One of the most fundamental questions in biology is: “How do cells differentiate in the right place, at the right time, into the right kinds?” Understanding the phenomenon of cell differentiation in its spatial and temporal framework is a prelude to understanding the development and physiology of all multicellular systems, including the immune system. Insights over the past 2300 years, since Aristotle, suggest that biological differentiation is guided by the interplay between genetic programs and specific environmental signals. This is exemplified by the mammalian immune response to pathogens, where qualitatively different types can emerge. Although it is appreciated that this type immunity is critical for optimal defense against different pathogens, the early “decision-making mechanisms” are largely obscure. Recent developments in innate immunity and genomics, especially in the biology of dendritic cells (DCs) and pathogen recognition receptors, have stimulated intense research in understanding the mechanisms guiding the differentiation of Th1, Th2, and T regulatory responses. In this study, I summarize recent findings which suggest that activation of DCs via distinct pathogen recognition receptors stimulate different gene expression programs and signaling networks in DCs that guide the variegation of immune responses.

The mammalian immune system can be said to have four fundamental properties: 1) a highly diverse repertoire of Ag-binding B and T cell receptors, clonally distributed on lymphocytes (1), that enables the system to recognize virtually any Ag with high specificity; 2) memory, which allows the host to remember the initial antigenic encounter, even for a lifetime (2); 3) immunological tolerance which encompasses a set of checkpoints against self-destructive immune responses (3); and 4) the ability to launch qualitatively different responses against different pathogens (4). Although there is considerable understanding of 1) and increasing awareness of the mechanisms underlying 2) and 3), the factors that determine “decision making” have remained an abiding puzzle. Clearly, the solution of this problem would not only illuminate our conceptual framework of the immune system, but also provide deep insights into a universal biological phenomenon: how cells differentiate into diverse phenotypes, guided by their own genes vs nurture. In addition, such knowledge is likely to be of critical importance in the design of novel vaccines and drugs that can generate optimally effective immune responses against a multitude of emerging and re-emerging infections. Thus, understanding the molecular mechanisms and players that regulate decision making in the immune response might be considered a Holy Grail of 21st century immunology and a grand challenge for biology. Recent advances in the biology of dendritic cells (DCs),3 TLRs, and other pathogen recognition receptor (PRR) systems are beginning to reveal the secrets of this decision-making process to such an extent that many of the critical molecular players within DCs that regulate Th1/Th2 and T regulatory responses are becoming known (5). The present review will summarize recent discoveries that point to a role for distinct DC subsets and PPRs, such as TLRs and C-type lectins, in orchestrating the type of immunity. Although the microenvironmental milieu is also known to play a major role, this will not be discussed here.

The existence of phenotypically distinct subsets of DCs that are localized in distinct microenvironments (4, 5, 6, 7) raises the question of whether they too, like distinct subsets of lymphocytes, may have evolved to serve distinct functions. As indicated in Table I, there is evidence for functional specialization of distinct DC subsets, but there is also compelling evidence for functional plasticity of a given DC subset. Distinct DC subsets are known to exhibit intrinsic differences in their ability to: 1) regulate the quality of the Th response (Th1, Th2, or CTL); 2) produce antiviral type I IFNs; and 3) cross-present exogenous Ags to CD8+ T cells.

Table I.

Evidence for functional specialization of DC subsets vs flexibility in programming DC function

YearEvidence for Functional Specialization of DC SubsetsEvidence for Functional Plasticity of DCs
1995–1996 Mouse CD8α +ve DCs in the spleen induce much weaker CD4+ and CD8+ T cell priming than CD8α −ve DCs (89 
1997–1998 Mouse CD8α +ve DCs can be induced to secrete much higher levels of IL-12 p70 than CD8α −ve DCs (1011Resting respiratory tract DCs preferentially stimulate Th2 responses and require obligatory cytokine signals for induction of Th1 immunity (24 ). 
  Prostaglandin E2 suppresses IL-12 production from human MDDCs and promotes Th2 responses (2223 ); IL-10 modulates DC function of murine splenic DCs (21
1999 Distinct DC subsets in mouse spleen and human blood differentially induce Th1 vs Th2 responses (121314— 
 Plasmacytoid DCs in human blood secrete much higher levels of IFN-α than MDDCs (16— 
2000 Distinct DC subsets in murine Peyer’s patches differentially bias Th1 vs Th2 responses (15Murine bone marrow-derived DCs conditioned by helminth components vs LPS to differentially induce Th2 vs Th1 responses (26
 Murine CD8α +ve DCs rather than CD8α −ve DCs cross-present Ags to CD8+ T cells (99Human MD DCs or murine bone marrow-derived DCs can be conditioned by environmental signals or prolonged culture to induce Th2 responses (252729
2001 E. coli LPS preferentially induces IL-12p70 in mouse CD8α +ve vs CD8α −ve DCs (33Murine bone marrow-derived DCs are conditioned by SEA to induce Th2 responses (30
 Plasmacytoid DCs in mice secrete much higher levels of IFN-α than MDDCs (17181920E. coli LPS induces IL-12p70 from CD8α+ DCs and primes the Th1 response, whereas P. gingivalis LPS induces no IL-12p70 and stimulates Th2 responses (33
  Distinct TLR ligands differentially induce IL-12p70 from human MDDCs (78
  Global gene expression analysis of human MDDCs activated with different stimuli reveals plasticity of gene expression (28
2002 Rat splenic DC subsets differentially induce Th1 vs Th2 responses (101Ability of murine CD8α +ve and CD8α −ve DCs to cross-present Ags can be modulated by external stimuli (100
 Murine splenic CD8α CD4+ CD11b+ DC subset cannot be induced to produce IL-12p70 by any stimulus tested (3435Murine splenic DC subsets exhibit some degree of plasticity in response to different stimuli and Ag dose (343538 ). 
  Similar findings with human MDDCs (32 ). 
2003  Different TLR ligands condition human MDDCs to induce distinct Th responses (36
  Electroporation of RNA induces IFN-α from non-plasmacytoid DCs (39
2004 E. coli LPS induces IL-12p70, preferentially from the murine CD8α +ve subset; while Pam-3-cys, a synthetic TLR2 ligand induces IL-10 preferentially from the CD8α-negative subset (37E. coli LPS induces IL-12p70 from murine CD8α +ve DCs, whereas Pam-3-cys does not; Pam-3-cys. induces IL-10 from the CD8α −ve subset, whereas LPS does not (37
 Global gene expression analysis of human papillomavirus type 16 virus-like particles on murine splenic DC subsets; human papillomavirus 16 virus-like particles activate CD8α DCs to express Th1-associated genes, while they activate CD8α −ve DCs to express Th2-associated genes (40 
YearEvidence for Functional Specialization of DC SubsetsEvidence for Functional Plasticity of DCs
1995–1996 Mouse CD8α +ve DCs in the spleen induce much weaker CD4+ and CD8+ T cell priming than CD8α −ve DCs (89 
1997–1998 Mouse CD8α +ve DCs can be induced to secrete much higher levels of IL-12 p70 than CD8α −ve DCs (1011Resting respiratory tract DCs preferentially stimulate Th2 responses and require obligatory cytokine signals for induction of Th1 immunity (24 ). 
  Prostaglandin E2 suppresses IL-12 production from human MDDCs and promotes Th2 responses (2223 ); IL-10 modulates DC function of murine splenic DCs (21
1999 Distinct DC subsets in mouse spleen and human blood differentially induce Th1 vs Th2 responses (121314— 
 Plasmacytoid DCs in human blood secrete much higher levels of IFN-α than MDDCs (16— 
2000 Distinct DC subsets in murine Peyer’s patches differentially bias Th1 vs Th2 responses (15Murine bone marrow-derived DCs conditioned by helminth components vs LPS to differentially induce Th2 vs Th1 responses (26
 Murine CD8α +ve DCs rather than CD8α −ve DCs cross-present Ags to CD8+ T cells (99Human MD DCs or murine bone marrow-derived DCs can be conditioned by environmental signals or prolonged culture to induce Th2 responses (252729
2001 E. coli LPS preferentially induces IL-12p70 in mouse CD8α +ve vs CD8α −ve DCs (33Murine bone marrow-derived DCs are conditioned by SEA to induce Th2 responses (30
 Plasmacytoid DCs in mice secrete much higher levels of IFN-α than MDDCs (17181920E. coli LPS induces IL-12p70 from CD8α+ DCs and primes the Th1 response, whereas P. gingivalis LPS induces no IL-12p70 and stimulates Th2 responses (33
  Distinct TLR ligands differentially induce IL-12p70 from human MDDCs (78
  Global gene expression analysis of human MDDCs activated with different stimuli reveals plasticity of gene expression (28
2002 Rat splenic DC subsets differentially induce Th1 vs Th2 responses (101Ability of murine CD8α +ve and CD8α −ve DCs to cross-present Ags can be modulated by external stimuli (100
 Murine splenic CD8α CD4+ CD11b+ DC subset cannot be induced to produce IL-12p70 by any stimulus tested (3435Murine splenic DC subsets exhibit some degree of plasticity in response to different stimuli and Ag dose (343538 ). 
  Similar findings with human MDDCs (32 ). 
2003  Different TLR ligands condition human MDDCs to induce distinct Th responses (36
  Electroporation of RNA induces IFN-α from non-plasmacytoid DCs (39
2004 E. coli LPS induces IL-12p70, preferentially from the murine CD8α +ve subset; while Pam-3-cys, a synthetic TLR2 ligand induces IL-10 preferentially from the CD8α-negative subset (37E. coli LPS induces IL-12p70 from murine CD8α +ve DCs, whereas Pam-3-cys does not; Pam-3-cys. induces IL-10 from the CD8α −ve subset, whereas LPS does not (37
 Global gene expression analysis of human papillomavirus type 16 virus-like particles on murine splenic DC subsets; human papillomavirus 16 virus-like particles activate CD8α DCs to express Th1-associated genes, while they activate CD8α −ve DCs to express Th2-associated genes (40 

