In adaptive immunity, Th17 lymphocytes produce the IL-17 and IL-22 cytokines that stimulate mucosal antimicrobial defenses and tissue repair. In this study, we observed that the TLR5 agonist flagellin induced swift and transient transcription of genes encoding IL-17 and IL-22 in lymphoid, gut, and lung tissues. This innate response also temporarily enhanced the expression of genes associated with the antimicrobial Th17 signature. The source of the Th17-related cytokines was identified as novel populations of CD3negCD127+ immune cells among which CD4-expressing cells resembling lymphoid tissue inducer cells. We also demonstrated that dendritic cells are essential for expression of Th17-related cytokines and so for stimulation of innate cells. These data define that TLR-induced activation of CD3negCD127+ cells and production of Th17-related cytokines may be crucial for the early defenses against pathogen invasion of host tissues.
Toll-like receptors are key players in innate immunity and are essential for sensing microbial components and triggering the host defense (1). At the luminal interface, the TLR response is mediated by the epithelium and mainly consists of neutrophil recruitment and activation (2). After microbes cross the epithelium, sensing occurs within the lamina propria. However, the nature of the TLR-mediated innate cells and defense factors that are triggered by microbial desequestration has yet to be defined.
Recent studies highlighted the contribution of IL-17A, IL-17F, and IL-22 to defensive reactions within the mucosa (3–6). These cytokines help orchestrate innate immunity by stimulating epithelial cells to produce defense molecules, matrix proteases, and tissue repair molecules (7, 8). The source of IL-17A, IL-17F, and IL-22 varies. During an adaptive response, the lymphocytes that differentiate into Th17 cells are the main producers of cytokines (9). IL-17A can rapidly be produced during innate responses to bacteria or microbial molecular patterns by γδ T lymphocytes in a TLR4-dependent manner, NKT cells activated with α-galactosylceramide, or lymphoid tissue inducer (LTi)-like cells following stimulation with the TLR2/Dectin-1 agonist zymosan (10–12). NK-like and LTi-like innate lymphocytes expressing IL-7Rα, NKp46, the transcription factor RORγt, and eventually CCR6 are sources of IL-22 and/or IL-17 in mucosa under steady-state conditions (13–18). Interestingly, microbial flora-colonizing mucosa are required to switch on lasting IL-17 and IL-22 production (15, 17). In absence of these innate lymphocytes, infectious colitis is exacerbated, suggesting an operational role of IL-22 and IL-17 in the gut’s innate immunity (15, 17). However, the link between TLR-mediated signaling, Th17-related cytokine production by innate immune cells, and mucosal defenses has not been defined.
The ability of TLR5 signaling to induce mucosal production of IL-17 and IL-22 and thereby promote antimicrobial defense has never been investigated. TLR5 detects flagellins—the main protein of bacterial flagella (19). Flagellins are expressed by bacteria, particularly pathogenic bacteria, in the gut and the lung and activate epithelial TLR5 signaling (19–21). Flagellin expression is switched off as soon as bacteria translocate into the lamina propria (22). Detection of flagellin molecules represents therefore an alarm signal for subepithelial invasion and/or disruption of the epithelial barrier function. TLR5 signaling is rapidly induced in the lamina propria dendritic cells (DCs) of the small intestine (23). In the current study, we show that flagellin activates (via DCs) the splenic and mucosal production of IL-17 and IL-22 and the subsequent expression of target genes. This TLR5-mediated response was associated with a unique population of immune cells expressing CD127 but not CD3 that resembles LTi cells, LTi-like, or NK-like innate lymphocytes. Our findings suggest that CD3negCD127+ innate immune cells may be instrumental to the host’s mucosal defense through the early production of Th17-related cytokines.
