IF YOU think comparing a fetus to a parasite is unkind, think again. The puzzle as to why a pregnant woman’s immune system doesn’t attack the fetus and placenta – both of which contain genetic material from the father – may finally have been cracked.
It seems the placenta produces hitherto unknown hormones containing the same molecule some parasitic worms use to avoid detection by the immune system. As well as helping the fetus avoid immune attack, the hormones may also summon extra blood and nutrients to the aid of an undernourished fetus.
“The placenta produces hormones containing the same molecule that some parasitic worms use to avoid the immune system”
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The discovery of this possible chemical control system could herald new ways to prevent recurrent miscarriages and pre-eclampsia, a condition which can lead to convulsions, coma and death in pregnant women. It also sheds some light on why rheumatoid arthritis, an autoimmune disease, eases when women become pregnant.
The placental hormones have avoided detection until now because they are virtually identical to neurotransmitter chemicals that convey signals in the brain and nervous system. But adding a molecule called phosphocholine changes their function completely.
“When the penny finally dropped, it was very exciting,” says Philip Lowry of the University of Reading, UK. Analysing fresh placental tissue from women and from female rats, his team found phosphocholine attached to the neurotransmitter neurokinin B and to the precursor of another neurotransmitter called hemokinin (Journal of Molecular Endocrinology, ).
Other researchers warn that extensive work is needed to establish exactly what the phosphocholine-linked hormones do. “It isn’t shown in this study that these molecules have any functions whatsoever,” notes Anne Croy of Queen’s University in Kingston, Ontario, Canada, who studies miscarriages.
Based on parasites’ successful use of phosphocholine as a “cloaking device”, Lowry thinks its main function in pregnant women is to hide the placenta and the fetus from the immune system. His previous research, which links pre-eclampsia to neurokinins, suggests that high levels of the placental version of neurokinin B might have an additional function – acting as a distress signal for an underfed or struggling fetus. By obtaining extra blood and nutrients for the fetus, Lowry says, the hormone might somehow trigger pre-eclampsia – the precise cause of which is unknown. Ultimately, Lowry hopes to use drugs based on phosphocholine as a new way to prevent miscarriages, which may result from an overactive immune system. The finding may also enable more benign immunosuppressant drugs to be developed for rheumatoid arthritis, he says.
