hare Lepus europaeus populations are poorly understood, but it has been recognized that the decline of hares in Europe is associated with an increasing intensity of agricultural activity. Many mechanisms have been suggested. We propose another mechanism; a negative impact arising from ingestion of the infective larvae of ruminant livestock. We dosed juvenile hares from a worm-free colony with a conservative dose from a mixed culture of infective larvae of the nematode parasites of sheep and cattle. We examined the hares post-mortem for the establishment of those ruminant nematodes, differences in weight changes, and the shedding of eggs. We found that under the circumstances of our trial, Trichostrongylus colubriformis and to a lesser extent T. rugatus, T. vitrinus, and Teladorsagia circumcincta were able to establish as adults in the dosed animals. We found strongyle eggs in the faeces of the dosed hares, and were able to culture larvae from those eggs. However, the ecological significance of our findings, if any, remains to be elucidated. Because of their mobility, hares may transmit resistant strains of parasites between grazing properties.

Parasite loss and inhibited development of Teladorsagia circumcincta in relation to the kinetics of the local IgA response in sheep

Groups of yearling sheep, which had been trickle infected with Teladorsagia circumcincta for 8 weeks and then drenched, were challenged with 50 000 T. circumcincta larvae together with groups of worm-free controls. Fewer parasites and a greater proportion of early fourth stage larvae were recovered from previously infected sheep compared to controls. Worm loss and arrested development were evident by 5 days after challenge whereas growth retardation of developing worms was observed by day 10. In the previously infected sheep a secondary IgA response was observed in the efferent gastric lymph from 5 days post-infection. Western blot analysis showed the lymph IgA to be predominantly dimeric and nonsecretory in nature and that the somatic antigens recognized were predominantly in the 100-250 kDa range. The concentration of IgA in lymph was always higher than in blood and in the previously infected sheep increased fivefold 8 days post-challenge in contrast to blood where IgA levels were unchanged. The timing of the response suggested that it occurred too late to have been the cause of worm loss or arrested development, though it may have retarded the growth of developing parasites.