Flk2⁺ common lymphoid progenitors possess equivalent differentiation potential for the B and T lineages

Hematopoietic stem cells (HSCs) have the unique ability to both self renew for the lifespan of an organism and to differentiate into more restricted progenitors.¹  Through multiple rounds of proliferation and irreversible steps of lineage commitment, HSCs ultimately drive the production of all mature blood cells. In the current hierarchical model of hematopoiesis, the first lineage decision is made at the bifurcation between the myeloid/erythroid/megakaryocytic and the lymphoid lineages.²  This first step in lineage commitment is evidenced by the existence of 2 distinct progenitor populations, the common myeloid/erythroid progenitor (CMP)²  and common lymphoid progenitor (CLP). CLPs were originally defined as Lin⁻Sca-1^intc-Kit^intIL-7Rα⁺ cells and as a population gave rise to T cells, B cells, natural killer (NK) cells, CD8⁺ and CD8⁻ dendritic cells (DCs), Langerhans cells, and interferon-α producing DCs but not to macrophages, erythrocytes, or platelets.^3-6 

Both the lineage potential and physiologic relevance of CLPs have been questioned recently on the basis of results from several studies. Some studies have suggested limited or negligible T-cell production from CLPs, and have suggested that CLPs are primarily progenitors for B cells.^7-9  In contrast, other studies have reported that CLP or their downstream progenitors retain myeloid potential, and are therefore not lymphoid committed (examples are Allman et al,⁷  Balciunaite et al,¹⁰  Perry et al,¹¹  and Rumfelt et al,¹²  and reviewed in Ye and Graf,¹³  Pelayo et al,¹⁴  and Bhandoola et al¹⁵ ). If the CLP does not possess physiologically relevant levels of T-cell potential, or is not actually lymphoid committed, our current models of myeloid, T- and B-cell development will need to be reconsidered. Therefore, it will be an important advance in the field to unambiguously prove or disprove the existence of the CLPs, and to determine, whether CLPs are robust sources of all lymphoid lineages in vivo.

Expression of the cytokine receptor Flt3/Flk2 is restricted to early hematopoietic progenitors and dendritic cells.^16,17  We and others have previously shown that short-term (ST)-HSCs and multipotent progenitors (MPPs) express Flk2 whereas long-term (LT)-HSCs do not.^18,19  Surprisingly, when CLPs were reanalyzed following publication of Flk2 as a multipotent cell marker (recently confirmed by Forsberg et al²⁰ ), we discovered that distinct subsets of CLPs could be identified on the basis of Flk2 expression,¹⁷  here labeled CLP^F and CLP^F− (following the convention we established in Forsberg et al²⁰ ). We analyzed the developmental potential of CLP^F and CLP^F− and demonstrated that while the Flk2⁻ subfraction of CLP contain only B-restricted progenitors, Flk2⁺ CLP possess robust T-cell, B-cell, NK, and DC potential, and completely lack myeloid potential. Furthermore, this oligopotency can be observed at the single cell level, definitively demonstrating that this population contains a common lymphoid-restricted progenitor. CLP^F, when injected in vivo, robustly reconstitute both the B and T lineages. Therefore, CLP^F represent the lymphoid branch of the hematopoietic tree, and Flk2 expression is a defining hallmark of these oligopotent progenitors.

All animal procedures were approved by the Institutional Animal Care and Use Committee and in compliance with the Public Health Service Policy for Humane Care and Use of Laboratory Animals. Balb/c, C57BL/Ka-Thy1.1CD45.2⁺ (BA), congenic C57BL/Ka-Thy1.1CD45.1⁺ (HZ), and C57BL/Ka rag2^−/−γ_c^−/− mice were bred and maintained at the Stanford Animal Facility according to the University's guidelines.

