W. I. Morrison

Host susceptibility to African trypanosomiasis: trypanotolerance.

Publisher Summary The exploitation of genetic resistance to disease is an important consideration in livestock development programs, where conventional disease control measures are not effective or are too costly. Such an approach may be directly applicable to African animal trypanosomiasis. In African trypanosomiasis, the control measures currently in use include diagnosis and treatment, chemoprophylaxis, and control or eradication of tsetse with insecticides. At present, there is no effective field vaccine available against African trypanosomiasis. The major constraints to the development of a vaccine include the existence of the different species of trypanosomes and of different serodemes within the same species, all with the capability of producing different repertoires of variable antigen types (VATS). This chapter discusses the available information on the aspects of trypanotolerance, with regard to cattle, sheep, goats, wildlife, and man. It describes experimental results derived from mouse models and the relevance of these models to trypanosomiasis of livestock. The genetic resistance to trypanosomiasis is not necessarily associated with low productivity. Trypanotolerance is not a stable characteristic, and evidence suggests that it can be supplemented or reduced by a number of factors affecting the host and its environment, which include age and sex; maternally-derived immunity, intensity of challenge, virulence, previous exposure, stress, and susceptibility to other diseases. Investigations of trypanotolerance also represent an exciting approach to the important interactions between host and parasite. Trypanotolerance exists as an innate characteristic and that it is probably inherited as a dominant trait. While the level of trypanotolerance can be reduced under certain circumstances, it can also be enhanced, for example, by previous exposure. Therefore, there is hope that it may be possible in the future to supplement the level of trypanotolerance both by genetic selection and by immunological or therapeutic procedures.

CYTOTOXIC T-CELLS ELICITED IN CATTLE CHALLENGED WITH THEILERIA-PARVA (MUGUGA) - EVIDENCE FOR RESTRICTION BY CLASS-I MHC DETERMINANTS AND PARASITE STRAIN SPECIFICITY

Summary The MHC restriction and parasite strain specificity of cytotoxic cells elicited in a group of Theileria parva (Muguga)‐immunized cattle following homologous challenge, were investigated. The cytotoxic cells were specific for parasitized target cells and in 9 of the 10 animals examined, they were clearly genetically restricted. Cytotoxicity could be inhibited by monoclonal antibodies (MoAb) to class I MHC molecules but not by MoAb to class II molecules, indicating that a large component of the response was restricted by class I MHC determinants. Low levels of inhibition of cytotoxicity were also obtained with a MoAb to the T‐cell subset marker BoT8, suggesting that at least part of the response was mediated by BoT8+ lymphocytes. When cytotoxic cells from individual cattle were assayed on panels of parasitized target cells, there was a close correlation between susceptibility of the target cells to lysis and sharing of BoLA‐A locus‐encoded specificities with the effectors. This observation, taken together with the knowledge that within several of the sets of BoLA‐A‐matched targets the relevant BoLA‐A specificities were on different MHC haplotypes, indicated that the responses were restricted predominantly by BoLA‐A products.

Trypanosoma congolense: inheritance of susceptibility to infection in inbred strains of mice.

Abstract Inbred strains of mice have shown marked differences in susceptibility to infection with Trypanosoma congolense, as judged by survival and levels of parasitemia. The underlying genetic basis of the susceptibility was examined with F1 hybrids and backcrosses derived from mouse strains of high and low susceptibility. The influence of H-2 haplotype on susceptibility was studied using H-2 congenic resistant strains of mice. F1 hybrids between the most susceptible strain (A/J) and the least susceptible strain (C57B l /6) showed similar survival to that of the C57B l /6 parent. This was reflected in a similar undulating pattern of parasitemia, although the level of parasitemia was consistently higher in the F1 hybrids than in the C57B l /6. Backcrosses of the F1 hybrids with C57B l /6 also had a similar pattern of parasitemia although there was a greater scatter in survival times so that a few animals survived longer than either of the parental strains. Backcrosses of F1 hybrids with A/J showed a range of survival times; approximately 25% of these animals died during the period when the A/J mice died, approximately 25% had a similar survival to that of C57B l /6, while the remaining animals showed an intermediate duration of survival. All these backcrosses had a high initial peak of parasitemia; in about 70% of the mice the early parasitemia showed a distinct undulating pattern. F1 hybrids of A/J and C57B l /6 with C3H/He mice, which are known to be of intermediate susceptibility, were also examined. The degree of dominance for low susceptibility was much less pronounced in these hybrid combinations than in the A/J × C57B l /6 hybrids. The H-2 congenic resistant strains, all of which were on a C57B l /10 genetic background, showed a similar pattern of parasitemia and survival. However, although the majority of all these strains survived for more than 100 days, there was a significant difference in survival between the C57B l /10 mice and the H-2 congenic resistant strains. It was concluded that susceptibility of mice to T. congolense infection is likely to be under complex genetic control and that, at least in C57B l /mice, H-2 haplotype has little influence on susceptibility.

