Deviation from a strong Th1-dominated to a modest Th17-dominated CD4 T cell response in the absence of IL-12p40 and type I IFNs sustains protective CD8 T cells

A tessellated lymphoid network provides whole-body T cell surveillance in zebrafish

Homeostatic trafficking to lymph nodes allows T cells to efficiently survey the host for cognate antigen. Nonmammalian jawed vertebrates lack lymph nodes but maintain diverse T cell pools. Here, we exploit in vivo imaging of transparent zebrafish to investigate how T cells organize and survey for antigen in an animal devoid of lymph nodes. We find that naïve-like T cells in zebrafish organize into a previously undescribed whole-body lymphoid network that supports streaming migration and coordinated trafficking through the host. This network has the cellular hallmarks of a mammalian lymph node, including naïve T cells and CCR7-ligand expressing nonhematopoietic cells, and facilitates rapid collective migration. During infection, T cells transition to a random walk that supports antigen-presenting cell interactions and subsequent activation. Our results reveal that T cells can toggle between collective migration and individual random walks to prioritize either large-scale trafficking or antigen search in situ. This lymphoid network thus facilitates whole-body T cell trafficking and antigen surveillance in the absence of a lymph node system.

A tessellated lymphoid network provides whole-body T cell surveillance in zebrafish

Homeostatic trafficking to lymph nodes allows T cells to efficiently survey the host for cognate antigen. Non-mammalian jawed vertebrates lack lymph nodes but maintain similarly diverse T cell pools. Here, we exploit in vivo imaging of transparent zebrafish to investigate how T cells organize and survey for antigen in an animal devoid of lymph nodes. We find that naïve-like T cells in zebrafish organize into a previously undescribed whole-body lymphoid network that supports streaming migration and coordinated trafficking through the host. This network has the cellular hallmarks of a mammalian lymph node, including naïve T cells and CCR7-ligand expressing non-hematopoietic cells, and facilitates rapid collective migration. During infection, T cells transition to a random walk that supports antigen presenting cell interactions and subsequent activation. Our results reveal that T cells can toggle between collective migration and individual random walks to prioritize either large-scale trafficking or antigen search in situ . This novel lymphoid network thus facilitates whole-body T cell trafficking and antigen surveillance in the absence of a lymph node system.

Significance Statement

In mammals, lymph nodes play a critical role in the initiation of adaptive immune responses by providing a dedicated place for T cells to scan antigen-presenting cells. Birds, reptiles, amphibians, and fish all maintain diverse repertoires of T cells but lack lymph nodes, raising questions about how adaptive immunity functions in lower jawed vertebrates. Here, we describe a novel network of lymphocytes in zebrafish that supports whole-body T cell trafficking and provides a site for antigen search, mirroring the function of mammalian lymph nodes. Within this network, T cells can prioritize large-scale trafficking or antigen scanning by toggling between two distinct modes of migration. This network provides valuable insights into the evolution of adaptive immunity.

Bovine Peripheral Blood Derived Lymphocyte Proteome and Secretome Show Divergent Reaction of Bovine Immune Phenotypes after Stimulation with Pokeweed Mitogen

We recently identified a deviant bovine immune phenotype characterized by hyperproliferation of lymphocytes after polyclonal stimulation. This phenotype was first discovered in dams that responded to PregSure BVD vaccination by producing pathological antibodies, triggering the fatal disease “bovine neonatal pancytopenia” in calves. The aim of the study was to gain deeper insights into molecular processes occurring in lymphocytes of immune phenotypes and the effect on their secretome after immune stimulation. Two discovery proteomic experiments were performed with unstimulated and Pokeweed Mitogen (PWM) stimulated lymphocytes, using label-free LC-MS/MS. In lymphocytes, 2447 proteins were quantified, and 1204 proteins were quantified in the secretome. Quantitative proteome analysis of immune deviant and control samples after PWM stimulation revealed clear differences. The increase in abundance of IL17A, IL17F, IL8, CCL5, LRRC59, and CLIC4 was higher in controls through mitogenic stimulation. In contrast, the abundance of IFNγ, IL2, IL2RA, CD83, and CD200 increased significantly more in immune deviant lymphocytes. Additional pathway enrichment analysis of differentially secreted proteins also yielded fundamental differences between the immune phenotypes. Our study provides a comprehensive dataset, which gives novel insights into proteome changes of lymphocytes from different bovine immune phenotypes. These differences point to the development of diverse immune responses of bovine immune phenotypes after immune stimulation.

Change of Dendritic Cell Subsets Involved in Protection Against Listeria monocytogenes Infection in Short-Term-Fasted Mice

The gastrointestinal tract is the first organ directly affected by fasting. However, little is known about how fasting influences the intestinal immune system. Intestinal dendritic cells (DCs) capture antigens, migrate to secondary lymphoid organs, and provoke adaptive immune responses. We evaluated the changes of intestinal DCs in mice with short-term fasting and their effects on protective immunity against Listeria monocytogenes (LM). Fasting induced an increased number of CD103⁺CD11b⁻ DCs in both small intestinal lamina propria (SILP) and mesenteric lymph nodes (mLN). The SILP CD103⁺CD11b⁻ DCs showed proliferation and migration, coincident with increased levels of GM-CSF and C-C chemokine receptor type 7, respectively. At 24 h post-infection with LM, there was a significant reduction in the bacterial burden in the spleen, liver, and mLN of the short-term-fasted mice compared to those fed ad libitum. Also, short-term-fasted mice showed increased survival after LM infection compared with ad libitum-fed mice. It could be that significantly high TGF-β2 and Aldh1a2 expression in CD103⁺CD11b⁻ DCs in mice infected with LM might affect to increase of Foxp3⁺ regulatory T cells. Changes of major subset of DCs from CD103⁺ to CD103⁻ may induce the increase of IFN-γ–producing cells with forming Th1-biased environment. Therefore, the short-term fasting affects protection against LM infection by changing major subset of intestinal DCs from tolerogenic to Th1 immunogenic.

Immunoadjunctive Therapy against Bacterial Infections Using Herbal Medicines Based on Th17 Cell-mediated Protective Immunity

One of the major health concerns in the world is the global increase in intractable bacterial infectious diseases due to the emergence of multi- and extensively drug-resistant bacterial pathogens as well as an increase in compromised hosts around the world. Particularly, in the case of mycobacteriosis, the high incidence of tuberculosis in developing countries, resurgence of tuberculosis in industrialized countries, and increase in the prevalence of Mycobacterium avium complex infections are important worldwide health concerns. However, the development of novel antimycobacterial drugs is currently making slow progress. Therefore, it is considered that devising improved administration protocols for clinical treatment against refractory mycobacteriosis using existing chemotherapeutics is more practical than awaiting the development of new antimycobacterial drugs. The regulation of host immune responses using immunoadjunctive agents may increase the efficacy of antimicrobial treatment against mycobacteriosis. The same situations also exist in cases of intractable infectious diseases due to common bacteria other than mycobacteria. The mild and long-term up-regulation of host immune reactions in hosts with intractable chronic bacterial infections, using herbal medicines and medicinal plants, may be beneficial for such immunoadjunctive therapy. This review describes the current status regarding basic and clinical studies on therapeutic regimens using herbal medicines, useful for the clinical treatment of patients with intractable bacterial infections. In particular, we focus on immunoadjunctive effects of herbal medicines on the establishment and manifestation of host antibacterial immunity related to the immunological roles of Th17 cell lineages.

