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Cutting Edge |

* Departments of Medicine and Microbiology-Immunology, University of California Medical Center, San Francisco, CA 94143; and
Ceretek, LLC, Alameda, CA 94502
| Abstract |
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| Introduction |
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Blood unstimulated CD4 T cells express high levels of LPA2 (Edg-4) receptors constitutively, whereas CD8 T cells from the same source show only traces of LPA1 (Edg-2) (8). In studies of regulation of expression of LPA receptors, mitogen activation of blood CD4 T cells down-regulated expression of LPA2 and up-regulated that of LPA1, but such activation of CD8 T cells did not alter expression of any LPA receptor (9). IL-2 production by blood CD4 T cells was suppressed through LPA2 and enhanced through LPA1. In contrast, LPA2 transduced LPA-evoked chemotaxis and chemokinesis of CD4 T cells, whereas LPA1 failed to signal migration directly but mediated LPA suppression of chemokine-elicited chemotaxis of CD4 T cells (10). Thus, the effects of activation of blood CD4 T cells on LPA receptors converted LPA from a migration-enhancing and cytokine production-inhibiting factor to a migration-inhibiting and cytokine production-enhancing factor for effector T cells.
In the course of studies of mouse splenic T cell LPA and S1P receptors, S1P1 and S1P4 were found to be the vastly predominant S1P receptors of CD4 and CD8 T cells and LPA receptors were detected at only very low levels (11). S1P was noted to elicit T cell chemotactic responses at concentrations well below those typical of plasma (11). It was also discovered that the high level of expression of S1P1 and S1P4 receptors by mouse splenic CD4 and CD8 T cells is suppressed nearly completely by TCR-dependent activation (11). Chemotactic responses of both unstimulated CD4 and CD8 T cells were elicited by 1 nM S1P, attained a maximum at 0.1 µM S1P, and were reduced in parallel with suppression of the levels of S1P receptors by TCR-dependent activation (11). A pharmacological probe of S1P receptors (12, 13) also inhibited chemotactic responses of T cells to S1P in vitro and in vivo (11).
It is now reported that the major effect of plasma concentrations of S1P on T cells is suppression of their responses to other chemotactic factors, such as chemokines, in a basement membrane transmigration assay. TCR-dependent activation of the T cells, resulting in decreased expression of S1P receptors, reduced the extent of suppression of chemotactic responses by S1P. S1P1 receptor is the major transducer of S1P effects based on findings that S1P suppressed chemokine-evoked chemotaxis of transfectants expressing only S1P1 receptors, but not that of transfectants expressing only S1P4 receptors.
| Materials and Methods |
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Mouse CD4 T cells were isolated from splenocytes of 6- to 8-wk-old C57BL/6 female mice at a minimum purity of 97% using metallic microbeads bearing anti-CD4 mAbs and two cycles of magnetic retention chromatography (Miltenyi Biotec, Auburn, CA), as described elsewhere (8, 9). For some studies, 1-ml suspensions of purified T cells in RPMI 1640-50 µg/ml fatty acid-free BSA (FAF-BSA) (Calbiochem, La Jolla, CA) with 100 U/ml penicillin G and 50 µg/ml streptomycin (RPMI 1640-BSA) were activated by incubation for 24 h on 2 µg each of adherent anti-CD3 plus anti-CD28 mAbs (BD PharMingen, San Diego, CA) in six-well plates. Stock solutions of 50 µg of methylbenzamido-1,1'-dioctadecyl-3,3,3',3'-tetramethylindo-dicarbocyanine (FM-DiI; Molecular Probes, Eugene, OR) in 50 µl of DMSO were prepared immediately before labeling each preparation of CD4 T cells. Suspensions of 2 x 106 CD4 T cells from normal C57BL/6 mice/ml Ca2+- and Mg2+-free PBS were incubated with 2 µg/ml FM-DiI for 5 min at 37°C and 15 min at 4°C, washed three times in PBS, and resuspended in 1 ml of RPMI 1640-FAF-BSA for incubation with S1P for 1 h at 37°C before i.p. injection into test mice. The fluorescent labeling did not affect T cell viability or chemotactic responses in vitro nor was any fluorescent label transferred to mouse adherent peritoneal macrophages by labeled T cells, at a cellular ratio of 1:5, after 6 h of incubation at 37°C.
Generation of HTC4 cell-S1P receptor transfectants
The cDNAs encoding human S1P1 and S1P4 were subcloned into pLXSN and introduced into the EcoHEK-293 packaging cell line (Clontech Laboratories, Palo Alto, CA). Viral particles secreted by this line were filtered, treated with polybrene, and substituted for culture medium over HTC4 rat hepatoma cells. HTC4 cells resistant to 400 µg/ml geneticin were subcloned, washed, and resuspended in MEM with 10% FBS and 4 mM L-glutamine, and then incubated for 14 h. The mRNAs encoding S1P1 and S1P4 extracted from the respective HTC4 cell lines were quantified by real-time PCR.
