Variant Surface Glycoproteins (VSGs) of Trypanosomes: Difference between revisions

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==Trypanosomes==
==Trypanosomes==
[[Image: T.brucei_SEM.jpg‎|thumb|600px|right|Figure 1. Colorized SEM of <I>T. brucei</I>, and erythrocytes [6]]]
[[Image: T.brucei_SEM.jpg‎|thumb|600px|right|Figure 1. Colorized SEM of <I>T. brucei</I>, and erythrocytes [1].]]
<br>By Kelly Wahl<br>
<br>By Kelly Wahl<br>
The trypanosomes shown in Figure 1 are single-celled, flagellated, eukaryotic parasites responsible for African sleeping sickness and Chagas disease. The African tsetse fly (subgenus <I>Glossina morsitans</I>) as seen in Figure 2, transmits the strains <I>Trypanosoma brucei gambiense</I> and <I>Trypanosoma brucei rhodesiense</I>  through its bite [2]. In central and South America the kissing bug (subfamily <I>Triatominae</I>) as shown in Figure 3, spreads <I>Trypanosoma cruzi</I> through its feces. People become infected when feces enter the body through mucosal membranes, or broken skin (bite marks or scratches). Vertical transmission from mother to child, and infection via infected blood or organs are also routes of transmission but they are very rare. There are many other species of trypanosomes but only <I>T. brucei</I>, and <I>T. cruzi</I> are relevant to humans [3].
[[Image: Tsetsefly.jpg‎‎|thumb|200px|right|Figure 2. Tsetse fly with a blood meal, vector of <I>T. brucei</I> [5].]]
[[Image: Kissingbug.jpg|thumb|400px|right|Figure 3. Kissing bug, arthropod vector of <I>T. cruzi</I> [6].]]
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Trypanosomes are single-celled flagellated eukaryotic parasites causing trypanosomiasis in humans and livestock or as it is more commonly known as, sleeping sickness in Africa, and Chagas disease in the Americas. These parasites are usually introduced into the body via an arthropod vector.  The tsetse fly (subgenus <I>Glossina morsitans</I>), transmits the strains <I>Trypanosoma brucei gambiense</I> (95% of reported cases and found exclusively in West Africa) and <I>Trypanosoma brucei rhodesiense</I>  (5% of reported cases, only seen in East Africa) responsible for sleeping sickness through its bite(figure 2).  Oddly the geographic range of these two strains does not overlap [3]. In central and South America the kissing bug (subfamily <I>Triatominae</I>) spreads <I>Trypanosoma cruzi</I> when its  feces enter the body through the mucosal membranes, or broken skin (bite marks or scratches)(figure 3). Besides <I>T. brucei</I>, and <I>T. cruzi</I>, there are many other strains of trypanosomes that affect mainly domesticated animals like; cows, camels, horses, donkey but can also cause disease in deer, and antelope.
These parasites are part of the order Kinetoplastidia, which broke off from the common eukaryotic ancestor fairly early on. Kinetoplastids are named after the kinetoplast at the base of their flagellum. A kinetoplast is a mitochondrion with coiled mitochondrial and genomic DNA. The kinetoplast changes size and shape throughout the parasite’s life cycle and is the target site of the anti-parasitic drug ethidium [3,4].
[[Image: Tsetsefly.jpg‎‎|thumb|400px|left|Figure 2. Tsetse fly with a blood meal, vector of <I>T. brucei</I>[7]]]
[[Image: Kissingbug.jpg|thumb|400px|right|Figure 3. Kissing bug, arthropod vector of <I>T. cruzi</I>]]
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These parasites fall under the order Kinetoplastidia and are related to another important parasite, Leishmania. Kinetoplasts are called such because of the kinetoplast at the base of their flagellum. A kinetoplast is a mitochondrion with coiled mitochondrial and genomic DNA. The kinetoplast changes size and shape throughout the parasites life cycle and is the target site of some anti-parasitic drugs such as ethidium. Kinetoplasts broke off from the common eukaryotic ancestor fairly early on and consequently transcription of their genome is unique and the target of potential drugs [2, 11].
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==Trypanosome Infection; Sleeping Sickness, and Chagas Disease==
==Trypanosome Infection; Sleeping Sickness, and Chagas Disease==
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<b>Sleeping Sickness</b>
<b>Sleeping Sickness</b>
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Stage one of <I>T. brucei</I> infection is the haemolymphatic stage. Where parasites circulate and multiply by binary fission in the blood and lymph systems. Clinical infection with <I>T. b. rhodesiense</I>  often begins as a sore called a chancre and is followed a week or two of headaches, fevers, muscle pain, joint pain and possibly a rash. Patients may also show Winterbottom’s sign; enlarged, tender lymph nodes in the neck. <I>T. b. gambiense</I> infection shows the same symptoms but they are not as severe and last for a longer period of time [3, 4].
After a person is bitten they may develop a sore called a chancre and have headaches, fevers, muscle pain, joint pain, enlarged, tender lymph nodes (Winterbottom’s sign) and possibly a rash that last for about two weeks. During this first stage of infection known as the haemolymphatic stage, parasites are actively multiply by binary fission in the blood and lymph circulatory systems. Both <I>T. b. rhodesiense</I>  and <I>T. b. gambiense</I> show the same clinical symptoms but symptoms are much milder in <I>T. b. gambiense</I> infections and usually last for a longer period of time [2,7].
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The second stage of infection is neurological. The parasites have crossed the blood-brain barrier and are now multiplying in the central nervous system.  classic symptoms include rapid weight loss, impaired motor skills, seizures, confusion, and changes in personality. A disrupted circadian rhythm is the source of this disease’s name. Patients lie awake at night but sleeps during the day. Without treatment <I>T. b. rhodesiense</I> infection progress to coma and death within a few months. Often times <I>T. b. rhodesiense</I>  infection moves so quickly that malnutrition, heart failure, and pneumonia cause death before the neurological problems ever manifest themselves. The progression of <I>T. b. gambiense</I> infection takes much longer and a person can take years to show neurological symptoms. The different clinical manifestations of the two strains stems from differences in the two strains stimulation of the immune system [3].  
The second stage of infection is neurological. Parasites have crossed the blood-brain barrier and are now multiplying in the central nervous system.  classic symptoms include rapid weight loss, impaired motor skills, seizures, confusion, and changes in personality, coma, and death. A disrupted circadian rhythm is the source of this disease’s name. Patients lie awake at night but sleep during the day. Without treatment <I>T. b. rhodesiense</I> infections can progress so rapidly that malnutrition, heart failure, or pneumonia kill the patient before neurological symptoms ever manifest. In  <I>T. b. gambiense</I> infections disease progression is much slower and it can take years for a person to show any neurological symptoms [2].  
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<b>Chagas Disease</b>
<b>Chagas Disease</b>
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Chagas disease is similar to sleeping sickness in many ways. There is an acute stage when the parasite is circulating in the bloodstream and the initial site of infection swells and a patient may or may not present with signs of infection such as a fever. One of the classic signs of acute Chagas infection is Romana’s sign, the swelling of the area around the eye where the conjunctiva (mucosal tissue around the eye) or a site near the eye was the point of infection. Serious symptoms are rare in this stage of infection but possible. They include heart failure, chest pain, seizures, or paralysis.
Chagas disease is similar to sleeping sickness in many ways. There is an acute stage and a chronic stage. One of the classic signs of acute Chagas infection is Romana’s sign, the swelling of the area around the eye where the conjunctiva (mucosal tissue around the eye) or a site near the eye was the point of infection. Patients may not present with signs of infection such as a fever as <I>T. cruzi</I> parasites are multiplying in the bloodstream. Serious symptoms are rare in this stage of infection but can include; heart failure, chest pain, seizures, or paralysis [8].
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Unlike sleeping sickness, Chagas disease only progresses to the chronic form of the disease in 20 to 30% or people infected. The remaining 70 to 80% of people infected remain asymptomatic for life and may or may never know they are infected. As the parasite replicates in the target tissues; cardiac muscle and digestive smooth muscle those infected show a variety of cardiac problems such as irregular heartbeat, blood flow disorders, issues with heart muscle function, and blood clots. As well as problems with the digestive tract such as ulcers [4].
Unlike sleeping sickness, Chagas disease only progresses to the chronic form in 20 to 30% or people infected. The remaining 70 to 80% of people infected remain asymptomatic for life and may be unaware they are infected. As the parasite replicates in cardiac muscle and digestive smooth muscle patients develop cardiac problems such as irregular heartbeat, blood flow disorders, issues with heart muscle function, and blood clots as well as ulcers in the digestive tract [8]. Without treatment most chronic cases of Chagas disease are fatal [7].
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<b>Treatment</b>
<b>Treatment</b>
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Symptomatic trypanosome infection is usually fatal if left untreated. While there is no vaccine or prophylactic drugs to prevent <I>T. brucei</I> infection there are drugs available for treatment of asymptomatic, acute, and chronic infections. The earlier the diagnosis is made the easier it is to manage symptoms with less toxic drugs that target the parasite outside of the central nervous system. once the infection has become symptomatic, stronger drugs with much higher toxicity must be used to cross the blood-brain barrier and attack the parasite in the CNS. If the infection has progressed to this point it is likely that a lengthy hospital stay is required to clear up infection. There is a need for new drugs to combat this disease because the toxicity associated with some of the drugs currently used to treat the infection are almost as bad as the infection itself.