Sorted progenitor cells were cultured in Iscove-modified Dulbecco medium (IMDM) supplemented with 10% fetal bovine serum (FBS; Invitrogen, Carlsbad, CA), L-Glutamine (5 mM; Invitrogen), 50 μM 2-mercaptoethanol, 1× nonessential amino acids, and penicillin G (100 U/mL)/streptomycin (100 μg/mL; Lonza Walkersville, Walkersville, MD). Recombinant murine IL-3, IL-7, IL-15, macrophage colony-stimulating factor (M-CSF), granulocyte-macrophage colony-stimulating factor (GM-CSF), stem cell factor (SCF), and human Flt3L were purchased from R&D Systems (Minneapolis, MN) and reconstituted in sterile phosphate-buffered saline (PBS)/1% bovine serum albumin (BSA). Cell staining and sorting were done in PBS with 2% FBS.

The following monoclonal antibodies were produced in our laboratory: M1/70 (anti–Mac-1/CD11b), 8C5 (anti–Gr-1), 6B2 (anti-B220), KT-31 (anti-CD3), GK1.5 (anti-CD4), 53-7.3 (anti-CD5), 53-6.7 (anti-CD8), A7R34 (anti–IL-7Rα/CD127), 2B8 (anti-c-Kit/CD117), 3C11 (anti-c-Kit/CD117), E13-161-7 (anti–Sca-1), 19XE5 (anti-Thy-1.1), A20.1.7 (anti-CD45.1), AL1-4A2 (anti-CD45.2). Goat antirat IgG (phycoerythrin [PE] or Cy5-PE conjugated) was purchased from Invitrogen. Monoclonal antibodies to I-A^b, CD3, CD4, CD8, CD11b, CD11c, CD19, CD24, CD25, CD27, CD34, CD43, CD44, CD62L, CD93/AA4.1, Flk2, NK1.1, TCRβ, and isotype controls were purchased from BD Biosciences (San Jose, CA), eBioscience (San Diego, CA) or Biolegend (San Diego, CA). For visualization of biotinylated antibodies, streptavidin-conjugated PE, Cy5-PE, Cy7-PE, and Texas Red (Invitrogen, Carlsbad, CA) were used.

Progenitor populations were isolated from bone marrow (BM) or thymus based on phenotype as previously described: hematopoietic stem cells (HSC; Lin⁻/CD127⁻/c-Kit^hi/Sca^hi/Thy1.1^lo Flk2⁻); multipotent pogenitors (MPP^F; Lin⁻/CD127⁻/c-Kit^hi/Sca^hi/Thy1.1⁻Flk2⁺); common lymphoid progenitors (CLP; Lin⁻/CD127⁺/c-Kit^int/Sca^int/Thy1.1⁻); thymic progenitors c-Kit^hi DN1 (Lin^−/low/CD44^hi/c-Kit^hi/CD25⁻); c-Kit^hi DN2 (Lin^−/low/CD44^hi/c-Kit^hi/CD25⁺); and pro B cells (B220⁺/CD43⁺/CD19⁺/IgM⁻). The lineage cocktail included antibodies directed against CD3, CD4, CD5, CD8, CD19, B220, Gr-1, Mac-1, NK1.1, and Ter-119. All populations were double sorted on a BD FACSVantage SE or BD FACSAria to high purity (> 99%; BD Biosciences). Lineage-specificity of progenitor populations was confirmed by flow cytometric analysis of spleen and thymus of mice that received transplants using a combination of donor- and host-specific anti-CD45 antibodies in combination with lineage specific antibodies, including CD19, TCRβ, NK1.1, Mac-1, Gr-1, and Ter-119.

For isolation of CLP^F from progenitors with myeloid potential, high quality IL-7Rα stains are essential and require optimization. Anti-B220 should also be used in its own channel to distinguish CLP from early B lineage–committed progenitors. If these components are optimal, c-Kit and Sca-1 stains are confirmatory but not essential.

We have observed that CLP^F are far more sensitive to serum and media differences, temperature fluctuations, and long delays after sorting than upstream MPP or downstream B progenitors (see Figure S1, available on the Blood website; see the Supplemental Materials link at the top of the online article). Maintaining cells in ice-cold, serum-containing medium throughout preparation, sorting, and before transplantation or culturing is critical to preserving the oligopotency of CLP^F.

