Toll-like receptor 8-mediated reversal of CD4+ regulatory T cell function

CD8+ T-cell memory in tumor immunology and immunotherapy

The cellular and molecular mechanisms underlying the formation of distinct central, effector, and exhausted CD8+ T-cell memory subsets were first described in the setting of acute and chronic viral diseases. The role of these T-cell memory subsets are now being illuminated as relevant to the tumor-bearing state. The generation and persistence of productive CD8+ T-cell memory subsets is determined, in part, by antigen clearance, costimulation, responsiveness to homeostatic cytokines, and CD4+ T-helper cells. By contrast, chronic exposure to antigen, negative costimulation, and immunomodulation by CD4+ T regulatory cells corrupt productive CD8+ T memory formation. It has become clear from human and mouse studies that the mere generation of CD8+ T-cell memory is not a 'surrogate marker' for cancer vaccine efficacy. Some current cancer vaccine strategies may fail because they amplify, rather than correct or reset, the corrupted CD8+ memory population. Thus, much of the present effort in the development of vaccines for cancer and chronic infectious diseases is aimed at creating effective memory responses. Therapeutic vaccines for cancer and chronic infectious diseases may achieve consistent efficacy by ablation of the dysfunctional immune state and the provision of newly generated, non-corrupted memory cells by adoptive cell transfer.

TLR8-mediated NF-kappaB and JNK activation are TAK1-independent and MEKK3-dependent

TLR8-mediated NF-kappaB and IRF7 activation are abolished in human IRAK-deficient 293 cells and IRAK4-deficient fibroblast cells. Both wild-type and kinase-inactive mutants of IRAK and IRAK4, respectively, restored TLR8-mediated NF-kappaB and IRF7 activation in the IRAK- and IRAK4-deficient cells, indicating that the kinase activity of IRAK and IRAK4 is probably redundant for TLR8-mediated signaling. We recently found that TLR8 mediates a unique NF-kappaB activation pathway in human 293 cells and mouse embryonic fibroblasts, accompanied only by IkappaBalpha phosphorylation and not IkappaBalpha degradation, whereas interleukin (IL)-1 stimulation causes both IkappaBalpha phosphorylation and degradation. The intermediate signaling events mediated by IL-1 (including IRAK modifications and degradation and TAK1 activation) were not detected in cells stimulated by TLR8 ligands. TLR8 ligands trigger similar levels of IkappaBalpha phosphorylation and NF-kappaB and JNK activation in TAK1(-/-) mouse embryo fibroblasts (MEFs) as compared with wild-type MEFs, whereas lack of TAK1 results in reduced IL-1-mediated NF-kappaB activation and abolished IL-1-induced JNK activation. The above results indicate that although TLR8-mediated NF-kappaB and JNK activation are IRAK-dependent, they do not require IRAK modification and are TAK1-independent. On the other hand, TLR8-mediated IkappaBalpha phosphorylation, NF-kappaB, and JNK activation are completely abolished in MEKK3(-/-) MEFs, whereas IL-1-mediated signaling was only moderately reduced in these deficient MEFs as compared with wild-type cells. The differences between IL-1R- and TLR8-mediated NF-kappaB activation are also reflected at the level of IkappaB kinase (IKK) complex. TLR8 ligands induced IKKgamma phosphorylation, whereas IKKalpha/beta phosphorylation and IKKgamma ubiquitination that can be induced by IL-1 were not detected in cells treated with TLR8 ligands. We postulate that TLR8-mediated MEKK3-dependent IKKgamma phosphorylation might play an important role in the activation of IKK complex, leading to IkappaBalpha phosphorylation.

Emerging possibilities in the development and function of regulatory T cells

CD25+CD4+ Regulatory T cells (Treg) represent a unique population of lymphocytes capable of powerfully suppressing immune responses. A large body of experimental data have now confirmed the essential role played by these cells in a host of clinically relevant areas such as self-tolerance, transplantation, allergy and tumor/microbial immunity. Despite this mass of knowledge, significant gaps in our understanding of fundamental Treg biology remain, particularly regarding their development and mechanisms of suppression. In this review we attempt to highlight the current controversies and directions in which this exciting field is moving.

