- Open Access
Endogenous analgesia mediated by CD4+ T lymphocytes is dependent on enkephalins in mice
- Lilian Basso†1,
- Jérôme Boué†1,
- Karim Mahiddine2,
- Catherine Blanpied1,
- Sébastien Robiou-du-Pont1,
- Nathalie Vergnolle1,
- Céline Deraison1 and
- Gilles Dietrich1Email author
© The Author(s). 2016
Received: 20 January 2016
Accepted: 20 May 2016
Published: 1 June 2016
T cell-derived opioids play a key role in the control of inflammatory pain. However, the nature of opioids produced by T cells is still matter of debate in mice. Whereas β-endorphin has been found in T lymphocytes by using antibody-based methods, messenger RNA (mRNA) quantification shows mainly mRNA encoding for enkephalins. The objective of the study is to elucidate the nature of T cell-derived opioids responsible for analgesia and clarify discrepancy of the results at the protein and genetic levels.
CD4+ T lymphocytes were isolated from wild-type and enkephalin-deficient mice. mRNA encoding for β-endorphin and enkephalin was quantified by RT-qPCR. The binding of commercially available polyclonal anti-endorphin antibodies to lymphocytes from wild-type or enkephalin knockout mice was assessed by cytofluorometry. Opioid-mediated analgesic properties of T lymphocytes from wild-type and enkephalin-deficient mice were compared in a model of inflammation-induced somatic pain by measuring sensitivity to mechanical stimuli using calibrated von Frey filaments.
CD4+ T lymphocytes expressed high level of mRNA encoding for enkephalins but not for β-endorphin in mice. Anti-β-endorphin polyclonal IgG antibodies are specific for β-endorphin but cross-react with enkephalins. Anti-β-endorphin polyclonal antibodies bound to wild-type but not enkephalin-deficient CD4+ T lymphocytes. Endogenous regulation of inflammatory pain by wild-type T lymphocytes was completely abolished when T lymphocytes were deficient in enkephalins. Pain behavior of immune-deficient (i.e., without B and T lymphocytes) mice was superimposable to that of mice transferred with enkephalin-deficient lymphocytes.
Rabbit polyclonal anti-β-endorphin serum IgG bind to CD4+ T lymphocytes because of their cross-reactivity towards enkephalins. Thus, staining of T lymphocytes by anti-β-endorphin polyclonal IgG reported in most of studies in mice is because of their binding to enkephalins. In mice, CD4+ T lymphocytes completely lose their analgesic opioid-mediated activity when lacking enkephalins.
Here, we show that the intracytoplasmic staining of activated mouse T lymphocytes by anti-β-endorphin polyclonal IgG is due to cross-reactivity towards enkephalin. Moreover, we show that the endogenous regulation of inflammatory somatic pain by CD4+ T lymphocytes in mice is completely abrogated when T lymphocytes are deficient in enkephalins.
C57BL/6 mice were provided from Janvier (Le Genest Saint Isle, France) and recombination-activating gene 2-deficient C57BL/6 (RAG2−/−) mice were from ANEXPLO platforms (UMS 006, Toulouse, France). Pre-proenkephalin knockout (PENK−/−) mice were the B6.129-Penk-rstm1Pig/J strain with a genetic background C57BL/6 (MHC H-2 Haplotype b) provided by The Jackson Laboratory (Bar Harbor, Maine, USA). All mice used in the study were 8–10-week-old male weighing 20–25 g. Mice were housed at a temperature between 20 and 22 °C and maintained under a 12-h light/dark cycle in sawdust coated transparent cages. Animals were housed by three or two in ventilated cages with chow and water ad libitum. All experiments involving animals were performed in accord with ethical guidelines (INSERM) and were approved by the Midi-Pyrénées (France) ethics committee (application Number MP/06/73/10/12).
Isolation and activation of CD4+ T lymphocytes
CD4+ T lymphocytes were isolated from splenocytes using cell negative isolation kits according to the manufacturer’s instructions (Invitrogen Dynal AS, Oslo, Norway). Twenty-four-well cell culture plates (Corning, Life Sciences, Amsterdam, Netherlands) previously coated with 2.5 μg mL−1 of anti-CD3 (clone 145-2C11) and 2.5 μg mL−1 of anti-CD28 (clone 37.51) monoclonal antibodies (mAbs) (BD Biosciences, San Jose, CA) were seeded with 5 × 105 purified naive CD4+ T cells (more than 92 % pure) in RPMI-1640 medium (GIBCO Life Technologies, Paisley, UK) supplemented with 10 % heat inactivated fetal calf serum (GIBCO Life Technologies), 1 % non-essential amino acids, 4-mM l-glutamine, 1-mM sodium pyruvate, 100-IU/ml penicillin, 100-μg/ml streptomycin (GIBCO-BRL), 10-mM HEPES (4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid), and 2 × 10-5 M 2-β-Mercapto-ethanol.