Mice typically received 400 rad x-irradiation before intravenous injection of progenitors in 150 μL sterile PBS through the retro-orbital venous plexus. Rag2^−/−γ_c^−/− mice were used without conditioning. Intrathymic injections were performed as previously described.²¹ 

Cells were plated out in triplicates in 1 mL MethoCult M3231 methyl cellulose medium (StemCell Technologies, Vancouver, BC) containing SCF, Flt3L, IL-3, IL-6, IL-11, thrombopoietin (TPO), erythropoietin (Epo), and GM-CSF as previously described.²  Cultures were incubated for 12 days in a humidified chamber at 37°C, 5% CO₂ and colonies counted on day 7 and day 12 using an inverted microscope.

For limiting dilution assays, CLP^F were clone sorted directly onto 96-well plates plated with 5 × 10³ to 2.5 × 10⁴ of either S17 (B-cell assay) or OP9-DL1 (T-cell assay) stromal cell lines. Prior to plating, stroma cells were growth arrested by mitomycin C (Sigma-Aldrich, St Louis, MO) treatment (10 μg/mL for 3 hours). Cultures were supplemented with SCF, Flt3L, and IL-7. After 10 days, cultures were analyzed by flow cytometry to score for B cells (B220⁺CD19⁺) and/or T cells (Thy1.1⁺CD25⁺). Wells were scored positive when at least 20 B220⁺CD19⁺ and/or Thy1.1⁺CD25⁺ cells were detectable by flow cytometry. For clonal assays, cells were clone-sorted onto Terasaki wells plated with 200 cells/well OP9 stromal cells and supplemented with SCF, Flt3L, and IL-7. Single-cell deposit was confirmed right after sorting by microscopy, and only wells containing exactly one cell were further analyzed. After 3 days of culture, wells containing 4 or more cells were harvested and equal aliquots seeded into 96-well plates containing either B cell–inducing (OP9 stroma plus SCF, Flt3L, and IL-7) or T cell–inducing (OP9-DL1 stroma plus SCF, Flt3L, and IL-7). Clones that contained 8 or more cells were split into 4 wells, 1 with B cell–inducing, 1 with T cell–inducing, 1 with NK-inducing (OP9 stroma plus IL-2, IL-7, IL-15, SCF, and Flt3L) and 1 with DC/myeloid–inducing (AC6 stroma plus IL-3, GM-CSF, M-CSF, SCF, and Flt3L) conditions. All 3 stroma cell lines were left untreated in these experiments (no growth inhibition). After 8 days in culture, wells were analyzed by flow cytometry to identify B cells (CD19⁺B220⁺), T cells (Thy1.1⁺CD25⁺), NK cells (NK1.1⁺DX5⁺), DCs (I-Ab⁺CD11c⁺) and myeloid cells (Mac-1⁺Gr-1⁺). Only clones that produced 1 or more lineages were scored.

We originally described the CLP as Lin⁻IL-7Rα⁺c-Kit^loSca-1^lo and showed more recently that this population expressed variable levels of Flk2.¹⁷  In 6-week-old Bl/6 mice, depending on the strictness of lineage marker exclusion, roughly 60% of the CLPs express high levels of Flk2, whereas the remaining 40% were negative (Figure 1A). We previously showed that most of the dendritic-cell potential is retained within the Flk2⁺ fraction.^4,5  Nevertheless, the full lymphoid developmental potential of these 2 CLP fractions was never formally investigated. After altering the sequence of the gating strategy by first gating on Lin⁻ live cells and then gating on the IL-7Rα⁺ Flk2^−/+ fractions, we found that the Flk2⁺ cells were a homogenous population with respect to Sca-1 and c-Kit expression and matched the original Sca-1 and c-Kit expression of conventionally isolated CLPs (Figure 1B). In contrast, the Flk2⁻ fraction was more heterogeneous and these cells only partially overlapped with the original Sca-1/c-Kit gate (Figure 1B). We also tested the expression of several other surface molecules that have been used to describe early lymphoid or multipotent progenitors including CD24/HSA, CD27, CD34, CD43, CD44, CD62L/L-selectin, and CD93/AA4.1, and found that all Flk2⁺ CLPs expressed homogeneous levels of these markers (Figure 1C and data not shown). Interestingly, we found that Flk2⁺ CLP expressed lower levels of CD93 (also known as AA4.1) than downstream B220⁺ populations (Figure 1D), calling into question results from other studies that identified putative CLP based upon high expression of AA4.1 and low or absent expression of Sca1.^7-9,12,22-25  We also tested Balb/c mice and found a population with nearly identical phenotype, suggesting that Flk2⁺ and Flk2⁻ fractions of CLPs is not unique to the Bl/6 background (see Figure S2).