Regulatory T Cells and Toll-Like Receptors in Cancer Therapy: Figure 1

T regulatory (Treg) cells that suppress immune responses may limit the efficiency of cancer immunotherapy. Recent findings indicate that Toll-like receptors (TLR) directly regulate the suppressive activity of Treg cells. Linking TLR signaling to the functional control of Treg cells may offer new opportunities to improve the outcome of cancer immunotherapy by coadminstration of certain TLR ligands.

CD4+ T Regulatory Cell Induction and Function in Transplant Recipients after CD154 Blockade Is TLR4 Independent

Although the role of CD4(+) T regulatory cells (Treg) in transplantation tolerance has been established, putative mechanisms of Treg induction and function in vivo remain unclear. TLR4 signaling has been implicated in the regulation of CD4(+)CD25(+) Treg functions recently. In this study, we first examined the role of recipient TLR4 in the acquisition of operational CD4(+) Treg following CD154 blockade in a murine cardiac transplant model. Then, we determined whether TLR4 activation in allograft tolerant recipients would reverse alloimmune suppression mediated by CD4(+) Treg. We document that donor-specific immune tolerance was readily induced in TLR4-deficient recipients by a single dose of anti-CD154 mAb, similar to wild-type counterparts. The function and phenotype of CD4(+) Treg in both wild-type and TLR4 knockout long-term hosts was demonstrated by a series of depletion experiments examining their ability to suppress the rejection of secondary donor-type test skin grafts and to inhibit alloreactive CD8(+) T cell activation in vivo. Furthermore, TLR4 activation in tolerant recipients following exogenous LPS infusion in conjunction with donor-type skin graft challenge, failed to break Treg-mediated immune suppression. In conclusion, our data reveals a distinctive property of CD4(+) Treg in tolerant allograft recipients, whose induction and function are independent of TLR4 signaling.

Toll-like receptor 2 signaling modulates the functions of CD4+ CD25+ regulatory T cells

Toll-like receptors (TLRs) are primary sensors of both innate and adaptive immune systems and play a pivotal role in response against structurally conserved components of pathogens. Synthetic bacterial lipoprotein (BLP) Pam3Cys-SK4 is a TLR2 agonist that is capable of modulating T cell immune responses. We show here that BLP, together with anti-CD3 antibody [T cell receptor (TcR) activation], induced proliferation of both CD4+ CD25+ regulatory T cells (Tregs) and CD4+ CD25- (effector) T cells in the absence of antigen-presenting cells. The expanded Tregs showed a transient loss of suppressive activity. Moreover, BLP rendered effectors resistant to the suppression of Tregs by increasing IL-2 secretion. BLP also transiently suppressed the induction of Foxp3 (X-linked forkhead/winged helix transcription factor) mRNA in Tregs at the first 8-15 h after T cell receptor activation. Consistent with this observation, BLP-stimulated Tregs regained their inhibitory activity and prevented spontaneous colitis induced by effectors in severe combined immunodeficient mice. Our results demonstrate a previously unrecognized pathway by which TLR expressed on T cells may directly modulate the immune response. Thus, during an acute bacterial infection, BLP may rapidly increase the host's adaptive immunity by expanding effectors and also by attenuating the suppressive activity of Tregs. In the process, BLP also expands the Tregs, which recover their suppressive activity when the infection has subsided, in time to limit potential autoimmunity that might result from the overactivated effectors.

Effects of adenoviral gene transfer of mutated IkappaBalpha, a novel inhibitor of NF-kappaB, on human monocyte-derived dendritic cells

AIM

To investigate the effects of adenoviral gene transfer of IkappaBalpha mutant (IkappaBalphaM), a novel inhibitor of nuclear factor kappaB (NF-kappaB), on apoptosis, phenotype and function of human monocyte-derived dendritic cells (DC).

