The immune system has evolved mechanisms to recognize and eliminate threats, as well as to protect against self-destruction. Tolerance to self-antigens is generated through two fundamental mechanisms: (a) elimination of self-reactive cells in the thymus during selection and (b) generation of a variety of peripheral regulatory cells to control self-reactive cells that escape the thymus. It is becoming increasing apparent that a population of thymically derived CD4+ regulatory T cells, exemplified by the expression of the IL-2Rα chain, is essential for the maintenance of peripheral tolerance. Recent work has shown that the forkhead family transcription factor Foxp3 is critically important for the development and function of the regulatory T cells. Lack of Foxp3 leads to development of fatal autoimmune lymphoproliferative disease; furthermore, ectopic Foxp3 expression can phenotypically convert effector T cells to regulatory T cells. This review focuses on Foxp3 expression and function and highlights...

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FOXP3: Of Mice and Men

Classical genetic methods, driven by phenotype rather than hypotheses, generally permit the identification of all proteins that serve nonredundant functions in a defined biological process. Long before this goal is achieved, and sometimes at the very outset, genetics may cut to the heart of a biological puzzle. So it was in the field of mammalian innate immunity. The positional cloning of a spontaneous mutation that caused lipopolysaccharide resistance and susceptibility to Gram-negative infection led directly to the understanding that Toll-like receptors (TLRs) are essential sensors of microbial infection. Other mutations, induced by the random germ line mutagen ENU (N-ethyl-N-nitrosourea), have disclosed key molecules in the TLR signaling pathways and helped us to construct a reasonably sophisticated portrait of the afferent innate immune response. A still broader genetic screen--one that detects all mutations that compromise survival during infection--is permitting fresh insight into the number and types of proteins that mammals use to defend themselves against microbes.

Genetic analysis of host resistance: Toll-like receptor signaling and immunity at large

The recovery of an intact epithelium following lung injury is critical for restoration of lung homeostasis. The initial processes following injury include an acute inflammatory response, recruitment of immune cells, and epithelial cell spreading and migration upon an autologously secreted provisional matrix. Injury causes the release of factors that contribute to repair mechanisms including members of the epidermal growth factor and fibroblast growth factor families (TGF-α, KGF, HGF), chemokines (MCP-1), interleukins (IL-1β, IL-2, IL-4, IL-13), and prostaglandins (PGE2), for example. These factors coordinate processes involving integrins, matrix materials (fibronectin, collagen, laminin), matrix metalloproteinases (MMP-1, MMP-7, MMP-9), focal adhesions, and cytoskeletal structures to promote cell spreading and migration. Several key signaling pathways are important in regulating these processes, including sonic hedgehog, Rho GTPases, MAP kinase pathways, STAT3, and Wnt. Changes in mechanical forces may also affect these pathways. Both localized and distal progenitor stem cells are recruited into the injured area, and proliferation and phenotypic differentiation of these cells leads to recovery of epithelial function. Persistent injury may contribute to the pathology of diseases such as asthma, chronic obstructive pulmonary disease, and pulmonary fibrosis. For example, dysregulated repair processes involving TGF-β and epithelial-mesenchymal transition may lead to fibrosis. This review focuses on the processes of epithelial restitution, the localization and role of epithelial progenitor stem cells, the initiating factors involved in repair, and the signaling pathways involved in these processes.

Epithelial repair mechanisms in the lung

Scurfy mice, which are deficient in a functional Foxp3, exhibit a severe lymphoproliferative disorder and display generalized over-production of cytokines. Here, we show that, among the Foxp transcriptional factor family, which includes Foxp1, Foxp2, and Foxp3, only Foxp3 has the ability to inhibit IL-2, IL-4, and IFN-γ production by primary T helper cells. We found that Foxp3 physically associates with the Rel family transcription factors, nuclear factor of activated T cells (NFAT) and NF-κB, and blocks their ability to induce the endogenous expression of their target genes, including key cytokine genes. More importantly, T cells derived from scurfy mice have a dramatic increase in nuclear factor of activated T cells (NFAT) and NF-κB transcriptional activity compared with the T cells derived from WT mice. Furthermore, complementation of Foxp3 in scurfy-derived T cells lowers the NFAT and NF-κB transcriptional activity to the physiological level. Finally, we show that myelin proteolipid protein-specific autoreactive T cells transduced with Foxp3 cannot mediate experimental autoimmune encephalomyelitis, providing further support that Foxp3 suppresses the effector function of autoreactive T cells. Foxp3 has already been associated with the generation of CD4+CD25+ regulatory T cells; our data additionally demonstrate that Foxp3 suppresses the effector functions of T helper cells by directly inhibiting the activity of two key transcription factors, NFAT and NF-κB, which are essential for cytokine gene expression and T cell functions.

https://www.pnas.org/content/pnas/102/14/5138.full.pdf

Foxp3 interacts with nuclear factor of activated T cells and NF-κB to repress cytokine gene expression and effector functions of T helper cells