Materials and Methods
Specific pathogen-free mouse strains C57BL/6J, C57BL/6J-Ly5.1, BALB/c, and Tcrb−/−, Tcrd−/−, Tcrb−/−Tcrd−/−, Tlr5−/− (24); Myd88−/− (25); transgenic animals for pre-TCRα, Cd11c-DTR-EGFP (Itagx-DTR/EGFP) (26); Rag2−/−Il2rg−/− backcrossed on C57BL/6J mice; Cd1d−/− backcrossed on BALB/c background; and C.B-17 scid (SCID) mice were purchased from Charles River Laboratories (Wilmington, MA), The Jackson Laboratory (Bar Harbor, ME), or Janvier (St. Berthevin, France) or bred in an accredited establishment (number A59107; Institut Pasteur de Lille, Lille, France; Transgenose Institute Centre National de la Recherche Scientifique, Orleans, France; RCHCI at Eidgenössiche Technische Hochschule Zurich, Zurich, Switzerland; Ludwig Institute for Cancer Research, Brussels, Belgium; and Ludwig Institute for Cancer Research, Lausanne, Switzerland). Animals (6–16 wk old) were used according to national regulations and ethical guidelines.
For bone marrow (BM) chimera, recipient mice were irradiated (1000–1500 rad) and reconstituted 2–24 h later with BM cells (4–20 × 106 cells i.v.). These mice were used at 10–16 wk posttransplantation, and the degree of chimerism was assessed by measuring CD45.1 and CD45.2 surface expression by leukocytes. The current protocol yielded 96.7% reconstitution for Cd11c-DTR/EGFP→C57BL/6 (wild-type [WT]) in spleen and 98.2% for Tlr5−/−→WT and 97.2% for WT→Tlr5−/− in lung. Depletion of CD11c+ cells was achieved by injecting i.p. diphtheria toxin (DTX) as described previously (26). Depletion of γδ T cells (∼90% depletion) and NK cells (∼72% depletion) was performed by injecting i.p., 24 h prior to flagellin treatment, 100 μg mAb specific for TCRδ-chain (GL3 clone) or NK1.1 (PK136) or irrelevant mAb HB152 as control.
LPS-depleted flagellin FliC from Salmonella typhimurium (5 μg) produced as previously described (21), ultrapure LPS from Escherichia coli (serotype 0111:B4, 5 μg; InvivoGen, Toulouse, France), or phosphorothioate CpG oligonucleotide (5′-TCCATGACGTTCCTGATGCT-3′, 5 μg; Eurogentec, Angers, France) diluted in PBS were injected i.v. or i.p to mice.
Flow cytometry and sorting
DCs were differentiated from BM as described previously (25). On day 7 or 11, BMDCs were stimulated for 2 h and analyzed.
Determination of cytokine production
−ΔΔCt) were determined by comparing 1) the cycle thresholds (Ct) for the gene of interest and Actb (ΔCt) and 2) ΔCt values for treated and control groups (ΔΔCt). Ct upper limit was fixed to 33 cycles.
27). Data were analyzed using the NeONORM method, and heat maps were created as described previously (27, 28). Gene Ontology was analyzed using the Panther Protein Classification System (www.pantherdb.org).
The Mann-Whitney U test and the Graphpad Prism software 5.0 were used in analyses. The Limma test with Benjamini-Hochberg false discovery rate (FDR) correction was used for high-throughput PCR with TaqMan Low Density Arrays. Results were considered significant for p < 0.05 indicated by an asterisk. Results are expressed as arithmetic means ± SD.
Systemic TLR5 signaling enhances Il17a, Il17f, and Il22 gene expression in lymphoid tissues
To establish whether TLR stimulation promotes the rapid expression of the Th17-related cytokines, mice were treated i.p. or i.v. with a TLR4 agonist (LPS), a TLR5 agonist (flagellin), or a TLR9 agonist (CpG). Gene expression in spleen and lymph nodes was then monitored (Fig. 1A, 1B). Flagellin administration triggered within 2 h ∼1000-fold increase of Il22 mRNA levels. Similarly, Il17a and Il17f gene expression was upregulated. A TLR5-mediated, Th17-related innate response was also observed in the mediastinal and inguinal lymph nodes and, to a lesser extent, in the liver (Fig. 1, Supplemental Fig. 1). LPS was initially shown to promote Il22 expression in many tissues (29). We found that LPS also enhanced the Th17-related innate response but to a lesser extent than flagellin did (Fig. 1A, 1B, Supplemental Fig. 1). Although TLR9-mediated signaling activated the response in lymph nodes, it was devoid of any effect in the spleen.
