Overview of the development, characterization, and function of human types 1, 2, and 3 innate lymphoid cells

Hematopoiesis is characterized by the differentiation and maturation of multipotent stem cells into hematopoietic cells. Common lymphoid progenitor cells differentiate into B and T lymphocytes; natural killer cells can also originate from common lymphoid progenitors. In recent years, a cellular subtype of lymphocytes, called innate lymphocytes, has been described. Innate lymphoid cells (ILCs) play an important effector and regulatory role in innate immunity, and similar to natural killer cells, depend on the γc and Id2 chains for their development. These cells are divided into three main subtypes according to their characteristics, namely type 1 innate lymphocytes (ILC1), type 2 (ILC2), and type 3 (ILC3); the production of cytokines and transcription factors is essential for this classification. Furthermore, these cells have high plasticity, which allows them to change their phenotype in response to the environment. ILCs have recently been characterized further and emerged as a family of effectors and regulators of innate immune responses. Uncontrolled activation of these cells can contribute to inflammatory, autoimmune diseases and cancer. The current review aimed to describe their main characteristics, immunophenotypes, and plasticity, and based on the existing literature, suggested a phenotypic analysis to differentiate innate lymphocytes from natural killer cells, and across the subsets.