Fc gamma RII/III and CD2 expression mark distinct subpopulations of immature CD4-CD8- murine thymocytes: in vivo developmental kinetics and T cell receptor beta chain rearrangement status

Stepwise progression of β-selection during T cell development involves histone deacetylation

During T cell development, the first step in creating a unique T cell receptor (TCR) is genetic recombination of the TCRβ chain. The quality of the new TCRβ is assessed at the β-selection checkpoint. Most cells fail this checkpoint and die, but the coordination of fate at the β-selection checkpoint is not yet understood. We shed new light on fate determination during β-selection using a selective inhibitor of histone deacetylase 6, ACY1215. ACY1215 disrupted the β-selection checkpoint. Characterising the basis for this disruption revealed a new, pivotal stage in β-selection, bookended by up-regulation of TCR co-receptors, CD28 and CD2, respectively. Within this "DN3b^Pre" stage, CD5 and Lef1 are up-regulated to reflect pre-TCR signalling, and their expression correlates with proliferation. These findings suggest a refined model of β-selection in which a coordinated increase in expression of pre-TCR, CD28, CD5 and Lef1 allows for modulating TCR signalling strength and culminates in the expression of CD2 to enable exit from the β-selection checkpoint.

Salt inducible kinases 2 and 3 are required for thymic T cell development

Salt Inducible Kinases (SIKs), of which there are 3 isoforms, are established to play roles in innate immunity, metabolic control and neuronal function, but their role in adaptive immunity is unknown. To address this gap, we used a combination of SIK knockout and kinase-inactive knock-in mice. The combined loss of SIK1 and SIK2 activity did not block T cell development. Conditional knockout of SIK3 in haemopoietic cells, driven by a Vav-iCre transgene, resulted in a moderate reduction in the numbers of peripheral T cells, but normal B cell numbers. Constitutive knockout of SIK2 combined with conditional knockout of SIK3 in the haemopoietic cells resulted in a severe reduction in peripheral T cells without reducing B cell number. A similar effect was seen when SIK3 deletion was driven via CD4-Cre transgene to delete at the DP stage of T cell development. Analysis of the SIK2/3 Vav-iCre mice showed that thymocyte number was greatly reduced, but development was not blocked completely as indicated by the presence of low numbers CD4 and CD8 single positive cells. SIK2 and SIK3 were not required for rearrangement of the TCRβ locus, or for low level cell surface expression of the TCR complex on the surface of CD4/CD8 double positive thymocytes. In the absence of both SIK2 and SIK3, progression to mature single positive cells was greatly reduced, suggesting a defect in negative and/or positive selection in the thymus. In agreement with an effect on negative selection, increased apoptosis was seen in thymic TCRbeta high/CD5 positive cells from SIK2/3 knockout mice. Together, these results show an important role for SIK2 and SIK3 in thymic T cell development.

CD2 Immunobiology

The glycoprotein CD2 is a costimulatory receptor expressed mainly on T and NK cells that binds to LFA3, a cell surface protein expressed on e.g., antigen-presenting cells. CD2 has an important role in the formation and organization of the immunological synapse that is formed between T cells and antigen-presenting cells upon cell-cell conjugation and associated intracellular signaling. CD2 expression is upregulated on memory T cells as well as activated T cells and plays an important role in activation of memory T cells despite the coexistence of several other costimulatory pathways. Anti-CD2 monoclonal antibodies have been shown to induce immune modulatory effects in vitro and clinical studies have proven the safety and efficacy of CD2-targeting biologics. Investigators have highlighted that the lack of attention to the CD2/LFA3 costimulatory pathway is a missed opportunity. Overall, CD2 is an attractive target for monoclonal antibodies intended for treatment of pathologies characterized by undesired T cell activation and offers an avenue to more selectively target memory T cells while favoring immune regulation.

ISWI ATPase Smarca5 Regulates Differentiation of Thymocytes Undergoing β-Selection

Development of lymphoid progenitors requires a coordinated regulation of gene expression, DNA replication, and gene rearrangement. Chromatin-remodeling activities directed by SWI/SNF2 superfamily complexes play important roles in these processes. In this study, we used a conditional knockout mouse model to investigate the role of Smarca5, a member of the ISWI subfamily of such complexes, in early lymphocyte development. Smarca5 deficiency results in a developmental block at the DN3 stage of αβ thymocytes and pro-B stage of early B cells at which the rearrangement of Ag receptor loci occurs. It also disturbs the development of committed (CD73⁺) γδ thymocytes. The αβ thymocyte block is accompanied by massive apoptotic depletion of β-selected double-negative DN3 cells and premitotic arrest of CD4/CD8 double-positive cells. Although Smarca5-deficient αβ T cell precursors that survived apoptosis were able to undergo a successful TCRβ rearrangement, they exhibited a highly abnormal mRNA profile, including the persistent expression of CD44 and CD25 markers characteristic of immature cells. We also observed that the p53 pathway became activated in these cells and that a deficiency of p53 partially rescued the defect in thymus cellularity (in contrast to early B cells) of Smarca5-deficient mice. However, the activation of p53 was not primarily responsible for the thymocyte developmental defects observed in the Smarca5 mutants. Our results indicate that Smarca5 plays a key role in the development of thymocytes undergoing β-selection, γδ thymocytes, and also B cell progenitors by regulating the transcription of early differentiation programs.

Modeling the development of the post-natal mouse thymus in the absence of bone marrow progenitors

Many mathematical models have been published with the purpose of explaining aspects of T-cell development in the thymus. In this manuscript we adapted a four-compartment model of the thymus and used a range of mathematical approaches with the aim of explaining the dynamics of the four main thymocyte populations in the mouse thymus, from the emergence of the first fetal thymocyte until the death of the animal. At various pre-natal and post-natal stages we investigated experimentally the number and composition of thymocytes populations, their apoptosis and proliferation, along with data from literature, to create and validate the model. In our model the proliferation processes are characterized by decreasing proliferation rates, which allows us to model the natural involution of the thymus. The best results were obtained when different sets of parameters were used for the fetal and post-natal periods, suggesting that birth may induce a discontinuity in the modeled processes. Our model is able to model the development of both pre-natal and post-natal thymocyte populations. Also, our findings showed that the post-natal thymus is able to develop in the absence of the daily input of bone marrow progenitors, providing more evidence to support the autonomous development of the post-natal thymus.