The flagellin-dependent response was transient and peaked at 2 h; mRNA levels returned to baseline levels after 24 h (Fig. 1C, 1D). Gene profiling showed that the expression of genes specific for TLR-, IL-17R–, and IL-22R–mediated signaling was significantly enhanced in spleen (Fig. 1E). These genes encode pleiotropic and Th17-promoting cytokines (TNF-α, IL-1β, and IL-6), chemokines that are specific for neutrophils, monocytes, and lymphocytes (CXCL-1, -2, -5, -9, and -10), antimicrobial molecules like CAMP and HAMP, lipocalin 2, S100A9, and tissue remodeling proteases matrix metalloproteinases 3 and 13. Strikingly, the expression of chemokine CCL20, which is specific for the recruitment of DC precursors, Th17 lymphocytes, LTi-like or NK-like cells (12, 14, 30, 31), was significantly upregulated. The transcription of the IFN-γ encoding gene was also upregulated by flagellin treatment. However, we did not observe any change in the expression of genes coding for IL-21, RORγt, or TGF-β (i.e., other factors involved in Th17 differentiation). As shown in Fig. 1F, IL-22 levels rose significantly in serum and spleen from flagellin-treated animals, whereas IL-17A was hardly detectable in serum but rose 3-fold in the spleen. In conclusion, TLR5 signaling in lymphoid tissues promotes the rapid production of the innate cytokines IL-17A, IL-17F, and IL-22—a pattern that resembles a Th17-related innate response.
TLR5-mediated innate responses require common γ-chain–dependent immune cells
TLR5 is expressed by monocyte/macrophage/DC lineages, NK cells, CD4+ lymphocytes, and radioresistant stromal cells but not B lymphocytes (25, 32–35). To define the cells involved in the early production of IL-17 and IL-22, we used BM chimera expressing or not Tlr5 and tested their ability to respond to flagellin. As shown in Fig. 2A and 2E, TLR5-competent hematopoietic cells were required to trigger flagellin-mediated Il17 and Il22 gene expression. By using SCID, pre-TCRα animals harboring enhanced number of γδ lymphocytes, Tcrb−/− and Tcrd−/− mice, we demonstrated that neither B cells nor TCRαβ-/TCRγδ-expressing T lymphocytes were required for flagellin-mediated Il17 and Il22 expression (Fig. 2B, Supplemental Fig. 2A, 2B, 2D). In contrast, the response was impaired in Rag2−/−Il2rg−/− mice that have almost normal DCs but lack B and T lymphocytes as well as NK, NKT, LTi, NK-like, and LTi-like cells, all of which depend on the IL receptor common γ-chain (γc) encoded by Il2rg gene (Fig. 2C, 2E) (12, 15, 17, 36). The impairment was not a collapse of TLR5 signaling, because the liver was still responsive to flagellin (Fig. 2D). Our experiments using genetically deficient animals and depleting Abs suggested that CD1d-restricted NKT cells or NK cells were not drivers of the TLR5-mediated response (Supplemental Fig. 2C, 2D). Therefore, our data showed that the cells expressing Th17-related cytokines after flagellin stimulation require the IL receptor γc for differentiation or activation but are not NK, NKT, or TCRγδ-expressing innate lymphocytes.
