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| Abstract |
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Fas ligand (L) belongs to the TNF superfamily, a group of type II transmembrane or secreted molecules whose interactions with their receptors, members of the TNF receptor superfamily, regulate important functions in immune cell proliferation, activation, differentiation, and survival (1–3). FasL exists mainly in membrane-bound form, is generally expressed on cells of the T cell lineage, and is induced during T cell activation (1). FasL and members of this ligand superfamily primarily interact with their receptors by direct cell–cell contact (3). This observation, coupled with the cross-species sequence conservation of the cytoplasmic domains of these ligands, has led to the suggestion that signaling occurs in both directions for this family of ligand–receptor pairs (3). In support of this argument, family members including CD27L, 4-1BBL, OX40L, CD30L, and CD40L have recently been shown to transduce internally costimulatory signals to activated peripheral lymphocytes (2, 4, 5). As of now, no data exist concerning the reverse signaling capacity of ligands with known negative regulatory functions, such as FasL. To explore this added twist to the picture of reverse signaling, we used CTL lines generated from B6 wildtype and B6.lpr mice, which both express FasL, and from B6.gld mice, which lack FasL function, to demonstrate that FasL shares this capacity for reverse signaling in CD8+ but not in CD4+ T cells. Our data reveal that FasL, a molecule long associated with the induction of apoptosis in Fas+ target cells (6) can also positively regulate the proliferative capacity of CD8+ T cells that express it. These data also offer some insight into how CD4+ and CD8+ T cells may be differentially regulated during an immune response.
Generation and Maintenance of T cell lines.
Cytotoxicity and Proliferation Assays.
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Materials and Methods
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Abstract
Materials and Methods
Results and Discussion
References
Mice and Reagents.
C57BL/6 (B6), B6.MRL-Faslpr (B6.lpr), B6Smn.C3H-Faslgld (B6.gld), B6.C-H2bm1/By (B6.bm1), B6.C- H2bm12/KhEg (B6.bm12), C3H/HeJ (C3H), C3H/HeJ-Faslgld (C3H.gld), and C57BL/6-scid/SzJ (B6.SCID) mice were all purchased from The Jackson Laboratory (Bar Harbor, ME) and used at 6–9 wk of age. Reagents include rat antibodies specific for murine CD4 (RL172.4R6; reference 7) and for murine CD8 (3.168.8; reference 8), a hamster antibody specific for murine CD3-
(145-2C11; PharMingen, San Diego, CA), and a mouse antibody specific for murine H-2Kk (16-3-1N; reference 9). NIH (Harvard Medical School, Boston, MA) 3T3 transfectants expressing the FasIgG fusion protein, which consists of the extracellular domain of murine Fas joined to the hinge and constant regions of human IgG1 (10), were derived by Dr. Ben Stanger and provided by Dr. A. Marshak-Rothstein (Boston University, Boston, MA). Experiments included either the supernatant of this cell line or sera from B6.SCID mice injected 5 wk previously with the transfectants.
Alloreactive H-2k– specific CTLs were originally generated from age-matched B6 wild-type, B6.lpr, and B6.gld mice by incubating donor spleen and LN cells with an equal number of 3,000 rad C3H splenocytes in RPMI 1640 supplemented with 10% FCS (RP10). Lines were maintained by restimulation every 8–10 d for 1–12 rounds. RP10 was supplemented after the third stimulation with 50 mM
-methyl mannoside and 5% supernatant from rat cells stimulated for 2 d with 3 µg/ml Con A. CD8+ T cells for generating anti-Kbm1 CTLs were purified from spleen and LN cells from age-matched B6 wild-type and B6.gld mice by nonadherence to nylon wool, treatment with anti-CD4 plus complement, and adherence to plate-bound anti-CD8 antibodies. Purity as monitored by flow cytometry was >99%. All CTL lines were routinely monitored by flow cytometry and CTL assay. CD4+ T cells for generating primary anti–H-2bm12 responders were similarly nylon wool–purified, anti-CD8 killed, anti-CD4 panned spleen and LN cells, with >95% purity by flow cytometry.
Serial dilutions of effector cells and 104 tumor targets labeled with 51Cr sodium chromate were plated in round-bottomed 96-well plates in a final volume of 200 µl. After a 4 h incubation at 37°C, 100 µl of supernatant was collected and specific lysis was determined as follows: percentage of specific lysis = 100 x [(cpm released by CTL – spontaneous release)/(cpm released by detergent – spontaneous release)]. Spontaneous release in the absence of CTL was <20% in all experiments. For antigen-specific proliferation, 2 x 104 CTL from long-term lines or 2 x 105 primary T cells were cultured with 5 x 105 irradiated allogeneic stimulator cells in a total volume of 200 µl of RP10. For long-term lines, media were supplemented with 5% rat Con A supernatant or 50 U/ml recombinant human IL-2. For optimal anti-CD3 stimulation, responders were added to wells coated at 4°C overnight with 15 µg/ml goat anti– hamster IgG (GahIg) in PBS and then for 4 h with 5 µg/ml anti-CD3 antibodies. For proliferation with FasL costimulation, responders were added to wells coated overnight at 4°C with both 15 µg/ml GahIg and 15 µg/ml goat anti–human IgG and then for 4 h with a mixture of 1.0 µg/ml anti-CD3 (suboptimal dose) with either 4.6 µg/ml FasIgG or isotype-matched human IgG1 (huIgG1). Proliferation levels were measured by thymidine incorporation 18 h after pulsing with 1 µCi [3H]TdR/well on day 3. Background counts (responders with only first stage antibodies) were subtracted from the values. Antigen-specific proliferation was also assayed for viable cells by trypan blue exclusion over the course of the culture period. Where appropriate, APCs were removed before counting viable cells by anti–H-2k plus complement. For the FasIgG blocking assay, FasIgG was added to cultures at 2.8–10 µg/ml. HuIgG1 and/or B6.SCID preimmune sera were used as negative controls.
