IL-22 mediates mucosal host defense against Gram-negative bacterial pneumonia

Emerging evidence supports the concept that T helper type 17 (T(H)17) cells, in addition to mediating autoimmunity, have key roles in mucosal immunity against extracellular pathogens. Interleukin-22 (IL-22) and IL-17A are both effector cytokines produced by the T(H)17 lineage, and both were crucial for maintaining local control of the Gram-negative pulmonary pathogen, Klebsiella pneumoniae. Although both cytokines regulated CXC chemokines and granulocyte colony-stimulating factor production in the lung, only IL-22 increased lung epithelial cell proliferation and increased transepithelial resistance to injury. These data support the concept that the T(H)17 cell lineage and its effector molecules have evolved to effect host defense against extracellular pathogens at mucosal sites.     

Loss of Function of Intestinal IL-17 and IL-22 Producing Cells Contributes to Inflammation and Viral Persistence in SIV-Infected Rhesus Macaques

In HIV/SIV-infected humans and rhesus macaques (RMs), a severe depletion of intestinal CD4⁺ T-cells producing interleukin IL-17 and IL-22 associates with loss of mucosal integrity and chronic immune activation. However, little is known about the function of IL-17 and IL-22 producing cells during lentiviral infections. Here, we longitudinally determined the levels and functions of IL-17, IL-22 and IL-17/IL-22 producing CD4⁺ T-cells in blood, lymph node and colorectum of SIV-infected RMs, as well as how they recover during effective ART and are affected by ART interruption. Intestinal IL-17 and IL-22 producing CD4⁺ T-cells are polyfunctional in SIV-uninfected RMs, with the large majority of cells producing four or five cytokines. SIV infection induced a severe dysfunction of colorectal IL-17, IL-22 and IL-17/IL-22 producing CD4⁺ T-cells, the extent of which associated with the levels of immune activation (HLA-DR⁺CD38⁺), proliferation (Ki-67⁺) and CD4⁺ T-cell counts before and during ART. Additionally, Th17 cell function during ART negatively correlated with residual plasma viremia and levels of sCD163, a soluble marker of inflammation and disease progression. Furthermore, IL-17 and IL-22 producing cell frequency and function at various pre, on, and off-ART experimental points associated with and predicted total SIV-DNA content in the colorectum and blood. While ART restored Th22 cell function to levels similar to pre-infection, it did not fully restore Th17 cell function, and all cell types were rapidly and severely affected—both quantitatively and qualitatively—after ART interruption. In conclusion, intestinal IL-17 producing cell function is severely impaired by SIV infection, not fully normalized despite effective ART, and strongly associates with inflammation as well as SIV persistence off and on ART. As such, strategies able to preserve and/or regenerate the functions of these CD4⁺ T-cells central for mucosal immunity are critically needed in future HIV cure research.     

Marked expansion of CD11c+CD8+ T-cells in melanoma-bearing mice induced by anti-4-1BB monoclonal antibody

4-1BB (CD137), a member of the tumor necrosis factor receptor superfamily, is expressed on activated T-cells, and 4-1BB signaling due to interaction with 4-1BB ligand or ligation with anti-4-1BB monoclonal antibody (mAb) costimulates T cells. It has been shown that administration of anti-4-1BB mAb induces anti-tumor immunity in mice, but the nature of the cellular subsets responsible for this immunity is uncertain. In this study we found that anti-4-1BB mAb administration to B16F10 melanoma-bearing mice induced marked expansion of CD11c+CD8+ T-cells in parallel with suppression of pulmonary tumors. The mAb-treated mice produced higher levels of IFN- in their tumor tissues, spleen and lymph nodes than mice exposed to control antibody. When the CD11c+CD8+ T-cells were purified and re-stimulated in vitro, they produced high levels of the Th1 cytokines, IFN- and IL-2, but low levels of the Th2 cytokines, IL-4 and IL-10. Furthermore, they expressed high levels of 4-1BB and CD107a, a marker of activated cytotoxic T-lymphocytes. Our results suggest that CD11c+CD8+ T-cells play a role in the anti-tumor immunity induced by anti-4-1BB mAb.     

