Emerging functions of amphiregulin in orchestrating immunity, inflammation, and tissue repair

Abstract

Type 2 inflammatory responses can be elicited by diverse stimuli, including toxins, venoms, allergens, and infectious agents, and play critical roles in resistance and tolerance associated with infection, wound healing, tissue repair, and tumor development. Emerging data suggest that in addition to characteristic type 2-associated cytokines, the epidermal growth factor (EGF)-like molecule Amphiregulin (AREG) might be a critical component of type 2-mediated resistance and tolerance. Notably, numerous studies demonstrate that in addition to the established role of epithelial- and mesenchymal-derived AREG, multiple leukocyte populations including mast cells, basophils, group 2 innate lymphoid cells (ILC2s), and a subset of tissue-resident regulatory CD4(+) T cells can express AREG. In this review, we discuss recent advances in our understanding of the AREG-EGF receptor pathway and its involvement in infection and inflammation and propose a model for the function of this pathway in the context of resistance and tissue tolerance.

Figure 1. Amphiregulin is a bi-functional growth factor that may induce either cell proliferation or differentiation

The EGFR is a growth factor receptor well known for its capacity to induce mitogenic stimuli. Nevertheless, it is also well established that the EGFR is important for the differentiation of specific cell types. In recent years it has been revealed that binding characteristics of EGF ligands and qualitative differences in signaling downstream of the receptor dictate the dynamics of gene expression and the fate of recipient cells. Sustained signals downstream of the receptor induces stable ERK activation and leads to growth arrest and cell differentiation, while an oscillating signal downstream of the receptor induces a repetitive activation and inactivation of ERK, leading to proliferation of the target cell. AREG is distinct from most other EGF-like growth factors in that it can not only induce a mitogenic signal but can also induce cell differentiation. This is explained by a single amino acid difference in the receptor-binding domain, which decreases the stability of the AREG-EGFR. Such interaction fails to induce receptor internalization and enables AREG to induce a sustained signal downstream of the receptor. In contrast, most other EGFR ligands, such as EGF, TGFα or HB-EGF, bind EGFR with high affinity and thereby induce internalization and degradation of the receptor, and induce negative feedback loops within the MAP-kinase signaling cascade. This activation and termination of an induced signal by the high-affinity ligands of the EGFR in effect lead to a ligand-specific oscillation of the MAP-kinase signaling pathway and thus a mitogenic signal in the recipient cell.

Figure 2. Amphiregulin enhances regulatory T cell function

AREG has been shown to be of critical importance for efficient CD4⁺ regulatory T (Treg) cell function, and in the absence of AREG, CD4⁺ Treg cells cannot suppress local inflammation. As CD4⁺ Treg cells become activated they up-regulate the EGFR, in part via STAT5 mediated signaling. These activated regulatory T cells migrate to the site of inflammation, where they are exposed to AREG, which enhances their suppressive capacity, possibly through secretion of immunosuppressive exosomes. Depending on the type and the site of inflammation, CD4⁺ Treg cells are dependent on different cell types for AREG; in chronic inflammation mast cell derived Amphiregulin enhances CD4⁺ Treg cell function, while in an acute inflammatory response basophils are described as the main source of AREG. In addition, some organs contain tissue-specific CD4⁺ Treg cells. In response to inflammation, these tissue-resident Treg cells can produce Amphiregulin that may act in an autocrine fashion to enhance their suppressive capacity. The extent that AREG derived from these different cell types contributes to immune regulation within inflamed tissues is an ongoing area of study.

Figure 3. Leukocyte-derived Amphiregulin in health and disease

Recent evidence implicates hematopoietic-derived AREG expression as an important component of pathogen resistance and tissue tolerance mechanisms. Type 2-associated immune cells of both the innate and adaptive lineages have been reported to express AREG. (a) In the context of helminth infection, CD4⁺ T cells and AREG promote epithelial cell turnover to expedite helminth eradication, although the specific cellular sources of AREG in this process remain undefined. (b) Tissue damage can cause up-regulation of the alarmin IL-33, a strong positive stimulus for ILC2 accumulation and cytokine production that can support tissue-protective M2 macrophage differentiation. Recent data suggest the possibility that ILC2, M2 MΦ, AREG and IGF-1 may collaborate to promote wound healing at mucosal barrier surfaces. (c) Uncontrolled wound healing responses can result in pathologic tissue fibrosis. It is possible that the same cells (ILC2 and M2 MΦ) and the same cytokines (IL-13 and AREG) that promote tissue repair can become dysregulated in the presence of chronic inflammation and an altered cytokine milieu to promote fibrotic responses, including myofibroblast differentiation, proliferation and ECM deposition. (d) Epithelial-associated solid tumor microenvironments can have high local concentrations of AREG and tumor infiltrating EGFR⁺ CD4⁺ Treg cells. Treatment with EGFR antagonists or Treg cell depleting agents can promote tumor regression. Tissue-resident CD4⁺ Treg cells can express and respond to AREG, suggesting the possibility that in addition to growth factor starvation of the tumor, EGFR antagonists may function in part to inhibit local CD4⁺ Treg cell function, thereby alleviating immunosuppression and enhancing antitumor immune responses, including the activity and function of tumor specific CD8⁺ cytotoxic T lymphocytes (CTLs). Much work remains to define the cellular sources of AREG and the mechanisms regulating its expression in inflamed tissues. Increased understanding of these pathways could reveal new therapeutic targets for multiple disease states.