TCR Affinity Controls the Dynamics but Not the Functional Specification of the Antimycobacterial CD4+ T Cell Response

The quality of T cell responses depends on the lymphocytes' ability to undergo clonal expansion, acquire effector functions, and traffic to the site of infection. Although TCR signal strength is thought to dominantly shape the T cell response, by using TCR transgenic CD4⁺ T cells with different peptide:MHC binding affinity, we reveal that TCR affinity does not control Th1 effector function acquisition or the functional output of individual effectors following mycobacterial infection in mice. Rather, TCR affinity calibrates the rate of cell division to synchronize the distinct processes of T cell proliferation, differentiation, and trafficking. By timing cell division-dependent IL-12R expression, TCR affinity controls when T cells become receptive to Th1-imprinting IL-12 signals, determining the emergence and magnitude of the Th1 effector pool. These findings reveal a distinct yet cooperative role for IL-12 and TCR binding affinity in Th1 differentiation and suggest that the temporal activation of clones with different TCR affinity is a major strategy to coordinate immune surveillance against persistent pathogens.

Insights into the mechanisms of Th17 differentiation and the Yin-Yang of Th17 cells in human diseases

Th17 cells are a lineage of CD4⁺ T helper cells with Th17-specific transcription factors RORγt and RoRα. Since its discovery in 2005, research on Th17 has been in rapid progress, and increasing cytokines or transcription factors have been uncovered in the activation and differentiation of Th17 cells. Furthermore, growing evidence proves there are two different subsets of Th17 cells, namely non-pathogenic Th17 (non-pTh17) and pathogenic Th17 (pTh17), both of which play important roles in adaptive immunity, especially in host defenses, autoimmune diseases, and cancer. In this review, we summarize and discuss the mechanisms of Th17 cells differentiation, and their roles in immunity and diseases.

A Perspective Of Intestinal Immune-Microbiome Interactions In Alcohol-Associated Liver Disease

Uncovering the intricacies of the gut microbiome and how it interacts with the host immune system has opened up pathways in the search for the treatment of disease conditions. Alcohol-associated liver disease is a major cause of death worldwide. Research has shed light on the breakdown of the protective gut barriers, translocation of gut microbes to the liver and inflammatory immune response to microbes all contributing to alcohol-associated liver disease. This knowledge has opened up avenues for alternative therapies to alleviate alcohol-associated liver disease based on the interaction of the commensal gut microbiome as a key player in the regulation of the immune response. This review describes the relevance of the intestinal immune system, the gut microbiota, and specialized and non-specialized intestinal cells in the regulation of intestinal homeostasis. It also reflects how these components are altered during alcohol-associated liver disease and discusses new approaches for potential future therapies in alcohol-associated liver disease.

Persistent Zika Virus Clinical Susceptibility despite Reduced Viral Burden in Mice with Expanded Virus-Specific CD8+ T Cells Primed by Recombinant Listeria monocytogenes

Vaccines against Zika virus (ZIKV) infection that target CD8⁺ T cells are of considerable interest because Abs may enhance infection susceptibility. However, whether CD8⁺ T cells are protective or promote susceptibility to clinical infection symptoms remains uncertain. To more precisely investigate ZIKV-specific CD8⁺ T cells in isolation, we engineered a Listeria monocytogenes-based vector to express a single MHC class I-restricted immune dominant peptide, E294-302, from ZIKV envelope protein. We show accumulation of activated ZIKV-specific CD8⁺ T cells primed by recombinant L. monocytogenes is associated with reductions in circulating virus levels after ZIKV challenge in type I IFN receptor-deficient mice and wildtype mice administered neutralizing Abs against type I IFN receptor. Interestingly, susceptibility to ZIKV clinical infection including weight loss and mortality each persists and is neither significantly improved nor worsened compared with isogenic L. monocytogenes-primed control mice. These data demonstrating persistent ZIKV clinical susceptibility despite reduced viral burden in mice with expanded virus-specific CD8⁺ T cells highlights the need for targeting other adaptive immune components in developing vaccines against ZIKV infection.

The protective efficacy of recombinant hypoxic response protein 1 of Nocardia seriolae in largemouth bass (Micropterus salmoides)

Nocardia seriolae has become one of the major pathogens affecting the aquaculture industry and causes Nocardiosis, a highly devastating disease of marine and freshwater fish that leads to severe economic losses. Therefore, research efforts towards developing efficacious vaccines to control this disease are of high importance. In this study, the hypoxic response protein 1 (HRP1) cloned into pET32a vector was expressed, and produced in Escherichia coli strain BL21 (DE3). The antigenicity of purified recombinant TRX-tagged HRP (rHRP1) was analysed by western blotting using largemouth bass anti-N. seriolae sera. The results showed that largemouth bass anti-N. seriolae sera could specifically detect a 33 kDa rHRP1 protein. Further, the vaccine efficacy of rHRP1 was evaluated in a largemouth bass fish model by calculating the relative percent survival (RPS). rHRP1 incurred an RPS of 73.33% as compared to the control group. Immunological analysis showed that rHRP1 could produce significantly higher serum concentrations of anti-N. seriolae antibodies and serum lysozyme activity as compared to the control groups. Quantitative reverse transcription polymerase chain reaction (qRT-PCR) analysis showed that rHRP1 significantly enhanced the expression of immune-related genes, such as IL-12p40, IL-8, IL-1β, TNFα, IFNγ, NKEF, MHCIα, MHCIIα, CD4-1, CD8α, IgM, NF-κβ, STAT3, IRF4, RORα, and CCL20. These results indicate that rHRP1 may be a promising vaccine candidate against nocardiosis.

Essential role of IκBNS for in vivo CD4+ T-cell activation, proliferation, and Th1-cell differentiation during Listeria monocytogenes infection in mice

Acquisition of effector functions in T cells is guided by transcription factors, including NF‐κB, that itself is tightly controlled by inhibitory proteins. The atypical NF‐κB inhibitor, IκB_NS, is involved in the development of Th1, Th17, and regulatory T (Treg) cells. However, it remained unclear to which extend IκB_NS contributed to the acquisition of effector function in T cells specifically responding to a pathogen during in vivo infection. Tracking of adoptively transferred T cells in Listeria monocytogenes infected mice antigen‐specific activation of CD4⁺ T cells following in vivo pathogen encounter to strongly rely on IκB_NS. While IκB_NS was largely dispensable for the acquisition of cytotoxic effector function in CD8⁺ T cells, IκB_NS‐deficient Th1 effector cells exhibited significantly reduced proliferation, marked changes in the pattern of activation marker expression, and reduced production of the Th1‐cell cytokines IFN‐γ, IL‐2, and TNF‐α. Complementary in vitro analyses using cells from novel reporter and inducible knockout mice revealed that IκB_NS predominantly affects the early phase of Th1‐cell differentiation while its function in terminally differentiated cells appears to be negligible. Our data suggest IκB_NS as a potential target to modulate specifically CD4⁺ T‐cell responses.

