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Department of Immunology, University of Toronto, Toronto, Ontario, Canada
| Abstract |
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| Introduction |
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We recently identified a later stage in fetal thymic ontogeny, marked by the expression of NK1.1 (CD161) on CD117+ thymocytes, which characterizes a population of progenitors committed to the T and NK cell fates (8), termed fetal TNK progenitors. These cells are present as early as day 13 of fetal thymic ontogeny and also can be generated on reconstitution of alymphoid fetal thymic lobes in vitro with sorted NK1.1-CD117+ TLPs or fetal liver-derived hemopoietic precursors (8). In addition, we have recently characterized a similar progenitor population in the fetal circulation (14), indicating that commitment of precursors to the T and NK cell lineages can occur before thymic entry.
To conclusively determine whether individual CD117+NK1.1+ TNK cells can give rise to descendants in both the T and NK cell lineages, we performed single-cell reconstitution assays in alymphoid fetal thymic organ cultures (FTOCs) with fetal TNK progenitors. Our results provide direct evidence for the existence of a common lineage-restricted T and NK cell progenitor. Thus, these findings establish that bipotent T and NK cell progenitors within the fetal thymus have a CD117+NK1.1+CD90lowCD25- phenotype.
| Materials and Methods |
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Timed-pregnant Swiss.NIH and C57BL/6 mice were obtained from the National Cancer Institute, Frederick Cancer Research and Development Center (Frederick, MD).
Isolation of fetal thymocytes
Fetal thymuses were harvested at day 14 of gestation, washed three times in 5 ml FTOC medium (DMEM supplemented with 12% FCS, 2 mM glutamine, 10 U/ml penicillin, 100 µg/ml streptomycin, 100 µg/ml gentamicin, 110 µg/ml sodium pyruvate, 50 µM 2-ME, and 10 mM HEPES, pH 7.4) (GibcoBRL, Burlington, Canada), and disrupted through 70-µm pore size nylon mesh with a syringe plunger. CD24lowCD25- thymocytes were obtained by Ab/complement-mediated lysis, as described previously (8). Briefly, 50200 µl of each anti-CD24 (J11d.2) and anti-CD25 (7D4) culture supernatant and a 1/10 dilution of Low-Tox rabbit complement (CedarLane, Hornby, ON, Canada) were added to single-cell suspensions in 23 ml medium, and cells were incubated at 37°C for 30 min. After incubation, viable cells were recovered by discontinuous density gradient centrifugation over Lympholyte-Mammal (CedarLane) and washed before analysis.
Flow cytometric analysis and cell sorting
FITC-, PE-, biotin-, and APC-conjugated mAbs and
streptavidin-APC were obtained from PharMingen (San Diego, CA). Cell
suspensions were stained in 50 µl FACS buffer (HBSS without phenol
red, plus 1% BSA and 0.05% NaN3) for 20 min on
ice and washed twice before analysis. Stained cells were analyzed with
a FACSCalibur flow cytometer using CellQuest software (Becton
Dickinson, Mountain View, CA); data were live-gated by forward/side
light scatter and lack of propidium iodide uptake. Frequencies in each
quadrant are given as percent of total in the upper right corner. For
cell sorting, single-cell suspensions were prepared and stained for
FACS as described above, except that no NaN3 was
added to the FACS buffer. Cells were sorted using a Coulter Elite
cytometer (Hialeah, FL); sorted cells were
99% pure, as determined
by post sort analysis. Cells were sorted into FTOC media supplemented
with 10 ng/ml stem cell factor (SCF) (R&D Systems, Minneapolis,
MN).
Single-cell reconstitution of FTOCs
Sorted donor cells (CD117+NK1.1+Ly-9.1+) were resuspended in media containing 10 ng/ml SCF at 0.3 cell/25 µl and then transferred to a Terasaki plate (Nalge Nunc, Naperville, IL) at 25 µl/well. Each well was checked by examination under an inverted microscope to verify that only a single cell was present. Day 15 host fetal thymic lobes from timed-pregnant C57BL/6 mice (Ly-9.2+) were irradiated with 30 Gy (Gammacell-1000, MDS Nordion, Kanata, ON, Canada), and one lobe was then transferred to a well of a Terasaki plate containing a single donor cell. Terasaki plates were inverted ("hanging drop") and cultures were incubated at 37°C in a humidified incubator containing 5% CO2 for 24 h. Lobes were then transferred to standard FTOC for 14 days, in the absence of exogenously added cytokines. A similar procedure was performed for bulk reconstitutions of FTOCs, in which 2000 sorted donor cells were added to each well before transferring the irradiated fetal thymic lobe. Cell suspensions from reconstituted thymic lobes were analyzed by flow cytometry.
