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Searching for hematopoietic stem cells: evidence that Thy-1.1lo Lin- Sca-1+ cells are the only stem cells in C57BL/Ka-Thy-1.1 bone marrow.

Hematopoietic stem cells (HSCs) are defined in mice by three activities: they must rescue lethally irradiated mice (radioprotection), they must self-renew, and they must restore all blood cell lineages permanently. We initially demonstrated that HSCs were contained in a rare (approximately 0.05%) subset of bone marrow cells with the following surface marker profile: Thy-1.1lo Lin- Sca-1+. These cells were capable of long-term, multi-lineage reconstitution and radioprotection of lethally irradiated mice with an enrichment that mirrors their representation in bone marrow, namely, 1,000-2,000-fold. However, the experiments reported did not exclude the possibility that stem cell activity may also reside in populations that are Thy-1.1-, Sca-1-, or Lin+. In this article stem cell activity was determined by measuring: (a) radioprotection provided by sorted cells; (b) long-term, multi-lineage reconstitution of these surviving mice; and (c) long- term, multi-lineage reconstitution by donor cells when radioprotection is provided by coinjection of congenic host bone marrow cells. Here we demonstrate that HSC activity was detected in Thy-1.1+, Sca-1+, and Lin- fractions, but not Thy-1.1-, Sca-1-, or Lin+ bone marrow cells. We conclude that Thy-1.1lo Lin- Sca-1+ cells comprise the only adult C57BL/Ka-Thy-1.1 mouse bone marrow subset that contains pluripotent HSCs.

H ematopoietic stem cells (HSCs) 1 are intriguing because of their ability to perpetuate themselves (self-renewal) and to give rise to progeny that differentiate into all blood cell types. Hematopoiesis commences with clonogenic pluripotent HSCs, which pass through several stages of differentiation, finally producing functionally mature blood cells (reviewed in reference 1). Historically, HSC have also been described as the constituents of bone marrow transplantation essential for saving lethally irradiated hosts, presumably by self-renewal and long-term, multi-lineage blood cell repopulations (2)(3)(4).
The concept and definition of HSCs was first developed by Till and McCulloch (5). They developed a quantitative assay for donogenic bone marrow precursors of myeloerythroid cells: namely, the assay for spleen CFU (CFU-S) (5). While it was initially believed that CFU-S were equivalent to HSCs, it is now accepted that CFU-S are heterogeneous (6)(7)(8). The CFU-S activity of a cell inoculum can be used as a quantitative measure of the proliferative and differentiative capacities 1 Abbreviations used in this paper: CFU-GM, granulocyte/macrophage CFU; CFU-S, spleen CFU; CFU-T, thymic CFU; HSC, hematopoietic stem cell; Lin, lineage markers; PD, radioprotective bone marrow cell dose; Rh-123, rhodamin~123; Sca-1, stem cell antigen-I; WBM, whole bone marrow. of several classes of hematopoietic progenitors (8). Earlyforming (day 8) CFU-S are derived from more committed progenitors, while late-forming (day 12) CFU-S are derived from more primitive hematopoietic cells (7). Further, a subset of cells, defined as pr~CFU-S, are capable of forming CFU-S after secondary bone marrow transplantation (9,10). However, the relationships between HSCs and any subsets of CFU-S/pre-CFU-S are unclear.
Radioprotection is defined as the ability of transplanted hematopoietic cells to prevent lethally irradiated animals from hematopoietic death. Although the mechanisms underlying radioprotection are poorly understood, radioprotection often refers to a property of HSC activity. The radioprotective capacity of HSCs has to satisfy two criteria: (a) self-renewal capacity, which leads to long-term blood cell production; and (b) differentiation potential, which generates all blood cell types.
In an attempt to purify HSCs using physical and biological separation methods, Jones et al. used counter-flow centrifugal elutriation to fractionate cells on the basis of size and density (11). In their experiment, one fraction of bone marrow cells (representing 25% of the bone marrow) was relatively depleted of CFU-S and granulocyte-macrophage CFU (CFU-GM), yet contained precursor cells for long-term engraftment.
They concluded that long-term engraftment was a property of pre-CFU-S HSCs that could be separated from CFU-S on day 8 and day 12 (11).