METHODS

Monocytes, cocultured with granulocyte/macrophage colony-stimulating factor (GM-CSF; 900 ng/mL) and interleukin (IL)-4 (300 ng/mL) for 5 d, followed by stimulation with lipopolysaccharide (LPS; 100 ng/mL) for 2 d differentiated into mature DC. Monocytes were either left untransfected or were transfected with AdIkappaBalphaM or AdLacZ. The transcription and expression of the IkappaBalphaM gene, and the inhibitory effect of IkappaBalphaM on tumor necrosis factor (TNF)-alpha-induced NF-kappaB activation in mature DC were detected by polymerase chain reaction (PCR), reverse transcription-polymerase chain reaction (RT-PCR), Western blot analysis, and electrophoretic mobility shift assays, respectively. The phenotype, apoptosis, IL-12 secretion level of DC, and ability to stimulate the proliferation of T cells were determined by flow cytometry, enzyme-linked immunosorbent assay and mixed leukocyte reaction.

RESULTS

PCR and RT-PCR were used to detect a unique 801 bp band in AdIkappaBalphaM-transfected mature DC, and also a dose- and time-dependent expression of the IkappaBalphaM gene, which peaked at a multiplicity of infection of 100 pfu/cell and at 48 h. Furthermore, AdIkappaBalphaM significantly suppressed the TNF-alpha-induced NF-kappaB activation, augmented apoptosis, downregulated CD80, CD83, and CD86 surface molecules, IL-12 secretion levels and the ability to stimulate the proliferation of T cells in mature DC.

CONCLUSION

AdIkappaBalphaM effectively transfected and potently inhibited NF-kappaB activation in monocyte-derived mature DC. Overexpression of the IkappaBalphaM gene in mature DC may contribute to T-cell immunosuppression through induction of DC apoptosis and downregulation of B7 molecules, providing a potential strategy for future DC-based immunotherapy of asthma.

Epstein-Barr virus (EBV) latent membrane protein-1-specific cytotoxic T lymphocytes targeting EBV-carrying natural killer cell malignancies

Epstein-Barr virus (EBV)-encoded latent membrane protein (LMP) 1 is a potential target for immunotherapy of some proportion of Hodgkin's disease cases, nasopharyngeal carcinomas, EBV-associated natural killer (NK)/T lymphomas, and chronic active EBV infection (CAEBV). Since it is unknown whether EBV-infected NK/T cells are susceptible to lysis by LMP1-specific cytotoxic T lymphohcytes (CTL), we here tested the ability of mRNA-transduced antigen-presenting cells (APC) to stimulate rare LMP1-specific CTL. A 43-amino acid N-terminal deletion mutant LMP1 (DeltaLMP1) could be efficiently expressed in dendritic cells and CD40-activated B cells upon mRNA electroporation. DeltaLMP1-expressing APC were found to stimulate LMP1-specific CTL from a healthy donor and a CTL clone recognized a peptide, IIIILIIFI, presented by HLA-A*0206 molecules. Processing and presentation of the antigenic peptide proved dependent on expression of an immunoproteasome subunit, low-molecular-weight protein-7, as confirmed by RNA interference gene silencing. Furthermore, an EBV-infected NK cell line derived from a patient with CAEBV, and another from an NK lymphoma with enforced HLA-A*0206 expression, were specifically lysed by the CTL. Overall, these data suggest that immunotherapy targeting LMP1 in EBV-associated NK lymphomas and CAEBV might serve as an alternative treatment modality.

Activation of the mammalian immune system by siRNAs

Inhibition of gene expression through RNA interference (RNAi) is emerging as a powerful experimental tool for gene function and target validation studies. The potential uses of this technology seem unlimited, extending to the prevention and therapy of human diseases. However, recent work demonstrating that there are unanticipated, different nonspecific effects associated with the use of small interfering RNAs in mammals has raised concerns about the safe use of RNAi in vivo. These nonspecific effects include activation of the immune system, potentially harming the individual. The application of screening assays for nonspecific activation of both innate and acquired immunity will be necessary for further development of RNAi as a therapeutic tool.

