Published by the International Laboratory for Research on Animal Diseases
Volume 10 Number 1
Transfecting modified trypanosome genes into trypanosomes
Transfecting cattle genes into mouse cells
Confirmation of a second MHC class I locus in cattle
Use of transfected cells to characterize monoclonal antibodies
Isolating genes that encode parasite antigens recognized by monoclonal antibodies
New tools for better diagnosis of African trypanosomiasis
Clearance of antibody–VSG complexes by trypanosomes
The central challenge of today's studies of disease-causing parasites is to understand the workings of the parasites and the cells of their mammalian hosts in molecular detail. Making use of technical breakthroughs in molecular biology and the natural plasticity of genes, ILRAD scientists are transferring DNA from one cell and organism to another to discover the molecules responsible for the lethal effects of parasite infection and the molecules that may be used in safe and effective vaccines against diseases.
IN THE LAST THREE DECADES, our knowledge of the biology of organisms has increased remarkably due to advances made in molecular biology. To understand how a single- or multiple-celled organism functions, scientists are studying how molecules—specifically proteins, the building blocks of all cells—function. Proteins play crucial roles in virtually all biological processes, from catalyzing chemical reactions, to transporting important molecules, regulating cell growth and differentiation, and protecting an organism from foreign pathogens. Although scientists have long been able to break up cells in ways that allow them to sort sub-cellular components, identifying the functions of these components has remained difficult, largely because the amount of a purified protein that could be obtained has been too small for analysis. Today, with technical breakthroughs in recombinant DNA technology, the genes that 'encode' the proteins are being used to produce sufficient amounts of even rare proteins so that their structures and functions can be determined for the first time.
Proteins are built from a repertoire of 20 amino acids. Each gene present in a DNA molecule in the cell nucleus consists of small subunits called nucleotides. The arrangement of nucleotides acts as a code that specifies the sequence of amino acids needed for the cell to build a particular protein. When deciphered in the mid-1960s this genetic code was found to be nearly the same for all organisms, from bacteria to mammals. This suggested that a gene coding for a particular molecule in one organism should code for the same molecule when inserted into another organism.
Molecules of three types of cells—a white blood cell (lymphocyte) of cattle (top), a trypanosome parasite (middle) and a thieileria parasite-infected bovine lymphocyte (bottom; the parasite = (P))—are being studied at ILRAD by removal and alteration of selected genes and their 'transfection' back into the original cell or into other cells.
The transfected cells will produce the proteins encoded by the foreign genes in quantities large enough to determine the functions of the molecules.
In this way, scientists will be able to discover which parasite molecules to target for attack and to determine which molecules of the bovine immune system help protect animals against disease.
DEVELOPMENT of a range of new laboratory techniques, particularly the recombinant DNA technologies which first appeared in the early 1970s, revolutionized studies in molecular biology. These techniques provide scientists with powerful means of analysing and altering genes and the proteins they encode.
Scientists are now able to clone particular genes from an organism by breaking up the organism's DNA and inserting the fragments into plasmid DNA. The plasmids are then introduced into bacteria, where they replicate many thousands-fold. Scientists can screen the plasmid-containing bacteria to pull out a gene of interest. As predicted, genes thus isolated from one organism can be altered and either reinserted into the original cell or transferred to cells of other organisms and expressed in those cells.
This group of techniques, collectively called molecular genetics, has become indispensable in studies of many basic biological questions, such as how different proteins function and how the cell regulates protein synthesis through gene expression. (Of the thousands of genes carried in each mammalian or parasite cell, only a fraction at any one time are 'expressed', that is, transcribed into RNA and then translated into protein.)
The power and success of this new technology have greatly increased our understanding of the biology of pathogenic organisms and have raised prospects that this knowledge will benefit people in many very different ways. At ILRAD, scientists are using this technology to advance understanding of protozoan parasites that cause fatal livestock diseases. The researchers are manipulating the genetic material of these single-celled parasites—as well as the genes in cattle that control the animals' immune response to parasite infection—to discover molecules of the parasites that can be used effectively in vaccines, to discern 'weak spots' in the parasites that may be targeted for chemical or immunological attack, and to determine which components of the bovine immune system may be strengthened by administration of a vaccine. Improving the control of economically important tropical livestock diseases in such novel and cost-efficient ways will benefit both mixed farmers and nomadic pastoralists who in many parts of the world depend on their livestock for life as well as livelihood. —Ed.