This result was in apparent conflict with the findings of Spangrude et al. (12,13), that HSCs could be identified and enriched according to cell surface phenotype. HSC activity was found in cells that expressed low but significant levels of Thy-l.1 (Thy-l.ll~ high levels of stem cell-associated antigen (Sea-l), and undetectable levels of blood cell lineagespecific differentiation markers (Lin). This rare subset of bone marrow calls (~0.05%) was highly enriched by using antibodies in a combination of magnetic bead selection and FACS ~ The Thy-l.1 l~ Lin-Sea-1 + calls contained a 1,000-2,000fold enrichment of (a) late-forming CFU-S, (b) thymic CFU (CFU-T), (c) progenitor activity giving rise to long-term hematolymphoid cultures, and (d) radioprotective activity. Thus these cells were proposed as candidate HSCs (12,14). Thy-l.1 l~ Lin-Sca-1 + cells are uniform in size as shown by Spangrude et al. (12). Recently Spangrude and Johnson showed that Thy-l.1 l~ Lin-Sea-1 ~ calls can be further divided into rhodamine-123 low and high (Rh123 l~ and Rh123 hi) cell types (15). The Kh1231~ Thy-11~ Lin-Sca-1 + subset was also over 100-fold enriched for day 13 CFU-S activity (23 CFU-S/1,000 calls injected), but were about fourfold less potent than the Kh123 ~ subset (105 CFU-S/1,000 cells injected) in this assay. The Kh123 ~~ were most highly enriched for pre-CFU-S and long-term, multi-lineage reconstituting activity, as tested by secondary bone marrow transfer into lethally irradiated mice (15).
Nevertheless, cells that are not of the Thy-l.11~ Sea-1 + Lin-phenotype might also contain a rare stem cell population, perhaps that described by Jones et al. (11,13). We therefore sought in this study to test whether radioprotection, stir-renewal, and/or long-term, multi-lineage blood cell reconstituting capacity could be found in Thy-l.l-, Sea-l-, or Lin + cells. We found that Thy-l.1 + but not Thy-l.l-, Seal + but not Sea-l-and Lin-but not Lin + ceils contained all three HSC activities detected in the C57BL/Ka-Thy-l.1 bone marrow.
CellPrelmration. Bone marrow cells were obtained by flushing tibias and femurs as described (12,16). Cells were stained sequentially as follows: (a) cells were first incubated with lineage marker rat antibodies to B220 (RA3-6B2), CD4 (GK-1.5), CD8 (53.6.72), Gr-1 (RB6-8C5), Mac-l(M1/70. 15.11.5), and erythrocytes (antibody TER-119); (b) the washed cells were then exposed to phycoerythrin-conjugated goat antiserum to rat immunoglobulin (Biomeda, Foster City, CA); (c) the washed cells were then incubated in 20% normal rat serum to block free binding sites, followed by addition of biotinylated rat antibody to Sea-1 (antibody E13 161-7) and directly fluoresceinated mouse antibody to Tby-l.1 (antibody 19XE5); and (d) the washed cells were then exposed to Texas redconjugated avidin (Cappel Laboratories, Malvem, PA). The cells were incubated for 20 min on ice for each step, followed by a wash through a FCS cushion. After the final wash, cells were rgsuspended in HBSS containing 1 ~g/ml propidium iodide. The hbded cells were analyzed and sorted with a dual laser FACS | (Becton Dickinson Immunocytometry Systems, Mountain View, CA), modified as described (17), and made available through the FACS | shared user group at Stanford University. Whole bone marrow (WBM) cells were separated on the basis of background levels of fluorescein (Thy-l.1-) from intermediate and high levels of fluorescein (Thy-1.1+), background low levels of Texas red (Sea-l-) from high levds of Texas red (Sca-l+), and background and low levels of phycoerythrin (Lin-) from high levels of phycoerythrin (Lin +). Dead cells were excluded from analysis by propidium iodide staining detected by the allophycocyanin channel. After sorting, each bone marrow fraction was reanalyzed by FACS | Radioprotection and Long-Term Reconstitution Assays. Recipient mice were lethally irradiated (9 Gy) by a 250-kV x-ray machine at 100 rad/min in two split doses (4.5 Gy each) with a 3-h interval. After irradiation, mice were maintained on antibiotic water containing 106 U/liter of polymyxin B sulfate and 1.1 g/liter of neomycin sulfate. The next day, sorted bone marrow subsets or WBM cells were injected (200/~l/mouse) intravenously into the retro-orbital plexus of anesthetized mice. Surviving animals were monitored for 100 d (radioprotection assay).
