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.
Innate immunity protects the host from pathogenic infectious agents. Every infectious microorganism possesses conserved molecular structures, for example, lipopolysaccharide, peptidoglycan, flagellin, microbial nucleic acids and these are collectively referred to as pathogen-associated molecular patterns (PAMPs) [
At least 5 classes of PRRs have been characterized: Toll-like receptors (TLRs), retinoic-acid-inducible gene-I- (RIG-I-) like receptors (RLRs), nucleotide-binding domain and leucine-rich repeat containing gene family (alternatively named NOD-like receptors, NLRs), C-type lectin receptors (CLRs) and cytosolic DNA receptors (CDRs) [
Upon recognition of foreign antigen for DCs via the TLR-PAMP interaction [
The effects of TLR on T-cell activation. PAMPs from invading pathogens bind with TLRs expressed in DCs, which causes DC activation. Activated DCs migrate to the draining lymph nodes where, in the presence of co-stimulatory signals and instructing cytokines, they present the antigen epitope with MHC molecules to activate naive T cells. DCs also induce iTreg in the presence of TGF-
T-cell development and differentiation. It is believed that thymic lymphoid progenitor cells are derived from circulating hematopoietic stem cells originating from the bone marrow. The initial CD4/CD8 double-negative (DN) thymocytes migrate from the corticomedullary junction to the subcapsular region of the cortex and sequentially transform into DN1 (CD44+CD25
Thymic T-cell progenitors are believed to come from circulating hematopoietic stem cells originating from bone marrow. All peripheral T cells are developed from these progenitor cells [
Positively selected thymocytes migrate to the medulla via CCR7-mediated chemotaxis [
Self-tolerance is induced in thymus either by negative selection or by natural regulatory T cells (nTreg) development. Most of the nTreg cells are derived from CD4+ SP thymocytes residing in the medullary compartment of the thymus [
Peripheral T-cell tolerance in lymph node. All the subsets of LNSC can express PTA. AIRE and Deaf1 are involved in the regulation of this expression. Both the LNSC and follicular DC in lymph node can serve as APC to present or cross-present self-epitopes to T cells. Lymph contains abundant-processed protein fragments and peptides from draining organs or tissues and serves as a significant pool of self-antigen for the induction of peripheral tolerance. LNSC can upregulate co-stimulatory molecules to induce T-cell lineage deletion. The autoreactive T-cell lineage deletion is mediated by apoptosis mediated by Fas or Bim signals when inflammation is absent. The engagement of Fas ligand with Fas on T-cell surface triggers the apoptosis of activated T cell through caspase-dependent pathway. T-cell stimulation causes downregulation of Bcl-2 and a transient slight upregulation of Bim and this results in increased uncomplex Bim which is combined with Bcl-2 in resting status. This then activates Bcl-2 homologous antagonist/killer (Bak) and Bcl-2–associated X protein (Bax). Consequently, the integrity of mitochondria is damaged and this culminates in cell death. The tolerogenic DCs induce T-cell functional tolerance, that is, anergy by upregulation of either CTLA-4 or PD-1 expression in T cells. Augmented expression of CTLA-4 can block co-stimulatory signals by binding to CD80/86 in competition with CD28 to induce T-cell anergy. In recognition of self-antigen, PD-L1 on tolerogenic DCs interacts with PD-1 on T cells to limit T-cell activity in peripheral tissues and maintain T cell in unresponsiveness. PD-1 suppresses the PI3K induction and Akt activation. This disturbs cellular glucose metabolism and impairs T-cell survival. PD-1 activation also inhibits the cell-survival factor Bcl-xL production. CTLA-4 engagement blocks Akt phosphorylation by activation of protein phosphatase 2. Engagement of both PD-1 and CTLA-4 can significantly decrease gene transcriptions of T cell being activated.