The transfer of genes 'laterally', as distinct from reproductively (from one generation to the next), is not solely an invention of molecular geneticists: the phenomenon occurs naturally, for example, in bacteria, which pass a plasmid—a short, circular piece of DNA—from one individual to another. Gene transfer in bacteria was first unexpectedly observed in 1928 in a series of simple and elegant experiments conducted by the English microbiologist Frederick Griffith.
The form of the pneumococcus bacterium that causes pneumonia in animals is surrounded by a capsule responsible for the bacterium's pathogenicity. There also exists a mutant form of the bacterium that, lacking this capsule, does not cause disease.
It was known that mice infected with the live virulent form of the bacterium died of septicaemia, whereas neither the virulent form when killed by heat nor the avirulent mutant live form were lethal to mice. Griffith injected mice with a mixture of live avirulent and heat-killed virulent pneumococci. Two striking results followed: the mice died and the blood of the dead mice was found to contain live virulent pneumococci. The heat-killed virulent bacteria had thus somehow 'transformed' the live avirulent form into live virulent pneumococci. The change, moreover, was permanent: the transformed bacteria yielded pathogenic progeny of the virulent form. This suggested that the heat that killed the pathogenic cells did not damage those cells' genetic components, which, liberated somehow from their original cells, passed through the cell wall of the living recipient cells and subsequently underwent genetic recombination with the recipients' genetic apparatus.
Griffith named the factor responsible for the transformation from avirulent to virulent strain the 'transforming' principle. The significance of this discovery was not fully understood until 1944, when Oswald Avery, Colin MacLeod and Maclyn McCarty at the Rockefeller Institute published their landmark discovery that the active principle effecting this transformation is deoxyribonucleic acid (DNA).
Transformation of nonpathogenic (small colonies) to pathogenic pneumococci (large) by DNA from heat-killed pathogenic pneumococci. (From Avery, MacLeod, McCarty, Journal of Experimental Medicine 79,1944, p. 158.)
The above account illustrates how studies of a mutant form of an organism can produce important information about biological processes in the normal form. In a similar way, studies of mutant genes often yield insights into the functions of normal genes. ILRAD scientists working on improved control methods for the animal disease trypanosomiasis are developing 'transfection' systems for trypanosomes, the causative parasites. (The term 'transfection' is used to describe the artificial transfer of DNA from one organism to another.)
After extracting a gene of potential interest from trypanosomes, the scientists modify the gene using molecular biology techniques and then reintroduce the gene into the parasite to examine the effects of its alteration. Use of reliable transfection systems will help the researchers locate important receptor molecules on the parasite surface.
ILRAD scientists have isolated several genes in two species, Trypanosoma brucei and Trypanosoma congolense, that are expressed, or 'turned on', in particular rather than all stages of the parasites' life cycle. This selective gene expression suggests that the proteins encoded by these genes play important roles in the development and growth of the parasites. The protein products of these genes are thus potential targets for chemotherapeutic or immunological interventions aimed at stopping the parasites from proliferating in their animal hosts. ILRAD scientists are using transfection systems to discover the functions of these developmentally regulated proteins.
Transfection technology is also being used to discover the mechanisms responsible for development of parasite resistance to commonly used trypanocides. ILRAD scientists recently began to use transfection techniques to identify the genes responsible for the resistance phenotype and to establish the role these genes play in conferring resistance. Results of these studies will enable them to identify the targets of the drugs and should help disease control workers make more effective use of drugs.