Overexpression of RhoH Permits to Bypass the Pre-TCR Checkpoint

RhoH, an atypical small Rho-family GTPase, critically regulates thymocyte differentiation through the coordinated interaction with Lck and Zap70. Therefore, RhoH deficiency causes defective T cell development, leading to a paucity of mature T cells. Since there has been no gain-of-function study on RhoH before, we decided to take a transgenic approach to assess how the overexpression of RhoH affects the development of T cells. Although RhoH transgenic (RhoH^tg) mice expressed three times more RhoH protein than wild-type mice, β-selection, positive, and negative selection in the thymus from RhoH^tg mice were unaltered. However, transgenic introduction of RhoH into Rag2 deficient mice resulted in the generation of CD4⁺CD8⁺ (DP) thymocytes, indicating that overexpression of RhoH could bypass β-selection without TCRβ gene rearrangement. This was confirmed by the in vitro development of DP cells from Rag2^-/-RhoH^tg DN3 cells on TSt-4/Dll-1 stroma in an Lck dependent manner. Collectively, our results indicate that an excess amount of RhoH is able to initiate pre-TCR signaling in the absence of pre-TCR complexes.

De novo generation of cells within human nurse macrophages and consequences following HIV-1 infection

Nurse cells are defined as those that provide for the development of other cells. We report here, that in vitro, human monocyte-derived macrophages can behave as nurse cells with functional capabilities that include de novo generation of CD4+ T-lymphocytes and a previously unknown small cell with monocytoid characteristics. We named these novel cells “self-renewing monocytoid cells” (SRMC), because they could develop into nurse macrophages that produced another generation of SRMC. SRMC were not detectable in blood. Their transition to nurse behavior was characterized by expression of CD10, a marker of thymic epithelium and bone marrow stroma, typically absent on macrophages. Bromodeoxyuridine labeling and immunostaining for cdc6 expression confirmed DNA synthesis within nurse macrophages. T-cell excision circles were detected in macrophages, along with expression of pre-T-cell receptor alpha and recombination activating gene 1, suggesting that genetic recombination events associated with generation of the T-cell receptor were occurring in these cells. SRMC expressed CCR5, the coreceptor for R5 HIV-1 isolates, and were highly susceptible to HIV-1 entry leading to productive infection. While expressing HIV-1, SRMC could differentiate into nurse macrophages that produced another generation of HIV-1-expressing SRMC. The infected nurse macrophage/SRMC cycle could continue in vitro for multiple generations, suggesting it might represent a mechanism whereby HIV-1 can maintain persistence in vivo. HIV-1 infection of nurse macrophages led to a decline in CD4+ T-cell production. There was severe, preferential loss of the CCR5+ CD4+ T-cell subpopulation. Confocal microscopy revealed individual HIV-1-expressing nurse macrophages simultaneously producing both HIV-1-expressing SRMC and non-expressing CD3+ cells, suggesting that nurse macrophages might be a source of latently infected CD4+ T-cells. Real-time PCR experiments confirmed this by demonstrating 10-fold more HIV-1-genome-harboring T-cells, than virus-expressing ones. These phenomena have far-reaching implications, and elicit new perspectives regarding HIV pathogenesis and T-cell and hematopoietic cell development.

Early T cell development and the pitfalls of potential

The long-standing model for hematopoiesis, which features a dichotomy into separate lymphoid and myeloid branches, predicts that progenitor T cells arise from a lymphocyte-restricted pathway. However, experiments that have detected myeloid potential in progenitor T cells have been reported as evidence to question this model. Mapping physiological differentiation pathways has now led to opposite conclusions, by showing that T cells and thymic myeloid cells have distinct origins and that, in vivo, T cell progenitors lack significant potential for myeloid lineages including dendritic cells. Here, we review the underlying experiments that have led to such fundamentally different conclusions. The current controversy might reflect a need to distinguish between cell fates that are possible experimentally from physiological fate choices, to build a map of immunological differentiation pathways.

LKB1 is essential for the proliferation of T-cell progenitors and mature peripheral T cells

The serine/threonine kinase LKB1 has a conserved role in Drosophila and nematodes to co-ordinate cell metabolism. During T lymphocyte development in the thymus, progenitors need to synchronize increased metabolism with the onset of proliferation and differentiation to ensure that they can meet the energy requirements for development. The present study explores the role of LKB1 in this process and shows that loss of LKB1 prevents thymocyte differentiation and the production of peripheral T lymphocytes. We find that LKB1 is required for several key metabolic processes in T-cell progenitors. For example, LKB1 controls expression of CD98, a key subunit of the L-system aa transporter and is also required for the pre-TCR to induce and sustain the regulated phosphorylation of the ribosomal S6 subunit, a key regulator of protein synthesis. In the absence of LKB1 TCR-beta-selected thymocytes failed to proliferate and did not survive. LBK1 was also required for survival and proliferation of peripheral T cells. These data thus reveal a conserved and essential role for LKB1 in the proliferative responses of both thymocytes and mature T cells.

The serine/threonine kinase LKB1 controls thymocyte survival through regulation of AMPK activation and Bcl-XL expression

LKB1 is a serine/threonine kinase that directly activates the energy sensor AMP-activated protein kinase (AMPK) in response to bioenergetic stress, and mainly acts as a tumor suppressor that controls cell polarity and proliferation. Although LKB1 is expressed in multiple tissues including the thymus and the spleen, its roles in T-cell development and function remain unknown. Here, we show that T-cell-specific deletion of LKB1 resulted in reduced survival of double-positive (DP) thymocytes and impaired generation of both CD4 and CD8 single-positive thymocytes. Disruption of LKB1 not only prevented the activation of AMPK but also impaired the expression of anti-apoptotic protein Bcl-XL. Importantly, ectopic expression of either Bcl-XL or the constitutively active AMPK mutant significantly rescued DP thymocytes from LKB1 deficiency-induced cell death. Moreover, ectopic expression of the constitutively active AMPK mutant was found to restore the expression of Bcl-XL in LKB1-deficient DP thymocytes. These findings identify LKB1 as a critical factor for the survival of DP thymocytes through regulation of AMPK activation and Bcl-XL expression.

