Dendritic Cells Exposed to Triiodothyronine Deliver Pro-Inflammatory Signals and Amplify IL-17-Driven Immune Responses
Vanina Alejandra Alaminoa María del Mar Montesinosa María Florencia Solera
Lucila Giusianoa Nicolás Gigenaa Laura Fozzattia Sebastián Matías Mallerb
Santiago Patricio Méndez-Huergob Gabriel Adrián Rabinovichb
Claudia Gabriela Pellizasa
aCentro de Investigaciones en Bioquímica Clínica e Inmunología (CIBICI-CONICET) and Departamento de Bioquímica Clínica, Facultad de Ciencias Químicas, Universidad Nacional de Córdoba, Córdoba, Argentina, bLaboratorio de Inmunopatología, Instituto de Biología y Medicina Experimental (IBYME-CONICET) and Departamento de Química Biológica, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, Buenos Aires, Argentina
Key Words
Dendritic cells • Adaptive immunity • Triiodothyronine • Pro-inflammatory signals • Th17 • γδ-T cells
Abstract
Background/Aims: Although a cross-talk between immune and endocrine systems has been well established, the precise pathways by which these signals co-regulate pro- and anti-inflammatory responses on antigen-presenting cells remain poorly understood. In this work we investigated the mechanisms by which triiodothyronine (T3) controls T cell activity via dendritic cell (DC) modulation. Methods: DCs from wild-type (WT) and IL-6-deficient mice were pulsed with T3. Cytokine production and programmed death protein ligands (PD-L) 1 and 2 expression were assayed by flow cytometry and ELISA. Interferon-regulatory factor-4 (IRF-4) expression was evaluated by RT-qPCR and flow cytometry. The ability of DCs to stimulate allogenic splenocytes was assessed in a mixed lymphocyte reaction and the different profile markers were analyzed by flow cytometry and ELISA. For in vivo experiments, DCs treated with ovalbumin and T3 were injected into OTII mice. Proliferation, cytokine production, frequency of FoxP3+ regulatory T (Treg) cells and PD-1+ cells were determined by MTT assay, ELISA and flow cytometry, respectively. Results: T3 endows DCs with pro-inflammatory potential capable of generating IL-17-dominant responses and down-modulating expression of PD-L1 and 2. T3-stimulated WT-DCs increased the proportion of IL-17-producing splenocytes, an effect which was eliminated when splenocytes were incubated with T3-treated DCs derived from IL-6-deficient mice. Enhanced IL-17 expression was recorded in both, CD4- and CD4+ populations and involved the IRF-4 pathway. Particularly, γδ-T cells but not natural killer (NK), NKT, B lymphocytes nor CD8+ T cells were the major source of IL-17-production from CD4- cells. Moreover, T3-conditioned DCs promoted a decrease of the FoxP3+ Treg population. Furthermore, T3 down-modulated PD-1 expression on CD4- cells thereby limiting inhibitory signals driven by this co-inhibitory pathway. Thus, T3 acts at the DC level to drive pro-inflammatory responses in vitro. Accordingly, we found that T3 induces IL-17 and IFN-γ-dominant antigen-specific responses in vivo. Conclusion: These results emphasize the relevance of T3 as an additional immune-endocrine checkpoint and a novel therapeutic target to modulate IL-17-mediated pro-inflammatory responses.
Introduction
Dendritic cells (DCs) are highly specialized antigen presenting cells (APC) that recognize, process, and present antigens to resting T cells for induction of antigen-specific immune responses. Moreover, they play key roles in polarization of CD4+ T cells into T helper (Th) subtypes, including Th1, Th2, Th17, Th22, follicular T-helper (fTh) effector cells, as well as regulatory T cells (Treg) [1]. These CD4+ T cell subsets tailor adaptive immune responses to control pathogen clearance, autoimmune and allergic processes, tissue homeostasis and antitumor responses. This specialization process is determined by innate immune microenvironments and the cytokine milieu. Whereas interferon (IFN)-g, IFN-α, and interleukin (IL)-12 favor Th1 differentiation and IL-4 is the primary inducer of Th2 cell differentiation, transforming growth factor (TGF)-β1 is a key cytokine required for the induction of Treg cells, but paradoxically, it can also promote pro-inflammatory responses when IL-6, IL-23 and IL-1β are present, driving the differentiation of pro-inflammatory Th17 cells [2]. Although significant progress has been made in identifying DC-derived cytokines and transcription factors leading to CD4+ T cell polarization, the extracellular signals endowing DCs with specialized polarizing activity are still poorly understood.
