Imagine you’re a bacterium looking for a nice warm respiratory tract to grow in. Eventually, you find one. But it’s not quite that simple, because the immune system might have something to say about your new choice of home. Can you imagine that? No? Well if not, this video shows a bacterial view of what it might look like:
An estimated 2 billion people (one third of the world’s population) are infected with Mycobacterium tuberculosis. Each year, approximately 9 million people become ill with TB and nearly 2 million die from the disease, with new infections occurring at a rate of one every second. The current standard vaccine for TB is Bacille Calmette-Guerin, or BCG, which provides some protection in children but is unreliable against pulmonary TB, the most common type of infection. The BCG vaccine is prepared from a strain of the attenuated (weakened) bovine tuberculosis bacillus, Mycobacterium bovis, and was first used in the 1920s.
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The problem with BCG is that its effectiveness is in some doubt. Various clinical trials appear to show that its efficacy depends on geography. Clinical trials conducted in the UK have consistently shown a protective effect of 60 to 80%, but trials conducted elsewhere have shown no protective effect, and efficacy seems to fall the closer to the equator the vaccine is tested. Although the BCG vaccine seems to be effective in childhood, even in the best trials, the protection offered falls from 84% 5 years after immunization to 59% after 15 years and to zero after 20 years. Moreover, BCG seems to have its greatest effect in preventing miliary TB but is far less effective against pulmonary tuberculosis (infection of the lungs), the most common form of the disease.
Currently, the first new TB vaccine for over 80 years is being tested in clinical trials in South Africa. The new vaccine is based on a genetically modified vaccinia virus (MVA) which has been widely used as a vaccine against smallpox and which is safe for HIV negative individuals. There is also good reason to believe that MVA is safe for HIV positive individuals, a major target population for future TB vaccines. The new MVA85A vaccine works in tandem with BCG and is designed to amplify a pre-existing T cell response. MVA has been passaged more than 570 times though avian cells, is replication incompetent in human cells and has an excellent safety record, having been administered to more than 120,000 vaccinees as part of the smallpox eradication programme, with no adverse effects, despite the deliberate vaccination of high risk groups. This virus vector has six major genome deletions compared to the parental vaccinia virus genome and these block its ability to replicate in mammalian cells. Although replication is blocked, virus and recombinant protein synthesis is unimpaired even during this abortive infection.
The new vaccination strategy is called prime-boost immunization, where the BCG vaccine is given to prime a CD8+ T cell immune response and the recombinant MVA85A vaccine is administered several weeks later to boost it. Since Mycobacterium tuberculosis is an intracellular pathogen, protective immunity is dependent on the cell-mediated immune response rather than on the synthesis of antibodies. Previous phase I trials have already shown that the new vaccine is safe and produces a high cell-mediated immune response, but the key to the present trial is to show it can actually prevent tuberculosis.
This strategy requires an antigen that is common to both BCG and boosting vector MVA: antigen 85A. The MVA85A poxvirus is alive but does not replicate in mammals, including humans. Instead, its function is to express the tuberculosis 85A antigen to boost the immune response. Antigen 85A is highly conserved among all mycobacterial species and is present in all strains of BCG. 85A is a secreted antigen which forms a major portion of the secreted proteins of both M. tuberculosis and BCG.
Unfortunately, this is still only a phase II trial. And even if it works, getting the recombinant vaccine into clinical use is at least eight years away. Still, after waiting 80 years, eight more years is better than nothing.
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Although there are still a few people who deny that human immunodeficiency virus (HIV) infection causes AIDS, they no longer have any scientific credibility. The best way to avoid AIDS is not to become infected with HIV, but that’s not much help to the 39 million people worldwide who already are infected with the virus. If we are to find a cure for AIDS, we need to understand the mechanisms by which the virus causes the disease. Although the basic biology of HIV is well understood (Basics of the virology of HIV-1 and its replication. J Clin Virol. 2005 34: 233-244), and there have been many theories about how HIV infection may result in AIDS, scientists have never had a complete understanding of the processes by which CD4+ T helper cells are depleted in HIV infection, and therefore have never been able to fully explain why HIV destroys the body’s supply of these vital cells.
