Abstract
Recent studies have identified novel lymphocyte subsets named innate lymphoid cells (ILCs) lacking antigen-specific receptors. ILCs are present in a wide variety of epithelial compartments and occupy an intermediate position between acquired immune cells and myeloid cells. ILCs are now classified into three groups: group 1 ILC, group 2 ILC, and group 3 ILC based on their cytokine production patterns that correspond to the helper T cell subsets Th1, Th2, and Th17, respectively. ILCs play important roles in protection against various invading microbes including multicellular parasites, and in the maintenance of homeostasis and repair of epithelial layers. Excessive activation of ILCs, however, leads to various inflammatory disease conditions. ILCs have thus attracted interests of many researchers in the fields of infectious immunity, inflammatory diseases, and allergic diseases. Because epithelial cells sense alterations in environmental cues, it is important to understand the functional interaction between epithelial cells, ILCs, and environmental factors such as commensal microbiota. We discuss in this review developmental pathways of ILCs, their functions, and contribution of commensal microbiota to the differentiation and function of ILCs.
Highlights
Recent studies have identified novel lymphocyte subsets named innate lymphoid cells (ILCs) lacking antigenspecific receptors
ILCs were divided into three groups: group 1 ILC (ILC1 and Natural killer (NK) cells), group 2 ILC (ILC2s), and group 3 ILC (ILC3s and lymphoid tissue inducer (LTi)) based on their cytokine production patterns that correspond to the helper T cell subsets Th1, Th2, and Th17, respectively
ILC1, ILC2s, and ILC3s are all dependent on a transcription factor promyelocytic leukemia zinc finger (PLZF), but differentiation of conventional NK (cNK) cells and LTi cells is independent of PLZF
Summary
Immune cells are divided into two categories, acquired immune cells and innate immune cells, based on their ability to recognize specific antigens. Innate immune cells do not have specific antigen receptors and can be divided into two types, myeloid cells and innate lymphoid cells (ILCs). In the late 1990s, another type of lymphocyte-lacking antigen receptors was found in fetal tissues These cells play crucial roles in the differentiation of lymphoid organs such as lymph nodes and Peyer’s patches and were named lymphoid tissue inducer (LTi) cells. ILCs were divided into three groups: group 1 ILC (ILC1 and NK cells), group 2 ILC (ILC2s), and group 3 ILC (ILC3s and LTi) based on their cytokine production patterns that correspond to the helper T cell subsets Th1, Th2, and Th17, respectively This classification has been generally accepted by researchers in this field, but as developmental pathways and functions of ILC subsets continue to be extensively studied, we faced some incongruities with these descriptions. ILC1, ILC2s, and ILC3s are all dependent on a transcription factor promyelocytic leukemia zinc finger (PLZF), but differentiation of cNK cells and LTi cells is independent of PLZF
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