Since the discovery and characterization of Th17 cells, several studies reported their pro-inflammatory activity in a wide variety of physiologic and pathologic settings. However, αβ-T cells do not represent the only source of IL-17; it is also produced by other cell types including γδ-T cells, natural killer (NK) cells and invariant natural killer T (iNKT) cells [3]. In addition, B cells produce IL-17 in response to challenge with infectious pathogens [4]. Moreover, a cytokine milieu containing TGF-β1 and IL-6 may also drive the differentiation of IL-17-producing CD8+ T cells (Tc17) with limited cytotoxic activity [5].
Tolerogenic mechanisms including differentiation of Treg cells, production of anti-inflammatory cytokines and expression of co-inhibitory receptors, act in concert to counterbalance pro-inflammatory signals and prevent development of autoimmune and chronic inflammatory diseases. Programmed Death protein-1 (PD-1; CD279), a co-inhibitory receptor expressed on the surface of antigen-activated and exhausted T and B cells, interacts with PD-L1 (B7-H1; CD274) and PD-L2 (B7-DC; CD273) ligands. While PD-L1 is expressed on a wide range of non-hematopoietic cells, the expression of PD-L2 is restricted mostly to APC [6]. Interaction of PD-1 with PD-L1 triggers activation of the SHP2 phosphatases leading to inhibition of T-cell responses. This immune inhibitory checkpoint serves to maintain T-cell homeostasis by restricting T cell activation and proliferation [7]. Tumor cells co-opt PD-L1 to attenuate immune responses by triggering PD-1-dependent inhibitory signals that promote T-cell exhaustion. This inhibitory effect prompted the development of checkpoint blockade therapies by reinforcing anti-tumor T-cell responses [8].
The cross-regulation of endocrine and immune signals in the balance of pro- and anti-inflammatory signals, remain poorly understood. In this regard, we identified a critical role of triiodothyronine (T3), the most active thyroid hormone (TH), in stimulating DC maturation and function. Moreover, we found that the expression of TH receptor (TR) β1 contributes to DC maturation and Th1-type cytokine secretion induced by physiologic levels of T3 [9,10]. These effects involved activation of Akt and NF-κB pathways [10] and were counteracted by glucocorticoids [11]. Moreover, we identified a major role of T3 in induction of cytotoxic T cells and protection against tumor development via DC activation [12, 13]. Recently, we reported the scarce effect of the main circulating TH, thyroxine (T4) on DC functionality, and described the mechanisms of TH transport across DC surface as well as TH metabolism in these cells [14].
Here, we aimed to understand the mechanisms by which TH controls T cell activity via DC modulation. In this study, we report the effects of T3-conditioned DCs in promoting the differentiation of IL-17-producing cells. These results emphasize the importance of THs as an additional immune-endocrine checkpoint and a novel therapeutic target to attenuate IL-17-mediated pro-inflammatory responses.
Materials and Methods
Mice
Wild type (WT) female C57BL/6 (B6; H-2b) and BALB/c mice were obtained from Ezeiza Atomic Center, Buenos Aires, Argentina. C57BL/6 IL-6 knockout (KO) mice (B6.129S2- Il6tm1Kopf/J) and C57BL/6 OTII transgenic mice (B6.Cg-Tg (TcraTcrb) 425Cbn/J) were from The Jackson Laboratory. Mice were maintained under specific pathogen-free conditions and used at 6–10 weeks old. Animal protocols were in compliance with the Guide for the Care and Use of Laboratory Animals published by the NIH and the local institutional animal care committee (Universidad Nacional de Cordoba).
Cell Preparation and Culture
Immature dendritic cells (iDCs) were obtained from bone marrow progenitors from C57BL/6 WT or IL-6-deficient mice as described [9]. At day 10 of culture, cells were treated with T3 (5 nM, Sigma) or lipopolysaccharides (LPS, 100 ng/ml; E. coli strain 0111:B4; Sigma). To rule out endotoxin contamination of T3 preparations, endotoxin content was checked (levels lower than 0.03 IU/mL, limit of detection) by the Limulus amebocyte lysate assay.
