CpG DNA inhibits CD4+CD25+ Treg suppression through direct MyD88-dependent costimulation of effector CD4+ T cells

Toll-Like Receptor Signaling and Its Role in Cell-Mediated Immunity

Innate immunity is the first defense system against invading pathogens. Toll-like receptors (TLRs) are well-defined pattern recognition receptors responsible for pathogen recognition and induction of innate immune responses. Since their discovery, TLRs have revolutionized the field of immunology by filling the gap between the initial recognition of pathogens by innate immune cells and the activation of the adaptive immune response. TLRs critically link innate immunity to adaptive immunity by regulating the activation of antigen-presenting cells and key cytokines. Furthermore, recent studies also have shown that TLR signaling can directly regulate the T cell activation, growth, differentiation, development, and function under diverse physiological conditions. This review provides an overview of TLR signaling pathways and their regulators and discusses how TLR signaling, directly and indirectly, regulates cell-mediated immunity. In addition, we also discuss how TLR signaling is critically important in the host's defense against infectious diseases, autoimmune diseases, and cancer.

Toll-Like Receptor Signaling and Its Role in Cell-Mediated Immunity

Innate immunity is the first defense system against invading pathogens. Toll-like receptors (TLRs) are well-defined pattern recognition receptors responsible for pathogen recognition and induction of innate immune responses. Since their discovery, TLRs have revolutionized the field of immunology by filling the gap between the initial recognition of pathogens by innate immune cells and the activation of the adaptive immune response. TLRs critically link innate immunity to adaptive immunity by regulating the activation of antigen-presenting cells and key cytokines. Furthermore, recent studies also have shown that TLR signaling can directly regulate the T cell activation, growth, differentiation, development, and function under diverse physiological conditions. This review provides an overview of TLR signaling pathways and their regulators and discusses how TLR signaling, directly and indirectly, regulates cell-mediated immunity. In addition, we also discuss how TLR signaling is critically important in the host's defense against infectious diseases, autoimmune diseases, and cancer.

Toll-Like Receptors in Adaptive Immunity

The immune (innate and adaptive) system has evolved to protect the host from any danger present in the surrounding outer environment (microbes and associated MAMPs or PAMPs, xenobiotics, and allergens) and dangers originated within the host called danger or damage-associated molecular patterns (DAMPs) and recognizing and clearing the cells dying due to apoptosis. It also helps to lower the tissue damage during trauma and initiates the healing process. The pattern recognition receptors (PRRs) play a crucial role in recognizing different PAMPs or MAMPs and DAMPs to initiate the pro-inflammatory immune response to clear them. Toll-like receptors (TLRs) are first recognized PRRs and their discovery proved milestone in the field of immunology as it filled the gap between the first recognition of the pathogen by the immune system and the initiation of the appropriate immune response required to clear the infection by innate immune cells (macrophages, neutrophils, dendritic cells or DCs, and mast cells). However, in addition to their expression by innate immune cells and controlling their function, TLRs are also expressed by adaptive immune cells. We have identified 10 TLRs (TLR1-TLR10) in humans and 12 TLRs (TLR1-TLR13) in laboratory mice till date as TLR10 in mice is present only as a defective pseudogene. The present chapter starts with the introduction of innate immunity, timing of TLR evolution, and the evolution of adaptive immune system and its receptors (T cell receptors or TCRs and B cell receptors or BCRs). The next section describes the role of TLRs in the innate immune function and signaling involved in the generation of inflammation. The subsequent sections describe the expression and function of different TLRs in murine and human adaptive immune cells (B cells and different types of T cells, including CD4⁺T cells, CD8⁺T cells, CD4⁺CD25⁺T_regs, and CD8⁺CD25⁺T_regs, etc.). The modulation of TLRs expressed on T and B cells has a great potential to develop different vaccine candidates, adjuvants, immunotherapies to target various microbial infections, including current COVID-19 pandemic, cancers, and autoimmune and autoinflammatory diseases.

Cancer Vaccines: Adjuvant Potency, Importance of Age, Lifestyle, and Treatments

Although the discovery and characterization of multiple tumor antigens have sparked the development of many antigen/derived cancer vaccines, many are poorly immunogenic and thus, lack clinical efficacy. Adjuvants are therefore incorporated into vaccine formulations to trigger strong and long-lasting immune responses. Adjuvants have generally been classified into two categories: those that ‘depot’ antigens (e.g. mineral salts such as aluminum hydroxide, emulsions, liposomes) and those that act as immunostimulants (Toll Like Receptor agonists, saponins, cytokines). In addition, several novel technologies using vector-based delivery of antigens have been used. Unfortunately, the immune system declines with age, a phenomenon known as immunosenescence, and this is characterized by functional changes in both innate and adaptive cellular immunity systems as well as in lymph node architecture. While many of the immune functions decline over time, others paradoxically increase. Indeed, aging is known to be associated with a low level of chronic inflammation—inflamm-aging. Given that the median age of cancer diagnosis is 66 years and that immunotherapeutic interventions such as cancer vaccines are currently given in combination with or after other forms of treatments which themselves have immune-modulating potential such as surgery, chemotherapy and radiotherapy, the choice of adjuvants requires careful consideration in order to achieve the maximum immune response in a compromised environment. In addition, more clinical trials need to be performed to carefully assess how less conventional form of immune adjuvants, such as exercise, diet and psychological care which have all be shown to influence immune responses can be incorporated to improve the efficacy of cancer vaccines. In this review, adjuvants will be discussed with respect to the above-mentioned important elements.

Effects of CpG oligodeoxynucleotides on the differentiation of Treg/Th17 cells

AIM

The balance between Th17 cells and T regulatory (Treg) cells has emerged as a prominent factor in regulating cancer development. However, the effect of CpG oligodeoxynucleotides (ODNs) on the differentiation of Treg/Th17 cells has not been well studied. We sought here to explore the function of CpG ODNs in the differentiation of Tregs and Th17 cells in vitro and in vivo.

METHODS

Mouse spleen cells were cultured with anti-CD3 monoclonal antibodies in vitro. Tregs and Th17 cell differentiation was induced by transforming growth factor (TGF)-β and interleukin (IL)-2, or TGF-β, IL-6, and IL-23, respectively. Then cells were treated with two CpG ODNs, CpG 1982, or CpG 1826. FBL-3-inoculated C57Bl/6 mice were treated with CpG 1826, tumor vaccine, or combination of CpG 1826 and tumor vaccine. After treatment, spleen cells and serum were isolated, and Tregs/Th17 cells were detected by flow cytometry. The expression of forkhead box P3 (Foxp3), retinoid-related orphan receptor gamma-t (RORγt), IL-10, and IL-17 mRNA was measured by real-time PCR, and protein levels were measured by Western blot and enzyme-linked immunosorbent assay.

RESULTS

The frequency of Treg cells increased significantly (p < 0.05) in the FBL-3-inoculated leukemia mouse model compared with control mice, whereas the frequency of Th17 cells did not change. Median survival of mice after treatment with CpG 1826 and tumor vaccine was significantly prolonged compared with that of control mice (p < 0.05). The frequency of induced Treg cells decreased after treatment with CpG 1826, whereas the frequency of Th17 cells induced by cytokines in vitro and in the murine leukemia model increased following treatment with CpG 1826. Furthermore, after treatment with CpG 1826, the mRNA and protein levels of Foxp3 and IL-10 decreased significantly both in vitro and in vivo (p < 0.05), whereas those of RORγt and IL-17 increased significantly (p < 0.05).