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Results and Discussion
Top
Abstract
Materials and Methods
Results and Discussion
References
Antigenic Stimulation of B6.gld CTL Lines Is Characterized by Depressed Proliferation and Cell Recoveries Relative to CTL Lines Expressing Functional FasL.
Proliferation assays using CTL lines derived from wild-type B6, B6.lpr, and B6.gld mice revealed that FasL+ CTLs proliferate better in response to allogeneic stimulation than do FasL– CTLs (Fig. 1, a and c). APC titration from 5 x 104 to 1 x 106 cells cocultured per well with CTLs did not alter the relative depression in proliferation of B6.gld CTL lines (data not shown). In addition, kinetic analysis of viable cell counts revealed that the lack of FasL function also influences the ability of CTLs to increase in number over the course of an MLC (Fig. 1 b), and that this expansion coincides with FasL expression by the responding T cells. As determined by flow cytometry, FasL expression on the CTL lines is upregulated by 3 h after antigenic stimulation, peaks on day 3, and begins to decrease by day 4 (data not shown). This pattern of depressed proliferative capacity and number of recovered cells was reproducible for three independent CTL lines and was not reversed by the addition of exogenous IL-2. The depressed proliferation and recovered cell counts were also seen upon coculture of B6.gld CTLs with APCs from C3H.gld mice, demonstrating that this phenomenon is not caused by Fas-mediated killing of gld CTLs by FasL expressed on the APCs (Fig. 1 d). Autocrine Fas-mediated suicide (10–12) can also be ruled out as a mechanism for depressed proliferation because gld CTLs do not express functional cell surface FasL. Analysis by flow cytometry (data not shown) also ensured that there is not an accumulation of Thy1+CD8–CD4– cells analogous to those found in vivo in gld and lpr mice (1).
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In contrast to their CD8+ counterparts, CD4+ T cells lacking functional FasL proliferate vigorously upon alloantigenic stimulation. Purified CD4+ cells from B6.gld mice cultured with class II different B6.bm12 stimulators proliferated better (Fig. 2 a) and generated more cells over the course of the 5 d of MLC (Fig. 2 b) than did CD4+ cells from wild-type B6 mice. This observation demonstrates the differential requirement for functional FasL expression in the promotion of maximal proliferation by CD8+ but not CD4+ T cells.
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The role of TCR engagement in conjunction with the FasL signal remains unclear. Other molecules known for their positive signaling capabilities have recently been implicated in the death of cells in the absence of a concomitant antigen receptor signal. For example, signaling through CD40 without concurrent engagement of the B cell receptor leads to Fas-mediated cell death (21, 22), and may serve an immunoregulatory role by removing nonspecific B cells. It will be interesting to determine if FasL can still signal without engagement of the CD3/TCR complex and to analyze the consequences of such uncoupled signaling. Should FasL fail to deliver a positive signal without the coengagement of the TCR, the death signal through Fas may serve to remove activated T cells after antigen has been eliminated. In light of the discovery that CD40 signals can direct germinal center B cells to become memory B cells (23), one could speculate on the role of FasL in the clonal expansion of antigen-specific CTLs and the generation of memory T cells. Adoptive transfer of OT-1 (reference 24; class I–restricted OVA-specific TCR transgenic B6) spleen and LN cells into B6 wild-type, B6.lpr, and B6.gld mice followed by intraperitoneal injection of OVA peptide daily for 3 d revealed that antigen-specific donor cells injected into B6.lpr mice were defective in their expansion compared to that demonstrated in the other two recipients (Martin, S., and M. Bevan, unpublished data). These data are consistent with the inability of the Fas– B6.lpr host to deliver a positive signal through FasL for clonal expansion. It will be interesting to determine whether this difference in expansion is related to the development of memory cells and if the FasL signal during an immune response influences the entry of CTLs into the memory pathway.
Several models that allow arrangement of the positive/ negative cell surface receptors into three categories have recently emerged: (a) surface receptors capable of both positive (survival or proliferation) and negative (death) signals such as Fas (25, 26) and TNFR1 (27); (b) positively signaling receptors that augment the death receptor signal (28, 29); and (c) induced receptors paired with constitutive receptors with the opposite signaling capacity, such as CD28 and CTLA-4 (30). Our data place Fas–FasL signaling into this third category. FasL, induced upon activation of CD8+ T cells, sends a negative signal outward through its constitutively expressed receptor while also receiving a positive signal.
| Acknowledgments |
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This work was supported by the following grants from the National Institutes of Health: research grant AG-13078 and Basic Immunology training grant CA-09537.
Submitted: 4 August 1997
Revised: 29 October 1997
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