Cytokine determination
IL-6, IL-23, IL-1β, TGF-β1, IL-17 and IFN-γ detection was performed in cell culture supernatants using standard capture ELISA (BD Biosciences PharMingen).
Flow cytometry
Cell suspensions were incubated with fluorochrome labeled-Abs for 30 min at 4°C. Different combinations of the following anti-mouse Abs (BD Biosciences, eBioscience, Biolegend) were used: PerCP-Cy5.5-conjugated anti-CD4, FITC-conjugated anti-CD8 and anti-CD3, PE-conjugated anti-CD25, APC-conjugated anti-gd-TCR and anti-CD19, PE-Cy7-conjugated anti-NK1.1, APC-Cy7-conjugated anti-B220, PE-conjugated anti-PD-1, anti-PD-L1 and anti-PD-L2. Transcription factors Forhead box P3 (FoxP3) and interferon-regulatory factor-4 (IRF-4) were detected after cell fixation and permeabilization with FoxP3 Staining Buffer Set (eBioscience) using the following antibodies: APC-conjugated anti-FoxP3 and eFluor660-conjugated anti-IRF4 (eBioscience). For intracellular cytokine staining, DCs or allogenic cultures were exposed to brefeldin A (10 μg/ml; Sigma) for the last 4 h of cell culture. Cells were fixed and permeabilized with Cytofix/Cytoperm and Perm/Wash (BD Biosciences) and incubated with surface staining antibodies and PE-conjugated anti-IL-17 and APC-conjugated anti-IFN-γ (Biolegend). Cells were acquired on BD FACSCanto II and analyzed using the FlowJo software.
RNA Isolation and quantitative real-time RT-PCR
Total RNA purification, cDNA synthesis and quantitative PCR (qPCR) were performed as previously described [12]. Gene specific primer sets were as follow: mouse IRF-4 (150 bp), 5´-TTTATGCTTGTGCCCCACCT-3´ (forward) and 5´-TCGTCGTGGTCAGCTCTTTC-3´ (reverse); mouse β-actin (223 bp), 5´-CTACAATGAGCTGCGTGTGG-3´ (forward) and 5´-GGGCACAGTGTGGGTGAC-3´ (reverse). Relative changes in gene expression were calculated using the 2-ΔΔCt method normalized against the housekeeping gene β-actin.
Allogenic cultures
Allogenic cultures were performed to assess the ability of DCs to stimulate allogenic splenocytes in vitro as described [9]. Briefly, allogenic splenocytes (BALB/c, responder cells) were incubated for 3 days with irradiated DCs (C57BL/6; 30 Gy, stimulator cells) at a ratio of 1:15 (DC/splenocytes) in 96-well round-bottom plates.
In vivo model
5x106 iDCs treated with 100 µg/ml ovalbumin (OVA) and 5 nM T3 (OVA+T3-DCs) for 18 h, were injected i.v. into OTII transgenic mice that express an OVA-specific TCR, exhibit a 4-fold increase in peripheral T CD4+/CD8+ and T cells from lymph nodes and proliferate in an OVA dose-dependent manner [15]. One week later, splenocytes were restimulated ex vivo with OVA323 (50 ng/ml) and evaluated for proliferation (Methyl thiazolyl tetrazolium, MTT assay), cytokine production (ELISA), Treg frequency and PD-1 expression (flow cytometry) 4 days later.
Methyl thiazolyl tetrazolium (MTT) assay
Splenocytes were seeded in 96-well plates (4x105, 200 μl per well). Twenty μl of MTT solution (5 mg/ml) were added into each well for 4 h incubation at 37°C, and culture supernatant was discarded. Dimethyl sulfoxide (DMSO, 50 μl) was added to each well, and gently mixed with the precipitate. Optical density was measured at 540 nm (A540).
Statistical analysis
Analysis of intergroup differences was conducted by one-way analysis of variance (ANOVA), followed by Student–Newman–Keuls test. For the analysis of differences between two groups, the statistical significance was determined using Student’s t test for unpaired observations. P values less than 0.05 were considered statistically significant. All experiments were performed at least in triplicate.