In addition to the tolerance induced in thymus, autoreactive T cells that have escaped from negative selection in thymus due to low avidity of TCR to self-peptide-MHC complex [
Lymph nodes are a primary location where peripheral tolerance takes place. It has been demonstrated that lymph node stromal cells (LNSCs), similar to mTECs in thymus, are able to express a variety of TSAs to induce immune tolerance of T cells [
LNSCs are reported capable of upregulating co-stimulatory molecules to induce T-cell lineage deletion rather than activation [
Mucosa discussed here are those that line the gastrointestinal system and the respiratory system including nasal passages. The largest immune organ of the body is the gut-associated lymphoid tissue (GALT) consisting of Peyer’s patches and isolated lymphoid follicles [
Intestinal commensal microbiota is essential for adaptive and innate immunity. In germ-free mice, the absence of these bacteria results in impaired local and systemic immune responses. This is evidenced by a reduced number and smaller sized Peyer’s patches, a reduced number of mesenteric lymph nodes and diminished IgA and IgG production [
Metabolites of intestinal microbiota, for example, in mice with dextran-sulfate-sodium- (DSS-) induced colitis, short-chain fatty acids such as acetate, a fermented product of
The orally ingested antigen can be taken up by a variety of mechanisms. Microfold cells (M cells) are specialized epithelial cells without microvilli and thick glycocalyx in the small intestine overlying Peyer’s patches and lymphoid follicles and are responsible for transcytosis [
A variety of regulatory mechanisms are involved in oral tolerance. The amount of ingested antigen is a major factor that determines the mechanism of oral tolerance. Generally, low amounts of antigen result in Treg induction while higher doses lead to immune cell anergy or clonal deletion [
All major types of regulatory T cells are involved in oral tolerance, including thymic-derived nTreg, mucosally induced iTreg, IL-10 secreting CD4+CD25lowCD45RBlow type 1 regulatory T cell (Tr1 cell), TGF-
Nasal administration of antigen preferentially induces IL-10-dependent Treg cell development, for example, Tr1 cell and CD4+CD25−LAP+ Treg cell [
CD4+ T cells play critical roles in the functioning of the host immune system. Upon stimulation, peripheral CD4+ T cells can differentiate into T helper (Th) cells or inducible Treg cells (iTreg). Currently, at least 4 Th cell subsets have been identified, Th1, Th2, Th17 and iTreg [
APCs take up antigen and digest it in the cytosol to process the epitope. The epitope is then presented together with MHC molecules to TCR on the T-cell surface. Simultaneously, APCs also secrete co-stimulatory molecules for example, CD80, CD86 that bind the co-stimulatory receptor of T cells, for example, CD28. Thus all 3 elements are required for T-cell activation, that is, epitope, MHC molecules and costimulation signals. Upon TCR activation, T cells produce CD154 (alternatively named CD40L) to bind CD40 on the cell surface of APCs to further activate APCs. The lineage commitment of Th cells is determined by the cytokine milieu, transcription factors and co-stimulatory molecules such as CD28, CD154. The transcription factors involved in this process are activated by TCR signaling [
Newly primed CD4+ T cells are programmed by various cytokines and other factors from DCs to produce transcription factors. T box expressed in T cells (T-bet) is a major factor for Th1 cell differentiation and IFN-
GATA3, a member of GATA transcription factor family capable of binding to the DNA sequence “GATA,” is the master regulator of Th2 [
STAT6 and STAT5 are essential in Th2 cell differentiation and expansion [
The master regulator of Th17 cell is retinoic acid receptor related orphan receptor-
Treg cell development is controlled by the transcription factor Foxp3 [
Lineage commitment of Tfh cells is controlled by transcriptional factor Bcl-6, identified by the transcriptional profiles obtained from microarray analysis in Tfh cells that was Bcl-6 upregulated [