OP9 stromal cell line coculture
Sorted cell populations were prepared as described above and used in parallel with FTOC reconstitution assays; 1, 3, 10, 30, or 100 donor cells were cocultured in medium for 10 days on confluent monolayers (96-well flat-bottom plates) of OP9 cells (8) in the presence of IL-3, IL-6, IL-7, and SCF (50 ng/ml each). Cells were then harvested for flow cytometric analysis.
| Results and Discussion |
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ß-TCR and NK1.1 on distinct
populations of cells (Fig. 2
ß-TCR and therefore corresponded to mature
and functional NK cells, rather than the unique subset of T cells
expressing NK1.1 (16). Moreover, further analysis showed
that donor-derived NK1.1+ cells also lacked
surface expression of CD3
, indicating these cells do not express
TCR/CD3 complex on the cell surface (data not shown) (8, 15). Together with our previous findings, these results indicate
that both T and NK cells can be generated in FTOCs and that this assay
serves to identify the lineage potential of progenitor thymocytes.
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ß-TCR+NK1.1- T cells
and
ß-TCR-NK1.1+NK
cells. Two representative FTOCs are shown to indicate the varying
levels of reconstitution observed in this assay (Fig. 3
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To determine whether TNK progenitors were potentially biased toward the
NK cell lineage, we performed in vitro clonogenic assays, in which
progenitors were cocultured with the bone marrow-derived stromal cell
line, OP9. This stromal cell line has been shown to support the
differentiation of TLPs (Fig. 1
; R2) toward the B and NK cell lineages
(8). In particular, TNK progenitors cocultured on OP9
cells failed to generate B cells (up to 300 cells/well), while even at
the clonal level mature functional NK cells were routinely recovered
(data not shown) (8). Thus, taken together with the data
shown in Table I
, these results further establish that TNKs cells in
the absence of continued thymic influence proceed down an NK cell
differentiation pathway. Indeed, these results are consistent with the
fact that NK cell differentiation generally occurs extrathymically
(17, 18), thus supporting our notion that in vitro
manipulation of TNK progenitors promotes the default pathway of NK cell
differentiation.
Although the existence of a common precursor for both T and NK cells present within the mouse thymus was suggested many years ago (8, 19, 20), its identification at the clonal level has never been demonstrated. Nevertheless, a common T and NK cell progenitor has been characterized within a subset of human fetal thymocytes (21, 22). In this regard, our results close the unresolved dilemma pertaining to the existence of a similar bipotent progenitor within the mouse fetal thymus.
This report uses a previously characterized population of T and NK cell-restricted progenitor thymocytes (8) to determine the existence of a common T and NK cell progenitor. Indeed, our findings conclusively demonstrate that cells within this population contain a true common bipotent progenitor for both lineages. Therefore, this report sheds light on the events that occur during lymphocyte lineage commitment by finally identifying, at the clonal level, a mouse progenitor thymocyte capable of giving rise to T cells and NK cells. This new understanding should facilitate the further molecular characterization of key genetic events responsible for T and NK cell lineage commitment.
| Acknowledgments |
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| Footnotes |
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2 Address correspondence and reprint requests to Dr. J. C. Zúñiga-Pflücker, Department of Immunology, University of Toronto, Toronto, Ontario, M5S 1A8, Canada. E-mail address: ![]()
3 Abbreviations used in this paper: TLP, thymic lymphoid progenitors; FTOC, fetal thymus organ culture; SCF, stem cell factor; TNK, T and NK progenitors. ![]()
Received for publication September 23, 1999. Accepted for publication December 2, 1999.
| References |
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and IL-1
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ß T lymphocyte lineage commitment. Immunity 9:187.[Medline]
RII/III contains precursors of T lymphocytes and natural killer cells. Cell 69:139.[Medline]
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