Theileria parva: Kinetics of infection in the lymphoid system of cattle

Abstract The kinetics of infection with Theileria parva in cattle were studied by examining the total cellularity and numbers of parasites in a range of lymphoid organs from animals killed at intervals during the course of the infection. With the dose of T. parva stabilate used, macroschizonts were initially detected in the drainage lymph node about 7 days after inoculation and death of the host resulted on Day 18–19. Associated with the initial detection of parasites, there was a marked increase in cellularity of the drainage lymph node and a more gradual and less pronounced increase in cellularity of the other lymphoid organs. From about Day 12 onward, there was a gradual decrease in the cellularity in all of the lymphoid organs, so that in animals examined in the terminal stages of the infection there was often cellular depletion. The pattern of these cellular changes was similar in groups of Boran and Friesian cattle, although both the increase in cellularity and the terminal depletion were more marked in the Friesians. Blood leukocyte counts in infected Boran started to drop as early as Day 7 of infection and by Day 14 had reached values less than 25% of normal. Quantitation of parasitic schizonts indicated that the numbers of parasites in the lymphoid organs do not increase in a simple exponential manner. Rather, there appears to be an early rapid increase in parasite numbers followed by a phase of less rapid multiplication. Because of the marked changes which occured in total cellularity of the lymphoid organs during the course of the infection, a significant discrepancy was found between the replication rate of the parasite as calculated using total numbers of parasites and that obtained using schizont index (SI). These results indicated that the use of SI, as described in previous studies, is not a reliable method of determining the replication rate of the parasite.

Protective Immune Mechanisms against Theileria parva: Evolution of Vaccine Development Strategies

Theileria parva is an intracellular sporozoan parasite that infects and transforms bovine lymphocytes, causing a severe lymphoproliferative disease known as East Coast fever in eastern, central and southern Africa. In this article, Declan McKeever and colleagues summarize the current understanding of immune mechanisms provoked by the parasite with regard to their role in both pathogenesis and protection. In particular, the influence of genomic polymorphism in parasite and host on the development of immunity is discussed, along with the evolution of current vaccine development strategies as a result of immunological research on the disease.

Non-specific induction of increased resistance in mice to Trypanosoma congolense and Trypanosoma brucei by immunostimulants

Administration of the immunostimulants Corynebacterium parvum, Bacillus Calmette-Guérin (BCG) or Bordetella pertussis prior to, or at the same time as, challenge with Trypanosoma congolense significantly increased survival times in mice, both of trypano-susceptible (A/J) and trypano-resistant (C57Bl) strains. The increased survival time was associated with significant alterations in parasitaemia, which included lengthening of the pre-patent period, a delay in the time taken to reach the first peak of parasitaemia and a reduction in the level of parasitaemia. Similar results were obtained when these strains of mice were challenged with Trypanosoma brucei following pre-treatment with C. parvum. Thus, by the use of immunostimulants it was possible to reduce the susceptibility of mice to trypanosomiasis and the hope is that this can also be achieved with domestic livestock.

INTERFERENCE IN THE ESTABLISHMENT OF SUPERINFECTIONS WITH TRYPANOSOMA CONGOLENSE IN CATTLE

To examine the influence of an established infection on subsequent challenge with another unrelated trypanosome serodeme, cattle were subjected to two challenges, 5 to 6 wk apart, with unrelated isolates of Trypanosoma congolense. The primary infection inhibited the establishment of the second infection despite the initial absence of detectable antibody to the trypanosomes used for the second challenge. This was true whether the second challenge consisted of bloodstream forms of the parasite or metacyclics from infected Glossina m. morsitans. Rechallenge with bloodstream forms resulted in a slight antibody response, which was only detectable by immunofluorescence and was much less than in the challenge controls. Although animals subjected to the second challenge by tsetse flies showed no appreciable increase in parasitemia and, in most instances, no chancre reaction at the site where the tsetse bit, they developed readily detectable neutralizing antibody to the metacyclic trypanosomes. That this interference effect was not the result of specific immunity and required an active infection was confirmed by the finding that when infected animals were treated with Berenil prior to rechallenge, they were fully susceptible to the infection.