Table I presents a chronological perspective of some important developments in this area. In the mid-1990s, Shortman and colleagues (8, 9) demonstrated that the CD8α+ve DCs from the spleens of mice were weaker at stimulating CD4+ and CD8+ T cell proliferation compared with CD8α-negative DCs (8, 9), thus suggesting that a specific subset of DC might be dedicated for keeping the immune response in check. Contrary to this, in 1997, it was demonstrated that microbial stimuli could induce murine CD8α +ve DCs to secrete much higher levels of the biologically active form of IL-12p70 relative to the CD8α–ve subset (10, 11). Given the pivotal role of IL-12p70 in inducing Th1 responses, the question of whether the distinct subsets might differentially stimulate Th1 vs Th2 response arose. In 1999, it was demonstrated that the CD8α +ve and CD8α–ve subsets in the spleen differentially primed Th1 vs Th2-biased responses (12, 13). Similar results with distinct human DC subsets in vitro, were obtained shortly thereafter (14). Taken together, these three reports provided evidence for the notion that distinct DC subsets might indeed regulate Th responses differentially. Subsequent work with the equivalent DC subsets from Peyer’s patches confirmed these findings (15). Then, Siegal et al. (16) demonstrated that human plasmacytoid DCs could be induced to make much higher levels of IFN-α than monocyte-derived DCs). This was later confirmed with murine plasmacytoid DCs (17, 18, 19, 20). Collectively, these observations suggested that distinct subpopulations of DCs may have some intrinsic biases in their ability to stimulate qualitatively different types of immune responses.

This notion was challenged by a series of reports from various groups showing an impressive degree of flexibility or “plasticity” of DCs in response to different microbial stimuli. For example, various environmental factors such as IL-10 (21), prostaglandins and steroids (22, 23), tissue of DC residence (15, 24), or duration of in vitro culture (25), affect the ability of DCs to produce cytokines and to induce Th1 and Th2 responses. Furthermore, the nature of the microbial stimulus was also shown to exert a potent influence; thus early reports suggested that DCs exposed to helminth products could stimulate a Th2-like response, while the same DCs when exposed to LPS induced a Th1-biased response (26, 27). This was followed by several other reports suggesting that specific subsets of murine or human DCs cultured in vitro with different stimuli, or different TLR ligands, responded with great plasticity of gene expression and cytokine secretion (28, 29, 30, 31, 32, 33, 34, 35, 36, 37). Furthermore, a specific murine splenic DC subset cultured with different doses of OVA peptide has been shown to induce distinct Th responses (38), and one report claimed that electroporation of RNA into DCs could induce nonplasmacytoid, CD11c+ splenic DCs to secrete IFN-α (39).

Clearly, these reports have highlighted the considerable degree of functional plasticity in DCs. However, a central question that must be asked is: How flexible are DCs? Recent work suggests that there might be restraints on DC flexibility. For example, our work suggests that stimuli such as Escherichia coli LPS and Staphylococcus aureus Cowan I bacteria preferentially induce IL-12p70 from the splenic CD8α+ subset (10, 33, 37); in contrast, stimuli such as Pam-3-cys induce IL-10, preferentially from the CD8α subset (33). Moreover, recent work from Reis e Sousa’s group (34, 35) suggests that the CD4+CD8αCD11b+ splenic DC subset appears to be very resistant to IL-12p70 induction by any of the stimuli tested, but a potent producer of IL-10. Finally, a recent report suggests that human papillomavirus type 16 virus-like particles increase transcription of IFN-γ and numerous Th1-related cytokines and chemokines in CD8α+CD11c+ DCs, but induce Th2-associated cytokines and chemokines in the CD8αCD11b+CD11c+ DCs (40). Taken together, these studies suggest that microbial stimuli induce distinctive responses in different DC subsets.

The reader will appreciate that these seemingly contradictory findings have been the source of many a spirited debate in the field! However, DC biologists might well spare ourselves any agony, by remembering that this issue of “genetic programming” vs plasticity is a universal biological theme, whether one considers pattern formation and lineage fate decisions during embryogenesis (41), or functional specialization vs plasticity of different areas of the cerebral cortex (42), or in the learning of language (43). In each case, it appears that developmental fate decisions are governed by mechanisms that rely on a superimposition of a basic genetic program (“hardwiring”) on a dynamic environment.

For more than 100 years since Ellie Metchinkoff’s pioneering observations of phagocytic cells in starfish larvae (44), most immunologists have relegated the innate immune system to a “second tier” system of nonspecific host responses of limited influence (Table II). Burnet’s clonal selection theory (1), and the subsequent demonstration that bacterial Ags such as flagellin could be directly recognized by Ag receptors on B lymphocytes (45), helped reinforce the perceived supremacy of the adaptive immune system. However, spectacular advances in innate immunity over the past 6 years have established a new paradigm in which the burden of pathogen sensing is now placed on the innate immune system (46, 47, 48). It is now clear that the innate immune system recognizes microbial components directly through various specific PRRs expressed on innate immune cells, such as DCs. TLRs constitute an evolutionarily conserved family of PRRs, of which 11 have yet been described and are widely expressed on a variety of innate immune cells, including DCs, macrophages, mast cells, neutrophils, and endothelial cells. Several excellent articles have reviewed their biology extensively (49, 50, 51, 52); therefore, in this review this topic will be discussed only briefly.

Table II.

Role of PRRs in modulation of Th responses

PRR FamilyPRRsLigandDC or Macrophage Cytokine ResponseAdaptive Immune Response
TLRs TLR2 (heterodimer with TLR1 or 6) Lipopeptides Low IL-12p70 Th1 (74
  Pam-3-cys (TLR 2/1) High IL-10 Th2 (33363777
  MALP (TLR 2/6) IL-6 (33363775767778798081T regulatory (798081
 TLR3 dsRNA IL-12p70 Th1 (4849505152
   IFN-α  
   IL-6 (4849505152 
 TLR-4 E. coli LPS High IL-12p70 Th1 (33363776
   Intermediate IL-10  
   IL-6 (3336377678 
 TLR5 Flagellin High IL-12p70 (36102Th1 (36102
   Low IL-12p70 (103104Th2 (103104
 TLR7/8 ssRNA High IL-12p70 Th1 (4849505152105
  Imidazoquinolines IFN-α  
   IL-6 (4849505152105 
 TLR 9 CpG DNA High IL-12p70 Th1 (4849505152106
   Low IL-10  
   IL-6  
   IFN-α (4849505152106 
 TLR10 
C-type lectins DC-SIGN Env of HIV; core protein of HCV; Components of M. tubercolosis; H. pylori Lewis Ag H. pylori Lewis Ag Suppresses IL-12p70 (95Th2 (95
   Suppression of TLR signaling in DCs (9596T regulatory (96
NOD NOD2 Muramyl dipeptide of peptidoglycan Induces IL-10 in DCs (97Weak T cell response (tolerogenic?) 
Mannose receptor Mannose receptor Mannosylated lipoarabinomannans from bacillus Calmette-Guerin and M. tuberculosis Suppression of IL-12 and TLR signaling in DCs (98Weak T cell response? (tolerogenic?) 
PRR FamilyPRRsLigandDC or Macrophage Cytokine ResponseAdaptive Immune Response
TLRs TLR2 (heterodimer with TLR1 or 6) Lipopeptides Low IL-12p70 Th1 (74
  Pam-3-cys (TLR 2/1) High IL-10 Th2 (33363777
  MALP (TLR 2/6) IL-6 (33363775767778798081T regulatory (798081
 TLR3 dsRNA IL-12p70 Th1 (4849505152
   IFN-α  
   IL-6 (4849505152 
 TLR-4 E. coli LPS High IL-12p70 Th1 (33363776
   Intermediate IL-10  
   IL-6 (3336377678 
 TLR5 Flagellin High IL-12p70 (36102Th1 (36102
   Low IL-12p70 (103104Th2 (103104
 TLR7/8 ssRNA High IL-12p70 Th1 (4849505152105
  Imidazoquinolines IFN-α  
   IL-6 (4849505152105 
 TLR 9 CpG DNA High IL-12p70 Th1 (4849505152106
   Low IL-10  
   IL-6  
   IFN-α (4849505152106 
 TLR10 
C-type lectins DC-SIGN Env of HIV; core protein of HCV; Components of M. tubercolosis; H. pylori Lewis Ag H. pylori Lewis Ag Suppresses IL-12p70 (95Th2 (95
   Suppression of TLR signaling in DCs (9596T regulatory (96
NOD NOD2 Muramyl dipeptide of peptidoglycan Induces IL-10 in DCs (97Weak T cell response (tolerogenic?) 
Mannose receptor Mannose receptor Mannosylated lipoarabinomannans from bacillus Calmette-Guerin and M. tuberculosis Suppression of IL-12 and TLR signaling in DCs (98Weak T cell response? (tolerogenic?) 

