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The development of T cells from pluripotent stem cells involvesa coordinated series of lineage-commitment steps. Commonlymphoid precursors in the fetal liver or adult bone marrowmust first choose between a T, B or NK cell fate. CommittedT cell precursors in the thymus then differentiate into cellscommitted to the αβ or γδ lineages. Recent advances havebeen made in our understanding of the mechanisms underlyingT cell fate specification and αβ/γδ lineage divergence.

AddressesLudwig Institute for Cancer Research, Lausanne Branch, University ofLausanne, CH-1066 Epalinges, Switzerland*e-mail: HughRobson.MacDonald@isrec.unil.ch†e-mail: Freddy.Radtke@isrec.unil.ch‡e-mail: Anne.Wilson@isrec.unil.ch

Current Opinion in Immunology 2001, 13:219–224

0952-7915/01/$ — see front matter© 2001 Elsevier Science Ltd. All rights reserved.

AbbreviationsBM bone marrowckitR ckit receptorCLP common lymphoid precursorDC dendritic cellE10 embryonic day 10Eββ TCRβ enhancerHSC hematopoietic stem cell

IntroductionT lymphocytes, like other cells of the blood-forming sys-tem, are derived from pluripotent hematopoietic stem cells(HSCs) present in the fetal liver or adult bone marrow(BM). During T cell development, precursor cells are con-fronted with three distinct cell fate specification events[1–6]. First, a common lymphoid precursor (CLP), whichhas lost erythroid and myeloid potential but is still capableof giving rise to lymphocytes (T, B and NK cells) and pos-sibly thymic dendritic cells (DCs), must decide whether toadopt a T cell fate. Once the T cell lineage is specified, apro-T (or pre-T) cell in the thymus must choose betweenthe αβ and γδT cell lineages. Finally, thymocytes that arecommitted to the αβ lineage must differentiate alongeither the CD4+ or CD8+ mature T cell lineages.

In this brief review, we will cover recent advances con-cerning T cell fate specification from CLPs (withparticular emphasis on the inter-relationships betweenbipotential T/B, T/NK and T/DC precursors) and themechanism of αβ/γδ lineage commitment. Readers inter-ested in the CD4/CD8 lineage choice are recommended toconsult the review by Hogquist (this issue, pp 225−231).

T/B precursorsT and B lymphocytes share a remarkable number of devel-opmental properties, including ordered rearrangement oftheir antigen-receptor genes, obligatory expression of a

surrogate (invariant) component of their pre-receptors, andligand-dependent positive and negative selection of theirmature antigen-receptor repertoires. It might, therefore,be anticipated that T and B cells originate from a commonprecursor and indeed such a CD117+CD127+ (ckit recep-tor [ckitR]+IL-7Rα+) bipotential T/B progenitor has beendemonstrated at the clonal level in adult BM [7].Surprisingly, however, recent studies from Katsura and col-leagues [8•,9•], using a novel multilineage progenitor assay,have been unable to confirm the existence of clonal T/Bprecursors during fetal development. These authors wereable to detect bipotential precursors giving rise to eitherT/myeloid or B/myeloid (but not T/B) progeny as early asembryonic day 10 (E10) in the aorta-gonad-mesonephrous(AGM) region in the mouse embryo. By E12 the fetal liverwas found to contain a high frequency of T but a relative-ly low frequency of B and no detectable T/B progenitors.In contrast, E14 fetal liver contained much lower levels ofT and higher levels of B cell precursors; bipotential prog-enitors were still absent.

The decrease in numbers of T cell precursors in fetal liverbetween E12 and E14 is consistent with another recentstudy showing a selective immigration of T cell progeni-tors into the fetal thymus during the same time period[10•]. Thus it seems likely that T cell precursors migratefrom the fetal liver to the fetal thymus (presumably via thefetal blood [11]) preferentially between E12 and E14.

