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CUTTING EDGE |
Department of Immunology, Institute for Frontier Medical Sciences, Kyoto University, Kyoto, Japan
| Abstract |
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| Introduction |
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Using a recently established clonal assay system, named multilineage progenitor (MLP)3 assay (4), which is able to determine the developmental potential of each progenitor toward T, B, and myeloid cells, we have succeeded in obtaining direct evidence for the presence of progenitors (p-T) capable of generating T cells but not B or myeloid cells in the fetal liver (FL). However, the possibility has not been ruled out that multipotent progenitors (p-Multi) or stem cells migrate into the thymus, since the most immature cell population in fetal thymus (FT) is shown to be able to generate not only T but also B and myeloid cells, when cocultured with a deoxyguanosine (dGuo)-treated FT lobe or with stromal cells (5, 6). The present study was performed to clarify the developmental capability of individual cells in the earliest population of FT with the MLP assay system. The results indicated that all progenitors in FT were lineage restricted, of which a large majority were p-T.
| Materials and Methods |
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Pregnant C57BL/6 (B6) mice were purchased from SLC (Shizuoka, Japan), and B6Ly5.1 mice were maintained in our animal facility. FT and FL were obtained from B6Ly5.1 mice at 12 days postcoitum (dpc) and were used as the source of progenitors. B6 fetuses (15 dpc) were used as the source of dGuo-treated FT lobes.
Reagents
The following Abs were used: anti-Ly5.1 (A20-1.7, donated by
Dr. Y. Saga, Banyu Seiyaku, Tokyo, Japan), anti-Ly5.2 (ALI-4A2,
donated by Dr. I. L. Weissman, Stanford University, San Francisco,
CA), anti-B220 (RA3-6B2, obtained from American Type Culture
Collection (ATCC), Manassas, VA), phycoerythrin (PE)-anti-Mac-1
(M1/70, Caltag, South San Francisco, CA), PE-anti-Gr-1 (RA3-8C5,
PharMingen, San Diego, CA), PE-anti-B220 (RA3-6B2, Caltag),
allophycocyanin (APC)-anti-Thy-1.2 (5a-8, Caltag), anti-CD25
(PC61.5.3, ATCC), FITC-anti-CD45 (30F11.1, PharMingen),
PE-anti-CD45 (30F11.1, PharMingen), PE-anti-Sca-1 (E13-161.7,
PharMingen), anti-Fc
RII/III (FcR) (2.4G2, PharMingen), and
anti-c-kit (ACK-2, donated by Dr. S.-I. Nishikawa, Kyoto
University, Japan). Anti-Ly5.1 and anti-FcR were labeled with FITC.
Anti-c-kit, anti-B220, anti-CD25, and anti-Ly5.2
were labeled with cyanine 5 (Cy5)(Cy5 labeling kit; Biologic Detection
Systems, Pittsburgh, PA).
Recombinant murine (rm) IL-7 was kindly donated by Dr. T. Sudo (Basic
Research Laboratory, Toray, Kanagawa, Japan). Commercially available rm
stem cell factor (SCF) (Genzyme, Cambridge, MA), rm IL-3 (Genzyme), rm
granulocyte-macrophage (GM)-CSF (Life Technologies, Gaithersburg, MD),
rm IL-1
(Genzyme), recombinant human granulocyte (G)-CSF (Life
Technologies), and recombinant human macrophage (M)-CSF (Cellular
Products, Buffalo, NY) were also used.
