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

Toll-like receptors are critical for clearance of Brucella and play different roles in development of adaptive immunity following aerosol challenge in mice

Brucella spp. cause undulant fever in humans and brucellosis in variety of other animals. Both innate and adaptive immunity have been shown to be important in controlling Brucella infection. Toll-like receptors (TLRs) represent a group of pattern recognition receptors (PRRs) that play critical roles in the host innate immune response, as well as development of adaptive immunity. In the current report, we investigated the role of TLR signaling in the clearance of Brucella and development of adaptive immunity in TLR2(-/-), TLR4(-/-), or MyD88(-/-) mice following aerosol exposure to B. melitensis 16 M. Consistent with previous reports, MyD88 is required for efficient clearance of Brucella from all three organs (lung, spleen, and liver). The results reveal Th2-skewed immune responses in TLR2(-/-) mice late in infection and support a TLR2 requirement for efficient clearance of Brucella from the lungs, but not from the spleen or liver. Similarly, TLR4 is required for efficient clearance of Brucella from the lung, but exhibits a minor contribution to clearance from the spleen and no demonstrable contribution to clearance from the liver. Lymphocyte proliferation assays suggest that the TLRs are not involved in the development of cell-mediated memory response to Brucella antigen. Antibody detection reveals that TLR2 and 4 are required to generate early antigen-specific IgG, but not during the late stages of infection. TLR2 and 4 are only transiently required for IgM production and not at all for IgA production. In contrast, MyD88 is essential for antigen specific IgG production late in infection, but is not required for IgM generation over the course of infection. Surprisingly, despite the prominent role for MyD88 in clearance from all tissues, MyD88-knockout mice express significantly higher levels of serum IgA. These results confirm the important role of MyD88 in controlling infection in the spleen while providing evidence of a prominent contribution to protection in other tissues. In addition, although TLR4 and TLR2 contribute little to control of spleen infection, a significant contribution to clearance of lung infection is described.

Differential cytokine expression in T-cell subsets of chicken caecal tonsils co-cultured with three species of Lactobacillus

Members of the intestinal microbiota play an important role in the development of T-cells. Little is known about responses of intestinal T-cell subsets of chickens to commensal bacteria. Therefore, we set out to characterise cytokine responses in T-cells after exposure to lactobacilli. Caecal tonsil mononuclear cells were isolated and co-cultured with Lactobacillus acidophilus, Lactobacillus reuteri and Lactobacillus salivarius for 12 hours. Subsequently the CD4+ and CD8+ cells were fractionated by flow cytometry and the expression of pro- and anti-inflammatory cytokines as well as Toll-like receptor 21 (TLR21) was determined. The results demonstrated that chicken CD4+ and CD8+ T-cells express TLR21 and that the various isolates of lactobacilli differentially induces the expression of interleukin 10, interferon-gamma and transforming growth factor beta. Our results demonstrate that different Lactobacillus species have the capacity to regulate intestinal T-cell responses and that these responses may be important to intestinal homeostasis.

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.

Stronger Toll-like receptor 1/2, 4, and 7/8 but less 9 responses in peripheral blood mononuclear cells in non-infectious exacerbated asthmatic children

Toll-like receptors (TLR) initiate innate and often affect adaptive immune response. This study aimed to determine if TLR response and T regulatory cell (Treg) function in peripheral blood mononuclear cells (PBMC) correlate with clinical severity in non-infectious asthma. TLR1-9 expression and representative response cytokine TNF-α, IL-6, and IFN-β secretions were analyzed after stimulation by TLR1-9 ligands from 17 non-infectious asthmatic children. TNF-α production was higher in TLR1/2 (median 385.4 vs. 250.3 pg/ml in 1 μg/ml Pam3CSK4, p=0.0078), TLR4 (2392.4 vs. 1355.9 in 1 μg/ml LPS; p=0.0005), and TLR7/8 (10,776.2 vs. 4237.0 pg/ml in 1 μg/ml R848, p=0.0079) of patients in exacerbation than those in convalescence and healthy controls despite equal TLR expression. TNF-α production stimulated by TLR9 agonist was significantly lower in exacerbation (17.7 vs. 34.9 pg/ml in 1 μg/ml ODN2216, p=0.0175), while IL-6 production had similar patterns but was significantly lower in TLR3 signaling (119.7 vs. 245.0 pg/ml in 0.1 μg/ml poly(I:C), p=0.0033). IFN-β production by TLR3 agonist also decreased in exacerbation but not statistically significant. Six older children showed decreased FOXP3 percentage in CD4+CD25(high) and decreased suppression capability in exacerbation but restored in stabilization (82.8% vs. 90.0%, p=0.0061 and 60.9% vs. 81.7%, p=0.0071; respectively). In conclusion, normalizing imbalanced TLR signaling and enhancing Treg cell capability may guide possible therapeutic strategies for non-infectious asthma in exacerbation.

Small heat shock protein αA-crystallin prevents photoreceptor degeneration in experimental autoimmune uveitis

The small heat shock protein, αA-crystallin null (αA−/−) mice are known to be more prone to retinal degeneration than the wild type mice in Experimental Autoimmune Uveoretinitis (EAU). In this report we demonstrate that intravenous administration of αA preserves retinal architecture and prevents photoreceptor damage in EAU. Interestingly, only αA and not αB-crystallin (αB), a closely related small heat shock protein works, pointing to molecular specificity in the observed retinal protection. The possible involvement of αA in retinal protection through immune modulation is corroborated by adaptive transfer experiments, (employing αA−/− and wild type mice with EAU as donors and Rag2−/− as the recipient mice), which indicate that αA protects against the autoimmune challenge by modulating the systemic B and T cell immunity. We show that αA administration causes marked reduction in Th1 cytokines (TNF-α, IL-12 and IFN-γ), both in the retina and in the spleen; notably, IL-17 was only reduced in the retina suggesting local intervention. Importantly, expression of Toll-like receptors and their associated adaptors is also inhibited suggesting that αA protection, against photoreceptor loss in EAU, is associated with systemic suppression of both the adaptive and innate immune responses.