Sonic hedgehog negatively regulates pre-TCR-induced differentiation by a Gli2-dependent mechanism

Hedgehog signaling regulates differentiation, survival, and proliferation of the earliest double-negative (DN) thymocytes, but its importance at later stages of T-cell development is controversial. Here we use loss- and gain-of-function mouse models to show that Shh, by signaling directly to the developing thymocyte, is a negative regulator of pre-TCR-induced differentiation from DN to double-positive (DP) cell. When hedgehog signaling was reduced, in the Shh(-/-) and Gli2(-/-) thymus, or by T lineage-specific transgenic expression of a transcriptional-repressor form of Gli2 (Gli2DeltaC(2)), differentiation to DP cell after pre-TCR signal transduction was increased. In contrast, when Hh signaling was constitutively activated in thymocytes, by transgenic expression of a constitutive transcriptional-activator form of Gli2 (Gli2DeltaN(2)), the production of DP cells was decreased. Gene expression profiling showed that physiologic Hh signaling in thymocytes maintains expression of the transcription factor FoxA2 on pre-TCR signal transduction.

Adoptive precursor cell therapy to enhance immune reconstitution after hematopoietic stem cell transplantation in mouse and man

Hematopoietic stem cell transplantation is a curative therapy for hematological malignancies. T cell deficiency following transplantation is a major cause of morbidity and mortality. In this review, we discuss adoptive transfer of committed precursor cells to enhance T cell reconstitution and improve overall prognosis after transplantation.

B cells modulate T cells so as to favour T helper type 1 and CD8+ T-cell responses in the acute phase of Trypanosoma cruzi infection

In this study, we have evaluated the production of pro- and anti-inflammatory cytokines and the formation of central and effector memory T cells in mice lacking mature B cells (mu MT KO). The results show that Trypanosoma cruzi infection in C57Bl/6m mu MT KO mice is intensified in relation to control mice and this exacerbation is related to low levels of inflammatory cytokines produced during the acute infection and the lower numbers of central and effector memory CD4(+) and CD8(+) T cells generated during the acute phase of the infection. In addition, a marked reduction in the CD8(+) T-cell subpopulation was observed in mu MT KO infected mice. In agreement to this, the degree of tissue parasitism was increased in mu MT mice and the tissue inflammatory response was much less intense in the acute phase of the infection, consistent with a deficit in the generation of effector T cells. Flow cytometry analysis of the skeletal muscle inflammatory infiltrate showed a predominance of CD8(+) CD45Rb low in B-cell-sufficient C57Bl/6 mice, whereas the preponderant cell type in mu MT KO skeletal muscle inflammatory infiltrate was CD4(+) T cells. In addition, CD8(+) T cells found in skeletal muscle from mu MT KO infected mice were less activated than in control B-cell sufficient infected mice. These results suggest that B cells may participate in the generation of effector/memory T cells. In addition and more importantly, B cells were crucial in the maintenance of central and effector memory CD8(+) T cell, as well as the determination of the T cell cytokine functional pattern, and they may therefore account for critical aspects of the resistance to intracellular pathogens, such as T. cruzi.

PKB Rescues Calcineurin/NFAT-Induced Arrest of Rag Expression and Pre-T Cell Differentiation

Protein kinase B (PKB), an Ag receptor activated serine-threonine kinase, controls various cellular processes including proliferation and survival. However, PKB function in thymocyte development is still unclear. We report PKB as an important negative regulator of the calcineurin (CN)-regulated transcription factor NFAT in early T cell differentiation. Expression of a hyperactive version of CN induces a profound block at the CD25+CD44- double-negative (DN) 3 stage of T cell development. We correlate this arrest with up-regulation of Bcl-2, CD2, CD5, and CD27 proteins and constitutive activation of NFAT but a severe impairment of Rag1, Rag2, and intracellular TCR-beta as well as intracellular TCR-gammadelta protein expression. Intriguingly, simultaneous expression of active myristoylated PKB inhibits nuclear NFAT activity, restores Rag activity, and enables DN3 cells to undergo normal differentiation and expansion. A correlation between the loss of NFAT activity and Rag1 and Rag2 expression is also found in myristoylated PKB-induced CD4+ lymphoma cells. Furthermore, ectopic expression of NFAT inhibits Rag2 promoter activity in EL4 cells, and in vivo binding of NFATc1 to the Rag1 and Rag2 promoter and cis-acting transcription regulatory elements is verified by chromatin immunoprecipitation analysis. The regulation of CN/NFAT signaling by PKB may thus control receptor regulated changes in Rag expression and constitute a signaling pathway important for differentiation processes in the thymus and periphery.

Stepwise Development of Committed Progenitors in the Bone Marrow That Generate Functional T Cells in the Absence of the Thymus

We identified committed T cell progenitors (CTPs) in the mouse bone marrow that have not rearranged the TCRbeta gene; express a variety of genes associated with commitment to the T cell lineage, including GATA-3, T cell-specific factor-1, Cbeta, and Id2; and show a surface marker pattern (CD44+ CD25- CD24+ CD5-) that is similar to the earliest T cell progenitors in the thymus. More mature committed intermediate progenitors in the marrow have rearranged the TCR gene loci, express Valpha and Vbeta genes as well as CD3epsilon, but do not express surface TCR or CD3 receptors. CTPs, but not progenitors from the thymus, reconstituted the alphabeta T cells in the lymphoid tissues of athymic nu/nu mice. These reconstituted T cells vigorously secreted IFN-gamma after stimulation in vitro, and protected the mice against lethal infection with murine CMV. In conclusion, CTPs in wild-type bone marrow can generate functional T cells via an extrathymic pathway in athymic nu/nu mice.

Extrathymic Hemopoietic Progenitors Committed to T Cell Differentiation in the Adult Mouse

The role of the thymus in T cell commitment of hemopoietic precursor is yet controversial. We previously identified a major T cell progenitor activity in precursor cells isolated from bone marrow-derived spleen colonies. In this study, we characterize the properties of these pre-T cells. We demonstrate that they have unique phenotype and can be generated in a total absence of any thymic influence. Indeed, even when studied at the single-cell level, extrathymic T cell-committed precursors express T cell-specific genes. Moreover, these cells are not committed to a particular T cell differentiation pathway because they can generate both extrathymic CD8alphaalpha+ intraepithelial lymphocytes and thymus-derived conventional thymocytes. We also compared these pre-T cells with fully T cell-committed thymic progenitors. When tested in vitro or by direct intrathymic transfer, these cells have a low clonogenic activity. However, after i.v. transfer, thymus repopulation is efficient and these precursors generate very high numbers of peripheral T cells. These results suggest the existence of extra steps of pre-T cell maturation that improve thymus reconstitution capacity and that can be delivered even after full T cell commitment. Consequently, our studies identify a source of extrathymic progenitors that will be helpful in defining the role of the thymus in the earliest steps of T cell differentiation.

