Lynette A. Fouser

Interleukin (IL)-22 and IL-17 are coexpressed by Th17 cells and cooperatively enhance expression of antimicrobial peptides

Th17 cells are a distinct lineage of effector CD4+ T cells characterized by their production of interleukin (IL)-17. We demonstrate that Th17 cells also expressed IL-22, an IL-10 family member, at substantially higher amounts than T helper (Th)1 or Th2 cells. Similar to IL-17A, IL-22 expression was initiated by transforming growth factor β signaling in the context of IL-6 and other proinflammatory cytokines. The subsequent expansion of IL-22–producing cells was dependent on IL-23. We further demonstrate that IL-22 was coexpressed in vitro and in vivo with both IL-17A and IL-17F. To study a functional relationship among these cytokines, we examined the expression of antimicrobial peptides by primary keratinocytes treated with combinations of IL-22, IL-17A, and IL-17F. IL-22 in conjunction with IL-17A or IL-17F synergistically induced the expression of β-defensin 2 and S100A9 and additively enhanced the expression of S100A7 and S100A8. Collectively, we have identified IL-22 as a new cytokine expressed by Th17 cells that synergizes with IL-17A or IL-17F to regulate genes associated with skin innate immunity.

Border patrol: regulation of immunity, inflammation and tissue homeostasis at barrier surfaces by IL-22

The maintenance of barrier function at exposed surfaces of the mammalian body is essential for limiting exposure to environmental stimuli, preventing systemic dissemination of commensal and pathogenic microbes and retaining normal homeostasis of the entire body. Indeed, dysregulated barrier function is associated with many infectious and inflammatory diseases, including psoriasis, influenza, inflammatory bowel disease and human immunodeficiency virus, which collectively afflict millions of people worldwide. Studies have shown that interleukin 22 (IL-22) is expressed at barrier surfaces and that its expression is dysregulated in certain human diseases, which suggests a critical role in the maintenance of normal barrier homeostasis. Consistent with that, studies of mouse model systems have identified a critical role for signaling by IL-22 through its receptor (IL-22R) in the promotion of antimicrobial immunity, inflammation and tissue repair at barrier surfaces. In this review we will discuss how the expression of IL-22 and IL-22R is regulated, the functions of the IL-22–IL-22R pathway in regulating immunity, inflammation and tissue homeostasis, and the therapeutic potential of targeting this pathway in human disease.

PD-1:PD-L inhibitory pathway affects both CD4(+) and CD8(+) T cells and is overcome by IL-2.

Programmed death‐1 (PD‐1) is an immunoreceptor tyrosine‐based inhibitory motif (ITIM)‐containing receptor expressed upon T cell activation. PD‐1–/– animals develop autoimmune diseases, suggesting an inhibitory role for PD‐1 in immune responses. Members of the B7 family, PD‐L1 and PD‐L2, are ligands for PD‐1. This study examines the functional consequences of PD‐1:PD‐L engagementon murine CD4 and CD8 T cells and shows that these interactions result in inhibition of proliferation and cytokine production. T cells stimulated with anti‐CD3/PD‐L1.Fc‐coated beads display dramatically decreased proliferation and IL‐2 production, while CSFE analysis shows fewer cells cycling and a slower division rate. Costimulation with soluble anti‐CD28 mAb can overcome PD‐1‐mediated inhibition by augmenting IL‐2 production. However, PD‐1:PD‐L interactions inhibit IL‐2 production even in the presence of costimulation and, thus, after prolonged activation, the PD‐1:PD‐L inhibitory pathway dominates. Exogenous IL‐2 is able to overcome PD‐L1‐mediated inhibition at all times, indicating that cells maintain IL‐2 responsiveness. Experiments using TCR transgenic CD4+ or CD8+ T cells stimulated with antigen‐presenting cells expressing PD‐L1 show that both T cell subsets are susceptible to this inhibitory pathway. However, CD8+ T cells may be more sensitive to modulation by the PD‐1:PD‐L pathway because of their intrinsic inability to produce significant levels of IL‐2.

