Cellular and Molecular Dynamics of Th17 Differentiation and its Developmental Plasticity in the Intestinal Immune Response

Abstract

After emerging from the thymus, naive CD4 T cells circulate through secondary lymphoid tissues, including gut-associated lymphoid tissue of the intestine. The activation of naïve CD4 T cells by antigen-presenting cells offering cognate antigen initiate differentiation programs that lead to the development of highly specialized T helper (Th) cell lineages. Although initially believed that developmental programing of effector T cells such as T helper 1 (Th1) or T helper 2 (Th2) resulted in irreversible commitment to a fixed fate, subsequent studies have demonstrated greater flexibility, or plasticity, in effector T cell stability than originally conceived. This is particularly so for the Th17 subset, differentiation of which is a highly dynamic process with overlapping developmental axes with inducible regulatory T (iTreg), T helper 22 (Th22), and Th1 cells. Accordingly, intermediary stages of Th17 cells are found in various tissues, which co-express lineage-specific transcription factor(s) or cytokine(s) of developmentally related CD4 T cell subsets. A highly specialized tissue like that of the intestine, which harbors the largest immune compartment of the body, adds several layers of complexity to the intricate process of Th differentiation. Due to constant exposure to millions of commensal microbes and periodic exposure to pathogens, the intestinal mucosa maintains a delicate balance between regulatory and effector T cells. It is becoming increasingly clear that equilibrium between tolerogenic and inflammatory axes is maintained in the intestine by shuttling the flexible genetic programming of a developing CD4 T cell along the developmental axis of iTreg, Th17, Th22, and Th1 subsets. Currently, Th17 plasticity remains an unresolved concern in the field of clinical research as targeting Th17 cells to cure immune-mediated disease might also target its related subsets. In this review, we discuss the expanding sphere of Th17 plasticity through its shared developmental axes with related cellular subsets such as Th22, Th1, and iTreg in the context of intestinal inflammation and also examine the molecular and epigenetic features of Th17 cells that mediate these overlapping developmental programs.

Keywords: T helper cells; Th17; developmental plasticity; immune response; intestine.

Figure 1

Shared axes of Th17 differentiation. Developmental axis of Th17 subset substantially overlaps with developmental axes of iTreg, Th22, and Th1 subsets of T helper cells. While the origin of Th17 differentiation is intrinsically linked with iTreg cells due to their common requirement of TGFβ signaling, Th17 differentiation is also linked with Th22 subset due to the shared requirement of IL-6 signaling. Although proximal signaling events guiding Th17 differentiation are distinct from the Th1 subset, late developmental axis of Th17 is overlapping with Th1 cells as chronic TCR stimulation or action of IL-23 or IL-12 readily converts mature Th17 cells to IFNγ-producing “Th1-like” cells. Accordingly, along the entire developmental axis of Th17 and its related subsets, intermediate phenotypes co-producing FoxP3/IL-17, IL-17/IFNγ, and IL-17/IL-22 are found in vivo that can perform beneficial or pathogenic functions depending on the nature of the disease. While IL-2 and retinoic acid (RA) are negative regulators of iTreg–Th17 axis that oppose Th17 differentiation while promoting iTreg differentiation, TGFβ negatively regulates the Th17–Th22 axis as well as the Th17–Th1 axis by suppressing Th22 and Th1 cellular differentiations while facilitating iTreg and Th17 differentiations.