TLRs may be expressed as homodimers or heterodimers (TLR1 plus TLR2 or TLR6 plus TLR2) and have broad specificity for conserved molecular structures of pathogens (reviewed in Refs.49, 50, 51, 52). For example, LPS from E. coli signals through TLR4, whereas TLR2 appears to have several ligands, including peptidoglycan of Gram-positive bacteria, lipoproteins from Mycobacterium tuberculosis, and certain components of Saccharomyces cerevisiae zymosan, as well as highly purified Porphyromonas gingivalis LPS. TLR3 recognizes dsRNA, TLR5 recognizes flagellin, TLR7 can be triggered by the synthetic compounds imidazoquinolines, as well as ssRNA, and may thus be important for viral recognition; TLR9 recognizes certain types of CpG-rich DNA found in bacteria and some viruses (49, 50, 51, 52).

Although initial studies suggested that all TLRs signal via a common downstream signaling pathway, resulting in a common biological response, it is now amply clear that triggering different TLRs result in distinct but overlapping signaling networks and biological responses (Fig. 1). For example, signaling through any TLR results in the recruitment of IL-1R1-associated protein kinases 1 and 4 to the TLR complex and their phosphorylation. This process results in an association with the TNF receptor-associated factor 6, leading to the activation of the NF-κB and MAPK signaling pathways, which mediate certain core aspects of immune activation, including the induction of inflammatory cytokines such as TNF-α and IL-6. However, the induction of other responses such as type I IFNs and Th2 cytokines appears to be mediated by a specific subset of TLRs. Moreover, even signaling via a single TLR can result in distinct biological responses, depending on the dose of ligand used (53). Understanding the molecular basis for this diversity of biological responses is in its infancy, but as discussed below, already some insights are evident.

FIGURE 1.

Three mechanisms by which TLR adaptor proteins might generate immune diversity. Mechanism 1, Different adaptor proteins that mediate distinct signaling pathways and function might be associated with different TLRs; Mechanism 2, a single TLR might be associated with multiple adaptor proteins with bifurcating signaling pathways. Precisely which pathway is actually triggered might be determined by several factors such as the affinity/avidity of ligand binding to the TLR, and the accessory receptors involved; Mechanism 3: different adaptor proteins might be geographically or temporally segregated, either in different subsets of DCs (e.g., splenic CD8α+ vs CD8α DCs), or DCs present in distinct microenvironments such as the gut or spleen, or in DCs in different stages of an immune response.

FIGURE 1.

Three mechanisms by which TLR adaptor proteins might generate immune diversity. Mechanism 1, Different adaptor proteins that mediate distinct signaling pathways and function might be associated with different TLRs; Mechanism 2, a single TLR might be associated with multiple adaptor proteins with bifurcating signaling pathways. Precisely which pathway is actually triggered might be determined by several factors such as the affinity/avidity of ligand binding to the TLR, and the accessory receptors involved; Mechanism 3: different adaptor proteins might be geographically or temporally segregated, either in different subsets of DCs (e.g., splenic CD8α+ vs CD8α DCs), or DCs present in distinct microenvironments such as the gut or spleen, or in DCs in different stages of an immune response.

Close modal

One theoretical possibility of generating immune diversity appears to be by segregated expression of distinct TLRs in functionally different subsets of DCs. Thus, the expression of TLR9 and TLR7 in the endosomes of human plasmacytoid DCs, but not on human monocyte-derived DC (MDDCs) (54, 55), might facilitate the generation of antiviral type I IFNs in response to intracellular pathogens such as viruses. In contrast, TLR4 appears not to be expressed on human plasmacytoid DCs, but is expressed on the surface of human MDDCs, consistent with the induction of IL-12 in such DCs by LPS, perhaps important in generating Th1 responses during bacterial infections. Although this is an interesting hypothesis, it is based on TLR expression patterns in DC subsets, which are in the resting state (not activated) and is not supported by TLR expression patterns in the equivalent DC subsets in mice.

The next conceivable method of generating diversity in TLR signaling appears to be with a limited number of proximal adaptor proteins which themselves contain the Toll/IL-1R (TIR) domain that facilitates their association with TLRs (Fig. 1). In this study, at least three possible mechanisms can be envisioned—mechanism 1: different adaptor proteins that mediate distinct signaling pathways and function might be associated with different TLRs; mechanism 2: a single TLR might be associated with multiple adaptor proteins with bifurcating signaling pathways. Precisely which pathway is actually triggered might be determined by several factors such as the affinity/avidity of ligand binding to the TLR and the accessory receptors involved; and mechanism 3: different adaptor proteins might be segregated spatially, either in different subsets of DCs, or DCs present in distinct microenvironments (e.g., CD8α–ve and CD8α +ve DCs in the marginal zones and T cell areas of the spleen, or in DCs in mucosal tissues vs splenic DCs), or segregated temporally (e.g., in resting vs activated vs “exhausted” (25) DCs).

In practice, as described below, there is now evidence for 1) and 2), but no evidence as yet for 3). However, all TLRs (except perhaps TLR3) are associated with at least one common adaptor, MyD88, which likely mediates the common gene expression programs and functional outputs of most TLRs. Thus, macrophages and DCs from MyD88 knockout mice appear to have impaired activation of NF-κB or JNK MAPK, or secretion of TNF-α or IL-12 in response to various TLR ligands (48, 49, 50, 51, 52). However, DCs from MyD88−/− up-regulate costimulatory molecules, acquire immune stimulatory capacity and produce type 1 IFNs and related cytokines and chemokines in response to LPS (56, 57, 58).

TIR domain-containing adaptor protein (TIRAP), another TIR domain-containing protein, is known to associate with TLR4 and TLR2, but not with other TLRs (59, 60). Initial studies suggested that overexpression of a dominant negative form of TIRAP was shown to inhibit NF-κB activation and up-regulation of costimulatory molecules in DCs (59, 60). Furthermore, an inhibitory peptide was shown to suppress IFN-γ-inducible protein 10 (IP-10) and IFN-γ-related gene induction by LPS (61). These observations led to the belief that TIRAP may be part of a MyD88-independent pathway. However, more recent studies using TIRAP-deficient mice (62, 63) have not yet revealed any significant differences in the biological functions mediated by TIRAP or MyD88. As with MyD88−/− mice, DCs from TIRAP-deficient mice are able to up-regulate costimulatory molecules in response to TLR4 and TLR2 ligands, and cells from TIRAP−/− mice do not appear to have any defects in the production of IP-10 or IFN-αβ-related proteins (62, 63). Taken together, these data suggest that both the MyD88 and TIRAP adaptor proteins may lie in the same pathway downstream of TLR2 and TLR4. What unique biological functions it might mediate is yet to be discovered.

TIR domain-containing adaptor-inducing IFN-β (TRIF) (64) or TIR-containing adaptor molecule 1 (65) is an adaptor protein that appears to mediate unique biological functions different from those of MyD88 or TIRAP. Studies with TRIF-deficient mice (66) or with Lps2 mice, which have a distal frameshift mutation in the Trif gene (67), revealed strong deficiencies in both TLR3- and TLR4-mediated expression of IFN-β and activation of IFN regulatory factor 3 and type I IFNs and hypersusceptibility to mouse CMV infections. In fact, none of the effects of poly(IC) could be detected in TRIF−/− mice, suggesting that TRIF is vital for TLR3 signaling. In the case of LPS, however, there was normal activation of IL-1R-assocaited kinase 1, NF-κB, and MAPK, indicating that TRIF is not involved in the LPS-induced activation of the MyD88-dependent signaling. Surprisingly, inflammatory cytokine production (TNF, IL-6, and IL-12p40) in response to the TLR4 ligand but not to other TLR ligands was impaired in TRIF-deficient macrophages (66). Thus, there is likely some cooperation between the MyD88 and TRIF pathways. Mice deficient in both MyD88 and TRIF showed complete loss of NF-κB activation in response to TLR4 stimulation (66). These studies suggest that TRIF is selectively expressed downstream of TLR3 and TLR4 and that it mediates certain unique functions, such as up-regulation of costimulatory molecules on APCs and the induction of IFN-αβ-related genes. Because TRIF is not associated with TLR2, or TLR9 or TLR7, which also induce type I IFNs, and because MyD88 and TIRAP do not mediate up-regulation of costimulatory molecules, there must be additional adaptors to mediate these processes.