Quantification of mouse T cell and HTC4 cell-S1P receptor transfectant chemotaxis
Migration of mouse purified CD4 T cells was analyzed in Transwell chambers (Costar, Cambridge, MA) with 8-µm pore width polycarbonate filters, as described previously (14), except that the layer of growth factor-depleted Matrigel was reduced from 15 to 8 µl, T cell suspensions were 1 x 107/ml, and Exodus-2 (CCL-21) and RANTES (CCL-5) chemokines(PeproTech, Rocky Hill, NJ) were the positive chemotactic stimuli. In selected assays of intrinsic mobility of T cells and of HTC4-S1P receptor transfectants, the filters lacked Matrigel and were instead coated with 8 µl of a 0.05 µg/ml solution of type IV collagen (Sigma-Aldrich, St. Louis, MO), and migration was for 6 h instead of 36 h. The significance of differences between stimulated or inhibited and control migration in both of these chambers and the in vivo model were calculated with a standard two-sample t test.
Assessment of effects of S1P on mouse CD4 T cell recruitment to dorsal s.c. air pouches
Air pouches were established on the back of each mouse by s.c. injection of 5 ml of micropore-filtered air. Two days later, pouch airspaces were irrigated twice with 1 ml of PBS and then received 1.0 ml of 0.5 µM CCL-21 (Exodus-2) or CCL-5 (RANTES) in RPMI 1640-FAF-BSA as in vivo chemotactic stimuli 15 min before i.p. injection of FM-DiI fluorescently labeled CD4 cells that had been preincubated with 0.3 or 1 µM S1P or with medium alone. Pouch cells were harvested in 2 ml of PBS and peritoneal cells in two washes of 2 ml of PBS each after 24 h for performance of microscopic counts and determination of fluorescence.
| Results and Discussion |
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Another important variable in this regulatory formula is the level of
expression of the S1P1 (Edg-1) and/or
S1P4 (Edg-6) GPCRs and the importance of each as
transducers of effects of S1P on T cells (11).
TCR-dependent activation of T cells reduces substantially the levels of
expression of both S1P1 (Edg-1) and
S1P4 (Edg-6) with concomitant reduction in S1P
signaling of T cell migration (11) (Fig. 2
). Thus, it is
not surprising that such activation of CD4 T cells eliminates both the
augmentation by subplasma S1P levels and the suppression by plasma S1P
levels of T cell responses to chemokines (Fig. 2
). The relative
contributions of signaling by S1P1 (Edg-1) and
S1P4 (Edg-6) to the observed effects of S1P on T
cell chemotaxis cannot be easily dissected with the presently available
tools. It was assumed that S1P1 (Edg-1) would
predominate at concentrations of S1P below 3 µM as the affinity for
S1P and resultant transductional efficiency is nearly 30-fold higher
than for S1P4 (Edg-6) (15).
Transfectants expressing solely S1P1 (Edg-1) are
susceptible both to S1P stimulation of their chemotaxis and
chemokinesis and to S1P inhibition of their chemotactic responses to
chemokines, whereas otherwise identical transfectants solely expressing
S1P4 (Edg-6) exhibited neither response (Fig. 3
).
These data suggest that Edg-1 is the principal unit for cellular
transmission of S1P signals to T cells.
The findings now reported answer for immunity a fundamental question in the field of lysophospholipid mediators. Polypeptide and all other lipid chemotactic factors, whose signals are transduced by high-affinity GPCRs, are generated and degraded rapidly in immune responses, such that their optimally active picomolar to nanomolar concentrations are maintained most often for only very brief periods. So why then are the fluid-phase concentrations of lysophospholipid T cell migration-directed factors, which act through similarly high-affinity GPCRs, sustained at near micromolar concentrations in physiological fluids? It appears for S1P that its high concentrations in T cell corridors serve the primary homeostatic function of regulating the threshold of responsiveness of naive and memory T cells to other chemotactic factors, such as chemokines, and efficiently preventing responses to minimal perturbations of these powerful effector systems. Many other aspects of the question remain to be addressed, including the immune implications of fluctuations in the fluid-phase concentrations of S1P and the involvement of many other possible regulators of expression of S1P receptors on T cells. More than ever, we will need S1P receptor pharmacological agonists and antagonists of useful potency and single receptor selectivity to elucidate the range of regulatory immune activities of S1P and related lysophospholipid mediators.
| Acknowledgments |
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| Footnotes |
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2 Address correspondence and reprint requests to Dr. Edward J. Goetzl, University of California, UB8B, UC Box 0711, 533 Parnassus at 4th, San Francisco, CA 94143-0711. E-mail address: egoetzl{at}itsa.ucsf.edu ![]()
3 Abbreviations used in this paper: S1P, sphingosine 1-phosphate; LPA, lysophosphatidic acid; Edg, endothelial differentiation gene-encoded; FAF, fatty acid free; FM-DiI, methylbenzamido-1,1'-dioctadecyl-3,3,3',3'-tetramethylindo-dicarbocyanine; GPCR, G protein-coupled receptor. ![]()
Received for publication July 23, 2002. Accepted for publication August 22, 2002.
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