Prospective isolation and global gene expression analysis of the erythrocyte colony-forming unit (CFU-E)

The erythrocyte colony-forming unit (CFU-E) is a rare bone marrow (BM) progenitor that generates erythrocyte colonies in 48 hours. The existence of CFU-Es is based on these colonies, but CFU-Es have not been purified prospectively by phenotype. We have separated the "nonstem," "nonlymphoid" compartment (lineage marker [lin]-c-Kit+Sca-1-IL-7Ralpha-) into interleukin 3 receptor alpha negative (IL-3Ralpha-) and IL-3Ralpha+ subsets. Within IL-3Ralpha- but not IL-3Ralpha+ cells we have identified TER119-CD41-CD71+ erythrocyte-committed progenitors (EPs). EPs generate CFU-E colonies at about 70% efficiency and generate reticulocytes in vivo. Depletion of EPs from BM strongly reduces CFU-E frequencies. EPs lack potential for erythrocyte burst-forming unit, megakaryocyte, granulocyte (G), and monocyte (M) colonies, and for spleen colony-forming units. Chronically suppressed erythropoiesis in interferon consensus sequence-binding protein (ICSBP)-deficient BM is associated with reduced frequencies of both the EP population and CFU-E colonies. During phenylhydrazine-induced acute anemia, numbers of both the EP population and CFU-E colonies increase. Collectively, EPs (lin-c-Kit+Sca-1-IL-7Ralpha-IL-3Ralpha-CD41-CD71+) account for most, if not all, CFU-E activity in BM. As a first molecular characterization, we have compared global gene expression in EPs and nonerythroid GM progenitors. These analyses define an erythroid progenitor-specific gene expression pattern. The prospective isolation of EPs is an important step to analyze physiologic and pathologic erythropoiesis.

Deranged Early T Cell Development in Immunodeficient Strains of Nonobese Diabetic Mice

NOD mice exhibit defects in T cell functions that have been postulated to contribute to diabetes susceptibility in this strain. However, early T cell development in NOD mice has been largely unexplored. NOD mice with the scid mutation and Rag1 deficiency were analyzed for pre-T cell development in the NOD genetic background. These strains reveal an age-dependent, programmed breakdown in beta selection checkpoint enforcement. At 5-8 wk of age, even in the absence of TCRbeta expression, CD4+ and CD4+CD8+ blasts appear spontaneously. However, these breakthrough cells fail to restore normal thymic cellularity. The breakthrough phenotype is recessive in hybrid (NODxB6)F1-scid and -Rag1null mice. The breakthrough cells show a mosaic phenotype with respect to components of the beta selection program. They mimic normal beta selection by up-regulating germline TCR-Calpha transcripts, CD2, and Bcl-xL and down-regulating Bcl-2. However, they fail to down-regulate transcription factors HEB-alt and Hes1 and initially express aberrantly high levels of Spi-B, c-kit (CD117), and IL-7Ralpha. Other genes examined distinguish this form of breakthrough from previously reported models. Some of the abnormalities appear first in a cohort of postnatal thymocytes as early as the double-negative 2/double-negative 3 transitional stage. Thus, our results reveal an NOD genetic defect in T cell developmental programming and checkpoint control that permits a subset of the normal outcomes of pre-TCR signaling to proceed even in the absence of TCRbeta rearrangement. Furthermore, this breakthrough may initiate thymic lymphomagenesis that occurs with high frequency in both NOD-scid and -Rag1null mice.

The serine kinase phosphoinositide-dependent kinase 1 (PDK1) regulates T cell development

T lymphocyte activation is associated with activation of diverse AGC serine kinases (named after family members protein kinase A, protein kinase G and protein kinase C). It has been difficult to assess the function of these molecules in T cell development with simple gene-deletion strategies because different isoforms of AGC kinases are coexpressed in the thymus and have overlapping, redundant functions. To circumvent these problems, we explored the consequences of genetic manipulation of phosphoinositide-dependent kinase 1 (PDK1), a rate-limiting 'upstream' activator of AGC kinases. Here we analyzed the effect of PDK1 deletion on T lineage development. We also assessed the consequences of reducing PDK1 levels to 10% of normal. Complete PDK1 loss blocked T cell differentiation in the thymus, whereas reduced PDK1 expression allowed T cell differentiation but blocked proliferative expansion. These studies show that AGC family kinases are essential for T cell development.

Intracellular Location and Cell Context-Dependent Function of Protein Kinase D

Protein kinase D (PKD) is an antigen receptor-activated serine kinase localized at either the plasma membrane or the cytosol of lymphocytes. To probe PKD function at these different locations, transgenesis was used to target active PKD either to the membrane or cytosol of pre-T cells. In recombinase gene null pre-T cells, membrane and cytosolic active PKD both induced differentiation reminiscent of beta selection: downregulation of CD25 and upregulation of CD2 and CD5. Active PKDs also induced pre-T cell proliferation, although this response was not universal to all thymocyte subsets. There were two striking differences between the actions of the differentially localized PKDs. Membrane but not cytosolic PKD could induce expression of CD8 and CD4 in recombinase null mice; cytosolic but not membrane PKD suppressed Vbeta to DJbeta rearrangements of the TCRbeta chain locus in wild-type T cells. PKD function is thus determined by its intracellular location and cell context.

Microenvironmental regulation of Notch signalling in T cell development

T cells develop in the thymus from blood-borne progenitors derived from haematopoietic tissues. Amongst the mechanisms by which stromal cells in thymic and prethymic tissues influence lymphoid progenitors, recent attention has focussed on the importance of Notch signalling in early T cell development. Here, we review evidence that developing T cells and their progenitors receive signals through Notch receptors as a result of interactions with Notch ligands expressed by stromal cells. In particular, we focus on the role of Notch ligand-expressing stromal cells in regulating key control points during pre- and intrathymic phases of T cell development.

