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Is Critical for T Cell Chemotaxis1


* Department of Medicine,
Department of Pharmacology,
Department of Pathology and Laboratory Medicine, Abramson Family Cancer Research Institute,
Department of Biology, University of Pennsylvania, and
¶ Department of Pediatrics, Division of Pathology, Childrens Hospital of Philadelphia, Philadelphia, PA 19104; and
|| Department of Genetics and Developmental Biology, University of Connecticut Health Center, Farmington, CT 06030
| Abstract |
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i protein-coupled receptor-mediated neutrophil and lymphocyte chemotaxis. Although PLC
is not essential for the chemotactic response of neutrophils, its role in lymphocyte migration has not been clearly defined. We compared the chemotactic response of peripheral T cells derived from wild-type mice with mice containing loss-of-function mutations in both of the two predominant lymphocyte PLC
isoforms (PLC
2 and PLC
3), and demonstrate that loss of PLC
2 and PLC
3 significantly impaired T cell migration. Because second messengers generated by PLC
lead to a rise in intracellular calcium and activation of PKC, we analyzed which of these responses was critical for the PLC
-mediated chemotaxis. Intracellular calcium chelation decreased the chemotactic response of wild-type lymphocytes, but pharmacologic inhibition of several PKC isoforms had no effect. Furthermore, calcium efflux induced by stromal cell-derived factor-1
was undetectable in PLC
2
3-null lymphocytes, suggesting that the migration defect is due to the impaired ability to increase intracellular calcium. This study demonstrates that, in contrast to neutrophils, phospholipid second messengers generated by PLC
play a critical role in T lymphocyte chemotaxis. | Introduction |
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Chemokines are chemotactic cytokines that signal through seven transmembrane receptors coupled to pertussis toxin-sensitive Gi proteins (6). Stromal cell-derived factor-1
(SDF-1
)3 is one of the most extensively investigated chemokines, with regard to signal transduction mechanisms, and binds exclusively to its receptor, CXCR4. CXCR4 is uniformly expressed in all types of mature blood cells, including monocytes, granulocytes, T and B cells, and platelets (7, 8, 9). The signal transduction pathways initiated by the binding of SDF-1
to CXCR4 are not fully understood. In human T cells, SDF-1
stimulates the activation of PI3K, the phospholipase C/protein kinase C (PLC/PKC) cascade, and MAPK p42/44 (ERK1/2) (10, 11).
Publications over the past several years have greatly advanced our understanding of the contribution of inflammatory chemokines in the control of inflammation and autoimmune diseases (12). By contrast, we are only recently beginning to appreciate the subtle network of homeostatic chemokines that orchestrates the basal, inflammation-unrelated leukocyte traffic. Furthermore, although it is well established that chemokine-induced signaling is mediated by G protein-coupled cell surface receptors, the mechanisms underlying lymphocyte chemotaxis are largely unknown.
Two of the pathways that culminate in intracellular calcium fluxes, cytoskeletal reorganization, directional movement, and activation of specialized leukocyte functions depend on lipid-derived second messengers produced by PI3K and PLC (13, 14). PI3K and PLC are widely expressed enzymes that modify the membrane-bound phosphoinositol, phosphatidylinositol 4,5-bisphosphate (PIP2). PI3K
phosphorylates PIP2 to generate the second messenger, phosphatidylinositol 3,4,5-trisphosphate (PIP3). This second messenger, in turn, has been demonstrated to play a vital role in actin organization, proliferation, and survival within a variety of different cell types (15), and has a clear role in lymphocyte and neutrophil chemotaxis (1, 16, 17, 18). PLC hydrolyzes PIP2 to produce inositol 1,4,5-trisphosphate (IP3) and diacylglycerol (DAG) (19). IP3 leads to a rise in intracellular calcium, whereas DAG activates some isoforms of PKC (20). PLC
2 and PLC
3 are the most abundant isoforms of PLC in hemopoietic cells (21). Together, PLC
2 and PLC
3 are critical to chemoattractant-induced responses in neutrophils, such as calcium efflux, superoxide production, and regulation of protein kinases, but are not required for chemotaxis of these cells (17).
In this study, we analyzed the contribution of PLC
-synthesized second messengers in T cell migration. This analysis was accomplished using peripheral node lymphocytes derived from genetically altered mice lacking the predominant isoforms of these enzymes (17), in conjunction with a variety of pharmacologic inhibitors. We find that in contrast to granulocytes, PLC
is fundamental for T cell chemotaxis that appears to be dependent upon the generation of IP3 and the resultant increase of intracellular calcium, but independent of PKC activity.
| Materials and Methods |
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Cell culture media and supplements were obtained from Invitrogen Life Technologies. SDF-1
and fMLP were obtained from Sigma-Aldrich. The calcium chelators BAPTA-AM and quin-2/AM, the PKC inhibitor GF109203x, and 7-aminoactinomycin D were obtained from EMD Biosciences. Fluorescence-conjugated Abs were obtained from BD Biosciences. PLC
2
3 knockout mouse lines were previously described (17).