Next, we determined the full lineage potential of these 2 CLP subpopulations in vivo. Both populations were isolated by 2 rounds of fluorescence-activated cell sorting (FACS) to achieve high purity (> 99%) from the bone marrow of Bl/6 Ly5.2 mice, and 10³ cells were transplanted intravenously into sublethally irradiated (400 rad) congenic Bl/6 Ly5.1 mice. Three weeks after transplantation, a time when B-cell lineage production is robust and T-cell export from the thymus into the secondary lymphoid organs has just begun, we analyzed donor cell contributions to the spleen and thymus of recipient mice. In the spleen, the overall magnitude of reconstitution from CLP^F was much higher than from the Flk2⁻ fraction (4% vs 0.02%; Figure 2A). In addition, we found that all progeny derived from the Flk2⁻ fraction were B-lineage cells as determined by expression of CD19⁺, whereas no T, NK or myeloid cells (Mac-1⁺Gr-1⁺) were detected (Figure 2B-D). In contrast, CLP^F produced TCRβ⁺ T cells and Nk1.1⁺ NK cells as well as large numbers of CD19⁺ B cells. Similar results were obtained when bone marrow and blood were analyzed (data not shown). Within the thymus CLP^F, but not the Flk2⁻ fraction, gave rise to donor-derived T-lineage cells, contributing nearly 70% of thymocytes at 3 weeks after transplantation (Figure 2E). Even when both subpopulations were injected directly into the thymus, only CLP^F gave rise to T-lineage progeny (Figure 2F).

While all groups have recognized the ability of CLP to produce B cells, some have questioned whether CLP have enough T-cell potential to be considered a potential major source of these cells. Because T-cell potential segregates with Flk2 expression within the original CLP population, we performed a time course experiment to determine the kinetics and magnitude of T- and B-cell production from CLP^F and the Flk2⁻ fraction (Figure 3A). As our initial results predicted, the Flk2⁻ fraction gave rise only to B cells in peripheral tissues at all time points. While few T cells were detected in the spleen at 3 weeks or earlier from Flk2⁺ CLP, a much larger number were observed at 4 weeks and beyond, suggesting a maturation period of approximately 3 weeks within the thymus before donor T-cell emigration to secondary lymphoid tissues (Figure 3A left). Underscoring the robustness of the T-cell potential of CLP, CLP^F-derived splenic T cells outnumbered splenic B cells by week 4. In the blood, CLP^F derived B- and T-cell contributions ranged from 0.5% to 2% at 3 weeks after transplantation and were roughly parallel over the time course, strongly supporting the idea of CLP^F being a major contributor to both B and T lineages (Figure 3A right).

Within the thymus, donor chimerism exceeded 50% by 3 weeks (Figure 2E), with the vast majority (99%) of donor-derived cells belonging to the α/β T-cell lineage (as determined by expression of either or both CD4 and CD8), with limited TCRγδ, NK, and B220⁺ cells (Figure 3B). T-cell development of CLP^F-derived cells proceeded sequentially from the CD4⁻CD8⁻ (DN) stage to the CD4⁺CD8⁺ (DP) and CD4⁺CD8⁻ or CD8⁺CD4⁻ (SP) stages, and more mature SP cells were predominant by 3 weeks post injection (Figure 3C top row). The very earliest thymocyte stages (DN1 and DN2) express CD44 and c-Kit, and these were not observed, even at the 1 week time point. Instead, CLP^F-derived cells at the DN3 and DN4 stages (CD44⁻CD25⁺ and CD44⁻CD25⁻, respectively) predominated at 1 week, suggesting that CLP^F either directly seeded the thymus, or gave rise to thymus seeding cells rapidly after injection, and that these cells had differentiated beyond the DN2 stage by 1 week (Figure 3C bottom panels). We tested whether CLP could give rise to the earliest thymocyte stages (DN1 and DN2), by directly injecting CLP^F intrathymically into nonirradiated recipients, and determining the differentiation status of the donor cells 4 days after injection (Figure 3D). We found that intrathymically injected CLP^F differentiate into DN1 and DN2 at this time point, consistent with the intravenous injection time course, indicating that CLP can develop down the T lineage through the classic T-cell development pathway.