Although important for the measurement of developmen-tal potential at the clonal level, in vitro precursor assayshave the limitation that they cannot assess whether a prog-enitor cell would ever adopt a corresponding fate in vivo. Inthis respect, alternative approaches to investigate in vivocell fate determination are necessary. One such approachinvolves the identification of genes that are responsible forcell fate specification in a particular developmental lin-eage. Insofar as the T/B lineage choice is concerned, twoindependent types of evidence indicate that Notch-1 (atransmembrane receptor involved in multiple cell-fatedecisions in invertebrates [12]) plays an essential role.Thus, over-expression of Notch-1 in HSCs leads to ectopicdevelopment of immature T cells in the BM and concomi-tant inhibition of B cell development at an early stage [13].In reciprocal experiments, inactivation of Notch-1 in BMstem cells led to an early block in T cell development inthe thymus that was accompanied by ectopic developmentof immature thymic B cells [14].

These complementary loss-of-function and gain-of-functionexperiments provide compelling evidence that Notch-1plays an obligatory role in the commitment of a bipotentialT/B precursor to the T cell lineage. In the absence ofNotch-1 signalling, bipotential T/B precursors cannot be

T cell fate specification and ααββ/γγδδ lineage commitmentH Robson MacDonald*, Freddy Radtke† and Anne Wilson‡

induced to adopt a T cell fate in the thymus and insteaddevelop as B cells (Figure 1). Interestingly, although themajority of B lymphopoiesis in the adult occurs in BM, a low(but significant) level of steady-state B lymphopoiesis canbe detected in the normal adult thymus [15•], raising thepossibility that an inductive mechanism for determiningT cell fate may be inherently slightly leaky.

An inductive mechanism of T cell fate specification (assuggested by studies of Notch-1) could, in theory, providean explanation for certain apparent discrepancies betweenfetal and adult lymphoid development. As mentionedabove, bipotential T/B precursors could not be detected inthe AGM or fetal liver although they have been demon-strated in adult BM. Moreover, a reciprocal quantitativerelationship between T and B cell precursors was observedbetween E12 and E14 in the fetal liver.

Although these findings were interpreted by the authors[8•,9•] to mean that T and B cell development occurs inde-pendently of a bipotential precursor during fetaldevelopment, it remains possible that bipotential T/B pre-cursors present in the fetal liver become committed to theT cell lineage only during a narrow time-window betweenE12 and E14. According to this scenario, the high frequen-cy of T and low frequency of B cell precursors in the fetalliver on E12 would reflect efficient T cell commitment ofbipotential progenitors. However, the reverse situation onE14 would result from a lack of appropriate inductive sig-nals for T cell lineage commitment and consequentaccumulation of cells committed to the B lineage.

Obviously this model would imply that commitment ofbipotential T/B precursors to the T cell lineage can occurextrathymically (i.e. in the fetal liver) during embryonicdevelopment. In contrast, definitive T cell fate specifica-tion in the adult may only occur after CLPs have migratedfrom the BM to the thymus.

T cell fate specification in the gutAlthough the thymus is the major anatomical site of T celldevelopment, a distinct lineage of extrathymically derivedT cells is present in the adult gut mucosa. Theseextrathymic T cells, which can be distinguished by theirsurface phenotype and selection requirements, are derivedfrom CD117+ precursors present in specialized stucturesknown as ‘cryptopatches’ in the lamina propria [16,17•].Interestingly, Notch-1-deficient BM precursor cells areunable to reconstitute extrathymic T cell generation in thegut, indicating that both conventional and extrathymicT cell development are critically dependent upon induc-tive Notch-1 signalling [18•]. In contrast to the thymus,however, immature B cells are not seen in the gut in theabsence of Notch-1, suggesting that the gut microenviron-ment (unlike that of the thymus) is not permissive forectopic B cell development. Moreover Notch-1–/– putativelymphoid precursors expressing CD117 are present at rel-atively much higher levels in the gut than in the thymus[18•]. Collectively these data suggest that bipotential T/Bprecursors that do not receive an inductive Notch-1 signaladopt a B cell fate in the thymus but become develop-mentally arrested in the gut (Figure 1).