Symptomatic trypanosome infection is usually fatal if left untreated. There are no vaccines or prophylactic drugs to prevent infection but there are treatments available for both stages of the disease. The earlier the diagnosis, the easier it is to manage symptoms with less toxic drugs that can kill the parasite before it reaches the central nervous system.
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==Why Are Trypanosomes of Public health and Economic Interest?==
==Why Are Trypanosomes an Economic and Public Health Concern==
African trypanosomes not only affects people but also livestock animals like cows, camels, horses, donkey. In animals the disease is called <I>Nagana</I>. In endemic areas in sub-Saharan Africa where human and animal trypanosome infections are common farmers are unable to raise cattle further contributing to malnutrition in the area. Animals can also serve as reservoirs for <I>T. b. rhodesiense</I> making it impossible to eradicate the endemic trypanosomiasis in humans if there are also infected cattle nearby. The behavior of the Tsetse fly vector also affects the distribution of those who are stricken by the disease. Those who rely on farming, fishing, and hunting are most at risk to be bitten by an infected tsetse fly and are also commonly the breadwinners in poor, rural populations [7]. There are estimated to be more than 70,000 cases annually in sub-Saharan Africa, which encompasses the entire geographic range of the Tsetse fly [2,3,7,9].
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African trypanosomiasis is of public health and economic importance for a number of reasons. Besides affecting humans other strains of trypanosomes also infect domesticated cattle making cattle sick with disease called <I>Nagana</I>. The inability to raise cattle because of trypanosome infection in sub-Saharan African further contributes to malnutrition in the area. Animals also serve as reservoirs to the <I>T. b. rhodesiense</I> strain making it difficult to eradicate the parasite in an area with infected cattle.
African sleeping sickness and Chagas disease are part of a broader group of neglected tropical diseases that have no available vaccines and the drugs used to treat these diseases either have documented drug resistance against them or are very toxic. This group of neglected diseases is projected to account for 534,000 deaths a year and millions of years of functional life lost due to disability from disease. Developing countries bore the brunt of this burden, as the human and animal tolls of these diseases stunt economic development in affected areas [10].
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These parasites are only found within the tsetse flies’ geographic range, sub-Saharan Africa and have devastating economic effects on the people in that area because of the length and nature of trypanosomiasis. Those who rely on farming, fishing, and hunting are most at risk to be bitten by an infected tsetse fly and are also commonly the breadwinners in poor, rural populations [4]. While the WHO only reports about 10,000 new cases a year this number is most likely a gross underestimate of the true number of cases and other estimates put the number around 100,000 although others still caution that these numbers are underestimated when close to 50,000,000 are at risk [2,3,4].
Domestically, Chagas is a potential risk for blood and organ transplant recipients. It is estimated that more than 300,000 people living in the U.S. are infected with <I>T. cruzi</I>. Most infections were picked up from countries in the Americas where the disease is endemic but, there may be cases of natural transmission in North America because of the presence of triatome species [8].
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Both African trypanosomiasis and Chagas disease are considered neglected tropical diseases under which a projected 534,000 deaths a year and account for millions of year of functional life lost due to disability or years of life lost due to ill health. Those affected are primarily low and middle income countries in areas where the disease is most prevalent. Under international public health, trypanosomiasis is important because of the number of lives lost and emerging rural economies stunted by this disease [13]. Domestically, Chagas presents a threat to blood and organ transplant recipients. It is estimated that there are more than 300,000 people living with <I>T. cruzi</I> infections in the U.S. Although natural transmission through triatomines found in North America is possible it is very rare and iatrogenic (through blood or organ transplants) exposure represents the greatest risk of route of domestic exposure [12].
In this increasingly globalized world diseases that once hid in the jungles of South America and the plains of sub-Saharan Africa have ended up in the foyer of industrialized nations. Although trypanosomiasis is still considered a third-world disease, infections are continually being diagnosed in the first world and represent a risk to anyone that can get on a plane and travel to an affected area [2,7,8].
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In this increasingly globalized world diseases that once hid in the jungles of South America and the plains of sub-Saharan Africa have ended up on the doorstep of industrialized nations. Although trypanosomiasis is still considered a third-world disease infections are continually diagnosed in the first world and represent a real risk to anyone that can get on a plane and travel to an affected area.
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==Transmission and Life Cycle==
==Transmission and Life Cycle==
[[Image: Lifecycyle_t.brucei.JPG‎|thumb|800px|right|Figure 4. Life cycle of <I>T. brucei</I> [3]]]
[[Image: Lifecycyle_t.brucei.JPG‎|thumb|800px|right|Figure 4. Life cycle of <I>T. brucei</I> [2].]]
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<b>Transmission</b>
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Although the most common route of infection is for <I>T. brucei</I>, and <I>T. cruzi</I> are run-ins with the arthropods  <I>Glossina morsitans</I> and <I>Triatominae</I> . These parasites can be passed from mother to child during pregnancy. Blood transfusion and sex can also spread these parasites but this is very rare [3, 4].
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<b>Life cycle</b>
<b>Life cycle</b>
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Over the course of its lifetime a trypanosome cycles through five different morphological stages between arthropod vector and mammalian host. These different forms allow these parasites to live in very different host environments; from the gut of an insect to the central nervous system of a human [1].
Trypanosomes cycle through five different morphological stages during their movement between arthropod vector and mammalian host [11].The morphology of trypomastigotes within the mammalian body are distinctly different from the trypomastigotes found in insects vectors because of their different metabolic needs and the presence or absence of variant surface glycoproteins (VSGs).  In the midgut of the tsetse fly, after the ingestion of an infected blood meal procyclic trypomastigotes proliferate and move to the salivary gland where they differentiate into epimastigotes. The epimastigotes become VSG positive metacyclic epimastigotes. When the Tsetse fly bites a new mammalian host the metacyclic epimastigotes differentiate into a “slender” form and travel through the lymph and blood systems reproducing by binary fission in almost all organs of the body. Some of these dividing slender forms become “stumpy” and stop dividing. Although the stumpy form is no longer dividing within the mammalian host this form can survive in the fly midgut where it differentiates into the procyclic form again and completes the cycle of infection. When the parasites are in the mammalian host, numbers stages 1 through 5 in Figure 4, they are VSG positive and when they are in the Tsetse fly vector they are VSG negative [11, 3].
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The morphology of trypomastigotes within the mammalian body is distinctly different from the trypomastigotes found in insects because of differences in metabolic needs and the presence of variant surface glycoproteins (VSGs). These sugar-proteins are the secret weapon of trypanosomes that allow them to remain undetected by the immune system almost indefinitely. Ten million VSGs form homodimers with each other and stick out of the cell membrane like trees in a dense forest. Although VSGs amino acid sequences and attached sugars vary wildly from variant to variant their secondary and tertiary structures are highly conserved. In a way, VSGs are similar to the T cell receptors found on the surface of T cells in the immune system. Allelic exclusion ensures that only one VSG is expressed at a time. This monotypic surface coat is recognizable to B cells independent of signals B cells to differentiate. Even though the B cells are able to attack a particular VSG variant of the circulating trypomastigotes population there are always members of the population that have already silenced expression of that particular VSG and begun expressing a new VSG that has yet to be seen by the immune system [9].
While <I>T. brucei</I> multiplies in the extracellular space between cells, <I>T. cruzi</I> does not have VSGs and hides from the immune system by living within host cells. <I>T. cruzi</I> is taken up by phagocytosis and escapes from the lysozyme into the cytoplasm with the help of a special protein that is activated at low pH [3]. Stage 3 in Figure 5 shows parasites dividing intracellularly in infected muscle cells [8].
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[[Image: T.cruzi_lifecycle.jpg‎|thumb|800px|center|Figure 5. Life cycle of <I>T. cruzi</I> [8]]]
