Hidden talents of natural killers: NK cells in innate and adaptive immunity

Natural killer (NK) cells are innate immune lymphocytes capable of killing target cells and producing immunoregulatory cytokines. Herein, we discuss recent studies that indicate that NK cells span the conventional boundaries between innate and adaptive immunity. For example, it was recently discovered that NK cells have the capacity for memory‐like responses, a property that was previously thought to be limited to adaptive immunity. NK cells have also been identified in multiple tissues, and a subset of cells that specialize in the production of the T_H17 cytokine IL‐22, NK‐22s, was recently described in mucosal‐associated lymphoid tissue. Finally, we review work that shows that NK cells develop at sites that were traditionally thought to be occupied only by adaptive immune cells, including the thymus and lymph nodes.     

Innate endogenous adjuvants prime to desirable immune responses via mucosal routes

Vaccination is an effective strategy to prevent infectious or immune related diseases, which has made remarkable contribution in human history. Recently increasing attentions have been paid to mucosal vaccination due to its multiple advantages over conventional ways. Subunit or peptide antigens are more reasonable immunogens for mucosal vaccination than live or attenuated pathogens, however adjuvants are required to augment the immune responses. Many mucosal adjuvants have been developed to prime desirable immune responses to different etiologies. Compared with pathogen derived adjuvants, innate endogenous molecules incorporated into mucosal vaccines demonstrate prominent adjuvanticity and safety. Nowadays, cytokines are broadly used as mucosal adjuvants for participation of signal transduction of immune responses, activation of innate immunity and polarization of adaptive immunity. Desired immune responses are promptly and efficaciously primed on basis of specific interactions between cytokines and corresponding receptors. In addition, some other innate molecules are also identified as potent mucosal adjuvants. This review focuses on innate endogenous mucosal adjuvants, hoping to shed light on the development of mucosal vaccines.     

Deletion of flagellin's hypervariable region abrogates antibody-mediated neutralization and systemic activation of TLR5-dependent immunity

TLRs trigger immunity by detecting microbe-associated molecular patterns (MAMPs). Flagellin is a unique MAMP because it harbors 1) an antigenic hypervariable region and 2) a conserved domain involved in TLR5-dependent systemic and mucosal proinflammatory and adjuvant activities. In this study, the contribution of the flagellin domains in TLR5 activation was investigated. We showed that TLR5 signaling can be neutralized in vivo by flagellin-specific Abs, which target the conserved domain. However, deletions of flagellin's hypervariable region abrogated the protein's intrinsic ability to trigger the production of neutralizing Abs. The fact that MAMP-specific Abs block TLR-mediated responses shows that this type of neutralization is a novel mechanism for down-regulating innate immunity. The stimulation of mucosal innate immunity and adjuvancy to foreign Ag was not altered by the hypervariable domain deletions. In contrast, this domain is essential to trigger systemic innate immunity, suggesting that there are distinct mechanisms for TLR5 activation in systemic and mucosal compartments. In summary, specific MAMP determinants control the production of neutralizing Abs and the compartmentalization of innate responses.     

Regulated MIP-3alpha/CCL20 production by human intestinal epithelium: mechanism for modulating mucosal immunity

Human intestinal epithelial cells secrete an array of chemokines known to signal the trafficking of neutrophils and monocytes important in innate mucosal immunity. We hypothesized that intestinal epithelium may also have the capacity to play a role in signaling host adaptive immunity. The CC chemokine macrophage inflammatory protein (MIP)-3alpha/CCL20 is chemotactic for immature dendritic cells and CD45RO(+) T cells that are important components of the host adaptive immune system. In these studies, we demonstrate the widespread production and regulated expression of MIP-3alpha by human intestinal epithelium. Several intestinal epithelial cell lines were shown to constitutively express MIP-3alpha mRNA. Moreover, MIP-3alpha mRNA expression and protein production were upregulated by stimulation of intestinal epithelial cells with the proinflammatory cytokines tumor necrosis factor-alpha or interleukin-1alpha or in response to infection with the enteric bacterial pathogens Salmonella or enteroinvasive Escherichia coli. In addition, MIP-3alpha was shown to function as a nuclear factor-kappaB target gene. In vitro findings were paralleled in vivo by increased expression of MIP-3alpha in the epithelium of cytokine-stimulated or bacteria-infected human intestinal xenografts and in the epithelium of inflamed human colon. Mucosal T cells, other mucosal mononuclear cells, and intestinal epithelial cells expressed CCR6, the cognate receptor for MIP-3alpha. The constitutive and regulated expression of MIP-3alpha by human intestinal epithelium is consistent with a role for epithelial cell-produced MIP-3alpha in modulating mucosal adaptive immune responses.     