Cell isolation
Peripheral inguinal, axillary, and cervical lymph nodes were removed from donor mice and pressed through a cell strainer to generate single-cell suspensions. T cells were isolated using a negative-selection magnetic-sorting approach. In brief, single-cell suspensions were incubated with anti-B220-conjugated Dynabeads (Dynal Biotech) for 30 min at 4°C, and B cells were removed using a Bio-Mag magnetic stand (Polysciences). T cell purity was generally >98%. T cells were activated with 5 nM PMA and 0.1 µM ionomycin.
Bone marrow-derived neutrophils were isolated as previously described (22). Briefly, bone marrow was flushed from the femur and tibia of mice with HBSS Prep (5.4 mM KCl, 0.3 mM Na2HPO4, 0.8 mM KH2PO4, 4.2 mM NaHCO3, 137 mM NaCl, 5.6 mM dextrose, 20 mM HEPES, 0.5% FBS). RBC were lysed in hypotonic sodium chloride, followed by addition of an equal volume of hypertonic sodium chloride solution to restore normal tonicity. Cells were resuspended in 5 ml of HBSS Prep, layered over a 62.5% Percoll gradient, and centrifuged at 2200 rpm for 30 min. Mature neutrophils were harvested from the bottom layer of the gradient. The purity of the neutrophil preparation was assessed using flow cytometry; standard preparations yielded neutrophils of >90% purity.
Flow cytometry
Expression of TCR and BCR (CD4, CD8, CD3, CD45R/B220, CD25, CD69, CD44, CD62L, CD45RB, CXCR4) on peripheral lymphocytes and Gr-1 on bone marrow-derived neutrophils was determined by direct immunofluorescence. Cells were incubated with murine fluorescence-conjugated mAbs in cell-appropriate medium for 30 min at 37°C and washed three times. Cell-bound fluorescence was determined in a FACSCalibur flow cytometer (BD Biosciences) using the CellQuest software.
Chemotaxis
Assays for T lymphocyte and neutrophil chemotaxis were performed in polycarbonate membrane (6.5-mm diameter, 10-µm thickness, 5-µm diameter pore size) Transwell cell culture chambers (Costar). T cells (100 µl at 2 x 106/ml) suspended in RPMI 1640 containing 0.5% BSA (fraction V; Sigma-Aldrich) and 25 mM HEPES, and neutrophils (100 µl at 1 x 106/ml) suspended in serum-free HBSS with 0.5% BSA, were added to the upper chamber; SDF-1
as a T cell chemokine or fMLP as a neutrophil chemokine was added to the lower chamber. Pharmacologic inhibitors, calcium chelators, or DMSO (<1%) were preincubated with the cells 15–30 min before their addition to the upper Transwell chamber. Cells were allowed to migrate for 3 h at 37°C in 5% CO2 atmosphere. T cell migration was quantified by collecting T cells in both the upper and lower chambers and counting by a FACSCalibur flow cytometer. For neutrophil migration, filters were removed and at least 10 fields of cells adherent to the bottom part of the polycarbonate membrane were counted. Visual inspection of the membranes in the lymphocyte assay verified that few lymphocytes were adherent to the filter. The percentage of migration was determined for T cells by the number of cells collected from the lower chamber divided by the sum of the number of cells collected in lower and upper chambers, multiplied by 100. The chemotaxis index for neutrophils was determined by dividing the migration in the presence of fMLP by migration in the absence of fMLP. Cell viability was verified by staining with 7-aminoactinomycin D and trypan blue.
Measurement of cytoplasmic calcium concentration
Purified T cells were suspended in RPMI 1640 containing 1% FBS at a concentration of 1 x 107/ml. After loading with 5 µM fura 2-AM (Molecular Probes) for 40 min at 37°C in the dark, T cells were sedimented at 833 x g for 15 min at room temperature, and the pellet resuspended in RPMI containing 0.5% BSA at a final cell concentration of 1 x 106/ml. Aliquots of fura 2-loaded T cells were transferred to a 10 x 10 mm cuvette and prewarmed to 37°C. The cells were stimulated with 300 ng/ml SDF-1
. Subsequent measurements of fura 2 fluorescence were performed under continuous stirring using an SLM/Aminco model AB2 fluorescence spectrophotometer with excitation at dual wavelengths of 340 and 380 nm, and emission spectra measured at 510 nm.
| Results |
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2
3-null lymphocytes are phenotypically similar to wild-type (WT) lymphocytes
Mice lacking PLC
2 and PLC
3 develop spontaneous multifocal skin ulcers starting around 6 mo of age (17). In view of this ulcerative phenotype, our studies to evaluate resting T cell migration have been performed with mice younger than 6 mo, before the development of skin ulceration. At this age, no gross phenotypic differences are seen between the PLC
2
3-null (PLC
2
3–/–) and WT mice.