A fourth TIR domain-containing adaptor, TRIF-related adaptor molecule (TRAM), has been discovered recently (68). TRAM-deficient mice show defects in cytokine production in response to the TLR4 ligand but not to other TLR ligands. TLR4- but not TLR3-mediated MyD88-independent IFN-β production and activation of signaling cascades are impaired in TRAM-deficient cells. An independent study using dominant negative and short-interfering RNA approaches confirmed that TRAM is restricted to the TLR4 pathway (69). These studies suggest that TRIF and TRAM both function in LPS-TLR4 signaling to regulate the MyD88-independent pathway. However, the question of what adaptor proteins mediate the induction of type I IFN proteins in response to TLR7/8 or 9 triggering remains to be determined.

It would thus appear that segregation of different adaptor proteins with distinct TLRs (mechanism 1) facilitates the generation of unique gene expression programs and functional responses. It is also likely that signaling via a single TLR might generate distinct functional responses, depending on the dose of the ligand, or strength of signal used (mechanism 2). Thus, recent work suggests that although low doses of E. coli LPS induce Th2 responses, high doses favor Th1 responses in vivo (53). Consistent with such a model, our recent data suggest that different synthetic derivatives of the LPS of Neisseria meningitis, all of which signal via TLR4, are able to differentially induce type 1 IFNs (TRIF pathway) or proinflammatory cytokines (MyD88 pathway). There is currently no evidence for the geographical segregation of different adaptor proteins in distinct DC subsets, or DCs in different microenvironments (mechanism 3). However, given the strikingly different responsiveness of distinct DC subsets to the same PAMP (e.g., E. coli LPS induces much higher levels of IL-12p70 from murine CD8α+ve DCs than from CD8α–ve DCs (33, 37); Pam-3-cys induces much higher levels of IL-10 from CD8αCD11b+ DCs than from CD8α+ DCs (37); DCs in the Peyer’s patch secrete much lower levels of IL-12p70 than splenic DCs (15)). It is possible that mechanism 3 might be operating.

Initial studies established the importance of TLRs in inducing Th1 responses (Table III). For example, the TLR4 ligand LPS (70), CpG DNA, poly(IC), and TLR7 ligands induce IL-12p70 and IFN-α from DCs and stimulate Th1 responses (4, 5, 6, 7, 33, 36, 37, 47, 48, 49, 50, 51, 52). Certain TLR2 ligands also induce weak IL-12p70, and MyD88−/− mice appear to have a selective defect in Th1 responses (58, 71, 72, 73, 74), suggesting that all TLRs preferentially mediate Th1 responses and not Th2 responses. However, there is increasing evidence that certain TLR ligands may also mediate Th2 responses (Table III). For example, while E. coli LPS (a TLR4 ligand) induces a Th1 response, highly purified LPS preparations from P. gingivalis LPS, a putative TLR2 ligand (75), favors a Th2 response (33). Escherichia coli LPS but not P. gingivalis LPS induces IL-12p70 in the splenic CD8α+ DCs (33). This finding is consistent with other work that suggests that P. gingivalis LPS fail to induce IL-12p70 in murine macrophages (75) and human MDDCs in vitro (76). Similar results have been obtained with the synthetic TLR2 ligand Pam-3-cys (36, 37). Thus, Pam-3-cys, E. coli LPS, flagellin, and schistosome egg Ags (SEA) activate human MDDCs to express enhanced levels of costimulatory molecules, but differ in the cytokines they induce. Although E. coli LPS and flagellin induce abundant IL-12p70, Pam-3-cys and SEA do not do so but can induce the Th2-inducing or regulatory cytokine IL-10. Although E. coli LPS and flagellin induce strong Th1 responses, Pam-3-cys and SEA favor a Th2 bias (36). In the mouse system, almost identical results are evident; Pam-3-cys induces very little IL-12p70 in splenic CD8α+ DCs compared with E. coli LPS (37). In contrast, it induces much higher levels of IL-10 in the CD8α DCs, relative to E. coli LPS (37). Interestingly, the induction of IL-10 was largely abrogated in DCs from MyD88−/− mice (37). Consistent with their differential cytokine induction in DCs, Pam-3-cys and E. coli LPS induced Th2- and Th1-biased responses, respectively (36, 37). These studies are supported by several other reports, which suggest that signaling via TLR2 may result in Th2 or T regulatory responses (77, 78, 79, 80, 81).

Table III.

Evidence for a role for TLRs in induction of Th2 and T regulatory responses

YearTLRObservation
2001 TLR2 TLR2 ligands (unlike TLR4 ligands) do not induce IL-12p70, IP-10, or INF-α from human MDDCs (78
 TLR2 P. gingivalis LPS, a TLR2 ligand, does not stimulate Th1-associated genes in murine macrophages (75
 TLR2 P. gingivalis LPS does not induce IL-12p70 in murine splenic DCs and induces Th2 responses; E. coli LPS stimulates IL-12p70 in DCs and induces Th1 responses (33
2002 TLR4 Intranasal administration of low vs high doses of E. coli LPS induces Th2 and Th1 responses, respectively (53
 TLR2 Schistosomal lysophosphatidylserine stimulates human MDDCs via TLR2 and induces T regulatory responses (81
 TLR2 Yersinia pestis V Ag stimulates macrophages via TLR2 and induces T regulatory responses (80
2003 TLR2 Pam-3-cys, a synthetic TLR2 ligand, induces little or no IL-12, but induces high IL-10 from human MDDCs and stimulates a Th2-biased response via a mechanism involving ERK and c-Fos signaling in DCs (36
 TLR2 P. gingivalis LPS induces human MDDCs to stimulate Th2 responses (76
 TLR2 Pam-3-cys and CpG DNA induce Th2 and Th1 responses, respectively, in vivo (77
 TLR4 TLR4-mediated innate IL-10 stimulates T regulatory cells and confers resistance to B. pertussis (107
2004 TLR2 Pam-3-cys induces IL-12p70low, IL-10high splenic DC Th2-biased responses in vivo (37
 TLR2 C. albicans signals through TLR2 to stimulate IL-10 and T regulatory cells (79
 TLR5 Flagellin promotes Th2 responses in vivo (103104
YearTLRObservation
2001 TLR2 TLR2 ligands (unlike TLR4 ligands) do not induce IL-12p70, IP-10, or INF-α from human MDDCs (78
 TLR2 P. gingivalis LPS, a TLR2 ligand, does not stimulate Th1-associated genes in murine macrophages (75
 TLR2 P. gingivalis LPS does not induce IL-12p70 in murine splenic DCs and induces Th2 responses; E. coli LPS stimulates IL-12p70 in DCs and induces Th1 responses (33
2002 TLR4 Intranasal administration of low vs high doses of E. coli LPS induces Th2 and Th1 responses, respectively (53
 TLR2 Schistosomal lysophosphatidylserine stimulates human MDDCs via TLR2 and induces T regulatory responses (81
 TLR2 Yersinia pestis V Ag stimulates macrophages via TLR2 and induces T regulatory responses (80
2003 TLR2 Pam-3-cys, a synthetic TLR2 ligand, induces little or no IL-12, but induces high IL-10 from human MDDCs and stimulates a Th2-biased response via a mechanism involving ERK and c-Fos signaling in DCs (36
 TLR2 P. gingivalis LPS induces human MDDCs to stimulate Th2 responses (76
 TLR2 Pam-3-cys and CpG DNA induce Th2 and Th1 responses, respectively, in vivo (77
 TLR4 TLR4-mediated innate IL-10 stimulates T regulatory cells and confers resistance to B. pertussis (107
2004 TLR2 Pam-3-cys induces IL-12p70low, IL-10high splenic DC Th2-biased responses in vivo (37
 TLR2 C. albicans signals through TLR2 to stimulate IL-10 and T regulatory cells (79
 TLR5 Flagellin promotes Th2 responses in vivo (103104

Specific TLRs may also induce cross-presentation and CTLs. Thus, signaling via TLR3, 7, and 9 induces cross-presentation, whereas TLR2 and TLR4 ligands do not (82). Interestingly, TLR3, 7, and 9 also induce abundant type I IFNs, which are known to enhance cross-presentation by DCs (83). Thus, TLR3, 7, and 9 ligands may induce CTL activity by stimulating IFN-α.

Given that certain TLRs might bias toward the Th2 pathway (Fig. 2), by what signaling mechanisms do they act? Our recent work suggests that Pam-3-cys and SEA induce enhanced duration and magnitude of ERK signaling in DCs (36, 37). Interestingly, DCs from ERK1−/− mice produced enhanced levels of IL-12p70 and greatly diminished levels of IL-10 in response to Pam-3-cys (37), and human MDDCs treated with a synthetic inhibitor of MEK1/2 produced enhanced levels of IL-12p70 in response to TLR4, 5, or 2 stimulation (36). This is consistent with previous reports that ERK suppresses the induction of IL-12 and enhances IL-10 induction (84), as well as with a subsequent report that SEA induces enhanced ERK signaling in DCs (85).

FIGURE 2.

A model for “DC1/DC2” regulation by MAPKs and c-Fos. TLR4 or TLR5 ligands induce strong activation of p38 and JNK, but only a transient and modest activation of ERK1/2. This results in the production of IL-12p70, which skews the Th balance toward the Th1 end of the spectrum. In contrast, certain TLR2 ligands, as well as the classic Th2 stimulus SEA, induce sustained and enhanced phosphorylation of ERK, which stabilizes c-Fos, which suppresses the production of IL-12p70. As a result, the balance is shifted toward the Th2 end of the spectrum. Potential therapeutic targets and approaches to modulating the Th balance are highlighted (3637 ).