CONCLUSION

CpG 1826 may inhibit the differentiation of Treg cells induced by cytokines, promote the differentiation of Th17 cells in vitro and in murine leukemia models, and prolong the median survival of mice with leukemia.

Regulatory T cells isolated from endometriotic peritoneal fluid express a different number of Toll-like receptors

OBJECTIVE

To analyze and compare the expression of Toll-like receptors by regulatory T cells present in the peritoneal fluid of patients with and without endometriosis.

METHODS

Regulatory T cells were isolated from peritoneal fluid of women with and without endometriosis, collected during surgery, and mRNA was extracted for analysis of Toll-like receptors expression by reverse-transcriptase polymerase chain reaction.

RESULTS

Patients with endometriosis presented regulatory T cells expressing a larger number and variety of Toll-like receptors when compared to regulatory T cells from patients in the Control Group. Toll-like receptor-1 and Toll-like receptor-2 in regulatory T cells were expressed in both groups. All other expressed Toll-like receptors types were only found in regulatory T cells from the Endometriosis Group.

CONCLUSION

Patients with endometriosis had peritoneal regulatory T cells expressing various Toll-like receptors types.

The Role of TLRs in Anti-cancer Immunity and Tumor Rejection

In recent years, a lot of scientific interest has focused on cancer immunotherapy. Although chronic inflammation has been described as one of the hallmarks of cancer, acute inflammation can actually trigger the immune system to fight diseases, including cancer. Toll-like receptor (TLR) ligands have long been used as adjuvants for traditional vaccines and it seems they may also play a role enhancing efficiency of tumor immunotherapy. The aim of this perspective is to discuss the effects of TLR stimulation in cancer, expression of various TLRs in different types of tumors, and finally the role of TLRs in anti-cancer immunity and tumor rejection.

Turning the Tide Against Regulatory T Cells

Regulatory T (Treg) cells play crucial roles in health and disease through their immunosuppressive properties against various immune cells. In this review we will focus on the inhibitory role of Treg cells in anti-tumor immunity. We outline how Treg cells restrict T cell function based on our understanding of T cell biology, and how we can shift the equilibrium against regulatory T cells. To date, numerous strategies have been proposed to limit the suppressive effects of Treg cells, including Treg cell neutralization, destabilizing Treg cells and rendering T cells resistant to Treg cells. Here, we focus on key mechanisms which render T cells resistant to the suppressive effects of Treg cells. Lastly, we also examine current limitations and caveats of overcoming the inhibitory activity of Treg cells, and briefly discuss the potential to target Treg cell resistance in the context of anti-tumor immunity.

Recent advances in the development of vaccines for chronic inflammatory autoimmune diseases

Chronic inflammatory autoimmune diseases leading to target tissue destruction and disability are not only causing increase in patients' suffering but also contribute to huge economic burden for the society. General increase in life expectancy and high prevalence of these diseases both in elderly and younger population emphasize the importance of developing safe and effective vaccines. In this review, at first the possible mechanisms and risk factors associated with chronic inflammatory autoimmune diseases, such as rheumatoid arthritis (RA), multiple sclerosis (MS), systemic lupus erythematosus (SLE) and type 1 diabetes (T1D) are discussed. Current advances in the development of vaccines for such autoimmune diseases, particularly those based on DNA, altered peptide ligands and peptide loaded MHC II complexes are discussed in detail. Finally, strategies for improving the efficacy of potential vaccines are explored.

MyD88 in antigen-presenting cells is not required for CD4+ T-cell responses during peptide nanofiber vaccination

Self-assembled peptide nanofibers raise significant antibody and T cell responses without adjuvants, but the mechanism by which they achieve this has not been fully elucidated. Myeloid differentiation primary response gene 88 (MyD88) previously has been shown to be critical for the antibody response to antigens presented by peptide nanofibers. The present study sought to determine the cell subset in which MyD88 is essential for T cell responses. Mice deficient in MyD88 or CD11c⁺ cells had severely attenuated T cell responses. However, mice lacking MyD88 in only CD11c⁺ cells remained capable of internalizing, processing, and presenting nanofiber-derived epitopes to stimulate T cell responses. The necessity of inflammasome pathway was ruled out. Using adoptive transfer models where MyD88 was eliminated in CD4⁺ T cells or in the host, we observed that deficiency only in T cells or only in the host had no impact on the T cell response to nanofiber vaccines. Therefore, knocking out MyD88 in either antigen presenting cells (APCs) or CD4 T cells could not compromise the CD4 T cell responses, suggesting that self-assembled peptide nanofibers trigger redundant MyD88-dependent and MyD88-independent signaling pathways in APCs and T cells. Similar redundancy has been observed for other adjuvants, and this is discussed.

Breaking Free of Control: How Conventional T Cells Overcome Regulatory T Cell Suppression

Conventional T (Tcon) cells are crucial in shaping the immune response, whether it is protection against a pathogen, a cytotoxic attack on tumor cells, or an unwanted response to self-antigens in the context of autoimmunity. In each of these immune settings, regulatory T cells (Tregs) can potentially exert control over the Tcon cell response, resulting in either suppression or activation of the Tcon cells. Under physiological conditions, Tcon cells are able to transiently overcome Treg-imposed restraints to mount a protective response against an infectious threat, achieving clonal expansion, differentiation, and effector function. However, evidence has accumulated in recent years to suggest that Tcon cell resistance to Treg-mediated suppression centrally contributes to the pathogenesis of autoimmune disease. Tipping the balance too far in the other direction, cancerous tumors utilize Tregs to establish an overly suppressive microenvironment, preventing antitumor Tcon cell responses. Given the wide-ranging clinical importance of the Tcon/Treg interaction, this review aims to provide a better understanding of what determines whether a Tcon cell is susceptible to Treg-mediated suppression and how perturbations to this finely tuned balance play a role in pathological conditions. Here, we focus in detail on the complex array of factors that confer Tcon cells with resistance to Treg suppression, which we have divided into two categories: (1) extracellular factor-mediated signaling and (2) intracellular signaling molecules. Further, we explore the therapeutic implications of manipulating the phosphatidylinositol-3 kinase (PI3K)/Akt signaling pathway, which is proposed to be the convergence point of signaling pathways that mediate Tcon resistance to suppression. Finally, we address important unresolved questions on the timing and location of acquisition of resistance, and the stability of the “Treg-resistant” phenotype.

Microbiota, immunity and the liver

The gut harbors a complex community of over 100 trillion microbial cells known to exist in symbiotic harmony with the host influencing human physiology, metabolism, nutrition and immune function. It is now widely accepted that perturbations of this close partnership results in the pathogenesis of several major diseases with increasing evidence highlighting their role outside of the intestinal tract. The intimate proximity and circulatory loop of the liver and the gut has attracted significant attention regarding the role of the microbiota in the development and progression of liver disease. Here we give an overview of the interaction between the microbiota and the immune system and focus on their convincing role in both the propagation and treatment of liver disease.

Toll-Like Receptors in the Pathogenesis of Autoimmune Diseases

Human Toll-like receptors (TLRs) are a family of transmembrane receptors, which play a key role in both innate and adaptive immune responses. Beside of recognizing specific molecular patterns that associated with different types of pathogens, TLRs may also detect a number of self-proteins and endogenous nucleic acids. Activating TLRs lead to the heightened expression of various inflammatory genes, which have a protective role against infection. Data rising predominantly from human patients and animal models of autoimmune disease indicate that, inappropriate triggering of TLR pathways by exogenous or endogenous ligands may cause the initiation and/or perpetuation of autoimmune reactions and tissue damage. Given their important role in infectious and non-infectious disease process, TLRs and its signaling pathways emerge as appealing targets for therapeutics. In this review, we demonstrate how TLRs pathways could be involved in autoimmune disorders and their therapeutic application.