IL-17 production by thymocytes upon CD3 stimulation and costimulation with microbial factors

IL-17 is a potent proinflammatory cytokine produced by activated memory T cells. Recent studies in both human autoimmune diseases and in their animal models have indicated that IL-17 rather than IFN-gamma might be the essential T-cell effector cytokine in the T-cell mediated autoimmune process. Since the thymus has a central role in maintaining T-cell self-tolerance and disturbance of thymic self-tolerance is implied in various autoimmune diseases, we here investigated the capability of murine thymocytes to produce IL-17. Our results indicate that thymocytes are a potent source of IL-17 in response to CD3 stimulation and various microbial immune stimuli and thereby show different patterns in the expression of the proinflammatory cytokines IFN-gamma and IL-17. In addition, strong differences between thymocytes and splenocytes were detected. Altered IL-17 production by thymocytes upon contact with foreign pathogens might be a key regulator in the education of adaptive immunity.     

Cytokines and the regulation of fungus-specific CD4 T cell differentiation

CD4 T cells play important and non-redundant roles in protection against infection with diverse fungi. Distinct CD4 T cell subsets can mediate protection against fungal disease where Th1 and Th17 CD4 T cell subsets have been found to promote fungal clearance and protective immunity against diverse fungal pathogens. The differentiation of na?ve CD4 T cells into Th1 or Th17 cells is crucially controlled by their interaction with dendritic cells and instructed by cytokines. IL-12 and IFN-γ promote Th1 differentiation while TGF-β, IL-6, IL-1, IL-21 and IL-23 promote Th17 differentiation and maintenance. The production of these cytokines by DCs is in turn regulated by innate receptors triggered in response to fungal infection. In this review we will discuss the contributions of cytokines found to influence fungus-specific CD4 T cell differentiation and their role in defense against fungal disease. We will also highlight the contributions of innate receptors involved in recognition of fungi and how they shape cytokine secretion and CD4 T cell differentiation.     

IL-4 and IL-13 receptors: Roles in immunity and powerful vaccine adjuvants

The roles of interleukin (IL)-4 and IL-13 during both innate and adaptive Th2 mediated immunity have received considerable scrutiny, however, mechanisms by which these cytokines influence the cellular interactions involved in negatively modulating the development of effective Th1 immunity are poorly characterized. In this article we discuss the recent advances in IL-4/IL-13 biology, mainly (i) role of these cytokines in allergic inflammation, atopic dermatitis, cancer, transplant rejection, bacterial/viral infections, and specifically the therapeutic potential of IL-13Rα2, (ii) insights into how “alarmin” stimulation activate IL-4/IL-13 at the lung mucosae, (iii) how these two cytokines modulate antigen-specific CD8⁺ T cell quality/avidity in a vaccine route dependent manner and (iv) finally discuss the potential of using transient inhibition of IL-4 and/or IL-13 at the vaccination site as a platform vaccine technology to induce strong sustained high quality CD8⁺ T cell immunity for protection against many chronic mucosal pathogens such as HIV-1.     

Cytokine-mediated regulation of antimicrobial proteins

Antimicrobial proteins constitute a phylogenetically ancient form of innate immunity that provides host defence at skin and mucosal surfaces. Although some components of this system are constitutively expressed, new evidence reviewed in this Progress article shows that the production of certain antimicrobial proteins by epithelial cells can also be regulated by cytokines of the innate and adaptive immune systems. In particular, the effector cytokines interleukin-17 and interleukin-22, which are produced by the T-helper-17-cell subset, are emerging as crucial regulators of antimicrobial-peptide production in the gut and the lungs. This suggests that this T-cell lineage and its cytokines have important roles in skin and mucosal immunity.     

