Heterogeneity and diversity of group 3 innate lymphoid cells: new cells on the block.

Innate lymphoid cells (ILCs) are a newly identified subset of innate cells that play fundamentally crucial roles for early immune defense at mucosal and non-mucosal sites. ILCs consist of ILC1s, ILC2s and ILC3s, which each have distinct transcription factors controlling their development and function. Interestingly, each of the ILC subsets represents the innate counterparts of CD4(+) helper T-cell subsets T(h1), T(h2) and T(h17) on the basis of transcriptional regulation. ILC1s that produce IFN-γ or TNF-α, ILC2s that produce T(h2)-type cytokines mainly such as IL-5 or IL-13 and ILC3s have been recently reported and reviewed in terms of IL-22- or IL-17-producing function and cell development. However, in this relatively new field, it remains likely that additional functional and regulatory mechanisms remain to be explored. More recent findings show that ILC3s are regulated by RORγt, which plays an important role for the mucosal barrier and surface protection against pathogenic bacterial infection. ILC3s might cooperate with other cells (e.g. T cells or dendritic cells) directly or indirectly, and subsequently ILC3s have impact on tissues with prompt regulation. Especially, ILC3s in mucosal site are well known to protect the intestinal surface barrier through inducible anti-microbial peptides via IL-22. Here, I will summarize and discuss the roles, function and heterogeneity of ILC3s in mucosal tissues.