Origin and functions of pro-inflammatory cytokine producing Foxp3+ regulatory T cells

CD4(+)CD25(+)Foxp3(+) regulatory cells (Tregs) are a special lineage of cells central in the maintenance of immune homeostasis, and are targeted for human immunotherapy. They are conventionally associated with the production of classical anti-inflammatory cytokines such as IL-10, TGF-β and IL-35, consistent to their anti-inflammatory functions. However, emerging evidence show that they also express effector cytokines such as IFN-γ and IL-17A under inflammatory conditions. While some studies reveal that these pro-inflammatory cytokine producing Foxp3(+) regulatory cells retain their suppressive ability, others believe that these cells are dys-regulated and are associated with perpetuation of immunopathology. Therefore the development of these cells may challenge the efficacy of human Treg therapy. Mechanistically, toll-like receptor (TLR) ligands and the pro-inflammatory cytokine milieu have been shown to play important roles in the induction of effector cytokines in Tregs. Here we review the mechanisms of development and the possible functions of pro-inflammatory cytokine producing Foxp3+ Tregs.

Treg functional stability and its responsiveness to the microenvironment

Regulatory T cells (Tregs) prevent autoimmunity and tissue damage resulting from excessive or unnecessary immune activation through their suppressive function. While their importance for proper immune control is undeniable, the stability of the Treg lineage has recently become a controversial topic. Many reports have shown dramatic loss of the signature Treg transcription factor Forkhead box protein 3 (Foxp3) and Treg function under various inflammatory conditions. Other recent studies demonstrate that most Tregs are extremely resilient in their expression of Foxp3 and the retention of suppressive function. While this debate is unlikely to be settled in the immediate future, improved understanding of the considerable heterogeneity within the Foxp3(+) Treg population and how Treg subsets respond to ranging environmental cues may be keys to reconciliation. In this review, we discuss the diverse mechanisms responsible for the observed stability or instability of Foxp3(+) Treg identity and function. These include transcriptional and epigenetic programs, transcript targeting, and posttranslational modifications that appear responsive to numerous elements of the microenvironment. These mechanisms for Treg functional modulation add to the discussion of Treg stability.

Immunologic characterization suggests reduced alloimmunization in a murine model of thalassemia intermedia

BACKGROUND

Transfusion therapy remains a mainstay of treatment for patients with thalassemia major and to a lesser extent for the less anemic patients with thalassemia intermedia. We have previously reported a role for regulatory T cells (Tregs) in the control of antibody responses in wild-type C57BL/6 (WT) mice exposed to allogeneic red blood cell transfusions. As an initial step to study and characterize immune regulation in thalassemias, we performed an immunologic cell-type characterization of C57BL/6 Hbb(th-1)/Hbb(th-1) mouse model of thalassemia intermedia (Thal) in steady state as well as after transfusions with allogeneic blood.

STUDY DESIGN AND METHODS

The myeloid and lymphocyte compartments including Tregs and T helper (Th) responses were analyzed in transfusion naive Thal and WT mouse spleens. The effect of allogeneic transfusions on Treg and global T helper responses was also measured.

RESULTS

We found elevated levels and activity of splenic Tregs in Thal mice with lower Th type 1/Th type 2 ratios before as well as after transfusion. Furthermore, pretransfused Thal mice had altered ratios of the splenic myeloid compartment with increased proportion of macrophages but lower frequency of conventional dendritic cells. Surprisingly, transfusions resulted in lower alloimmunization levels in Thal compared to WT mice.

CONCLUSION

These data suggest that this experimental model of thalassemia intermedia has an intrinsic alteration in splenic immunoregulation with an increased resistance to alloimmunization, raising the possibility that studying this animal model may help to identify potential immunoregulatory networks to inhibit alloimmunization.

Suppressive oligodeoxynucleotides promote the generation of regulatory T cells by inhibiting STAT1 phosphorylation

Suppressive oligodeoxynucleotides (Sup ODN) express repetitive TTAGGG motifs that have proven useful in the treatment/prevention of numerous inflammatory and autoimmune diseases. The mechanism underlying the immunosuppressive activity of Sup ODN is incompletely understood. Regulatory T cells (Treg) play a key role in controlling a variety of pathologic autoimmune responses. Treg are generated from activated CD4(+) T cells in a process that involves the phosphorylation of STAT family members. Current studies demonstrate that Sup ODN promote the differentiation of CD4(+)CD25(-) T cells into functionally active iTreg in vitro. When administered in vivo, Sup ODN promote the generation of iTreg in response to peptide challenge. Central to this effect is the ability of Sup ODN to block the phosphorylation of STAT1. These findings clarify the mechanism underlying the therapeutic activity of Sup ODN and support their use in Treg-based immunotherapy.

HBHA vaccination may require both Th1 and Th17 immune responses to protect mice against tuberculosis

Almost one century after the discovery of the BCG vaccine, tuberculosis remains a major cause of global mortality and morbidity, emphasizing the urgent need to design more efficient vaccines. The heparin-binding haemagglutinin (HBHA) appears to be a promising vaccine candidate, as it was shown to afford protection to mice against a challenge infection with Mycobacterium tuberculosis when combined with the strong adjuvant DDA/MPL (dimethyldioctadecyl-ammonium bromide/monophosphoryl lipid A), a TLR4 ligand. In this study, we investigated the immunological response and protection of mice immunized with HBHA formulated in lipid-containing nanoparticles and adjuvanted with CpG, a TLR9 ligand. Subcutaneous immunization with this HBHA formulation led to a marked Th1 response, characterized by high IFN-γ levels, but no significant IL-17 production, both in spleen and lung, in contrast to DDA/MPL MPL-formulated HBHA, which induced both IFN-γ and IL-17. This cytokine profile was also observed in BCG-primed mice and persisted after M. tuberculosis infection. No significant protection was obtained against challenge infection after vaccination with the nanoparticle-CpG formulation, and this was associated with a failure to mount a memory immune response. These results suggest the importance of both Th1 and Th17 immune responses for vaccine-induced immunity.

Ubiquitous points of control over regulatory T cells

Posttranslational modification by ubiquitin tagging is crucial for regulating the stability, activity and cellular localization of many target proteins involved in processes including DNA repair, cell cycle progression, protein quality control, and signal transduction. It has long been appreciated that ubiquitin-mediated events are important for certain signaling pathways leading to leukocyte activation and the stimulation of effector function. Now it is clear that the activities of molecules and pathways central to immune regulation are also modified and controlled by ubiquitin tagging. Among the mechanisms of immune control, regulatory T cells (or Tregs) are themselves particularly sensitive to such regulation. E3 ligases and deubiquitinases both influence Tregs through their effects on the signaling pathways pertinent to these cells or through the direct, posttranslational regulation of Foxp3. In this review, we will summarize and discuss several examples of ubiquitin-mediated control over multiple aspects of Treg biology including the generation, function and phenotypic fidelity of these cells. Fully explored and exploited, these potential opportunities for Treg modulation may lead to novel immunotherapies for both positive and negative fine-tuning of immune restraint.