High oxygen submersion (HOS) culture and MLP assay
HOS culture of FT organs was performed as described elsewhere (7, 8). The MLP assay culture, which is able to examine the capability of a progenitor to give rise not only to T but also to B and myeloid cells, is a modification of the HOS culture. The detailed method for MLP assay has previously been described (4). Briefly, single dGuo-treated lobes were submerged in 0.2 ml of RPMI 1640 medium supplemented with FCS (10%), SCF (10 ng/ml), IL-3 (3 ng/ml), and IL-7 (200 U/ml) in a well of a 96-well V-bottom plate (Costar, Cambridge, MA). CD44+CD25-FcR- (FcR-) cells in 12-dpc FT and c-kit+CD45+Sca-1+(Sca-1+) cells in 12-dpc FL were sorted as described (4, 6). Single cells were lifted using a micropipet under direct microscopic visualization and seeded into the wells. Plates were centrifuged at 150 x g for 5 min at room temperature, placed into a plastic bag (Ohmi Oder Air Service, Hikone, Japan), and the air was exchanged with a gas mixture of 5% CO2, 70% O2, and 25% N2. The plastic bag was then incubated at 37°C.
Cells grown both inside and outside the cultured lobe were harvested from each well after 10 days of culture, and single cell suspensions were made. One third of each cell sample was stained with FITC-anti-Ly5.1 and Cy5-anti-Ly5.2, and analyzed by flow cytometry. The samples containing Ly5.1+ cells were selected for further analysis. The remaining two thirds of cells from the selected samples were divided into two batches. One batch was stained with FITC-anti-Ly5.1, PE-anti-B220, and allophycocyanin-anti-Thy1.2, and the other stained with FITC-anti-Ly5.1, PE-anti-Mac-1, PE-anti-Gr-1, and Cy5-anti-B220. The flow cytometric analysis of surface phenotypes is detailed in the previous paper (4).
Long-term culture initiation assay
FT or FL cells were plated on confluent layers of stromal cell
line FBMD-1 (9) in 48-well plates coated with rat tail collagen type I
(Becton Dickinson Labware, Bedford, MA). The culture medium was IMDM
(Life Technologies) containing 10% FCS, 5% horse serum, and
hydrocortisone (10-5 M). The plates were incubated at
33°C in 10% CO2. On day 35, the number of colonies with
or without cobblestone area (CA) (10) were counted. To assess long-term
culture-initiating cells (LTC-IC), cells in wells that contained
colonies were trypsinized and plated individually in semisolid medium
to examine the presence of colony forming unit-culture (CFU-C) (11).
Wells with replatable CFU-C were scored as LTC-IC. In CFU-C assay,
-MEM (Life Technologies) containing 30% FCS, 1% methylcellulose,
1% BSA, 2-ME (5 x 10-5 M), L-glutamine
(1 mM), SCF (10 ng/ml), IL-3 (10 ng/ml), granulocyte-macrophage-CSF (10
ng/ml), IL-1
(10 ng/ml), granulocyte-CSF (10 ng/ml), and
macrophage-CSF (10 U/ml) were used. After 14 days of incubation,
colonies consisting of more than 50 cells were scored as CFU-C.
| Results and Discussion |
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All lymphoid cells in 12-dpc FT are
Thy-1-CD4-CD8-CD3-c-kit+CD45+CD44+CD25-
(CD44+CD25-), which is known to be the most
immature subset of thymocytes (12). Our previous investigation
indicated that FcR- subpopulation of the
CD44+CD25- FT cells (Fig. 1
) retains the multipotent progenitor
activity (6). A total of 150 individual FcR- FT cells from
12-dpc fetuses of the B6Ly5.1 strain were cultured under MLP assay
conditions. Cells were harvested on the 10th day from each well and
assayed for expression of T, B, and myeloid cell markers. The type of
progenitors seeded in each well was determined from the phenotype of
the cells grown in the well, and representative flow cytometric
profiles are shown in Figure 2
B. p-T, and progenitors
restricted to B (p-B) and myeloid (p-M) lineages, were detected, but no
p-Multi or bipotent progenitors were found. Profiles of cells derived
from p-T, p-B, p-M, and p-Multi of FL are also shown (Fig. 2
C). Thymic p-T are able to produce a large number of T
cells, almost to the same extent as FL p-T. On the other hand, thymic
p-B and p-M generated much smaller numbers of cells than p-B and p-M in
FL.