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AIDS is defined as the presence of HIV infection, plus one or both of the following:
Soon after HIV was discovered in the 1980s it was shown that the virus could kill CD4+ cells in culture. Early experiments suggested there might not be enough virus present in AIDS patients to account for all the damage seen. More recently, sensitive techniques such as PCR suggest that with the amount of virus now known to be present in infected individuals, the CD4+ cell count should in fact decline much faster and AIDS develop much earlier than it does after HIV infection. A paper recently published in PLoS Medicine addresses this important question (Understanding the Slow Depletion of Memory CD4+ T Cells in HIV Infection. 2007 PLoS Medicine 4, 5, e177).
For over a decade, researchers have used a “tap and drain” analogy to describe CD4+ cell loss in HIV infection. In this description, CD4+ cells (like water in a sink) are constantly being eliminated by HIV (the drain), while the body is constantly replacing them with new ones (the tap). Over time, the tap cannot keep up with the drain, and CD4 counts begin to drop, leaving the body susceptible to the infections that define AIDS. CD4+ cells that are activated in response to invading microbes (including HIV itself) are highly susceptible to infection with the virus, and following infection these cells may produce many new copies of HIV before dying. A popular explanation for CD4+ cell loss is the “runaway” hypothesis, in which CD4+ cells infected by HIV produce more virus particles, which activate more CD4+ cells that in turn become infected, leading to a positive feedback cycle of CD4+ cell activation, infection, HIV production, and cell destruction.
Using a mathematical model containing a series of equations to describe the processes by which CD4+ cells are produced and eliminated, the authors showed that if the “runaway” hypothesis was correct, then CD4+ cells in HIV infected individuals would fall to low levels over a few months, not over several years as usually happens. Therefore they conclude that the “runaway” hypothesis cannot explain the slow pace of CD4+ cell depletion in HIV infection.
Of course that leaves open the question of what exactly is going on between the time someone becomes infected with HIV and the time that they develop AIDS. Although they have no definitive answer, the paper does summarize some alternative mechanisms by which the CD4+ T cells might slowly diminish. While virus adaptation (antigenic variation) is important in the biology of HIV, this alone cannot explain the whole story. This study highlights how understanding CD4+ T cell dynamics in chronic HIV infection requires a quantitative description of T cell maintenance in health, as well as understanding how HIV infection affects the turnover and differentiation of T cell subsets.
I spent ten years of my scientific career trying to find out how HIV causes AIDS, and the ongoing debate shows that we still don’t completely understand this vital issue. Until we do, the prospects for curing AIDS remain bleak.
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West Nile Virus (WNV) is a single stranded RNA flavivirus. WNV infection of humans typically results in subclinical or non-specific, mild febrile illnesses. However, about 1 in 150 patients will develop encephalitis and meningitis with high lethality rate due to virus invasion into the central nervous system. Since 1999, the virus has increasingly gained importance in North America as it caused an epizootic among birds and horses and an epidemic of meningitis and encephalitis in humans. To date, no pharmacological treatment options for WNV-infected patients exist. Apoptosis is a highly conserved mode of programmed cell death, commonly mediated by the activation of caspases. Although neurons are regarded as the major target of WNV in vivo, WNV infection has been shown to induce apoptosis in different cell lines in a similar manner in vitro. Recently, it was shown that WNV-infection induces caspase-3 activation and apoptosis in brains of wild-type mice and in primary CNS-derived mouse neurons. WNV infection-induced cell death may contribute to fatal WNV disease. Therefore, thorough knowledge of the molecular mechanism of WNV-induced neural cell death will allow us to better understand the progression of WNV infection and the associated neurological pathology.
We used WNV-infected glioma cells to study WNV-replication and WNV-induced apoptosis in human brain-derived cells. We found that WNV infection induces cell death in the brain-derived tumour cell line T98G by apoptosis under involvement of constituents of the extrinsic as well as the intrinsic apoptotic pathways. Our results illuminate the molecular mechanism of WNV-induced neural cell death.