Peripheral lymph nodes from WT and PLC
2
3–/– mice were of comparable size and produced a similar total number of lymphocytes ranging from 15 to 20 million cells per mouse. As shown in Figs. 1A and 2A, the ratios of CD3+ and B220+ lymphocytes were similar between WT and PLC
2
3–/– mice (p < 0.5). Furthermore, markers of T cell activation (CD25 and CD69) were comparable in both WT and PLC
2
3–/– mice (p < 0.5), as shown in Figs. 1, B and C, and 2A. Additionally, the ratio of CD4+ to CD8+ cells, and the percentage of the naive and memory T cell markers CD44, CD45RB, and CD62L was the same between WT and PLC
2
3-null lymphocytes (p < 0.1) as shown in Fig. 2B. These data demonstrate that the lymphocytes from WT and PLC
2
3–/– mice are similar with respect to their subtype distribution and level of activation, and appear to display a predominantly naive phenotype.
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2 and PLC
3 impair T cell migration to SDF-1
Granulocytes from mice lacking the two dominant isoforms of PLC
, PLC
2, and PLC
3 migrate at least as well as WT cells in response to fMLP or MIP-1
(17) (Fig. 3A). To determine whether migration of T cells was dependent on the PLC
isoform, we analyzed T cell migration in response to the CXCR4 ligand, SDF-1
. Using a Transwell assay, migration of lymphocytes toward SDF-1
was quantitated after 3 h (the time demonstrated by previous studies to be optimal for migration of both WT and PLC
2
3-null T cells). We found that after 3 h, T cells isolated from WT mice exhibited a 20-fold increase in migration with SDF-1
stimulation compared with baseline. The maximal response concentration to SDF-1
(300 ng/ml) was determined by a titrated dose response for both WT and PLC
2
3-null T cells (p < 0.05) (Fig. 3B). The loss of PLC
2
3 decreased chemokine-stimulated migration over 3 h in T cells by 60 ± 7% (p < 0.0001) (Fig. 3C). The impaired response in PLC
2
3-null T cells was not attributable to impaired surface expression of CXCR4, the SDF-1
receptor, as demonstrated in Fig. 3D (p < 0.5). These results demonstrate that, in contrast to the findings with neutrophils, loss of both PLC
2 and PLC
3 significantly impairs the migration of T cells.
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-induced T cell migration is largely chemotactic
Chemokinesis is the process of random cell migration, while chemotaxis is defined as directional cell migration in response to a gradient of chemoattractant. To evaluate whether the results demonstrated in Fig. 3 were due to an effect on chemokinesis or chemotaxis, we examined the effect of PLC
on chemokinesis, when the same concentration of chemoattractant was placed above and below the filter. If the migration of the T cells between the two Transwell chambers is due solely to chemotaxis, elimination of the chemoattractant gradient should ablate this migration. However, if the cell migration is due to chemokinesis, the cell migration should be unaffected by elimination of the chemoattractant gradient. The results shown in Fig. 4 demonstrate that most of the WT T cell migration depends on the gradient, and only a small component of their movement is due to random migration. The migration of PLC
2
3-null T cells was too small to accurately determine whether it was predominantly chemotactic or chemokinetic.
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-induced T cell migration through PLC
2
3 is calcium-dependent
Hydrolysis of PIP2 by PLC
liberates IP3, which contributes to signaling required for release of cytoplasmic calcium stores, and DAG, which binds to and activates both classical and novel isoforms of PKC. We investigated whether one or both of these second messengers generated by PLC
contributed to T cell migration. Pharmacologic chelation of intracellular calcium release by BAPTA-AM (p < 0.05) or quin-2/AM (p < 0.05) eliminated the chemotactic response of WT lymphocytes, suggesting that the release of IP3 from PIP2 is critical for T lymphocyte migration (Fig. 5A). No impairment was seen with DMSO use as a carrier control (data not shown). Consistent with these studies, we found that SDF-1
-induced calcium efflux was deficient in the PLC
2
3-null T cells when compared with those of WT (Fig. 5B). Together, these data suggest that the T cell migration defect seen in the PLC
2
3 knockout T cells is likely due to an impaired ability to increase intracellular calcium.
