The breakthrough was made while investigating when antibodies that line the vertebrate gut, stomach and lungs of rainbow trout evolved the dual abilities to defend against pathogens while also cultivating beneficial microbes. In the process they found evidence that these mucosal immunoglobulins have long been supporting a healthy balance of "bad" and "good" microbes.
The mucosal surfaces of vertebrates, which line the stomach, gut and lungs, are simultaneously inhabited by beneficial microbes and invaded by pathogens. In humans, immunoglobulin A (IgA) antibodies coat a large portion of both the “good” and “bad” microbes at these surfaces, where they help to eliminate the bad and cultivate the good.
But which role came first? To answer this decades-old question, fish immunologists Zhen Xu and colleagues developed a novel rainbow trout model, in which they were able to temporarily deplete IgT, the trout equivalent of IgA, in adult fish. By contrast, in most mammalian models, antibodies are usually depleted from birth. Without IgT, the fish became highly susceptible to Ich. At the same time, IgT depletion caused a profound microbial imbalance and inflammation in the gills, a microbially rich surface, marked by the loss of beneficial microbes, like Baccillales, and the expansion of pathogens like Flavobacteriales. Restoring IgT in the fish reversed tissue damage and returned microbial balance to the gill surfaces. Together, these results suggest the concurrent evolution of both microbe-regulating roles in ancient vertebrates, a scenario that might have occurred again during the evolution of IgA antibodies in humans.
The research, published under the title "Specialization of mucosal immunoglobulins in pathogen control and microbiota homeostasis occurred early in vertebrate evolution," is available here.