Inhibition of apoptosis prevents West Nile virus induced cell death
BMC Microbiology 2007 7: 49
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Kaposi sarcoma-associated herpesvirus (KSHV) is a gamma-herpesvirus associated with Kaposi sarcoma. Recently, it was found that KSHV encodes 12 microRNAs (miRNAs) within its latency-associated region. miRNAs are small 22 nucleotide-long single-stranded RNA molecules that act to inhibit gene expression by binding to target messenger RNAs (mRNAs). Human cells express several hundred miRNAs which regulate important biological pathways such as development, proliferation, and apoptosis. Because miRNAs bind to their targets with limited base pairing, it has been difficult to find the targets. Microarray analysis of cells expressing the KSHV miRNAs revealed a set of 81 genes that were changed. Several genes are regulators of important functions such as blood vessel growth, cell proliferation, and cell death. One target, thrombospondin 1, is a potent inhibitor of blood vessel growth and is known to be down-regulated in Kaposi sarcoma tumors. We confirmed miRNA-dependent regulation for three of these genes and found that protein levels of thrombospondin 1 (THBS1) were decreased >10-fold. THBS1 has previously been reported to be down-regulated in Kaposi sarcoma lesions and has known activity as a strong tumor suppressor and anti-angiogenic factor, exerting its anti-angiogenic effect in part by activating the latent form of TGF-beta. We show that reduced THBS1 expression in the presence of viral miRNAs translates into decreased TGF-beta activity. These data suggest that KSHV-encoded miRNAs may contribute directly to pathogenesis by down-regulation of THBS1, a major regulator of cell adhesion, migration, and angiogenesis. To our knowledge, our data describe the first targets for tumorvirus-encoded miRNAs and suggest that these novel regulators may have roles in pathogenesis.
Identification of Cellular Genes Targeted by KSHV-Encoded MicroRNAs.
PLoS Pathogens 3(5): e65
Hepatitis is inflammation of the liver and can be caused by a variety of environmental or infectious agents. There are at least five different viruses which specifically infect human livers and cause hepatitis. These days, they are named hepatitis viruses A to E (HAV, HBV, …).
Hepatitis C virus (HCV) was first identified by molecular cloning of the virus genome in 1989 (Isolation of a cDNA clone derived from a blood-borne non-A, non-B viral hepatitis genome. Science 244: 359-62, 1989).
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The HCV genome consists of a positive-sense RNA molecule approximately 9.6 kb in length. HCV is a member of the Flaviviridae and is the only member of it’s genus (Hepacivirus).
It has been estimated that 2.2% of the world’s population, approximately 170 million people, are infected with hepatitis C.
Blood-borne infections are most prevalent, with high rates seen in intravenous drug abusers, and recipients of unscreened blood transfusions and blood products. The possibility of sexual transmission has not been eliminated, but if it occurs, the risk seems to be very low. Vertical transmission of HCV from mother to child occurs at a rate of 5-10%. This risk can be significantly lowered if babies are delivered by caesarean section. However, breast feeding does not appear to be a significant risk factor to children with infected mothers. Over 4 million people in the USA are infected with HCV, a prevalence rate of 1.6 %. The peak prevalence of HCV infection occurs among people of 40 to 49 years of age and a history of injection drug use is the strongest risk factor (The prevalence of hepatitis C virus infection in the United States, 1999 through 2002. Ann Intern Med. 2006 144: 705-714).
The UK Health Protection Agency’s annual reports on HCV estimate that in England and Wales around 4,500 people are suffering from severe liver disease due to chronic HCV infection, and that this could rise to around 7,000 by 2010. Over 200,000 UK residents have chronic HCV infection, but five out of six are unaware of this. It is believed that around 80% of UK HCV infections are linked to the use of injected drugs, and 50% of injecting drug users are infected with HCV.
The majority of cases of HCV infection give rise to an acute illness, but up to 80% may then develop into chronic hepatitis. Almost all patients develop a vigorous antibody and cell-mediated immune response which fails to clear the virus infection but may contribute to liver damage. Spontaneous resolution of chronic liver disease is very rare (<2%) and patients with chronic disease are at risk of developing hepatocellular carcinoma (HCC). However, some studies have suggested that infection may have a more benign outcome, at least in some populations (Pathophysiology of hepatitis C virus infection and related liver disease. Trends Microbiol. 2004 12: 96-102).