Cytofluorometric analysis of antibody activity
106 cells were incubated with anti-CD16/CD32 (mouse Fc block™, clone 2.4G2, BD Biosciences), fixed and permeabilized with BD Cytofix/Cytoperm solution before being incubated with 100 μL of either rabbit anti-β-endorphin polyclonal IgG (10 μg mL−1, Merck-Millipore (Chemicon International), Temecula, CA), rabbit anti-Met-enkephalin polyclonal IgG (10 μg mL−1, Merck-Millipore (Chemicon International), Temecula, CA), or rabbit non-immune serum IgG (10 μg mL−1, Jackson Immunoresearch Lab) diluted in PBS containing 1 % fetal calf serum and 2 mM EDTA. After washing, cell-bound antibodies were revealed using fluorescein isothiocyanate (FITC)-conjugated goat anti-rabbit IgG secondary antibodies (BD Biosciences). Data were collected on 20,000 cells by forward and side scatter intensity on an FACs calibur flow cytometer (Becton Dickinson, Franklin Lakes, NJ) and were subsequently analyzed using the Flow Jo software (Tree Star Inc., Ashland, OR). The neuroepithelioma cell line SK-N-MC was grown in DMEM supplemented with 10 % fetal-calf serum at 37 °C with 5 % CO2 in air atmosphere until the cells reach 80 % confluence .
For inhibition experiments, rabbit anti-β-endorphin or anti-Met-enkephalin antibodies were incubated with 10 μM of soluble β-endorphin (Sigma-Aldrich, St. Louis, MO) or Met-enkephalin (Sigma-Aldrich, St. Louis, MO) for 20 min at 37 °C before being added to the cells overnight at 4 °C . After washing, cell-bound antibodies were revealed using fluorescein isothiocyanate (FITC)-conjugated goat anti-rabbit IgG secondary antibodies as described above.
Real-time PCR analysis
Total RNA was isolated by TRIzol™ Reagent and reverse-transcribed with Moloney murine leukemia virus reverse transcriptase using random hexamers for priming. Transcripts encoding hypoxanthine phosphoribosyl transferase (HPRT), proenkephalin (PENK), and proopiomelanocortin (POMC) were quantified by real-time PCR in mouse CD4+ T cells. Forward and reverse primers were 5′-GTTCTTTGCTGACCTGCTGGAT-3′ and 5′-CCCCGTTGACTGATCATTACAG-3′ for HPRT, 5′-CGACATCAATTTCCTGGCGT-3′ and 5′-AGATCCTTGCAGGTCTCCCA-3′ for PENK, and 5′-TGGCCCTCCTGCTTCAGAC-3′ and 5′-CAGCGAGAGGTCGAGTTTGC-3′ for POMC, . The target gene expression was normalized to the HPRT mRNA and quantified relative to a standard complementary DNA (cDNA) (calibrator sample) prepared from mouse brain using the 2−ΔΔCT method, where ΔΔC T = ΔC T sample − ΔC T calibrator .
Spleen cell transfer, immunization, and measurement of somatic nociception in mice
Splenocytes (30 × 106) isolated from either wild-type C57BL/6 (PENK+/+) or pre-proenkephalin knockout (PENK−/−) mice were intravenously (i.v.) injected into immunodeficient RAG2 −/− mice. Mice were housed by three or two in ventilated cages. The next day, recipient mice were immunized by injecting subcutaneously (s.c.) into hind footpads ovalbumin (OVA, Sigma Chemical Co., St Louis, MO) emulsified in complete Freund’s adjuvant (CFA) (50 μl at 1 mg mL−1).