Several groups have reported that CLP, or progenitors presumed downstream of CLP, have significant myeloid potential. These studies contradict the original description of CLP, which showed that CLP lack myeloid potential. Because the CLP^F subset has robust B- and T-lineage potential and is a more homogeneous population than has been previously identified, it enabled us to address this issue in a more convincing manner than previous studies. We therefore examined myeloid readout in intravenous transplants of CLP^F. We did observe a small number of Mac1⁺ cells that others have proposed represent myeloid cells (Figure 2D). However we found that nearly all these cells were CD11c⁺, and thus were of the DC lineage, consistent with the lymphoid restriction of CLP (Figure 4A). We did not observe any Mac1⁺Gr-1⁺ cells derived from CLP^F (Figure 2D). To exclude low levels of early myeloid cell production, we analyzed mice 2, 4 and 7 days after transplantation of 10⁴ cells and stained bone marrow, spleen, and blood for Mac-1⁺Gr-1⁺ cells but found no donor-derived granulocytes at any time point (data not shown). At 4 weeks after transplantation, we also observed Mac1⁺ B-1b cells (Figure 4B). In addition to the in vivo transplantations, we used colony formation assays in methylcellulose, which are very sensitive assays for detection of myeloid or erythroid potential. We used various numbers of CLP^F cells and compared them to the more primi-tive c-Kit^hiLin⁻Sca-1^hiFlk2⁺ (MPP^F) cells. Using a cocktail of 8 different cytokines to enable simultaneous readout of erythroid, myeloid, and megakaryocytic potential, we detected approximately 1 colony per every 10³ seeded CLP^F cells (Figure 4C), which were 99% pure after double sorting (data not shown). In stark contrast, 100 MPP^F cells had a plating efficiency of above 80%, giving rise to mostly mixed colonies. Therefore, CLP^F do not give rise to myeloid cells at frequencies above the calculated impurity levels in our sorted preparations.

It is clear from the data above that only the Flk2⁺ fraction of CLP has any significant combined B- and T-cell potential. We therefore sought to determine the clonal frequency of T- and B-cell progenitor activity within this population by in vitro and in vivo limiting dilution assays. When limiting dilutions of Flk2⁺ CLP were cultured on S17 stroma in the presence of SCF, Flt3L, and IL-7, the frequency of CLP^F that had B-cell progenitor potential was approximately 1 of 3 (Figure 5A top left). When we counted the total number of progeny from a single CLP^F after 10 days we calculated an average burst size of approximately 1900 per CLP^F, demonstrating the large proliferative capacity of these cells when differentiating down the B lineage (Figure 5A top middle). To determine the clonal T-cell potential of CLP^F, we cultured CLP^F in SCF, Flt3L, and IL-7 on OP9 stromal cells expressing delta-like 1 (OP9-DL1), a ligand for Notch.²⁶  We found that on average 1 of 2 CLP^F cells gave a T-cell readout (Figure 5A bottom left). Aliquots of some of the positive wells were further cultured for an additional 2 weeks on OP9-DL1 stroma and positive staining for CD4 and CD8 confirmed their T-cell lineage commitment (data not shown). We also measured the proliferative potential of single CLP^F cells that gave a positive readout and found expansion capacities of up to 8000 T cells within 10 days with an average of approximately 3500 cells generated per seeded CLP^F (Figure 5A bottom middle).