Signaling through the adaptor molecule MyD88 in CD4+ T cells is required to overcome suppression by regulatory T cells

Innate immune recognition controls adaptive immune responses through multiple mechanisms. The MyD88 signaling adaptor operates in many cell types downstream of Toll-like receptors (TLRs) and interleukin-1 (IL-1) receptor family members. Cell-type-specific functions of MyD88 signaling remain poorly characterized. Here, we have shown that the T cell-specific ablation of MyD88 in mice impairs not only T helper 17 (Th17) cell responses, but also Th1 cell responses. MyD88 relayed signals of TLR-induced IL-1, which became dispensable for Th1 cell responses in the absence of T regulatory (Treg) cells. Treg cell-specific ablation of MyD88 had no effect, suggesting that IL-1 acts on naive CD4(+) T cells instead of Treg cells themselves. Together, these findings demonstrate that IL-1 renders naive CD4(+) T cells refractory to Treg cell-mediated suppression in order to allow their differentiation into Th1 cells. In addition, IL-1 was also important for the generation of functional CD4(+) memory T cells.

Functions of invariant NK T cells are modulated by TLR ligands and IFN-α

Invariant NK T (iNKT) cells perform numerous immunoregulatory functions. In mice, they express a unique and invariant Vα14-Jα18 rearrangement of α chain in their TCR recognizing glycolipid Ags presented by CD1d. This recognition results in the rapid release of both Th1- and Th2-type cytokines, making them early mediators of the immune response. Owing to their rapid activation and genetic rigidity of their TCR, iNKT cells share characteristics with innate lymphocytes. Therefore, we investigated whether iNKT cells could be induced to express TLRs, a class of pathogen recognition receptor. Mouse iNKT cells were stimulated with anti-CD3 monoclonal Ab and IFN-α, resulting in an increase in the transcription of TLRs 3, 5, 7 and 9, and increased surface expression of TLR3. These cells were subsequently stimulated with TLR ligands, resulting in an increase in the production of IFN-γ, IL-4 and TNF-α. Supernatants from these cells also increased macrophage production of IL-6 and prostaglandin E2, and increased their phagocytic activity and CD80 expression. These supernatants also reduced vesicular stomatitis virus-GFP replication in fibroblasts. This study demonstrates the role of IFN-α in iNKT cell activation, as well as the direct modulatory effects of TLR ligands on iNKT cell function, including antiviral activity.

Toll-like receptor 9 agonist enhances anti-tumor immunity and inhibits tumor-associated immunosuppressive cells numbers in a mouse cervical cancer model following recombinant lipoprotein therapy

Background

Although cytotoxic T lymphocytes (CTLs) play a major role in eradicating cancer cells during immunotherapy, the cancer-associated immunosuppressive microenvironment often limits the success of such therapies. Therefore, the simultaneous induction of cancer-specific CTLs and reversal of the immunosuppressive tumor microenvironment may be more effectively achieved through a single therapeutic vaccine. A recombinant lipoprotein with intrinsic Toll-like receptor 2 (TLR2) agonist activity containing a mutant form of E7 (E7m) and a bacterial lipid moiety (rlipo-E7m) has been demonstrated to induce robust CTL responses against small tumors. This treatment in combination with other TLR agonists is able to eliminate large tumors.

Methods

Mouse bone marrow-derived dendritic cells (DCs) were employed to determine the synergistic production of pro-inflammatory cytokines upon combination of rlipo-E7m and other TLR agonists. Antigen-specific CTL responses were investigated using immunospots or in vivo cytolytic assays after immunization in mice. Mice bearing various tumor sizes were used to evaluate the anti-tumor effects of the formulation. Specific subpopulations of immunosuppressive cells in the tumor infiltrate were quantitatively determined by flow cytometry.

Results

We demonstrate that a TLR9 agonist (unmethylated CpG oligodeoxynucleotide, CpG ODN) enhances CTL responses and eradicates large tumors when combined with rlipo-E7m. Moreover, combined treatment with rlipo-E7m and CpG ODN effectively increases tumor infiltration by CTLs and reduces the numbers of myeloid-derived suppressor cells (MDSCs), tumor-associated macrophages (TAMs) and regulatory T cells (Tregs) in the tumor microenvironment.

Conclusion

These findings suggest that the dramatic anti-tumor effects of the recombinant lipoprotein together with CpG ODN may reflect the amplification of CTL responses and the repression of the immunosuppressive environment. This promising approach could be applied for the development of additional therapeutic cancer vaccines.

Inhibition of CD4+CD25+ regulatory T cell function and conversion into Th1-like effectors by a Toll-like receptor-activated dendritic cell vaccine

Despite the success of vaccines against some microbial pathogens, their utility in the prevention and treatment of cancer has thus far been limited. We have previously demonstrated that vaccination with dendritic cells activated with the TLR-4 ligand LPS and IFN-γ promotes an antigen-specific anti-tumor response that prevents tumor recurrence. To evaluate this mechanistically, we here studied the effects of this TLR-activated DC on regulatory T cell activity. Dendritic cells activated with LPS and IFN- γ negated the effects of regulatory T cells on responder cell proliferation. Restoration of responder cell proliferation was noted when TLR-activated dendritic cells were separated from both regulators and responders by a semi-permeable membrane. The effect is therefore mediated by a soluble factor but was independent of both IL-6 and IL-12. Furthermore, the soluble mediator appeared to act at least in part on the regulators themselves rather than responder cells exclusively. Because recent studies have demonstrated conversion of T regulatory cells into IL-17-producing effectors, we further questioned whether the TLR-activated dendritic cell would induce cytokine production and effector function in our system. We found that regulators produced a substantial amount of IFN- γ in the presence of TLR-activated dendritic cells but not immature dendritic cells. IFN-γ production was associated with upregulation of the Th1 transcriptional regulator T-bet, and a significant fraction of IFN-γ-producing regulators coexpressed T-bet and FoxP3. While the effects of the LPS-activated dendritic cell on responder cell proliferation were IL-12 independent, upregulation of T-bet was inhibited by a neutralizing anti-IL-12 antibody. Collectively, these and prior data suggest that varying innate immune signals may direct the phenotype of the immune response in part by inhibiting suppressor T cells and promoting differentiation of these regulators into particular subsets of effectors.

CpG oligodeoxynucleotide ligand potentiates the activity of the pVAX1-Sj26GST

Schistosomiasis is considered one of the most important neglected tropical diseases and remains a major public health problem in endemic countries. Toll-like receptor (TLR) ligands have been investigated as potential vaccine adjuvants for tumor and virus immunotherapy. However, few TLR ligands affecting schistosoma vaccines have been characterized. In this study, we evaluated a TLR9 ligand (CpG oligodeoxynucleotide 1826, CpG) as an adjuvant for a partially protective DNA vaccine encoding a 26-kDa glutathione S-transferase of Schistosoma japonicum (pVAX1-Sj26GST). Vaccination with pVAX1-Sj26GST in combination with CpG inhibited Treg immunosuppressive function, upregulated the production of interferon (IFN)-γ, tumor necrosis factor (TNF)-α, interleukin (IL)-4, IL-10, IL-2 and IL-6, and decreased CD4⁺CD8⁺Foxp3⁺ expression in vitro, which may contribute to the escape from Treg-mediated suppression during vaccination, allowing expansion of antigen-specific T cells against pathogens. In conclusion, our data demonstrated that selective TLR ligand combination may increase protective efficacy against schistosomiasis, which may synergistically antagonize Treg-mediated suppression.

Combined TLR7/8 and TLR9 ligands potentiate the activity of a Schistosoma japonicum DNA vaccine

Background

Toll-like receptor (TLR) ligands have been explored as vaccine adjuvants for tumor and virus immunotherapy, but few TLR ligands affecting schistosoma vaccines have been characterized. Previously, we developed a partially protective DNA vaccine encoding the 26-kDa glutathione S-transferase of Schistosoma japonicum (pVAX1-Sj26GST).