CD3negCD127+ LTi-like cells produce IL-17 and IL-22 in response to TLR5 signaling
We next sought to determine which γc-dependent innate immune cells are involved in TLR5-mediated response. Recent work has suggested that innate lymphocytes expressing IL-7R (i.e., the CD127 or IL-7Rα–chain and the CD132 or γc) and LTi/NK cell markers are sources of IL-22 and IL-17 (12–17). To determine whether innate immune cells produce Th17-related cytokines, splenic cells from mock- and flagellin-treated animals were sorted on the basis of lineage markers (CD11b, CD11c, Gr1, CD3, and B220), NK1.1, CD4, and CD127 (Fig. 3A). After TLR5 stimulation, LinnegNK1.1negCD127+CD4neg and LinnegNK1.1negCD127+CD4+ cells were found to strongly upregulate Il17f and Il22 expression ∼80- and 500-fold, respectively (Fig. 3B). These subsets account for ∼0.5–1% of LinnegNK1.1neg cells and 0.02–0.1% of splenocytes in a C57BL/6 mouse. Similar observations were done using CD3negCD127+CD4+ cells from SCID mice (Fig. 3D). Both subsets were found to express CD45, indicating a hematopoietic origin and their classification as immune cells (Supplemental Fig. 3A). We also found that the CD3negCD127+CD4+ and CD3negCD127+CD4neg cells are mostly absent in the Rag2−/−Il2rg−/− mouse (Supplemental Fig. 3D). This is consistent with previous observations (12, 17). In spleen, the frequency of LinnegNK1.1negCD127+CD4+ matched the number of LTi cells (12, 37). We also detected upregulation of Il22 transcript levels in CD4 lymphocytes (Lin+CD4+) and NK cells (Lin+NK1.1+) after flagellin administration. However, the LinnegNK1.1negCD127+CD4+ cells were the most potent producers, because Il17f and Il22 mRNA levels were 100- to 1000-fold higher in this population than in LinnegNK1.1negCD127+CD4neg, CD4, or NK cells (Fig. 3C). Like the NK-like or LTi-like cells, LinnegNK1.1negCD127+CD4+ innate lymphocytes express Ccr6, but we were unable to detect any mRNA for Rorc or Ncr1 encoding RORγt and NKp46. In addition, we observed that, upon flagellin stimulation, Ifng expression was mainly upregulated in NK cells rather than LinnegCD127+ cells (data not shown). Our data provide evidence that LinnegCD127+ innate immune cells, and especially the CD4+ fraction, which resembles LTi cells, can rapidly produce IL-17 and IL-22 cytokines following TLR activation.
TLR5-mediated activation of Th17-related innate responses requires DCs
To determine whether the TLR5-mediated upregulation of Th17-related innate response is a direct innate lymphocyte activation process or requires DC stimulation, DTX-mediated ablation of CD11c+ cells was performed in a Cd11c-DTR-EGFP BM chimera. The DTX treatment depleted 93.8 ± 2.2% of CD11c+MHCIIhigh DCs (Fig. 4A). In addition, we found that DTX treatment did not eliminate the CD3negCD4+CD127+ cells in the spleen of a Cd11c-DTR-EGFP chimera and did not alter the TLR5-mediated production of IL-22 (Supplemental Fig. 3). As shown in Fig. 4B, systemic administration of flagellin to DC-depleted animals resulted in impaired Il17f or Il22 transcription, compared with controls. Thus, our experiments demonstrated that DCs are necessary for TLR5-mediated expression of Th17-related cytokines.
Th17 differentiation depends on tissue-derived TGF-β and IL-1β, and IL-6 produced by DCs and the maintenance of Th17 phenotype have been associated to DC-derived IL-23 (9). In vivo, DC cell ablation was found to attenuate the upregulation of Il6 transcription in response to flagellin (Fig. 4C). We did not observe any alteration in Il1b, Il12b coding for the p40 chain of IL-12 or IL-23, and Tgfb gene expression (data not shown). Intriguingly, Il23a (coding for the p19 chain of IL-23) transcription was enhanced by DTX treatment; it is possible that CD11clowMHCIIneg cells having infiltrated the spleen in DTX-treated mice support Il23a upregulation (Fig. 4A, 4C). In this study, we found that flagellin promoted the expression of both Il12b and Il23a in BM-derived DCs (Fig. 4D). In response to flagellin, DCs can therefore produce both IL-6 and IL-23, with the subsequent expression of an innate, Th17-related signature. However, in vivo, IL-23 alone appears unable to induce gene activation. In response to flagellin, DC-mediated IL-6 production could therefore promote the Th17-related signature by LinnegCD127+ cells.
Previous studies suggested that DCs produce IL-22 (4, 38). In this study, we found that flagellin promoted the expression of Il12b in BM-derived DCs but did not have any effect on Il17a, Il17f, and Il22 transcription (Fig. 4D). LPS, which strongly increased Il12b and Il23a transcription, did not upregulate expression of Il17a and Il17f in DCs but did enhance Il22 mRNA levels ∼8-fold, compared with untreated cells. Analysis of transcription in splenic DCs sorted from mice treated with flagellin also showed that DCs were not potent source of IL-22 (Fig. 3D, 3E). Taken as a whole, our data suggested that DCs are not a major source of TLR5-mediated, Th17-related cytokine production but contribute to induction of the latter.