Toll-like receptors and graft rejection

Innate immunity plays a role in fighting against invading microorganisms. Emerging evidence suggests that in addition to responding to pathogen-associated molecular patterns of microorganisms, Toll-like receptors (TLRs) can be activated by endogenous ligands expressed by mammalian cells. Clinical and laboratory studies have shown that TLRs may participate in organ graft rejection and transplant immune tolerance, which are briefly reviewed in the present manuscript.

Toll-like receptors in central nervous system glial inflammation and homeostasis

Toll-like receptors (TLRs) are a family of pattern-recognition receptors expressed on cells of the innate immune system that allow for the recognition of conserved structural motifs on a wide array of pathogens, referred to as pathogen-associated molecular patterns, as well as some endogenous molecules. The recent emergence of studies examining TLRs in the central nervous system (CNS) indicates that these receptors not only play a role in innate immunity in response to infectious diseases but may also participate in CNS autoimmunity, neurodegeneration, and tissue injury. This review summarizes the experimental evidence demonstrating a role for TLRs in the context of CNS inflammation in both infectious and noninfectious conditions.

Direct costimulatory effect of TLR3 ligand poly(I:C) on human gamma delta T lymphocytes

TLR3 recognizes viral dsRNA and its synthetic mimetic polyinosinic-polycytidylic acid (poly(I:C)). TLR3 expression is commonly considered to be restricted to dendritic cells, NK cells, and fibroblasts. In this study we report that human gammadelta and alphabeta T lymphocytes also express TLR3, as shown by quantitative real-time PCR, flow cytometry, and confocal microscopy. Although T cells did not respond directly to poly(I:C), we observed a dramatic increase in IFN-gamma secretion and an up-regulation of CD69 when freshly isolated gammadelta T cells were stimulated via TCR in the presence of poly(I:C) without APC. IFN-gamma secretion was partially inhibited by anti-TLR3 Abs. In contrast, poly(I:C) did not costimulate IFN-gamma secretion by alphabeta T cells. These results indicate that TLR3 signaling is differentially regulated in TCR-stimulated gammadelta and alphabeta T cells, suggesting an early activation of gammadelta T cells in antiviral immunity.

Translating innate immunity into immunological memory: implications for vaccine development

Vaccination is the most effective means of preventing infectious diseases. Despite the success of many vaccines, there is presently little knowledge of the immunological mechanisms that mediate their efficacy. Such information will be critical in the design of future vaccines against old and new infectious diseases. Recent advances in immunology are beginning to provide an intellectual framework with which to address fundamental questions about how the innate immune system shapes adaptive immunity. In this review, we summarize current knowledge about how the innate immune system modulates the quantity and quality of long-term T and B cell memory and protective immune responses to pathogens. In addition, we point out unanswered questions and identify critical challenges, the solution of which, we believe, will greatly facilitate the rational design of novel vaccines against a multitude of emerging infections.

Human CD4+ T cells express TLR5 and its ligand flagellin enhances the suppressive capacity and expression of FOXP3 in CD4+CD25+ T regulatory cells

Germline encoded pattern recognition receptors, such as TLRs, provide a critical link between the innate and adaptive immune systems. There is also evidence to suggest that pathogen-associated molecular patterns may have the capacity to modulate immune responses via direct effects on CD4+ T cells. Given the key role of both CD4+CD25+ T regulatory (Treg) cells and the TLR5 ligand flagellin in regulating mucosal immune responses, we investigated whether TLR5 may directly influence T cell function. We found that both human CD4+CD25+ Treg and CD4+CD25- T cells express TLR5 at levels comparable to those on monocytes and dendritic cells. Costimulation of effector T cells with anti-CD3 and flagellin resulted in enhanced proliferation and production of IL-2, at levels equivalent to those achieved by costimulation with CD28. In contrast, costimulation with flagellin did not break the hyporesponsiveness of CD4+CD25+ Treg cells, but rather, potently increased their suppressive capacity and enhanced expression of FOXP3. These observations suggest that, in addition to their APC-mediated indirect effects, TLR ligands have the capacity to directly regulate T cell responses and modulate the suppressive activity of Treg cells.