Results
T3 endows DCs with a pro-inflammatory phenotype
Previously, we demonstrated that DCs exposed to T3 augmented IL-12 secretion, whereas production of IL-10 was not altered [9]. To further understand the precise nature of pro-inflammatory responses triggered by T3, we broaden the study of the cytokine profile induced by this treatment. As observed in Fig. 1A, IL-6, IL-23, IL-1β and TGF-β1 were significantly increased by T3 stimulation (6.5, 84.4, 293.7 and 1.9-fold, respectively, compared with control [DC]). As a positive control, we exposed DCs to LPS which stimulated the secretion of all measured cytokines, as previously reported [16]. Thus, in addition to its role in promoting Th1 responses [9], T3 also endows DCs with the ability to produce Th17-polarizing cytokines.
The co-inhibitory ligands PD-L1 and PD-L2 are well known for their ability to dampen T-cell responses [17]. Whether TH may impact on immune checkpoint pathways is still unknown. As shown in Fig. 1B and C, T3 decreased expression levels of both, PD-L1 and PD-L2 on DCs, as evidenced by the percentage of PD-L1 and PD-L2-expressing cells (48.4% and 33.8% reduction vs control [DC], respectively). These results suggest that T3 may limit the tolerogenic capacity of DCs by favoring a pro-inflammatory cytokine profile and down-regulating PD-L1 and PD-L2 expression.
T3-stimulated DCs instruct IL-17-mediated responses
Th17 cells differentiate in the presence of pro-inflammatory cytokines including IL-6, IL-23, IL-1β in combination with TGF-β1 [2]. Although IL-17 is considered mainly a T cell-secreted cytokine, recent evidence showed that much of the IL-17 released during an inflammatory response is produced by other immune cells [18]. Since T3 stimulates the secretion of Th17-polarizing cytokines in DCs (Fig. 1A), we then analyzed whether this hormone modulates DC ability to generate IL-17-producing T cells. For this purpose, allogenic cultures were performed and 72 h later, IL-17 production was evaluated. As depicted in Fig. 2A-D, T3-treated DCs induced not only a higher frequency of IL-17+ cells (Fig. 2A and B: 47.5% over control [DC]), but also a significant increase in intracellular IL-17 production by splenocytes (Fig. 2C: mean fluorescence intensity, MFI 58.2% vs splenocytes exposed to control [DC]), as well as higher levels of secreted IL-17 (Fig. 2D, white bars: 226.7% vs splenocytes exposed to control [DC]). Consistent with the critical role of IL-6 in driving Th17 responses [19], T3-treated DCs obtained from IL-6-deficient mice co-cultured with allogenic WT splenocytes secreted considerably lower levels of IL-17 as compared to control DCs (Fig. 2D, black bars: 51.3% over control [DC]). Thus, IL-6 contributes to the Th17 polarizing activity of T3-conditioned DCs.
Interestingly, increased IL-17 production by splenic cells exposed to T3-stimulated DCs (Fig. 2A) was evident in both CD4- and CD4+ cells (Fig. 2A, E and F). In this regard, CD4- and CD4+ IL-17+ cells significantly increased their frequency (Fig. 2E, CD4-: 184% and CD4+: 123% over control [DC]) and intracellular levels of IL-17 (Fig. 2F, CD4-: 27% and CD4+: 61% over control [DC]) after co-culture with T3-treated DCs compared to control DCs. Notably, results from Fig. 2A and E show that the main source of IL-17 production was the CD4- population. However, an increase of IL-17-producing CD4+ cells was also found when we cultured splenocytes with T3-conditioned DCs (Fig. 2A and E). A comparable result was obtained in the frequency of Th17 cells, evaluated by co-culture of purified allogenic and syngenic CD4+ cells with DCs stimulated by T3 (data not shown). This result suggests that different cell types within the splenocyte population may contribute to IL-17 production induced by T3-treated DCs.
IRF-4 is a transcription factor closely related to IL-17-mediated responses [20, 21]. Interestingly, T3-stimulated DCs showed enhanced expression of IRF-4 mRNA and intracellular protein (Fig. 2G, 4.5, and Fig. 2H and I; 0.7 fold increase over control [DC], respectively). These results show that T3-stimulated DCs direct the differentiation of IL-17-producing CD4- and CD4+ T cells through an effect accompanied by higher IRF-4 mRNA and protein expression.