TLR5 signaling triggers an intestinal, Th17-related, innate response
Because Th17-related cytokines are important in the control of inflammation and infection in the mucosa (3–6, 38, 39), we next assessed the impact of flagellin administration on intestinal tissues. Flagellin strongly enhanced the production of IL-22 within 2 h of administration from the duodenum to the proximal colon; the level then returned to the baseline at 8 h (Fig. 5). CCL20 synthesis was also strongly induced in the small intestine (Supplemental Fig. 4A). In contrast, gut IL-17A production changed moderately following flagellin treatment. In any intestine segments, TLR5 signaling strongly enhanced transcription of the Il17 and Il22 genes and those encoding antimicrobial peptides, neutrophil-specific chemokines and growth factors, and tissue remodeling/repair molecules (Fig. 5C, Supplemental Fig. 4B–E). A similar pattern was also observed in lung tissue, but regression to baseline levels was not observed at 8 h, suggesting that the kinetic may be different in the respiratory tract (Supplemental Fig. 5). In conclusion, systemic flagellin administration promotes mucosal Th17-related innate responses.
We used the same type of analysis than for the response in spleen and lymph nodes to investigate the effects of flagellin on mucosa. The gut and pulmonary flagellin-mediated Th17-like innate response was stronger in WT→Tlr5−/− than in Tlr5−/−→WT animals (Fig. 5D, Supplemental Fig. 5). These findings indicated that TLR5-competent hematopoietic cells are also instrumental in the mucosal response. DC depletion impaired the upregulation of the intestinal Th17-related signature (Supplemental Fig. 4F). Last, we compared Th17-related gene expression in Rag2−/−Il2rg−/− and WT animals (Fig. 5E). As previously reported, steady-state levels of Il22, Il17, and Ifng transcripts in the ileum were significantly lower in Rag2−/−Il2rg−/− than in WT animals (17). After TLR5 stimulation, transcription of these genes was not enhanced in the Rag2−/−Il2rg−/− intestine. Taken as a whole, our data suggest that TLR5 signaling elicits a mucosal Th17-related response by innate immune cells that require the γc and help from DCs.
Finally, the expression of many genes was upregulated in the gut of Rag2−/−Il2rg−/− animals, suggesting that TLR5 signaling promotes the activation of non-lymphoid cells (Fig. 5E). Experiments with the Tlr5−/− chimera reinforced these conclusions because radioresistant cells, likely structural/stromal cells, contributed to TLR5-mediated responses, especially in lung tissue (Fig. 5D, Supplemental Fig. 5).
The flagellin-induced transcriptional signature has features of Th17- and TLR-mediated responses
To further characterize the response to flagellin, we performed a microarray time course analysis of gene expression in the distal ileum of treated mice. Biological processes including signaling and defense pathways, cytokine- and chemokine-mediated immunity, NF-κB signaling, granulocyte- and macrophage-mediated immunity, cell proliferation and differentiation, and apoptosis, were significantly modulated 2–8 h after flagellin administration (Supplemental Fig. 6, Supplemental Table II). On the basis of these data, we arbitrarily defined five groups of genes according to their potential role in the gut immune response (Table I). The first group of genes encodes modulators of IL-17 an IL-22 production as well as differentiation of Th17, LTi, LTi-like, NK, and NK-like cells. The second class was involved in the positive or negative transcriptional control of IL-17R (i.e., C/EBPβ and C/EBPδ), IL-22R (i.e., SOCS-1, SOCS-3, and STAT5A) and TLR (i.e., activating transcription factor 3 and I-κBα) signaling. The third group includes TLR, IL-17R, and IL-22R signaling target genes coding for antimicrobial molecules or factors regulating epithelium barrier function. Interestingly, our analysis also identified antiviral genes. In contrast to previous studies of Th17 cytokine-mediated signaling, we did not find any transcriptional modulation of the mucin and β-defensin genes. The fourth group corresponds to genes encoding factors involved in recruitment, development, or function of various immune cells. Relative to controls, chemokines specific for innate immune cells like neutrophils, NK, NKT, or LTi cells, T and B lymphocytes, or monocytes were significantly enriched—suggesting that several cell types may enter the tissues and participate in the immune response. The last group includes genes required for TLR4 or TLR2 signaling (i.e., LBP, CD14, and MAL). In conclusion, intestinal gene expression profiling showed that flagellin promotes a transient immune response involving regulators and effectors of both TLR- and Th17-mediated immunity.