Differential expression of CD126 and CD130 mediates different STAT-3 phosphorylation in CD4+CD25− and CD25high regulatory T cells

IL-6 is a pleiotropic cytokine involved in T-lymphocyte biology. Following IL-6 binding, the soluble IL-6R (CD126)-IL-6 complex can directly activate cells that express the signal-transducing gp130 (CD130) molecule, which mediates two distinct signals, mitogenesis by mitogen-activated protein kinase (MAPK) activation and anti-apoptosis by signal transducer and activator of transcription 3 (STAT-3) activation. This 'trans-signaling', also mediated by the soluble CD126/IL-6 fusion protein hyper-IL-6 (H-IL-6), contributes to the perpetuation of autoimmune diseases such as Morbus Crohn or rheumatoid arthritis. On the other hand, the homeostasis of cellular immune reactions and its failure leading to autoimmune diseases are critically controlled by regulatory T cells (Tregs). Here, we investigated the differential expression of CD126 and CD130 on subsets of human leukocytes in blood, tonsil and spleen. Among CD4+ T cells, differential expression of CD126 and CD130 was observed on the basis of CD25 expression. CD4+CD25- T cells were strongly CD126+ and CD130+, whereas CD25(high) Tregs expressed CD126 but little CD130. Both CD126 and CD130 were down-modulated on CD4+CD25- T cells following ligand binding, whereas only marginal modulation was observed on Tregs. Interestingly, we observed a correlation between CD126 and CD130 expression with STAT-3 phosphorylation in CD4+CD25- T cells compared with Tregs after stimulation with IL-6 or H-IL-6, whereas the MAPK extracellular signal-regulated kinase 1/2 were not activated by CD130 dimerization. The differential expression of CD126 and CD130 and subsequent STAT-3 phosphorylation might be relevant for the recently described role of IL-6 in the control of Treg activity.

Vaccines for tumour prevention

Despite tremendous progress in basic and epidemiological research, effective prevention of most types of cancer is still lacking. Vaccine use in cancer therapy remains a promising but difficult prospect. However, new mouse models that recapitulate significant features of human cancer progression show that vaccines can keep precancerous lesions under control and might eventually be the spearhead of effective and reliable ways to prevent cancer.

Regulatory T cells and Toll-like receptors in tumor immunity

Regulatory T (Treg) cells induce immune tolerance by suppressing host immune responses against self- or non-self-antigens, thus playing critical roles in preventing autoimmune diseases. However, tumor cells may take advantage of Treg cells to protect themselves from immune attack elicited by vaccines. Recent studies demonstrate the presence of Treg cells in various types of cancers and their suppressive function. Therefore, Treg cells at tumor sites have detrimental effects on immunotherapy directed to cancer and infectious diseases. This review will discuss antigen specificity of Treg cells, the factors that contribute to Treg cell generation and suppressive function, and their regulation through Toll-like receptor signaling. It was generally though that TLR-mediated recognition of specific structures of invading pathogens initiate innate as well as adaptive immune responses through dendritic cells. New evidence suggests that TLR signaling may directly regulate the suppressive function of Treg cells. Linking TLR signaling to the functional control of Treg cells opens intriguing opportunities to shift the balance between CD4(+) T-helper and Treg cells, in ways that may improve the outcome of cancer immunotherapy.

Immune regulation in tumor-bearing hosts

A wealth of data indicates that tumor immunity directed against a wide variety of malignancies is suppressed in cancer patients. Recent studies have explored the role of 'natural' CD4(+)CD25(+) regulatory T cells (Tregs) in the suppression of tumor immunity in cancer patients. It is now clear, using multiple phenotypic and functional criteria, that the frequency of Tregs is increased in the peripheral blood of cancer patients as well as within the tumor microenvironment. Human Tregs with specificity for tumor-associated antigens have recently been identified, and murine studies have demonstrated that vaccination with tumor-associated antigens can expand Tregs, posing a challenge to cancer vaccine strategies. However, a variety of approaches, including depletion of Tregs or modulation of their activity in vivo, might soon enhance the efficacy of existing cancer vaccines directed against a variety of malignancies.