Until recently, HCV could not be cultured in vitro and this hampered investigation of the virus. As an example of how difficult HCV is to study, this virus has still not been definitively imaged by electron microscopy and its fine structure remains to be determined. However, during the last few years, powerful model systems have been developed to study the HCV replication cycle (Replication of hepatitis C virus. 2007 Nature Reviews Microbiology 5: 453). Much work still remains to be done with respect to understanding virion assembly and structure, the stages of replication and the mechanism by which the RNA genome is replicated.
Treatment of HCV infection is very difficult and the current standard therapy, pegylated interferon-alpha combined with ribavirin, is often difficult for patients to tolerate and results in a sustained virus reduction in only about half of patients. HCV can be classified into several genotypes based on variations in the nucleotide sequence of its genome. The infecting HCV type has been shown to be clinically important because it predicts responses to antiviral therapy, with infection by type 1 being associated with the most resistance to treatment. Spontaneous virus clearance may occur more often with HCV type 1 infection than with other genome types, although type 1 infection may also be more aggressive than genome types 2 or 3 (Does the clinical outcome of hepatitis C infection vary with the infecting hepatitis C virus type? J Viral Hepat. 2007 14: 213-20).
Compared with other types of viral hepatitis such as hepatitis A and hepatitis B, development of vaccines against HCV infection has been difficult. Some of the reasons for this are that HCV seems to elicit a weak immune response which does not protect against infection, and also the considerable genetic and antigenic variability of the virus. In these respects, HCV is reminiscent of HIV. Nevertheless, some progress has been made and HCV vaccine candidates are being tested in humans. One of these consists of a recombinant form of the HCV envelope proteins (E1 and E2) expressed in mammalian cells. This vaccine has been shown to offer partial protection to chimpanzees, specifically, in protecting them from becoming chronically infected. Other candidate vaccines are composed of proteins from the nonstructural region of the HCV genome, which stimulate more of a cellular rather than a humoral immune response. Finally, some therapeutic vaccines are being tested for their possible value in the treatment of chronic HCV infection.
Although some progress has been made in attempts to prevent HCV infection, a truly effective vaccine still appears to be a considerable way off. With the number of diagnosed cases still increasing, we have a mountain to climb.
Update: Mitchell Shiffman of Virginia Commonwealth University has recently announced a “cure” for HCV. Standard therapy with pegylated interferon and ribavirin is said to have removed all detectable virus in 99% of patients for up to seven years. This result conflicts with other trials of this therapy, where response rates are encouraging but lower than 99%, and so needs to be treated with optimistic caution until it is confirmed.
Researchers from NYU School of Medicine in New York say they have developed a vaccine which stops mice developing prion disease after experimental infection. Mice were orally vaccinated with an attenuated Salmonella strain which expressed the prion protein. Some (but not all) of the animals responded well to the vaccine and developed high levels of antibodies in their blood. These mice showed no symptoms of prion disease 400 days after infection (it normally takes 120 days for mice to develop the disease after deliberate inoculation). The mice with low levels of antibodies also had a significant delay in the onset of the disease, although symptoms did eventually appear in these animals. Much more work will be needed before the vaccine could be considered for use in humans.
Source: American Academy of Neurology
There’s a lot of stuff on the internet about how good negative ions are for you, from improving your health to just making you feel good. You can Google it for yourself. If you look at the search results, you might notice that most of them are to sites which are trying to sell some sort of ionization product. Hmm … interesting, especially in light of a paper just published:
Bactericidal action of positive and negative ions in air. BMC Microbiology 2007, 7: 32
Background: In recent years there has been renewed interest in the use of air ionisers to control of the spread of airborne infection. One characteristic of air ions which has been widely reported is their apparent biocidal action. However, whilst the body of evidence suggests a biocidal effect in the presence of air ions the physical and biological mechanisms involved remain unclear. In particular, it is not clear which of several possible mechanisms of electrical origin (i.e. the action of the ions, the production of ozone, or the action of the electric field) are responsible for cell death. A study was therefore undertaken to clarify this issue and to determine the physical mechanisms associated with microbial cell death.