T/NK precursorsAnother potential fate that can be adopted by CLPs is NKcells. Indeed, earlier studies [19] have shown that a subsetof mouse fetal thymocytes expressing the Fc receptor cangive rise to both T cells and NK cells, and more recent dataindicate that CD44+CD25–NK1.1+CD117+ fetal thymo-cytes likewise exhibit bipotential T/NK precursor activity[20]. However, neither of these studies was confirmed atthe clonal level and it remained possible that T and NKcells are derived from distinct (although phenotypicallyindistinguishable) progenitors in the fetal thymus. Thesedoubts have now been eliminated by two recent studiesdemonstrating a clonal origin of bipotential T/NK precur-sors in the fetal thymus. Thus, Ikawa et al. [21•] used anadaptation of their multilineage progenitor assay (involv-ing addition of exogenous cytokines) to reveal clonalT/NK precursors among CD44+CD25– E14 fetal thymo-cytes. Likewise Michie et al. [22•] used a fetal thymicorgan culture reconstitution assay to demonstrate thatCD117+NK1.1+CD25– E14 fetal thymus progenitors couldgive rise clonally to both T and NK cells. It should benoted that the existence of a bipotential T/NK precursorhad already been demonstrated clonally some time ago inthe human fetal thymus [23].

Although bipotential T/NK precursors are clearly demon-strable in E14 murine fetal thymus, no comparable

220 Lymphocyte development

Figure 1

Role of Notch-1 in T/B cell fate specification in the thymus and gut.HSCs give rise to CLPs in the bone marrow. CLPs migrate to thethymus or gut, where they receive an instructive signal from Notch-1 toadopt a T cell fate. In the absence of Notch-1 signalling, CLPs in thethymus adopt a B cell fate by default whereas CLPs in the gut cannotadopt a B cell fate (due to the absence of appropriatemicroenvironmental cues) and therefore persist as developmentallyarrested cells.

T

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+ Notch-1

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Current Opinion in Immunology

progenitor cell has yet been isolated in the adult. Indeed,it is clear that the vast majority of adult NK cells arise inthe BM and only a very minor subset (0.1%) of cells withan NK phenotype can be detected in the adult thymus.The relationship of these latter cells to the T cell lineageremains unclear. In this regard, it is of interest that thymicNK cells develop normally from Notch-1-deficient BMprogenitors (A Wilson, F Radtke, HR MacDonald, unpub-lished data) despite the fact that putative bipotentialthymic T/NK precursors (CD44+CD25−CD117+) areessentially absent. Thus, it remains possible that bipoten-tial T/NK precursors represent a developmental stage thatis restricted to fetal development.

T/DC precursorsPerhaps the most controversy in the field of lymphoid cell fatedetermination concerns whether a CLP can become a thymicDC. Although a number of studies have demonstrated thatpopulations of CD44+CD25−CD117+ early intrathymic pre-cursors can give rise to thymic DCs as well as T cells, bothin vivo and in vitro [24,25], it has not been possible to validatethe concept of a T/DC precursor at the clonal level.

Two recent studies have now challenged the concept of acommon origin of thymic DCs and T cells, by examiningthe effect of mutations that block intrathymic T cell devel-opment at a very early stage. In one report, Rodewald et al.[26•] have analyzed mice doubly deficient for the growthfactor receptors, CD117 and CD132 (γc [common cytokineγ chain]). Despite the fact that such mice have no demon-strable CD44+CD25–CD117+ intrathymic precursors,thymic DC development is relatively normal (althoughCD8α expression is lower than usual). In an independentapproach, Radtke et al. [27•] have shown that radiationchimeras reconstituted with BM precursors deficient inNotch-1 give rise to normal thymic DC populations (interms of both absolute number and phenotype), despitethe fact that they are totally unable to generate T cells. Aswith the CD117/CD132-deficient mice, early intrathymicprecursors are essentially absent in the Notch-1-deficientchimeras. Taken together, these two studies argue strong-ly that the intrathymic development of T cells and DCscan be dissociated and hence (by implication) that a com-mon T/DC precursor may not exist.

Cell fate plasticity of CLPsVery little is known about the mechanism of cell fate spec-ification during development. One plausible model wouldbe that maturation of a multipotential precursor cell alonga particular developmental pathway is accompanied byprogressive repression of expression of genes compatiblewith other cell fates [28]. In this context, it is of consider-able interest that Kondo et al. [29•] have shown thatclonogenic CLPs in the adult BM (which normally giverise exclusively to T, B and NK cells) can be redirected tothe myeloid lineage by stimulation through an ectopicallyexpressed cytokine receptor, IL-2Rβ. Myeloid differentia-tion of CLPs in this model correlates with IL-2-induced

upregulation of expression of myeloid-lineage-specificgrowth-factor receptors (GM-CSFR and M-CSFR) that arenormally extinguished as HSCs mature into CLPs. Animportant implication of this study is that specification oflymphoid cell fates in CLPs may depend upon the repres-sion of expression of myeloid-lineage-specific genes.

