SCIENCE: 'Killer' B cells demonstrate evolutionary link between fish and mammal immune systems
EcoPolis
'Killer' B cells demonstrate evolutionary link between fish and mammal
immune systems
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PHILADELPHIA - Researchers from the University of Pennsylvania School
of Veterinary Medicine have discovered a unique evolutionary link
between the immune systems of fish and mammals in the form of a
primitive version of B cells, white blood cells of the immune system.
Their studies link the evolution of the adaptive immune system in
mammals, where B cells produce antibodies to fight infection, to the
more primitive innate immunity in fish, where they found that B cells
take part in phagocytosis (literally: cell eating), the process by
which cells of the immune system ingest foreign particles and microbes.
The finding, which appears in the online version of Nature Immunology
and will be featured on the cover of the October issue, represents a
sizeable evolutionary step for the mammalian immune system and offers a
potential new strategy for developing much-needed fish vaccines.
"When examining fish B cells we see them actively attacking and eating
foreign bodies, which is a behavior that, according to the current
dogma, just shouldn't happen in B cells," said J. Oriol Sunyer, a
professor in Penn Vet's Department of Pathobiology. "I believe it is
evidence for a very real connection between the most primitive forms of
immunological defense, which has survived in fish, and the more
advanced, adaptive immune response seen in humans and other mammals."
About 400 million years ago, the earliest ancestors of modern fish
split off of the evolutionary pathway that became the earliest
ancestors of modern mammals. In modern mammals, the B cell is a highly
adapted part of the immune system chiefly responsible for, among other
things, the creation of antibodies that tag foreign particles and
microbes for destruction. Mammals have phagocytic cells, but they are a
specialized few cells identified apart from the complex interactions
that drive other white blood cells.
Sunyer and his colleagues discovered this previously unsuspected B cell
activity while examining the immune cells of rainbow trout and catfish.
The researchers determined that these attack B cells account for more
than 30-40% of all immune cells in fish, while phagocytic cells only
make up a small portion of the total number of immune cells in mammals.
Further research also showed that a significant portion of amphibian B
cells retained their digestive traits.
"The immune systems of amphibians and fish are far less advanced than
ours," Sunyer said. "When you only have a rudimentary adaptive immune
system, it helps to have more phagocytic cells to compensate, which is
what has served fish so well over the last 400 million years."
In the past, research on the immune systems of more "primitive" species
has paved the way to the discovery of new molecules and pathways that
are critical to the immune response in humans and other mammals. B
cells themselves, for example, were first discovered in chickens in the
1960s. According to Sunyer, the Penn findings are not only important
for understanding the evolution and function of immune cells in fish
but also may point out to novel roles of B cells in mammals.
"At this point, we cannot rule out the possibility that small
subpopulations of phagocytic B cells, perhaps remnants of those present
in fish, are still present in mammals," Sunyer said.
Their findings also have an agricultural implication. The current
vaccines given to farmed salmon, for example, appeal to the fish's
adaptive immune response, which this research has now shown to be a
smaller part of the overall fish immune system than previously thought.
"If we work to create vaccines that encourage phagocytic B cell to
respond to infection, then we would play to the strengths of fish
immunity," Sunyer said. "In the long term, farming is a better, more
environmentally sound approach to fishing, so better vaccines may make
the practice more financially attractive to fisherman and less
destructive to fish populations."
There is little doubt that, despite the behavioral differences, the
fish B cells represent a less advanced version of mammalian B cells.
Sunyer found the very cellular structures that medical science has used
to define B cells in humans to be present in fish B cells, which is why
they are able to label them as B cells in the first place.
"Here we have a clear picture of where one part of the immune system,
primitive phagocytes, adapted over time to serve a more complex role as
part of the immune system that humans enjoy today," Sunyer said. "There
is still much we can learn about our own health through the ongoing
study of immune system evolution among all organisms."
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Funding for this research was provided by the National Science
Foundation and United States Department of Agriculture.
(Source: University of Pennsylvania)