The Immunoregulation of Th17 in Host against Intracellular Bacterial Infection

T helper 17 cells (Th17) constitute a distinct subset of helper T cells with a unique transcriptional profile (STAT3, RORγ, and RORα), cytokine production pattern (IL17 family), and requirement of specific cytokines for their differentiation (TGF-β, IL6, IL21, and IL23). Recent studies involving experimental animals and humans have shown that Th17/IL17 plays a crucial role in host defense against a variety of pathogens, including bacteria and viruses. The underlying mechanisms by which Th17 performs include dendritic cell (DC) regulation, neutrophil recruitment, Th1 modulation, and T regulatory cell (Treg) balance. In recent years, researchers have generated an accumulating wealth of evidence on the role of Th17/IL17 in protective immunity to intracellular bacterial pathogens, such as Mycobacterium tuberculosis and Chlamydia trachomatis, which are one of the most important pathogens that inflict significant socioeconomic burden across the globe. In this article, we reviewed the current literature on the functions and mechanisms by which Th17/IL17 responds to intracellular bacterial infections. A better understanding of Th17/IL17 immunity to pathogens would be crucial for developing effective prophylactics and therapeutics.

Yin and yang of interleukin-17 in host immunity to infection

The interleukin-17 (IL-17) family cytokines, such as IL-17A and IL-17F, play important protective roles in host immune response to a variety of infections such as bacterial, fungal, parasitic, and viral. The IL-17R signaling and downstream pathways mediate induction of proinflammatory molecules which participate in control of these pathogens. However, the production of IL-17 can also mediate pathology and inflammation associated with infections. In this review, we will discuss the yin-and-yang roles of IL-17 in host immunity to pathogens.

Chlamydia pneumoniae induces interleukin-12 responses in peripheral blood mononuclear cells in asthma and the role of toll like receptor 2 versus 4: a pilot study

BACKGROUND

Chlamydia pneumoniae causes respiratory infection in adults and children, and has been associated with asthma exacerbations and induction of Immunoglobulin (Ig) E responses. We previously reported that C. pneumoniae enhances T helper (Th) 2 responses of peripheral blood mononuclear cells (PBMC) from asthmatic patients. It is likely that toll like receptor (TLR)-2 and TLR-4 mediate cytokine responses and host defense against C. pneumoniae. Thus, we sought to determine whether engagement of TLR-2 or TLR-4 may induce IL-12 production in our C. pneumoniae model.

METHODS

PBMC (1.5 × 10⁶) from asthmatic patients (N = 10) and non-asthmatic controls (N = 5) were infected or mock-infected for 1 h ± C. pneumoniae TW183 at a multiplicity of infection (MOI) = 1 and MOI = 0.1, and cultured for 48 h ± anti- TLR-2 and TLR-4 antibodies (Abs) (1 mg/mL). Interleukin (IL)-12 (48 h p.i.) and total IgE levels (day 10) were measured in supernatants (ELISA).

RESULTS

High IgE levels were detected in supernatants of C. pneumoniae- infected PBMC from asthmatics on day 10, compared with mock-infected PBMC (p < 0.03). In contrast, IgE was not detected (<0.3 ng/mL) in either C. pneumoniae infected or mock-infected PBMC from non-asthmatics. IL-12 production by C. pneumoniae-infected asthmatic and non-asthmatic PBMC were similar. When anti-TLR4, but not anti-TLR2, was included in culture, IL-12 production by C. pneumoniae- infected asthmatic PBMC decreased.

CONCLUSIONS

C. pneumoniae infection induces IgE production and modulates IL-12 responses in patients with asthma, which may be caused, in part, by differences in TLR-2 and TLR-4 stimulation.

Characterization of host responses induced by Toll-like receptor ligands in chicken cecal tonsil cells

The innate responses of cecal tonsils against invading microorganisms are mediated by conserved pattern recognition receptors (PRRs) such as the Toll-like receptors (TLRs). TLRs expressed by mammalian and avian immune system cells have the capability to recognize pathogen-associated molecular patterns (PAMPs). Although, the role of TLR ligands in innate and adaptive responses in chickens has been characterized in spleen and bursa of Fabricius, considerably less is known about responses in cecal tonsils. The aim of the current study was to assess responses of mononuclear cells from cecal tonsils to treatment with the TLR2, TLR4 and TLR21 ligands, Pam3CSK4, lipopolysaccharide (LPS), and CpG oligodeoxynucleotide (ODN), respectively. All three ligands induced significant up-regulation of interferon (IFN)-γ, interleukin (IL)-1β, IL-6 and CxCLi2/IL-8, whereas no significant changes were observed in expression of IL-13 or the antimicrobial peptides, avian β-defensin (AvBD) 1, AvBD2 and cathelicidin 3 (CATHL-3). In general, CpG ODN elicited the highest cytokine responses by cecal tonsil mononuclear cells, inducing significantly higher expression compared to LPS and Pam3CSK4, for IFNγ, IL-1β, IL-6 and CxCLi2 at various time points. These findings suggest the potential use of TLR21 ligands as mucosal vaccine adjuvants, especially in the context of pathogens of the intestinal tract.

Multiple Serum Cytokine Profiling to Identify Combinational Diagnostic Biomarkers in Attacks of Familial Mediterranean Fever

The precise cytokine networks in the serum of individuals with familial Mediterranean fever (FMF) that are associated with its pathogenesis have been unknown. Here, we attempted to identify specific biomarkers to diagnose or assess disease activity in FMF patients. We measured serum levels of 45 cytokines in 75 FMF patients and 40 age-matched controls by multisuspension cytokine array. FMF in "attack" or "remission" was classified by Japan College of Rheumatology-certified rheumatologists according to the Tel Hashomer criteria. Cytokines were ranked by their importance by a multivariate classification algorithm. We performed a logistic regression analysis to determine specific biomarkers for discriminating FMF patients in attack. To identify specific molecular networks, we performed a cluster analysis of each cytokine. Twenty-nine of the 45 cytokines were available for further analyses. Eight cytokines' serum levels were significantly elevated in the FMF attack versus healthy control group. Nine cytokines were increased in FMF attack compared to FMF remission. Multivariate classification algorithms followed by a logistic regression analysis revealed that the combined measurement of IL-6, IL-18, and IL-17 distinguished FMF patients in attack from the controls with the highest accuracy (sensitivity 89.2%, specificity 100%, and accuracy 95.5%). Among the FMF patients, the combined measurement of IL-6, G-CSF, IL-10, and IL-12p40 discriminated febrile attack periods from remission periods with the highest accuracy (sensitivity 75.0%, specificity 87.9%, and accuracy 84.0%). Our data identified combinational diagnostic biomarkers in FMF patients based on the measurement of multiple cytokines. These findings help to improve the diagnostic performance of FMF in daily practice and extend our understanding of the activation of the inflammasome leading to enhanced cytokine networks.