Methodology/Principal Findings

In this study, we evaluated a TLR7/8 ligand (R848) and a TLR9 ligand (CpG oligodeoxynucleotides, or CpG) as adjuvants for pVAX1-Sj26GST and assessed their effects on the immune system and protection against S. japonicum. We show that combining CpG and R848 with pVAX1-Sj26GST immunization significantly increases splenocyte proliferation and IgG and IgG2a levels, decreases CD4⁺CD25⁺Foxp3⁺ regulatory T cells (Treg) frequency in vivo, and enhances protection against S. japonicum. CpG and R848 inhibited Treg-mediated immunosuppression, upregulated the production of interferon (IFN)-γ, tumor necrosis factor (TNF)-α, interleukin (IL)-4, IL-10, IL-2, and IL-6, and decreased Foxp3 expression in vitro, which may contribute to prevent Treg suppression and conversion during vaccination and allow expansion of antigen-specific T cells against pathogens.

Conclusions

Our data shows that selective TLR ligands can increase the protective efficacy of DNA vaccines against schistosomiasis, potentially through combined antagonism of Treg-mediated immunosuppression and conversion.

There is evidence that TLR activation can block Treg cell responses and thereby break tolerance to self-antigens. It is expected that the use of TLR ligands as vaccine adjuvants will induce potent anti-pathogen immune responses and simultaneously overcome immune inhibition mediated by Tregs. However, the impact of TLR ligands on schistosomiasis vaccines is unclear. Here, we demonstrate that the use of a TLR7/8 ligand (R848) and a TLR9 ligand (CpG) as adjuvants in combination with the S. japonicum vaccine pVAX1-Sj26GST improves disease protection. The combination of CpG and R848 administered after vaccination causes an immune response marked by an upregulation of splenocyte proliferation and IgG and IgG2a levels that also coincides with a decreased proportion of CD4⁺CD25⁺ Tregs in mice. We also show that combined adjuvant use of CpG and R848 may impair Treg development and function by promoting the secretion of proinflammatory cytokines and reducing Foxp3 expression. Our findings suggest that in combination with the vaccine, TLR ligands may protect the effector response from Treg-mediated suppression, thereby eliciting the appropriate immune response to improve vaccine efficacy. Immunization combined with the TLR ligands CpG and R848 thus represents a promising new approach for the design of schistosoma vaccines.

Profound tumor-specific Th2 bias in patients with malignant glioma

Background

Vaccination against tumor-associated antigens is one promising approach to immunotherapy against malignant gliomas. While previous vaccine efforts have focused exclusively on HLA class I-restricted peptides, class II-restricted peptides are necessary to induce CD4⁺ helper T cells and sustain effective anti-tumor immunity. In this report we investigated the ability of five candidate peptide epitopes derived from glioma-associated antigens MAGE and IL-13 receptor α2 to detect and characterize CD4⁺ helper T cell responses in the peripheral blood of patients with malignant gliomas.

Methods

Primary T cell responses were determined by stimulating freshly isolated PBMCs from patients with primary glioblastoma (GBM) (n = 8), recurrent GBM (n = 5), meningioma (n = 7), and healthy controls (n = 6) with each candidate peptide, as well as anti-CD3 monoclonal antibody (mAb) and an immunodominant peptide epitope derived from myelin basic protein (MBP) serving as positive and negative controls, respectively. ELISA was used to measure IFN-γ and IL-5 levels, and the ratio of IFN-γ/IL-5 was used to determine whether the response had a predominant Th1 or Th2 bias.

Results

We demonstrate that novel HLA Class-II restricted MAGE-A3 and IL-13Rα2 peptides can detect T cell responses in patients with GBMs as well as in healthy subjects. Stimulation with a variety of peptide antigens over-expressed by gliomas is associated with a profound reduction in the IFN-γ/IL-5 ratio in GBM patients relative to healthy subjects. This bias is more pronounced in patients with recurrent GBMs.

Conclusions

Therapeutic vaccine strategies to shift tumor antigen-specific T cell response to a more immunostimulatory Th1 bias may be needed for immunotherapeutic trials to be more successful clinically.

IL-7 abrogates suppressive activity of human CD4+CD25+FOXP3+ regulatory T cells and allows expansion of alloreactive and autoreactive T cells

CD4(+)CD25(+)FOXP3(+) regulatory T cells (Tregs) control the activation and expansion of alloreactive and autoreactive T cell clones. Because uncontrolled activation and expansion of autoreactive T cells occur in an IL-7-rich environment, we explored the possibility that IL-7 may affect the function of Treg. We show that the functional high-affinity IL-7R is expressed on both naive and memory Tregs, and exposure to IL-7 results in STAT-5 phosphorylation. Naive, but not memory, Tregs proliferated greatly and acquired a memory phenotype in the setting of a suppression assay when IL-7 was present. Importantly, the presence of IL-7 abrogated the capacity of Tregs to suppress proliferation of conventional T cells in response to TCR activators, including alloantigens and autoantigens. Removal of IL-7 restored the suppressive function of Tregs. Preblocking of the IL-7R on the Tregs also restored suppressor function, indicating that IL-7 directly affected Treg function. Thus, prolonged periods of homeostatic expansion can temporarily release natural regulatory brakes on T cells, thereby providing an additional mechanism for activating and expanding alloreactive and autoreactive T cells.

CD4+CD25+ Treg derived from hepatocellular carcinoma mice inhibits tumor immunity

CD4+CD25+ regulatory T cells (Tregs) play an essential role in the establishment and persistence of tumor immune suppression. Tregs can prevent anti-tumor-specific T cells from clearing the tumor, making Tregs a significant barrier for effective immunotherapy. An increase in the number of Tregs has been detected in the peripheral blood and tumor infiltrating lymphocytes of patients with hepatocellular carcinoma. Dendritic cells (DCs) are antigen-presenting cells that play a pivotal role in the initiation of immune responses. The evidence for their ability to act as natural adjuvant in the stimulation of specific anti-tumor cytotoxic T lymphocytes and in the induction of protective and therapeutic anti-tumor immunity is now overwhelming. The aim of our study was to investigate the variation of Tregs in hepatocellular carcinoma mice and how Tregs derived from the tumor mice affect DCs' function. We found that Tregs derived from the tumor mice down-regulated the expression of costimulatory molecules CD80/CD86 on DCs and inhibited the production of TNF-α and IL-12 from DCs. The suppressive function of Tregs was mediated by cell-to-cell contact, CTLA-4 expression and IL-10 secretion. In conclusion, these mechanisms acting in hepatocellular carcinoma may be necessary to better understand the immunosuppression of Tregs and helpful to the tumor immunotherapy.

Sterile inflammation - do innate lymphoid cell subsets play a role?

The recent identification of several novel innate lymphoid cell (iLC) subsets has increased our understanding of the mechanisms which link the innate and adaptive immune systems. While the contribution of these subsets toward the pathogenesis of human disease remains largely to be determined, it seems likely that they will play a particularly important role in sterile inflammatory settings where the innate response is seen as a critical mediator of inflammation. Several recent studies have highlighted the role of endogenous damage-associated molecular patterns such as IL-33, IL-1α, and IL-1β in promoting lymphoid cell responses. This review discusses the influence of such endogenous danger signals on novel iLCs such as lymphoid tissue-inducer cells, innate type 2 helper cells, and γδ T cells and explores how these responses may contribute to the development of an inflammatory response in a sterile setting.