Comparison of influenza and SIV specific CD8 T cell responses in macaques

Macaques are a potentially useful non-human primate model to compare memory T-cell immunity to acute virus pathogens such as influenza virus and effector T-cell responses to chronic viral pathogens such as SIV. However, immunological reagents to study influenza CD8(+) T-cell responses in the macaque model are limited. We recently developed an influenza-SIV vaccination model of pigtail macaques (Macaca nemestrina) and used this to study both influenza-specific and SIV-specific CD8(+) T-cells in 39 pigtail macaques expressing the common Mane-A*10(+) (Mane-A01*084) MHC-I allele. To perform comparative studies between influenza and SIV responses a common influenza nucleoprotein-specific CD8(+) T-cell response was mapped to a minimal epitope (termed RA9), MHC-restricted to Mane-A*10 and an MHC tetramer developed to study this response. Influenza-specific memory CD8(+) T-cell response maintained a highly functional profile in terms of multitude of effector molecule expression (CD107a, IFN-γ, TNF-α, MIP-1β and IL-2) and showed high avidity even in the setting of SIV infection. In contrast, within weeks following active SIV infection, SIV-specific CD8(+) effector T-cells expressed fewer cytokines/degranulation markers and had a lower avidity compared to influenza specific CD8(+) T-cells. Further, the influenza specific memory CD8 T-cell response retained stable expression of the exhaustion marker programmed death-marker-1 (PD-1) and co-stimulatory molecule CD28 following infection with SIV. This contrasted with the effector SIV-specific CD8(+) T-cells following SIV infection which expressed significantly higher amounts of PD-1 and lower amounts of CD28. Our results suggest that strategies to maintain a more functional CD8(+) T-cell response, profile may assist in controlling HIV disease.     

Primary and Chronic HIV Infection Differently Modulates Mucosal Vδ1 and Vδ2 T-Cells Differentiation Profile and Effector Functions

Gut-associated immune system has been identified as a major battlefield during the early phases of HIV infection. γδ T-cells, deeply affected in number and function after HIV infection, are able to act as a first line of defence against invading pathogens by producing antiviral soluble factors and by killing infected cells. Despite the relevant role in mucosal immunity, few data are available on gut-associated γδ T-cells during HIV infection. Aim of this work was to evaluate how primary (P-HIV) and chronic (C-HIV) HIV infection affects differentiation profile and functionality of circulating and gut-associated Vδ1 and Vδ2 T-cells. In particular, circulating and mucosal cells were isolated from respectively whole blood and residual gut samples from HIV-infected subjects with primary and chronic infection and from healthy donors (HD). Differentiation profile and functionality were analyzed by multiparametric flow cytometry. P-HIV and C-HIV were characterized by an increase in the frequency of effector Vδ1-T cells both in circulating and mucosal compartments. Moreover, during P-HIV mucosal Vδ1 T-cells expressed high levels of CD107a, suggesting a good effector cytotoxic capability of these cells in the early phase of infection that was lost in C-HIV. P-HIV induced an increase in circulating effector Vδ2 T-cells in comparison to C-HIV and HD. Notably, P-HIV as well as HD were characterized by the ability of mucosal Vδ2 T-cells to spontaneously produce IFN-γ that was lost in C-HIV. Altogether, our data showed for the first time a functional capability of mucosal Vδ1 and Vδ2 T-cells during P-HIV that was lost in C-HIV, suggesting exhaustion mechanisms induced by persistent stimulation.     

IL-12 and NK cells are required for antigen-specific adaptive immunity against malaria initiated by CD8+ T cells in the Plasmodium yoelii model.

CD8+ T cells have been implicated as critical effector cells in protection against preerythrocytic stage malaria, including the potent protective immunity of mice and humans induced by immunization with radiation-attenuated Plasmodium spp. sporozoites. This immunity is directed against the Plasmodium spp. parasite developing within the host hepatocyte and for a number of years has been presumed to be mediated directly by CD8+ CTL or indirectly by IFN-gamma released from CD8+ T cells. In this paper, in BALB/c mice, we establish that after immunization with irradiated sporozoites or DNA vaccines parasite-specific CD8+ T cells trigger a novel mechanism of adaptive immunity that is dependent on T cell- and non-T cell-derived cytokines, in particular IFN-gamma and IL-12, and requires NK cells but not CD4+ T cells. The absolute requirement for CD8+ T cells to initiate such an effector mechanism, and the requirement for IL-12 and NK cells in such vaccine-induced protective immunity, are unique and underscore the complexity of the immune responses that protect against malaria and other intracellular pathogens.     