Caspase-12 modulates NOD signaling and regulates antimicrobial peptide production and mucosal immunity

Bacterial sensing by intracellular Nod proteins and other Nod-like receptors (NLRs) activates signaling pathways that mediate inflammation and pathogen clearance. Nod1 and Nod2 associate with the kinase Rip2 to stimulate NF-kappaB signaling. Other cytosolic NLRs assemble caspase-1-activating multiprotein complexes termed inflammasomes. Caspase-12 modulates the caspase-1 inflammasome, but unlike other NLRs, Nod1 and Nod2 have not been linked to caspases, and mechanisms regulating the Nod-Rip2 complex are less clear. We report that caspase-12 dampens mucosal immunity to bacterial infection independent of its effects on caspase-1. Caspase-12 deficiency enhances production of antimicrobial peptides, cytokines, and chemokines to entric pathogens, an effect dependent on bacterial type III secretion and the Nod pathway. Mechanistically, caspase-12 binds to Rip2, displacing Traf6 from the signaling complex, inhibiting its ubiquitin ligase activity, and blunting NF-kappaB activation. Nod activation and resulting antimicrobial peptide production constitute an early innate defense mechanism, and caspase-12 inhibits this mucosal antimicrobial response.     

Caspase-1-processed IL-1 family cytokines play a vital role in driving innate IL-17

The interleukin (IL)-1 cyokine family plays a vital role in inflammatory responses during infection and in autoimmune diseases. The pro-inflammatory cytokines, IL-1β and IL-18 are members of the IL-1 family that require cleavage by caspase-1 in the inflammasome to generate the mature active cytokines. Cells of the innate immune system, including γδ T cells and invariant natural killer T (iNKT) cells respond rapidly to invading pathogens by producing inflammatory cytokines, such as IFN-γ and IL-17. IL-1β or IL-18 in combination with IL-23 can induce IL-17 production by γδ T cells without T cell receptor (TCR) engagement. IL-1β and IL-23 can also synergize to induce IL-17 production by iNKT cells. Furthermore, CD4+ αβ effector memory T cells secrete IL-17 in response to IL-23 in combination with either IL-1β or IL-18, in the absence of any TCR stimulation. The early IL-17 produced by innate cells induces recruitment of neutrophils to the site of infection, stimulates local epithelial cells to secrete anti-microbial proteins, such as lipocalins and calgranulins, induces production of structural proteins important in tight junction stability, and promotes production of matrix metalloproteinases. Caspase-1 processed IL-1 family cytokines therefore play a vital role in the innate immune response and induction of IL-17 from innate immune cells which functions to fight infections and promote autoimmunity.     

The role of Th17 cytokines in primary mucosal immunity

The T helper type 17 (Th17) lineage of CD4+ T-cells produce several effector molecules including IL-17A, IL-17F, IL-21, and IL-22. In addition to CD4+, αβ T-cells, these cytokines can be produced by natural killer and γδ T-cells. These effector cytokines can be produced rapidly upon infection at mucosal sites and evidence to date strongly implicates that this arm of the immune system plays a critical role in mucosal immunity to many extracellular pathogens. Moreover these cytokines can also coordinate adaptive immunity to some intracellular pathogens. In this review, we will highlight recent progress in our understanding of these cytokines, and mechanisms of their effector function in the mucosa.     

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.     