IL-22 is required for Th17 cell–mediated pathology in a mouse model of psoriasis-like skin inflammation

Psoriasis is a chronic skin disease resulting from the dysregulated interplay between keratinocytes and infiltrating immune cells. We report on a psoriasis-like disease model, which is induced by the transfer of CD4(+)CD45RB(hi)CD25(-) cells to pathogen-free scid/scid mice. Psoriasis-like lesions had elevated levels of antimicrobial peptide and proinflammatory cytokine mRNA. Also, similar to psoriasis, disease progression in this model was dependent on the p40 common to IL-12 and IL-23. To investigate the role of IL-22, a Th17 cytokine, in disease progression, mice were treated with IL-22-neutralizing antibodies. Neutralization of IL-22 prevented the development of disease, reducing acanthosis (thickening of the skin), inflammatory infiltrates, and expression of Th17 cytokines. Direct administration of IL-22 into the skin of normal mice induced both antimicrobial peptide and proinflammatory cytokine gene expression. Our data suggest that IL-22, which acts on keratinocytes and other nonhematopoietic cells, is required for development of the autoreactive Th17 cell-dependent disease in this model of skin inflammation. We propose that IL-22 antagonism might be a promising therapy for the treatment of human psoriasis.

Innate Lymphoid Cells Promote Anatomical Containment of Lymphoid-Resident Commensal Bacteria

Protecting Against a Barrier Breach In order to coexist peacefully, a “firewall” exists that keeps the commensal bacteria that reside in our intestines and associated lymphoid tissue contained. Several diseases and infections, however, lead to a breach in this barrier, which leads to chronic inflammation and pathology. Sonnenberg et al. (p. 1321) found that in mice, innate lymphoid cells (ILCs) are critically important for the anatomical containment of commensal bacteria in an interleukin-22 (IL-22)–dependent manner. ILC depletion or blockade of IL-22 led to loss of bacterial containment and systemic inflammation. Lymphocytes prevent bacteria from spreading beyond gut-associated lymphoid tissues and causing systemic inflammation. The mammalian intestinal tract is colonized by trillions of beneficial commensal bacteria that are anatomically restricted to specific niches. However, the mechanisms that regulate anatomical containment remain unclear. Here, we show that interleukin-22 (IL-22)–producing innate lymphoid cells (ILCs) are present in intestinal tissues of healthy mammals. Depletion of ILCs resulted in peripheral dissemination of commensal bacteria and systemic inflammation, which was prevented by administration of IL-22. Disseminating bacteria were identified as Alcaligenes species originating from host lymphoid tissues. Alcaligenes was sufficient to promote systemic inflammation after ILC depletion in mice, and Alcaligenes-specific systemic immune responses were associated with Crohn’s disease and progressive hepatitis C virus infection in patients. Collectively, these data indicate that ILCs regulate selective containment of lymphoid-resident bacteria to prevent systemic inflammation associated with chronic diseases.

CD4+ Lymphoid Tissue-Inducer Cells Promote Innate Immunity in the Gut

Fetal CD4(+) lymphoid tissue inducer (LTi) cells play a critical role in the development of lymphoid tissues. Recent studies identified that LTi cells persist in adults and are related to a heterogeneous population of innate lymphoid cells that have been implicated in inflammatory responses. However, whether LTi cells contribute to protective immunity remains poorly defined. We demonstrate that after infection with Citrobacter rodentium, CD4(+) LTi cells were a dominant source of interleukin-22 (IL-22) early during infection. Infection-induced CD4(+) LTi cell responses were IL-23 dependent, and ablation of IL-23 impaired innate immunity. Further, depletion of CD4(+) LTi cells abrogated infection-induced expression of IL-22 and antimicrobial peptides, resulting in exacerbated host mortality. LTi cells were also found to be essential for host protective immunity in lymphocyte-replete hosts. Collectively these data demonstrate that adult CD4(+) LTi cells are a critical source of IL-22 and identify a previously unrecognized function for CD4(+) LTi cells in promoting innate immunity in the intestine.

An IL-17F/A Heterodimer Protein Is Produced by Mouse Th17 Cells and Induces Airway Neutrophil Recruitment

IL-17A and IL-17F are related homodimeric proteins of the IL-17 family produced by Th17 cells. In this study, we show that mouse Th17 cells also produce an IL-17F/A heterodimeric protein. Whereas naive CD4+ T cells differentiating toward the Th17 cell lineage expressed IL-17F/A in higher amounts than IL-17A/A homodimer and in lower amounts than IL-17F/F homodimer, differentiated Th17 cells expressed IL-17F/A in higher amounts than either homodimer. In vitro, IL-17F/A was more potent than IL-17F/F and less potent than IL-17A/A in regulating CXCL1 expression. Neutralization of IL-17F/A with an IL-17A-specific Ab, and not with an IL-17F-specific Ab, reduced the majority of IL-17F/A-induced CXCL1 expression. To study these cytokines in vivo, we established a Th17 cell adoptive transfer model characterized by increased neutrophilia in the airways. An IL-17A-specific Ab completely prevented Th17 cell-induced neutrophilia and CXCL5 expression, whereas Abs specific for IL-17F or IL-22, a cytokine also produced by Th17 cells, had no effects. Direct administration of mouse IL-17A/A or IL-17F/A, and not IL-17F/F or IL-22, into the airways significantly increased neutrophil and chemokine expression. Taken together, our data elucidate the regulation of IL-17F/A heterodimer expression by Th17 cells and demonstrate an in vivo function for this cytokine in airway neutrophilia.