Nasal vaccination stimulates CD8(+) T cells for potent protection against mucosal Brucella melitensis challenge

Brucellosis remains a significant zoonotic threat worldwide. Humans and animals acquire infection via their oropharynx and upper respiratory tract following oral or aerosol exposure. After mucosal infection, brucellosis develops into a systemic disease. Mucosal vaccination could offer a viable alternative to conventional injection practices to deter disease. Using a nasal vaccination approach, the ΔznuA B. melitensis was found to confer potent protection against pulmonary Brucella challenge, and reduce colonization of spleens and lungs by more than 2500-fold, with more than 50% of vaccinated mice showing no detectable brucellae. Furthermore, tenfold more brucellae-specific, IFN-γ-producing CD8⁺ T cells than CD4⁺ T cells were induced in the spleen and respiratory lymph nodes. Evaluation of pulmonary and splenic CD8⁺ T cells from mice vaccinated with ΔznuA B. melitensis revealed that these expressed an activated effector memory (CD44^hiCD62L^loCCR7^lo) T cells producing elevated levels of IFN-γ, TNF-α, perforin, and granzyme B. To assess the relative importance of these increased numbers of CD8⁺ T cells, CD8^−/− mice were challenged with virulent B. melitensis, and they showed markedly increased bacterial loads in organs in contrast to similarly challenged CD4^−/− mice. Only ΔznuA B. melitensis- and Rev-1-vaccinated CD4^−/− and wild-type mice, not CD8^−/− mice, were completely protected against Brucella challenge. Determination of cytokines responsible for conferring protection showed the relative importance of IFN-γ, but not IL-17. Unlike wild-type mice, IL-17 was greatly induced in IFN-γ^−/− mice, but IL-17 could not substitute for IFN-γ’s protection, although an increase in brucellae dissemination was observed upon in vivo IL-17 neutralization. These results show that nasal ΔznuA B. melitensis vaccination represents an attractive means to stimulate systemic and mucosal immune protection via CD8⁺ T cell engagement.

Regulatory T cells and the immune pathogenesis of prenatal infection

Pregnancy in placental mammals offers exceptional comprehensive benefits of in utero protection, nutrition, and metabolic waste elimination for the developing fetus. However, these benefits also require durable strategies to mitigate maternal rejection of fetal tissues expressing foreign paternal antigens. Since the initial postulate of expanded maternal immune tolerance by Sir Peter Medawar 60 years ago, an amazingly elaborate assortment of molecular and cellular modifications acting both locally at the maternal-placental interface and systemically have been shown to silence potentially detrimental maternal immune responses. In turn, simultaneously maintaining host defense against the infinite array of potential pathogens during pregnancy is equally important. Fortunately, resistance against most infections is preserved seamlessly throughout gestation. On the other hand, recent studies on pathogens with unique predisposition for prenatal infections have uncovered distinctive holes in host defense associated with the reproductive process. Using these infections to probe the response during pregnancy, the immune suppressive regulatory subset of maternal CD4 T cells has been increasingly shown to dictate the inter-workings between prenatal infection susceptibility and pathogenesis of ensuing pregnancy complications. Herein, the recent literature suggesting a necessity for maternal regulatory T cells (Tregs) in pregnancy-induced immunological shifts that sustain fetal tolerance is reviewed. Additional discussion is focused on how expansion of maternal Treg suppression may become exploited by pathogens that cause prenatal infections and the perilous potential of infection-induced immune activation that may mitigate fetal tolerance and inadvertently inject hostility into the protective in utero environment.

The role of T helper (TH)17 cells as a double-edged sword in the interplay of infection and autoimmunity with a focus on xenobiotic-induced immunomodulation

Extensive research in recent years suggests that exposure to xenobiotic stimuli plays a critical role in autoimmunity induction and severity and that the resulting response would be exacerbated in individuals with an infection-aroused immune system. In this context, heavy metals constitute a prominent category of xenobiotic substances, known to alter divergent immune cell responses in accidentally and occupationally exposed individuals, thereby increasing the susceptibility to autoimmunity and cancer, especially when accompanied by inflammation-triggered persistent sensitization. This perception is learned from experimental models of infection and epidemiologic studies and clearly underscores the interplay of exposure to such immunomodulatory elements with pre- or postexposure infectious events. Further, the T_H17 cell subset, known to be associated with a growing list of autoimmune manifestations, may be the “superstar” at the interface of xenobiotic exposure and autoimmunity. In this review, the most recently established links to this nomination are short-listed to create a framework to better understand new insights into T_H17's contributions to autoimmunity.

Bacterial load and inflammation in fetal tissues is not dependent on IL-17a or IL-22 in 10-14 day pregnant mice infected with Listeria monocytogenes

In this study, we first assessed the effect of intragastric infection of pregnant mice with Listeria monocytogenes on relative expression of select genes associated with T cell subsets. Relative gene expression was moderately increased in placental tissues for IFNγ, IL-4, IL-17a, IL-22, CD3, and FoxP3. To assess the roles of IL-17a and IL-22 in resistance to listeriosis during pregnancy, we compared the severity of maternal and fetal infection in IL-17a((-/-)), IL-22((-/-)), and IL-17a((-/-))/IL-22((-/-)) mice with that of wild type C57BL/6 mice. Intragastric infection with modest numbers of bacterial cells (10(5) CFU) caused reproducible maternal and fetal infection in all four mouse strains. We recovered greater numbers of CFU from the bloodstream of pregnant IL-22((-/-)) mice than pregnant wild type mice. Otherwise we found no significant difference in bacterial load in maternal or fetal tissues (spleen, liver, fetoplacental units) from pregnant IL-17a((-/-)), IL-22((-/-)), or IL-17a((-/-))/IL-22((-/-)) or wild type mice. Nor did we observe histopathologic differences in severity of inflammation in maternal or fetal tissues from the various groups of mice. Although IL-17a and IL-22 are up-regulated in placental tissue, our study suggests that antibacterial resistance and the host inflammatory response are not dependent on IL-17a or IL-22 during infection of mice with L. monocytogenes at 10-14 days of gestation.

Pathogenesis of listeriosis during pregnancy

Listeria monocytogenes causes several clinical manifestations in humans and domestic animals. This bacterium is a saprophyte in soil and ensiled feeds, which are sources of infection for food producing animals (i.e. ruminants). The most common route of infection for people is via ingestion of contaminated ready-to-eat food products such as produce, soft cheeses and deli meats. In the United States, L. monocytogenes causes relatively few cases of clinical disease compared to other food-borne pathogens. However, clinical listeriosis is associated with high mortality, especially in immunocompromised patients, pregnant women, neonates, and the elderly. Listeria is an intracellular pathogen, which has been widely used in basic research to elucidate mechanisms of molecular pathogenesis and protective cell-mediated immunity. Despite the sizeable knowledge on L. monocytogenes pathogenesis, key points regarding listeriosis during pregnancy and the perinatal period remain unknown. This review summarizes listeriosis in humans and domestic animals during pregnancy, and animal models used to study the pathogenesis and immune response to L. monocytogenes infection during these periods.