Developmental dissociation of T cells from B, NK, and myeloid cells revealed by MHC class II-specific chimeric immune receptors bearing TCR-zeta or FcR-gamma chain signaling domains

The T-cell receptor zeta (TCR-zeta) and FcR-gamma chains play a critical role in mediating signal transduction. We have previously described HIV glycoprotein 120 (gp120)-specific chimeric immune receptors (CIRs) in which the extracellular domain of CD4 is linked to the signaling domain of zeta (CD4zeta) or gamma (CD4gamma). Such CIRs are efficiently expressed following retroviral transduction of mature T cells and specifically redirect effector functions toward HIV-infected targets. In this report, we examine development of CD4zeta- or CD4gamma-expressing T cells from retrovirally transduced hematopoietic stem cells following bone marrow transplantation. Although CD4zeta/gamma-expressing myeloid, NK, and B cells were efficiently reconstituted, parallel development of CD4zeta/gamma-expressing T cells was blocked prior to the CD25(+)CD44(+) prothymocyte stage. In contrast, T cells expressing a signaling-defective CIR were efficiently generated. When major histocompatibility complex (MHC) class II-deficient mice were used as transplant recipients, development of CD4zeta/gamma-expressing T cells was restored. We conclude that CD4zeta/gamma signaling generated following engagement of MHC class II selectively arrests T-lineage development.

Heterogeneity of the DN4 (CD44-CD25-) subset of CD4-CD8- double negative thymocytes; dependence on CD3 signaling

Recent studies have shown that apoptotic cell death associated with selection for thymocytes that express clonotypic TCRbeta or TCRgammadelta proteins takes place in the DN4 (CD44-CD25-) subset of CD4-CD8- double negative (DN) thymocytes. A detailed analysis of the DN4 subset is therefore of interest. Using intracellular (IC) staining for clonotypic TCR and CD3varepsilon proteins we find that DN4 cells consist of five subpopulations: TCRbetaIC(high)/CD3varepsilonIC(high)/TCRgammadeltaIC-, TCRbetaI-C-/CD3varepsilonIC(high)/TCRgammadeltaIC(+), TCRbetaIC(high)/CD3varepsilonIC(high)/TCRgammadeltaIC(+), TCRbetaIC(low)/CD3varepsilonIC(low)/TCRgammadeltaIC(-), and TCRbetaIC(-)/CD3varepsilonIC(-)/TCRgammadeltaIC(-). Expression levels of IC TCRbeta/CD3varepsilon, and of Thy1.2, CD2, and CD69 at the cell surface suggest that the TCRbetaIC(low)/CD3varepsilonIC(low)/TCRgammadeltaIC(-) subset harbors the direct precursors of DP cells, and is critical for life/death decisions in early thymic selection. TCRbeta/CD3varepsilon downregulation is less pronounced in DN4 and DP cells of mice deficient for CD3zeta or for p56(lck), suggesting that the dynamics of TCR protein regulation in the DN4 subset is dependent on CD3 signaling.

Major T cell progenitor activity in bone marrow-derived spleen colonies

Common lymphoid progenitors (CLP) are generated in adult bone marrow (BM), but the intermediate steps leading to T cell commitment are unknown, and so is the site at which this commitment occurs. Here, we show that colonies arising in the spleen 12 days after BM injection harbor T cell precursors that are undetectable in BM. These precursors did not generate myeloid cells in vivo but repopulated the thymus and the peripheral T cell compartment much faster than did CLP. Two lineage negative (Lin(-)) subpopulations were distinguished, namely CD44(+) Thy1(-) cells still capable of natural killer generation and transient low-level B cell generation, and T cell-restricted CD44(-) Thy1(+) cells. At a molecular level, frequency of CD3epsilon and preTalpha mRNA was very different in each subset. Furthermore, only the CD44(-) Thy1(+) subset have initiated rearrangements in the T cell receptor beta locus. Thus, this study identifies extramedullary T cell progenitors and will allow easy approach to T cell commitment studies.

Characterization of T cell differentiation in the murine gut

Gut intraepithelial CD8 T lymphocytes (T-IEL) are distinct from thymus-derived cells and are thought to derive locally from cryptopatch (CP) precursors. The intermediate stages of differentiation between CP and mature T-IEL were not identified, and the local differentiation process was not characterized. We identified and characterized six phenotypically distinct lineage-negative populations in the CP and the gut epithelium: (a) we determined the kinetics of their generation from bone marrow precursors; (b) we quantified CD3-epsilon, recombination activating gene (Rag)-1, and pre-Talpha mRNAs expression at single cell level; (c) we characterized TCR-beta, -gamma, and -alpha locus rearrangements; and (d) we studied the impact of different mutations on the local differentiation. These data allowed us to establish a sequence of T cell precursor differentiation in the gut. We also observed that the gut differentiation varied from that of the thymus by a very low frequency of pre-Talpha chain mRNA expression, a different kinetics of Rag-1 mRNA expression, and a much higher impact of CD3 epsilon/delta and pre-Talpha deficiencies. Finally, only 3% of CP cells were clearly involved in T cell differentiation, suggesting that these structures may have additional physiological roles in the gut.

CD2 facilitates differentiation of CD4 Th cells without affecting Th1/Th2 polarization

The role of CD2 in murine CD4 helper T cell differentiation and polarization was examined using TCR-Cyt-5CC7-I transgenic recombination activating gene-2-/- H-2(a) mice on CD2+/+ or CD2-/- backgrounds. In the absence of CD2, thymic development was abnormal as judged by reduction in the steady state number of total, double-positive, and CD4 single-positive (SP) thymocytes, as well as a defect in their restorative dynamics after peptide-induced negative selection in vivo. In addition, in CD2-/- animals, lymph node CD4 SP T cells manifest a 10- to 100-fold attenuated activation response to cytochrome c (CytC) agonist peptides as judged by induction of CD25 and CD69 cell surface expression or [(3)H]TdR incorporation; differences in the magnitude of responsiveness and requisite molar peptide concentrations were even greater for altered peptide ligands. Although the presence or absence of CD2 did not impact the final Th1 or Th2 polarization outcome, CD2 expression reduced the CytC peptide concentration threshold necessary to facilitate both Th1 and Th2 differentiation. In vivo administration of CytC peptide to CD2-/- animals yielded an impaired CD4 SP T cell effector/memory phenotype compared with similarly treated CD2+/+ mice. Analysis of TCR-Cyt-5CC7-I human CD2 double-transgenic mice similarly failed to reveal a preferential Th1 vs Th2 polarization. Collectively, these results indicate that CD2 is important for the efficient development of CD4 SP thymocytes and TCR-dependent activation of mature CD4 lymph node T cells, but does not direct a particular helper T cell subset polarity.