Interleukin-17 as an effector molecule of innate and acquired immunity against infections

Interleukin (IL)-17 is a proinflammatory cytokine which induces differentiation and migration of neutrophils through induction of cytokines and chemokines including granulocyte-colony stimulating factor and CXCL8/IL-8. IL-17-producing CD4(+) T cells (Th17) have pivotal role in pathogenesis of autoimmune diseases. IL-17 is also involved in protective immunity against various infections. IL-17 has important role in induction of neutrophil-mediated protective immune response against extracellular bacterial or fungal pathogens such as Klebsiella pneumoniae and Candida albicans. Importance of IL-17 in protection against intracellular pathogens including Mycobacterium has also been reported. Interestingly, not only CD4(+) T cells but atypical CD4(-)CD8(-) T cells expressing T cell receptor (TCR) gammadelta produce IL-17, and IL-17 producing cells participate in both innate and acquired immune response to infections. Furthermore, neutrophil induction may not be the only mechanism of IL-17-mediated protective immunity. IL-17 seems to participate in host defense through regulation of cell-mediated immunity or induction of antimicrobial peptides such as beta-defensins. In this review, we summarize recent progress on the role of IL-17 in immune response against infections, and discuss possible application of IL-17 in prevention and treatment of infectious diseases.     

A role for IL-15 in the migration of effector CD8 T cells to the lung airways following influenza infection

The cytokines generated locally in response to infection play an important role in CD8 T cell trafficking, survival, and effector function, rendering these signals prime candidates for immune intervention. In this paper, we show that localized increases in the homeostatic cytokine IL-15 induced by influenza infection is responsible for the migration of CD8 effector T cells to the site of infection. Moreover, intranasal delivery of IL-15-IL-15Rα soluble complexes (IL-15c) specifically restores the frequency of effector T cells lost in the lung airways of IL-15-deficient animals after influenza infection. Exogenous IL-15c quantitatively augments the respiratory CD8 T cell response, and continued administration of IL-15c throughout the contraction phase of the anti-influenza CD8 T cell response magnifies the resultant CD8 T cell memory generated in situ. This treatment extends the ability of these cells to protect against heterologous infection, immunity that typically depreciates over time. Overall, our studies describe what to our knowledge is a new function for IL-15 in attracting effector CD8 T cells to the lung airways and suggest that adjuvanting IL-15 could be used to prolong anti-influenza CD8 T cell responses at mucosal surfaces to facilitate pathogen elimination.     

Th17 cells in immunity to Candida albicans

Our understanding of immunity to fungal pathogens has advanced considerably in recent years. Particularly significant have been the parallel discoveries in the C-type lectin receptor family and the Th effector arms of immunity, especially Th17 cells and their signature cytokine, IL-17. Many of these studies have focused on the most common human fungal pathogen, Candida albicans, which is typically a commensal microbe in healthy individuals but causes various disease manifestations in immunocompromised hosts, ranging from mild mucosal infections to lethal disseminated disease. Here, we discuss emerging fundamental discoveries with C.?albicans that have informed our overall molecular understanding of fungal immunity. In particular, we focus on the importance of pattern recognition receptor-mediated fungal recognition and subsequent IL-17 responses in host defense against mucosal candidiasis. In light of these recent advances, we also discuss the implications for anticytokine biologic therapy and vaccine development.     