Anti-cancer versus cancer-promoting effects of the interleukin-17-producing T helper cells

Research on T helper 17 (Th17) cells with regard to immunoediting has revealed elusive results. Whereas enhanced Th17 response and related molecules such as interleukin (IL)-17, IL-21, IL-22, IL-23 and STAT3 accompanied tumor induction and progression, finding that tumor growth/stage was negatively correlated with increased infiltration of Th17 cells in the tumor mass has prompted elucidation of various antitumor mechanisms elicited by Th17 and their related molecules. The pro-tumor efficacy of Th17 response included promotion of neutrophilia and induction of angiogenic (e.g. VEGF, MMP2 and MMP9) and anti-apoptotic factors (e.g. Bcl-XL), as well as expansion and activation of myeloid-derived suppressor cells, which facilitate generation of tumor-specific regulatory T cells. Other tumor immunogenic settings revealed anti-tumor pathways including induction of cytotoxic activity, expression of MHC antigens, the ability Th17 cells to reside within the tumor, and to convert into IFN-γ producers. Notably, Th17 cell related molecules exert indirect pro- or anti-tumor effects via inducing viral persistence or mediating protective mechanisms against bacterial and viral infection. Herein, the recent literature revealing such immunoediting events mediated by Th17 cells and their associated molecules as delivered by various experimental regimens and observed in cancer patient are revised, with a focus on some proposed anti-cancer therapies.

Innate IFN-γ is essential for programmed death ligand-1-mediated T cell stimulation following Listeria monocytogenes infection

Although best characterized for sustaining T cell exhaustion during persistent viral infection, programmed death ligand-1 (PDL-1) also stimulates the expansion of protective T cells after infection with intracellular bacterial pathogens. Therefore, establishing the molecular signals that control whether PDL-1 stimulates immune suppression or activation is important as immune modulation therapies based on manipulating PDL-1 are being developed. In this study, the requirement for PDL-1 blockade initiated before infection with the intracellular bacterium Listeria monocytogenes in reducing pathogen-specific T cell expansion is demonstrated. In turn, the role of proinflammatory cytokines triggered early after L. monocytogenes infection in controlling PDL-1-mediated T cell stimulation was investigated using mice with targeted defects in specific cytokines or cytokine receptors. These experiments illustrate an essential role for IL-12 or type I IFNs in PDL-1-mediated expansion of pathogen-specific CD8(+) T cells. Unexpectedly, direct stimulation by neither IL-12 nor type I IFNs on pathogen-specific CD8(+) cells was essential for PDL-1-mediated expansion. Instead, the absence of early innate IFN-γ production in mice with combined defects in both IL-12 and type I IFNR negated the impacts of PDL-1 blockade. In turn, IFN-γ ablation using neutralizing Abs or in mice with targeted defects in IFN-γR each eliminated the PDL-1-mediated stimulatory impacts on pathogen-specific T cell expansion. Thus, innate IFN-γ is essential for PDL-1-mediated T cell stimulation.

Current investigational drugs in psoriasis

INTRODUCTION

The advent of biologic therapies has revolutionized the treatment of psoriasis. Increased understanding of immunogenetic pathways has allowed for the development of more selective targeted biologic therapies. Multiple new treatments are currently in development for the treatment of psoriasis. Preliminary data for many of these agents, particularly with regard to agents targeting the IL-23/Th17 pathway, are promising. Proven long-term safety, however, is an absolute necessity with newly developed drugs, and should, therefore, still be considered second-line agents to current established treatments with long-term safety data.

AREAS COVERED

This review details the mechanisms of action of drugs currently in development or in clinical trials for the treatment of psoriasis, using clinical trial registries and associated publications. Readers will gain a comprehensive overview about the mechanism of action of emerging treatments targeting various immune pathways deeply involved in psoriasis. Pathogenesis, clinical efficacy and safety data for these treatments are discussed where available.

EXPERT OPINION

Psoriasis remains a heavily undertreated systemic immune-mediated disease despite increased understanding of immunopathogenesis of the disease and advent of a multitude of novel therapeutic agents with potentially improved bioavailability and safety profiles. Limitations, however, remain in the realm of topical agents for treatment of mild to moderate psoriasis, which has seen little progress over the years. A concerted effort will need to be made among researchers, clinicians and patient advocacy groups to ensure new therapeutic agents are developed and gain proper exposure.

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 naïve 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.

Interleukin (IL)-21-independent pathogen-specific CD8+ T-cell expansion, and IL-21-dependent suppression of CD4+ T-cell IL-17 production

Although interleukin-21 (IL-21) potently activates and controls the differentiation of immune cells after stimulation in vitro, the role for this pleiotropic cytokine during in vivo infection remains poorly defined. Herein, the requirement for IL-21 in innate and adaptive host defence after Listeria monocytogenes infection was examined. In the innate phase, IL-21 deficiency did not cause significant defects in infection susceptibility, or in the early activation of natural killer and T cells. In the adaptive phase, L. monocytogenes-specific CD8(+) T cells expand to a similar magnitude in IL-21-deficient mice compared with control mice. Interestingly, the IL-21-independent expansion of L. monocytogenes-specific CD8(+) T cells was maintained even in the combined absence of IL-12 and type I interferon (IFN) receptor. Similarly, L. monocytogenes-specific CD4(+) T cells expanded and produced similar levels of IFN-γ regardless of IL-21 deficiency. Unexpectedly however, IL-21 deficiency caused significantly increased CD4(+) T-cell IL-17 production, and this effect became even more pronounced after L. monocytogenes infection in mice with combined defects in both IL-12 and type I IFN receptor that develop a T helper type 17-dominated CD4(+) T-cell response. Despite increased CD4(+) T-cell IL-17 production, L. monocytogenes-specific T cells re-expanded and conferred protection against secondary challenge with virulent L. monocytogenes regardless of IL-21 deficiency, or combined defects in IL-21, IL-12, and type I IFN receptor. Together, these results demonstrate non-essential individual and combined roles for IL-21, IL-12 and type I IFNs in priming pathogen-specific CD8(+) T cells, and reveal IL-21-dependent suppression of IL-17 production by CD4(+) T cells during in vivo infection.

Fidelity of pathogen-specific CD4+ T cells to the Th1 lineage is controlled by exogenous cytokines, interferon-gamma expression, and pathogen lifestyle

The degree of lineage stability achieved by pathogen-specific CD4(+) T cells in vivo, and how this impacts host defense against infection, remains unclear. We demonstrate that in response to Th1-polarizing intracellular bacterial or viral pathogens, only 80%-90% of responding polyclonal T cells become indelibly committed to this lineage. Th1 commitment was nearly invariant in cells that proliferated extensively, but perturbations to the extrinsic cytokine milieu or the pathogen's ability to enter the cytosol impeded commitment and promoted plasticity for future IL-17 expression. Conversely, cell-intrinsic interferon-gamma expression and acquisition of permissive chromatin at the Ifng gene during priming predicted heritable Th1 commitment. Importantly, CD4(+) T cells that retained plasticity conferred protection against Mycobacterium tuberculosis, while these protective effects were abolished with Th17 polarization. These findings illustrate the immune signals that induce memory CD4(+) T cell responses required for maintaining host defense against infection yet are adaptable in novel environmental contexts.

