Promyelocytic leukemia zinc finger controls type 2 immune responses in the lungs by regulating lineage commitment and the function of innate and adaptive immune cells

Type 2 immune responses are critical for host defense, mediate allergy and Th2-high asthma. The transcription factor, promyelocytic leukemia zinc finger (PLZF), has emerged as a significant regulator of type 2 inflammation in the lung; however, its exact mechanism remains unclear. In this review, we summarized recent findings regarding the ability of PLZF to control the development and function of innate lymphoid cells (ILCs), iNKT cells, memory T cells, basophils, and other immune cells that drive type 2 responses. We discussed the important role of PLZF in the pathogenesis of Th2-high asthma. Collectively, prior studies have revealed the critical role of PLZF in the regulation of innate and adaptive immune cells involved in type 2 inflammation in the lung. Therefore, targeting PLZF signaling represents a promising therapeutic approach to suppress Th2-high asthma.

ILC3: a case of conflicted identity

Innate lymphoid cells type 3 (ILC3s) are the first line sentinels at the mucous tissues, where they contribute to the homeostatic immune response in a major way. Also, they have been increasingly appreciated as important modulators of chronic inflammatory and autoimmune responses, both locally and systemically. The proper identification of ILC3 is of utmost importance for meaningful studies on their role in immunity. Flow cytometry is the method of choice for the detection and characterization of ILC3. However, the analysis of ILC3-related papers shows inconsistency in ILC3 phenotypic definition, as different inclusion and exclusion markers are used for their identification. Here, we present these discrepancies in the phenotypic characterization of human and mouse ILC3s. We discuss the pros and cons of using various markers for ILC3 identification. Furthermore, we consider the possibilities for the efficient isolation and propagation of ILC3 from different organs and tissues for in-vitro and in-vivo studies. This paper calls upon uniformity in ILC3 definition, isolation, and propagation for the increased possibility of confluent interpretation of ILC3's role in immunity.

IRF2 is required for development and functional maturation of human NK cells

Natural killer (NK) cells are cytotoxic and cytokine-producing lymphocytes that play an important role in the first line of defense against malignant or virus-infected cells. A better understanding of the transcriptional regulation of human NK cell differentiation is crucial to improve the efficacy of NK cell-mediated immunotherapy for cancer treatment. Here, we studied the role of the transcription factor interferon regulatory factor (IRF) 2 in human NK cell differentiation by stable knockdown or overexpression in cord blood hematopoietic stem cells and investigated its effect on development and function of the NK cell progeny. IRF2 overexpression had limited effects in these processes, indicating that endogenous IRF2 expression levels are sufficient. However, IRF2 knockdown greatly reduced the cell numbers of all early differentiation stages, resulting in decimated NK cell numbers. This was not caused by increased apoptosis, but by decreased proliferation. Expression of IRF2 is also required for functional maturation of NK cells, as the remaining NK cells after silencing of IRF2 had a less mature phenotype and showed decreased cytotoxic potential, as well as a greatly reduced cytokine secretion. Thus, IRF2 plays an important role during development and functional maturation of human NK cells.

Circulating T cells and resident non-T cells restrict type 2 innate lymphoid cell expansion in the small intestine

Interaction among various adaptive, circulating cells and tissue-resident cells including innate lymphocytes during the establishment and maintenance of the barrier-tissue immune system has only recently started to be explored. Here, we show that the cellular crosstalk with circulating T cells and other resident cells regulated the population size of type 2 innate lymphoid cells (ILC2s) in the small intestine lamina propria. Rag1^-/- mice had excessive numbers of both ILC2s and ILC3s, and such an over-expansion was corrected by establishing parabiosis with wild type mice or by adoptively transferring wild type CD4⁺ T cells. In contrast, anti-CD3 antibody-mediated T cell depletion in wild type mice increased ILC2 but not ILC3 numbers. Unconventional CD4^-CD8^- αβ T and γδ T cells could also restrict ILC2 expansion as the numbers of ILC2s were not altered even in mice treated with anti-CD4/anti-CD8 antibodies. ILC3 restriction seemed to be through the control of proliferation, but that for ILC2s did not. In addition, elevation in ILC2 numbers seen in mice lacking the transcription factors RORγt and STAT6 was found to be T cell-independent. Our current findings altogether uncovered the homeostatic 'quota' restriction imposed on intestinal ILC2s in the steady state by resident non-T cells via RORγt- and STAT6-dependent mechanisms as well as by conventional and nonconventional T cells.

Differential Expression of the Transcription Factor GATA3 Specifies Lineage and Functions of Innate Lymphoid Cells

Lymphoid tissue inducer (LTi) cells are regarded as a subset of innate lymphoid cells (ILCs). However, these cells are not derived from the ILC common progenitor, which generates other ILC subsets and is defined by the expression of the transcription factor PLZF. Here, we examined transcription factor(s) determining the fate of LTi progenitors versus non-LTi ILC progenitors. Conditional deletion of Gata3 resulted in the loss of PLZF+ non-LTi progenitors but not the LTi progenitors that expressed the transcription factor RORγt. Consistently, PLZF+ non-LTi progenitors expressed high amounts of GATA3, whereas GATA3 expression was low in RORγt+ LTi progenitors. The generation of both progenitors required the transcriptional regulator Id2, which defines the common helper-like innate lymphoid progenitor (ChILP), but not cytokine signaling. Nevertheless, low GATA3 expression was necessary for the generation of functionally mature LTi cells. Thus, differential expression of GATA3 determines the fates and functions of distinct ILC progenitors.