γδ-T cells are the main source of IL-17 induced by T3-stimulated DCs
Given that increased IL-17 production induced by T3-stimulated DCs is mainly driven by a CD4- population, further phenotypic analysis of this subset was conducted. As shown in Fig. 3, T3-stimulated DCs induced a significant increase in the frequency of IL-17-expressing γδ-T cells (CD3+, gd-TCR+, Fig. 3A, DC vs DC-T3: 33% vs 40%). Neither NK (CD3-, NK1.1+; Fig. 3B) nor NKT (CD3+, NK1.1+; Fig. 3B) cells showed significant modifications in IL-17 secretion. Of note, the recorded decrease in the percentage of IL-17+ B cells (CD19+, B220+; Fig. 3C) could result from the increase in IL-17+-expressing γδ-T cells, as the absolute number was not altered (data not shown).
To explore the possible contribution of Tc17 to increased IL-17 production induced by T3-conditioned DCs, we further evaluated CD8+ cells as a source of this cytokine. As shown in Fig. 3D, Tc17 cells were almost absent within the splenocyte population co-cultured with either control or T3-stimulated DCs. Thus, γδ-T cells are a major source of IL-17 induced by T3-educated DCs.
T3-stimulated DCs restrain Treg cell differentiation and induction of PD-1+ CD4- T cells
To further understand the differential regulation of tolerogenic and immunogenic circuits triggered by T3-stimulated DCs, we then analyzed their influence on the Treg cell compartment. Results revealed a significant reduction in the frequency of CD25+, FoxP3+ Treg cells when splenocytes were co-cultured with T3-stimulated DCs compared with control DCs (DC-T3: 24% under DC; Fig. 4A, right panel). Along with the induction of IL-17+ cells (Fig. 2 and 3) and Th1 cells [9], these findings support the notion that T3 acts at the DC level to drive pro-inflammatory responses.
Next, we evaluated the capacity of T3-treated DCs to modulate the expression of PD-1 on T cells. As shown in Fig. 4B, PD-1 expressing cells were significantly reduced when the CD4- population was exposed to T3-treated DCs (Fig. 4B, left and middle panel, frequency: 33% of control value) with a tendency towards reduction in CD4high cells (Fig. 4B, left and right panel). In accordance, the MFI of PD-1-expressing cells was also lower in the CD4- population (data not shown). Surprisingly, PD-1 expression on CD4- cells did not correspond to the CD8 population since the frequency of PD-1+ CD8+ cells was not altered by co-culture with T3-stimulated DCs (Fig. 4C). This result suggests that T3-conditioned DCs could limit PD-1 expression on CD4- γδ-T cells as well as other CD4- cells. Thus, T3 counteracts immune inhibitory pathways through modulation of DC function.
T3 tilts the balance towards a pro-inflammatory T cell response in vivo
Given the effects of T3-stimulated DCs in development of pro-inflammatory responses in vitro, we then examined their pathophysiologic relevance in vivo. For this purpose, OTII transgenic mice that express an OVA-specific TCR were used as an antigen-specific experimental model. DCs from syngeneic mice pulsed with OVA and treated with T3 (OVA+T3-DC) for 18 h, or not (control, OVA-DC), were injected i.v. into OTII transgenic mice. One week later, splenocytes were isolated and restimulated ex vivo with OVA323 peptide. We found a significant increase in the proliferation of splenocytes from mice inoculated with OVA+T3-DC in the absence (Fig. 5A, white bars, 3.3-fold vs OVA-DC) or in the presence (Fig. 5A, black bars, 5.2 times vs OVA-DC) of OVA323 restimulation. In turn, IL-17 and IFN-γ levels were increased in culture medium from OVA+T3-DC vs OVA-DC (Fig. 5B and E, white bars). Moreover, after restimulation with OVA323, both IL-17 and IFN-γ augmented in medium from splenocytes isolated from mice inoculated with T3-conditioned, OVA-pulsed DCs (Fig 5.B and E, black vs white bars OVA+T3-DC group: 2.1-fold and 2.0-fold, respectively). In addition, an increase in IL-17-producing CD4+ T cells was recorded in the OVA+T3-DC group after restimulation with OVA323 (Fig. 5C, black vs white bars, 1.5-times over unstimulated cells). Of note, although the rise in IL-17+ CD4- cells did not reach statistical significance in the OVA+T3-DC group, there was a tendency towards an increase when compared to control mice treated with OVA-DC not exposed to T3 (Fig. 5D). Besides, and in agreement with IL-17 released, IFN-γ+-secreting CD4+ and CD8+ cells increased in the spleens from mice inoculated with OVA+T3-DC vs OVA-DC (Fig. 5F and 5G, respectively), although CD4+ cells increased only after restimulation with OVA323. In turn, the frequency of IL-17/IFN-γ double-positive cells was very low in both the CD4- and CD4+ populations (data not shown).