When exposed to foreign antigens, peripheral naive CD8+ T cells differentiate into two reciprocal subsets: short-lived effector T cells, that is, CTLs and long-lived memory T cells [
It has been suggested that the CD8+ effector and memory T cell develops from a single precursor cell when instructed by distinct TCR signals, cytokines [
Transcriptional factors, T-bet, eomesodermin (Eomes), Bcl-6 and Blimp-1 are involved in CD8+ T-cell differentiation. T-bet is the master regulator of CD8+ T cells [
Bcl-6 and Blimp-1 are transcriptional repressors. Blimp-1 expression is required for the terminal differentiation of effector CD8+ T cells, that is, the short-lived CD8+ CTLs [
STAT5 plays a critical role in the maintenance of phenotype of effector CD8+ T cells. It is also required in the induction of the anti-apoptotic molecule Bcl-2 expression by IL-7 and IL-15 and the maintenance of Bcl-2 expression in effector CD8+ T cells [
Toll was initially identified as an essential protein that plays a central role in the establishment of dorsoventral polarity in the embryo of Drosophila [
Thirteen TLRs have been currently identified, TLR1 to TLR13, of which TLR1 to TLR9 are conserved both in human and mice. TLR10 is not functional in mice while TLR11, TLR12 and TLR13 are absent from human genome [
TLRs can be classified as cell-surface TLRs or intracellular TLRs. The former group consists of TLR1, TLR2, TLR4, TLR5, TLR6, TLR10, TLR11 and TLR12, and it is largely expressed on the cell surface and recognizes molecules mainly from microbial membrane, for example, lipid, lipoprotein, or lipopeptide and protein. The latter group is composed of TLR3, TLR7, TLR8, TLR9, and perhaps TLR13 in mice localized in intracellular compartments like endoplasmic reticulum (ER), endosomes, lysosomes, and endolysosomes to detect microbial nucleic acids [
Intracellular TLRs are present in the ER in resting cells and move to endosomes upon stimulation of the cells (Figure
Intracellular TLRs traffic. Intracellular TLRs are present in the ER in resting cells and migrate to endosomes upon stimulation. Chaperone proteins, for example, UNC93B1 are required for their residence in ER and for their intracellular trafficking. When the ligands are taken into the cell, TLRs exit the ER through Golgi complex by conventional secretory pathways and reach the endolysosome where they interact with the ligands. TLR9 is cleaved by lysosomal cysteine proteases within their ectodomains in the endolysosome. TLR3 does not appear to be required for proteolysis during intracellular trafficking.
Chaperone proteins are required for maintaining the retention of these TLRs in ER in resting cells and their intracellular trafficking. UNC93B1, a highly conserved multiple membrane-spanning protein in ER, is involved in trafficking of nucleotide-sensing TLRs (Figure
Upon binding ligands, TLRs dimerize to form homodimer or heterodimer (e.g., TLR2/TLR1, TLR2/TLR6 and perhaps TLR2/TLR10) and recruit adaptor molecules through the interaction of their intracellular TIR domain and the TIR domain of adaptor molecules [
MyD88 is the essential adaptor for most TLRs. Upon ligand recognition, TLR recruits MyD88 to its cytoplasmic TIR domain by association with the TIR domain of the adaptor molecule (Figure
MyD88 signal pathway. MyD88 is the universal adaptor of all the identified TLRs except TLR3. In this figure, TLR1/TLR2 is used to illustrate the MyD88 signal pathway. TLR1/TLR2 uses triacryl lipopeptide as the ligand to recruit MyD88 via its cytoplasmic TIR domain. MyD88 interacts with DD to associate with IRAK4. IRAK4 then phosphorates IRAK1 and IRAK2 activates TRAF6. TRAF6 induces the synthesis of polyubiquitin chains that links TRAF6, NEMO, IRAK1 and TAB2, 3, 4. The ubiquitination of TAB2/3/4 in association with TAB1 activates TAK1. This induces phosphorylation of IKK complex resulting in the dissociation of I
TRIF is the sole adaptor of TLR3 and the adjunctive adaptor of TLR4. After sensing dsRNA, the TIR domain of TLR3 associates TRIF TIR, then TRIF interacts with receptor-interacting protein 1 (RIP1) through the RIP homotypic interaction motif (RHIM) present in both proteins (Figure