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Although progenitors generating T and B cells, or p-TB, have been reported to be present in adult bone marrow (14), we have not succeeded in detecting such p-TB type progenitors in either FT or FL. As we have discussed elsewhere (4), it is probable that the frequency of p-TB in FL, should they exist, is too low to be detected, or that the p-TB stage is skipped over in the hemopoiesis in fetuses.
Further investigations for searching earlier progenitors or stem cells in FT
The following experiments were conducted to verify with different
systems the finding made with the MLP assay (Fig. 3
) that no p-Multi
are present in FT. Early hemopoietic progenitors are known to form long
lasting colonies with CA (10) on a stromal cell monolayer, and the most
primitive progenitors or stem cells can be detected as LTC-IC (11). A
large number of nonfractionated FT cells (12 dpc), equivalent to 3 FT
lobes, were assayed for CA-forming cells (CAFC) and LTC-IC activities.
As shown in Table I
, only three CAFC were
detected among 1,440 FT cells, and no LTC-IC were found. This is in
marked contrast with the detection of 37 CAFC and 24 LTC-IC among
144 Sca-1 high positive (Sca-1high) FL cells (prepared as
in Figure 1
), where p-Multi are highly enriched.4
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Characterization of the progenitors migrating into the thymus to
produce T cells has been one of the most basic problems in the study of
the mechanisms of T cell development. The present results, in
combination with our previous findings (4) that restriction of stem
cells to T, B, and myeloid lineages occurs in FL (see also Fig. 3
of
this paper), strongly suggested that it is the p-T that migrates into
the thymus.
It has been shown that NK cells and lymphoid dendritic cells are derived from thymic progenitors (15, 16, 17). Since the MLP assay system used in the present investigation is not contrived for determining the differentiation and/or growth of these cells, it is unclear whether the thymic p-T, p-B, and p-M determined here are exactly unipotent or are also able to generate NK or dendritic cells. So far, we have not yet tried to modify the MLP assay cultures to generate dendritic cells. On the other hand, very recently it became possible to induce NK cells in thymic organ cultures (HOS culture) by adding IL-2, and we found that a large majority of p-T in FcR- FT cells retained the capability of giving rise to NK cells (our unpublished observation). It is not yet known, however, whether other types of progenitors such as p-M, p-B, or prethymic p-T are also able to generate NK cells. More detailed investigation is necessary to clarify the exact process of commitment, including NK and DC lineages.
| Acknowledgments |
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| Footnotes |
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2 Address correspondence and reprint requests to Dr. Yoshimoto Katsura, Department of Immunology, Institute for Frontier Medical Sciences, Kyoto University, Sakyo-ku, Kyoto 606-8507, Japan. E-mail address: ![]()
3 Abbreviations used in this paper: MLP, multilineage progenitor; CA, cobblestone area; CAFC, cobblestone area-forming cells; CFU-C, CFU-culture; dGuo, deoxyguanosine; dpc, days postcoitum; FcR, Fc
RII/III; FL, fetal liver; FT, fetal thymus; HOS culture, high oxygen submersion culture; LTC-IC, long-term culture-initiating cells; p-B, progenitors capable of generating B cells but not T or myeloid cells; p-M, progenitors capable of generating myeloid cells but not T or B cells; p-Multi, multipotent progenitor; p-T, progenitors capable of generating T cells but not B or myeloid cells; B6, C57BL/6; PE, phycoerythrin; Cy5, cyanine 5; rm, recombinant murine; SCF, stem cell factor. ![]()
4 H. Kawamoto, K. Ohmura, S. Fujimoto, and Y. Katsura. Submitted for publication. ![]()
Received for publication May 26, 1998. Accepted for publication July 30, 1998.
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ß-bearing T cells in the submersion organ culture of murine fetal thymus at high oxygen concentrations. Eur. J. Immunol. 23:200.[Medline]
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