ααββ//γγδδ lineage commitmentFollowing definitive commitment to the T cell lineage,intrathymic T cell precursors (CD44+CD25–CD117+)express CD25 and begin to rearrange and express theirTCR β, γ and δ genes. Cells that successfully rearrangeTCRγ and TCRδ express a γδTCR and can proceed alongthe γδ lineage pathway. Similarly, cells that successfullyrearrange TCRβ express a pre-TCR (formed by associa-tion of TCRβ with the invariant pTα chain) and are ableto differentiate along the αβ lineage. The extent to whichthe γδ TCR and pre-TCR play an instructive (as opposedto a selective) role in the αβ/γδ lineage commitmentprocess remains controversial [1–4]. According to instruc-tive models, expression of a γδ TCR or pre-TCR per se issufficient to direct a bipotential precursor cell to adopt a γδor αβ T cell fate. In contrast, selective models assume thatαβ/γδ cell fate is determined independently of the TCRbut that appropriate TCR (or pre-TCR) signalling isrequired for survival and subsequent differentiation alongthe predetermined lineage pathway.

Evidence favoring the selective model of αβ/γδ lineagecommitment has previously been obtained from the analy-sis of mutant mouse strains unable to assemble a pre-TCR.Thus, in TCRβ-deficient mice, a small number of thymo-cytes committed to the αβ lineage (readily identified bytheir CD4+CD8+ phenotype) still develop; nevertheless,the development of these cells apparently requires anintact γδTCR, since it is abolished in TCRβ×TCRδ-defi-cient mice [30]. More direct evidence that thymocytescommitted to the αβ lineage in TCRβ–/– mice develop viaa γδ TCR has been obtained by quantitating the frequen-cy of productive TCRγ and TCRδ rearrangements in thesecells. Whereas TCRγ and TCRδ rearrangements are large-ly nonproductive in CD4+CD8+ thymocytes of wild-typemice [31–33], such rearrangements are mainly productivein CD4+CD8+ thymocytes of TCRβ–/– mice [31].

These findings have been confirmed and extended in arecent study of mice deficient for the TCRβ enhancer(Eβ) [34•]. Like TCRβ–/– mice, Eβ-deficient mice show asevere developmental block at the pre-T (CD25+CD44–)stage due to an inability to rearrange TCRβ genes. As inTCRβ–/– mice, thymocytes committed to the αβ lineage(CD4+CD8+) arise at low levels in Eβ-deficient mice.Development of these cells requires a γδ TCR (i.e. it isabsent in Eβ×TCRδ-deficient mice) and analysis of TCRγand TCRδ rearrangements in CD4+CD8+ thymocytes ofEβ-deficient mice again reveals a marked enrichment forproductive rearrangements. Taken together with earlierstudies, these data are consistent with the hypothesis that

T cell fate specification and ααββ/γγδδ lineage commitment MacDonald, Radtke and Wilson 221

the γδTCR can (at least to some extent) substitute for thepre-TCR during αβ lineage development.

Although it is relatively well established that the γδ TCRcan promote αβ lineage development, reciprocal evidencethat the αβ TCR can promote γδ lineage development ismore limited [35,36] due to the fact that there is no defin-itive phenotypic marker (other than the γδTCR itself) forthymocytes committed to the γδ lineage. This concept ofαβ TCR promotion of γδ lineage development has beenreinforced by a recent report by Terrence et al. [37•]. Theseauthors demonstrate that CD4–CD8– thymocytes develop-ing in several strains of TCRαβ-transgenic mice probablyrepresent cells committed to the γδ lineage, as assessed bya variety of phenotypic and functional parameters (includ-ing co-expression of αβ and γδ TCRs in some instances).On the basis of these data, the authors conclude that earlyexpression of a TCRαβ transgene suppresses TCRγ andTCRδ rearrangements but still allows precursor cells com-mitted to the γδ lineage to proceed along a (presumablypre-determined) developmental pathway. If confirmed,these data, in conjunction with other findings, would beconsistent with a model in which αβ/γδ lineage commit-ment is determined prior to (or independently of) TCRgene rearrangement.