In collaboration with scientists from the Netherlands Cancer Institute (Amsterdam), ILRAD workers are developing systems for the permanent (stable) transfection of trypanosomes. Vectors are constructed by inserting a selectable marker—a gene encoding resistance to an antibiotic—into a cloned parasite gene. The vector is introduced into live trypanosomes by a method known as electroporation. High-voltage electric currents are passed through the parasite medium, causing a temporary disruption of parasite cell membranes, through which the vectors enter the cells. The parasites that are resistant to the antibiotic are then selected.
These experiments have shown that the transfected trypanosomes contain the antibiotic resistance gene inserted at the genomic location of the cloned parasite gene. Using this 'targeted insertion', the ILRAD scientists are able to alter cloned trypanosome genes, target them back to their original genomic locations and examine the effects of the alteration. This technique is being used to examine the effects of altering genes identified as playing important roles in trypanosome cell division.
Foreign DNA can be introduced into mammalian cells using one of several methods. The choice of method depends largely on the cell type to be transfected and whether or not the gene of interest has been isolated. The following discussion illustrates how some of these methods have been employed at ILRAD for use in studies of important molecular functions in the immune responses of cattle to infection with Theileria parva, a protozoan parasite that causes East Coast fever.
East Coast fever is a virulent form of theileriosis. The disease is endemic in 11 countries of Africa where it is a severe constraint not only to the production of milk and beef, but also to mixed farming: keeping at least one or two cattle is essential for the survival of many of Africa's small mixed crop–livestock farms.
Some cattle naturally survive an infection with T. parva and these animals are thereafter immune to East Coast fever. Evidence suggests that this immunity is mediated by a subpopulation of bovine white blood cells known as cytotoxic T lymphocytes (CTLs). These cells bind to and kill host cells that are infected with T. parva by recognizing foreign (parasite) proteins that appear on the surface of the infected cells.
Research on other disease systems has shown that CTLs recognize foreign proteins on host cells only when these appear in close association with a particular type of molecule produced by the host cell and known as an MHC class I molecule. These molecules are encoded by genes located in an area of the mammalian genome designated the major histocompatibility complex (MHC). With a couple of exceptions, each mammalian species has a pool of many MHC class I molecules. Each individual within a species inherits a very small proportion of the available MHC class I pool. Thus, there can be significant differences in the composition of expressed MHC class I molecules from one individual to another.
The darkly stained cells in this group of mouse cells have been 'transfected' at ILRAD by insertion of a cattle gene that encodes a class I MHC molecule.
The great advantage of using transfection technology is that it allows scientists to study a single gene and its protein product in isolation.
These MHC differences among individuals may help to prevent the eradication of an entire species by a lethal pathogen. Today, these variations must be considered when interpreting results of immunization trials and when conducting experiments aimed at identifying parasite molecules that may be used in vaccines. To gain a better understanding of the complexity of the bovine MHC class I gene system, ILRAD scientists wanted to know how many types of MHC class I molecules are expressed on the surface of a single bovine cell.
Past results obtained at ILRAD and other laboratories suggested that cattle have more than one genetic locus expressing MHC class I molecules. To obtain clear evidence of this, ILRAD scientists transfected total (genomic) DNA from an animal into mouse fibroblast cells using a calcium-phosphate-mediated transfection system.
Calcium-phospate-mediated gene transfer is one of the most widely used transfection procedures. In essence, the DNA to be transfected is precipitated by the addition of calcium phosphate and added to the recipient cells. The precipitated DNA particles are taken up by the cells and the DNA is permanently incorporated into the chromosomes of the transfected cells. The unique advantage of this technique is that it can be used to transfect genes that have not been isolated but are known to be present in the genomic DNA from a particular cell type.
Cells transfected with the genomic DNA may express one or more of the foreign genes. Those transfected cells that express the gene of interest can be identified and isolated. This is particularly straightforward if the product of the gene of interest is an antigenic molecule located on the surface of the original cell. Using monoclonal anti-bodies that bind to this surface antigen and a technique known as fluorescence-activated cell sorting, scientists can isolate the transfected cell population. The disadvantages of this technique are that it is successful only for certain cell types, and that it does not allow easy recovery of the gene of interest.