FIGURE 2.

A model for “DC1/DC2” regulation by MAPKs and c-Fos. TLR4 or TLR5 ligands induce strong activation of p38 and JNK, but only a transient and modest activation of ERK1/2. This results in the production of IL-12p70, which skews the Th balance toward the Th1 end of the spectrum. In contrast, certain TLR2 ligands, as well as the classic Th2 stimulus SEA, induce sustained and enhanced phosphorylation of ERK, which stabilizes c-Fos, which suppresses the production of IL-12p70. As a result, the balance is shifted toward the Th2 end of the spectrum. Potential therapeutic targets and approaches to modulating the Th balance are highlighted (3637 ).

Close modal

By what mechanism does enhanced ERK signaling impair IL-12 induction? We were struck by a report which suggested that sustained duration and magnitude of ERK signaling results in phosphorylation and stabilization of the early growth transcription factor c-Fos in fibroblasts (86). We thus investigated this question and indeed c-Fos was found to be expressed at enhanced levels, and in a phosphorylated form, in DCs stimulated by TLR2 ligands and SEA, relative to DCs stimulated with E. coli LPS or flagellin (36, 37). Furthermore, c-Fos appears to control the induction of IL-12p70 and IL-10 in DCs, because DCs from c-Fos−/− mice have a dramatic impairment of IL-12p70 and an enhancement of IL-10 (37). Consistently, human MDDCs pretreated with short-interfering RNA against c-Fos and then stimulated with SEA produced high levels of IL-12p70 (36). Thus, the impairment of c-Fos, a single transcription factor within DCs, was sufficient to convert a classic Th2 stimulus into a Th1 stimulus. This raises the possibility that targeting specific transcription factors within DCs might provide novel avenues for controlling the quality of immunity. Indeed, there is now emerging evidence for other transcription factors in playing important roles in DC1/DC2 polarization. Thus, T-bet (87) and NF-κB (88), and c-Rel (89) are required for optimal induction of Th1 responses by DCs, whereas c-Maf appears to suppress IL-12 and enhance IL-10 in macrophages (90). Finally, two recent reports suggest that different Notch receptors on DCs can instruct distinct Th responses (91, 92). Thus, Amsen et al. (91) suggest that DCs can be induced to express Jagged or Delta by Th1-inducing stimuli (LPS) and Th2-inducing stimuli (cholera toxin), respectively, and that the ligands mediate the induction Th1 vs Th2 responses. Precisely how such ligands interact with the ERK-c-Fos pathway described above is yet to be determined.

Although TLRs have enjoyed center stage, other important candidates are waiting in the aisle. For example, C-type lectins bind carbohydrates from pathogens and from self-glycoproteins, and thus likely are important not only in pathogen sensing, but also in cell adhesion, in migration, and in maintaining self-tolerance (93, 94). DC-specific ICAM3-grabbing nonintegrin (DC-SIGN), which is involved in the capture of different pathogens, is expressed by dermal DCs, as well as in the mucosal tissues by interstitial DCs (93, 94). Emerging evidence suggests that several viruses (HIV, CMV, hepatitis C virus (HCV), and dengue), bacteria (Helicobacter pylori, M. tuberculosis ManLAM (lipoarabinomannan) and Klebsiella pneumonia), and yeasts interact with DC-SIGN. A common feature of several of these pathogens is that they cause chronic infections, in which the T regulatory responses are a critical determinant of pathogen persistence. Therefore, many pathogens might specifically target DC-SIGN to suppress DC function and modulate immune responses by inducing Th2 or T regulatory responses that are beneficial for their persistence in the host (95, 96). Thus, learning how to reset the balance toward the Th1 end of the spectrum may offer novel strategies to treat chronic infections such as HIV and HCV.

Another potential intracellular PRR is NOD2, a member of the NOD-leucine-rich repeat protein family, which recognizes the muramyl dipeptide component of peptidoglycans (97). Mutations in CARD15, the gene encoding NOD2, are observed in a significant fraction of patients with inflammatory bowel disease, suggesting that NOD2 might act as an immune brake. Consistent with this, NOD2−/− mice appear to have enhanced Th1 responses to stimulation with certain TLR2 ligands (97). Thus, NOD2 might partly account for the Th2 or T regulatory responses observed previously with TLR2 signaling (33, 36, 37, 75, 76, 77, 78, 79, 80, 81). Exactly how NOD2 signaling interacts with the ERK/c-Fos Th2 pathway described above (36, 37) remains to be seen.

Finally, there is some evidence that signaling through the mannose receptor also results in inhibition of TLR signaling. Thus, mannose-capped lipoarabinomannans from Mycobacterium bovis, bacillus Calmette-Guérin, and M. tuberculosis inhibit LPS-induced IL-12 production by DCs (98).

The past decade has witnessed a paradigm shift in immunology, triggered by Janeway (46), that places innate immunity at the helm. It is now clear that DCs and PRRs play key instructive roles in the variegation of adaptive immunity. Today in the field of DC biology, as there has been in so many other fields, there is a great debate on the respective roles of genetic preprogramming vs functional plasticity. Here, it is perhaps instructive for us to gain a wider biological perspective by considering mechanisms of lineage fate decisions in other biological systems. In such cases, it appears that developmental fate decisions are governed by mechanisms that rely both on genetic hardwiring and dynamic environmental cues. This debate notwithstanding, the sequencing of the human genome, and the ensuing era of “genomics,” have provided an opportune platform to discover the molecular minutia of signaling networks within DCs that control adaptive immunity. However, although these molecular details are of great importance, it is perhaps of even greater import to develop an understanding of the whole “system” and of the fundamental biological processes that shape immunity.

The authors have no financial conflict of interest.

Much of the work discussed in this review was performed by two outstanding colleagues in my laboratory, Stephanie Dillon and Sudhanshu Agrawal.

The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked advertisement in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

1

This work was supported by grants from the National Institutes of Health, and from Emory Vaccine Center and the Department of Pathology, Emory University.

3

Abbreviations used in this paper: DC, dendritic cell; PRR, pathogen recognition receptor; TIR, Toll/IL-1R; MDDC, monocyte-derived DC; IP-10, IFN-γ-inducible protein 10; HCV, hepatitis C virus; TIRAP, TIR domain-containing adaptor protein; TRIF, TIR domain-containing adaptor-inducing IFN-β; IRF, IFN regulatory factor; TRAM, TRIF-related adaptor molecule; SEA, schistosome egg Ag; DC-SIGN, DC-specific ICAM3-grabbing nonintegrin.