We next determined if this high progenitor potential and enormous burst size accurately reflected in vivo repopulating frequencies. We transplanted between 1 and 4 CLP^F cells intravenously into sublethally irradiated congenic mice in a limiting dilution experiment (Figure 5B left). Based on these data, we determined the progenitor frequency among CLP^F to be roughly 1 of 2 cells, thus confirming the in vitro results (Figure 5B). When very low numbers of CLP^F were transplanted, donor cells were largely limited to the B lineage in the spleen and bone marrow. Occasionally, small numbers of NK cells were detectable in the spleen, but T cells were not. To more sensitively detect T-cell potential in vivo, we transplanted CLP^F by intrathymic injection and analyzed thymi 3 weeks later by FACS. Using this assay we determined the T-cell progenitor frequency among CLP^F to be 1 of 11 cells (Figure 5B right).

Although the high frequency of T- and B-cell readout from CLP^F suggests that at least some CLP^F are bipotent, we sought to show this directly at the clonal level. Using automated cell deposition, single CLP^F cells were cultured in Terasaki wells on top of OP9 cells in the presence of SCF, Flt3L, and IL-7 (experimental layout in Figure S3A). After 3 days of culture, wells containing 4 or more cells were split into separate wells containing either B cell– or T cell–promoting differentiation conditions. Clones that contained 8 or more cells were split into 4 wells containing, B-cell, T-cell, NK-cell, or DC/myeloid conditions. After 8 additional days of culture, wells were analyzed by flow cytometry using a panel of antibodies to identify committed cells including B, T, NK, DC, and myeloid cells (see Figure S3B for representative stains). Under these conditions, 35% of single CLP^F-derived colonies gave rise to both B and T cells (Figure 5C), thus directly proving that a substantial fraction of CLP^F contains combined clonal B- and T-cell potential. When we analyzed clones that were additionally cultured under NK cell– and DC-inducing conditions we found that 28% (2/7) showed a combined readout of all 4 lymphoid lineages stemming from a single CLP^F. Under the DC promoting conditions, which also supports myeloid cell generation (compare MPP control sample in Figure S3B), no CD11c⁻ myeloid cells were detectable. Although the technical limitations of these experiments may cause lineage restriction decisions to be made during the first 3 days of culture and thus prevent 100% of CLP^F from differentiating into all 4 lineages, it clearly demonstrates that at least a portion of single CLP^F cells exist that can differentiate into all 4 lymphoid lineages, but not the myeloid lineage. To be precise, these experiments do not prove that every single CLP^F cell possesses the full lymphoid potential but it demonstrates that true clonogenic CLPs exist within this population and at least that all cells are restricted to one of the 4 lymphoid lineages.

The identification of the CLP, one of the key intermediates in lymphoid differentiation, has been a point of contention within the recent literature, with much debate as to (1) whether the T-cell potential of this population is physiologically relevant, and (2) whether or not this population is truly lymphoid committed. In this study, we address both of these issues, first by identifying key CLP markers that enable unprecedented purification of this population, and second, by performing a series of in vivo and clonal in vitro assays of lineage potential using these highly purified cells.

We demonstrate that the originally defined CLP population can be separated into 2 major subsets on the basis of Flk2 expression, and that while the Flk2⁺ subset (CLP^F) contains lymphoid oligopotent cells, the Flk2⁻ subset is mainly composed of weak B progenitors that lack T-cell potential. When transplanted in vivo, CLP^F have all the hallmarks of a lymphoid committed progenitor that gives rise equally well to both B and T lineages. Even a single intravenously transplanted CLP^F can give rise to B cells in the spleen, and as few as 10 CLP^F intrathymically injected give rise to T cells in the thymus. These numbers illustrate a remarkably high efficiency of CLP^F to engraft and differentiate into both B and T lineages in vivo.

While in vivo assays are the gold standard for determining the lineage potential of progenitor populations, in vitro assays on single cells must be performed to determine clonal lineage potentials. When plated as single cells in conditions that enabled differentiation into multiple lymphoid and myeloid lineages, single CLP^F gave rise to T, B, NK, and DC lineages, but did not give rise to any macrophages or neutrophils. Approximately 50% of single CLP^F clones gave rise to both B- and T-lineage cells in a clonal assay, and while this does not definitively prove that every single phenotypic CLP^F contains the full lymphoid potential, it is likely that the frequency of oligopotent cells is much higher than this assay reveals. Nevertheless, this assay indicates that the CLP^F population contains a significant proportion of cells that are lymphoid committed and lymphoid oligopotent at the single cell level.