More direct evidence that the αβ/γδ lineage decisionoccurs independently of TCR signalling comes fromrecent analysis of the developmental potential ofCD44+CD25+CD117+ pro-T cells. Immature thymocytesat this developmental stage have undergone very limitedTCRβ, TCRγ and TCRδ rearrangements, and are veryheterogeneous in their expression of CD127. Taking

advantage of the latter parameter, Raulet and Kang (assummarized in [38]) sorted CD44+CD25+ thymocytes intoCD127hi and CD127lo subsets and assessed the ability ofthe sorted cells to give rise to αβ or γδ lineages followingintrathymic transfer or repopulation of fetal thymic organcultures. In both assays, the CD127lo subset was biasedtowards αβ T cell development whereas the CD127hi sub-set gave rise preferentially to γδ cells. Since thesedevelopmental outcomes did not correlate with levels ofTCR rearrangement in the sorted cells, the authors con-clude that αβ/γδcell fate determination is (at least in part)independent of TCR-mediated signals. An obvious impli-cation of this study is that αβ and γδ lineages alreadydiverge at the pro-T cell stage (Figure 2). In this regard itis interesting that a detailed analysis of intracellularTCRγδstaining in immature thymocytes [39•] identified asubset of CD25–CD44– intracellular TCRγδ+ cells butfailed to detect intracellular TCRγδ in less mature pro-T(CD44+CD25+) or pre-T (CD44–CD25+) cells. Thus itremains possible that pro-T cells that are committed to theγδ lineage rapidly downregulate CD44 and CD25 and dif-fentiate into γδ cells without passing through aconventional pre-T stage (Figure 2).

Irrespective of the mechanism of lineage commitment, thefact that T cell maturation along the αβ and γδlineages canoccur (at least in some instances) in the absence of the ‘cor-rect’ TCR raises the question of the specificity of γδTCRand pre-TCR signalling during development. In this con-text, recent evidence suggests that the pre-TCR differsfrom the γδ TCR in that the former (but not the latter)spontaneously localizes in membrane rafts and initiatessignalling independently of ligation [40•].

Although these data provide a biochemical rationale for thefact that the pre-TCR has no apparent ligand, they fail toaddress the question of how γδ TCR and pre-TCR sig-nalling could lead to distinct developmental outcomes.Indeed, Petrie et al. [41•] argue in a recent study that thepre-TCR is not required for certain hallmarks of αβ lineagecommitment, such as proliferation and progression to theCD4+CD8+ stage of thymic development. According totheir model, pre-TCR signalling would only be required forcells committed to the αβ lineage to pass a critical cell-sur-vival checkpoint at the pre-T cell (CD25+CD44–) stage.Obviously, similar arguments could be applied to the role ofγδTCR signalling in γδlineage progression. Nonetheless, itis clear that γδ TCR and pre-TCR signalling cannot bedevelopmentally equivalent since the rescue of αβ lineagecells by the γδTCR is extremely inefficient.

ConclusionsThe molecular basis of cell fate determination remains oneof the fundamental unsolved questions of developmentalbiology. The lymphoid system is an excellent model toaddress this issue since a single precursor (a CLP) can giverise to at least three independent lineages (T, B and NKcells), which are very well characterized both phenotypically

222 Lymphocyte development

Figure 2

Divergence of the αβ and γδ lineages during thymus development.According to this model, pro-T (CD44+CD25+) cells expressing highor low levels of CD127 (filled squares) are already biased to the γδorαβ lineages, respectively, prior to TCR rearrangement. Progressionalong the γδor αβ lineage pathways is normally assured by signallingvia the γδTCR or pre-TCR (intracellular [ic] γδ+ or ic β+, respectively)although inefficient development can be rescued by ‘inappropriate’TCR molecules. Note that cells committed to the γδ lineage may notpass through a pre-T (CD44—CD25+) stage.