Conclusion: The results presented in this paper suggest that the bactericidal action attributed to negative air ions by previous researchers may have been overestimated.
The responses of the immune system can be divided into two broad types – innate and adaptive immunity. Adaptive immune responses depend on clones of B and T lymphocytes which are specific for particular antigens. This is a powerful system and one of the main mechanisms the body uses to fight disease, but it has the limitation that these clones of cells take time to develop and respond to infections, typically something like four to seven days. In many infections, that would give an invading pathogen enough time to take over the entire body and inflict fatal damage, so to control infections during vital the first few days, the body relies on the evolutionarily ancient and more universal innate immune system. These innate immune responses include opsonization, phagocytosis, activation of complement and apoptosis.
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The adaptive immune system recognizes foreign attackers through T and B cell receptors on the surface of white blood cells which allow them to respond to individual antigens. In contrast, innate immunity works through a more general set of recognition molecules called pattern recognition receptors (PRRs). These are evolutionarily ancient proteins, which seem to have evolved originally to allow the body to distinguish “self” from “non-self” during development, but are also important in fighting diseases. There are various groups of pattern recognition receptors, some of which are secreted from cells, while others expressed on the cell surface or in intracellular compartments.
The Toll protein was first identified in the fruit fly Drosophila melanogaster, where it was shown to play a role in embryonic development. In 1996, Toll was also found play an essential part in the fly’s immunity to fungal infections, which it does by activating the synthesis of antimicrobial peptides. Similar proteins known as Toll-like receptors (TLRs) are present in all vertebrates as well as in invertebrates all the way back to nematodes. TLRs seem to be one of the most ancient and conserved parts of the immune system. It has been estimated that most mammalian species have between ten and fifteen types of Toll-like receptor.
So how do they work?
Unlike B and T cell receptors which are constantly changing, pattern recognition receptors such as Toll do not alter, so they are directed against key pathogen associated molecules which are evolutionarily conserved. Such features in pathogens include bacterial cell-surface lipopolysaccharides (LPS), proteins such as flagellin from bacterial flagella, double-stranded RNA of virus genomes, and the unmethylated CpG islands found in bacterial and viral DNA. Each TLR respond to a small set of different but unchanging targets.
When the molecule (or ligand in immunology jargon) to which they respond binds to the receptor, this causes intracelluar signals to be sent through a complicated cascade reaction which alters the pattern of gene expression in the cell. Adjuvants are substances which stimulate the effect an antigen such as a vaccine component by stimulating the immune system to respond to the vaccine. Many adjuvants are believed to work by mimics of TLR targets, so TLRs turn out to be important in vaccine development as well as in fighting disease directly.
Ten different TLRs have been identified in humans and the overlap between them allows recognition of a diverse range of pathogens. The functional molecules work in pairs known as dimers. Sometimes these pairs consist of two identical TLR molecules (these are called homodimers), and sometimes two different TLRs (known as heterodimers) join together to form a working receptor. This further extends the range of targets the limited number of proteins is able to recognize and respond to.
Because it is not possible to demonstrate direct biochemical interactions between them, it has been suggested that mammalian TLRs do not bind to pathogen-associated molecular patterns directly, but instead recognize the more basic building blocks from which their targets are constructed. For example, TLR4 recognizes lipid A, a core component of bacterial lipopolysaccharide (LPS). This makes sense as this makes it even harder for pathogens to evolve in order to avoid recognition.
Generally, the number of TLR molecules on the cell surface is rather low, varying from a few hundred to a few thousand molecules per cell. Some TLRs such as TLR1 are found on a wide range of different cell types, whereas others are only found in certain places, e.g. TLR3 is mostly found on immature dendritic cells. The amount of TLR present on a cell does seem to respond to stimulation by the presence of the targets they can detect, but TLR expression is also extremely variable between different individuals, which might correlate with individual differences in susceptibility to different pathogens. Sadly for me, TLR expression declines with age, which is a possible partial explanation for the increased susceptibility of elderly people to infections. In spite of this, as our understanding of how TLRs work continues to develop, we will find new ways to advance the fight against infectious diseases
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