In the midgut of the tsetse fly procyclic trypomastigotes proliferate and move to the salivary gland where they become morphologically distinct epimastigotes.  After three weeks from the initial introduction of the trypanosomes into the midgut of the tsetse fly there are mature non-dividing metacyclic epimastigotes with VSGs ready for another round of mammalian infection. When the metacyclic epimastigotes enter the mammalian body through the bite of the fly they differentiate into “slender” form and travel in the lymph and blood systems reproducing by binary fission in almost all organs of the body. Some of these dividing slender forms become “stumpy” and stop dividing. Although the stumpy form is no longer dividing within the host this form can survive in the fly midgut where it differentiates into the procyclic form and complete the cycle of infection [1, 2].
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While <I>T. brucei</I> multiplies in the extracellular space between cells, <I>T. cruzi</I> actually hides intracellularly and does not undergo the same antigenetic switching of VSGsInstead it induces host cells to phagocytose it and escapes from the lysozyme into the cytoplasm before it is digested. A special protein is secreted that is only activated at low pH helps the parasite escape [2].
[[Image: T.cruzi_lifecycle.jpg‎|thumb|800px|center|Figure 5. Life cycle of <I>T. cruzi</I> [12]]]
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==Variant Surface Glycoproteins (VSGs)==
==Variant Surface Glycoproteins (VSGs)==
<b>VSGs</b>
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<b>VSGs</b>
VSGs allow trypanosomes to remain undetected by the adaptive immune system almost indefinitely and escape the complement system. Ten million identical VSGs form homodimers with one another  and cover the parasite’s cell membrane like a dense homogenous forest.  Although amino acid sequences and attached sugars vary from one VSG variant to another  their secondary and tertiary structures are highly conserved. Only one VSG variant is expressed at a time. There are at least 200 active VSG alleles and more than 1000 silent alleles and psuedogenes also encoding VSG variants. Active genes can recombine with silent genes to form new unique active genes. This antigenetic switching allows trypomastigotes to continually change their “appearance” to the immune system and remain undetected through the course of infection [12]. This monotypic surface coat is eventually recognized by the immune system but by the time the body is ready to react the parasite has already silenced expression of that particular VSG and begun expressing a new VSG [13]. The expression of only a single VSG also makes it difficult for complement proteins to bind and help the innate immune system catch the parasite [14].
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Variant surface glycoproteins (VSGs) are sugar-proteins that coat the surface of trypomastigotes. There are at least 200 active VSG alleles and more than 1000 silent alleles and psuedogenes. Active genes can recombine with silent genes to form new unique active genes. This process is called antigenetic switching and allows the blood-stream, slender form of the parasite to continually change its “appearance” to the immune system and remain undetected through the course of infection. While the immune system eventually detects and makes antibodies to a particular VSG it cannot make antibodies to every type of VSG present on the surface of all the different circulating trypomastigotes. The immune response continually selects for the expression of new VSG variants. It is a case of the Red Queen’s race only the parasite eventually wins because it has more possible VSG variants than the immune system could ever catch before parasite proliferation in vital organs causes death. VSGs can also actively modulate various components of the immune system to further multiply throughout the body undetected. The mechanism that regulates this entire process is fairly unique but has been studied in yeast [1, 8, 9].
The host immune response continually selects for the expression of new VSG variants. It is a case of the Red Queen’s race only <I>T. brucei</I>  eventually wins as the host’s resources burn out and increasingly virulent variants are selected. There are more VSG variants than the immune system could ever recognize. VSGs can also actively modulate various components of the immune system to further avoid detection [11,12,13,14].
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<b>VSG expression</b>
<b>VSG expression</b>
[[Image: VSGexpression.jpg‎|thumb|800px|right|Figure 6. Two mechanisms for gene expression of variant surface glycoproteins (VSGs)]]
[[Image: VSGexpression.jpg‎|thumb|800px|right|Figure 6. Two mechanisms for mono-telomeric expression of variant surface glycoproteins (VSGs)[12].]]
Only one of the 10-20 VSG telomeric sites expression sites is actively transcribed while the rest are silenced. Oddly enough these expression sites are transcribed by RNA polymerase I. <I>T. brucei</I>  and other strains associated with Nangana or African sleeping sickness are the only eukaryotes that transcribe anything other than ribosomal RNA [12]. All other eukaryotes including <I>T. cruzi</I>  only used RNA Pol I to transcribe ribosomal RNA [9].
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Only one of 10-20 VSG telomeric sites expression sites is available for transcription at a time while the rest are silenced. This strategy allows only one VSG coat with a given set of epitopes to be expressed at a time. The adaptive immune system eventually forms antibodies that bind to these epitopes and elicit an immune response but by the time this happens the parasite has silenced that expression site and begins expressing antigenetically different VSGs from another telomeric site. Oddly enough these expression sites are transcribed by RNA polymerase I. In eukaryotes RNA pol I is responsible only for the transcription of ribosomal RNA while RNA pol II is normally used for the transcription of other types of RNA [8].
VSGs can be mono-telomerically expressed in one of two ways. The lower left panel of Figure 6 shows transcription switching. An actively transcribed telomeric site is silenced and another telomeric site begins active transcription.
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There are two ways that VSGs are mono-telomerically expressed (only one site/one VSG is transcribed). Transcription switching is where an active telomeric site that was being actively transcribed is repressed and another active telomeric site that was silenced begins active transcription (figure 6). Recombination switching is the other way switch VSG expression. One of the thousand or so pseudogenes that cannot be transcribed on its own recombines with an active telomeric site so it can be expressed (figure 6). Active expression sites are flexible in their recombination. Psuedogenes are found in silent expression sites, the telomeres of chromosomes, and in subtelomeric VSG arrays. When recombining with active expression sites, either whole genes or segments of genes can be moved increasing the possible number of VSGs that can be expressed. The exact proteins involved and the mechanism by which they are able to recombine is still under study [8].
Recombination switching as shown in the lower right panel of Figure 6, is the other way <I>T. brucei</I>  switches VSG expression [12]. One of the thousand or so silent pseudogenes recombines with an active telomeric site so it can be expressed. This type of switching greatly increases the number of possible variants. The exact proteins involved and the mechanism by which they are able to recombine is still under study but there are a number of homologs involved in homologous recombination and DNA repair that are common to trypanosomes and other eukaryotes such as yeast [12,15].
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The mechanism that allows one telomeric expression site to be actively transcribed while the rest are silenced is generally known some of the details need to be worked out. It seems to involve the coiling and uncoiling of DNA from nucleosomes and other chromatin proteins. Nucleosomes are the spooling proteins that help structurally organize DNA and play a role in gene expression. Expression of certain genes is repressed when these proteins twist DNA up in such a way that RNA polymerase cannot physically bind the promoter region of the gene and begin transcription [8].
The coiling and uncoiling of chromatin determines which expression site is open for transcription while the rest are silenced. The nucleosomes, or spooling proteins that help structurally organize DNA are important for VSG expression. Expression of certain VSGs are repressed when these proteins twist DNA up in such a way that RNA polymerase cannot get to the promoter region to bind the expression site and begin transcription [12,15].
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Expression site silencing requires at least six protein factors and operates by two different mechanism; short-range and long-range telomeric silencing. In short-range silencing, the modifications by the histone deacetylase SIR2rp1 is within 5 kbp of the expression site silence expression. HAT1 is the histone acetylase that reverses silencing and allows the histones to relax enough so active transcription can take place. Long-range silencing is controlled farther away from the expression site (about 50 kbp upstream) most likely through chromatin remodeling with a variety of histone deacetylases, acetylases, methylases, chromatin remodelers (assemblers and disassemblers of histones). The default for both of these mechanisms is silencing and almost all are required for growth. One of the factors being considered as a drug target is the deacetylase DAC3 [8].
Expression site silencing requires at least six protein factors and operates by a long or short-range mechanism. In short-range silencing, acetyl groups are removed from histones within 5 KBP of the expression site by histone deacetylase SIR2rp1. The histone acetylase HAT1 reverses silencing by adding acetyl groups to histones so they bind DNA less tightly and active transcription can take place. Long-range silencing is controlled about 50 kbp upstream of the expression site most likely through chromatin remodeling with a combination of histone deacetylases, acetylases, methylases, and other enzymes.The default state for VSG expression, is silence [12].
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While the mechanism of silencing expression itself is well understood the factors that control which genes are silenced or activated remains to be studied. Indeed it is amazing that with all of the potential VSG alleles available for transcription only one allele is transcribed at a time (monoallelic expression) [8].