1
Burnet, F. M..
1957
. A modification of Jerne’s theory of antibody production using the concept of clonal selection.
Austr. J. Sci.
20
:
67
.
2
Kaech, S. M., E. J. Wherry, R. Ahmed.
2002
. Effector and memory T-cell differentiation: implications for vaccine development.
Nat. Rev. Immunol.
2
:
251
.
3
Nossal, G. J..
1995
. Choices following antigen entry: antibody formation or immunologic tolerance?.
Annu. Rev. Immunol.
13
:
27
.
4
Pulendran, B., K. Palucka, J. Banchereau.
2001
. Sensing pathogens and tuning immune responses.
Science
29
:
253
.
5
Pulendran, B..
2004
. Modulating vaccine responses with dendritic cells and Toll-like receptors.
Immunol. Rev.
199
:
227
.
6
Banchereau, J., F. Briere, C. Caux, J. Davoust, S. Lebecque, Y. J. Liu, B. Pulendran, K. Palucka.
2000
. Immunobiology of dendritic cells.
Annu. Rev. Immunol.
18
:
767
.
7
Shortman, K., Y. J. Liu.
2002
. Mouse and human dendritic cell subtypes.
Nat. Rev. Immunol.
2
:
151
.
8
Suss, G., K. Shortman.
1996
. A subclass of dendritic cells kills CD4 T cells via Fas/Fas-ligand-induced apoptosis.
J. Exp. Med.
183
:
1789
.
9
Kronin, V., K. Winkel, G. Suss, A. Kelso, W. Heath, J. Kirberg, H. von Boehmer, K. Shortman.
1996
. A subclass of dendritic cells regulates the response of naive CD8 T cells by limiting their IL-2 production.
J. Immunol.
157
:
3819
.
10
Pulendran, B., J. Lingappa, M. K. Kennedy, J. Smith, M. Teepe, A. Rudensky, C. R. Maliszewski, E. Maraskovsky.
1997
. Developmental pathways of dendritic cells in vivo: distinct function, phenotype, and localization of dendritic cell subsets in FLT3 ligand-treated mice.
J. Immunol.
159
:
2222
.
11
Reis, E., C. Sousa, S. Hieny, T. Scharton-Kersten, D. Jankovic, H. Charest, R. N. Germain, A. Sher.
1997
. In vivo microbial stimulation induces rapid CD40 ligand-independent production of interleukin 12 by dendritic cells and their redistribution to T cell areas.
J. Exp. Med.
186
:
181
.
12
Pulendran, B., J. L. Smith, G. Caspary, K. Brasel, D. Pettit, E. Maraskovsky, C. R. Maliszewski.
1999
. Distinct dendritic cell subsets differentially regulate the class of immune response in vivo.
Proc. Natl. Acad. Sci. USA
96
:
1036
.
13
Maldonado-Lopez, R., T. De Smedt, P. Michel, J. Godfroid, B. Pajak, C. Heirman, K. Thielemans, O. Leo, J. Urbain, M. Moser.
1999
. CD8α+ and CD8α subclasses of dendritic cells direct the development of distinct T helper cells in vivo.
J. Exp. Med.
189
:
587
.
14
Rissoan, M. C., V. Soumelis, N. Kadowaki, G. Grouard, F. Briere, R. de Waal Malefyt, Y. J. Liu.
1999
. Reciprocal control of T helper cell and dendritic cell differentiation.
Science
283
:
1183
.
15
Iwasaki, A., B. L. Kelsall.
1999
. Freshly isolated Peyer’s patch, but not spleen, dendritic cells produce interleukin 10 and induce the differentiation of T helper type 2 cells.
J. Exp. Med.
190
:
229
.
16
Siegal, F. P., N. Kadowaki, M. Shodell, P. A. Fitzgerald-Bocarsly, K. Shah, S. Ho, S. Antonenko, Y. J. Liu.
1999
. The nature of the principal type 1 interferon-producing cells in human blood.
Science
284
:
1835
.
17
Asselin-Paturel, C., A. Boonstra, M. Dalod, I. Durand, N. Yessaad, C. Dezutter-Dambuyant, A. Vicari, A. O’Garra, C. Biron, F. Briere, G. Trinchieri.
2001
. Mouse type I IFN-producing cells are immature APCs with plasmacytoid morphology.
Nat. Immunol.
2
:
1144
.
18
Nakano, H., M. Yanagita, M. D. Gunn.
2001
. CD11c+B220+Gr-1+ cells in mouse lymph nodes and spleen display characteristics of plasmacytoid dendritic cells.
J. Exp. Med.
194
:
1171
.
19
Bjorck, P..
2001
. Isolation and characterization of plasmacytoid dendritic cells from Flt3-ligand and granulocyte-macrophage colony stimulating factor treated mice.
Blood
98
:
3520
.
20
Dalod, M., T. P. Salazar-Mather, L. Malmgaard, C. Lewis, C. Asselin-Paturel, F. Briere, G. Trinchieri, C. A. Biron.
2002
. Interferon α/β and interleukin 12 responses to viral infections: pathways regulating dendritic cell cytokine expression in vivo.
J. Exp. Med.
195
:
517
.
21
De Smedt, T., M. Van Mechelen, G. De Becker, J. Urbain, O. Leo, M. Moser.
1997
. Effect of interleukin-10 on dendritic cell maturation and function.
Eur. J. Immunol.
27
:
1229
.
22
Kalinski, P., C. M. Hilkens, A. Snijders, F. G. Snijdewint, M. L. Kapsenberg.
1997
. IL-12-deficient dendritic cells, generated in the presence of prostaglandin E2, promote type 2 cytokine production in maturing human naive T helper cells.
J. Immunol.
159
:
28
.
23
Kalinski, P., J. H. Schuitemaker, C. M. Hilkens, M. L. Kapsenberg.
1998
. Prostaglandin E2 induces the final maturation of IL-12-deficient CD1a+CD83+ dendritic cells: the levels of IL-12 are determined during the final dendritic cell maturation and are resistant to further modulation.
J. Immunol.
161
:
2804
.
24
Stumbles, P. A., J. A. Thomas, C. L. Pimm, P. T. Lee, T. J. Venaille, S. Proksch, P. G. Holt.
1998
. Resting respiratory tract dendritic cells preferentially stimulate T helper cell type 2 (Th2) responses and require obligatory cytokine signals for induction of Th1 immunity.
J. Exp. Med.
188
:
2019
.
25
Langenkamp, A., M. Messi, A. Lanzavecchia, F. Sallusto.
2000
. Kinetics of dendritic cell activation: impact on priming of TH1, TH2 and nonpolarized T cells.
Nat. Immunol.
1
:
311
.
26
Whelan, M., M. M. Harnett, K. M. Houston, V. Patel, W. Harnett, K. P Rigley.
2000
. A filarial nematode-secreted product signals dendritic cells to acquire a phenotype that drives development of Th2 cells.
J. Immunol.
15
:
645
. 164.
27
Vieira, P. L., E. C. de Jong, E. A. Wierenga, M. L. Kapsenberg, P. Kalinski.
2000
. Development of Th1-inducing capacity in myeloid dendritic cells requires environmental instruction.
J. Immunol.
164
:
4507
.
28
Huang, Q., D. Liu, P. Majewski, L. C. Schulte, J. M. Korn, R. A. Young, E. S. Lander, N. Hacohen.
2001
. The plasticity of dendritic cell responses to pathogens and their components.
Science
294
:
870
.
29
d’Ostiani, C. F., G. Del Dero, A. Bacci, C. Montagnoli, A. Spreca, A. Mencacci, P. Riccardi-Castagnoli, L. Romani.
2000
. Dendritic cells discriminate between yeasts and hyphae of the fungus Candida albicans: implications for initiation of T helper immunity in vitro and in vivo.
J. Exp. Med.
191
:
1661
.
30
MacDonald, A. S., A. D. Straw, B. Bauman, E. J. Pearce.
2001
. CD8-dendritic cell activation status plays an integral role in influencing Th2 response development.
J. Immunol.
167
:
1982
.
31
Aliberti, J., S. Hieny, C. Reis e Sousa, C. N. Serhan, A. Sher.
2002
. Lipoxin-mediated inhibition of IL-12 production by DCs: a mechanism for regulation of microbial immunity.
Nat. Immunol.
3
:
76
.
32
de Jong, E. C., P. L. Vieira, P. Kalinski, J. H. Schuitemaker, Y. Tanaka, E. A. Wierenga, M. Yazdanbakhsh, M. L. Kapsenberg.
2002
. Microbial compounds selectively induce Th1 cell-promoting or Th2 cell-promoting dendritic cells in vitro with diverse Th cell-polarizing signals.
J. Immunol.
168
:
1704
.
33
Pulendran, B., P. Kumar, C. W. Cutler, M. Mohamatzadeh, T. Van Dyke, J. Banchereau.
2001
. Lipopolysaccharides from distinct pathogens induce different classes of immune responses in vivo.
J. Immunol.
167
:
5067
.
34
Edwards, A. D., S. P. Manickasingham, R. Sporri, S. S. Diebold, O. Schulz, A. Sher, T. Kaisho, S. Akira, C. Reis e Sousa.
2002
. Microbial recognition via Toll-like receptor-dependent and -independent pathways determines the cytokine response of murine dendritic cell subsets to CD40 triggering.
J. Immunol.
169
:
3652
.
35
Manickasingham, S. P., A. D. Edwards, O. Schulz, C. Reis e Sousa.
2003
. The ability of murine dendritic cell subsets to direct T helper cell differentiation is dependent on microbial signals.
Eur. J. Immunol.
33
:
101
.
36