The way in which TLR signaling activates the host’s innate defenses during mucosal invasion by pathogens is subject to debate. In the current study, we showed that TLR5 signaling induces systemic and mucosal innate expression of the Th17-related IL-17 and IL-22 cytokines by stimulating LinnegCD127+ cells in a DC-dependent manner. Overall, our data suggest that LinnegCD127+ cells may play a major role as innate lymphocytes in the early orchestration of a TLR-dependent, protective response to mucosal invasion by pathogens.
Th17-related cytokines contribute to adaptive immunity in response to various inflammatory and infectious diseases (5, 6, 30, 39); however, their impact on the early phase of infection is poorly understood. The effect of TLR signaling on IL-17 and IL-22 was previously suggested because their synthesis was enhanced after administration of TLR2 and TLR4 agonists (12, 29). Very recently, TLR5 signaling was also associated to such response (40). In this study, we analyzed the immune response to the TLR5 activator flagellin, with a focus on mucosa. Our rationale was that because flagellin expression is specifically restricted to luminal compartment, its desequestration is likely to be an alarm signal for mucosal invasion (22). We found that systemic flagellin administration promotes the swift, intense, transient production of IL-17A, IL-17F, and IL-22 and factors controlling Th17 differentiation. For example, flagellin modulates expression of the genes coding for the aryl hydrogen receptor repressor, the aryl hydrogen receptor nuclear translocator-like factor ARNTL and the activating transcription factor-like factor BATF—all of which are involved in Th17 differentiation (41, 42). Moreover, the flagellin-induced innate response and the Th17 adaptive response share many effectors, such as chemokines, antimicrobial peptides, antiapoptotic factors, and tissue remodeling factors. Therefore, we hypothesized that flagellin-mediated response enables the rapid and transient recruitment of systemic and mucosal defenses.
Our results suggest that the LinnegCD127+ cells, especially the CD4+ fraction, have a pivotal role in the TLR-mediated response via the production of IL-17 and IL-22. These cells are similar to LTi (CD3negCD127+CD4+), LTi-like (RORγt+LinnegCD127+CD4+), and NK-like (RORγt+NKp46+LinnegCD127+NK1.1+/neg) cells (12, 14–17). LTi-like/NK-like cells constitutively produce Th17-related cytokines in a process that depends on gut flora, γc, and RORγt (15, 17). Moreover, the LTi-like cells were shown to upregulate the production of IL-17 and IL-22 in response to microbial products. The development of LinnegCD127+CD4+ and LinnegCD127+CD4neg cells identified in this study requires the γc. These subsets also express the CCR6 but not the NKp46 encoding gene, suggesting a common ontogeny with LTi and LTi-like cells (12, 13). We were unable to detect any expression of RORγt in the LinnegCD127+ cells; expression below our assay’s detection threshold is one possible explanation for this failure. Furthermore, Rorc expression was downregulated in gut after flagellin injection. Interestingly, we noted enhanced intestinal expression of the gene encoding NFIL3—a factor that is essential for NK cell development (43). Recent studies demonstrated that increased expression of IL-7 enhances the number of LTi cells (37) and that deficiency in IL-7 affects the number of NK-like IL-22 expressing population (44). Additional work will be needed to define the ontogeny and transcriptional factors involved in the differentiation of LinnegCD127+ cells.
DCs have an important role in integrating microbial signals and activating immune cells like Th17 lymphocytes (9). When DCs were depleted, the Th17-related innate response to flagellin was impaired, indicating that DCs are necessary for the activation of LinnegCD127+ cells. Similarly, transcription of Il6 was attenuated, which suggests that, in Th17 differentiation, DC-derived IL-6 like may contribute to LinnegCD127+ cell activation (9). Our findings suggested that IL-1β, IL-23, or TGF-β (or at least the amounts produced by DCs) are not required for LinnegCD127+ cell activation. However, IL-23 might be important for an alternative activating pathway for production of IL-17 or IL-22 by LTi-like and NK-like cells because 1) cells are activated in vitro by supplementing the culture medium with IL-23 and 2) CD3negCD4+ cells express IL-23R (12, 13, 15, 45). In the intestine, TLR5 signaling activates lamina propria DCs, which then promote Th17 differentiation (23). In contrast, intestinal DCs do not respond to TLR4 stimulation (23, 46). Flagellin treatment enhanced the transcription of the genes coding for CD14, LPB, MAL, and TLR2 (Supplemental Fig. 2, Table I). These findings suggest that responsiveness to TLR2 and TLR4 agonists may be reactivated or amplified after TLR5 stimulation, allowing the production of a second wave of effectors.