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==Current Research==
==Current Research==
<I>T. brucei</I>’s use of ever-changing VSGs is simple and elegant but there some confounding details in the expression and alternate function of VSGs that still need to be explored. One issue is the transition from one VSG surface coat to the next. What happens as a new coat is forming and the old coat is shedding from the cell surface? There is also the question of what other tricks trypanosomes have to avoid detection by the immune system besides their VSG coats [13,10].
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<b>Expression of a Mosaic VSG Coat to Escape Early Detection by B-cells</b>
<b>Expression of a Mosaic VSG Coat to Escape Early Detection by B-cells</b>
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Although only one VSG gene is expressed at a time the surface coat isn’t always made up of one VSG. Even though a new gene is being expressed the existing VSG coat doesn’t just disappear overnight.  It takes about 48 hours for the VSG coat to completely switch from one variant to another. This lag period is due to the time is takes to transcribe, translate, and transport the new VSGs to the surface and get rid of residual mRNA and protein from the previous coat [9].
Although only one VSG gene is expressed at a time, the surface coat isn’t always monotypic. Even though a new gene is being expressed the existing VSG coat doesn’t just disappear overnight because it takes time to transcribe, translate, and transport the new VSGs to the surface and destroy residual mRNA and protein from the previous coat. It takes about 48 hours for the VSG coat to completely switch from one variant to another [13].
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[[Image: VSG_coexpression_on_cell_surface.jpg‎|thumb|400px|right|Figure 7. Fluorescence micrograph of trypanosome co-expressing LouTat1 VSG (wild-type)panel A and 117 VSG (transformed) panel B. Panel C represent an overlay of panels A and B, while Panel D shows a micrograph of the parasite without staining [9].]]
[[Image: VSG_coexpression_on_cell_surface.jpg‎|thumb|400px|right|Figure 7. Fluorescence micrograph of trypanosome co-expressing LouTat1 VSG (wild-type)panel A and 117 VSG (transformed) panel B. Panel C represent an overlay of panels A and B. Panel D shows the parasite without florescence [13].]]
Recently Dubois et al. (2005) found that this 48 period of a mosaic VSG coat during antigenetic variation actually helps prevent B cells independent T cells from recognizing trypanosomes as new VSGs that are appearing on the surface of the cell amongst VSGs that have already been recognized by B cells. It is not until the surface coat again becomes monotypic with a single VSG present are B cells once again able to recognize the parasite and amount an immune response [9].
Dubois et al. (2005) found the 48 hour transition period where more than one VSG variant is present on the cell surface actually prevents B-cells from recognizing either the old or new VSGs. It’s like the trypomastigote call a timeout from the B-cell response while it’s changing clothes [13].
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[[Image: B-cell_response.jpg|thumb|400px|center|Figure 8. B cell response in athymic mice after inoculation with wildtype and transformed <I>T. b. rhodesiense</I> strain. Upward pointing triangles represent the wildtype strain expressing a single VSG, downward pointing triangles show transformed strain expressing both LouTat1 and 117 VSGs, the open circles are uninfected control mice. The X-axis shows days into the infection and the Y-axis are counts for the number of activated B cells harvested from the spleen [9].]]
[[Image: B-cell_response.jpg|thumb|400px|center|Figure 8. B cell response in athymic mice after inoculation with wildtype and transformed <I>T. b. rhodesiense</I> strain. Upward pointing triangles represent the wildtype strain expressing a single VSG, downward pointing triangles show transformed strain expressing both LouTat1 and 117 VSGs, the open circles are uninfected control mice. The X-axis shows days into the infection and the Y-axis is the number of activated B cells found in the spleen [13].]]
Athymic mice (lacking T-cells) were infected with a strain of <I>T. b. rhodesiense</I> that had been transformed to express two VSGs at the same time. Figure X shows dual VSG expression of transformed trypanosomes. Panel A of figure 7 shoes fluorescence labeled LouTat1 VSG while panel B shows labeled 117 VSG. Panel C shoes the overlay of both panels A and B, and panel D shows the same parasite under differential, interference, contrast microscopy independent of VSG labeling.
To prove this, athymic mice (lacking T-cells) were infected with a strain of <I>T. b. rhodesiense</I> that had been transformed to express two VSGs at the same time. Panel A of Figure 7 shows fluorescence labeled LouTat1 VSG while panel B shows labeled 117 VSG. Panel C is an overlay of panels A and B. Wild-type trypanosomes could not be used because only a few parasites in a given population are switching coats at any one time and have mixed VSG coats. An ELISPOT assay measured the number of activated splenic B-cells producing antibodies. Figure 8 shows almost no B-cell activity in mice infected with trypanosomes co-expressing VSG 117 and LouTat1. One explanation is that B-cells are fooled by the insertion of variant VSGs in a monotypic coat during switching because the new VSGs breakup the pattern of identical epitopes required for the direct activation of B-cells void of any interactions with T-cells [13].
Wild-type trypanosomes could not be used because very few parasites in a given population are switching coats at any one time and dual expression of VSGs on the surface of cells is brief. An ELISPOT assay was done on B-cells from the spleen. They found that fewer B-cells were active and producing antibodies to either expressed VSG for the five says of infection. Figure 8 shows that the number of B-cells activated by VSG-co-expressing trypanosomes was near the level of B-cells activated in uninfected mice. It seems that B-cells are fooled by the insertion of variant VSGs in a monotypic coat during switching because the new VSGs breakup the pattern of identical epitopes that are required for B-cell activation independent of T cells [9].
<br><br>
<br><br>
<b>Trypanosomes Effects on Immune Function</b>
<b>Trypanosomes Effects on Immune Function</b>
<br><br>
<br><br>
Besides hiding from the immune system with an ever-changing VSG surface coat trypanosomes also manage to down-regulate the function of various components of the immune system. one example of down-regulated function is the decreased ability of white blood cells to process and present newly encountered antigens after trypanosome infection [10].  
Besides hiding from the immune system with an ever-changing VSG surface coat, trypanosomes also manage to down-regulate the function of various components of the immune system. one example is the decreased ability of white blood cells to process and present newly encountered antigens after trypanosome infection [16].
<br><br>
[[Image: IL-2level.jpg|thumb|400px|center|Figure 9. Relative ability of T-cell hybridomas to present novel antigens after trypanosome infection [16].]]
In 2009, Dagenais et al. showed white blood cells from infected mice displayed a decreased ability to respond to newly encountered hen egg lysozyme (HEL) antigen <I>ex vivo</I>. Extracted T-cells were irradiated and combined with immortal cells to form hybridomas that could be grown in cell culture. The ability to process and present an antigen was measured by the amount of interlukin-2 (IL-2) produced. IL-2 was used as a measure of immune function because it is an important cytokine that helps stimulate cytoxic T-cells and affects the development of T-cell memory, among other functions. To measure IL-2, radiolabeled thymidine was added to the hybridoma medium and the amount of IL-2 produced with incorporated radiolabeled thymidine was measured with a scintillation counter. A scintillation counter measures the amount of radiation in a sample as counts (or decays) per minute (CPM) [16].
<br><br>
Naïve (uninfected) cells displayed a normal dose-dependent response to the introduction of HEL. As cells were exposed to more antigen, more IL-2 was produced. Figure 9 shows constant IL-2 production in cells from infected animals regardless of the concentration of HEL or length of time the animal had been infected before T-cells were harvested [16].
<br><br>
<br><br>
[[Image: IL-2level.jpg|thumb|400px|center|Figure 9. Relative ability of splenocytes to present novel antigens after trypanosome infection]]
==Conclusion==
In 2009, Dagenais et al. showed that white blood cells taken from infected mice displayed a decreased ability to respond to newly encountered hen egg lysozyme (HEL) antigen ex vivo. This assay had to be done ex vivo in order to measure this type of activity in a controlled way. Extracted T-cells were irradiated and hybridized with hybridomas so cells could be grown in cell culture. The ability to process and present antigen was measured by the amount of interlukin-2 (IL-2) that was produced. After two days of infection radiolabeled thymidine was added to the hybridoma medium and the amount of IL-2 produced with incorporated radiolabeled thymidine was measure with a scintillation counter. A scintillation counter measures the amount of radiation in a sample as counts (or decays) per minute (CPM).  IL-2 is an important cytokine that helps stimulate cytoxic T-cells and affects the process of T-cell immunologic memory, among other functions. Compared to naïve (uninfected) cells which displayed a dose-dependent response to the introduction of HEL. The relative amount of IL-2 produced by cells from infected animals did not change with concentration of HEL or day post infection (Figure 9)[10].
A better understanding of the interactions between <I>T. brucei</I> and immune could lead to the discovery of new anti-trypanosome drugs. Current drugs available for treatment are extremely toxic with side effects that can be almost as bad as trypanosomiasis itself [10]. New anti-parasite drugs could make a huge impact on areas where sleeping sickness and Nangana are endemic. Taking control of trypanosome diseases would be a huge win for international public health and give affected areas an economic boost from increased human productivity and the ability to raise livestock [3].
 