Agrawal, S., A. Agrawal, B. Doughty, A. Gerwitz, J. Blenis, T. Van Dyke, B. Pulendran.
2003
. Cutting edge: different Toll-like receptor agonists instruct dendritic cells to induce distinct Th responses via differential modulation of extracellular signal-regulated kinase-mitogen-activated protein kinase and c-Fos.
J. Immunol.
171
:
4984
. 9.
37
Dillon, S., A. Agrawal, T. Van Dyke, G. Landreth, L. McCauley, A. Koh, C. Maliszewski, S. Akira, B. Pulendran.
2004
. A Toll-like receptor 2 ligand stimulates Th2 responses in vivo, via induction of extracellular signal-regulated kinase mitogen-activated protein kinase and c-Fos in dendritic cells.
J. Immunol.
172
:
4733
.
38
Boonstra, A., C. Asselin-Paturel, M. Gilliet, C. Crain, G. Trinchieri, Y. J. Liu, A. O’Garra.
2003
. Flexibility of mouse classical and plasmacytoid-derived dendritic cells in directing T helper type 1 and 2 cell development: dependency on antigen dose and differential toll-like receptor ligation.
J. Exp. Med.
197
:
101
.
39
Diebold, S. S., M. Montoya, H. Unger, L. Alexopoulou, P. Roy, L. E. Haswell, A. Al-Shamkhani, R. Flavell, P. Borrow, C. Reis e Sousa.
2003
. Viral infection switches non-plasmacytoid dendritic cells into high interferon producers.
Nature
424
:
324
.
40
Lenz, P., P. M. Day, Y. Y. Pang, S. A. Frye, P. N. Jensen, D. R. Lowy, J. T. Schiller.
2001
. Papillomavirus-like particles induce acute activation of dendritic cells.
J. Immunol.
166
:
5346
.
41
Arnone, M. I., E. H. Davidson.
1997
. The hardwiring of development: organization and function of genomic regulatory systems.
Development
124
:
1851
.
42
Hubel, D. H..
1995
.
Eye, Brain and Vision
New York Scientific American Library, New York.
43
Sperry, R. W..
1982
. Some effects of disconnecting the cerebral hemispheres.
Science
217
:
1223
.
44
Metchnikoff, E..
1905
.
Immunity in Infective Disease
1905
. Cambridge Univ. Press, Cambridge, U.K.
45
Nossal, G. J., G. L. Ada, C. M. Austin.
1964
. Antigens in immunity. II. Immunogenic properties of flagella, polymerized flagellin and flagellin in the primary immune response.
Aust. J. Exp. Biol. Med. Sci.
42
:
283
.
46
Janeway, C. A., Jr.
1989
. Approaching the asymptote? Evolution and revolution in immunology.
Cold Spring. Harb. Symp. Quant. Biol.
54
:
1
.
47
Hoffmann, J. A., F. C. Kafatos, C. A. Janeway, R. A. Ezekowitz.
1999
. Phylogenetic perspectives in innate immunity.
Science
284
:
1313
.
48
Janeway, C. A., Jr, R. Medzhitov.
2002
. Innate immune recognition.
Annu. Rev. Immunol.
20
:
197
.
49
Medzhitov, R., C. Janeway.
2000
. Innate immune recognition: mechanisms and pathways.
Immunol. Rev.
173
:
89
.
50
Takeda, K., T. Kaisho, S. Akira.
2003
. Toll-like receptors.
Annu. Rev. Immunol.
21
:
335
.
51
Akira, S., K. Takeda, T. Kaisho.
2001
. Toll-like receptors: critical proteins linking innate and acquired immunity.
Nat. Immunol.
2
:
675
.
52
Akira, S., K. Takeda.
2004
. Toll-like receptor signalling.
Nat. Rev. Immunol.
4
:
499
.
53
Eisenbarth, S. C., D. A. Piggott, J. W. Huleatt, I. Visintin, C. A. Herrick, K. Bottomly.
2002
. Lipopolysaccharide-enhanced, Toll-like receptor 4-dependent T helper cell type 2 responses to inhaled antigen.
J. Exp. Med.
196
:
1645
.
54
Kadowaki, B., S. Ho, S. Antonenko, R. W. Malefyt, R. A. Kastelein, F. Bazan, Y. J. Liu.
2001
. Subsets of human dendritic cell precursors express different Toll-like receptors and respond to different microbial antigens.
J. Exp. Med.
194
:
863
.
55
Jarrossay, D., G. Napolitani, M. Colonna, F. Sallusto, A. Lanzavecchia.
2001
. Specialization and complementarity in microbial molecule recognition by human myeloid and plasmacytoid dendritic cells.
Eur. J. Immunol.
31
:
3388
.
56
Kawai, T., O. Adachi, T. Ogawa, K. Takeda, S. Akira.
1999
. Unresponsiveness of MyD88-deficient mice to endotoxin.
Immunity
11
:
115
.
57
Schnare, M., G. M. Barton, A. C. Holt, K. Takeda, S. Akira, R. Medzhitov.
2001
. Toll-like receptors control activation of adaptive immune responses.
Nat. Immunol.
2
:
947
.
58
Kaisho, T., K. Hoshino, T. Iwabe, O. Takeuchi, T. Yasui, S. Akira.
2002
. Endotoxin can induce MyD88-deficient dendritic cells to support Th2 cell differentiation.
Int. Immunol.
14
:
695
.
59
Horng, T., G. M. Barton, R. Medzhitov.
2001
. TIRAP: an adapter molecule in the Toll signaling pathway.
Nat. Immunol.
2
:
835
.
60
Fitzgerald, K. A., E. M. Palsson-McDermott, A. G. Bowie, C. A. Jefferies, A. S. Mansell, G. Brady, E. Brint, A. Dunne, P. Gray, M. T. Harte, et al
2001
. Mal (MyD88-adapter-like) is required for Toll-like receptor-4 signal transduction.
Nature
6
:
78
.
61
Toshchakov, V., B. W. Jones, P. Y. Perera, K. Thomas, M. J. Cody, S. Zhang, B. R. Williams, J. Major, T. A. Hamilton, M. J. Fenton, S. N. Vogel.
2002
. TLR4, but not TLR2, mediates IFN-β-induced STAT1α/β-dependent gene expression in macrophages.
Nat. Immunol.
3
:
392
.
62
Yamamoto, M., S. Sato, H. Hemmi, H. Sanjo, S. Uematsu, T. Kaisho, K. Hoshino, O. Takeuchi, M. Kobayashi, T. Fujita, et al
2002
. Essential role for TIRAP in activation of the signalling cascade shared by TLR2 and TLR4.
Nature
420
:
324
.
63
Horng, T., G. M. Barton, R. A. Flavell, R. Medzhitov.
2002
. The adaptor molecule TIRAP provides signalling specificity for Toll-like receptors.
Nature
420
:
329
.
64
Yamamoto, M., S. Sato, K. Mori, K. Hoshino, O. Takeuchi, S. Akira.
2002
. Cutting edge: a novel Toll/IL-1 receptor domain-containing adapter that preferentially activates the IFN-β promoter in the Toll-like receptor signaling.
J. Immunol.
169
:
6668
.
65
Oshiumi, H., M. Matsumoto, K. Funami, T. Akazawa, T. Seya.
2003
. TICAM-1, an adaptor molecule that participates in Toll-like receptor 3-mediated interferon-β induction.
Nat. Immunol.
4
:
161
.
66
Yamamoto, M., S. Sato, H. Hemmi, K. Hoshino, T. Kaisho, H. Sanjo, O. Takeuchi, M. Sugiyama, M. Okabe, K. Takeda, S. Akira.
2003
. Role of adaptor TRIF in the MyD88-independent Toll-like receptor signaling pathway.
Science
301
:
640
.
67
Hoebe, K., X. Du, P. Georgel, E. Janssen, K. Tabeta, S. O. Kim, J. Goode, P. Lin, N. Mann, S. Mudd, et al
2003
. Identification of Lps2 as a key transducer of MyD88-independent TIR signaling.
Nature
424
:
743
.
68
Fitzgerald, K.A, D. C. Rowe, B. J. Barnes, D. R. Caffrey, A. Visintin, E. Latz, B. Monks, P. M. Pitha, D. T. Golenbook.
2003
. LPS-TLR4 signaling to IRF-3/7 and NF-κB involves the toll adapters TRAM and TRIF.
J. Exp. Med.
198
:
1043
.
69
Yamamoto, M., S. Sato, H. Hemmi, S. Uematsu, K. Hoshino, T. Kaisho, O. Takeuchi, K. Takeda, S. Akira.
2003
. TRAM is specifically involved in the Toll-like receptor 4-mediated MyD88-independent signaling pathway.
Nat. Immunol.
4
:
1144
.
70
Poltorak, A., X. He, I. Smirnova, M. Y. Liu, C. Van Huffel, X. Du, D. Birdwell, E. Alejos, M. Silva, C. Galanos, et al
1998
. Defective LPS signaling in C3H/HeJ and C57BL/10ScCr mice: mutations in Tlr4 gene.
Science
28
:
2085
.
71
Takeuchi, O., K. Hoshino, T. Kawai, H. Sanjo, H. Takada, T. Ogawa, O. Takeda, S. Akira.
1999
. Differential roles of TLR2 and TLR4 in recognition of gram-negative and gram-positive bacterial cell wall components.
Immunity
11
:
443
.
72
Schnare, M., G. M. Barton, A. C. Holt, K. Takeda, S. Akira, R. Medzhitov.
2001
. Toll-like receptors control activation of adaptive immune responses.
Nat. Immunol.
2
:
94
.
73
Sugawara, I., H. Yamada, S. Mizuno, K. Takeda, S. Akira.
2003
. Mycobacterial infection in MyD88-deficient mice.
Microbiol. Immunol.
47
:
841
.
74
Brightbill, H. D., D. H. Libraty, S. R. Krutzik, R. B. Yang, J. T. Belisle, J. R. Bleharski, M. Maitland, M. V. Norgard, S. E. Plevy, S. T. Smale, et al
1999
. Host defense mechanisms triggered by microbial lipoproteins through Toll-like receptors.
Science
285
:
732
.
75