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Current Opinion in Immunology

and functionally. As outlined in this brief review, consider-able progress has been made in delineating thedevelopmental potential of CLPs in both fetal ontogeny andadult steady-state hematopoiesis. Convincing evidence nowexists that CLPs can give rise to either T and B or T and NKcells at the clonal level. Moreover, although characterizationof the molecular mechanisms responsible for committingCLPs to particular lineages remains for the most partobscure, a pivotal role for Notch-1 signalling in the T/B lineage choice has been identified in vivo.

With respect to the mechanism underlying the αβ/γδ lin-eage decision, data obtained within the past year favormodels in which lineage commitment occurs (at least inpart) independently of γδ TCR or pre-TCR signalling. Asignificant advance in this area is the demonstration thatpro-T cells can be separated (on the basis of levels ofCD127 expression) into subsets that are intrinsicallybiased in their developmental potential towards either theαβ or γδT cell lineages. Further refinement in the defini-tion of these subsets will hopefully lead to a basis forinvestigating the mechanism of αβ/γδlineage commitmentat the molecular level.

References and recommended readingPapers of particular interest, published within the annual period of review,have been highlighted as:

• of special interest••of outstanding interest

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2. Kang J, Raulet DH: Events that regulate differentiation of ααββ TCR++

and γγδδ TCR++ T cells from a common precursor. Semin Immunol1997, 9:171-179.

3. MacDonald HR, Wilson A: The role of the T-cell receptor (TCR) inααββ/γγδδ lineage commitment: clues from intracellular TCR staining.Immunol Rev 1998, 165:87-94.

4. Robey E, Fowlkes BJ: The ααββ versus γγδδ T-cell lineage choice. CurrOpin Immunol 1998, 10:181-187.

5. Rodewald HR, Fehling HJ: Molecular and cellular events in earlythymocyte development. Adv Immunol 1998, 69:1-112.

6. Rothenberg EV: Stepwise specification of lymphocytedevelopmental lineages. Curr Opin Genet Dev 2000, 10:370-379.

7. Kondo M, Weissman IL, Akashi K: Identification of clonogeniccommon lymphoid progenitors in mouse bone marrow. Cell 1997,91:661-672.

8. Ohmura K, Kawamoto H, Fujimoto S, Ozaki S, Nakao K, Katsura Y:• Emergence of T, B, and myeloid lineage-committed as well as

multipotent hemopoietic progenitors in the aorta-gonad-mesonephros region of day ten fetuses of the mouse. J Immunol1999, 163:4788-4795.

Using an elegant in vitro clonal assay, the authors show that at embryonicday 10 the AGM contains a small number of progenitors already committedto the myeloid, T or B cell lineages, as well as multipotent precursors.Therefore, in addition to being a site of stem cell production, the AGM iscapable of limited lymphopoiesis.

9. Kawamoto H, Ikawa T, Ohmura K, Fujimoto S, Katsura Y: T cell• progenitors emerge earlier than B cell progenitors in the murine

fetal liver. Immunity 2000, 12:441-450.Using the same assay as [8•], the authors demonstrate that a CD127+ (lin–)E12 (embryonic day 12) fetal liver population contains clonal precursors giv-ing rise to T/myeloid or B/myeloid progeny but no detectable bipotential T/Bprogenitors. Whereas T cell precursors predominate on E12, B cell precur-sors are more frequent on E14. The authors conclude that T and B lineagecommitment occurs independently of a bipotential T/B progenitor.

10. Douagi I, Andre I, Ferraz JC, Cumano A: Characterization of T cell• precursor activity in the murine fetal thymus: evidence for an

input of T cell precursors between days 12 and 14 of gestation.Eur J Immunol 2000, 30:2201-2210.

The authors show that, whereas embryonic day 14 (E14) thymic precursorscan efficiently give rise to T lineage cells both in vitro and in vivo, E12 thy-mocytes have a much lower precursor frequency. These results suggest thatthe earliest thymic immigrants (E12) do not significantly contribute to T cellgeneration whereas those entering the fetal thymus around E13 and E14have a much higher precursor potential.

11. Rodewald HR, Kretzschmar K, Takeda S, Hohl C, Dessing M:Identification of pro-thymocytes in murine fetal blood: T lineagecommitment can precede thymus colonization. EMBO J 1994,13:4229-4240.