==References==
==References==
[1] Parasite Museum (2009, June 4). <I>Trypanosoma brucei</I>. Retrieved April 25, 2011, from Parasite Museum: http://www.parasitemuseum.com/trypanosome/
<br><br>
<br><br>
[1] Foster, W. F., and Slonczewski, J. L. (2010). <I>Microbiology; An Evolving Science</I>. New York: W.W. Norton & Company, Inc.
[2] Centers for Disease Control and Prevention (2010, November 2). <I>Parasites - African Trypanosomiasis (also known as Sleeping Sickness) </I>. Retrieved April 1, 2011, from the CDC: http://www.cdc.gov/parasites/sleepingsickness/
<br><br>
<br><br>
[2] Hunt, R. C. (2010, February 15). Parasitology-Chapter Three; Molecular Parasitology: Trypanosomes <I>Eukaryotic cells with a different way of doing things</I>. Retrieved April 16, 2011, from Microbiology and Immunity On-line: http://pathmicro.med.sc.edu/lecture/trypanosomiasis.htm
[3] Hunt, R. C. (2010, February 15). Parasitology-Chapter Three; Molecular Parasitology: Trypanosomes <I>Eukaryotic cells with a different way of doing things</I>. Retrieved April 16, 2011, from Microbiology and Immunity On-line: http://pathmicro.med.sc.edu/lecture/trypanosomiasis.htm
<br><br>
<br><br>
[3] Centers for Disease Control and Prevention (2010, November 2). <I>Parasites - African Trypanosomiasis (also known as Sleeping Sickness) </I>. Retrieved April 1, 2011, from the CDC: http://www.cdc.gov/parasites/sleepingsickness/
[4] Rudenko, G. Epigenetics and transcriptional control in African trypanosomes. <I>Essays Biochem.</I>. 2010. 20;48(1):201-19.
<br><br>
<br><br>
[4] World Health Organization (2011). <I>Trypanosomiasis, African</I>. Retrieved April 1, 2011, from the WHO: http://www.who.int/topics/trypanosomiasis_african/en/
[5] (2007). <I>A well fed tse tse fly</I>. Retrieved April 25, 2011, from Tanzania 2007 Safari: http://www.accuratereloading.com/hr20071.html
<br><br>
<br><br>
[5] Pottinger, P. (2010, July 27). <I>History of Chagas Disease</I>. Retrieved April 20, 2011, from History of Infectious Diseases: http://idhistory.tumblr.com/
[6] Pottinger, P. (2010, July 27). <I>History of Chagas Disease</I>. Retrieved April 20, 2011, from History of Infectious Diseases: http://idhistory.tumblr.com/
<br><br>
<br><br>
[6] Parasite Museum (2009, June 4). <I>Trypanosoma brucei</I>. Retrieved April 25, 2011, from Parasite Museum: http://www.parasitemuseum.com/trypanosome/  
[7] World Health Organization (2011). <I>Trypanosomiasis, African</I>. Retrieved April 1, 2011, from the WHO: http://www.who.int/topics/trypanosomiasis_african/en/
<br><br>
<br><br>
[7] (2007). <I>A well fed tse tse fly</I>. Retrieved April 25, 2011, from Tanzania 2007 Safari: http://www.accuratereloading.com/hr20071.html
[8] Centers for Disease Control and Prevention (2010, November 2). <I>Parasites – American Trypanosomiasis (also known as Chagas Disease)</I>. Retrieved April 1, 2011, from the CDC: http://www.cdc.gov/parasites/chagas/
<br><br>
<br><br>
[8] Horn, D., and McCulloch, R. Molecular mechanisms underlying the control of antigenic
[9] Daniels, J.P., Gull, K. Wickstead, B. Cell biology of the trypanosome genome.  <I>Microbiology and Molecular Biology Reviews</I>. 2010. 74;(4):552-569.
<br><br>
[10] Wikipedia (2011, April 11). <I>Neglected tropical disease research and development</I>. Retrieved April 16, 2011, from Wikipedia: http://en.wikipedia.org/wiki/Neglected_Tropical_Disease_Research_and_Development
<br><br>
[11] Foster, W. F., and Slonczewski, J. L. (2010). <I>Microbiology; An Evolving Science</I>. New York: W.W. Norton & Company, Inc.
<br><br>
[12] Horn, D., and McCulloch, R. Molecular mechanisms underlying the control of antigenic
variation in African trypanosomes. <I> Current Opinion in Microbiology</I> 2010. 13:700–705.
variation in African trypanosomes. <I> Current Opinion in Microbiology</I> 2010. 13:700–705.
<br><br>
<br><br>
[9] Dubois, M. E., Demick, K. P., Mansfield, J. M.  Trypanosomes expressing a mosaic variant surface glycoprotein coat escape early detection by the immune system. <I>Infection and Immunity</I> 2005. 2690-2697.
[13] Dubois, M. E., Demick, K. P., Mansfield, J. M.  Trypanosomes expressing a mosaic variant surface glycoprotein coat escape early detection by the immune system. <I>Infection and Immunity</I> 2005. 2690-2697.
<br><br>
[10] Dagenais, T. R., Freeman, B. E., Freeman, Demick, K. M., Paulnock, D. M., Mansfield, J. M. Processing and presentation of variant surface glycoprotein molecules to T cells in African trypanosomiasis. <I>J Immunol</I> 2009. 183(5): 3344–3355.
<br><br>
<br><br>
[11] Rudenko, G. Epigenetics and transcriptional control in African trypanosomes. <I>Essays Biochem.</I>. 2010. 20;48(1):201-19.
[14] Paulnock, D.M., Freeman, B.E., Mansfield, J.M., Modulation of innate immunity by African Trypanosomes. <I>Parasitology</I>. 2010. 137:2051-2063.
<br><br>
<br><br>
[12] Centers for Disease Control and Prevention (2010, November 2). <I>Parasites – American Trypanosomiasis (also known as Chagas Disease)</I>. Retrieved April 1, 2011, from the CDC: http://www.cdc.gov/parasites/chagas/
[15] Stanne, T.M., Rudenko, G. Active VSG expression sites in <I>Trypanosoma brucei</I> are depleted of nucleosomes. <I>Eukaryotic Cell</I>. 2010. 9;(1):136-147.
<br><br>
<br><br>
[13] Wikipedia (2011, April 11). <I>Neglected tropical disease research and development</I>. Retrieved April 16, 2011, from Wikipedia: http://en.wikipedia.org/wiki/Neglected_Tropical_Disease_Research_and_Development
[16] Dagenais, T. R., Freeman, B. E., Freeman, Demick, K. M., Paulnock, D. M., Mansfield, J. M. Processing and presentation of variant surface glycoprotein molecules to T cells in African trypanosomiasis. <I>J Immunol</I> 2009. 183(5): 3344–3355.
<br><br>
<br><br>
Edited by student of [mailto:slonczewski@kenyon.edu Joan Slonczewski] for [http://biology.kenyon.edu/courses/biol238/biol238syl10.html BIOL 238 Microbiology], 2011, [http://www.kenyon.edu/index.xml Kenyon College].
Edited by student of [mailto:slonczewski@kenyon.edu Joan Slonczewski] for [http://biology.kenyon.edu/courses/biol238/biol238syl10.html BIOL 238 Microbiology], 2011, [http://www.kenyon.edu/index.xml Kenyon College].
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Trypanosomes

