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Archive for the ‘Measles’ Category

HIV & Measles – double hit pathogenesis?

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Despite ongoing worldwide eradication efforts, measles infection still results in significant morbidity and mortality. Although, throughout most of the developed world measles infection has been considerably reduced there still exists massive (and deadly) outbreaks in areas such as Africa and South-East Asia. Investigation of the reasons why this disparity occurs therefore  is of major medical, political and social interest.

Many factors are likely to be behind this major difference – and all of which deserve our attention if we are ever to remove measles from the human population. There exists problems in rolling out vaccines in countries with poor infrastructure such as roads and transport facilities; disruption to what is known as the vaccine ‘cold-chain’ (vaccines have to be kept cold to avoid rendering them unusable) is likely to occur; general poor health of the population in these regions and possible interference of vaccination in children with high levels of passively acquired maternal antibody.

Measles vaccination efforts in Africa may not be entirely effective

Today in PLoS Pathogens, Nilsson and Chiodi highlight in a featured opinion article, another possible source: the link between co-infection with HIV-1 and Measles infection. They point out that HIV-1 infection and replication may result in impaired immune responses in both mothers and children leaving open the possibility of measles infection (no immune system, no protection). HIV-1, as I’m sure you will all know, is a potentially deadly pandemic retrovirus – particularly a major problem in sub-Saharan Africa- which infects humans where it resides in the bodies own immune system: T cells, dendritic cells and macrophages. Viral replication results in the death of these immune cells and destruction of important lymphoid tissues resulting in an individual without key immune functions.

The authors note that children born to mothers who are HIV-1 positive or are HIV-1 positive themselves develop lower levels of anti-measles antibody upon vaccination -a big deal if we’re looking to protect these kids through vaccination. They show that memory B cells may be impaired and lower protection will result through failure to mount a B cell-generated antibody response. Immunity is a highly regulated system, if you remove one aspect-  in this case T cells – you will affect another pathway , in this case B cells. Thus there exists a major  problem with HIV-1 infected people and infection with other pathogens in the environment; HIV-1 infection significantly alters the host immune system weakening it to other invading pathogens such as measles which is endemic in these areas.

So how do we overcome this problem? Well, the authors suggest that on top of increasing vaccination coverage through catch-up programs it would be wise to administer anti-retroviral drugs  to mothers and children prior to vaccination to allow sufficient immune function; this should hopefully make a difference in combating both measles and HIV in the developing world, especially in an area where both cause so much pain. Hopefully, strategies such as this will aid treatment efforts for other pathogens rife in the developing world – targeting both HIV and the individual agents may be more effective.

Sadly, there exists another interaction between HIV and co-infection with other pathogens. Infection usually results in increased levels of immune cells in the blood and tissues yet these very cells are the target for HIV and if these cells increase, HIV replication will also. There exists a deadly interaction between multiple pathogens which must be broken.

Nilsson, A., & Chiodi, F. (2011). Measles Outbreak in Africa—Is There a Link to the HIV-1 Epidemic? PLoS Pathogens, 7 (2) DOI: 10.1371/journal.ppat.1001241

Written by Connor

February 11, 2011 at 3:05 pm

Can fluorescent-‘labelled’ viruses illuminate their mechanisms of pathogenesis?

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Have you ever wanted to visualise viral infection? Ever wanted to observe how they enter and spread throughout their host organism? Ever wanted to know how exactly they caused disease – at the cellular and whole-organism level? Well, this may be entirely possible using fluorescent-labeled recombinant viruses infecting a relevant model system.

GFP-virus infected cells

So how does it work?

Lemon et al recently report the continued investigation of measles virus pathogenesis in a non-human primate (Macaque) model utilising a green-fluorescent protein (GFP) expressing virus. Upon infection of host cells, viral transcription leads to the very high expression of GFP, flooding the cytoplasm with this fluorescent ‘tag’. Subsequent microscopy, imaging and immunohistochemistry allows for the identification and location of the infected cells, tissues and organs – see image above. Tracking of cellular infection allows us to decipher the development of MeV entry, spread and replication at both the cellular and whole-organism level throughout the entire infection. Studies such as these give an unprecedented view of viral infection in a means directed related to that of human infection. This model even allows for macroscopic real-time detection of fluorescence and hence viral infection.

Why is this important for measles?