Finally, the regulatory response induced by T3-treated DCs was also explored in vivo. Supporting the in vitro observations, we found a reduction of Treg cell frequency in spleens from OTII mice inoculated with T3-conditioned, OVA-pulsed DCs as evidenced by a reduction in CD4+ CD25+ FoxP3+ cells, both in basal conditions and following restimulation with OVA323 (Fig. 5H, basal: 5.6-fold and after OVA323: 4.6-fold from OVA-DC values, respectively). In addition, when mice were injected with OVA+T3-DC, a tendency towards a reduction in PD-1 expression on CD4+ cells was recorded (compared with splenocytes from mice treated with DC-OVA; Fig. 5I), particularly after restimulation with OVA323, without reaching statistical significance.
Thus, T3 endows DCs with the capacity to promote both IFN-γ- and IL-17-mediated antigen-specific responses and down-modulate Treg cells in vivo.
Discussion
Thyroid hormones play key roles linking immune and endocrine networks. Our laboratory has centered the attention in elucidating the role of T3 in the initiation and termination of adaptive immune responses with a special focus on its role within the DC compartment [9-12, 14]. Although an initial therapeutic impact of T3-conditioned DCs was revealed in vivo in cancer settings [12, 13], the precise characterization of the immune landscape modulated by these cells is still lacking.
Our results demonstrated that splenocytes co-cultured with T3-matured DCs secreted higher levels of IL-17. This finding correlated with increased production of Th17-promoting cytokines: IL-23, IL-1β, IL-6 and TGF-β1 by T3-stimulated DCs. These results integrate with our previous findings demonstrating that T3 endows DCs with enhanced ability to polarize Th1 responses in vitro [9], as well as cytotoxic and antitumor effects in vivo [12, 13]. Accordingly, in vivo studies revealed the ability of T3-conditioned DCs to promote IFN-γ and IL-17-dominant antigen-specific adaptive responses. Induction of simultaneous Th1 and Th17 responses were also reported for DCs stimulated by resuscitation-promoting factor (Rpf) E of Mycobacterium tuberculosis leading to production of pro-inflammatory cytokines IL-12, IL-1β, IL-6, IL-23 and TNF and instructing CD4+ T cells to secrete IFN-γ, IL-2 and IL-17 [22]. A similar pattern of Th cytokine production was also observed in co-cultures of CD4+ T cells with DCs treated with Loxoribine, a selective Toll-like receptor (TLR)-7 agonist [23].
Although Th1 cells have shown broad antitumor activity [24] and T3-conditioned DCs increase IFN-γ-producing CD8 responses in cancer [12, 13], the role of Th17 and other IL-17-producing cells in neoplastic settings is controversial. A growing body of evidence supports the notion that Th17 cells have pro-carcinogenic activity during inflammation-induced tumorigenesis [25]. However, it was reported that IL-17 and Th17 cells have both tumor-promoting as well as tumor-suppressing functions, and part of this controversy arose when different aspects of the IL-17 response were studied [26]. In this regard, a connection has been proposed between DC activation, their recruitment to the tumor microenvironment and the T helper cytokine profile. In humans, IL-17 produced by macrophages and Th17 cells were associated with high microvessel density and poor survival in colorectal carcinoma [27], whereas mast cells expressing IL-17 indicate a favorable prognosis in esophageal squamous cell carcinoma [28]. Of note, vaccination of melanoma patients with tumor lysate-pulsed DCs increases both Th1 and Th17 cells in the peripheral blood and this effect correlated with prolonged survival [29]. Interestingly, whereas IL-17 mainly promoted tumorigenesis, a subpopulation of IL-17-producing Th17 cells seems to have tumor-suppressing effects, emphasizing the importance of the IL-17 source as well as the type of tumor in delineating the function of this cytokine [26]. In this regard, recent studies highlighted the double-edge effects of cytokines acting either as anti- or pro-tumorigenic agents depending on the tumor type [30]. Notably, consistent with the increased CD8+ T-cell proliferation and antigen-specific cross-presentation induced by T3-exposed DCs [12] and the enhanced IL-17 production by γδ-T cells (this work), previous studies showed a central role for IL-17A-producing γδ-T cells during infection [31]. Moreover, given the paradoxical pathogenic and regulatory roles of IL-17-producing cells [32], further studies should be aimed at investigating the potential role of T3-conditioned DC in autoimmune disorders. Besides, the very low splenic CD4+ IFN-γ-IL-17 double-positive cells induced by T3-treated DCs disregards a major contribution of canonical pathogenic Th17 cells to this effect [33].