Figure 2

Non-cytokine Factors and Cytokine-Induced Transcription Factors Governing Th17 Plasticity. The fate of Th17 differentiation during the journey of a naive CD4 T cell towards becoming an antigen-specific Th17 cell can be broadly dictated by “non-cytokine” factors and “cytokine-induced” transcription factors (TFs). Among “non-cytokine” factors, strength of TCR–pMHC engagement, strength of co-stimulation, nature of pathogen-associated molecular pattern or PAMP–TLR interaction, and nature of APC impact plasticity of Th17 cells. Moreover, the microenvironment of APC–T cell interaction also guides Th17 lineage commitment. An environment rich in retinoic acid (RA) or TGFβ, which prevails in the intestine, also influences Th17 plasticity. The entire landscape of Th17 programing that is initiated by “cytokine-induced” TFs can be divided into three sequential terrains (orange, blue, and green). During its journey through the first terrain, TCR signaling coupled with co-stimulation and other “non-cytokine” factors cooperate with APC-generated cytokines to induce STAT proteins (STAT3, STAT1, STAT4, and STAT5) and “pioneer” TFs like BATF and IRF4 (Orange). The STAT proteins and the pioneer TFs then jointly initiate the lineage-specific developmental programing by inducing STAT-responsive and IRF4/BATF-responsive genes, which include activation of master TF (RORγt) of Th17 cells. During its transition through the second terrain, STAT protein-induced RORγt jointly cooperates with the “pioneer” TFs to alter chromatin accessibility of key Th17-specific genes by epigenetic modification (Blue). During this point of time, the lineage-associated TFs are also induced during Th17 differentiation by both “non-cytokine” and “cytokine-mediated” signaling. In the final terrain of Th17 programming, an orchestration of complex networking of signaling events modulated by the lineage-specific TF (RORγt) in association with lineage-associated TFs (e.g., c-Maf, AhR, Runx1, etc.) determine the stability of the Th17 developmental program through integration of various additional pro-inflammatory and anti-inflammatory environmental cues. Although STAT3-induced RORγt predominantly drives Th17 programing, other STAT proteins (like STAT1, STAT5) and TFs (like T-bet and FoxP3) that alters Th17 lineage stability are also induced contingent on the initial priming environment. Lineage commitment or plasticity of Th17 cells is the outcome of the interplay of these variable factors acting across all the three levels.

Figure 3

Development of Th17 cell and its related intermediary cells during intestinal homeostasis and breakdown of homeostasis during inflammation or infection. During intestinal homeostasis, CD103+ dendritic cells (DCs) of the intestine are conditioned by epithelial-cell-derived factors like thymic stromal lymphopoietin or TSLP, IL-10, and TGFβ. Conditioned DCs maintain tolerance by active secretion of retinoic acid (RA) that predominantly promote the differentiation of FoxP3-expressing inducible regulatory T (iTreg) cells. This process can also be facilitated by commensal microflora (green elongated and blue circular) like Clostridum sp. and Bacteroides fragilis that promote a tolerogenic phenotype in the DC by inducing secretion of TGFβ and other factors from intestinal epithelial cells. During intestinal homeostasis, iTreg cells are most abundant in the colonic lamina propria. Once intestinal homeostasis is broken by invasion of pathogenic microorganism (yellow elongated and red elongated) like Citrobacter rodentium or other bacterial species, the epithelial barrier is destroyed leading to loss or alteration of epithelial cell-derived immunosuppressive factors. As a result of which the tolerogenic state of DC is altered by activation of additional toll-like receptor (TLR) that induces DC to secrete proinflammatory cytokines like IL-6, IL-23, IL-1β, and IL-12. Accordingly, with decreasing spatio-temporal concentration of RA and TGFβ and increasing concentration of proinflammatory cytokines like IL-6, IL-23, and IL-1β, an infectious or inflammatory state allows induction of Th17, Th22, and Th1 cells. The induction of proinflammatory cytokines also induces the transition of iTreg cells to Th17 cells via intermediary FoxP3 and RORγt-coexpressing cells (iTreg–Th17). Reversion of Th17 cells to iTreg cells may also occur after resolution of inflammation. After maturation of Th17 cells, reduced TGFβ concentration may also lead to differentiation of Th22 cells via transitioning through AhR and RORγt co-expressing, intermediary Th17/Th22 cells that produce both IL-17 and IL-22 (Th17–Th22). Th22 cells can also transition to Th17 phenotype after being subjected to an environment supporting increased TGFβ concentration by shutting down IL-22 and initiating IL-17 production. During late Th17 development, chronic TCR stimulation of Th17 cells and/or the actions of IL-23 and IL-12 prompt the Th17 cells to transition into Th1 cells via T-bet and RORγt co-expressing Th17/Th1 cells that produce both IL-17 and IFNγ (Th17–Th1). Th1 cells are mostly fully committed, terminally differentiated cells that attain a “fixed” phenotype without any further fate alteration.