Characterization of the immunophenotype and the metastatic properties of a murine T-lymphoma cell line. Unexpected expression of cytoplasmatic CD4

We report the first characterization of a mouse T-lymphoma cell line that surprisingly expresses cytoplasmatic (cy) yCD4. Phenotypically, LBC cells are CD5+, CD8+, CD16+, CD24+, CD25+, CD2-/dim, CD3-/dim, TCRbeta-/dim, TCRgammadelta, CD154 , CD40-, and CD45R. Coexpress cyTCRbeta, cyCD3, cyCD4, and yet lack surface CD4 expression. Transplantation of LBC cells into mice resulted in an aggressive T-lymphoblastic lymphoma that infiltrated lymph nodes, thymus, spleen, liver, ovary, and uterus but not peripheral blood or bone marrow. LBC cells display a modal chromosome number of 39 and a near-diploid karyotype. Based on the characterization data, we demonstrated that the LBC cell line was derived from an early T-cell lymphocyte precursor. We propose that the malignant cell transformation of LBC cells could coincide with the transition stage from late double-negative, DN3 (CD4- CD8 CD44-/low, CD25+) or DN4 (CD4-low, CD8-/low, CD44-, CD25-) to double-positive (DP: CD4+CD8+) stage of T-cell development. LBC cells provide a T-lymphoblastic lymphoma model derived from a malignant early T-lymphocyte that can be potentially useful as a model to study both cellular regulation and differentiation of T-cells. In addition, LBC tumor provides a short latency neoplasm to study cellular regulation and to perform preclinical trials of lymphoma-relatel clisorders.

Clonable progenitors committed to the T lymphocyte lineage in the mouse bone marrow; use of an extrathymic pathway

We searched for clonable committed T cell progenitors in the adult mouse bone marrow and isolated rare (approximately 0.05%) cells with the Thy-1hiCD2-CD16+CD44hiCD25-Lin- phenotype. In vivo experiments showed that these cells were progenitors committed only to reconstituting the T cell lineage of irradiated Ly5 congenic hosts. Reconstitution of the thymus was minimal compared with that of the bone marrow, spleen, and lymph nodes. At limiting dilutions, donor T cell reconstitution of the spleen frequently occurred without detectable donor cells in the thymus. Progenitors were capable of rapidly reconstituting athymic hosts. In conclusion, the clonable bone marrow progenitors were capable of T cell reconstitution predominantly by means of an extrathymic pathway.

A critical role for CD2 in both thymic selection events and mature T cell function

To examine the function of CD2 in vivo, N15 TCR transgenic (tg) RAG-2(-/-) H-2(b) mice bearing a single TCR specific for the vesicular stomatitis virus octapeptide bound to the H-2K(b) molecule were compared on a wild-type or CD2(-/-) background. In N15tg RAG-2(-/-) CD2(-/-) mice, thymic dysfunction is evident by 6 wk with a pre-TCR block in the CD4(-)CD8(-) double-negative thymocytes at the CD25(+)CD44(-) stage. Moreover, mature N15tg RAG-2(-/-) CD2(-/-) T cells are approximately 100-fold less responsive to vesicular stomatitis virus octapeptide and unresponsive to weak peptide agonists, as judged by IFN-gamma production. Repertoire analysis shows substantial differences in Valpha usage between non-tg C57BL/6 (B6) and B6 CD2(-/-) mice. Collectively, these findings show that CD2 plays a role in pre-TCR function in double-negative thymocytes, TCR selection events during thymocyte development, and TCR-stimulated cytokine production in mature T cells.

Control of pre-T cell proliferation and differentiation by the GTPase Rac-I

The GTPase Rac-I has the potential for pleiotropic functions due to its ability to interact with multiple effectors. Here, activation of Rac-I is shown to potently regulate pre-T cell differentiation and proliferation at the point of T cell antigen receptor (TCR) beta selection. An activated Rac-I effector domain mutant that restricts signaling to particular actions on actin dynamics can drive pre-T cell differentiation. Rac-I activation cannot fully substitute for the pre-TCR complex but can fully correct defects in pre-T cell development in mice lacking the adapter molecule Vav-1. The present study identifies the subset of Rac-I responses that mediate Vav-1 action as critical regulators of TCR beta selection.

Different initiation of pre-TCR and gammadeltaTCR signalling

Lineage choice is of great interest in developmental biology. In the immune system, the alphabeta and gammadelta lineages of T lymphocytes diverge during the course of the beta-, gamma- and delta-chain rearrangement of T-cell receptor (TCR) genes that takes place within the same precursor cell and which results in the formation of the gammadeltaTCR or pre-TCR proteins. The pre-TCR consists of the TCRbeta chain covalently linked to the pre-TCRalpha protein, which is present in immature but not in mature T cells which instead express the TCRalpha chain. Animals deficient in pre-TCRalpha have few alphabeta lineage cells but an increased number of gammadelta T cells. These gammadelta T cells exhibit more extensive TCRbeta rearrangement than gammadelta T cells from wild-type mice. These observations are consistent with the idea that different signals emanating from the gammadeltaTCR and pre-TCR instruct lineage commitment. Here we show, by using confocal microscopy and biochemistry to analyse the initiation of signalling, that the pre-TCR but not the gammadeltaTCR colocalizes with the p56lck Src kinase into glycolipid-enriched membrane domains (rafts) apparently without any need for ligation. This results in the phosphorylation of CD3epsilon and Zap-70 signal transducing molecules. The results indicate clear differences between pre-TCR and gammadeltaTCR signalling.