Lack of TNFR p55 results in heightened expression of IFN-γ and IL-17 during the development of reactive arthritis

Reactive arthritis (ReA) is a type of arthritis originating from certain gastrointestinal or genitourinary infections. In previous studies, we reported the development of progressive Yersinia enterocolitica-induced ReA in mice lacking TNFR p55; however, the mechanisms underlying this effect are still uncertain. In this study, we investigated the impact of TNFR p55 deficiency in modulating Ag-specific Th1 and Th17 responses during this arthritogenic process. We found more severe ReA in TNFRp55(-/-) mice compared with their wild-type (WT) counterparts. This effect was accompanied by increased levels of Yersinia LPS in the joints of knockout mice. Analysis of the local cytokine profile revealed greater amounts of IFN-γ and IL-17 in arthritic joints of TNFRp55(-/-) mice compared with WT mice at day 21 postinfection. Moreover, altered IL-17 and IFN-γ production was observed in mesenteric and inguinal lymph nodes of Yersinia-infected TNFRp55(-/-) mice, as well as in spleen cells obtained from infected mice and restimulated ex vivo with bacterial Ags. Increased levels of cytokine secretion were associated with a greater frequency of CD4(+)IL-17(+), CD4(+)IFN-γ(+), and IL-17(+)IFN-γ(+) cells in TNFRp55(-/-) mice compared with WT mice. Remarkably, Ab-mediated blockade of IL-17 and/or IFN-γ resulted in reduced joint histological scores in TNFRp55(-/-) mice. A mechanistic analysis revealed the involvement of p40, a common subunit of heterodimeric IL-12 and IL-23, in the generation of augmented IFN-γ and IL-17 production under TNFR p55 deficiency. Taken together, these data indicate that, in the absence of TNFR p55 signaling, Th1 and Th17 effector cells may act in concert to sustain the inflammatory response in bacterial-induced arthritogenic processes.

The roles of IL-12 and IL-23 in CD8+ T cell-mediated immunity against Listeria monocytogenes: Insights from a DC vaccination model

Listeria monocytogenes infection induces a strong inflammatory response characterized by the production of IL-12 and IFN-gamma and protective immunity against this pathogen is dependent on CD8+ T cells (CTL). Recent studies have suggested that these inflammatory cytokines affect the rate of memory CD8+ T cell generation as well as the number of short-lived effector cells generated. The role of the closely related cytokine, IL-23, in this response has not been examined. We hypothesized that IL-12 and IL-23 produced by dendritic cells collectively enhance the generation and function of memory cells. To test this hypothesis, we employed a DC vaccination approach. Mice lacking IL-12 and IL-23 were vaccinated with wild-type (WT), IL-12(-/-), or IL-12/23(-/-) DC and protection to Lm was monitored. Mice vaccinated with WT and IL-12(-/-) DC were resistant to lethal challenge with Lm. Surprisingly, mice vaccinated with IL-12/23(-/-) DC exhibited significantly reduced protection when challenged. Protection correlated with the relative size of the memory pools generated. In summary, these data indicate that IL-23 can partially compensate for the lack of IL-12 in the generation protective immunity against Lm.

Inflammatory cytokines as a third signal for T cell activation

CD8 T cells require a third signal, along with Ag and costimulation, to make a productive response and avoid death and/or tolerance induction. Recent studies indicate that IL-12 and Type I IFN (IFNalpha/beta) are the major sources of signal 3 in a variety of responses, and that the two cytokines stimulate a common regulatory program involving altered expression of about 350 genes. Signal 3-driven chromatin remodeling is likely to play a major role in this regulation. Although less well studied, there is emerging evidence that CD4 T cells may also require a 'third signal' for a productive response and that IL-1 can provide this signal. Signal 3 cytokines can replace adjuvants in supporting in vivo T cell responses to peptide and protein antigens, and a better understanding of their activities and mechanisms should contribute to more rational design of vaccines.

MyD88 and interferon-alpha/beta are differentially required for dendritic cell maturation but dispensable for development of protective memory against Listeria

Signalling pathways mediated by MyD88 are important for sensing Toll-like receptor (TLR) ligands and directing an immune response. However, the influence of MyD88-derived cytokines and interferon (IFN)-alpha/beta, the latter being made by both MyD88-dependent and -independent pathways, in phenotypic and functional dendritic cell (DC) maturation during infection is poorly understood. Here we investigate the contribution of MyD88-dependent and -independent pathways to DC maturation, CD8 T-cell activation and the generation of protective memory against Listeria monocytogenes. We show that neither MyD88 deficiency alone nor MyD88/IFN-alphabetaR double deficiency alters Listeria-induced costimulatory molecule up-regulation on DCs in vivo. In contrast, DCs from infected IFN-alphabetaR(-/-) mice had higher CD80 and CD86 expression than wild-type DCs. We then examined the function of DCs matured in infected knockout mice. We found that DCs from Listeria-infected MyD88(-/-) and MyD88(-/-) IFN-alphabetaR(-/-) mice induced little or no IFN-gamma by CD8 T cells, respectively. In contrast, DCs from infected IFN-alphabetaR(-/-) mice had a greater capacity to induce IFN-gamma compared with DCs from infected wild-type mice. When the CD8 T-cell memory response was analysed, infected MyD88(-/-) and MyD88(-/- )IFN-alphabetaR(-/-) mice were found to have fewer bacteria-specific memory CD8 T cells than wild-type mice. However, the fraction of bacteria-specific CD8 T cells making IFN-gamma was similar in all mouse strains, and MyD88(-/-) and MyD88(-/- )IFN-alphabetaR(-/-) mice survived lethal challenge. Together the data suggest an inhibitory effect of IFN-alpha/beta on functional DC maturation during Listeria infection and reveal overlapping roles of MyD88-induced cytokines and IFN-alpha/beta in DC maturation and protective anti-Listeria immunity.

IL-23 is required for protection against systemic infection with Listeria monocytogenes

Listeria monocytogenes (LM) is a Gram-positive, intracellular bacterium that can induce spontaneous abortion, septicemia, and meningitis. Although it is known that neutrophils are required for elimination of the bacteria and for survival of the host, the mechanisms governing the recruitment of neutrophils to LM-infected tissues are not fully understood. We demonstrate here that IL-23 and the IL-17 receptor A (IL-17RA), which mediates both IL-17A and IL-17F signaling, are necessary for resistance against systemic LM infection. LM-infected IL-23p19 knockout (KO) mice have decreased production of IL-17A and IL-17F, while IFN-gamma production is not altered by the lack of IL-23. LM induces the production of IL-17A from gammadelta T cells, but not CD4, CD8, or NK cells. Furthermore, a lack of efficient neutrophil recruitment to the liver is evident in both IL-23p19 KO and IL-17RA KO mice during LM infection. Immunocytochemical analysis of infected livers revealed that neutrophils were able to localize with LM in IL-23p19 KO and IL-17RA KO mice, indicating that IL-23 and IL-17RA do not regulate the precise localization of neutrophils with LM. The importance of IL-23-induced IL-17A was demonstrated by injecting IL-23p19 KO mice with recombinant IL-17A. These mice had reduced LM bacterial burdens compared with IL-23p19 KO mice that did not receive IL-17A. These results indicate that during LM infection, IL-23 regulates the production of IL-17A and IL-17F from gammadelta T cells, resulting in optimal liver neutrophil recruitment and enhanced bacterial clearance.