Hirschfeld, M., J. J. Weiss, V. Toshchakov, C. A. Salkowski, M. J. Cody. D. C. Ward, N. Qureshi, S. M. Michalek, S. N. Vogel.
2001
. Signaling by Toll-like receptor 2 and 4 agonists results in differential gene expression in murine macrophages.
Infect. Immunity
69
:
1477
.
76
Jotwani, R., B. Pulendran, S. Agrawal, C. W. Cutler.
2003
. Human dendritic cells respond to Porphyromonas gingivalis LPS by promoting a Th2 effector response in vitro.
Eur. J. Immunol.
33
:
2980
.
77
Redecke, V., H. Hacker, S. K. Datta, A. Fermin, P. M. Pitha, D. H. Broide, E. Raz.
2004
. Cutting edge: activation of Toll-like receptor 2 induces a Th2 immune response and promotes experimental asthma.
J. Immunol.
172
:
2739
.
78
Re, F., J. L. Strominger.
2001
. Toll-like receptor 2 (TLR2) and TLR4 differentially activate human dendritic cells.
J. Biol. Chem.
276
:
37692
.
79
Netea, M. G., R. Sutmuller, C. Hermann, C. A. Van der Graaf, J. W. Van der Meer, J. H. van Krieken, T. Hartung, G. Adema, B. J. Kullberg.
2004
. Toll-like receptor 2 suppresses immunity against Candida albicans through induction of IL-10 and regulatory T cells.
J. Immunol.
172
:
3712
.
80
Sing, A., D. Rost, N. Tvardocskia, A. Roggenkamp, A. Wiedemann, C. J. Kirschning, M. Apefelbacher, J. Heesemann.
2002
. Yersinia V-antigen exploits Toll-like receptor 2 and CD14 for interleukin 10-mediated immunosuppression.
Exp. Med.
196
:
1017
.
81
van der Kleij, D., E. Latz, J. F. Brouwers, Y. C. Kruize, M. Schmitz, E. A. Kurt-Jones, T. Espevik, E. C. de Jong, M. L Kapsenberg, D. T. Golenbook, et al
2002
. A novel host-parasite lipid cross-talk: schistosomal lysophosphatidylserine activates Toll-like receptor 2 and affects immune polarization.
J. Biol. Chem.
277
:
48122
.
82
Datta, S. K., V. Redecke, K. P. Prilliman, K. Takabayashi, M. Corr, T. Tallant, J. Di Donato, R. Dziarski, S. Akira, S. P. Schoenberger, E. Raz.
2003
. A subset of Toll-like receptor ligands induces cross-presentation by bone marrow-derived dendritic cells.
J. Immunol.
170
:
4102
.
83
Le Bon, A, N. Etchart, C. Rossmann, M. Ashton, S. Hou, D. Giewert, P. Borrow, D. F. Tough.
2003
. Cross-priming of CD8+ T cells stimulated by virus-induced type I interferon.
Nat. Immunol.
4
:
1009
.
84
Feng, G. J., H. S. Goodridge, M. M. Hamett, X. O. Wei, A. V. Nikolaev, A. P. Higson, F. Y. Liew.
1999
. Extracellular signal-related kinase (ERK) and p38 mitogen-activated protein (MAP) kinases differentially regulate the lipopolysaccharide-mediated induction of inducible nitric oxide synthase and IL-12 in macrophages: Leishmania phosphoglycans subvert macrophage IL-12 production by targeting ERK MAP kinase.
J. Immunol.
163
:
6403
.
85
Thomas, P. G., M. R. Carter, O. Atochina, A. A. Da’Dara, D. Piskorska, E. McGuire, D. A. Harn.
2003
. Maturation of dendritic cell 2 phenotype by a helminth glycan uses a Toll-like receptor 4-dependent mechanism.
J. Immunol.
171
:
5837
.
86
Murphy, L. O., S. Smith, R. H. Chen, D. C. Fingar, J. Blenis.
2002
. Molecular interpretation of ERK signal duration by immediate early gene products.
Nat. Cell Biol.
4
:
556
.
87
Lugo-Villarino, G., R. Maldonado-Lopez, R. Possemato, C. Penararida, L. H. Glimcher.
2003
. T-bet is required for optimal production of IFN-γ and antigen-specific T cell activation by dendritic cells.
Proc. Natl. Acad. Sci. USA
100
:
7749
.
88
Laderach, D., D. Compagno, O. Danos, W. Vainchenker, A. Galy.
2003
. RNA interference shows critical requirement for NF-κB p50 in the production of IL-12 by human dendritic cells.
J. Immunol.
171
:
1750
.
89
Mason, N., J. Aliberti, J. C. Caamano, H. C. Liou, C. A. Hunter.
2002
. Cutting edge: identification of c-Rel-dependent and -independent pathways of IL-12 production during infectious and inflammatory stimuli.
J. Immunol.
168
:
2590
.
90
Cao, S., J. Liu, M. Chesi, P. L. Bergsagei, I. C. Ho, R. P. Donnelly, X. Ma.
2002
. Differential regulation of IL-12 and IL-10 gene expression in macrophages by the basic leucine zipper transcription factor c-Maf fibrosarcoma.
J. Immunol.
169
:
5715
.
91
Amsen, D., J. M. Blander, G. R. Lee, K. Tanigaki, T. Honjo, R. A. Flavell.
2004
. Instruction of distinct CD4 T helper cell fates by different notch ligands on antigen-presenting cells.
Cell
117
:
515
.
92
Tanigaki, K., M. Tsuji, N. Yamamoto, H. Han, J. Tsukada, H. Inoue, M. Kubo, T. Honjo.
2004
. Regulation of αβ/γδ T cell lineage commitment and peripheral T cell responses by Notch/RBP-J signaling.
Immunity
20
:
611
.
93
Gordon, S..
2002
. Pattern recognition receptors: doubling up for the innate immune response.
Cell
111
:
927
.
94
Figdor, C. G., Y. van Kooyk, G. J. Adema.
2002
. C-type lectin receptors on dendritic cells and Langerhans cells.
Nat. Rev. Immunol.
2
:
77
.
95
Bergman, M. P., A. Engering, H. H. Smits, S. J. Van Vliet, A. A. van Bodegraven, H. P. Wirth, M. L. Kapsenberg, C. M. Vandenbroucke-Grauls, Y. van Kooyk, B. J. Appelmelk.
2004
. Helicobacter pylori modulates the T helper cell 1/T helper cell 2 balance through phase-variable interaction between lipopolysaccharide and DC-SIGN.
J. Exp. Med.
200
:
979
.
96
Geijtenbeek, T. B., S. J. Van Vliet, E. A. Koppel, M. Sanchez-Hernandez, C. M. Vandenbroucke-Grauls, B Appelmelk, V. van Kooyk.
2003
. Mycobacteria target DC-SIGN to suppress dendritic cell function.
J. Exp. Med.
197
:
7
.
97
Watanabe, T., A. Kitani, P. J. Murray, W. Strober.
2004
. NOD2 is a negative regulator of Toll-like receptor 2-mediated T helper type 1 responses.
Nat. Immunol.
5
:
800
.
98
Nigou, J., C. Zelle-Rieser, M. Gilleron, M. Thurnher, G. Puzo.
2001
. Mannosylated lipoarabinomannans inhibit IL-12 production by human dendritic cells: evidence for a negative signal delivered through the mannose receptor.
J. Immunol.
166
:
7477
.
99
den Haan, J. M., S. M. Lehar, M. J. Bevan.
2000
. CD8+ but not CD8 dendritic cells cross-prime cytotoxic T cells in vivo.
J. Exp. Med.
192
:
1685
.
100
den Haan, J. M., M. J. Bevan.
2002
. Constitutive versus activation-dependent cross-presentation of immune complexes by CD8+ and CD8 dendritic cells in vivo.
J. Exp. Med.
196
:
817
.
101
Voisine, C., F. X. Hubert, B. Trinite, M. Heslan, R. Josien.
2002
. Two phenotypically distinct subsets of spleen dendritic cells in rats exhibit different cytokine production and T cell stimulatory activity.
J. Immunol.
169
:
228
.
102
McSorley, S. J., B. D. Ehst, Y. Yu, A. T. Gewirtz.
2002
. Bacterial flagellin is an effective adjuvant for CD4+ T cells in vivo.
J. Immunol.
169
:
3914
.
103
Didierlaurent, A., I. Ferrero, L. A. Otten, B. Dubois, M. Reinhardt, H. Carlsen, R. Blomhoff, S. Akira, J. P. Kraehenbuhl, J. C. Sirard.
2004
. Flagellin promotes myeloid differentiation factor 88-dependent development of Th2-type response.
J. Immunol.
172
:
6922
.
104
Adam, F., Cunningham, Mahmood, Khan, J. Ball, T. Kai-Michael, K. Serre, E. Mohr, and I.C. MacLennan. 2004. Responses to the soluble flagellar protein FliC are Th2, while those to FliC on Salmonella are Th1. Eur. J. Immunol. 13:2986.
105
Doxsee, C. L., T. R. Riter, M. J. Reiter, S. J. Gibson, J. P. Vasilakos, R. M. Kedl.
2003
. The immune response modifier and Toll-like receptor 7 agonist S-27609 selectively induces IL-12 and TNF-α production in CD11c+CD11b+CD8 dendritic cells.
J. Immunol.
171
:
1156
.
106
Krieg, A. M..
2002
. CpG motifs in bacterial DNA and their immune effects.
Annu. Rev. Immunol.
20
:
709
.
107
Higgins, S. C., E. C. Lavelle, C. McCann, B. Keogh, E. McNeela, P. Byrne, B. O’Gorman, A. Jarnicki, P. McGuirk, K. H. Mills.
2003
. Toll-like receptor 4-mediated innate IL-10 activates antigen-specific regulatory T cells and confers resistance to Bordetella pertussis by inhibiting inflammatory pathology.
J. Immunol.
171
:
3119
.