TRIF signal pathway. In TLR1-TLR13, TRIF is the sole adaptor of TLR3 and also an adjunct adaptor of TLR4. Here, the TLR3-TRIF signal is illustrated as an example of TRIF pathway. dsRNA that is internalized in endosome binds to TLR3, which possesses two dsRNA binding sites near the N-terminus and C-terminus, respectively. When combined with dsRNA, a sole dsRNA molecule associates two TLR3 molecules through four dsRNA binding sites in an “m” shape. TLR3 TIR domain combines with the TIR domain of TRIF. The interaction of TRIF with RIP1 or TRAF6 and Peli1 results in polyubiquitination of RIP1, the latter binds ubiquitin receptors TAB2 and TAB3 which activates TAK1. Activated TAK1 induces phosphorylation of IKK complex composed of IKK
Various viral infections through TLR interaction can induce type I IFN production. TLR3 recognizes ssRNA virus (West Nile virus), dsRNA virus (reovirus), respiratory syncytial virus, mouse cytomegalovirus (MCMV); TLR7 recognizes ssRNA viruses (vesicular stomatitis virus, influenza virus); TLR8 recognizes ssRNA from RNA virus; TLR9 recognizes dsDNA viruses (Herpes simplex virus, MCMV), CpG motifs from bacteria and viruses [
TLRs activation has been shown to bridge the innate and adaptive immunity [
On the other hand, a recent report indicated that signals from Th cells can govern the formation and function of specialized DC subsets, for example, Th1 and Th17 cells cause monocytes differentiation into Th1- or Th17-promoting DC subsets in psoriasis lesion, and Th2 cells induce the production of Th2-promoting DC subset in acute atopic dermatitis [
The expression and the activity of TLRs in T cells are related to the functional status, for example, effector or memory cells and central memory or effector memory cells as well as the activation status of T cells by TCR signals (Table
TLR expression and direct effects on T cells [
TLR | Location | Typical ligand | Expression in T-cell subsets | Direct effect on T cells | ||
---|---|---|---|---|---|---|
Naive | Activated/Memory | iTreg | ||||
TLR1 | Cell surface | Triacryl lipopeptide | ± | ++ | + | Increased effector T-cell proliferation and survival; abrogate the suppressive function of Treg cells |
TLR2 | Cell surface | Peptidoglycan | ± | ++ | + | Increased cell proliferation and survival; promote cytotoxic activity of CTL; generate efficient memory T cells; augment Treg cell proliferation with temporal loss of suppression |
TLR3 | Endosome | dsRNA | + | ++ | − | Promote activated CD4+ T-cell survival |
TLR4 | Cell surface | Lipopolysaccharide | ± | ++ | + | Induce Treg cell activation; enhance the suppressive function of Treg cells |
TLR5 | Cell surface | Flagellin | + | + | + | Augment the suppressive capacity of Treg cells |
TLR6 | Cell surface | Diacryl lipopeptide | + | + | + | Block the suppressive function of Treg cells |
TLR7 | Endosome | ssRNA | + | + | − | Augment activation/function of T cells; block the suppressive function of Treg cells |
TLR8 | Endosome | ssRNA | + | + | + | Augment activation/function of T cells; block the suppressive function of Treg cells |
TLR9 | Endosome | CpG DNA | + | ++ | − | Promote activated CD4+ T-cell survival; inhibit Treg cell suppression |
++: enhanced expression; +: normal expression; ±: weak or low expression; −: expression not detectable.
TLRs expressed in T cells have been suggested to act as co-stimulatory molecules involved in T-cell activation [
Costimulation of T cells. Antigen uptake by DCs is followed by epitope presentation by MHC complex molecules to TCR expressed on T-cells surface (signal 1). Upon TCR-activation signal, T cells produce CD154 to bind CD40 on the cell surfaces of DCs to further activate DCs. After interacting with TLRs, DCs express CD80 and CD86 which combine with CD28 in T cells for costimulation of T cells (signal 2). Activated DCs also produce cytokines to instruct T cells for polarized differentiation (signal 3). TLRs expressed in T cells act as co-stimulatory molecules in T-cell activation by reducing the activation requirements for signals 1 and 2 and generating efficient memory T cell in response to a weak signal 1. Some TLR ligands even can induce signal 2 in the absence of CD28 via activation of TLR expressed on T cells.