NOD1 cooperates with TLR2 to enhance T cell receptor-mediated activation in CD8 T cells

Pattern recognition receptors (PRR), like Toll-like receptors (TLR) and NOD-like receptors (NLR), are involved in the detection of microbial infections and tissue damage by cells of the innate immune system. Recently, we and others have demonstrated that TLR2 can additionally function as a costimulatory receptor on CD8 T cells. Here, we establish that the intracytosolic receptor NOD1 is expressed and functional in CD8 T cells. We show that C12-iEDAP, a synthetic ligand for NOD1, has a direct impact on both murine and human CD8 T cells, increasing proliferation and effector functions of cells activated via their T cell receptor (TCR). This effect is dependent on the adaptor molecule RIP2 and is associated with an increased activation of the NF-κB, JNK and p38 signaling pathways. Furthermore, we demonstrate that NOD1 stimulation can cooperate with TLR2 engagement on CD8 T cells to enhance TCR-mediated activation. Altogether our results indicate that NOD1 might function as an alternative costimulatory receptor in CD8 T cells. Our study provides new insights into the function of NLR in T cells and extends to NOD1 the recent concept that PRR stimulation can directly control T cell functions.

Expression of Toll-like receptor9 in diffuse large B-cell lymphoma: further exploring CpG oligodeoxynucleotide in NFκB pathway

Human Toll-like receptors (TLRs) that recognize a variety of pathogen-associated molecular patterns are associated with activation and immunogenic response in lymphoid neoplasms, but rarely explored in diffuse large B-cell lymphoma (DLBCL). We conducted this study to evaluate the expression of TLR9 in and potential treatment of DLBCL with TLR9 agonist - CpG oligodeoxynucleotide (ODN). The real-time quantitative reverse transcription-polymerase chain reaction was carried out to detect TLR9 expression in 41 formalin-fixed paraffin-embedded samples. The transformation of immunophenotype and NFκB pathway of DLBCL upon CpG ODN stimulation were investigated by a DLBCL cell line. TLR9 was commonly detected in DLBCL with relative mRNA levels above 1.0 × 10(-2) in 35 of 41 cases (85.36%). It was suspected that a high proportion of DLBCL to be activated by CpG stimulation. In vitro study with a DLBCL cell line revealed an increased CD20, but decreased BCL-6 and MUM1/IRF4 expression after treatment with CpG ODN. The NFκB pathway was initially activated, but finally suppressed upon CpG ODN stimulation. The proliferation of tumor cells was also inhibited by long time incubation. These findings provide new insights into the role of TLR9 in DLBCL and potential implication of TLR9 agonist in the treatment of DLBCL.

Toll-like receptor 5 deficiency protects from wasting disease in a T cell transfer colitis model in T cell receptor-β-deficient mice

BACKGROUND

Toll-like receptor 5 (TLR5) is implicated in the innate and adaptive immune responses that are associated with inflammatory bowel disease (IBD). In humans TLR5 is expressed on CD4(+) T cells and costimulation with flagellin potentiates effector and regulatory T cell responses. The aim of this study was to determine the role of TLR5 in CD4(+) T cell subsets versus other cells in induction of disease in a model of T cell-dependent colitis.

METHODS

TLR5 expression on CD4(+) T cells was assessed by real-time reverse-transcriptase polymerase chain reaction (RT-PCR). Wildtype (WT) or TLR5-deficient (5-/-) CD4(+) T conventional cells (Tconv) and T regulatory cells (Treg) were compared for their ability to induce and suppress T cell transfer colitis, respectively. In addition, the role of TLR5 expression in recipient mice was analyzed.

RESULTS

TLR5 is preferentially expressed on mouse Treg compared to Tconv, although expression levels were low. The colitogenic capacity of WT and 5-/- Tconv was found to be similar and Treg from WT or 5-/- donor animals both prevented T cell transfer colitis in TLR-competent hosts. TLR5 deficiency in recipient mice, however, did affect the disease process, as T cell receptor-β (TCRβ) 5-/- recipients had decreased weight loss compared to TCRβ recipient mice when WT Tconv were used.

CONCLUSIONS

TLR5 expression on T cells is not required for induction of or protection from T cell-dependent colitis. Expression of TLR5 in non-T cells has a pathogenic role, since TLR5 deficiency in recipient mice protects against weight loss induced by WT T cells.

The effects of TLR activation on T-cell development and differentiation

Invading pathogens have unique molecular signatures that are recognized by Toll-like receptors (TLRs) resulting in either activation of antigen-presenting cells (APCs) and/or costimulation of T cells inducing both innate and adaptive immunity. TLRs are also involved in T-cell development and can reprogram Treg cells to become helper cells. T cells consist of various subsets, that is, Th1, Th2, Th17, T follicular helper (Tfh), cytotoxic T lymphocytes (CTLs), regulatory T cells (Treg) and these originate from thymic progenitor thymocytes. T-cell receptor (TCR) activation in distinct T-cell subsets with different TLRs results in differing outcomes, for example, activation of TLR4 expressed in T cells promotes suppressive function of regulatory T cells (Treg), while activation of TLR6 expressed in T cells abrogates Treg function. The current state of knowledge of regarding TLR-mediated T-cell development and differentiation is reviewed.

Intestinal microbiota: shaping local and systemic immune responses

Recent studies have highlighted the fundamental role of commensal microbes in the maintenance of host homeostasis. For instance, commensals can play a major role in the control of host defense, metabolism and tissue development. Over the past few years, abundant experimental data also support their central role in the induction and control of both innate and adaptive responses. It is now clearly established that commensals are not equal in their capacity to trigger control regulatory or effector responses, however, the molecular basis of these differences has only recently begun to be explored. This review will discuss recent findings evaluating how commensals shape both effector and regulatory responses at steady state and during infections and the consequence of this effect on local and systemic protective and inflammatory responses.

Microbial infection-induced expansion of effector T cells overcomes the suppressive effects of regulatory T cells via an IL-2 deprivation mechanism

Foxp3(+) regulatory T (Treg) cells are a critical cell population that suppresses T cell activation in response to microbial and viral pathogens. We identify a cell-intrinsic mechanism by which effector CD4(+) T cells overcome the suppressive effects of Treg cells in the context of three distinct infections: Toxoplasma gondii, Listeria monocytogenes, and vaccinia virus. The acute responses to the parasitic, bacterial, and viral pathogens resulted in a transient reduction in frequency and absolute number of Treg cells. The infection-induced partial loss of Treg cells was essential for the initiation of potent Th1 responses and host protection against the pathogens. The observed disappearance of Treg cells was a result of insufficiency in IL-2 caused by the expansion of pathogen-specific CD4(+) T cells with a limited capacity of IL-2 production. Exogenous IL-2 treatment during the parasitic, bacterial, and viral infections completely prevented the loss of Treg cells, but restoration of Treg cells resulted in a greatly enhanced susceptibility to the pathogens. These results demonstrate that the transient reduction in Treg cells induced by pathogens via IL-2 deprivation is essential for optimal T cell responses and host resistance to microbial and viral pathogens.