Figure 1. Colorized SEM of T. brucei, and erythrocytes [1].


By Kelly Wahl
The trypanosomes shown in Figure 1 are single-celled, flagellated, eukaryotic parasites responsible for African sleeping sickness and Chagas disease. The African tsetse fly (subgenus Glossina morsitans) as seen in Figure 2, transmits the strains Trypanosoma brucei gambiense and Trypanosoma brucei rhodesiense through its bite [2]. In central and South America the kissing bug (subfamily Triatominae) as shown in Figure 3, spreads Trypanosoma cruzi through its feces. People become infected when feces enter the body through mucosal membranes, or broken skin (bite marks or scratches). Vertical transmission from mother to child, and infection via infected blood or organs are also routes of transmission but they are very rare. There are many other species of trypanosomes but only T. brucei, and T. cruzi are relevant to humans [3].

Figure 2. Tsetse fly with a blood meal, vector of T. brucei [5].
Figure 3. Kissing bug, arthropod vector of T. cruzi [6].



These parasites are part of the order Kinetoplastidia, which broke off from the common eukaryotic ancestor fairly early on. Kinetoplastids are named after the kinetoplast at the base of their flagellum. A kinetoplast is a mitochondrion with coiled mitochondrial and genomic DNA. The kinetoplast changes size and shape throughout the parasite’s life cycle and is the target site of the anti-parasitic drug ethidium [3,4].