Experimental Human Pneumococcal Carriage Augments IL-17A-dependent T-cell Defence of the Lung

Pneumococcal carriage is both immunising and a pre-requisite for mucosal and systemic disease. Murine models of pneumococcal colonisation show that IL-17A-secreting CD4⁺ T-cells (Th-17 cells) are essential for clearance of pneumococci from the nasopharynx. Pneumococcal-responding IL-17A-secreting CD4⁺ T-cells have not been described in the adult human lung and it is unknown whether they can be elicited by carriage and protect the lung from pneumococcal infection. We investigated the direct effect of experimental human pneumococcal nasal carriage (EHPC) on the frequency and phenotype of cognate CD4⁺ T-cells in broncho-alveolar lavage and blood using multi-parameter flow cytometry. We then examined whether they could augment ex vivo alveolar macrophage killing of pneumococci using an in vitro assay. We showed that human pneumococcal carriage leads to a 17.4-fold (p = 0.007) and 8-fold (p = 0.003) increase in the frequency of cognate IL-17A⁺ CD4⁺ T-cells in BAL and blood, respectively. The phenotype with the largest proportion were TNF⁺/IL-17A⁺ co-producing CD4⁺ memory T-cells (p<0.01); IFNγ⁺ CD4⁺ memory T-cells were not significantly increased following carriage. Pneumococci could stimulate large amounts of IL-17A protein from BAL cells in the absence of carriage but in the presence of cognate CD4⁺ memory T-cells, IL-17A protein levels were increased by a further 50%. Further to this we then show that alveolar macrophages, which express IL-17A receptors A and C, showed enhanced killing of opsonised pneumococci when stimulated with rhIL-17A (p = 0.013). Killing negatively correlated with RC (r = −0.9, p = 0.017) but not RA expression. We conclude that human pneumococcal carriage can increase the proportion of lung IL-17A-secreting CD4⁺ memory T-cells that may enhance innate cellular immunity against pathogenic challenge. These pathways may be utilised to enhance vaccine efficacy to protect the lung against pneumonia.     

Pertussis toxin by inducing IL-6 promotes the generation of IL-17-producing CD4 cells

Compelling evidence has now demonstrated that IL-17-producing CD4 cells (Th17) are a major contributor to autoimmune pathogenesis, whereas CD4+CD25+ T regulatory cells (Treg) play a major role in suppression of autoimmunity. Differentiation of proinflammatory Th17 and immunosuppressive Treg from naive CD4 cells is reciprocally related and contingent upon the cytokine environment. We and others have reported that in vivo administration of pertussis toxin (PTx) reduces the number and function of mouse Treg. In this study, we have shown that supernatants from PTx-treated mouse splenic cells, which contained IL-6 and other proinflammatory cytokines, but not PTx itself, overcame the inhibition of proliferation seen in cocultures of Treg and CD4+CD25- T effector cells. This stimulatory effect could be mimicked by individual inflammatory cytokines such as IL-1beta, IL-6, and TNF-alpha. The combination of these cytokines synergistically stimulated the proliferation of CD4+CD25- T effector cells despite the presence of Treg with a concomitant reduction in the percentage of FoxP3+ cells and generation of IL-17-expressing cells. PTx generated Th17 cells, while inhibiting the differentiation of FoxP+ cells, from naive CD4 cells when cocultured with bone marrow-derived dendritic cells from wild-type mice, but not from IL-6-/- mice. In vivo treatment with PTx induced IL-17-secreting cells in wild-type mice, but not in IL-6-/- mice. Thus, in addition to inhibiting the development of Treg, the immunoadjuvant activity of PTx can be attributable to the generation of IL-6-dependent IL-17-producing CD4 cells.     

Heterogeneity of multifunctional IL-17A producing S. Typhi-specific CD8+ T cells in volunteers following Ty21a typhoid immunization