Interleukin-22 Protects Intestinal Stem Cells from Immune-Mediated Tissue Damage and Regulates Sensitivity to Graft versus Host Disease

Little is known about the maintenance of intestinal stem cells (ISCs) and progenitors during immune-mediated tissue damage or about the susceptibility of transplant recipients to tissue damage mediated by the donor immune system during graft versus host disease (GVHD). We demonstrate here that deficiency of recipient-derived IL-22 increased acute GVHD tissue damage and mortality, that ISCs were eliminated during GVHD, and that ISCs as well as their downstream progenitors expressed the IL-22 receptor. Intestinal IL-22 was produced after bone marrow transplant by IL-23-responsive innate lymphoid cells (ILCs) from the transplant recipients, and intestinal IL-22 increased in response to pretransplant conditioning. However, ILC frequency and IL-22 amounts were decreased by GVHD. Recipient IL-22 deficiency led to increased crypt apoptosis, depletion of ISCs, and loss of epithelial integrity. Our findings reveal IL-22 as a critical regulator of tissue sensitivity to GVHD and a protective factor for ISCs during inflammatory intestinal damage.

Cutting Edge: Identification of GL50, a Novel B7-Like Protein That Functionally Binds to ICOS Receptor

By the genetic selection of mouse cDNAs encoding secreted proteins, a B7-like cDNA clone termed mouse GL50 (mGL50) was isolated encoding a 322-aa polypeptide identical with B7h. Isolation of the human ortholog of this cDNA (hGL50) revealed a coding sequence of 309 aa residues with 42% sequence identity with mGL50. Northern analysis indicated GL50 to be present in many tissues including lymphoid, embryonic yolk sac, and fetal liver samples. Of the CD28, CTLA4, and ICOS fusion constructs tested, flow cytometric analysis demonstrated only mouse ICOS-IgG binding to mGL50 cell transfectants. Subsequent phenotyping demonstrated high levels of ICOS ligand staining on splenic CD19+ B cells and low levels on CD3+ T cells. These results indicate that GL50 is a specific ligand for the ICOS receptor and suggest that the GL50-ICOS interaction functions in lymphocyte costimulation.

Pathological versus protective functions of IL-22 in airway inflammation are regulated by IL-17A

IL-22 has both proinflammatory and tissue-protective properties depending on the context in which it is expressed. However, the factors that influence the functional outcomes of IL-22 expression remain poorly defined. We demonstrate that after administration of a high dose of bleomycin that induces acute tissue damage and airway inflammation and is lethal to wild-type (WT) mice, Th17 cell–derived IL-22 and IL-17A are expressed in the lung. Bleomycin-induced disease was ameliorated in Il22−/− mice or after anti–IL-22 monoclonal antibody (mAb) treatment of WT mice, indicating a proinflammatory/pathological role for IL-22 in airway inflammation. However, despite increased bleomycin-induced IL-22 production, Il17a−/− mice were protected from airway inflammation, suggesting that IL-17A may regulate the expression and/or proinflammatory properties of IL-22. Consistent with this, IL-17A inhibited IL-22 production by Th17 cells, and exogenous administration of IL-22 could only promote airway inflammation in vivo by acting in synergy with IL-17A. Anti–IL-22 mAb was delivered to Il17a−/− mice and was found to exacerbate bleomycin-induced airway inflammation, indicating that IL-22 is tissue protective in the absence of IL-17A. Finally, in an in vitro culture system, IL-22 administration protected airway epithelial cells from bleomycin-induced apoptosis, and this protection was reversed after coadministration of IL-17A. These data identify that IL-17A can regulate the expression, proinflammatory properties, and tissue-protective functions of IL-22, and indicate that the presence or absence of IL-17A governs the proinflammatory versus tissue-protective properties of IL-22 in a model of airway damage and inflammation.