Identification of an early T cell progenitor for a pathway of T cell maturation in the bone marrow

We have identified a rare ( approximately 0.05-0.1%) population of cells (Thy-1(hi)CD16(+)CD44(hi)CD2(-)TCRalphabeta(-)B220(-)M ac-1(-)NK1. 1(-)) in the adult mouse bone marrow that generates CD4(+) and CD8(+) TCRalphabeta(+) T cells after tissue culture for 48 hr in the presence of Ly5 congenic marrow cells. The essential stages in the maturation of the progenitors were determined; the stages included an early transition from CD2(-)CD16(+)CD44(hi)TCRalphabeta(-) to CD2(+)CD16(int/-)CD44(int/-)TCRalphabeta(-) cells, and a later transition to CD4(+)CD8(+)TCRalphabeta(+) double-positive T cells that rapidly generate the CD4(+) and CD8(+) single-positive T cells. The maturation of the progenitors is almost completely arrested at the CD2(+)TCRalphabeta(-) stage by the presence of mature T cells at the initiation of cultures. This alternate pathway is supported by the marrow microenvironment; it recapitulates critical intermediary steps in intrathymic T cell maturation.

Signals Transduced by CD3ε, But Not by Surface Pre-TCR Complexes, Are Able to Induce Maturation of an Early Thymic Lymphoma In Vitro

Development of immature CD4−CD8− (double-negative) thymocytes to the CD4+CD8+ (double-positive) stage is linked to productive rearrangement of the TCRβ locus by signals transduced through the pre-TCR. However, the mechanism whereby pre-TCR signaling is initiated remains unclear, in part due to the lack of an in vitro model system amenable to both biochemical and genetic analysis. In this study, we establish the thymic lymphoma Scid.adh as such a model system. Scid.adh responds to Ab engagement of surface IL-2Ra (TAC):CD3ε molecules (a signaling chimera that mimics pre-TCR signaling in vivo) by undergoing changes in gene expression observed following pre-TCR activation in normal thymocytes. These changes include down-regulation of CD25, recombinase-activating gene (RAG)-1, RAG-2, and pTα; and the up-regulation of TCRα germline transcripts. We term this complete set of changes in gene expression, in vitro maturation. Interestingly, Scid.adh undergoes only a subset of these changes in gene expression following Ab engagement of the pre-TCR. Our findings make two important points. First, because TAC:CD3ε stimulation of Scid.adh induces physiologically relevant changes in gene expression, Scid.adh is an excellent cellular system for investigating the molecular requirements for pre-TCR signaling. Second, Ab engagement of CD3ε signaling domains in isolation (TAC:CD3ε) promotes in vitro maturation of Scid.adh, whereas engagement of CD3ε molecules contained within the complete pre-TCR fails to do so. Our current working hypothesis is that CD3ε fails to promote in vitro maturation when in the context of an Ab-engaged pre-TCR because another pre-TCR subunit(s), possibly TCRζ, qualitatively alters the CD3ε signal.

T Cell Development in PU.1-Deficient Mice

These studies address the role of PU.1 in T cell development through the analysis of PU.1−/− mice. We show that the majority of PU.1−/− thymocytes are blocked in differentiation prior to T cell commitment, and contain a population of thymocyte progenitors with the cell surface phenotype of CD44+, HSAbright, c-kitint, Thy-1−, CD25−, Sca-1−, CD4−, and CD8−. These cells correspond in both number and cell surface phenotype with uncommitted thymocyte progenitors found in wild-type fetal thymus. RT-PCR analysis demonstrated that PU.1 is normally expressed in this early progenitor population, but is down-regulated during T cell commitment. Rare PU.1−/− thymi, however, contained small numbers of thymocytes expressing markers of T cell commitment. Furthermore, almost 40% of PU.1−/− thymi placed in fetal thymic organ culture are capable of T cell development. Mature PU.1−/− thymocytes generated during organ culture proliferated and produced IL-2 in response to stimulation through the TCR. These data demonstrate that PU.1 is not absolutely required for T cell development, but does play a role in efficient commitment and/or early differentiation of most T progenitors.

Differentiation in Culture of Murine Primitive Lymphohematopoietic Progenitors Toward T-Cell Lineage

Earlier, we described a stromal cell-free two-step clonal culture system in which murine primitive lymphohematopoietic progenitors produce myeloid and B-lymphoid lineage cells. In the same culture T-cell potential of the progenitors was maintained. We now report that, in addition to myeloid and B-lymphoid cells, putative T-cell progenitors are also produced in culture. Lineage-negative (Lin−) Ly-6A/E+ c-kit+ bone marrow cells from 5-fluorouracil–treated mice were cultured in methylcellulose in the presence of SF (Steel factor), interleukin (IL)-11, and IL-7, and the resulting primary colonies were picked and pooled. When injected into severe combined immune deficiency (scid) mice, the pooled cells reconstituted the T-cell compartment of the scid mice earlier than freshly prepared primitive marrow cells. This reconstitution activity of the pooled primary colony cells was enriched in the Ly-6A/E+ and FcγRII/III−/low cell fractions. Reverse transcriptase-polymerase chain reaction (RT-PCR) and DNA-PCR analyses showed that some of the primary colony cells are differentiated sufficiently to express messenger RNA (mRNA) of T-cell receptor (TCR) β-chain and pre-TCR alpha (pT) and, although not frequently, to perform Dβ-Jβ rearrangement of the TCR gene. Micromanipulation studies confirmed the clonal origin of myeloid lineage cells and the cells positive for the T-cell–specific transcripts and D-J rearrangement of TCR β-chain. These results suggested that, in the presence of SF, IL-11, and IL-7, primitive lymphohematopoietic progenitors differentiate toward T-cell lineage in addition to myeloid and B-cell lineages.