Neutrophils and macrophages work in concert as inducers and effectors of adaptive immunity against extracellular and intracellular microbial pathogens

Emerging data suggest new facets of the concerted participation of neutrophils and macrophages in antimicrobial immunity. The classical view is that DCs and macrophages are the inducers of adaptive antimicrobial immunity, but there is evidence for neutrophil participation in this task as cytokine and chemokine producers and APCs. On the other hand, the concept that the T(H)1 response is only associated with control of infections by intracellular pathogens through activation of macrophages by IFN-gamma, and the T(H)17/IL-17 axis is only involved in protection against extracellular pathogens through mobilization and activation of neutrophils is simplistic: There is evidence suggesting that T(H)1 and T(H)17 responses, separately or in parallel, may use macrophages and neutrophils against infections by extracellular and intracellular microbial pathogens. Opsonization by pathogen-specific Igs enhances the antimicrobial capabilities of neutrophils and macrophages in infections by extracellular and intracellular microbes. The functional partnership between macrophages and neutrophils as inducers and effectors of adaptive antimicrobial immunity conforms to their affiliation with the myeloid phagocyte system and reveals a strategy based on the concurrent use of the two professional phagocytes in the adaptive defense mechanisms. Starting from a common myeloid precursor in the bone marrow, macrophages and neutrophils split during differentiation but come together at the infectious foci for a cooperative strategy that uses modulator and effector activities to attack invading microbial pathogens.

Naturally occurring altered peptide ligands control Salmonella-specific CD4+ T cell proliferation, IFN-gamma production, and protective potency

T cell activation required for host defense against infection is an intricately regulated and precisely controlled process. Although in vitro studies indicate that three distinct stimulatory signals are required for T cell activation, the precise contribution of each signal in regulating T cell proliferation and differentiation after in vivo infection is unknown. In this study, altered peptide ligands (APLs) derived from the protective Salmonella-specific FliC Ag and CD4+ T cells specific for the immune-dominant FliC(431-439) peptide within this Ag were used to determine how changes in TCR stimulation impact CD4+ T cell proliferation, differentiation, and protective potency. To explore the prevalence and potential use of altered TCR stimulation by bacterial pathogens, naturally occurring APLs containing single amino acid substitutions in putative TCR contact residues within the FliC(431-439) peptide were identified and used for stimulation under both noninfection and infection conditions. On the basis of this analysis, naturally-occurring APLs that prime proliferation of FliC-specific CD4+ T cells either more potently or less potently compared with the wild-type FliC(431-439) peptide were identified. Remarkably, despite these differences in proliferation, all of the APLs primed reduced IFN-gamma production by FliC(431-439)-specific CD4+ T cells after stimulation in vivo. Moreover, after expression of the parental FliC(431-439) peptide or each APL in recombinant Listeria monocytogenes, only CD4+ T cells stimulated with the wild-type FliC(431-439) peptide conferred significant protection against challenge with virulent Salmonella. These results reveal important and unanticipated roles for TCR stimulation in controlling pathogen-specific CD4+ T cell proliferation, differentiation, and protective potency.

Plasticity of T-cell phenotype and function: the T helper type 17 example

Mature T helper type 1 (Th1) and Th2 cells antagonize the development of the opposing subset to sustain lineage-specific responses. However, the recent identification of a third distinct subset of helper T cells - the Th17 lineage - collapses the established Th1/Th2 dichotomy and raises intriguing questions about T-cell fate. In this review, we discuss the Th17 subset in the context of the effector and regulatory T-cell lineages. Initial studies suggested reciprocal developmental pathways between Th17/Th1 subsets and between Th17/regulatory T-cell subsets, and identified multiple mechanisms by which Th1 and Th2 cells antagonize the generation of Th17 cells. However, recent observations reveal the susceptibility of differentiated Th17 cells to Th1 polarization and the enhancement of Th17 memory cells by the Th1 factors interferon-gamma and T-bet. In addition, new data indicate late-stage plasticity of a subpopulation of regulatory T cells, which can be selectively induced to adopt a Th17 phenotype. Elucidating the mechanisms that undermine cross-lineage suppression and facilitate these phenotype shifts will not only clarify the flexibility of T-cell differentiation, but may also shed insight into the pathogenesis of autoimmunity and cancer. Furthermore, understanding these phenomena will be critical for the design of immunotherapy that seeks to disrupt lineage-specific T-cell responses and may suggest ways to manipulate the balance between pathogenic and regulatory lymphocytes for the restoration of homeostasis.

Precarious balance: Th17 cells in host defense

Lineage-specific responses from the effector T-cell repertoire form a critical component of adaptive immunity. The recent identification of Th17 cells-a third, distinct lineage of helper T cells-collapses the long-accepted paradigm in which Th1 and Th2 cells distinctly mediate cellular and humoral immunity, respectively. In this minireview, we discuss the involvement of the Th17 lineage during infection by extracellular bacteria, intracellular bacteria, and fungi. Emerging trends suggest that the Th17 population bridges innate and adaptive immunity to produce a robust antimicrobial inflammatory response. However, because Th17 cells mediate both host defense and pathological inflammation, elucidation of mechanisms that attenuate but do not completely abolish the Th17 response may have powerful implications for therapy.

IL-23 promotes the production of IL-17 by antigen-specific CD8 T cells in the absence of IL-12 and type-I interferons

In contrast to CD4 T cells, CD8 T cells inherently differentiate into IFN-gamma-producing effectors. Accordingly, while generation of IFN-gamma-producing Th1 CD4 T cells was profoundly impaired in mice deficient for both type-I IFN and IL-12 signaling in response to infection with Listeria monocytogenes, generation of Ag-specific, IFN-gamma-producing CD8 T cells was unimpaired. However, a fraction of these CD8 T cells also produced IL-17 in an IL-23-dependent manner. Furthermore, the addition of IL-23 in vitro was sufficient for some naive CD8 T cells to differentiate into IFN-gamma/IL-17 dual-producing cells and was associated with increased expression of ROR-gammat and ROR-alpha. Addition of IL-6 and TGF-beta to IL-23 further augmented ROR-gammat and ROR-alpha expression and suppressed Eomes expression, thereby enhancing IL-17 production by CD8 T cells. A loss of cytotoxic function accompanied the production of IL-17, as the addition of IL-6 and TGF-beta resulted in a marked reduction of granzyme B and perforin expression. Thus, CD8 T cells retain sufficient plasticity to respond to environmental cues and can acquire additional effector functions in response to their environmental context.