Trypanosome Infection; Sleeping Sickness, and Chagas Disease

Sleeping Sickness

After a person is bitten they may develop a sore called a chancre and have headaches, fevers, muscle pain, joint pain, enlarged, tender lymph nodes (Winterbottom’s sign) and possibly a rash that last for about two weeks. During this first stage of infection known as the haemolymphatic stage, parasites are actively multiply by binary fission in the blood and lymph circulatory systems. Both T. b. rhodesiense and T. b. gambiense show the same clinical symptoms but symptoms are much milder in T. b. gambiense infections and usually last for a longer period of time [2,7].

The second stage of infection is neurological. Parasites have crossed the blood-brain barrier and are now multiplying in the central nervous system. classic symptoms include rapid weight loss, impaired motor skills, seizures, confusion, and changes in personality, coma, and death. A disrupted circadian rhythm is the source of this disease’s name. Patients lie awake at night but sleep during the day. Without treatment T. b. rhodesiense infections can progress so rapidly that malnutrition, heart failure, or pneumonia kill the patient before neurological symptoms ever manifest. In T. b. gambiense infections disease progression is much slower and it can take years for a person to show any neurological symptoms [2].

Chagas Disease

Chagas disease is similar to sleeping sickness in many ways. There is an acute stage and a chronic stage. One of the classic signs of acute Chagas infection is Romana’s sign, the swelling of the area around the eye where the conjunctiva (mucosal tissue around the eye) or a site near the eye was the point of infection. Patients may not present with signs of infection such as a fever as T. cruzi parasites are multiplying in the bloodstream. Serious symptoms are rare in this stage of infection but can include; heart failure, chest pain, seizures, or paralysis [8].

Unlike sleeping sickness, Chagas disease only progresses to the chronic form in 20 to 30% or people infected. The remaining 70 to 80% of people infected remain asymptomatic for life and may be unaware they are infected. As the parasite replicates in cardiac muscle and digestive smooth muscle patients develop cardiac problems such as irregular heartbeat, blood flow disorders, issues with heart muscle function, and blood clots as well as ulcers in the digestive tract [8]. Without treatment most chronic cases of Chagas disease are fatal [7].

Treatment

Symptomatic trypanosome infection is usually fatal if left untreated. There are no vaccines or prophylactic drugs to prevent infection but there are treatments available for both stages of the disease. The earlier the diagnosis, the easier it is to manage symptoms with less toxic drugs that can kill the parasite before it reaches the central nervous system.

Why Are Trypanosomes an Economic and Public Health Concern

African trypanosomes not only affects people but also livestock animals like cows, camels, horses, donkey. In animals the disease is called Nagana. In endemic areas in sub-Saharan Africa where human and animal trypanosome infections are common farmers are unable to raise cattle further contributing to malnutrition in the area. Animals can also serve as reservoirs for T. b. rhodesiense making it impossible to eradicate the endemic trypanosomiasis in humans if there are also infected cattle nearby. The behavior of the Tsetse fly vector also affects the distribution of those who are stricken by the disease. Those who rely on farming, fishing, and hunting are most at risk to be bitten by an infected tsetse fly and are also commonly the breadwinners in poor, rural populations [7]. There are estimated to be more than 70,000 cases annually in sub-Saharan Africa, which encompasses the entire geographic range of the Tsetse fly [2,3,7,9].

African sleeping sickness and Chagas disease are part of a broader group of neglected tropical diseases that have no available vaccines and the drugs used to treat these diseases either have documented drug resistance against them or are very toxic. This group of neglected diseases is projected to account for 534,000 deaths a year and millions of years of functional life lost due to disability from disease. Developing countries bore the brunt of this burden, as the human and animal tolls of these diseases stunt economic development in affected areas [10].

Domestically, Chagas is a potential risk for blood and organ transplant recipients. It is estimated that more than 300,000 people living in the U.S. are infected with T. cruzi. Most infections were picked up from countries in the Americas where the disease is endemic but, there may be cases of natural transmission in North America because of the presence of triatome species [8].

In this increasingly globalized world diseases that once hid in the jungles of South America and the plains of sub-Saharan Africa have ended up in the foyer of industrialized nations. Although trypanosomiasis is still considered a third-world disease, infections are continually being diagnosed in the first world and represent a risk to anyone that can get on a plane and travel to an affected area [2,7,8].

Transmission and Life Cycle

Figure 4. Life cycle of T. brucei [2].

Life cycle

Trypanosomes cycle through five different morphological stages during their movement between arthropod vector and mammalian host [11].The morphology of trypomastigotes within the mammalian body are distinctly different from the trypomastigotes found in insects vectors because of their different metabolic needs and the presence or absence of variant surface glycoproteins (VSGs). In the midgut of the tsetse fly, after the ingestion of an infected blood meal procyclic trypomastigotes proliferate and move to the salivary gland where they differentiate into epimastigotes. The epimastigotes become VSG positive metacyclic epimastigotes. When the Tsetse fly bites a new mammalian host the metacyclic epimastigotes differentiate into a “slender” form and travel through the lymph and blood systems reproducing by binary fission in almost all organs of the body. Some of these dividing slender forms become “stumpy” and stop dividing. Although the stumpy form is no longer dividing within the mammalian host this form can survive in the fly midgut where it differentiates into the procyclic form again and completes the cycle of infection. When the parasites are in the mammalian host, numbers stages 1 through 5 in Figure 4, they are VSG positive and when they are in the Tsetse fly vector they are VSG negative [11, 3].

While T. brucei multiplies in the extracellular space between cells, T. cruzi does not have VSGs and hides from the immune system by living within host cells. T. cruzi is taken up by phagocytosis and escapes from the lysozyme into the cytoplasm with the help of a special protein that is activated at low pH [3]. Stage 3 in Figure 5 shows parasites dividing intracellularly in infected muscle cells [8].

Figure 5. Life cycle of T. cruzi [8]



Variant Surface Glycoproteins (VSGs)

VSGs

VSGs allow trypanosomes to remain undetected by the adaptive immune system almost indefinitely and escape the complement system. Ten million identical VSGs form homodimers with one another and cover the parasite’s cell membrane like a dense homogenous forest. Although amino acid sequences and attached sugars vary from one VSG variant to another their secondary and tertiary structures are highly conserved. Only one VSG variant is expressed at a time. There are at least 200 active VSG alleles and more than 1000 silent alleles and psuedogenes also encoding VSG variants. Active genes can recombine with silent genes to form new unique active genes. This antigenetic switching allows trypomastigotes to continually change their “appearance” to the immune system and remain undetected through the course of infection [12]. This monotypic surface coat is eventually recognized by the immune system but by the time the body is ready to react the parasite has already silenced expression of that particular VSG and begun expressing a new VSG [13]. The expression of only a single VSG also makes it difficult for complement proteins to bind and help the innate immune system catch the parasite [14].

The host immune response continually selects for the expression of new VSG variants. It is a case of the Red Queen’s race only T. brucei eventually wins as the host’s resources burn out and increasingly virulent variants are selected. There are more VSG variants than the immune system could ever recognize. VSGs can also actively modulate various components of the immune system to further avoid detection [11,12,13,14].

VSG expression

Figure 6. Two mechanisms for mono-telomeric expression of variant surface glycoproteins (VSGs)[12].