Triggering of toll-like receptor signaling pathways in T cells contributes to the anti-tumor efficacy of T cell responses

Traditionally, expression of toll-like receptors (TLRs) has been associated with innate immune cells in particular professional antigen presenting cells and natural killer cells. This led to the concept that the adjuvant effects of ligation of TLR in a host occur mainly in innate immune cells. However, this concept has been challenged by recent studies including ours demonstrating that T cells express appreciated levels of different TLRs, which can serve as costimulatory co-receptors during polyclonal and antigen-specific stimulation of T cells. Because T cells express low levels of TLRs as compared to innate immune cells, increasing the expression levels of TLRs in T cells can significantly maximize their responses to the costimulatory effects of TLR ligation. This review article focuses on the potential role of TLR expression in T cells in their responses to vaccination regimen containing TLR agonists and how it can be modulated to optimize anti-tumor immunity.

The adjuvant effect of TLR agonists on CD4(+) effector T cells is under the indirect control of regulatory T cells

TLR agonists have been suggested to directly impact Tregs, thereby enhancing or reversing their suppressive function. Here, in order to select TLR agonists leading to potent effector T-cell responses, while minimizing Treg inhibitory function, we used a model antigen, covalently linked to an inert delivery system, combined with a large panel of TLR agonists, for the immunization of mice with an attenuated/depleted or intact Treg subset. We observed that the negative modulation of effector CD4(+) T cells exerted by Tregs cannot be circumvented, whatever the TLR agonist used as adjuvant. To better understand the impact of TLR agonists on Tregs, we investigated (i) the TLR expression profile of highly purified CD4(+) Foxp3(+) Tregs, at steady state or subsequent to in vivo activation by TLR agonists and (ii) the Treg phenotype after in vivo and in vitro activation by TLR agonists. Our results demonstrate that TLR agonists, as single signal inducers, are not able to directly activate Tregs. The phenotypic Treg activation observed in vivo, following TLR administration, does not result from cross-talk with conventional T cells but is rather a consequence of the interaction with other immune cell type(s).

Lactobacillus gasseri OLL2809 and its RNA suppress proliferation of CD4(+) T cells through a MyD88-dependent signalling pathway

Recent studies have shown that probiotics are beneficial in prevention and improvement of inflammatory diseases. Accumulating evidence indicates that probiotics can modulate immune cell responses, although the specific molecular mechanism by which probiotics work remains elusive. Because T cells express receptors for microbial components, we examined whether the probiotic strain Lactobacillus gasseri OLL2809 (LG2809) and its components regulate murine CD4(+) T-cell activation. LG2809, as well as two other Lactobacillus strains, inhibited proliferation of CD4(+) T cells; LG2809 had the strongest suppressive activity among them. RNA isolated from LG2809 was also shown to have suppressive activity. We observed this suppressive effect in the culture of CD4(+) T cells stimulated with anti-CD3/CD28 treatment, suggesting a direct effect on CD4(+) T cells. In contrast, the suppressive effect was not observed for CD4(+) T cells from myeloid differentiation primary response gene 88 (MyD88) protein-deficient mice, and was abrogated in the presence of an anti-oxidant reagent, N-acetyl-cysteine (NAC). These results demonstrate that the suppressive effect of LG2809 and its RNA occurred through a MyD88-dependent signalling pathway and suggest involvement of a reactive oxygen species-dependent mechanism. LG2809 RNA injected subcutaneously suppressed delayed-type-hypersensitivity response in DO11.10 mice, and the suppression was abrogated by treatment with NAC. Collectively, these results suggest that suppression of T-cell proliferation by RNA may be one of the mechanisms when a probiotic bacterial strain exerts suppressive effects on inflammatory responses.

Engagement of TLR2 reverses the suppressor function of conjunctiva CD4+CD25+ regulatory T cells and promotes herpes simplex virus epitope-specific CD4+CD25- effector T cell responses

PURPOSE. The authors recently reported that Foxp3(+)CD4(+) CD25(+(Bright)) "natural" regulatory T cells (nT(reg) cells) are abundant in rabbit conjunctiva and suppress herpes simplex virus (HSV)-1-specific CD4(+) and CD8(+) effector T cells (T(eff) cells). However, little is known about the overall regulatory mechanisms of these nT(reg) cells. The authors investigate the regulation of conjunctiva-resident nT(reg) cells through Toll-like receptors (TLRs) and their effect on ocular mucosal T(eff) cell immunity. METHODS. CD4(+)CD25(+) nT(reg) cells were purified from naive rabbit conjunctivas, and their TLR expression profile was determined. The effects of TLR engagement on nT(reg) cell-mediated suppression of CD4(+) T(eff) cells were determined in vitro and in vivo. RESULTS. The authors found that conjunctiva-resident nT(reg) cells express high levels of TLR2 and TLR9; exposure to the TLR2 ligand lipoteichoic acid (LTA) led to the increased activation and proliferation of nT(reg) cells, and the addition of autologous APCs further increased nT(reg) cell expansion; in contrast, the TLR9 ligand CpG(2007) inhibited the proliferation of nT(reg) cells, and the addition of autologous APCs had no effect on such inhibition; nT(reg) cells treated with LTA, but not with CpG(2007), expressed IFN-γ and IL-10 mRNA, but not TGF-β; consistent with in vitro data, rabbits immunized by topical ocular drops of HSV-gD peptides + TLR2 ligand (LTA) displayed enhanced CD4(+)CD25(-) T(eff) cell immune responses when compared with HSV-gD peptides + TLR9 ligand (CpG(2007)). CONCLUSIONS. Although conjunctiva-resident CD4(+)CD25(+) nT(reg) cells express high level of TLR2 and TLR9, their suppressive function is more significantly reversed after the administration of TLR2 ligand (LTA; P < 0.005) than of TLR9 ligand (CpG(200); P > 0.005). These findings will likely help optimize the topical ocular administration of immunotherapies.

Immune adjuvants in early life: targeting the innate immune system to overcome impaired adaptive response

The neonatal phase is a transitory period characterized by an absence of memory cells, favoring a slow adaptive response prone to tolerance effects and the development of Th2-type responses. However, when appropriately stimulated, neonates may achieve an immune response comparable with adult counterparts. One strategy to stimulate the immunological response of neonates or children in early infancy has been to explore natural or synthetic ligands of cell receptors to stimulate innate immunity. The use of adjuvants for activating different cell receptors may be the key to enhancing neonatal adaptive immunity. This review highlights recent advances in the emerging field of molecular adjuvants of innate immune response and their implications for the development of immunotherapies, with particular focus on the neonatal period.

Distinct inflammatory signals have physiologically divergent effects on epigenetic regulation of Foxp3 expression and Treg function

Foxp3 expression in regulatory T cells (Treg) is required for their development and suppressive function. How different inflammatory signals affect Foxp3 chromatin structure, expression and Tregs plasticity are not completely known. In the present study, the Toll-like receptor 2 (TLR2) ligand peptidoglycan inhibited Foxp3 expression in both natural Treg (nTreg) and TGFβ-driven adaptive Treg (aTreg). Inhibition was independent of paracrine Th1, Th2 and Th17 cytokines. PGN-induced T cell-intrinsic TLR2-Myd88-dependent IFR1 expression and induced IRF1 bound to IRF1 response elements (IRF-E) in the Foxp3 promoter and intronic enhancers, and negatively regulated Foxp3 expression. Inflammatory IL-6 and TLR2 signals induced divergent chromatin changes at the Foxp3 locus and regulated Treg suppressor function, and in an islet transplant model resulted in differences in their ability to prolong graft survival. These findings are important for understanding how different inflammatory signals can affect the transplantation tolerance and immunity.