12. Artavanis-Tsakonas S, Rand MD, Lake RJ: Notch signaling: cell fatecontrol and signal integration in development. Science 1999,284:770-776.

13. Pui JC, Allman D, Xu L, DeRocco S, Karnell FG, Bakkour S, Lee JY,Kadesch T, Hardy RR, Aster JC, Pear WS: Notch1 expression inearly lymphopoiesis influences B versus T lineage determination.Immunity 1999, 11:299-308.

14. Radtke F, Wilson A, Stark G, Bauer M, van Meerwijk J, MacDonald HR,Aguet M: Deficient T cell fate specification in mice with an inducedinactivation of Notch1. Immunity 1999, 10:547-558.

15. Akashi K, Richie LI, Miyamoto T, Carr WH, Weissman IL: B• lymphopoiesis in the thymus. J Immunol 2000, 164:5221-5226.Although the thymus is primarily a site of T cell differentiation, a small num-ber of B cells are produced and exported from the normal adult thymus. Thisstudy identifies a small population of immature B cells located in the cortexwhereas more mature B cells are found at the corticomedullary junction andin the medulla.

16. Saito H, Kanamori Y, Takemori T, Nariuchi H, Kubota E, Takahashi-Iwanaga H, Iwanaga T, Ishikawa H: Generation of intestinal T cellsfrom progenitors residing in gut cryptopatches. Science 1998,280:275-278.

17. Oida T, Suzuki K, Nanno M, Kanamori Y, Saito H, Kubota E, Kato S,• Itoh M, Kaminogawa S, Ishikawa H: Role of gut cryptopatches in

early extrathymic maturation of intestinal intraepithelial T cells.J Immunol 2000, 164:3616-3626.

The authors demonstrate a role for gut lamina propria cryptopatches in sup-porting the generation of intestinal lymphocyte (IEL) populations from pre-cursors in the gut. In CD132-deficient mice that lack gut cryptopatches, onlya few immature precursors are present in the gut and extrathymic T celldevelopment is severely inhibited. In contrast, in SCID mice — where cryp-topatches develop normally — two major immature precursor populations(TCR–CD8αα+ and TCR–CD8αα–) can be identified in the gut, suggestinga role for cryptopatches in early maturation of CD8αα+ IELs.

18. Wilson A, Ferrero I, MacDonald HR, Radtke F: An essential role for• Notch-1 in the development of both thymus-independent and

-dependent T cells in the gut. J Immunol 2000, 165:5397-5400.These authors demonstrate that Notch-1 is essential for both thymic andextrathymic T cell fate specification and suggest that bipotential T/B precur-sors that do not receive a Notch-1 signal adopt a B cell fate in the thymusbut are developmentally arrested in the gut.

19. Rodewald HR, Moingeon P, Lucich JL, Dosiou C, Lopez P,Reinherz EL: A population of early fetal thymocytes expressingFcγγRII/III contains precursors of T lymphocytes and natural killercells. Cell 1992, 69:139-150.

20. Carlyle JR, Michie AM, Cho SK, Zuniga-Pflucker JC: Natural killercell development and function precede ααββ T cell differentiation inmouse fetal thymic ontogeny. J Immunol 1998, 160:744-753.

21. Ikawa T, Kawamoto H, Fujimoto S, Katsura Y: Commitment of• common T/natural killer (NK) progenitors to unipotent T and NK

progenitors in the murine fetal thymus revealed by a singleprogenitor assay. J Exp Med 1999, 190:1617-1625.

See annotation to [22•].

22. Michie AM, Carlyle JR, Schmitt TM, Ljutic B, Cho SK, Fong Q, Zuniga• Pflucker JC: Clonal characterization of a bipotent T cell and NK

cell progenitor in the mouse fetal thymus. J Immunol 2000,164:1730-1733.

This work, together with [21•], uses clonal assays to demonstrate that earlyfetal thymocytes (embryonic day 14) still contain bipotential T/NK precur-sors. The potential to produce NK cells is completely lost by the pro-T cell(CD44+CD25+) stage.

T cell fate specification and ααββ/γγδδ lineage commitment MacDonald, Radtke and Wilson 223

23. Sanchez MJ, Muench MO, Roncarolo MG, Lanier LL, Phillips JH:Identification of a common T/natural killer cell progenitor inhuman fetal thymus. J Exp Med 1994, 180:569-576.