Despite a highly effective vaccine and significant global control initiatives, measles infection still accounts for significant morbidity and mortality worldwide, mostly in the developing world (164,00 deaths in 2008). This is mostly attributable to the profound immunosuppression induced allowing for further infection with opportunistic pathogens. Currently, much is known about measles pathogenesis yet the molecular mechanisms of such are poorly understood and it is therefore of great interest to better understand these processes by which MeV infects and causes disease in humans. Knowledge of such may facilitate the development of more effective and safer vaccines for measles and indeed other viral pathogens.

Viruses being obligate intra-cellular parasites, must enter and exit cells in order to survive. Most of viral pathogenesis can therefore be attributed to the effects of viral replication of host cells and tissues; a major determinant of which is the expression of receptors on host cells surfaces allowing viral entry, infection and replication. Currently only a single receptor – CD150 – (otherwise known as signalling lymphocyte activation molecule SLAM) has been discovered that wild-type pathogenic MeV uses to enter host cells; the distribution of which only explains part of measles pathogenesis as epithelial and neuronal cells (important target cells) do not express the protein. As indicated by this receptor being expressed on lymphocytes and other immune cells, MeV is a highly lymphotropic virus! But if epithelial cells fail to express the receptor on their surface, how come its possible for MeV to enter via these cells?

The classical view of measles pathogenesis was that free-virus entered the host through the respiratory route, infecting and primarily replicating within the epithelial cell lining of the respiratory tract. Newly produced virus spreads to nearby lymph nodes where infected monocytes – a type of immune cell – facilitates viral dissemination throughout the host, resulting in the well-known symptoms of measles. The problem with this being that epithelial cells and unstimulated monocytes fail to express the MeV receptor CD150 and infection should therefore not occur. Recently, it has been shown (again using a GFP expressing virus in a macaque model) that MeV predominately infects dendritic cells during the peak of infection, ruling out a major role for monocytes. There is also however no direct evidence of MeV primary replication within the epithelium of the respiratory tract at the early stages of infection. So what exactly happens during the start of infection and does it develop? GFP-expressing viruses may shed light on this question.

Diagramtic representation of the cellular composition of the human respiratory tract - notice the epithelial cell lining and the alveolar macrophages. Dendritic cells are however not shown on this diagram.

Diagramatic representation of the cellular composition of the human respiratory tract - notice the epithelial cell lining and the alveolar macrophages. Dendritic cells are however not shown on this diagram.

So how can we study the early stages of infection?

The incubation period of  measles is about 2 weeks in humans making it particularly difficult to study the early events of viral infection – the kind of events like host entry, initial site of replication and subsequent intra-host dissemination – this is where we can use a non-human primate model.

Lemon et al  generated a highly virulent recombinant MeV based on viral isolates from an outbreak in Sudan; they engineered the viral genome so that it expressed GFP upon entry into cells – an addition that causes little or no replication defects to the virus. Groups of macaques were subsequently infected via the respiratory route allowing highly sensitive visualisation of GFP expressing cells following necropsy. The early time-points of around 5 days post infection were focussed on in this investigation allowing the determination of the early cell targets – epithelium? Immune cells?

So what did they find?

Their results suggest that at the early stages of MeV infection, GFP and hence viral replication is only found in immune cells within the respiratory tract and not the epithelial lining. Dendritic cells and alveolar macrophages are believed to capture viral particles in the lungs allowing spread via infected cells. This is known as a Trojan horse entry mechanism like that used by HIV to pass through mucosal tissues and infect humans – see below. This infection allows for spread and localised replication within nearby lymphoid tissues and then on to draining lymph nodes where massive lymphocyte cell infection may occur facilitating dissemination throughout the host, mainly within lymphoid tissues. Virus can be carried through host blood vessels to other lymphoid target tissues like the tonsils and adenoids and the gut-associated lymphoid tissue ‘ Peyer’s patches’.

HIV entry mechanisms utilising dendritic cells to pass through epithelial cell barriers - the 'Trojan horse' mechanism. This may be directly analogous to MeV entry and primary spread except in the respiratory tract.

What does this mean?

This study clearly demonstrates the importance of non-epithelial cells such as dendritic cells in MeV entry, early replication and subsequent systemic spread. It does not however, rule out a major role for epithelial cells in later stages and in transmission – MeV still infects non-CD150 expressing cells and currently the mechanisms of which are unknown. Focusing on the later stages of infection may allow us to appreciate the other cell targets in pathogenesis and viral transmission. As mentioned previously, the use of fluorescent-labeled viruses offers an unprecedented view of viral entry, spread and pathogenic mechanisms. We should look forward to the time when studies like these are applied to other viral and indeed non-viral pathogens.