TLR2 agonist Pam3Cys acts directly on purified Treg cells resulting in an augmented Treg cells proliferation. This is accompanied by a temporal loss of the suppressive Treg phenotype in the presence of TCR stimulation [
Pam3CSK4, a TLR1/TLR2 ligand can induce tumor remission in severe combined immunodeficiency (SCID) mice by diminishing the suppressive function of Foxp3+ Treg cells and enhancing the cytotoxicity of tumor-specific CTLs. Adoptive transfer of CTLs and Treg cells pretreated with Pam3CSK4 from wild-type mice into tumor-bearing SCID mice can restore antitumor immunity in SCID mice by reciprocal downregulation of Treg cells and upregulation of CTL function [
Activation of TLR4 in CD4+CD25+ Treg cells by LPS, in the absence of APC, can directly induce Treg cells activation. This activation involves the upregulation of activation markers, for example, CD69, CD44, CD38, as well as B7-1 and promotes cellular survival and proliferation [
Treg cells’ phenotypic plasticity is seen by their expression of proinflammatory cytokines such as IL-17, IFN-
Viral antigen taken up by APCs are processed into epitopes, loaded onto MHC-I molecules and cross-presented to CD8+ T cells eliciting an anti-virus CD8+ T-cell response. However, not all the potential epitopes can be equally cross-presented to CD8+ T cells. The epitopes recognized by the most abundant cognate T-cell populations are referred to as being immunodominant, while those recognized by less abundant T-cell populations are named as subdominant determinants. Thus, the immunodominant and subdominant determinants constitute a hierarchy (
The outcome of presentation by DCs depends on its activation status. DCs activated by PAMPs, for example, TLR ligands from invading pathogen will be capable of producing co-stimulatory molecules and proinflammatory cytokines immunogenic. On the other hand, self-antigen from apoptotic self-cells lack TLR ligands and cannot induce maturation of DCs and this eventually results in tolerance [
Human monocytes, when cultured with Wnt5a and subsequently stimulated by TLR ligands, can differentiate into DCs. Enhanced production of inhibitory ligands PD-L1 and PD-L2 rather than upregulation of CD83, HLA-DR, CD40, CD86, CD80 and CCR7 molecules would also occur [
Administration of TLR3 ligand poly(I:C) results in a strong expression of PD-1 ligand (PD-L1) in all subsets of LNSCs [
The discrimination of self or nonself antigen by DCs is also TLR dependent [
TLRs are directly involved in mucosal tolerance development. PAMPs from nonpathogenic commensal microorganisms in mucosa are also termed microbe-associated molecular patterns (MAMPs) [
TLR1, TLR2, TLR3, TLR4 and TLR5 as well as TLR9 proteins have been found expressed both in human small intestines and colon [
The mechanism of TLR in maintaining intestinal homeostasis is not fully understood. TLR hyporesponsiveness to commensal microbiota has been suggested to play an important role in keeping homeostasis in the gut. Several mechanisms to account for this hyporesponsiveness include downregulating TLR surface expression and upregulated inhibitory Toll interacting protein with reduced phosphorylation of IRAK [
A variety of DCs have been identified in intestine [
T cells play a central role in the cell-mediated immunity of the host. All subsets of T cells originate from thymocytes in thymus where they acquire their surface TCR repertoires and develop the primary phenotypic markers then migrate to peripheral lymphatic organ. Upon detection of infectious agents, T cells are activated and differentiate into effector T cells or Treg cells. TLRs are canonical members of PRRs capable of inducing T-cell activation through cross-presentation of APCs or directly acting on T cells. Activation of all the identified TLRs except TLR3 results in signaling through the MyD88-NF-
