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CUTTING EDGE |

* Division of Gastroenterology, University Hospitals of Cleveland, Case Western Reserve University School of Medicine, Cleveland, OH 44106; and
Department of Colorectal Surgery, Cleveland Clinic Foundation, Cleveland, OH 44195
| Abstract |
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| Introduction |
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| Materials and Methods |
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FITC-conjugated anti-CD40L, PE-conjugated anti-P-selectin (CD62P), PerCP-conjugated anti-CD42a, and FITC- and PE-conjugated mouse IgG1 isotype control were purchased from BD PharMingen (San Jose, CA); PE-conjugated anti-CD3 from DAKO (Carpinteria, CA), anti-CD40 from BD-PharMingen (San Diego, CA), thrombin from Sigma-Aldrich (St. Louis, MO), and the p38 inhibitor SB203580 from Calbiochem (La Jolla, CA). RANTES ELISA kits and human recombinant RANTES were purchased from R&D Systems (Minneapolis, MN) and phospho-p38 and phospho-extracellular signal-regulated kinase (ERK) Abs from Cell Signaling (Beverly, MA). Polystyrene beads coated with murine mAbs to human CD3 and CD28 were obtained from Dynal (Lake Success, NY). Trimeric soluble-CD40L (sCD40L), Abs against CD40 (M2 and M3) and CD40L (M90) were provided by Immunex (Seattle, WA), and the RANTES receptor antagonist met-RANTES by Dr. A. Proudfoot (Serono Pharmaceuticals, Geneva, Switzerland).
Platelet-T cell cocultures
Platelets were isolated from blood of healthy donors, resulting in a >99% pure population as assessed by expression of CD42b (13). Peripheral blood T cells (PBT) were cultured in RPMI 1640 with 10% FBS and activated for 16 h with IL-2 and anti-CD3 plus anti-CD28 Abs to induce CD40L (14). Resting platelets were mixed with various ratios of resting or activated autologous PBT, CD40L-negative Jurkat, or CD40L-positive D1.1 cells. In some experiments, T cells were fixed in 1% paraformaldehyde. In other experiments, platelets or T cells were preincubated with 10 µg/ml M2, M3, or M90 blocking Abs or 10 µg/ml SB203580. Platelet-T cell adhesion was assessed by flow cytometry measuring the concomitant expression of CD3 and CD42b in the aggregates. After 30 min at 37°C, cocultured cells were fixed in 1% paraformaldehyde and stained for flow cytometry (13). As a control, platelets were stimulated with sCD40L (0.01, 0.1, 1.0, and 10 µg/ml) or thrombin (0.5 U/ml) for 30 min at 37°C.
Flow cytometric analysis
For CD40, CD62P, and CD42b expression, platelets were fixed in 1% paraformaldehyde, stained with the appropriate Ab, examined by a Coulter Epics XL Flow Cytometer (Beckman Coulter, Fullerton, CA), and results were analyzed using Winlist (Verity Software House, Topsham, ME). For CD40L or CD3 expression, T cells were stained with FITC-conjugated anti-CD40L or PE-conjugated anti-CD3, respectively (13).
Assessment of phosphorylated p38 and ERK mitogen-activated protein (MAP) kinases and RANTES granules release
Induction of phosphorylated p38 and ERK MAP kinases was assessed by Western blotting using phospho-specific Abs. RANTES was measured in supernatants of paraformaldehyde-fixed platelets cocultured with T cells or sCD40L-stimulated platelets using a commercial ELISA (15).
Binding of platelet-derived RANTES to endothelial cells and T cell adhesion assay
Cell-free supernatants from degranulated platelets or recombinant RANTES (0.5 µg/ml) were added to human intestinal microvascular endothelial cell (HIMEC) monolayers for 60 min at 37°C (16). After washing, RANTES was detected on the HIMEC surface by flow cytometry and confocal microscopy (17).
MOLT4 T cells were used for the adhesion assay (16). Confluent HIMEC monolayers were left alone or stimulated with cell-free supernatants from 100 x 106 activated platelets or 0.5 µg/ml recombinant RANTES. In some experiments, MOLT4 cells were preincubated for 30 min at 37°C with 1 µg/ml met-RANTES. After 1 h, calcein (Molecular Probes, Eugene, OR)-labeled MOLT4 cells were overlaid and 1 h later nonadherent cells where removed by gentle washings with PBS. Adherent cells in duplicate wells were quantified using Image Pro Plus (Media Cybernetics, Silver Spring, MD) connected to a digital camera (Optronics, Olympus, Tokyo, Japan) and results were expressed as adherent cells per square millimeter.
Statistical analysis
Data were analyzed by GraphPad (San Diego, CA) using the Student t or ANOVA tests followed by the appropriate post hoc test and expressed as mean ± SEM. Significance was set at p < 0.05.
| Results and Discussion |
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We initially investigated whether activation could modulate the constitutive expression of CD40 on the surface of platelets. Stimulation with thrombin for up to 30 min revealed that, in contrast to the marked up-regulation of P-selectin, the expression of CD40 remained constant as demonstrated by flow cytometric analysis (Fig. 1A). These results were confirmed using two different Abs (M2 and M3) and by Western blot analysis (data not shown).