ResearchBlogging.orgLemon, K., de Vries, R., Mesman, A., McQuaid, S., van Amerongen, G., Yüksel, S., Ludlow, M., Rennick, L., Kuiken, T., Rima, B., Geijtenbeek, T., Osterhaus, A., Duprex, W., & de Swart, R. (2011). Early Target Cells of Measles Virus after Aerosol Infection of Non-Human Primates PLoS Pathogens, 7 (1) DOI: 10.1371/journal.ppat.1001263

Coombes, J., & Robey, E. (2010). Dynamic imaging of host–pathogen interactions in vivo Nature Reviews Immunology, 10 (5), 353-364 DOI: 10.1038/nri2746

Written by Connor

February 1, 2011 at 11:06 am

Measles, Papua New Guinea and the brain

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This post was chosen as an Editor's Selection for ResearchBlogging.orgYou may not have realised that – since most people nowadays have been vaccinated against it and have never seen it – but measles is a very serious illness. Generally an acute disease of children, measles is spread by the measles virus where it infects the body via the respiratory route and establishes a systemic infection – involving multiple organ systems – via your bodies own immune cells leading to the typical rash, mild to severe respiratory distress and immunosuppression (Rima and Duprex 2006).

Measles virus replicative cycle

In the ‘developed’ world we tend not to think about infectious disease in the same way as people in other parts of the world; national vaccination campaigns have largely removed the threat (not considering some minor outbreaks) of the some of the biggest human killers and we no longer worry ourselves over whether a family member will come down with these diseases.

Subacute Sclerosing Panencephalitis or SSPE is one of the most serious complications of measles resulting from viral infection of the central nervous system; SSPE is rare (1 in 10,000-25,000 measles infections) but is almost always fatal. Following infection at a particularly young age and on average 8 years following acute infection, a progressive deterioration of neurological function presents : loss of attention span, uncontrolled movements, behavioural changes, cognitive impairment and in all cases vegetative state is entered and death occurs.

It is caused by persistent measles infection i.e one that the isn’t removed when your immune system kicks in, which spreads throughout the  cells found within the brain causing cell death and inflammation. Strangely, no infectious virus can be recovered from infected brains and when this was investigated further they found that many mutations occurred throughout the genome rendering many of the genes nonfunctional. Although the major replicative functions (replication and gene expression) were left intact, the genes required for normal particles formation were those mutated suggesting that the virus may exploit the unique cellular environment in the CNS to spread, replicate and survive.

Green Fluorescent Protein expressing measles virus infection of neuronal cell

As I mentioned previously, due to increased transmission of virus, poverty and poor nutrition, measles infection is extremely serious in developing countries and it is no surprise that SSPE occurs here in higher numbers. In Papua New Guinea there exists a very high incidence of SSPE, THE highest incidence – roughly 3 – 20 times as many cases are reported (98 per million people versus 5 per million people). Manning et al (2011) have attempted to further characterise SSPE behaviour in this country between 1997 and 2008 and highlights the significant burden that measles is in many developing countries. They measured SSPE incidence, measles infection rates and time of birth of each patient presenting with SSPE finding a direct correlation between time of birth, measles epidemics and presenting with SSPE. The group emphasises the requirement

Why is SSPE incidence so high here and what can we do about it? SSPE rates are linked to measles infections in a population and hence have been significantly reduced following measles vaccination campaigns. Sadly, only half of children in Papua New Guinea receive two measles vaccines prior to 1st birthday – not enough to sufficiently protect an individual nor a population from measles infection and hence SSPE; there is insufficiently low-level of herd immunity in regions such as papua New Guinea. The level of vaccine effectiveness of measles vaccine in this region is also particularly low – possibly reflecting damage to the vaccine from cold-chain disruption (in tropical climates it is difficult to keep vaccines refrigerated), population genetic effects or persistence of low-level non-neutralising maternal antibody.

We can no longer afford to ignore the importance of measles in developing countries like Papua New Guinea and we must stress the need for adequate vaccine effectiveness and coverage in already susceptible human populations. Studies like these with SSPE emphasise the real-world need for the investigation of the molecular mechanisms of measles virus persistence and we should look forward to a time when we can adequatly treat measles CNS complications – or maybe with better vaccination coverage we may not have to worry about this.