Salmonella enterica serovar Typhi (S. Typhi), the causative agent of typhoid fever, continues to cause significant morbidity and mortality world-wide. CD8+ T cells are an important component of the cell mediated immune (CMI) response against S. Typhi. Recently, interleukin (IL)-17A has been shown to contribute to mucosal immunity and protection against intracellular pathogens. To investigate multifunctional IL-17A responses against S. Typhi antigens in T memory subsets, we developed multiparametric flow cytometry methods to detect up to 6 cytokines/chemokines (IL-10, IL-17A, IL-2, interferon-γ (IFN-γ), tumor necrosis factor-α (TNF-α) and macrophage inflammatory protein-1β (MIP-1β)) simultaneously. Five volunteers were immunized with a 4 dose regimen of live-attenuated S. Typhi vaccine (Ty21a), peripheral blood mononuclear cells (PBMC) were isolated before and at 11 time points after immunization, and CMI responses were evaluated. Of the 5 immunized volunteers studied, 3 produced detectable CD8+ T cell responses following stimulation with S. Typhi-infected autologous B lymphoblastoid cell lines (B-LCL). Additionally, 2 volunteers had detectable levels of intracellular cytokines in response to stimulation with S. Typhi-infected HLA-E restricted cells. Although the kinetics of the responses differed among volunteers, all of the responses were bi- or tri-phasic and included multifunctional CD8+ T cells. Virtually all of the IL-17A detected was derived from multifunctional CD8+ T cells. The presence of these multifunctional IL-17A+ CD8+ T cells was confirmed using an unsupervised analysis program, flow cytometry clustering without K (FLOCK). This is the first report of IL-17A production in response to S. Typhi in humans, indicating the presence of a Tc17 response which may be important in protection. The presence of IL-17A in multifunctional cells co-producing Tc1 cytokines (IL-2, IFN-γ and TNF-α) may also indicate that the distinction between Tc17 and Tc1 responses in humans is not as clearly delineated as suggested by in vitro experiments and animal models.     

Cutting edge: IL-5 primes Th2 cytokine-producing capacity in eosinophils through a STAT5-dependent mechanism

Both type-2 CD4(+) Th cells (CD4(+)Th2) and type-2 innate effector cells play critical roles in generating type-2 immunity that can either be protective against parasitic infection or cause tissue damage in allergy and asthma. How innate effector cells acquire the capacity to produce Th2 cytokines is not entirely known. We previously showed that IL-4 induced differentiation of Th2 cytokine-producing eosinophils. To determine whether other Th2 cytokines can also induce Th2 cytokine-producing capacity in innate effector cells, we cultured bone marrow progenitor cells in the presence of various Th2 cytokines. IL-5, but not IL-13 or IL-25, primed bone marrow progenitor cells to differentiate into robust IL-4-producing cells. The majority of IL-4-producing cells induced by IL-5 were eosinophils. Importantly, IL-5 completely depended on STAT5 to promote IL-4-producing capacity in eosinophils. Thus, our study demonstrates that IL-5 functions as a potent factor that drives bone marrow progenitor cells into IL-4-producing eosinophils.     

Synergistic role of CD4+ and CD8+ T lymphocytes in IFN-gamma production and protective immunity induced by an attenuated Toxoplasma gondii vaccine.

BALB/c mice vaccinated with a temperature-sensitive mutant (TS-4) of Toxoplasma gondii develop complete resistance to lethal challenge with a highly virulent toxoplasma strain (RH). This immunity is known to be dependent on IFN-gamma synthesis. In vitro and in vivo T cell depletions were performed in order to identify the subsets responsible for both protective immunity and IFN-gamma production. When stimulated with crude tachyzoite Ag in vitro, CD4+ cells from vaccinated mice produced high levels of TH1 cytokines (IL-2 and IFN-gamma) but not TH2 cytokines (IL-4 and IL-5). CD8+ cells, in contrast, produced less IFN-gamma and no detectable IL-2. Nevertheless, they could be induced to synthesize IFN-gamma when exposed in culture to exogenous IL-2. In vivo treatment with anti-CD4 plus anti-CD8 or anti-IFN-gamma antibodies during challenge infection completely abrogated resistance to T. gondii. In contrast, treatment with anti-CD4 alone failed to reduce immunity, whereas anti-CD8 treatment partially decreased vaccine-induced resistance. These results suggest that although IFN-gamma and IL-2-producing CD4+ lymphocytes are induced by vaccination, IFN-gamma-producing CD8+ T cells are the major effectors of immunity in vivo. Nevertheless, CD4+ lymphocytes appear to play a synergistic role in vaccine-induced immunity, probably through the augmentation of IFN-gamma synthesis by the CD8+ effector cells. This hypothesis is supported by the observation that when giving during vaccination, as opposed to after challenge, anti-CD4 antibodies are capable of blocking protective immunity.