Host genetics and disease resistance

1
Identification of gene networks associated with
lipid response to infection with Trypanosoma
congolense
Brass A3 Broadhead, A2 Gibson, JP1 Iraqi, FA1,
Kemp, SJ2 Musa, H1 Naessens, J1 Noyes, HA2
1International Livestock Research Institute, P.
O. Box 30709, Nairobi, Kenya 2School of
Biological Sciences, University of Liverpool, L69
7ZB, UK 3Department of Computer Science,
University of Manchester, UK
The innate immune response controls
infection African trypanosomes are best known for
their extreme antigenic variation. They generate
a new surface coat every seven to 14 days. Large
numbers of antibodies are generated to the
surface antigens, and control each wave of
parasitaemia. However, the control of the
disease appears to be T cell independent since
the disease in T cell deficient mice is no worse
than in wild type mice. Survival time after
infection varies substantially between different
inbred strains of mice and between different
breeds of African cattle. We have mapped QTL
associated with survival time in mice and with
parasitaemia, anaemia and growth rate in cattle.
Introduction Human African Trypanosomiasis (HAT)
or sleeping sickness is caused by subspecies of
the protozoan parasite Trypanosoma brucei. A very
similar disease of cattle (nagana) is caused by
Trypanosoma congolense. Both these parasites live
in the blood stream and are fatal unless treated.
Estimates of the number of humans infected vary
widely but over 2 million are believed to be
infected in The Democratic Republic of the Congo
alone. The cattle disease has been estimated to
cause economic losses of over 4 billion USD per
year and effectively restricts cattle production
to areas of Africa where numbers of the tsetse
files that transmit the disease are low. The
symptoms of infection are generally non-specific
and include cachexia, fever, anaemia in cattle
and neurological symptoms in humans.
Percentage survival of AJ, C57/BL6 mice and F2
AJxC57/BL6 by number of days post challenge.
A common mechanism controls HDL levels and
Trypanosomiasis? QTL for survival post infection
with T. congolense and for HDL levels have been
fine mapped to chromosomes 1,5,16 and 17
(Mammalian Genome 2000 11645-648 and Genome
Research 2003 131654-1664). Two of these QTL on
chromosomes One and Five overlap. Whilst there
are many genes under these QTL it is possible
that a common regulatory mechanism controls both
phenotypes.
Cholesterol levels correlate with survival after
infection. Total cholesterol, HDL and LDL
cholesterol and Triglycerides were all measured
in infected mice which were maintained on high
and low fat diets. HDL cholesterol and LDL
cholesterol levels both tracked total cholesterol
which is shown. Cholesterol levels declined
after infection, and absolute levels correlated
with susceptibility to infection. C57BL/6 mice
which survive longest after infection had highest
cholesterol levels and AJ mice which have the
shortest survival time had the lowest cholesterol
levels on both diets. There was an indication
that the Balb/c mice on the high fat diet
survived longer than the same mice on a low fat
diet, but numbers were not sufficient to
determine if this was significant. A further
experiment is currently underway to specifically
test the effect of high fat isocaloric diets on
survival time.
HDL QTL Trypanosomiasis QTL
Cyp7a1 and SRB1 may regulate the differences in
cholesterol levels. C57Bl/6 mice are known to
have relatively high cholesterol levels and are a
common model for atherosclerosis. Gene
expression was measured on Affymetrix microarrays
and a key difference in gene expression in
cholesterol metabolism pathways was found in
Cyp7a1 (cholesterol 7 alpha hydroxylase). Cyp7a1
is the rate limiting step in bile acid synthesis
and cholesterol secretion. Relative expression of
this enzyme was inversely correlated with
cholesterol levels at four time points post
infection. SRB1 is a selective cholesterol
receptor. Over-expression of SRBI is correlated
with to a reduction in plasma cholesterol and an
increase in biliary cholesterol (Nature 1997
387414-417). Plasma cholesterol increased in
infected C57BL6 but not other strains at day 7
post infection (the peak of parasitaemia) and
this correlated with a two fold drop in SRBI
expression in C57BL/6 at this time, consistent
with SRBI contributing to the increase in
cholesterol in C57BL/6. SRBI is under the
chromosome 5 QTL, as is ATP10d which may be
involved in cholesterol export from macrophages
and has a premature stop codon in C57BL6.
Lipids and inflammation Cholesterol synthesis and
inflammation are known to be linked by HmgCoA
which is the key step in cholesterol and
isoprenoid synthesis. Isoprenoids can regulate
inflammatory / anti-inflammatory switch (Journal
of Experimental Medicine 2006 203401-412). AJ
mice which have the lowest cholesterol levels and
the weakest inflammatory response also had
approximately two fold lower levels of HmgCoAr at
all time points post infection (not
shown). Overlay of gene expression post infection
on macrophage gene networks indicates that the
RXR/LXR transcription factors are down regulated
in susceptible AJ mice at day 9 post infection (E
on map). These are known to also be regulators
of inflammation as well as lipid metabolism
suggesting that these pathways may also be
involved.






E

Conclusion Lipids are known to be involved in the
control of trypanosomiasis in humans. There is a
difference in lipid responses between susceptible
and resistant mouse strains. There are also
differences in inflammatory responses between
mouse strains. The overlap of QTL for HDL and
survival time post infection suggests that the
lipids may be the key regulator of inflammation.
This may be via PPAR, LXR and RXR transcription
factors or via another as yet unidentified
mechanism.