The lymph node is the major peripheral lymph organ where antigen-specific responses or tolerance is triggered. As inflammation is a prerequisite to induce immune responses rather than tolerance, it is conceivable that delivery of inflammatory cytokines such as IL-12, IFN-
Current studies support the concept of reprogramming of TLR ligands, for example, CpG ODN on Treg cells. This raises the question of whether it might be possible to overcome the immunosuppressive effects of Treg cells, for example, in patients with disordered immunity. Indeed should the Th cell be reprogrammable, the roadmap of autoimmunity therapy and/or other types of therapy would have to be reevaluated. Some disorders of immunity requiring enhanced immunosuppression can occur in the context of liver transplantation [
Autoimmune regulator
Activator protein 1
Antigen-presenting cell
B cell lymphoma 6
Bcl-2-interacting mediator of cell death
B lymphocyte-induced maturation protein 1
C-C chemokine receptor
Conventional DC
C-type lectin receptor
Unmethylated cytosine preceding guanosine motif
Cortical thymic epithelial cell
Cytotoxic T lymphocyte
Cytotoxic T lymphocyte-associated antigen-4
C-X-C chemokine receptor
Mouse cytomegalovirus
DNAX-activating protein of molecular mass 12 kilodaltons
Dendritic cell
Death domain
Deformed epidermal autoregulatory factor 1
CD4/CD8 double-negative
CD4/CD8 double-positive
Double-stranded RNA
Dextran sulfate sodium
Eomesodermin
Endoplasmic reticulum
Fluorescence-activated cell sorting
Fas-associated cell death domain
Forkhead box P3
Gut-associated lymphoid tissue
A family of transcription factors capable of binding to the DNA sequence “GATA”
Granulocyte macrophage colony-stimulating factor
Heat shock protein
Indoleamine 2, 3-dioxygenase
Intestinal epithelial cells
Interferon
Inhibitor of
Interleukin
IL-12 receptor
Immunomagnetic cell sorting
IL-1R-associated kinase
Interferon regulatory factor
Inducible regulatory T cell
Inhibitor of
Immunoreceptor tyrosine-based activation motif
c-Jun N-terminal kinase
Latency-associated peptide
Lymphocytic choriomeningitis virus
Lymphatic endothelial cell
Lymph node stromal cell
Lipopolysaccharide
Leucine-rich repeat
MyD88-adapter-like
Microbe-associated molecular patterns
Mouse cytomegalovirus
Melanoma differentiation-associated gene 5
Myeloid DC
Mitogen-activated protein kinase phosphatase-1
Medullary thymic epithelial cell
Myeloid differentiation factor 88
NF-
NF-
NF-
Nuclear factor
Nucleotide binding domain and leucine-rich repeat containing gene family
Natural regulatory T cell
Oligodeoxynucleotide
Ovalbumin
Pathogen-associated molecular patterns
Programmed death 1
Plasmacytoid DC
Phosphatidylinositol 3-kinase
Polyinosinic-polycytidylic acid
Pattern recognition receptor
Peripheral tissue-restricted antigen
Receptor-interacting protein homotypic interaction motif
Retinoic acid-inducible gene-I
Receptor-interacting protein
Retinoic acid-inducible gene-I-like receptor
Retinoic acid receptor related orphan receptor
Recent thymic emigrants
Severe combined immunodeficiency
Son of sevenless gene 1
CD4/CD8 single-positive
Single-stranded RNA
Signal transducer and activator of transcription
Transforming growth factor
Transforming growth factor
T box expressed in T cell
TRAF family member-associated NF-
T-cell factor 1
Central memory T cell
T-cell receptor
Effector memory T cells
T follicular helper cell
Transforming growth factor
T helper cell
TIR domain containing adaptor molecule-1
Toll/IL-1 receptor domain
TIR domain-containing adapter protein
Toll-like receptor
Tumor necrosis factor
Tumor necrosis factor receptor associated factor
TRIF-related adaptor molecule
Triggering receptor expressed on myeloid cell-2
TIR domain-containing adaptor inducing interferon-
“Tissue-specific” antigen
Memory stem T cells.
The authors declare no conflict of interests.