Mechanical withdrawal thresholds were measured using ascending series calibrated von Frey filaments of binding forces ranging from 0.04 to 2 g (Stoelting, Wood Dale, IL, USA), applied onto the plantar surface of mice. Threshold to mechanical stimuli was calculated as the force value of the von Frey filament triggering three paw withdrawals over five applications. Responses to mechanical stimuli were recorded before, and daily after immunization. The groups of mice treated with naloxone methiodide (NLX) were injected (10 μl at 2 mg mL−1) into the ankle of the inflamed hind paw, 30 min before each pain assessment . Naloxone methiodide treatment was initiated on day 3 after immunization when T cells become the predominant immune cells at the inflammatory site .
Data are expressed as mean± SEM. Due to the skewed distribution of the responses, non-parametric tests were applied on longitudinal data using a ranked-based approach on factorial experiments [21, 22]. Statistics tests were computed using modified ANOVAs with the R software package “nparLD” for longitudinal data . Mann-Whitney-Wilcoxon tests were used for the post hoc analysis using R 3.0.2. p < 0.05 was considered as significant.
The statistical model assessed response over time between treatment groups including an interaction term between treatment groups and days of measurement as fixed effects and a random effect to account for the mice repeated measures over time. The variable treatment was considered as factorial experiment. We first tested whether response over time differed across the five treatment groups (RAG2−/−, PENK+/+, PENK+/+ NLX, PENK−/− and PENK−/− NLX). Significant (p < 0.05) differences of response between groups allowed performing subsequent analysis of a random effect to account for the mice repeated measures over time. All pairwise combinations of treatment were tested to assess their effect on response. Finally, we also compared response between treatments at each time point using a Mann-Whitney-Wilcoxon test.
Anti-β-endorphin polyclonal IgG antibodies cross-react with enkephalins expressed in activated T lymphocytes
Taken together, the data show that (1) activated CD4+ T lymphocytes do not express β-endorphin in mice and (2) the binding of rabbit polyclonal anti-β-endorphin serum IgG to activated mouse CD4+ T lymphocytes is because of their cross-reactivity towards enkephalins.
Analgesic activity of activated CD4+ T lymphocytes is dependent on enkephalins
During microbe-induced inflammation, leukocytes at the inflammatory site produce opioids and contribute to modulate the intensity of the inflammation-related pain . As shown in CFA-induced pain model in mice, the contribution of leukocytes in regulating pain varies at the different steps of the inflammatory process. At the early stage of inflammation, neutrophils and monocytes that first enter the site of inflammation produce opioids but the release of their opioid content is insufficient to spontaneously abolish painful perception [3, 19, 25]. From the third day of inflammation, effector CD4+ T lymphocytes generated in response to bacterial antigens contained in CFA migrate into the inflammatory site to enhance innate immune response against microbes. Effector CD4+ T lymphocytes, which become the predominant immune cell subset in the inflammatory site, release opioids. Activation of opioid receptors expressed on sensory neurons innervating the inflamed area by T lymphocyte-derived opioids is, at this time, sufficiently efficient to relieve from CFA-induced inflammatory pain [1, 2]. The ability of CD4+ T lymphocytes to produce opioids is acquired upon antigen priming in draining lymph nodes  and correlates with the stimulatory potency of antigen-presenting cells . Effector CD4+ T lymphocytes entering the inflammatory site release their opioid content upon stimulation by the cognate antigens in situ . A 6-day latency between immunization (i.e., CFA injection) and analgesia (Fig. 6) is required for antigen-specific priming, clonal expansion, and differentiation of CD4+ T lymphocytes within the draining lymph nodes and their recruitment in sufficient number at the site of inflammation [1, 26, 27]. The kinetics of the allodynic response to mechanical stimuli of immunodeficient mice transferred with PENK+/+ T lymphocytes was similar to that of wild-type immunocompetent mice [1, 8]. The production of enkephalins by effector T lymphocytes generated in response to CFA (mycobacterium tuberculosis) reduces almost by two the duration of somatic inflammatory pain in mice [1, 8, 26]. Given the anti-inflammatory properties of endogenous opioids [28–31], the analgesic effects of T cell-derived enkephalins in the chronic model of CFA-induced inflammation are certainly not restricted to their inhibitory action on sensory neurons .