In the ILRAD experiment, DNA was obtained from an animal known to have inherited identical MHC regions from each of its parents. (Such an individual is said to be homozygous at the MHC class I region.) When genomic DNA from this animal was transfected into mouse fibroblast cells, two distinct transfected cell lines were produced. Each line expressed a different MHC molecule as shown by the reactivities of the cells with monoclonal antibodies. This result was confirmed using genomic DNA from a second homozygous animal to produce another two transfected mouse cell lines expressing different MHC molecules.
These transfection experiments thus established that cells of each animal from which the DNA was obtained were expressing at least two types of MHC class I molecules. This means that each of the two MHC regions the animals inherited from their parents had two different class I genes.
Most animals are heterozygous rather than homozygous for MHC class I genes. The ILRAD results thus indicate that cattle can express at least four different MHC class I molecules on each cell. Parasite proteins could be presented to CTLs of the bovine immune system by any or all of these molecules.
Monoclonal antibodies that recognize and bind to surface antigens of lymphoid cells are essential reagents for the dissection of any immune response. For example, probing bovine cells with monoclonal antibodies allows scientists to identify subpopulations of immune cells, and thus to determine the contribution a particular subpopulation makes to an immune response. In this way, it was discovered that cytotoxic T cells play an important role in the protective immune response cattle mount against East Coast fever.
Monoclonal antibodies have traditionally been obtained by inoculating mice with cells expressing the appropriate antigen. Monoclonal antibodies derived from these mice are characterized by establishing their reactivities with subsets of cells or tissue sections, and by noting the size of antigens immunoprecipitated by the monoclonal antibodies. This procedure is often complicated by the fact that the expression of the antigen may not be clearly restricted to defined cell populations and that not all monoclonal antibodies will immunoprecipitate their respective antigens.
An alternative approach is available if the gene encoding the relevant antigen is isolated and transfected into cells that normally do not express the gene. The specificity of candidate monoclonal antibodies can be easily established by their reactivity with the transfected cell line. This approach was used at ILRAD to identify monoclonal antibodies specific for the bovine interleukin 2 (IL-2) receptor. IL-2 acts as a molecular messenger among lymphoid cells. Those lymphoid cells that are actively proliferating in response to an infection express elevated levels of the IL-2 receptor.
The gene encoding the IL-2 receptor had been isolated by workers elsewhere using the human IL-2 receptor gene as a probe. A mouse cell line permanently transfected with the bovine gene was produced at ILRAD and used to establish that several candidate monoclonal antibodies recognize the IL-2 receptor. These antibodies are now being used to obtain a clearer understanding of the roles that IL-2 and the IL-2 receptor play in the bovine immune response to trypanosomiasis.
The traditional way to isolate a gene that encodes an antigen is to construct gene 'library' in which all the DNA expressed by an organism is cloned into plasmid DNA, which is then introduced into bacteria for expression of the gene product. Scientists can identify which bacteria contain the gene of interest by probing the colonies with monoclonal antibodies that specifically recognize the protein product of the gene.
In a recently developed variation of this procedure, the gene library is transfected directly into mammalian cells. This modified approach is particularly useful to geneticists attempting to isolate a mammalian gene. Bacteria and mammalian cells process proteins differently. Thus, a mammalian gene expressed in a transfected mammalian cell may produce a protein that resembles the native gene product more closely than the same gene expressed in a transfected bacterium.
This approach has been used successfully in the transient expression COS cell system. COS cells are derivatives of an African green monkey kidney cell line (CV-1) infected with a replication-defective mutant of the simian virus 40 (SV40). These cells produce large amounts of the large tumour antigen of SV40. When specially designed plasmids are introduced into COS cells, the SV40 large tumour antigen present in the cells causes the plasmids to replicate as independent extrachromosomal elements to high copy number. Thus, any gene also inserted into the plasmid is expressed at very high levels if transfected into COS cells.
Scientists at the University Hospital in Utrecht, the Netherlands, working in collaboration with ILRAD, recently employed this procedure to isolate the gene that encodes an antigen produced by T.parva schizonts, the form of the parasite present in infected bovine lymphocytes. A library of genes expressed by schizonts was constructed in a plasmid specifically designed for expression of foreign genes in COS cells.