One of the disputed properties of the CLP is whether its T-lineage potential is physiologically relevant. Because Flk2 expression enriched for cells with T potential within the previously defined CLP population, it was important to revisit this issue. When 1000 CLP^F were transplanted intravenously, these cells contributed more than 50% of recipient's thymocytes and produced around 2 to 8 × 10⁷ thymocytes at 3 weeks after transplantation. At 4 weeks after transplantation, after CLP^F-derived thymic T cells have matured and exited the thymus, the contribution of CLP^F-derived T cells to the spleen was numerically similar to the B lineage contribution. Moreover, the clonal frequency and burst size of CLP^F that gave rise to B and T cells in vitro were similar. By these measures, the T and B potential of CLP^F, both in vitro and in vivo, are robust and of similar magnitude.

Nevertheless, these data do not address whether CLP^F are the major source or even a requisite step of T- or B-cell development. While the large proliferative capacity and differentiation potential suggest that CLP^F are a likely candidate for this role, this question will require more detailed analysis of thymus seeding, timing of development, and progenitor progeny relationships between CLP and other putative progenitors in both the B and T lineages. Other studies have attempted to address this issue in the past through limiting dilution analysis of CLP and other candidate populations, although the CLP that were used in those studies were isolated using markers that we show here were probably inadequate. Our isolation and characterization of CLP^F strongly suggest that the markers used to purify CLP in many previous studies, specifically those that used high expression of the marker AA4.1, would lead to isolation of a population of cells highly enriched for committed B progenitors (Figure 1D). Thus, the identification and characterization of a homogeneous CLP^F population provides an explanation for previous conflicting studies regarding the lineage potential of this progenitor.

While the CLP was originally described as being lymphoid committed, this has been subject to variable results from other groups. We addressed this issue in vitro and in vivo, using CLP^F, and found that CLP^F was lymphoid committed. It should be noted that most of the studies that have identified myeloid potential within the CLP have identified cells as myeloid based solely on expression of Mac-1, which is insufficient to unequivocally identify myeloid cells. CLP^F do give rise to Mac-1⁺ cells, however, these are all either CD11c⁺ dendritic cells, or B-1b cells that coexpress Mac1. Moreover, it is well known that NK cells express low levels of Mac1.²⁷ 

CLP^F represents a homogeneous, lymphoid-restricted oligopotent progenitor that gives rise equally well to both B and T lineages. Flk2 expression has heretofore been an underappreciated hallmark of these cells. Its use enriches for T potential by depleting B-committed cells from the previously purified populations. This has enabled the clarification of the lineage potential of these cells, and as a result should aid the field in moving forward from conflicting data, largely due to the prior difficulty of isolating pure CLP. With CLP^F at the branch point between all lymphoid lineages, this will also be an ideal population of cells to study the processes of lineage commitment.

The authors thank Christina Richter and Stephanie Smith-Berdan for antibody preparations and Libuse Jerabek for excellent laboratory management.

Support to T.S. was provided though a Ruth L. Kirschstein National Research Service Award (1 F32 AI 58521). D.B. was supported by a fellowship from the Cancer Research Institute and the National Institute of Health (T32AI0729022 and 5K01DK078318), and M.A.I. was supported by a fellowship from the California Institute for Regenerative Medicine (T1-00001). This work was supported by National Institutes of Health grant AI47458 to I.L.W.

National Institutes of Health

Contribution: H.K. and I.L.W. designed the experiments; H.K., T.S., D.B., and M.A.I. performed the experiments; H.K., D.B., M.A.I., and T.S. collected and analyzed data; all authors interpreted data; H.K. wrote the paper and all authors edited it.

Conflict-of-interest disclosure: The authors declare no competing financial interests.

Correspondence: Holger Karsunky, Cellerant Therapeutics, Inc., 1561 Industrial Road, San Carlos, CA 94070; e-mail: hkarsunky@cellerant.com.