For the long-term reconstitution assay, sorted donor bone marrow subsets were injected together with 10 s host congenic WBM cells to provide radioprotection.
Peripheral Blood Analysis. Peripheral blood was obtained from the retro-orbital sinus. Immunofluorescence staining and FACS | analysis were performed as described previously (12,16). Two-color staining was carried out using monoclonal antibodies specific for lineage marker for B cells (anti-B220), mydoid cells (anti-Gr-1 and anti-Mac-I), or T cells (anti-Thy-l.1) alone with antibodies to congenic markers specific for donor hematopoietic cells (anti-Ly-5.2, antibody A20.1).

Results
Experimental Design. We wanted to test whether Thy-1.1 l~ Lin-Sea-1 + cells were the only cells in the bone marrow that have pluripotent HSC activity. To avoid excluding any bone marrow cell populations from the analysis, we decided to determine how HSC activity was divided among Thy-1.1 + versus Thy-l.1-pools, Sea-1 + versus Sea-l-pools, and Lin-versus Lin § pools. To measure HSC activity we chose two experimental assays: (a) radioprotection, followed by longterm, multi-lineage reconstitution of T cells, B cells, and myelomonocytic cells in surviving mice; and (b) long-term, multilineage reconstitution by donor cells when radioprotection is provided by congenic host WBM cells.
The Ly-5.2 donor sorted cells were injected into C57BL/6-Ly-5.1 hosts so that in reconstituted animals all donor cells  (Although the intended dose of irradiation was 9 Gy, in one experiment mice were irradiated at 8 Gy due to an x-ray machine calibration error.) In the Lin-group (e), a single cage of animals all died on day 7, whereas the control group (no cells injected) did not start dying until later. We have shown the results both including (labeled Lin-, n = 20) and excluding (labeled Lin-*, n = 15) data from these animals.
and their progeny could be distinguished by a monoclonal antibody specific for Ly-5.2. Furthermore, Thy-l.1 and Thy-1.2 allelic markers allowed us to identify and follow donorderived T cells in reconstituted animals.

Which Bone Marrow Subsets Mediate Radiot~rotection of Lethally Irradiated
Mice? HSC activity was tested by injecting the sorted bone marrow subsets intravenously into lethally irradiated mice. These separated -versus + pools were examined for their ability to protect lethally irradiated mice from hematopoietic death for at least 100 d (i.e., radioprotection).
To provide the most sensitive assay for enrichment or depletion of radioprotective activity, we transferred these cells in numbers corresponding to their representation in WBM a predicted to be 50% radioprotective (PDs0 = 4 x 104 lO0-/ WBM cells) and 95-100% radioprotective (PDgs = 2 x 10 s 8o_| WBM cells). These were designated, respectively, the low dose group and the high dose group; the actual numbers of cells injected is given in Table 1. Fig. 2 shows the percentage 4 of surviving animals given low versus high doses of either 2o WBM or of the FACS| populations. When the lethally irradiated mice received no cells, the c majority of the mice died 10-18 d (90%) after irradiation. 100] In this study we observed survival of 1 in 30 animals injected 80~ with no cells. No animals survived that were injected with --~ t "~ 80 the PDs0 equivalent dose of Thy-l.l-, Sea-l-, or Lin + cells, o It should be noted that mice receiving Thy-l.1-or Sea-l-~ 40 cells, indicated by arrows in Fig. 2, c and e, survived ~6 d o~ 2o longer on average than mice receiving no cells. In contrast, "o 0 radioprotective activity was detected in the PDs0 equivalent "~ 9 e dose of Thy-l.l+, Sea-l+, Lin-cells: 45% by the Thy-l.1 + ~ 100. subset, 65% by the Sea-1 § subset, and 47% by the Lin-O_ subset.
It is evident from these experiments that Thy-l.l-, Seal-, or Lin + cell populations, which represent ,'o82-96% of WBM cells, were significantly depleted for their ability to 0. radioprotect lethally irradiated mice. The survival kinetics provided by Thy-l.1 *, Sea-1 +, and Lin-cell populations at both cell doses were similar to those with low or high doses of WBM cells. Thus the radioprotective activity of WBM cells is a property of Thy-l.l+, Sea-l+, and Lin-cells.