The production of endogenous opioids by effector CD4+ T lymphocytes in humans and rodents is commonly admitted, but the nature of the opioid peptides may differ across animal species [1, 4, 32, 33]. Contrasting with the experiments performed in humans and rats, which described at both genetic and protein levels [34–36] the expression of β-endorphin in immune cells including T lymphocytes, the studies performed in mice only used antibody-based methods. To our knowledge, all the studies describing β-endorphin in mouse T lymphocytes have been performed by immunochemistry methods using polyclonal anti-β-endorphin antibodies [11–16]. Accordingly, Met-enkephalin, which constitutes the first five amino acids of the NH2 terminus of the β-endorphin, may be recognized as an epitope by immune serum IgG raised against β-endorphin. Polyclonal anti-β-endorphin antibodies are specific for β-endorphin but cross-react with Met-enkephalin, and thereby, they bind to cells producing β-endorphin and/or Met-enkephalin. Their binding is always fully inhibited by soluble β-endorphin even if the cells only produce Met-enkephalin. However, the total inhibition of the anti-β-endorphin IgG binding by soluble Met-enkephalin indicates that the target cells only express Met-enkephalin (i.e., the anti-β-endorphin antibody staining is due to the recognition of Met-enkephalin). Thus, the absence of β-endorphin-encoding mRNA (POMC) in T lymphocytes [1, 6, 10], the full inhibition of the anti-β-endorphin antibody binding to wild-type CD4+ T lymphocytes by Met-enkephalin, and the inability of anti-beta-endorphin antibodies to bind to PENK−/− lymphocytes argue for the expression of enkephalins but not β-endorphin in mouse T lymphocytes.
All the three subclasses of opioid receptors are expressed on sensory neurons innervating skin and gastrointestinal tract [37, 38], and CD4+ T lymphocytes have been shown to be the most effective regulators of pain upon chronic inflammation including intestinal inflammation in mice [1, 6, 10, 13–15]. However, although opioid drugs specific for each of the three opioid receptors are efficient to relieve inflammatory pain when administered in periphery [6, 39–41], the molecular nature of the endogenous opioids produced by CD4+ T lymphocytes remains subject to controversy in mice . Because of the structural similarities between endogenous opioid neuropeptides, and thereby the poor reliability of anti-opioid immune sera, we used a genetic approach to show the pivotal role of enkephalins in the analgesic property of CD4+ T lymphocytes. We show, in CFA-induced pain model in mice, that CD4+ T lymphocytes lacking enkephalins completely lose their analgesic activity. Furthermore, the mechanical sensitivity of mice with enkephalin-deficient lymphocytes was superimposable to that of mice without lymphocytes indicating that the absence of enkephalins in T lymphocytes was not compensated by another opioid.
Endogenous regulation of CFA-induced inflammatory pain by CD4+ T lymphocytes is primarily mediated by enkephalins in mice suggesting that β-endorphin would be more relevant in rats and humans.
CFA, complete Freund’s adjuvant; DOR, delta-type opioid receptor; HPRT, hypoxanthine phosphoribosyl transferase; mAb, monoclonal antibody; NLX, naloxone methiodide; OVA, ovalbumin; PENK, proenkephalin; POMC, proopiomelanocortin; RAG2−/−, recombination-activating gene 2-deficient
The authors wish to thank ANEXPLO platforms (UMS 006) animal care facility (Y. Barreira and S. Appolinaire), the platform Aninfimip, and EquipEx (‘Equipement d’Excellence’) supported by the French government through the Investments for the Future program (ANR-11-EQPX-0003) and the U1043 flow cytometry facility (F. L’Faqihi-Olive and V. Duplan-Eche).
This work was supported by the Institut National de la Santé et de la Recherche Médicale (INSERM), the Université Paul Sabatier, Toulouse III, and the French Agence Nationale de la Recherche grant LYMPHOPIOID (201BLAN 1131 01). The sponsors had no role in the design of the study and collection, analysis, and interpretation of data and in writing the manuscript.
Availability of data and materials
The datasets supporting the conclusions of this article are included within the article.
LB and JB designed and performed the experiments and analyzed the data; KM, CD, and CB participated in the execution of the experiments and data analysis; SRdP performed the statistical analyses; NV contributed to the manuscript editing; GD conceived and supervised the study, designed the experiments, analyzed the data, and wrote the manuscript. All authors take public responsibility for the accuracy and integrity of the work and have read and approved the final manuscript.
The authors declare that they have no competing interests.
Consent for publication
Ethics approval and consent to participate
All experiments involving animals were performed in accord with ethical guidelines (INSERM) and were approved by the Midi-Pyrénées (France) ethics committee (application Number MP/06/73/10/12).
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