From virus to inflammation: mechanisms of RIG-I-induced IL-1β production

Early detection of viruses by the innate immune system is critical for host defense. Antiviral immunity is initiated by germline encoded pattern recognition receptors (PRRs) that recognize viral pathogen-associated molecular patterns (PAMPs) such as nucleic acids. Intracellular PRRs then drive the production of interferons and cytokines to orchestrate immune responses. One key host factor that is critical for antiviral immunity and for systemic inflammatory reactions including fever is interleukin-1beta (IL-1β). Here we discuss current insights into the molecular mechanisms how the cytosolic RNA helicase RIG-I triggers NF-κB signaling and inflammasome activation specifically for RNA virus-induced IL-1β production.     

Innate immunity to Paracoccidioides brasiliensis infection

Innate immunity is based in pre-existing elements of the immune system that directly interact with all types of microbes leading to their destruction or growth inhibition. Several elements of this early defense mechanism act in concert to control initial pathogen growth and have profound effect on the adaptative immune response that further develops. Although most studies in paracoccidioidomycosis have been dedicated to understand cellular and humoral immune responses, innate immunity remains poorly defined. Hence, the main purpose of this review is to present and discuss some mechanisms of innate immunity developed by resistant and susceptible mice to Paracoccidioides brasiliensis infection, trying to understand how this initial host-pathogen interface interferes with the protective or deleterious adaptative immune response that will dictate disease outcome. An analysis of some mechanisms and mediators of innate immunity such as the activation of complement proteins, the microbicidal activity of natural killer cells and phagocytes, the production of inflammatory eicosanoids, cytokines, and chemokines among others, is presented trying to show the important role played by innate immunity in the host response to P. brasiliensis infection.     

Autophagy and NLRP3 inflammasome crosstalk in neuroinflammation in aged bovine brains

NLRP3 inflammasome is a multiprotein complex that can sense several stimuli such as autophagy dysregulation and increased reactive oxygen species production stimulating inflammation by priming the maturation of proinflammatory cytokines interleukin-1β and interleukin-18 in their active form. In the aging brain, these cytokines can mediate the innate immunity response priming microglial activation. Here, we describe the results of immunohistochemical and molecular analysis carried out on bovine brains. Our results support the hypothesis that the age-related impairment in cellular housekeeping mechanisms and the increased oxidative stress can trigger the inflammatory danger sensor NLRP3. Moreover, according to the recent scientific literature, we demonstrate the presence of an age-related proinflammatory environment in aged brains consisting in an upregulation of interleukin-1β, an increased microglial activation and increased NLRP3 expression. Finally, we suggest that bovine may potentially be a pivotal animal model for brain aging studies.     

The protein kinase PKR is required for p38 MAPK activation and the innate immune response to bacterial endotoxin

Protein kinase RNA-regulated (PKR) is an established component of innate antiviral immunity. Recently, PKR has been shown to be essential for signal transduction in other situations of cellular stress. The relationship between PKR and the stress-activated protein kinases (SAPKs), such as p38 mitogen-activated protein kinase (MAPK), is not clear. Using embryonic fibroblasts from PKR wild-type and null mice, we established a requirement for PKR in the activation of SAPKs by double-stranded RNA, lipopolysaccharide (LPS) and proinflammatory cytokines. This does not reflect a global failure to activate SAPKs in the PKR-null background as these kinases are activated normally by anisomycin and other physicochemical stress. Activation of p38 MAPK was restored in immortalized PKR-null cells by reconstitution with human PKR. We also show that LPS induction of interleukin-6 and interleukin-12 mRNA is defective in PKR-null cells, and that production of these cytokines is impaired in PKR-null mice challenged with LPS. Our findings indicate, for the first time, that PKR is required for p38 MAPK signaling and plays a potentially important role in the innate response against bacterial endotoxin.     

Development and regulation of RORγt innate lymphoid cells

RORγt⁺ innate lymphoid cells (ILCs), or ILC3, play a fundamental role in the development of lymphoid tissues, as well as in homeostasis and defence of mucosal tissues. These cells produce IL-22, IL-17A and LTα1β2, key cytokines for the activation of epithelial defences and the recruitment of polymorphonuclear phagocytes. In the absence of ILC3, the early defence to infection and resistance to injury are compromised. Given the importance of ILC3 in mucosal immunity, significant efforts are made to discover their multiple functions and decipher their mode of action and regulation.     