The control of adaptive immune responses by the innate immune system

The mammalian immune system comprises an adaptive and an innate component. The innate immune system employs a limited number of germ-line-encoded pattern-recognition receptors (PRRs) that recognize invariant pathogen-associated molecular patterns (PAMPs). In contrast, the adaptive immune system depends on the generation of a diverse repertoire of antigen receptors on T and B lymphocytes and subsequent activation and clonal expansion of cells carrying the appropriate antigen-specific receptors. Induction of adaptive immunity not only depends on direct antigen recognition by the antigen receptors but also relies on essential signals that are delivered by the innate immune system. In recent years, we have witnessed the discovery of a still expanding array of different PRR systems that govern the generation of adaptive immunity. Here, we review our current understanding of innate control of adaptive immunity. In particular, we discuss how PRRs initiate adaptive immune responses in general, discuss specific mechanisms that shape the ensuing T and B cell responses, and highlight open questions that are still awaiting answers.

Viral infection in induction of Hashimoto's thyroiditis: a key player or just a bystander?

PURPOSE OF REVIEW

Viral infection activates both the innate and adaptive immunity and is implicated as a trigger of autoimmune diseases including Hashimoto's thyroiditis. This review summarizes our knowledge respecting the role of viral infection in the cause of Hashimoto's thyroiditis.

RECENT FINDINGS

Components of several viruses such as hepatitis C virus, human parvovirus B19, coxsackie virus and herpes virus are detected in the thyroid of Hashimoto's thyroiditis patients. Bystander activation of autoreactive T cells may be involved in triggering intrathyroidal inflammation. Signaling molecules associated with antiviral responses including Toll-like receptors may participate in Hashimoto's thyroiditis induction. However, studies have provided insufficient direct evidence for the viral hypothesis in Hashimoto's thyroiditis.

SUMMARY

Despite interesting circumstantial evidence, whether viral infection is responsible for Hashimoto's thyroiditis remains unclear. Studies addressing this issue are required to substantiate a contribution from viral infection to Hashimoto's thyroiditis and, consequently, the prospect for developing preventive modalities for Hashimoto's thyroiditis.

Regulatory T cells suppress natural killer cells during plasmid DNA vaccination in mice, blunting the CD8+ T cell immune response by the cytokine TGFbeta

Background

CD4⁺CD25⁺ regulatory T cells (Tregs) suppress adaptive T cell-mediated immune responses to self- and foreign-antigens. Tregs may also suppress early innate immune responses to vaccine antigens and might decrease vaccine efficacy. NK and NKT cells are the first responders after plasmid DNA vaccination and are found at the site of inoculation. Earlier reports demonstrated that NKT cells could improve plasmid DNA efficacy, a phenomenon not found for NK cells. In fact, it has been shown that under certain disease conditions, NK cells are suppressed by Tregs via their release of IL-10 and/or TGFβ. Therefore, we tested the hypothesis that NK cell function is suppressed by Tregs in the setting of plasmid DNA vaccination.

Methodology/Principal Findings

In this study we show that Tregs directly inhibit NK cell function during plasmid DNA vaccination by suppressing the potentially 10-fold, NK cell-mediated, augmentation of plasmid DNA antigen-specific CD8⁺ T cells. We found that this phenomenon is dependent on the secretion of cytokine TGFβ by Tregs, and independent of IL-10.

Conclusions

Our data indicate a crucial function for Tregs in blocking plasmid DNA vaccine-elicited immune responses, revealing potentially novel strategies for improving the efficiency of plasmid DNA vaccines including chemical- or antibody-induced localized blockage of Treg-mediated suppression of NK cells at the site of plasmid DNA vaccine inoculation.

Insights into the role of Toll-like receptors in modulation of T cell responses

The innate immune receptors, such as Toll-like receptors (TLRs), are intimately involved in the early sensing of invading microorganisms or their structural components. Engagement of TLRs with their ligands results in activation of several downstream intracellular pathways leading to activation of innate and adaptive immune system cells. It was initially thought that TLRs are primarily expressed by antigen-presenting cells (APCs), such as macrophages and dendritic cells, and that interactions between microbial ligands and TLRs in these cells will indirectly result in activation of cells of the adaptive immune system, especially T cells. However, it has now become evident that TLRs are also expressed by various T cell subsets, such as conventional αβT cells, regulatory T cells, and γδT cells as well as natural killer T cells. Importantly, it appears that at least in some of these T cell subsets, TLRs are functionally active, because stimulation of these cells with TLR agonists in the absence of APCs results in exertion of effector or regulatory functions of T cells. The present review attempts to summarize the recent findings related to TLR expression in different T cell subsets and the direct role of TLRs in the induction and regulation of T cell responses, including those responses that occur at mucosal surfaces. In addition, the potential use of TLR agonists for steering T cell responses as a prophylactic or therapeutic strategy in the context of infectious, allergic or autoimmune diseases is explored.

The contribution of direct TLR signaling to T cell responses

It is well established that Toll-like receptors (TLRs) play a critical role in the generation of innate immune responses and thereby also play an important, indirect role in the initiation of subsequent adaptive T cell responses. However, T cells also express certain TLRs, and we have focused on the physiological importance of direct TLR signaling in T cells. TLRs can function as co-stimulatory receptors that complement TCR-induced signals to enhance effector T cell proliferation, survival and cytokine production. We also found that TLR signaling pathways in T cells are required for the effective clonal expansion of antigen-specific T cells during infection in vivo. Thus, the importance of TLRs in T cell-mediated immunity reflects both T cell-extrinsic and T cell-intrinsic components, which warrants a reconsideration of the dogma that restricts germ-line encoded pattern recognition to cells of the innate immune system.

The immunopathogenesis of Entamoeba histolytica

Amebiasis is the disease caused by the enteric dwelling protozoan parasite Entamoeba histolytica. The WHO considers amebiasis as one of the major health problems in developing countries; it is surpassed by only malaria and schistosomiasis for death caused by parasitic infection. E. histolytica primarily lives in the colon as a harmless commensal, but is capable of causing devastating dysentery, colitis and liver abscess. What triggers the switch to a pathogenic phenotype and the onset of disease is unknown. We are becoming increasingly aware of the complexity of the host-parasite interaction. During chronic stages of amebiasis, the host develops an immune response that is incapable of eliminating tissue resident parasites, while the parasite actively immunosuppresses the host. However, most individuals with symptomatic infections succumb only to an episode of dysentery. Why most halt invasion and a minority progress to chronic disease remains poorly understood. This review presents a current understanding of the immune processes that shape the outcome of E. histolytica infections during its different stages.

Microbe-dendritic cell dialog controls regulatory T-cell fate

Each microenvironment is controlled by a specific set of regulatory elements that have to be finely and constantly tuned to maintain local homeostasis. These environments could be site specific, such as the gut environment, or induced by chronic exposure to microbes. Various populations of dendritic cells are central to the orchestration of this control. In this review, we discuss some new findings associating dendritic cells from defined compartments with the induction and control of regulatory T cells in the context of exposure to both commensal and pathogenic microbes.

Microbial control of regulatory and effector T cell responses in the gut

The human intestine harbors and is in constant contact with 1000 trillion microbes, composed of an estimated 15,000 strains. Recent studies have changed our perspective of commensal microbes from benign but inert passengers, to active participants in the processing of food into useful metabolic components, the postnatal development of mucosal and systemic immunity, and in its long-term steady state function. Although mucosal surfaces have to constitutively integrate a multitude of microbial derived signals, new evidence suggests that defined bacteria or microbial products can play a dominant role in the induction of distinct class of immune responses. In this review we will focus on recent findings associating microbes that colonize or invade the gut, specialized mucosal DCs, and induction of effector or regulatory response in the GI tract.