Immune responses can be destructive, and antiviral responses especially so. Because viruses are at their most dangerous when they\u2019re already inside a cell, most immune strategies that target viral infections work in part by damaging and killing infected cells.<\/p>\n

For that reason, cells cry \u201cVirus!\u201d at their own risk. In most tissues, Alu sequences are highly suppressed so that they never get a chance to mimic a viral attack. And yet that is the exact scenario the placenta seems to create on purpose. How does it balance the health of the growing embryo with a potentially risky immune response?<\/p>\n

In experiments with mice, Totary-Jain\u2019s team found that the placenta\u2019s double-stranded RNAs and ensuing immune response didn\u2019t seem to hurt the developing embryos. Instead they protected the embryos from Zika virus infection. The placental cells were able to toe the line \u2014 conferring protection on the embryos without cuing a self-destructive immune response \u2014 because they called in the gentler defenses of interferon lambda.<\/p>\n

Typically the first responders to double-stranded Alu RNA escapees are type I and type II interferons, which quickly recruit destructive immune cells to the site of an infection, leading to tissue damage and even autoimmune disease. Interferon lambda, on the other hand, is a type III interferon. It acts locally by communicating only with cells within the tissue, generating a milder immune response \u2014 one that can be sustained long term in the placenta.<\/p>\n

How placental cells manage to activate only interferon lambda, keeping the immune response simmering but never boiling over, is still a mystery. But Totary-Jain has an idea about why placental cells evolved this trick that other cells seemingly avoid: Since the placenta is discarded at birth, perhaps it can afford to take immune risks that other tissues can\u2019t.<\/p>\n

The findings reveal a new strategy the placenta has for protecting the fetus, apart from mom\u2019s immune system. Since the mother\u2019s immune response is dampened during pregnancy to prevent attacks on the genetically distinct embryonic cells, the placenta has had to develop extra defenses for the growing baby it supports.<\/p>\n