The surviving animals described above must have been radioprotected either by progeny of donor HSCs or by a transient population of circulating donor-derived blood cells followed by recovery of endogenous HSC activity. If long-term radioprotection was provided by progeny from donor HSCs, we should observe long-term, multi-lineage reconstitution by donor cells. We tested the levels of donor-derived, Ly-5marked peripheral blood cells in each lineage at 100 or 180 d after irradiation and cell injection (Fig. 3). We analyzed the mice injected with the PDgs equivalent cell dose. From past studies detection of sustained myelopoiesis is a good indicator of active hematopoiesis from HSCs and progenitor cells (18; Uchida, N., and I. L. Weissman, unpublished observations), because mature granulocytes are short-lived (on average <24 h) once they enter the blood circulation (18; Uchida, N., and I.L. Weissman, unpublished observations). It is clear that consistent long-term, multi-lineage reconstitution occurs only with WBM, Sea-1 + , Lin-, or Thy-l.1 + cells (Fig. 3, b, d, f, and h). It should be noted that in the experiment shown in Fig. 3 h, we were only able to inject 6 x 103 Thy-l.1 + cells into each recipient, a dose that represents only two-thirds of the expected PD9s equivalent cell dose.
We also tested the surviving animals that received no cells, or the PDgs equivalent dose of Thy-l.l-, Sea-l-, or Lin + cells. No multi-lineage reconstitution was detected except from one mouse injected with 2 x 10 s Thy-l.1-cells (Fig. 3, a, c, e, and g). We believe these surviving animals were almost certainly radioprotected due to repopulation by endogenous stem cells that survived the irradiation. As pointed out above, mice receiving Thy-l.l-, Sea-l-, or Lin + cells survive, on average, a few days longer than mice receiving no cells, implying that these cells might contain short-term engraftment activity. In that way, a few of these mice may have survived for sufficient time to allow repopulation by endogenous hematopoiesis. 1 of the 10 mice injected with 2 x 10 s Thy-l.1-cells in one experiment showed donor-derived, multi-lineage reconstitution (Fig. 3 g). This was the only mouse that had multilineage repopulation from the groups receiving Thy-l.l-, Sca-l-, or Lin + cells in this entire study (only 7 of 120 mice survived). While it is possible that the Thy-l.1-fraction may contain extremely rare stem cells, it is more likely to be due to the cross-contamination because of the technical difficulty in separating Thy-l.1 l~ cells from Thy-l.1-cells. To obtain the best sorting conditions for the Thy-l.1 antigen, another experiment was performed using C57BL/Ka-Thy-l.1 donors and C57BL/Ka (Thy-l.2) Thy-1 conic hosts (Fig. 4). T lineage reconstitution was detected when either    PDgs equivalent dose of Thy-1-cells. Thus we could detect radioprotective and T lineage repopulating activities in WBM and Thy-l.1 + cells, but not in Thy-l.1-cells.

Which Bone Marrow Subsets Mediate Long-Terra Multi-Lineage Reconstitution in Mice Radioprotected by Host IYBM Cells?
The experiments described above show that Thy-l.l-, Sca-l-, and Lin + cells do not radioprotect lethally irradiated mice. This assay does not exclude the possibility that cells contained in the Thy-l.l-, Sca-l-, or Lin + subsets do indeed have long-term engraftment activity, but this could not be detected if they failed to radioprotect in the short term. Jones et al. reported that they could isolate a bone marrow subset that contained long-term repopulating but not radioprotectire activity (11). We wanted to test whether similar activity was present among Thy-l.1-, Sca-l-, or Lin + cells. To provide these cells the opportunity to demonstrate long-term engraftment, 1 x 10 s host bone marrow cells were coinjected with each donor bone marrow fraction tested. As the long-term engraftment potential of Thy-l.l+, Sca-l+, and Lin-cells had already been tested, we limited our examination of possible long-term repopulating ability of Thy-l.l-, Sca-l-, and Lin + cells, and compared it with that of WBM cells. Tables 2, 3, and 4 show the results of long-term reconstitution assays of irradiated Ly-5 congenic hosts given sorted, donor cell populations plus 1 x 10 s host-type WBM cells for radioprotection. Under these conditions it is clear that Thy-l-, Sca-l-, and Lin + cells could not contribute to long-term, multi-lineage reconstitution ( Table 2). An equivalent dose of WBM cells could contribute to significant levels of long-term, multi-lineage reconstitution (Table 4). In three of four mice in this WBM group, 24-96% of peripheral mydoid cells were donor derived 14 wk after transplantation. Some short-term engraftment was observed with Thy-l.l-, Sca-l-, or Lin + cells (Tables 3 and 4). Thy-l.1-cells contained reproducible and significant B cell progenitor activity, while no long-term B cell progenitor activity was detected in Sca-1-cells (Tables 3 and 4). Thus, these progenitors must be Sca-1 + , but may be Lin + or Lin-cells. By the two approaches described above we found that radioprotective activity tightly correhted with long-term, multi-lineage repopulation activities.