Only one of the 10-20 VSG telomeric sites expression sites is actively transcribed while the rest are silenced. Oddly enough these expression sites are transcribed by RNA polymerase I. T. brucei and other strains associated with Nangana or African sleeping sickness are the only eukaryotes that transcribe anything other than ribosomal RNA [12]. All other eukaryotes including T. cruzi only used RNA Pol I to transcribe ribosomal RNA [9].

VSGs can be mono-telomerically expressed in one of two ways. The lower left panel of Figure 6 shows transcription switching. An actively transcribed telomeric site is silenced and another telomeric site begins active transcription.

Recombination switching as shown in the lower right panel of Figure 6, is the other way T. brucei switches VSG expression [12]. One of the thousand or so silent pseudogenes recombines with an active telomeric site so it can be expressed. This type of switching greatly increases the number of possible variants. The exact proteins involved and the mechanism by which they are able to recombine is still under study but there are a number of homologs involved in homologous recombination and DNA repair that are common to trypanosomes and other eukaryotes such as yeast [12,15].

The coiling and uncoiling of chromatin determines which expression site is open for transcription while the rest are silenced. The nucleosomes, or spooling proteins that help structurally organize DNA are important for VSG expression. Expression of certain VSGs are repressed when these proteins twist DNA up in such a way that RNA polymerase cannot get to the promoter region to bind the expression site and begin transcription [12,15].

Expression site silencing requires at least six protein factors and operates by a long or short-range mechanism. In short-range silencing, acetyl groups are removed from histones within 5 KBP of the expression site by histone deacetylase SIR2rp1. The histone acetylase HAT1 reverses silencing by adding acetyl groups to histones so they bind DNA less tightly and active transcription can take place. Long-range silencing is controlled about 50 kbp upstream of the expression site most likely through chromatin remodeling with a combination of histone deacetylases, acetylases, methylases, and other enzymes.The default state for VSG expression, is silence [12].

Current Research

T. brucei’s use of ever-changing VSGs is simple and elegant but there some confounding details in the expression and alternate function of VSGs that still need to be explored. One issue is the transition from one VSG surface coat to the next. What happens as a new coat is forming and the old coat is shedding from the cell surface? There is also the question of what other tricks trypanosomes have to avoid detection by the immune system besides their VSG coats [13,10].

Expression of a Mosaic VSG Coat to Escape Early Detection by B-cells

Although only one VSG gene is expressed at a time, the surface coat isn’t always monotypic. Even though a new gene is being expressed the existing VSG coat doesn’t just disappear overnight because it takes time to transcribe, translate, and transport the new VSGs to the surface and destroy residual mRNA and protein from the previous coat. It takes about 48 hours for the VSG coat to completely switch from one variant to another [13].

Figure 7. Fluorescence micrograph of trypanosome co-expressing LouTat1 VSG (wild-type)panel A and 117 VSG (transformed) panel B. Panel C represent an overlay of panels A and B. Panel D shows the parasite without florescence [13].

Dubois et al. (2005) found the 48 hour transition period where more than one VSG variant is present on the cell surface actually prevents B-cells from recognizing either the old or new VSGs. It’s like the trypomastigote call a timeout from the B-cell response while it’s changing clothes [13].

Figure 8. B cell response in athymic mice after inoculation with wildtype and transformed T. b. rhodesiense strain. Upward pointing triangles represent the wildtype strain expressing a single VSG, downward pointing triangles show transformed strain expressing both LouTat1 and 117 VSGs, the open circles are uninfected control mice. The X-axis shows days into the infection and the Y-axis is the number of activated B cells found in the spleen [13].

To prove this, athymic mice (lacking T-cells) were infected with a strain of T. b. rhodesiense that had been transformed to express two VSGs at the same time. Panel A of Figure 7 shows fluorescence labeled LouTat1 VSG while panel B shows labeled 117 VSG. Panel C is an overlay of panels A and B. Wild-type trypanosomes could not be used because only a few parasites in a given population are switching coats at any one time and have mixed VSG coats. An ELISPOT assay measured the number of activated splenic B-cells producing antibodies. Figure 8 shows almost no B-cell activity in mice infected with trypanosomes co-expressing VSG 117 and LouTat1. One explanation is that B-cells are fooled by the insertion of variant VSGs in a monotypic coat during switching because the new VSGs breakup the pattern of identical epitopes required for the direct activation of B-cells void of any interactions with T-cells [13].

Trypanosomes Effects on Immune Function

Besides hiding from the immune system with an ever-changing VSG surface coat, trypanosomes also manage to down-regulate the function of various components of the immune system. one example is the decreased ability of white blood cells to process and present newly encountered antigens after trypanosome infection [16].

Figure 9. Relative ability of T-cell hybridomas to present novel antigens after trypanosome infection [16].

In 2009, Dagenais et al. showed white blood cells from infected mice displayed a decreased ability to respond to newly encountered hen egg lysozyme (HEL) antigen ex vivo. Extracted T-cells were irradiated and combined with immortal cells to form hybridomas that could be grown in cell culture. The ability to process and present an antigen was measured by the amount of interlukin-2 (IL-2) produced. IL-2 was used as a measure of immune function because it is an important cytokine that helps stimulate cytoxic T-cells and affects the development of T-cell memory, among other functions. To measure IL-2, radiolabeled thymidine was added to the hybridoma medium and the amount of IL-2 produced with incorporated radiolabeled thymidine was measured with a scintillation counter. A scintillation counter measures the amount of radiation in a sample as counts (or decays) per minute (CPM) [16].

Naïve (uninfected) cells displayed a normal dose-dependent response to the introduction of HEL. As cells were exposed to more antigen, more IL-2 was produced. Figure 9 shows constant IL-2 production in cells from infected animals regardless of the concentration of HEL or length of time the animal had been infected before T-cells were harvested [16].

Conclusion

A better understanding of the interactions between T. brucei and immune could lead to the discovery of new anti-trypanosome drugs. Current drugs available for treatment are extremely toxic with side effects that can be almost as bad as trypanosomiasis itself [10]. New anti-parasite drugs could make a huge impact on areas where sleeping sickness and Nangana are endemic. Taking control of trypanosome diseases would be a huge win for international public health and give affected areas an economic boost from increased human productivity and the ability to raise livestock [3].

References

[1] Parasite Museum (2009, June 4). Trypanosoma brucei. Retrieved April 25, 2011, from Parasite Museum: http://www.parasitemuseum.com/trypanosome/

[2] Centers for Disease Control and Prevention (2010, November 2). Parasites - African Trypanosomiasis (also known as Sleeping Sickness) . Retrieved April 1, 2011, from the CDC: http://www.cdc.gov/parasites/sleepingsickness/

[3] Hunt, R. C. (2010, February 15). Parasitology-Chapter Three; Molecular Parasitology: Trypanosomes Eukaryotic cells with a different way of doing things. Retrieved April 16, 2011, from Microbiology and Immunity On-line: http://pathmicro.med.sc.edu/lecture/trypanosomiasis.htm

[4] Rudenko, G. Epigenetics and transcriptional control in African trypanosomes. Essays Biochem.. 2010. 20;48(1):201-19.

[5] (2007). A well fed tse tse fly. Retrieved April 25, 2011, from Tanzania 2007 Safari: http://www.accuratereloading.com/hr20071.html

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Edited by student of Joan Slonczewski for BIOL 238 Microbiology, 2011, Kenyon College.