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CD40L-positive T cells induce RANTES release by activated platelets
Having demonstrated that the platelet CD40 receptor is functional, we next investigated whether direct binding by CD40L-bearing T cells, as it may happen in immune-mediated or inflammatory conditions, could induce platelet activation. For this purpose, we cocultured platelets with different numbers of autologous resting or activated PBT expressing CD40L induced by anti-CD3 plus anti-CD28 stimulation. D1.1 and Jurkat cells were used as controls (Fig. 2A). As measured by P-selectin expression, resting PBT induced moderate, dose-dependent platelet activation, whereas CD40L-positive T cells triggered a significantly higher degree of activation (Fig. 2B). The effect of activated T cells was inhibited by blocking CD40 on platelets or CD40L on T cells. In cocultures with activated PBT, the number of platelet-T cell aggregates was three times higher than that present in cocultures with resting PBT and decreased by 4050% when M2, M3 (anti-CD40), or M90 (anti-CD40L) blocking Abs were added. CD40L-positive D1.1 cells mimicked the P-selectin up-regulation effect of activated PBT, whereas CD40L-negative Jurkat cells reproduced the results obtained with resting PBT (data not shown). The same results were also obtained when T cells were prefixed in 1% paraformaldehyde (data not shown).
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Critical involvement of p38 MAP kinase in RANTES release by CD40-activated platelets
It has been recently reported that the activation of platelets through the CD40 pathway is calcium independent (12), but no information exists on what signaling molecules are triggered downstream of the receptor. Since MAP kinases are involved in CD40-activated immune and nonimmune cells (21), we investigated the possible phosphorylation of p38 and ERK in platelets upon sCD40L ligation. Phosphorylated p38 was undetectable in resting platelets, but its induction was obvious in sCD40L-treated platelets and completely prevented by exposure to CD40 (M2 and M3) or CD40L (M90) Abs (Fig. 4A). In contrast, resting platelets exhibited a constitutive level of ERK phosphorylation which failed to be up-regulated by CD40-mediated signaling or inhibited by blocking Abs. The selectivity of this differential signaling was confirmed when thrombin was used instead of sCD40L, resulting in the expected up-regulation of both p38 and ERK phosphorylation (data not shown). To corroborate the involvement of p38 in CD40-dependent platelet activation, the selective inhibitor SB203580 was used. A marked inhibition of P-selectin up-regulation was observed (p < 0.001; Fig. 4B), pointing to a key role of p38 in platelet activation through CD40 engagement. Specificity was further confirmed by lack of inhibition of P-selectin up-regulation by SB203580 when thrombin was used (data not shown).
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Platelet-derived RANTES enhances T cell recruitment
Leukocyte recruitment at sites of inflammation is crucially dependent on chemokine presentation by local microvascular endothelial cells (19). We investigated whether platelet-derived RANTES could be immobilized on the surface of HIMEC and mediate T cell retention. Confocal microscopy revealed that after exposure to activated platelet-derived supernatants RANTES was readily detected on their surface, similarly to recombinant RANTES (Fig. 5A). Results were confirmed by flow cytometry (data not shown). We then tested whether RANTES released by platelets was able to mediate recruitment of T cells. Few MOLT4 cells bound to unstimulated HIMEC, but their number significantly (p < 0.01) increased after HIMEC exposure to activated platelet-derived or recombinant RANTES. Pretreatment of T cells with met-RANTES, a RANTES receptor antagonist, markedly reduced (p < 0.05) the number of adherent T cells (Fig. 5B).
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To the best of our knowledge, this is the first study demonstrating that activated T cells can stimulate platelets directly, resulting in CD40-mediated stimulation. Platelets are first activated through direct contact with T cells and, through the release of RANTES, secondarily recruit more T cells that lead to further platelet activation, creating an amplification loop that promotes leukocyte recruitment to sites of immune reactivity. Moreover, since P-selectin is up-regulated in activated platelets and facilitates binding to T cells constitutively expressing P-selectin glycoprotein ligand 1 (25), this could further contribute to intensification and maintenance of immune and inflammatory responses. These results reinforce the contribution of this unique cell type in immunity, bringing platelets close to the level of pathogenic relevance traditionally attributed to classical immune cells.
| Acknowledgments |
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| Footnotes |
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2 Address correspondence and reprint requests to Dr. Claudio Fiocchi, Division of Gastroenterology, University Hospitals of Cleveland, Case Western Reserve University School of Medicine (BRB 425), 10900 Euclid Avenue, Cleveland, OH 44106-4952. E-mail address: Claudio.Fiocchi{at}Case.edu ![]()
3 Abbreviations used in this paper: CD40L, CD40 ligand; sCD40L, soluble CD40L; PBT, peripheral blood T cell; ERK, extracellular signal-regulated kinase; MAP, mitogen-activated protein; HIMEC, human intestinal microvascular endothelial cell. ![]()
Received for publication October 22, 2003. Accepted for publication December 12, 2003.
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