It is most likely that the same population also has selfrenewal activity. Elsewhere we have shown that hosts demonstrating sustained, donor-derived myelopoiesis (>t6 mo) contain bone marrow cells that can transfer radioprotection and long-term, multi-lineage hematopoiesis to secondary hosts (15,18). In these experiments, clonogenic Thy-l.1 l~ Lin-Sca-1 + Ly-5 marked single cells (or donogenic units found at limit dilution) were transferred into Ly-5 congenic hosts along with either 1 x 10 s WBM or 100 Thy-l.11~ Lin-Sca-1 + host cells. Two outcomes were evident when donorderived cells could be detected in the blood: multi-lineage differentiation that fails to sustain myelopoiesis, and multilineage outcomes that sustain myelopoiesis (18; Uchida, N., and I. L. Weissman, unpublished observation). Only those chimeras with sustained donor-derived myelopoiesis contained donor-derived cells that could yield long-term, multi-lineage outcomes after secondary transplantation. Only WBM or Thy-l.11~ Lin-Sca-1 + cells contain such activity (18; Uchida, N., and I. L. Weissman, unpublished observations). Because virtually all mice given PDs0 or PDgs doses of Thy-l.1 + , Lin-, or Sca-1 + cells exhibited sustained myelopoiesis, we infer that self-renewal has also occurred in these cases. Because no mice given Thy-l.l-, Lin + , or Sca-1-cells (along with host WBM cells) demonstrated donor-derived, longterm, multi-lineage reconstitution, these populations do not contain pre-CFU-S or self-renewing HSCs.
We found no evidence for the existence of cells that augment or inhibit HSC activity, as mixing WBM cells with the sorted populations appeared to neither inhibit the Linfraction (12) nor help the Lin + fraction. It is interesting to note that Thy-l-, Sca-l-, and Lin + cells provided some degree of short-term or lineage-restricted engraftment. For example, the Thy-l.1-fraction gave rise to a large number of B cells in one of four mice injected with 2 x 104 cells, and in four of five mice injected with 2 x 10 s cells over the full length of assay. This is consistent with the presence of these progenitors in a Thy-1-fraction reported previously (19).
Jones et al. used counterflow centrifugal elutriation to separate bone marrow cells into four fractions on the basis of size and density (11). Progenitor activities of these cells were assayed by day 8 and day 12 CFU-S and by CFU-GM. They used male-spedfic DNA probes to trace donor-origin engraftment at days 11 and 60. One fraction of cells that elutriate at 25 ml/min (representing 25% of input cells) was enriched for morphologic lymphocytes and depleted of blast cells. This fraction was significantly depleted for progenitor cells that could produce CFU-GM and CFU-S; 1 x 10 s of these cells could not radioprotect lethally irradiated animals, but did contain a long-term progenitor activity (see below). Another fraction, representing '~25% of bone marrow cells, was highly enriched for blast cell types. This fraction contained CFU-GM and CFU-S activities, and could radioprotect 15 of 28 animals given 2 x 104 cells for at least 60 d. Donor-derived cells in bone marrow of the reconstituted animals, detected by dot blots probed for male-specific DNA, were decreased from 98% of male donor type at day 14 to 11% at day 60.
Interestingly, 2 x 104 male donor 25 ml/min cells contained multi-lineage reconstitution activity assayed at day 60 when the irradiated mice were radioprotected by 2 x 104 female "blast" fraction cells. While these hosts had no detectable male donor cells in bone marrow at day 14, male donor cells were detected in bone marrow at abundant levels ('~50% of normal) at day 60. Jones et al. concluded that two independent classes of bone marrow cells mediate two different phases of hematopoietic recovery in lethally irradiated animals. The "blastic" fraction would contain committed progenitors that provided radioprotection via rapid, but shortterm engraftment, while the 25 ml/min fraction contained pre-CFU-S or "true" HSC that could not contribute to radioprotection by early stage myeloerythroid development, but contained cells that could produce delayed, multi-lineage reconstitution (11).