Manning, L., Laman, M., Edoni, H., Mueller, I., Karunajeewa, H., Smith, D., Hwaiwhanje, I., Siba, P., & Davis, T. (2011). Subacute Sclerosing Panencephalitis in Papua New Guinean Children: The Cost of Continuing Inadequate Measles Vaccine Coverage PLoS Neglected Tropical Diseases, 5 (1) DOI: 10.1371/journal.pntd.0000932

Rima, B., & Duprex, W. (2006). Morbilliviruses and human disease The Journal of Pathology, 208 (2), 199-214 DOI: 10.1002/path.1873

Written by Connor

January 10, 2011 at 8:50 pm

Posted in Measles, Vaccines

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A mothers love declines – a measles vaccine problem?

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Worldwide, measles virus infection accounts for around 200,000 deaths annually; the importance of which is emphasized given the availability of a highly effective vaccine. Vaccine effectiveness, however, is a complex matter and is subject to many problems – a major one being transfer of maternal antibodies to children during early life, a form of natural passive immunity. Although these antibodies are there for a reason and do protect offspring from infections in early life, bridging the gap until they can synthesize their own antibodies, they have been shown to inhibit the activity to certain vaccines – measles vaccine is an example (see figure below).

Antibody concentrations in child following birth showing decline in attenuation (infection attenuation)

During early childhood, maternal antibody concentrations begin to wane and eventually reach such a level as to offer little protection from microbial challenge. These antibodies however are able to dampen the ability of a child to develop protective immunity following vaccination; it is this ‘window of opportunity’ that is responsible for a great number of measles virus infections and fatalities every year. The development of an effective vaccination strategy to get around this blocking effect would therefore be of great medical interest.

Recently, Kim et al (2010) publish their investigations into understanding how and why measles virus infection, in the presence of specific antibody results in the inhibition of a protective response following vaccination. Prior to this study it was unknown whether in this situation, MeV-specific B cells were being generated at all or whether they simply failed to secrete neutralizing antibody. The group used a rat model of MeV infection and simulated maternal antibody effects by passively transferring MeV specific antibodies and measuring the immunological outcomes. They demonstrated that there is a specific failure of B cells to secrete protective antibody in the presence of transferred antibody.

B cells will only secrete antibody when 3 signals are triggered:

1.     B-cell receptor/antigen interactions

2.     B – cell/ T-cell interactions

3.     Action of soluble mediators (for example: cytokines like interferon)

Kim et al hypothesized that in this model, where both signals 1 and 2 were active, inhibition of antibody secretion may be accounted for by the interference with certain soluble mediators. This idea was attractive providing the great deal of evidence showing MeV obstruction of interferon production – a pathway that normally results in the robust development of innate and adaptive immune responses. This results in two major problems involving antibody-specific inhibition of protective immune responses (maternal antibody) combined with MeV’s natural ability to inhibit the development of immunity; cases which are shared during the ‘window of opportunity’.

To this effect, the group developed a novel vaccine vector to circumvent wild-type measles interferon inhibition. Using reverse-genetics technology, they incorporated a MeV antigen gene, the haemagglutinin (HN) glycoprotein into the Newcastle Disease virus (NDV) genome as an extra gene, generating NDV-HN. NDV is an avian virus that induces high concentrations of IFNs upon infection allowing for the possibility of an effective measles vaccine in the presence of measles antibody.

The investigation confirmed the group’s predictions in that NDV-HN induced much higher levels of IFN in rat tissues when compared to MeV and that this led to development of MeV-specific neutralizing antibodies in the presence of transferred antibody. This work further verified that role that the restoration signal 3, in the form of alpha-IFN allows for B-cell secretion of antibody in in vivo and in vitro systems.

Given how medically important vaccination has been in protecting populations from often fatal and serious infectious disease and the troubles that arise when maternal antibody concentrations drop, any work developing vaccine technology to avoid these difficulties should be welcomed. Kim et al. confirmed the basis for the immunological blocks in generating MeV antibodies and began the development of a novel vector system to rationally provide protection. The results in this rat model, although not specifically applicable to the human situation, are promising in that it provides a logical framework to advance vaccine technology and prevent thousands of childhood deaths worldwide.

Kim D, Martinez-Sobrido L, Choi C, Petroff N, García-Sastre A, Niewiesk S, Carsillo T. (2011) Induction of Type I Interferon Secretion through Recombinant Newcastle Disease Virus Expressing Measles Virus Hemagglutinin Stimulates Antibody Secretion in the Presence of Maternal Antibodies. J Virol. 2011 Jan;85(1):200-7. PMID: 20962092

Written by Connor

December 17, 2010 at 10:43 pm