Successful transfer of frozen N'Dama embryos from the Gambia to Kenya

SummaryFrozen embryos from N’Dama cattle were successfully transferred from The Gambia to Kenya. Of the 26 N’Dama cows used 12 were successfully programmed to superovulate and of these seven produced 30 embryos that were collected seven days after oestrus/service. Five N’Dama bulls were used for natural service. In Kenya 29 embryos were implanted into 29 Boran heifers seven days (±1) after the induction of synchronised oestrus. Eleven pregnancies were established and after one abortion of unknown aetiology at seven and a half months five female and five male calves were born and subsequently reared. During programming the N’Dama cows showed prolonged anoestrus leading to the necessity of oestrus induction using intravaginal progesterone releasing coils; pregnant mare serum gonadotrophin gave better superovulation than follicle stimulating hormone. One N’Dama bull proved to be subfertile. The success of the project has demonstrated the potential of this technique to make disease-free N’Dama available for research purposes and for the promotion of livestock development programmes in tsetse-infested areas using trypanotolerant cattle.RésuméDes embryons congelés de bovins N’Dama ont été transférés avec succès de Gambie au Kenya. Douze vaches N’Dama sur vingt-six ont pu être programmées pour superovuler; sept d’entre elles ont produit trente embryons qui ont été récoltés sept jours après l’oestrus/insémination. Cinq taureaux N’Dama ont été utilisés en monte naturelle. Au Kenya, vingt-neuf embryons ont été implantés chez vingt-neuf génisses Boran sept jours (±1) après induction d’un oestrus synchronisé. Onze gestations se sont établies et, après un avortement d’étiologie inconnue à sept mois et demi, cinq veaux mâles et cinq veaux femelles sont nés et ont été élevés. Lors de la phase de programmation, les vaches N’Dama ont présenté un anoestrus prolongé nécessitant une induction oestrale avec des tampons imprégnés de progestérone; la gonadotrophine de jument gravide a induit une meilleure superovulation que l’hormone FSH. Un taureau N’Dama s’est montré être à la limite de la sous-fertilité. Le succès du projet a démontré le potentiel de cette technique pour la mise à disposition de bovins N’Dama sains à des fins de recherche et pour la promotion des programmes de développement de l’élevage avec du bétail trypanotolérant dans les régions infestées de tsé-tsé.ResumenSe transfirieron exitosamente embriones congelados de ganado N’Dama de Gambia a Kenia. De las 26 vacas N’Dama utilizadas, 12 fueron programadas exitosamente para superovular y de las cuales 7 produjeron 30 embriones que fueron colectados 7 días despues del servicio. Cinco toros N’Dama se utilizaron en servicio natural. En Kenia, 29 embriones fueron implantados en 29 vaquillonas Boran siete días (±I), despues de la inducción del cello sincronizado. Se establecieron II preñeces y despues de un aborto de etiología desconocida a los 7 1/2 meses, nacieron cinco hembras y cinco machos. Durante la programación de las vacas N’Dama, éstas mostraron anestro prolongado, surgiendo la necesidad de inducir estro mediante rollos intravaginales los cuales liberan progesterona; el suero de yeguas preñadas dió mejores resultados para inducir superovulación, que la hormona folículo estimulante. Un toro N’Dama fue subfertil. El éxito del proyecto ha demostrado plenamente la gran potencialidad de ésta técnica, la cual puede ser utilizada para obtener animales N’Dama libres de enfermedades para investigación, y para programas de desarrollo ganadero tendientes a utilizar ganado resistente a la tripanosomiasis en áreas de tsetse.

Influence of host and parasite genotypes on immunological control of Theileria parasites

Infections with Theileria parva in the African buffalo are invariably asymptomatic, whereas infections in cattle usually result in clinical disease, the severity of which varies in different populations of cattle. The parasite exhibits antigenic heterogeneity, which in cattle manifests as differences between parasite strains in their cross-protective properties. A series of studies on T cell responses to T. parva in cattle have demonstrated that class I MHC-restricted cytotoxic T lymphocytes (CTL), specific for parasitized lymphoblasts, are important mediators of immunity. Cytotoxic T cell responses frequently display parasite strain-restricted specificities which appear to correlate with the capacity of strains to cross-protect. The strain specificity of CTL responses varies in animals immunized with the same parasite strain and is influenced by both host and parasite genotype. Recent studies have provided evidence that there is competition between epitopes for induction of CTL responses, which can result in a bias to strain-specific epitopes. These properties of the CTL response have important implications for vaccination. Thus, in designing a vaccine, it may be possible, by selecting parasite proteins containing appropriate CTL epitopes, to generate CTL responses that protect against a wide range of parasite strains. Although there are no comparable data on CTL responses in the buffalo, it is considered that the features of the immune response described for cattle would be advantageous for survival of parasite populations in the buffalo. Specifically, a bias in the immune responses to strain-specific determinants should favor establishment of infection in buffalo already carrying the parasite and allow fluctuation in the levels of different parasite strains during the course of persistent infection.

Current status of vaccine development against Theileria parasites

The tick-borne protozoan parasites Theileria parva and Theileria annulata cause economically important diseases of cattle in tropical and sub-tropical regions. Because of shortcomings in disease control measures based on therapy and tick control, there is a demand for effective vaccines against these diseases. Vaccines using live parasites have been available for over two decades, but despite their undoubted efficacy they have not been used on a large scale. Lack of infrastructure for vaccine production and distribution, as well as concerns about the introduction of vaccine parasite strains into local tick populations have curtailed the use of these vaccines. More recently, research has focused on the development of subunit vaccines. Studies of immune responses to different stages of the parasites have yielded immunological probes that have been used to identify candidate vaccine antigens. Immunisation of cattle with antigens expressed in the sporozoite, schizont or merozoite stages has resulted in varying degrees of protection against challenge. Although the levels of protection achieved have not been sufficient to allow exploitation for vaccination, there are clearly further lines of investigation, relating to both the choice of antigens and the antigen delivery systems employed, that need to be pursued to fully explore the potential of the candidate vaccines. Improved knowledge of the molecular biology and immunology of the parasites gained during the course of these studies has also opened up opportunities to refine and improve the quality of live vaccines.