The explanation could be that bacteria in the bowel help regulate the immune system and stop it from launching an attack against the unfamiliar transplanted tissue. Finding the mechanism could lead to new medicines to stop organ rejection, says Jonathan Bromberg at the University of Maryland. “It’s a way to turn down the volume knob on the immune system.”
Organ transplants are given to people who have failing organs such as their heart, kidney or liver. Recipients have to take powerful medicines to stop their immune systems from rejecting the organ, but even so, after several years the organ is often slightly scarred and inflamed, because of low-level immune attack.
Previous work has suggested that people whose transplanted organs get rejected tend to have certain bacteria in their gut. But though bacteria causing the rejection is one explanation, so is the reverse – that rejection encourages growth of the bacteria.
To find out which, Bromberg’s team gave some mice a transplant of bacteria-loaded faeces from pregnant animals, as the immune system is known to be naturally suppressed during pregnancy, so it doesn’t react against the foetus. Other groups got a faecal transplant from either non-pregnant mice, or ones with colitis, a disease where the gut is inflamed.
Then the mice had a heart from an unrelated animal transplanted into their belly, without removing their original heart. This is a common way of testing new transplant drugs, and they were given a medicine to partly suppress their immune system.
The second heart survived for 40 days in all five animals that got the faecal transplant from pregnant animals. In the other two groups, survival rates were one out of five and three out four, respectively.
Bromberg says gut bacteria are in contact with immune cells in the walls of the bowel and can push its activity up or down. His next step is to find out more about how the faecal transplant had this effect and to see if it can be mimicked with drugs.
Source: New Scientist