Toll-like receptor 2 controls expansion and function of regulatory T cells

Tregs play a central role in the suppression of immune reactions and prevention of autoimmune responses harmful to the host. During acute infection, however, Tregs might hinder effector T cell activity directed toward the elimination of the pathogenic challenge. Pathogen recognition receptors from the TLR family expressed by innate immune cells are crucial for the generation of effective immunity. We have recently shown the CD4CD25 Treg subset in TLR2 mice to be significantly reduced in number compared with WT littermate control mice, indicating a link between Tregs and TLR2. Here, we report that the TLR2 ligand Pam3Cys, but not LPS (TLR4) or CpG (TLR9), directly acts on purified Tregs in a MyD88-dependent fashion. Moreover, when combined with TCR stimulation, TLR2 triggering augmented Treg proliferation in vitro and in vivo and resulted in a temporal loss of the suppressive Treg phenotype in vitro by directly affecting Tregs. Importantly, WT Tregs adoptively transferred into TLR2 mice were neutralized by systemic administration of TLR2 ligand during the acute phase of a Candida albicans infection, resulting in a 100-fold reduced C. albicans outgrowth. This demonstrates that in vivo TLR2 also controls the function of Tregs and establishes a direct link between TLRs and the control of immune responses through Tregs.

Discovering differential activation machinery of the Toll-like receptor 4 signaling pathways in MyD88 knockouts

To understand differential time activation of nuclear factor kappaB (NF-kappaB) and the temporal features of the downstream pro-inflammatory cytokines' [tumour-necrosis-factor-alpha (TNF-alpha) and IP-10] mRNA levels in myeloid differentiation primary-response protein 88 (MyD88) knockouts (KOs), I developed a computational model of the TLR4 pathway. The result suggests that the late phase expression of NF-kappaB activity observed in MyD88 KOs is possibly due to a number of novel intermediates acting along the MyD88-independent pathway. I also simulate that the TNF-alpha levels will increase at a longer time in MyD88 KOs, not previously mentioned.

Functional control of regulatory T cells and cancer immunotherapy

Regulatory T (Treg) cells induce immune tolerance by suppressing host immune responses against self- or non-self-antigens. Hence, they not only play critical roles in preventing autoimmune diseases, but also may have detrimental effects on vaccines directed to cancer and infectious diseases. Understanding the antigen specificity and functional control of Treg cells will be crucial to the development of effective cancer immunotherapy. This review will discuss different subsets of Treg cells, the factors that contribute to Treg cell generation and suppressive function, and the ability of signaling through Toll-like receptor 8 to reverse the suppressive function of Treg cells. Importantly, this TLR pathway does not depend on interaction with dendritic cells, but operates independently in Treg cells, relying on TLR8 (with MyD88 as its sole receptor-proximal adaptor) to transduce signals generated by TLR8 ligands. Linking TLR signaling to the functional control of Treg cells opens intriguing opportunities to shift the balance between CD4(+) T-helper and Treg cells, in ways that may improve the outcome of cancer immunotherapy.

Regulatory T cells in immune surveillance and treatment of cancer

Naturally occurring CD25(+)CD4(+) regulatory T cells (T(R) cells), which specifically express the transcription factor Foxp3, engage in the maintenance of immunological self-tolerance and suppressive control of aberrant or excessive immune responses to foreign antigens. They may, on the other hand, impede immune surveillance against cancer and hamper the development of effective immunity to autologous tumor cells. Indeed, natural T(R) cells have been observed to predominantly infiltrate tumor masses especially in the early phase of tumor progression. Depletion of natural T(R) cells by removing CD25(+) T cells prior to tumor challenge is therefore able to provoke effective tumor immunity in animals. Furthermore, attenuation of T(R) cell-mediated suppression in on-going anti-tumor immune responses, for example by altering signaling through CTLA-4 or GITR expressed by natural T(R) cells, can enhance the responses and thereby eradicate advanced cancers. A combination of depletion or attenuation of T(R) cells and concomitant stimulation of effector T cells, systemically or locally in tumors, may be a feasible immunotherapy for cancer.