COS cells were transfected with the gene library and probed with anti bodies specific for a schizont antigen. Using an enzyme-based-detection system, scientists identified the COS cells containing the relevant plasmids. The recombinant plasmids were recovered from the COS cell and the gene encoding the schizont antigen was isolated This gene will now be used to characterize the antigen. The scientists particularly want to know if the antigen is recognized by CTLs as well as antibodies from immune cattle (see below).
As mentioned earlier, CTLs are believed to be important in protecting cattle against East Coast fever. A major objective of ILRAD's theileriosis research is to identify the parasite antigens recognized by CTLs obtained from cattle that are immune to East Coast fever; such antigens will form the basis of an effective vaccine against the disease.
One way to determine if a particular parasite antigen is recognized by CTLs is to transfect the gene encoding the antigen into a cell line expressing the appropriate bovine MHC class I molecule, such as the mouse fibroblasts transfected with bovine MHC class I molecules, as described above. These cells can then be used as target cells in a cytotoxicity assay to determine whether the gene encodes an antigen recognized by CTLs from an immune animal. In this way, it may be possible to identify from an array of candidate antigens those that could induce a protective CTL response. These antigens would form the basis of an effective vaccine against East Coast fever.
This article is based on reports of ILRAD studies of gene transfection written by Noel Murphy (trypanosomes) and Philip Toye (mammalian cells). Other ILRAD scientists involved in the mammalian cell transfection studies were Niall MacHugh, Ivan Morrison (now at the Compton Laboratory, UK), Jan Naessens and Alan Teale. Collaborators in this work include Piet Borst, of the Netherlands Cancer Institute (Amsterdam), and Hans Clevers and colleagues at the University Hospital, Utrecht, the Netherlands. The following article (page 5) is based on a report by former staff member Vinand Nantulya. CORRECTION During the year, ILRAD scientists also identified a 24-kDa molecule present in infected bovine lymphocytes. The purified molecule was recognized in vitro by clones of helper T cells derived from immune cattle. The role of the 24-kDa antigen in inducing protective responses to T. parva is being investigated |
DIAGNOSIS is a critical element in the management of disease, both at the level of the individual animal when the decision to treat or not to treat has to be taken and for evaluating the success of disease control programs. The diagnostic tests used should be simple, rapid, specific and highly sensitive. They should also be able to differentiate between closely related parasite species if the disease syndromes they cause require different management approaches. Ideal tests should be suitable for field application and the cost should be within the means of the communities affected by the disease. ILRAD has developed simple tests for diagnosis of African trypanosomiasis that promise to fulfil many of these requirements.
Trypanosomiasis is characterized by severe anaemia, weight loss, reduced productivity, infertility and abortion, with death occurring in some animals during the acute phase of the disease. Animals that survive often remain infected with trypanosomes for several months or years and exhibit a low level of fluctuating parasitaemia. During this phase, the animals may manifest no overt clinical signs.
The standard laboratory method for confirmation of the diagnosis of African trypanosomiasis is to identify trypanosomes in tissues of the infected host, usually in peripheral blood. This method, however, is unsatisfactory, because a high proportion of infections are not detected. Alternative methods of diagnosis have therefore been developed, most of which are for the detection of antibody responses to the antigens of the infecting trypanosomes. Antibody detection tests, however, have several shortcomings: the antigens used are ill-defined, thus making standardization of the tests difficult with regard to sensitivity and specificity. Moreover, detection of antibody in serum does not necessarily reflect an existing infection, because antibodies may persist for several months following recovery.
Although undetectable in peripheral blood, trypanosomes can still be found sequestered in several other tissues of the infected host, such as the spleen, lymph-nodes, bone marrow and liver. Successive trypanosome variable antigen types in these populations are destroyed by the immune responses of the infected host as it attempts to eliminate the parasites. Several soluble antigens are thus released in the tissue fluids, including some that are trypanosome species-specific. Detection of these antigens thus provides direct evidence that an animal has a current infection. This is the rationale of the antigen-detection enzyme immunosorbent assays (antigen ELISAs) that have been developed for diagnosing African trypanosomiasis.