In turn, results revealed that T3-exposed DCs also promote expansion of IL-17-expressing γδ-T cells, which play an ambiguous role in antitumor immunity [34]. By contributing to IL-17A and IFN-γ secretion, these cells display cytotoxic functions in a broad range of tumor cells, including prostate cancer, melanoma, metastatic renal carcinoma, breast and ovarian cancer, colon carcinoma, hepatocellular carcinoma, lung cancer and myeloma [35]. Besides, a protective role of γδ-T cells in multiple mouse cancer models has been established in vivo, largely on the basis of their potent cytotoxic activity and IFN-γ production. These cells secrete substantial amounts of IFN-γ, which inhibits angiogenesis and enhances MHC class I expression by tumor cells, thus promoting CD8+ T cell responses [36]. IL-17-producing γδ-T cells promote recruitment of conventional Th17 cells, which in turn activate CD8+ T cells through IFN-γ-dependent mechanisms. Effectors Th1, Th17 and CD8+ T cells mediate antitumor activity and inhibit tumor growth. On the contrary, regulatory gd-T (gd-Treg) cells promote tumor growth by impairing the function of effector T cells [35]. Interestingly, in response to commensal microbiota and pro-inflammatory cytokines, gd-Treg cells amplify immunosuppressive circuits by secreting high amounts of galectin-1, an endogenous immunoregulatory lectin [37].
Treg cells play a central role in the control of autoimmunity, tissue homeostasis, microbial infection and tumor growth [28]. In malignant tumors, these cells promote tumor progression by suppressing effective antitumor immunity. High infiltration of Treg cells in tumors is associated with a poor prognosis in various cancer types [38]. Thus, stimulation of Th1/Th17 responses at the expense of contraction of Treg cell compartment in mice receiving T3-conditioned DCs could influence the clinical outcome of DC-based antitumor vaccines [12, 13].
In this regard, our findings also demonstrate the influence of T3 in modulating the PD-1/PD-L1 axis. We found that T3 reduced the expression of PD-L1 and PD-L2 on T3-conditioned DCs. In turn, T3-conditioned DCs trigger a decrease in the frequency of PD-1-expressing T cells both in vitro and in vivo. As immune checkpoint blockers are essential components of the arsenal of immunotherapeutic drugs [39], our results highlight the relevance of T3-conditioned DC vaccines as a possible complement to augment the clinical efficacy of anti-PD-1/PD-L1 therapy. These findings may also provide a rational explanation for the antitumor effects of T3-stimulated DC vaccination in B16 melanoma [12, 13], decreasing PD-L1, PD-L2 or PD-1 expression in the tumor microenvironment.
Conclusion
Taken together, the results presented in this work reinforce the critical role of T3 in the initiation of adaptive immunity and induction of pro-inflammatory responses. Thus, T3, at physiologic concentrations, provides a novel opportunity for manipulating the immunogenic potential of DCs to positively regulate the development of antimicrobial and antitumor immunity and limit the occurrence of autoimmune inflammation. These findings underlie the intimate connections between immune and endocrine systems in the modulation of inflammatory responses.
Acknowledgements
The authors thank Laura Gatica, Gabriela Furlan, Paula Abadie, Pilar Crespo and Alejandra Romero for technical assistance in flow cytometry analysis and cell culture, and Luis Navarro, Fabricio Navarro, Diego Luti, Victoria Blanco and Carolina Florit for helping in animal care. This study was supported by grants from Agencia Nacional de Promoción Científica y Tecnológica (ANPCyT), Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET), Secretaría de Ciencia y Tecnología de la Universidad Nacional de Córdoba (SeCyT) and Fundación Sales.
Disclosure Statement
The authors disclose no potential conflicts of interest.
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