To understand the pathogenesis of chronic inflammatory disease, we analyzed an experimental mouse model of chronic lung disease with pathology that resembles asthma and chronic obstructive pulmonary disease (COPD) in humans. In this model, chronic lung disease develops after an infection with a common type of respiratory virus is cleared to only trace levels of noninfectious virus. Chronic inflammatory disease is generally thought to depend on an altered adaptive immune response. However, here we find that this type of disease arises independently of an adaptive immune response and is driven instead by interleukin-13 produced by macrophages that have been stimulated by CD1d-dependent T cell receptor-invariant natural killer T (NKT) cells. This innate immune axis is also activated in the lungs of humans with chronic airway disease due to asthma or COPD. These findings provide new insight into the pathogenesis of chronic inflammatory disease with the discovery that the transition from respiratory viral infection into chronic lung disease requires persistent activation of a previously undescribed NKT cell-macrophage innate immune axis.

Persistent activation of an innate immune response translates respiratory viral infection into chronic lung disease.

Epithelial hyperplasia and metaplasia are common features of inflammatory and neoplastic disease, but the basis for the altered epithelial phenotype is often uncertain. Here we show that long-term ciliated cell hyperplasia coincides with mucous (goblet) cell metaplasia after respiratory viral clearance in mouse airways. This chronic switch in epithelial behavior exhibits genetic susceptibility and depends on persistent activation of EGFR signaling to PI3K that prevents apoptosis of ciliated cells and on IL-13 signaling that promotes transdifferentiation of ciliated to goblet cells. Thus, EGFR blockade (using an irreversible EGFR kinase inhibitor designated EKB-569) prevents virus-induced increases in ciliated and goblet cells whereas IL-13 blockade (using s-IL-13Ralpha2-Fc) exacerbates ciliated cell hyperplasia but still inhibits goblet cell metaplasia. The distinct effects of EGFR and IL-13 inhibitors after viral reprogramming suggest that these combined therapeutic strategies may also correct epithelial architecture in the setting of airway inflammatory disorders characterized by a similar pattern of chronic EGFR activation, IL-13 expression, and ciliated-to-goblet cell metaplasia.

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Blocking airway mucous cell metaplasia by inhibiting EGFR antiapoptosis and IL-13 transdifferentiation signals

Forkhead transcription factors play key roles in the regulation of immune responses. Here, we identify a role for one member of this family, Foxj1, in the regulation of T cell activation and autoreactivity. Foxj1 deficiency resulted in multiorgan systemic inflammation, exaggerated Th1 cytokine production, and T cell proliferation in autologous mixed lymphocyte reactions. Foxj1 suppressed NF-κB transcription activity in vitro, and Foxj1-deficient T cells possessed increased NF-κB activity in vivo, correlating with the ability of Foxj1 to regulate IκB proteins, particularly IκBβ. Thus, Foxj1 likely modulates inflammatory reactions and prevents autoimmunity by antagonizing proinflammatory transcriptional activities. These results suggest a potentially general role for forkhead genes in the enforcement of lymphocyte quiescence.

https://science.sciencemag.org/content/sci/303/5660/1017.full.pdf

Modulation of Th1 Activation and Inflammation by the NF-κB Repressor Foxj1

Establishment and maintenance of epithelial cell polarity depend on cytoskeletal organization and protein trafficking to polarized cortical membranes. ERM (ezrin, radixin, moesin) family members link polarized proteins with cytoskeletal actin. Although ERMs are often considered to be functionally similar, we found that, in airway epithelial cells, apical localization of ERMs depend on cell differentiation and is independently regulated. Moesin was present in the apical membrane of all undifferentiated epithelial cells. However, in differentiated cells, ezrin and moesin were selectively localized to apical membranes of ciliated airway cells and were absent from secretory cells. To identify regulatory proteins required for selective ERM trafficking, we evaluated airway epithelial cells lacking Foxj1, an F-box factor that directs programs required for cilia formation at the apical membrane. Interestingly, Foxj1 expression was also required for localization of apical ezrin, but not moesin. Additionally, membrane-cytoskeletal and threonine-phosphorylated ezrin were decreased in Foxj1-null cells, consistent with absent apical ezrin. Although apical moesin expression was present in null cells, it could not compensate for ezrin because ERM-associated EBP50 and the beta2 adrenergic receptor failed to localize apically in the absence of Foxj1. These findings indicate that Foxj1 regulates ERM proteins differentially to selectively direct the apical localization of ezrin for the organization of multi-protein complexes in apical membranes of airway epithelial cells.

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Melanie S. Spoor

Papers

Blocking airway mucous cell metaplasia by inhibiting EGFR antiapoptosis and IL-13 transdifferentiation signals

Modulation of Th1 Activation and Inflammation by the NF-κB Repressor Foxj1

Foxj1 is required for apical localization of ezrin in airway epithelial cells.