The immune system arose to protect us from infectious agents. This is most easily seen by the efficacy of vaccines, which induce antibodies that bind to and neutralize pathogenic viruses and bacteria before they can replicate and cause tissue damage. Much has been learned about the pathways necessary for the induction of successful immunity. Minimally, this includes the recognition of conserved constituents, such as bacterial cell wall moieties and bacterial and viral nucleic acids, which announce the invader as "foreign," together with the molecular details specific to each pathogen, which are recognized by the highly discriminatory antigen receptors of the adaptive immune cells, T cells and B cells. The communication between innate cells and adaptive cells is mediated by an array of cytokines that comprise the language of the immune system. Following these highly controlled cytokine communications provides a mechanism for probing how the immune system works and for determining what happens when things become dysregulated and cause pathology.
Vertebrate immunity is organized into modules such that stereotyped patterns of cytokines—the immune "language"—are used during responses to different types of organisms. For rapidly replicating pathogens, like most bacteria, viruses, and fungi, inflammatory cytokines mediate the recruitment and activation of cells to enhanced microbicidal states (represented by activated macrophages, for instance) that are necessary to kill organisms and limit infection. Although these inflammatory responses can lead to pathology, such as seen in septic shock, regulatory processes are also invoked that control the immune attack and re-establish homeostasis through mechanisms that lead to the establishment of protective memory T and B cells and antibody-producing plasma cells. The evolutionary importance of these immune responses is underscored by the consequences of human mutations in these pathways that lead to an inability to contain infectious organisms.
While these protective responses are fairly well understood, the immune response typified as "allergy" remains more puzzling. Allergic inflammation is characterized by the infiltration of tissues by eosinophils and basophils, which are rare myeloid cells that comprise only a few percent of circulating blood cells. The adaptive allergic response is characterized by increases in the numbers of Th2 cells that release interleukin-4 (IL-4), IL-5, and IL-13 and the development of plasma cells that secrete immunoglobulin E (IgE). When sustained, these responses can lead to alterations at mucosal epithelial surfaces, including increases in the numbers of mucus-secreting cells and increased deposition of collagen in the tissues. These responses can be protective in healing the epithelium from chronic attack by parasitic worms, such as hookworms and schistosomes. However, when this type of immunity becomes focused on common environmental exposures, such as inhaled dust mites or mold or consumed shellfish, the result can be allergies, including potentially life-threatening afflictions such as asthma and food allergy, which affect, respectively, more than 20 million and 2 million Americans.
In 2010, our laboratory and others discovered that rare innate lymphoid cells, now called Group 2 innate lymphoid cells, or ILC2s, are the major innate source of IL-13 and IL-5, key cytokines that comprise the “language” of allergy. These ILC2s are deposited in tissues during fetal development and, when activated, result in a stereotyped cytokine response that mediates the accumulation of eosinophils and "alternatively activated" macrophages in affected tissues. With continued stimulation, adaptive responses characterized by recruitment of Th2 cells and production of IgE occurs, thus exposing a fundamental pathway central to allergic immunity.
Our major focus remains how environmental perturbations are transmitted across barrier tissues to activate ILC2s and how the output from these cells is channeled to re-establish homeostasis, or, in pathologic conditions, to cause allergy. When aspirated into the lungs, chitin, a structural polysaccharide required for structural integrity in allergy-inducing insects, fungi and helminthes, induces focal areas of mucosal injury, leading to the production of epithelial cytokines TSLP, IL-33, and IL-25. ILC2s constitutively express receptors for these cytokines, and respond by secreting cytokines and growth factors. In the absence of these three epithelial cytokines, ILC2 cytokines are not released, and, when ILC2s are deleted, the infiltration of inflammatory cells and the extent of tissue injury are increased. A major target of IL-13 derived from activated ILC2s is the induction of epithelial mucins and chitinase enzymes that clear and degrade the insoluble polysaccharide, thus re-establishing airway homeostasis. We continue efforts to understand this epithelial-ILC2 circuit in maintaining lung homeostasis against environmental challenges and have begun to extend these findings to human lung diseases characterized by inflammation, tissue injury, and subepithelial fibrosis, such as asthma, to understand how such processes come to be dysregulated.
Using models of hookworm infection to induce small intestine injury, we discovered that IL-25, a key upstream ILC2-activating cytokine, is produced by tuft cells, among the rarest of mucosal epithelial cells. Tuft cells respond to helminth perturbation by activating lamina propria ILC2s via IL-25. ILC2s, in turn, release IL-13 which acts directly on crypt epithelial precursor cells to bias intestinal cell fates into the mucus-secreting goblet cell and tuft cell lineages; in the absence of tuft cell IL-25, ILC2s fail to activate and epithelial responses become attenuated. Thus, tuft cells transduce signals from the intestinal lumen to tissue ILC2s, which in turn release signals that alter the composition of the epithelium such that barrier integrity can be sustained. As in the lung injured by chitin deposition, barrier perturbation is transmitted by epithelial cytokines to local ILC2s, which activate cytokines that feedback on local tissue cells to re-establish barrier function through an epithelial-ILC2 circuit. We continue efforts to identify the constituents ‘sensed’ by healthy epithelia that activate cytokine expression from the lamina propria ILC2 cells, as well as mechanisms by which ILC2s respond to epithelial cell activation by producing cytokines that effect epithelial fate decisions. Additionally, the mechanisms by which thresholds become exceeded such that local homeostasis becomes dysregulated and leads to activation of adaptive type 2 allergic responses need to be defined. ILC2s are also necessary for metabolic homeostasis that sustains maintenance of healthy adipose tissue, raising the possibility that barrier surveillance is systemically integrated with host metabolism to foster a healthy host immune and microbiota interface. Together, our studies have uncovered basal physiologic conditions regulated by these rare "allergic" cells, and have opened new avenues of investigation into roles for these cells in vertebrate biology in sustaining interactions with the outside world. When dysregulated, allergies and asthma can result, but insights garnered by understanding the underlying homeostatic roles for these cells offer great promise for uncovering new strategies for control of these prevalent human afflictions.
Grants from the National Institutes of Health provided partial support for this project.
As of March 9, 2016