Endogenous TLR ligands and autoimmunity

Based on an evolutionary conserved repertoire Toll-like-receptors (TLRs) donate specificity to innate immune cells. Therefore, TLRs are considered as paradigmatic for "self" versus "non-self" discrimination. This view, however, needs to be modified since TLR's also appear to recognise "endogeneous", that is host-derived ligands, examples being host-derived DNA and -RNA. Here I discuss physiological and pathophysiological consequences of endogeneous ligand-recognition by TLRs. I conclude that endogeneous ligand recognition by TLRs drives sterile inflammation sustained by innate immune cells in certain autoimmune disorders.

Functional Dynamics of Naturally Occurring Regulatory T Cells in Health and Autoimmunity

A network of regulatory T (Treg) cells exists to downregulate immune responses in various inflammatory circumstances and ultimately assure peripheral T cell tolerance. Naturally occurring CD4(+)CD25(+) Treg cell represents a major lymphocyte population engaged in the dominant control of self-reactive T responses and maintenance of tolerance within this network. CD4(+)CD25(+) Treg cells differentiate in the normal thymus as a functionally distinct subpopulation of T cells bearing a broad T cell receptor repertoire endowing these cells with the capacity to recognize a wide spectrum of self-Ag and non-self-Ag specificities. The development of CD4(+)CD25(+) Treg cells is genetically determined, influenced by Ag-specific and nonspecific signals, costimulation, and cytokines that control their activation, expansion, and suppressive activity. Functional abrogation of these cells in vivo, or genetic defects that affect their development or function, unequivocally predisposes animals and humans to the onset of autoimmune and other inflammatory diseases. Studies have shed light in our understanding of the cellular and molecular basis of CD4(+)CD25(+) Treg cell-mediated immune regulation. In this chapter, we discuss the contribution of naturally occurring CD4(+)CD25(+) Treg cells in the induction of immunologic self-tolerance in animal models and humans and attempt to provide a comprehensive overview of recent findings regarding the phenotype, functional dynamics, and effector mechanism of these cells in autoimmune diseases.

TLR2 agonist ameliorates murine experimental allergic conjunctivitis by inducing CD4 positive T-cell apoptosis rather than by affecting the Th1/Th2 balance

Innate immune responses that operate through Toll-like receptors (TLRs) are actively involved in the development of diseases predominantly mediated by adaptive immune responses. This is true also for allergic disease, as TLRs have been found to be involved in the development of allergic airway inflammation. We investigated whether stimulating TLR2 also abrogates murine allergic conjunctivitis by upregulating Th1 responses. We found that treating mice during the efferent phase with the TLR2 agonist Pam3CSK4 significantly suppressed eosinophil infiltration into the conjunctiva. However, Pam3CSK4 treatment inhibited both the Th1 and Th2 responses in the mice, and also suppressed eosinophil infiltration in IFN-gamma knockout mice. Flow cytometric analysis demonstrated that Pam3CSK4 treatment significantly elevated the numbers of annexin V-positive splenocytes, especially CD4 positive T cells. Thus, the stimulation of TLR2 during the efferent phase of murine allergic conjunctivitis suppresses eosinophil infiltration by inducing CD4 positive T-cell apoptosis rather than upregulating Th1 responses.

Triggering TLR signaling in vaccination

Toll-like receptors (TLRs) are a family of pattern-recognition receptors that are an important link between innate and adaptive immunity. Many established, as well as experimental, vaccines incorporate ligands for TLRs, not only to protect against infectious diseases but also in therapeutic immunization against noninfectious diseases, such as cancer. We review the underlying mechanisms by which engagement of TLR signaling pathways might trigger an adaptive immune response after immunization. Although the engagement of TLR signaling pathways is a promising mechanism for boosting vaccine responses, questions of efficacy, feasibility and safety remain the subject of active investigation.