ILRAD scientists have developed three tests: one is specific for the parasite species Trypanosoma vivax, one for T. congolense and one for the brucei group of trypanosomes, which includes animal-infective T. brucei and T. evansi and human-infective T. rhodesiense and T. gambiense.
INITIAL EVALUATION of the tests for animal trypanosomiasis was conducted with staff from the Kenya Trypanosomiasis Research Institute (Muguga). Field sera were obtained from cattle in a trypanosomiasis-endemic area at Nguruman, Kenya. The three tests were able to detect trypanosome antigens in the sera of 121 (96.0%0) of 126 animals with parasitologically confirmed diagnosis. More importantly, the tests also detected antigens in 52.6% of animals in the same herd that had not been diagnosed as infected by parasite detection techniques, thus demonstrating the superior sensitivity of the antigen-ELISAs. Use of the latter tests enabled scientists also to detect mixed infections involving two or three trypanosome species. Furthermore, these assays proved highly specific: control sera obtained from cattle in a trypanosomiasis-free area of Kenya (at Kapiti) all tested negative using the three assays.
Applied to the diagnosis of T. evansi infections in camels and pigs, the test for Trypanozoon-specific antigens was positive in 92% of the animals with detectable parasitaemia. Once again, infections in a high proportion of animals undetected by parasitological techniques (55%) were detected using the antigen-ELISA. Control animals from a T. evansi-free area all tested negative with the latter.
FOLLOWING the initial successful field testing in Kenya, the tests were introduced for further evaluation in Ghana, Mali, Senegal, The Gambia, Tanzania, Zambia, Zimbabwe and Uganda. These field evaluation activities have been supported by the Government of the Netherlands in a collaborative project involving the Food and Agriculture Organization of the United Nations (FAO, Rome) and the International Atomic Energy Agency (IAEA, Vienna). The test specific for the T. brucei group has also been evaluated for its utility in diagnosing T. evansi infections in camels in Mali and Kenya and in diagnosing human trypanosomiasis in Côte d'Ivoire, Tanzania, Uganda, Zambia and Zaire. This evaluation was conducted under the auspices of the United Nations Development Programme/World Health Organization/World Bank Special Programme for Research and Training in Tropical Diseases (Geneva).
Results of these validation exercises were presented at an FAO/IAEA Research Coordination Meeting held in Côte d'Ivoire in May 1991. Participants of this meeting concluded that the two antigen-ELISAs that detect species-specific invariant trypanosome antigens have been successfully introduced and established at 10 of the 11 institutes involved in the validation exercise. The participants reported that in detecting active trypanosome infections, the antigen-ELISAs demonstrated a sensitivity 4–5 times greater than that of the more commonly used buffy coat technique. It was concluded that the antigen-ELISAs clearly have potential use in the development of strategies for monitoring and controlling animal trypanosomiasis.
CATTLE INFECTED with trypanosome parasites are known to produce antibodies against molecules known as VSGs that are located on the surface of trypanosomes. Host anti-bodies bound to parasite VSG mediate the killing of the parasites by other elements of the bovine immune system, such as macrophages and neutrophils. This is the main method by which the immune system clears trypanosomes from the body. It is also known, however, that trypanosomes can remove antibody bound to their VSG. Because this antibody removal has important implications for the control of the parasites, studies were undertaken to determine the trypanosome mechanisms responsible for cleaving VSG-antibody complexes.
The binding of antibodies that specifically recognize VSG molecules exposed on the surface of clones of Trypanosoma brucei, Trypanosoma congolense and Trypanosoma vivax was investigated using both immunofluorescence (light microscopy and a fluorescence-activated cell sorter) and electron microscopy. In addition, parasites of the T. brucei S427 clone 22, which were adapted to cultures containing no feeder cells, were incubated with specific F(ab)2 and Fab antibody fragments or with biotin.
