Non-canonical Activation of Human Group 2 Innate Lymphoid Cells by TLR4 Signaling

Group 2 innate lymphoid cells (ILC2) are emerging as a critical player in type 2 immunity at barrier sites in response to microbial infections and allergen exposures. Although their classical activators are known to be host epithelial-derived alarmin cytokines IL-33, IL-25 or TSLP, it remains elusive whether ILC2 cells can be activated by directly sensing microbial ligands via pattern-recognition receptors such as toll-like receptors (TLRs). Here we report that toll-like receptor 4 (TLR4) is a potent activating receptor of human ILC2. We found that among many microbial ligands examined, lipopolysaccharides (LPS) from multiple species of Gram-negative bacteria, was found to potently stimulate human, but not murine ILC2, to proliferate and produce massive amounts of type 2 effector cytokines IL-4, IL-5, and IL-13. LPS-activated ILC2 also had greatly enhanced the CD40 ligand (CD154) expression and were able to promote the proliferation and antibody production of human B cells in culture. In a humanized mouse model, LPS activated the adoptively transferred human ILC2 in mouse lungs. Both NF-kB and JAK pathways, but not the IL-33-ST2 pathway, were required for LPS to activate human ILC2. RNA-seq data further revealed that LPS induced a large set of genes overlapped significantly with those induced by IL-33. Collectively, these findings support a non-classical mode of activating human ILC2 cells via the LPS-TLR4 signaling axis. Thus, targeting TLR4 signaling pathway might be developed as a new approach by modulating ILC2 activation in treating various type 2 immunity-associated diseases.