GDNF and bacterial infectious disease: Both enteric neurons and glial cells express pattern recognition receptors including toll-like receptor (TLR)-2, TLR-3, TLR-4 and TLR-7.119–121 Interestingly, the activation of TLR2 and TLR4 on glial cells by commensal bacterial products can increase the release of glial cell line-derived neurotrophic factor (GDNF)-family ligands that bind to RET expressed on ILC3 (Fig. 2f), stimulating the secretion of IL-22 and providing protection against chemical-induced inflammation or pathogenic bacterial infection.18