To better understand the interaction between commensal bacteria and TLRs, the researchers used mice that lacked a key component of the TLR-signaling machinery, called MyD88. Without this protein, mice are unable to activate TLR signaling. In initial experiments, they gave the mice a drug called DSS, which is toxic to epithelial cells in the colon.
Normal mice recovered easily from DSS exposure, but the drug severely damaged the intestines of the MyD88-deficient mice. The researchers found essentially the same results when they tested the effects of DSS on mice lacking the TLR proteins themselves.
"This was the exact opposite of what we expected, because everything we know about TLRs in mammalian systems has to do with immune response to infection," said Medzhitov. "We thought that since these mice couldn't mount a TLR-dependent inflammatory response to commensal bacteria, they would show reduced injury from DSS. We had no idea that these receptors might be involved in protecting these tissues from damage."
To explore whether the overgrowth of commensal bacteria contributed to the damage, the researchers depleted the bacteria from the mice's intestines using antibiotics. These antibiotic-treated MyD88-deficient animals showed the same DSS-induced damage as MyD88-deficient animals in which bacteria were allowed to grow normally. The researchers also found no evidence that the infiltration of white blood cells triggered the increased injury in MyD88-deficient mice.