EASTERN BIOLOGY NETWORK MEETING 12.3.02
SPEAKER: Dr Emanuela Handman, from WEHI.
Dr Handman’s area of specialisation is intracellular pathogens, especially parasites.
She studies how they get into cells, how they live inside cells and what the host
response is, as well as their survival strategies.
The host-pathogen interaction determines ‘who’s in charge’.
In general, if there is an infectious disease, all individuals may be exposed but only
some will be infected, and of these only some will be diseased and even fewer of
these will die – as genetic component to susceptibility to infection.
Dr Handman is studying primarily leishmaniasis but also to a lesser extent malaria
at WEHI, and she talked to us more specifically on these diseases.
Note: AB = antibody, AG = antigen, RBC = red blood cell!
Malaria
Four pathogens:
Plasmodium falciparum
P. vivax
P. ovale
P. malariae
All cause malaria, P. falciparum is the killer. These species are specific to humans.
A significant component of the malaria research at WEHI by a large group led by
Alan Cowman, Brendan Crabb and Louis Schofield are investigating the mechanism
of parasite invasion of human red blood cells and the mechanisms which lead to the
pathology caused by malaria. Mouse malaria is used as a model for the study of
disease mechanisms but human malaria are used for the study of invasion.
Unfortunately, the Plasmodia that infect other mammals don’t infect humans so
there is no direct animal model for human malaria.
Complex lifecycles.
Parasites introduced by mosquito, move in blood to liver where they invade liver
cells, but without producing symptoms. P. falciparum multiply asexually over about
three weeks in humans then rupture the liver cells and move into the blood and
infect RBCs. Reproduce extensively asexually then break out to reinvade more
RBCs. Rupturing of the RBCs coincides with the symptoms. However, the precise
mechanism and molecules involved in the pathology of disease are still not clear.
Other types of Plasmodium can continue to survive in the liver and lie dormant for
years (relapses). Upon immune suppression or trauma, these may surge and
cause a relapse in disease.
The examination of the disease prevalence in an endemic population shows that
babies are protected from disease through antibodies from their mothers, become
infected at about three months of age, highest death rate at three to five years then
prevalence drops; usually low in an adult but never zero. Although these individuals
show no disease, they all harbour parasites despite having antibodies to them.
Immunity is not sterile. Rise in prevalence in pregnant women, especially first
pregnancy.
It is not possible to predict the onset or severity of malaria even though all hosts will
have the parasites in their blood.
Two questions: how to the parasites evade the immune response of the host and
what causes the disease manifestations in those individuals who do get the most
severe form of disease, cerebral malaria?
How do the plasmodia evade the host response?
1. Antigenic diversity – there will be many variants of parasites in the blood. Like
any population of organisms, they are not all alike. They may have a variety of
distinct alleles of many of their genes. They are diverse.
2. Antigenic variation is the programmed expression of genes belonging to a family
of about 50 VARIABLE genes encoding a surface protein which is exported by the
parasite to the outside of the infected red blood cell. It is a flag that the parasite puts
onto the host cell and it changes that cell drastically compared to the normal red
blood cell. (see below).
Each parasite in the population expresses only one of these VAR genes and puts
on to the RBC a unique member of the family. As the infected individual makes
antibodies to the dominant VAR, the RBC expressing it will be killed and eliminated.
However, new populations will expand over time. There is so much variation that
individual people can’t build a sufficiently large bank of antibodies, ie enough
immunity to protect against all the parasites, so some parasites survive.
This variation is one of the reasons why a vaccine difficult to make. The 50 or so
genes that allow for AGic variation appear to be stable – no new ones seem to be
appearing – worry that if a vaccine is developed, this will select for new versions.
3. Sequestration – the parasites stick to the capillaries so they bypass the spleen,
where most of the destruction of the infected RBCs occurs. If the capillaries are in
the brain, have cerebral malaria, the most severe form of the disease: the ‘killer
malaria’.
In infected RBCs, the pathogen exports the product of the VAR genes to the RBC
surface. The presence of this protein causes the appearance of knob-like
protrusions on the surface and makes such surfaces ‘sticky’. (Mechanism is a
research area – how do pathogens export whole proteins through several
membrane layers? (Parasite in vacuole hence extra membrane layer)). Infection
causes the production by the immune system of a protein called ICAM that makes
the endothelium sticky too, hence the sticky RBCs containing the parasite stick to
the sticky endothelium, initiating sequestration and reducing blood flow – may block
capillaries.
Another study area is how cerebral malaria kills. The production of soluble
cytokines calls all sorts of immune system cells into action. This is normally a very
important defense mechanism, but when it is excessive, it may actually damage the
host. One such example is septic shock syndrome. In this situation, the cytokine
Tumor Necrosis Factor alpha is produced and it shakes up the immune system so
much that it causes death. TNF-alpha is one of the cytokines that is up-regulated
by infection with malaria. There is evidence that there is a correlation between the
amount of TNF in the blood and the severity of disease. Polymorphism of TNFalpha genes has also been shown to correlate with the severity of disease. There is
a lot of work being done to understand the genetic basis of susceptibility to cerebral
malaria because not all infected individuals develop cerebral malaria.
4. Immunosuppression – especially in pregnant women.
5. Rapid rate of reproduction
Other areas of research in malaria include the study of drug resistance, with the
hope of developing new drugs.
Drug resistance has become a major problem; drug protection is no longer efficient,
especially in Africa. A few new drugs are available, being used in combination
therapy to reduce the chance of resistance – though still get some. Combination of
drugs and public health measures are what currently available.
Leishmania major
It is a zoonosis; the host is a wild animal eg rodent or dog. Humans are accidental
hosts. Means that have an animal model, unlike for malaria that is human-specific.
Macrophages are the main host cells, and dendritic cells. They internalise the
protozoa by phagocytosis so they are inside membrane-bound vacuole (a
phagosome), or more rarely in the cytosol. There can be more than one parasite /
macrophage. Lysosomes accumulate nearby and fuse with the phagosomes but
don’t digest the protozoans. The parasites have evolved mechanisms to avoid
killing in the macrophage and trying to understand this is an active area of research.
This is a similar interaction with a host to Mycobacteria (TB), except latter have thick
and waxy cell walls and their mechanism of survival in the macrophage is by
preventing the fusion of the phagosome with the lysosomes.
Intracellular replication – the parasites reproduce inside the phagocyte and are
released by exocytosis – one host cell can release many leishmanias in a few
hours.
Leishmaniasis is all round the world in tropical areas. It is not in Australia – yet – is
now in East Timor.
Spread by sand flies. If cutaneous leishmaniasis, skin is eaten away (lesion). If
visceral leishmaniasis, protozoans home in on liver, spleen and bone marrow. An
important question in leishmaniasis is why do the different parasites migrate and
home in on different organs to cause different diseases – do the infected
macrophages migrate from the skin at the bite site? Or do the parasites move?
Invasion involves specific receptor-mediated interaction between the parasite and
macrophage surfaces that allow the parasite to invade the macrophage. More than
one molecule is involved from each, and alternative receptors are possible. This
may explain how this parasite can infect humans as well as dogs or rodents.
Like in malaria, there is a genetic component to the ability of the host to resist
disease, ie to be infected but asymptomatic. Genetic resistance is polygenic, ie it is
due to the interaction of several genes – area of research, as is why some humans
are resistant.
Ref handouts – one on malaria, one on Leishmanias – both detailed but interesting.
Our thanks to Dr Handman for an informative and interesting talk – and for her
patience with questions!