The relevance of TLR5 signaling in defense has recently been assessed. Flagellin-mediated protection of rodents and nonhuman primates against lethal irradiation was associated with CSF3-mediated granulopoiesis and the antiapoptotic effect of superoxide dismutase 2 (47). Flagellin treatment has been linked to resistance against inflammatory colitis and gut infections (40, 48). The TLR5-induced circulating and local production of IL-17/IL-22 may be the main driving force behind these protective effects. The contribution of IL-22 was recently suggested as instrumental in the control of infection with enterococci (40). In response to IL-17R and IL-22R signaling, epithelial and stromal cells produce antimicrobial peptides (RegIII), CXC chemokines, and growth factors (CSF3) for neutrophils, all of which are involved in mucosal protection (3–7, 38). Flagellin treatment prompted the expression of similar factors. In addition, our study identified other potential effectors of the TLR/IL-17R/IL-22R axis, such as antiviral molecules (ISG15, ISG20, OAS2, and OAS3), acute-phase proteins (SAA2, SAA3, and PTX3), and superoxide-mediated killing (NCF1, NOX1, and superoxide dismutase 2).
The LinnegCD127+CD4+ cells described in this paper resemble the LTi cells that are instrumental in the development of secondary/tertiary lymphoid tissues (such as Peyer’s patches or isolated lymphoid follicles) (13, 49). Intestinal LTi cells express lymphotoxin β and CCR6 and CXCR5, the receptors for CCL20 and CXCL13, respectively (13, 49). CXCL13 and CCL20 are produced by epithelial/stromal cells and are involved in the clustering of lymphocytes, DCs, and LTi to form lymphoid follicles (50, 51). Ectopic epithelial expression of CXCL13 increases the number of LTi cells (which produce IL-22 constitutively) and isolated lymphoid follicles (13). Our work revealed that flagellin upregulated gut expression of genes coding for lymphotoxin β, CXCL13, and CCL20. We previously showed that flagellin triggers CCL20 production in intestinal epithelial cells (31). Hence, TLR5 signaling in both epithelial and hematopoeitic cells may increase the development of secondary/tertiary lymphoid tissues. Recent studies showed that TLR2, TLR4, and Nod1 are involved in the development of lymphoid follicles (50, 51). Pattern recognition receptors in general and TLRs in particular may have a pivotal role in simultaneously conditioning the antimicrobial environment and new ectopic sites for the development of mucosal adaptive immunity.
In conclusion, the current study found that LinnegCD127+ cells constitute a rapidly reacting, innate source of IL-17A, IL-17F, and IL-22 in response to TLR signaling. We hypothesize that this immune reaction occurs during microbial penetration into the lamina propria and stimulates innate effectors to locally clear the infection. Similar cell populations have been identified in humans (14, 16), and so, it remains to be seen whether TLR stimulation can promote activation of these innate immune cells.
We thank Michel Simonet and François Trottein for critical reading of the manuscript, Shizuo Akira for Tlr5−/− mice, David Dombrowicz for Rag2−/−Il2rg−/− mice, and Christelle Faveeuw and Laxmi Koodun for technical assistance.
Disclosures The authors have no financial conflicts of interest.
This work was supported by the Institut National de la Santé et de la Recherche Médicale (to. C.C., L.V.M., D.C., and J.-C.S.), the Institut Pasteur de Lille, the Université Lille Nord de France, and the Région Nord Pas de Calais (ARCir Europe). W.-D.H. and J.-C.S. are funded by the European Community (Grant INCO-CT-2006-032296).
Microarray data were deposited in the publicly available database (http://mace.ihes.fr) with accession number 2844328654.
The online version of this article contains supplemental material.
Abbreviations used in this paper:
- cycle threshold
- dendritic cell
- diphtheria toxin
- common γ-chain
- lymphoid tissue inducer
- Received January 15, 2010.
- Accepted May 11, 2010.
- Copyright © 2010 by The American Association of Immunologists, Inc.