The Immune Response to Trypanosoma cruzi: Role of Toll-Like Receptors and Perspectives for Vaccine Development

In the past ten years, studies have shown the recognition of Trypanosoma cruzi-associated molecular patterns by members of the Toll-like receptor (TLR) family and demonstrated the crucial participation of different TLRs during the experimental infection with this parasite. In the present review, we will focus on the role of TLR-activated pathways in the modulation of both innate and acquired immune responses to T. cruzi infection, as well as discuss the state of the art of vaccine research and development against the causative agent of Chagas disease (or American trypanosomiasis).

Peripheral immune contributions to the maintenance of central glial activation underlying neuropathic pain

Recent evidence implicates an adaptive immune response in the central nervous system (CNS) mechanisms of neuropathic pain. This review identifies how neuropathic pain alters CNS immune privilege to facilitate T cell infiltration. Once in the CNS, T cells may interact with the local antigen presenting cells, microglia, via the major histocompatibility complex and the costimulatory molecules CD40 and B7. In this way, T cells may contribute to the maintenance of neuropathic pain through pro-inflammatory interactions with microglia and by facilitating the activation of astrocytes in the spinal dorsal horn. Based on the evidence presented in this review, we suggest that this bidirectional, pro-inflammatory system of neurons, glia and T cells in neuropathic pain should be renamed the pentapartite synapse, and identifies the latest member as a potential disease-modifying therapeutic target.

Modulation of γδ T cell responses by TLR ligands

Toll-like receptors (TLR) are pattern-recognition receptors that recognize a broad variety of structurally conserved molecules derived from microbes. The recognition of TLR ligands functions as a primary sensor of the innate immune system, leading to subsequent indirect activation of the adaptive immunity as well as none-immune cells. However, TLR are also expressed by several T cell subsets, and the respective ligands can directly modulate their effector functions. The present review summarizes the recent findings of γδ T cell modulation by TLR ligands. TLR1/2/6, 3, and 5 ligands can act directly in combination with T cell receptor (TCR) stimulation to enhance cytokine/chemokine production of freshly isolated human γδ T cells. In contrast to human γδ T cells, murine and bovine γδ T cells can directly respond to TLR2 ligands with increased proliferation and cytokine production in a TCR-independent manner. Indirect stimulatory effects on IFN-γ production of human and murine γδ T cells via TLR-ligand activated dendritic cells have been described for TLR2, 3, 4, 7, and 9 ligands. In addition, TLR3 and 7 ligands indirectly increase tumor cell lysis by human γδ T cells, whereas ligation of TLR8 abolishes the suppressive activity of human tumor-infiltrating Vδ1 γδ T cells on αβ T cells and dendritic cells. Taken together, these data suggest that TLR-mediated signals received by γδ T cells enhance the initiation of adaptive immune responses during bacterial and viral infection directly or indirectly. Moreover, TLR ligands enhance cytotoxic tumor responses of γδ T cells and regulate the suppressive capacity of γδ T cells.

Innate immunity and transplantation tolerance: the potential role of TLRs/NLRs in GVHD

Graft-versus-host disease (GVHD) is a serious complication of allogeneic hematopoietic cell transplantation (HCT) and this occurs as donor T lymphocytes, activated by recipient antigen presenting cells (APC), attack the host tissues or organs. This APC activation is a crucial initial step of influencing the outcome of GVHD and is mediated by innate immune signaling. Toll-like receptors (TLRs) and nucleotide binding oligomerization domain (NOD)-like receptors (NLRs) are important components of innate immunity; both families of receptors are known for sensing various microbial ligands or danger signals. Signaling through TLRs/NLRs regulate activities of APCs, through phagocytosis, cytokine and chemokine release, delivery of APCs from peripheral tissues to draining lymph nodes, and antigen presentation. Several TLRs/NLRs have been identified and their ligands and signaling pathways have been described. Recent findings suggest a significant association of TLR/NLR polymorphisms with the increased risk for severe GVHD. Therefore, these TLR/NLR pathways likely contributing to immune response for GVHD may serve as novel therapeutic targets to facilitate allograft tolerance. This review summarizes the role of TLRs/NLRs innate immune receptors and signaling in GVHD pathophysiology.

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.

Bacterial infections, alloimmunity, and transplantation tolerance

Transplantation of solid organs across histocompatibility barriers in the absence of immunosuppression is invariably followed by acute allograft rejection. Although several immunosuppressive regimens have been developed to prevent allograft rejection, these global immunosuppressive agents effectively inhibit all T cells, leaving the host vulnerable to infections. Thus, a major goal in transplantation immunology is to induce donor-specific tolerance that results in the extended suppression of allograft-specific immune responses, while leaving the remainder of the immune system competent to fight infections and malignancies. Initial successes in identifying approaches that successfully induce transplantation tolerance in experimental models have led to a newer research focus of identifying potential barriers to the induction of such tolerance as well as events that may reverse established allograft tolerance. Both clinical and experimental studies have identified bacterial infections as a possible trigger of allograft rejection. Recently, experimental models of transplantation tolerance have identified that bacterial signals can promote acute allograft rejection either by preventing the induction of transplantation tolerance or by reversing tolerance after it has been stably established. This review summarizes experimental and clinical literature supporting the hypothesis that bacterial infections and innate immunity can qualitatively and quantitatively alter adaptive alloreactivity through effects on innate immune responses.