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2
3-null T cells for SDF-1
-mediated PKC activation by quantifying translocation of specific PKC isoforms into the high-speed particulate fraction of T cell lysates. Although SDF-1
induced a barely detectable activation of PKC-
, PKC-
1, and PKC-
2, the effect of this chemokine on PKC activation is quite modest (data not shown). Loss of PLC
2
3 does not produce a reliable effect on this nearly negligible response. There was no SDF-1
mediated activation of PKC-
, PKC-
, or PKC-
. Additionally, an anti-phospho-ERK1/2 immunoblot demonstrated only a minimal SDF-1
-mediated ERK1/2 activation in both WT and PLC
2
3-null T cells (data not shown). These data demonstrate that SDF-1
does not induce significant activation of classical or novel PKC isoforms. Furthermore, it shows that PKC is not an essential component of SDF-1
-mediated T cell signaling in general, and it specifically does not contribute to SDF-1
-induced T cell migration. | Discussion |
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, are constitutively expressed in discrete areas within lymphoid and nonlymphoid tissues, and control the physiological traffic and homing of leukocytes (6). Our experiments indicate that PLC
2
3 is required for SDF-1
-initiated signals in murine T lymphocytes. Both PI3K and PLC
have previously been shown to contribute to several aspects of lymphocyte function including 1) cell proliferation, 2) rescue from apoptosis, and 3) CD4+ and CD8+ T cell differentiation (18, 24). Thus, PLC and PI3K play a role in both early and late signaling events in T cells. This study highlights the important differences between the activation of lymphocytes and neutrophils by chemokines, in terms of receptor regulation and signaling cascades.
PI3K has been shown to be critical for chemotaxis of cells in general, but the particular isoform required appears to depend on the specific cell type. Neutrophils and lymphocytes use PI3K
during chemotaxis (16, 17, 25, 26). Mice deficient in PLC
2 and PLC
3 have neutrophils that migrate efficiently in response to chemokine, but are defective in their ability to produce superoxide (17). Our work demonstrates that T lymphocytes deficient in PLC
2 and PLC
3 have impaired migration in response to chemokines. Our findings are consistent with a recent publication by Smit et al. (27) that showed a sensitivity of T cell migration through an inflammatory chemokine receptor, CXCR3, using a relatively nonspecific inhibitor of PLC. These results taken together with our data demonstrate an important role for PLC in T cell migration in response to homeostatic and inflammatory stimuli.
In this study, we have begun to elucidate the mechanism by which PLC
contributes to chemotaxis. The T cell chemotactic defect in the PLC
2
3–/– mice can be mimicked in vitro by chelation of cytoplasmic calcium, but not by inhibition of PKC. This ability suggests that PLC
contributes to chemotaxis by transiently raising cytoplasmic calcium concentrations. It is not known at this point whether oscillations of cytoplasmic calcium concentration are important, and it remains to be determined which calcium-dependent signaling moieties of PLC are responsible for the control of the chemotactic process. Due to their known contribution to cytoskeletal dynamics (28) calmodulin kinase, myosin L chain kinase, SHIP-1, gelsolin, and calpain are some of the potential calcium-dependent candidates currently under investigation.
In summary, we have found fundamental differences between the signaling pathways required for T cell chemotaxis and those responsible for the cell migration of the more commonly studied granulocytes. The dissimilarities involve the alternative requirements for PLC
-generated second messengers. Neutrophils, and most other migrating leukocytes, are recruited to a wide variety of extravascular tissues through their ability to sense and polarize toward distinct chemokine gradients. T cells also directionally migrate toward established chemokine gradients, but this process occurs in more specialized regions, such as seen in peripheral lymph nodes. It is thus conceivable that second messengers generated by PLC
are uniquely suited for T cell microenvironmental localization within secondary lymphoid organs.
| Acknowledgments |
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| Disclosures |
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| Footnotes |
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1 This work was supported in part by funds from the National Institutes of Health. ![]()
2 Address correspondence and reprint requests to Dr. Charles S. Abrams, Department of Medicine, University of Pennsylvania School of Medicine, Philadelphia, PA 19104. E-mail address: abrams{at}mail.med.upenn.edu ![]()
3 Abbreviations used in this paper: SDF-1
, stromal cell-derived factor-1
; PLC, phospholipase C; PIP2, phosphatidylinositol 4,5-bisphosphate; IP3, inositol 1,4,5-trisphosphate; DAG, diacylglycerol; WT, wild type. ![]()
Received for publication May 31, 2007. Accepted for publication June 6, 2007.
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