24. Ardavin C, Wu L, Li CL, Shortman K: Thymic dendritic cells andT cells develop simultaneously in the thymus from a commonprecursor population. Nature 1993, 362:761-763.

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factor receptor mutant mice. Proc Natl Acad Sci USA 1999,96:15068-15073.

In a CD117—/—CD132—/— mouse model lacking all lymphoid cells in the thy-mus, thymic DCs are produced independently of pro-T cells. However,thymic DCs lack CD8α expression in the absence of more mature T cells.

27. Radtke F, Ferrero I, Wilson A, Lees R, Aguet M, MacDonald HR:• Notch1 deficiency dissociates the intrathymic development of

dendritic cells and T cells. J Exp Med 2000, 191:1085-1093.After bone marrow reconstitution with Notch-1-deficient precursors, thymicDCs (as well as all subsets of peripheral DCs) are fully reconstituted, despitethe absence of mature T cells and T cell progenitors. These results demonstratethat the development of DCs is totally independent of Notch-1 function andstrongly suggest a dissociation between intrathymic T cell and DC precursors.

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29. Kondo M, Scherer DC, Miyamoto T, King AG, Akashi K, Sugamura K,• Weissman IL: Cell-fate conversion of lymphoid-committed progenitors

by instructive actions of cytokines. Nature 2000, 407:383-386.The authors show that lymphocyte-committed bone marrow precursors thatgive rise exclusively to T, B and NK cells can be redirected to the myeloid lin-eage by stimulation through an ectopically expressed IL-2 receptor. WhereasGM-CSF receptors and M-CSF receptors are expressed on hematopoieticstem cells, they are absent on common lymphoid precursors (CLPs) but can beupregulated when CLPs are artificially induced by IL-2 to the myeloid lineage.The authors propose that cytokine signalling can regulate cell fate decisions andthat a first step in lymphoid commitment is downregulation of cytokine recep-tors that normally drive myeloid development.

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In Eβ-deficient mice, TCRβ rearrangement is abolished and the limited dif-ferentiation of precursors committed to the αβ lineage into CD4+CD8+ cellsdepends on cell-autonomous expression of γδ TCR. However, in contrast toTCRβ-deficient mice, a small percentage of surface-γδTCR+ CD4+CD8+

thymocytes (differing in T-cell-specific gene expression and RAG activityfrom αβ lineage CD4+CD8+ cells) are produced.

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rearrangement but permits development of γγδδ lineage T cells.J Exp Med 2000, 192:537-548.

In several different TCRαβ-transgenic mouse models, early expression ofTCRαβ suppresses TCRγδ gene rearrangements. In addition to mature αβT cells, CD4−CD8− cells expressing the transgenic TCR develop but exhibitproperties of γδ lineage cells. In contrast, co-expression of endogenousTCRγδ and transgenic TCRαβ can occur when the αβ TCR is expressedlater in development. The results suggest that TCRαβ can substitute forTCRγδ during γδ lineage commitment and/or maturation.

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development. Int Immunol 1999, 11:1641-1650.Using a combination of surface and intracellular FACS (fluorescence-acti-vated cell sorter) analysis, the authors show that intracellular TCRβ and(more surprisingly) intracellular CD3ε proteins are first detected at the pre-Tcell (CD44−CD25+) stage of thymic development. In addition, a CD44−

CD25− γδ precursor expressing intracellular TCRγδ is identified.

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Nature 2000, 406:524-527.Using biochemistry and confocal microscopy, the authors demonstrate thatthe pre-TCR (in contrast to the γδ-TCR) is localized in membrane rafts andundergoes constitutive signalling. These findings provide an explanation forthe fact that the pre-TCR has no apparent ligand.

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unrelated to the pre-TCR. J Immunol 2000, 165:3094-3098.The authors demonstrate that control of mitogenesis and differentiation ofimmature thymocytes occur independently of TCRβ gene recombination andpre-TCR expression but are absolutely dependent on normal thymic archi-tecture and cellular composition. They speculate that the pre-TCR regulatesa cell survival checkpoint at the pre-T cell stage.

224 Lymphocyte development