T-bet-dependent regulation of CD8+ T-cell expansion during experimental Trypanosoma cruzi infection

The transcription factor T-bet (T-box, expressed in T cells), promotes type I immunity to pathogens through effects involving T cells and dendritic cells. In CD8(+) T cells, many of the functions of T-bet are redundant with those of eomesodermin (Eomes), a paralogue of T-bet. We therefore investigated the role of T-bet in immunity to Trypanosoma cruzi, an intracellular pathogen that causes Chagas disease, and which requires CD8(+) T cells for resistance. T-bet-deficient mice (tbx21(-/-)) were highly susceptible to T. cruzi infection, marked by severe liver pathology. CD8(+) T cells from infected tbx21(-/-) mice expressed typical markers of activation, including CD44 and CD25. In striking contrast, there was a 10-fold reduction in the number of antigen-specific CD8(+) T cells in tbx21(-/-) mice. This reduction was not a consequence of increased apoptosis or altered tissue-specific migration. Further, antigen-presenting cell (APC) functions in tbx21(-/-) mice were normal as we observed comparable levels of B7-1, B7-2 and CD40 expression as well as normal antigen-driven proliferation of wild-type CD8(+) T cells in infected tbx21(-/-) mice. However, adoptive transfer of naïve T cells from tbx21(-/-) donors into infected Rag-2-deficient mice (tbx21(+/+)) demonstrated a similar quantitative defect in CD8(+) T-cell expansion. These data demonstrate that T-bet facilitates immunity to T. cruzi by promoting the expansion of T. cruzi-specific CD8(+) T cells in a T cell-intrinsic manner. They also serve to further illustrate the multifaceted functions of T-box proteins in regulating quantitative aspects of T-cell immunity, in addition to qualitative components such as cytokine production.

Programming for CD8 T cell memory development requires IL-12 or type I IFN

Inflammation can have both positive and negative effects on development of CD8 T cell memory, but the relative contributions and cellular targets of the cytokines involved are unclear. Using CD8 T cells lacking receptors for IL-12, type I IFN, or both, we show that these cytokines act directly on CD8 T cells to support memory formation in response to vaccinia virus and Listeria monocytogenes infections. Development of memory to vaccinia is supported predominantly by IL-12, whereas both IL-12 and type I IFN contribute to memory formation in response to Listeria. In contrast to memory formation, the inability to respond to IL-12 or type I IFN had a relatively small impact on the level of primary expansion, with at most a 3-fold reduction in the case of responses to Listeria. We further show that programming for memory development by IL-12 is complete within 3 days of the initial naive CD8 T cell response to Ag. This programming does not result in formation of a population that expresses killer cell lectin-like receptor G1, and the majority of the resulting memory cells have a CD62L(high) phenotype characteristic of central memory cells. Consistent with this, the cells undergo strong expansion upon rechallenge and provide protective immunity. These data demonstrate that IL-12 and type I IFN play an essential early role in determining whether Ag encounter by naive CD8 T cells results in formation of a protective memory population.

Selective priming and expansion of antigen-specific Foxp3- CD4+ T cells during Listeria monocytogenes infection

The Foxp3-expressing subset of regulatory CD4(+) T cells have defined Ag specificity and play essential roles in maintaining peripheral tolerance by suppressing the activation of self-reactive T cells. Similarly, during chronic infection, pathogen-specific Foxp3-expressing CD4(+) T cells expand and actively suppress pathogen-specific effector T cells. Herein, we used MHC class II tetramers and Foxp3(gfp) knockin mice to track the kinetics and magnitude whereby pathogen-specific Foxp3(+)CD4(+) and Foxp3(-)CD4(+) cells are primed and expand after acute infection with recombinant Listeria monocytogenes (Lm) expressing the non-"self"-Ag 2W1S(52-68). We demonstrate that Lm infection selectively primes proliferation, expansion, and subsequent contraction of Lm-specific Foxp3(-) effector CD4(+) cells, while the numbers of Lm-specific Foxp3(+)CD4(+) regulatory cells remain essentially unchanged. In sharp contrast, purified 2W1S(52-68) peptide primes coordinated expansion of both Foxp3(+) regulatory and Foxp3(-) effector T cells with the same Ag specificity. Taken together, these results indicate selective priming and expansion of Foxp3(-) CD4 T cells is a distinguishing feature for acute bacterial infection.

Interleukin-17 in host defence against bacterial, mycobacterial and fungal pathogens

The mammalian immune system is intricately regulated, allowing for potent pathogen-specific immunity to be rapidly activated in response to infection with a broad and diverse array of potential pathogens. As a result of their ability to differentiate into distinct effector lineages, CD4 T cells significantly contribute to pathogen-specific adaptive immune responses. Through the production of effector cytokines, CD4 T helper (Th) cells orchestrate the precise mobilization of specific immune cells to eradicate infection. The protective effects of the newly identified lineage of Th17 cells against pathogens like Klebsiella pneumoniae, Citrobacter rodentium and Candida albicans indicate the capacity of Th17 cells to confer protection against extracellular bacterial and fungal pathogens, filling a critical void in host immunity not covered by the classically described Th1 lineage that activates immunity to intracellular pathogens or the Th2 lineage that is important in protection against mucosal parasitic pathogens. Host defence by Th17 cells extends beyond protection against extracellular bacterial and fungal pathogens, as demonstrated in infections against intracellular bacteria like Listeria monocytogenes and Salmonella enterica, as well as Mycobacterium tuberculosis. Herein, we summarize both experimental data from mouse infection models and epidemiological studies in humans that demonstrate the protective effects of interleukin-17 and Th17 CD4 T cells in immunity to bacterial, mycobacterial and fungal pathogens.

Cytotoxic T-lymphocyte antigen 4 blockade augments the T-cell response primed by attenuated Listeria monocytogenes resulting in more rapid clearance of virulent bacterial challenge

Cytotoxic T-lymphocyte antigen 4 (CTLA-4) uniformly suppresses antigen-specific T cells during chronic infection with bacterial, parasitic or viral pathogens. However, the importance of CTLA-4 in controlling the T-cell response during acute infection or after priming with live attenuated vaccine vectors has not been well characterized. Since strategies aimed at blocking CTLA-4 are being actively developed to therapeutically augment T-cell-mediated immunity, the effects of CTLA-4 blockade on T-cell activation during these conditions need to be more clearly defined. We have examined the role of CTLA-4 in a prime-challenge model of acute bacterial infection using both attenuated and virulent strains of the intracellular bacterium Listeria monocytogenes. Although Foxp3(+) CD4(+) T cells are the predominant CTLA-4-expressing cell type in naïve mice, antigen-specific Foxp3(-) CD4(+) cells upregulate CTLA-4 expression after primary L. monocytogenes infection. Blockade of CTLA-4 results in increased numbers of L. monocytogenes-specific CD4 and CD8 T cells after primary infection with attenuated L. monocytogenes, and confers more rapid bacterial clearance after secondary challenge with virulent L. monocytogenes. Accordingly, CTLA-4 plays an important suppressive role in T-cell priming and protective immunity in a prime-challenge model of acute bacterial infection.