After incubation at 0°C, antibodies, antibody fragments and biotin molecules were observed over the whole parasite surface; fluorescence appeared strongest at the parasite's flagellum. Upon warming to 37°C, surface-bound antibody and antibody fragments were cleared from the parasite surface. Even in the absence of antibody-mediated crosslinking of VSG (i.e., Fab), clearance occurred through the movement of surface-bound Fab-VSG complexes toward the flagellar pocket. Studies of permeabilized trypanosomes using electron microscopy and immunofluorescence showed that after being cleared from the cell surface, small amounts of antibody were located intracellularly between the nucleus and the flagellar pocket. However, when a cocktail of protease inhibitors was added to the culture medium, large amounts of antibody or antibody fragments could be detected within vacuoles situated between the nucleus and the flagellar pocket, suggesting that proteases are required to break down antibody–VSG complexes. Different antibodies were cleared at different rates. Antibodies with both a higher molecular mass and more than one antigen-binding site were generally cleared most rapidly.
Movement of antibody-VSG complexes was inhibited at temperatures below 4°C and by adding 2-deoxy-Dglucose in lieu of D-glucose to the culture medium. Movement was immediately and reversibly inhibited by increasing the NaCl concentration in the medium to 200 mM. Antibody clearance was also inhibited by protein synthesis inhibitors and protease inhibitors. The process was not inhibited by microfilament (cytochalasin B and D) or microtubule (nocodazole) disrupters, nor was it altered by an increase in medium viscosity.
IN SUMMARY, the results of these studies showed that antibody clearance in trypanosomes is a directional, energy-dependent process. It is not dependent on crosslinking of VSG and it is selective: only VSG bound to antibody is cleared.
Antibody clearance may have an important role to play in the process of antigenic switching in trypanosomes in which the parasites periodically remove one coat of VSG and replace it with another of a different antigenic type. An antibody response to a specific variable antigen type may actually speed up the process of coat substitution. This would be a major advantage to the trypanosome: it would help the parasite evade the host immune response, thus prolonging its survival and increasing its chance of transmission.
David C.W. Russo,
Italian ILRAD Research Fellow.
Ph.D. thesis abstract submitted in 1992 to the Department of Biology and Biochemistry,
Brunel University,
Uxbridge, UK.
Ph.D. supervisors: Diana Williams and Dennis Grab.
ILRAD Reports is published quarterly by the International Laboratory for Research on Animal Diseases (ILRAD), in Nairobi, Kenya. The newsletter is written, designed and electronically laid out to camera-ready stage in house using a personal computer and laser printer. The newsletter is printed in Nairobi by Majestic Printing Works. It has a circulation of 3,500 English and 500 French readers and is available free upon request. Material in the Reports may be reproduced without prior permission, but we ask that credit be given and that two copies of the reprint be sent to ILRAD's Scientific Writer/Editor, Susan MacMillan: telephone: +254 (2) 632-311, telefax: 254 (2) 631-499, telex: 22040 ILRAD, electronic mail: (BT Tymnet) CGI017. ILRAD was founded in 1973 to conduct research into better ways of controlling livestock diseases. The current primary goal of the Laboratory is to develop safe, effective and economical methods to control two parasitic diseases that severely constrain animal production in Africa: trypanosomiasis, transmitted to animals by the bite of a tsetse fly, and East Coast fever, a virulent form of thelleriosis, transmitted to cattle by ticks. An international staff of about 50 scientists conducts basic research, much of it aimed at the development of vaccines, In the fields of biochemistry, cell biology, electron microscopy, epidemiology, genetics, immunology, molecular biology, pathology, parasitology and the socioeconomics of animal disease control. ILRAD is one of 17 international agricultural research centres sponsored by the Consultative Group on International Agricultural Research (CGIAR). The secretariat of the CGIAR is located in the World Bank headquarters, in Washington, D.C. The CGIAR is an informal umbrella organization of 40 national governments, international organizations and private foundations that together provide about US$300 million annually to the 17 centres for research, training and advisory services. The CGIAR aims to help farmers In developing countries increase their production of staple food crops, livestock, fish and trees in ways that improve the nutrition and well-being of low-income peoples and the management of natural resources. |