Immunology of term and preterm labor

During pregnancy there is an alteration in maternal immunity within the uterus where innate, proinflammatory immune responses are tightly regulated to prevent immunological rejection of the fetal allograft. Disruption of the delicate balance of cytokines by bacteria or other factors increases the production of proinflammatory cytokines at the maternal-fetal interface and activates the parturition mechanism prematurely. Despite years of searching, there is still no broadly effective strategy for preventing preterm labor and most therapies are directed at inhibiting myometrial contractions and improving neonatal outcome. Recent studies with progestins and interleukin-10 (IL-10), however, are showing promise in randomized clinical trials and animal studies. Furthermore, the identification of the Toll-like receptors as upstream mediators of inflammation may offer alternative therapeutic targets for preventing this common pregnancy complication.     

Defensins: antimicrobial peptides of innate immunity

The production of natural antibiotic peptides has emerged as an important mechanism of innate immunity in plants and animals. Defensins are diverse members of a large family of antimicrobial peptides, contributing to the antimicrobial action of granulocytes, mucosal host defence in the small intestine and epithelial host defence in the skin and elsewhere. This review, inspired by a spate of recent studies of defensins in human diseases and animal models, focuses on the biological function of defensins.     

CpG DNA-mediated induction of acute liver injury in D-galactosamine-sensitized mice: the mitochondrial apoptotic pathway-dependent death of hepatocytes

Unmethylated CpG motifs present in bacterial DNA (CpG DNA) induce innate inflammatory responses, including rapid induction of proinflammatory cytokines. Although innate inflammatory responses induced by CpG DNA and other pathogen-associated molecular patterns are essential for the eradication of infectious microorganisms, excessive activation of innate immunity is detrimental to the host. In this study, we demonstrate that CpG DNA, but not control non-CpG DNA, induces a fulminant liver failure with subsequent shock-mediated death by promoting massive apoptotic death of hepatocytes in D-galactosamine (D-GalN)-sensitized mice. Inhibition of mitochondrial membrane permeability transition pore opening or caspase 9 activity in vivo protects D-GalN-sensitized mice from the CpG DNA-mediated liver injury and death. CpG DNA enhanced production of proinflammatory cytokines in D-GalN-sensitized mice via a TLR9/MyD88-dependent pathway. In addition, CpG DNA failed to induce massive hepatocyte apoptosis and subsequent fulminant liver failure and death in D-GalN-sensitized mice that lack TLR9, MyD88, tumor necrosis factor (TNF)-alpha, or TNF receptor I but not interleukin-6 or -12p40. Taken together, our results provide direct evidence that CpG DNA induces a severe acute liver injury and shock-mediated death through the mitochondrial apoptotic pathway-dependent death of hepatocytes caused by an enhanced production of TNF-alpha through a TLR9/MyD88 signaling pathway in D-GalN-sensitized mice.     

Delineation of the innate and adaptive T-cell immune outcome in the human host in response to Campylobacter jejuni infection

Background

Campylobacter jejuni is the most prevalent cause of bacterial gastroenteritis worldwide. Despite the significant health burden this infection presents, molecular understanding of C. jejuni-mediated disease pathogenesis remains poorly defined. Here, we report the characterisation of the early, innate immune response to C. jejuni using an ex-vivo human gut model of infection. Secondly, impact of bacterial-driven dendritic cell activation on T-cell mediated immunity was also sought.

Methodology

Healthy, control paediatric terminal ileum or colonic biopsy tissue was infected with C. jejuni for 8–12 hours. Bacterial colonisation was followed by confocal microscopy and mucosal innate immune responses measured by ELISA. Marked induction of IFNγ with modest increase in IL-22 and IL-17A was noted. Increased mucosal IL-12, IL-23, IL-1β and IL-6 were indicative of a cytokine milieu that may modulate subsequent T-cell mediated immunity. C. jejuni-driven human monocyte-derived dendritic cell activation was followed by analyses of T cell immune responses utilising flow cytometry and ELISA. Significant increase in Th-17, Th-1 and Th-17/Th-1 double-positive cells and corresponding cytokines was observed. The ability of IFNγ, IL-22 and IL-17 cytokines to exert host defence via modulation of C. jejuni adhesion and invasion to intestinal epithelia was measured by standard gentamicin protection assay.