Can these results be reconciled with the data presented here and elsewhere describing activities of Thy-l.1 I~ Lin-Sca-1 + cells? Thy-l.lI~ + cells are similar in size to the 25 ml/min fraction, as shown by Spangrude et al. (12). Also, ~97% of this cell type are in the Go-G1 phase. Combining these findings, Thy-l.11~ Lin-Sca-1 + cells should not be included in a fraction enriched for blast cells. Yet they are highly enriched for day 12 CFU-S, in contrast to the 25 ml/min fraction of Jones et al. (11), and are responsible for both longand short-term reconstitution. It has been shown that several different cell types can contribute to CFU-S (8). Purified Thy-l.ll~ + cells have low day 8 CFU-S activity and high day 12-13 CFU-S activity (1:10 ratio), while three other FACS| populations give rise to CFU-S with varying ratios of day 8/day 12 activity (8). It is unclear which populations were deleted from the 25 ml/min fraction described by Jones et al. (11). The enrichment or depletion of CFU-S and long-term progenitor activities in fourfold enriched populations is, of course, much different than the highlyenriched activities described in a subset representing only 0.05% of WBM.
Nevertheless, the findings of Jones et al. (11) raise questions concerning the "homogeneity" of the Thy-l.11~ Lin-Sca-1 + cells. In fact, they appear to be heterogeneous. Spangrude and Johnson showed that Thy-l.1 l~ Lin-Sca-1 + cells can be further divided into rhodamine-123 low and high (Rh123 I~ Rh123 hi) cell types (15). While WBM contain "~0.15 day 13 CFU-S per 1,000 cells injected, R.h123 hl Thy-1.1 l~ Lin-Sca-1 + ceils were highly enriched for day 13 CFU-S activity (105 CFU-S per 1,000 cells injected), but were not correspondingly enriched for pre-CFU-S or long-term multilineage reconstituting activity in lethally irradiated hosts (15; Uchida, N., and I.L. Weissman, unpublished observations). The Rh123 l~ Thy-11~ + subset was also >100-fold enriched for day 13 CFU-S activity (23 CFU-S per 1,000 cells injected), but were about fourfold less potent than the R.h123 hi subset in this assay. Most striking, the Rh1231~ were most highly enriched for pre-CFU-S and long-term, multilineage reconstituting activity, as tested by secondary bone marrow or spleen transfer into lethally irradiated mice (15). Thus, Thy-l.ll~ + cells include both short-and long-term reconstituting activities. It is yet to be determined whether the Rh-123 defined subsets in fact correspond to the counterflow elutriation subsets.
There are several variables that could account for the apparent differences between Jones et al. (11) and our studies.
First, we have used C57BL/Ka-Thy-1.1 mice, mainly because our best anti-Thy-1 staining antibody detects the Thy-l.1 allele, and also because they are closely related to Thy-1 and Ly-5 congenic mouse lines. Jones et al. used B6D2F1 mice in their studies (11). Thus strain differences in HSC properties could exist. In fact, Spangrude has recently found that pre-CFU-S activity resides in both Thy-l.2 l~ and Thy-1.2-Sca-1 +Lin-cells in C57BL/Ka (Thy-l.2) bone marrow, whereas it is present only in Thy-l.1 l~ cells in C57BL/Ka-Thy-l.1 bone marrow (Spangrude, G., manuscript in preparation).
Recently Chandhary and Roninson showed that Rh-123 staining of human myeloerythroid progenitors was inversely correlated with the expression of the MDRI (multi-drug resistance) gene product P-glycoprotein (20). Low levels of Rh-123 staining of hematopoietic cells resulted from its export by the P-glycoprotein. It will be interesting to find out whether the heterogeneity of Rh-123 staining of Thy-l.1 l~ Lin-Sca-1 + subsets can be explained by the differential P-glycoprotein expression.
It is important to note that very low numbers (50-100 cells) of Thy-1.11~ + cells are capable of radioprotecting and reconstituting lethally irradiated animals. It is this combination of activities that will be required for human bone marrow transplantation to provide patients with longterm engraftment while preventing short-term hematopoietic failure.