Conclusions

Both innate and adaptive T cell-immunity to C. jejuni infection led to the release of IFNγ, IL-22 and IL-17A; suggesting a critical role for this cytokine triad in establishing host anti-microbial immunity during the acute and effectors phase of infection. In addition, to their known anti-microbial functions; IL-17A and IL-17F reduced the number of intracellular C. jejuni in intestinal epithelia, highlighting a novel aspect of how IL-17 family members may contribute to protective immunity against C. jejuni.     

Cytokines and cell adhesion receptors in the regulation of immunity to Trypanosoma cruzi

Pathophysiology of Chagas' disease is not completely defined, although innate and adaptative immune responses are crucial. In acute infection some parasite antigens can activate macrophages, and this may result in pro-inflammatory cytokine production, nitric oxide synthesis, and consequent control of parasitemia and mortality. Cell-mediated immunity in Trypanosoma cruzi infection is also modulated by cytokines, but in addition to parasite-specific responses, autoimmunity can be also triggered. Importantly, cytokines may also play a role in the cell-mediated immunity of infected subjects. Finally, leukocyte influx towards target tissues is regulated by cytokines, chemokines, and extracellular matrix components which may represent potential therapeutic targets in infected patients. Here we will discuss recent findings on the role of cytokines, chemokines and extracellular matrix components in the regulation of innate and adaptive immunity during T. cruzi infection.     

Modulation of immunity against herpes simplex virus infection via mucosal genetic transfer of plasmid DNA encoding chemokines

In this study, we examined the effects of murine chemokine DNA, as genetic adjuvants given mucosally, on the systemic and distal mucosal immune responses to plasmid DNA encoding gB of herpes simplex virus (HSV) by using the mouse model. The CC chemokines macrophage inflammatory protein 1beta (MIP-1beta) and monocyte chemotactic protein 1 (MCP-1) biased the immunity to the Th2-type pattern as judged by the ratio of immunoglobulin isotypes and interleukin-4 cytokine levels produced by CD4(+) T cells. The CXC chemokine MIP-2 and the CC chemokine MIP-1alpha, however, mounted immune responses of the Th1-type pattern, and such a response rendered recipients more resistant to HSV vaginal infection. In addition, MIP-1alpha appeared to act via the upregulation of antigen-presenting cell (APC) function and the expression of costimulatory molecules (B7-1 and B7-2), whereas MIP-2 enhanced Th1-type CD4(+) T-cell-mediated adaptive immunity by increasing gamma interferon secretion from activated NK cells. Our results emphasize the value of using the mucosal route to administer DNA modulators such as chemokines that function as adjuvants by regulating the activity of innate immunity. Our findings provide new insight into the value of CXC and CC chemokines, which act on different innate cellular components as the linkage signals between innate and adaptive immunity in mucosal DNA vaccination.     

Interrelationship between vitamins and cytokines in immunity

Cytokines are important proteins that modulate immunity and inflammation. Vitamins are also involved in immunity and inflammation. They are found to restore the ability of some cells to produce certain cytokines. Vitamin deficiency appears to affect the mechanism of immune cells, though the impact of reduced cytokine response in vitamin malnutrition is not clear. Vitamin D is involved in many medical conditions, such as infections and inflammation, and mediates innate immunity. Deficiency of vitamin D increases the risk of infectious and inflammatory diseases. In addition, this vitamin modulates Treg function and IL-10 production which is important for therapeutic treatment. Vitamin A increases inflammatory response and is involved in tissue damage; moreover, vitamin A is a key modulator of TGFbeta which can suppress several cytokines. Vitamin E, an anti-ageing compound, is associated with a defect of naive T cells and may inhibit some inflammatory compounds such as prostaglandin generation.