Differentiation in Culture of Murine Primitive Lymphohematopoietic Progenitors Toward T-Cell Lineage

Earlier, we described a stromal cell-free two-step clonal culture system in which murine primitive lymphohematopoietic progenitors produce myeloid and B-lymphoid lineage cells. In the same culture T-cell potential of the progenitors was maintained. We now report that, in addition to myeloid and B-lymphoid cells, putative T-cell progenitors are also produced in culture. Lineage-negative (Lin−) Ly-6A/E+ c-kit+ bone marrow cells from 5-fluorouracil–treated mice were cultured in methylcellulose in the presence of SF (Steel factor), interleukin (IL)-11, and IL-7, and the resulting primary colonies were picked and pooled. When injected into severe combined immune deficiency (scid) mice, the pooled cells reconstituted the T-cell compartment of the scid mice earlier than freshly prepared primitive marrow cells. This reconstitution activity of the pooled primary colony cells was enriched in the Ly-6A/E+ and FcγRII/III−/low cell fractions. Reverse transcriptase-polymerase chain reaction (RT-PCR) and DNA-PCR analyses showed that some of the primary colony cells are differentiated sufficiently to express messenger RNA (mRNA) of T-cell receptor (TCR) β-chain and pre-TCR alpha (pT) and, although not frequently, to perform Dβ-Jβ rearrangement of the TCR gene. Micromanipulation studies confirmed the clonal origin of myeloid lineage cells and the cells positive for the T-cell–specific transcripts and D-J rearrangement of TCR β-chain. These results suggested that, in the presence of SF, IL-11, and IL-7, primitive lymphohematopoietic progenitors differentiate toward T-cell lineage in addition to myeloid and B-cell lineages.

A regulatory role for Fc gamma receptors (CD16 and CD32) in hematopoiesis

Progenitor cells of the T- and B-lineages in mice express (CD32) and Fc gamma RIII (CD16) but as the developing lymphocytes begin to express clonal antigen receptors, CD16 and CD32 are downregulated in T-cells, and CD16 is downregulated in B-cells. Considering that counter-receptors for Fc gamma R occur on thymic and bone marrow stromal cells, the possibility exists that Fc gamma R might participate in some aspect of T- and B-lineage development prior to the stage of antigen receptor expression. Previous studies provided evidence that Fc gamma R can influence murine T-lineage development. In the present studies we found that anti-Fc gamma RII/III mAb accelerated B-lineage development in bone marrow cultures from normal mice, but not in cultures from CD16-/- or CD32-/- mice. Similar results were observed when FACS-purified B-progenitor cells were co-cultured with BMS2, a bone marrow stromal cell line. Fresh bone marrow from CD32-/- mice contained about two-fold more B-lineage cells compared to bone marrow from normal or CD16-/- mice. These studies indicate that the Fc gamma R on B-lineage progenitor cells can influence their further development and add to a growing body of evidence that implicates Fc gamma R as regulatory elements in hematopoiesis.

Surface Expression and Functional Competence of CD3-Independent TCR ζ-Chains in Immature Thymocytes

In recombinase-deficient (RAG-2−/−) mice, double-negative thymocytes can be stimulated to proliferate and differentiate by anti-CD3 Abs. CD3 molecules are expressed on the surface of these cells in association with calnexin. In this study, we show that ζ-chains can be recovered as phosphorylated proteins in association with phosphorylated ZAP-70 from anti-CD3-stimulated RAG-2−/− thymocytes, even though they are not demonstrably associated with the CD3/calnexin complex. The lack of a physical association of ζ dimers with the CD3 complex in RAG-2−/− thymocytes and also in a pre-TCR-expressing cell line, as well as the efficient association of ζ dimers with ZAP-70 in the RAG-2−/− thymocytes, suggest that these ζ-chain dimers could contribute to pre-TCR signaling. This idea is supported by the finding that in RAG-2−/− ζ-deficient thymocytes, ZAP-70 and p120cbl were only weakly phosphorylated.

The common cytokine receptor gamma chain and the pre-T cell receptor provide independent but critically overlapping signals in early alpha/beta T cell development

Intracellular signals emanating from cytokine and antigen receptors are integrated during the process of intrathymic development. Still, the relative contributions of cytokine receptor signaling to pre-T cell receptor (TCR) and TCR-mediated differentiation remain undefined. Interleukin (IL)-7 interactions with its cognate receptor complex (IL-7Ralpha coupled to the common cytokine receptor gamma chain, gammac) play a dominant role in early thymopoiesis. However, alpha/beta T cell development in IL-7-, IL-7Ralpha-, and gammac-deficient mice is only partially compromised, suggesting that additional pathways can rescue alpha/beta T lineage cells in these mice. We have investigated the potential interdependence of gammac- and pre-TCR-dependent pathways during intrathymic alpha/beta T cell differentiation. We demonstrate that gammac-dependent cytokines do not appear to be required for normal pre-TCR function, and that the rate-limiting step in alpha/beta T cell development in gammac- mice does not involve TCR-beta chain rearrangements, but rather results from poor maintenance of early thymocytes. Moreover, mice double mutant for both gammac and pre-Talpha show vastly reduced thymic cellularity and a complete arrest of thymocyte differentiation at the CD44(+)CD25(+) cell stage. These observations demonstrate that the pre-TCR provides the gammac-independent signal which allows alpha/beta T cell development in gammac- mice. Thus, a series of overlapping signals derived from cytokine and T cell receptors guide the process of alpha/beta thymocyte development.

A Regulatory Role for Fcγ Receptors CD16 and CD32 in the Development of Murine B Cells

Early in development, murine B-lineage progenitor cells express two classes of IgG Fc receptors (FcγR) designated as FcγRII (CD32) and FcγRIII (CD16), but mature B lymphocytes only express FcγRII (CD32), which functions as an inhibitor of B-cell activation when it is induced to associate with mIgM. The functions of CD16 and CD32 on B-lineage precursor cells have not previously been investigated. To search for FcγR functions on developing B-lineage cells, normal murine bone marrow cells were cultured in the presence of 2.4G2, a rat monoclonal antibody that binds to CD16 and CD32, or in the presence of control normal rat IgG, and then the B-lineage compartment was analyzed for effects. Cultures that contained 2.4G2 showed enhanced growth and differentiation of B-lineage cells compared with control cultures. The enhancing effect of 2.4G2 also occurred when fluorescence-activated cell-sorted B-cell precursors (B220+, sIgM−, HSAhigh, FcγR+) from normal bone marrow were cocultured with BMS2, a bone marrow stromal cell line, but not when they were cultured in BMS2-conditioned media. The enhancement of B-lineage development induced by 2.4G2 was CD16-dependent and CD32-dependent, because 2.4G2 did not effect B-lineage growth or differentiation in cultures of bone marrow from mice in which either the gene encoding CD16 or CD32 had been disrupted. Analysis of fresh bone marrow from the CD16 gene-disrupted mice showed normal numbers and distribution of cells within the B-cell compartment, but in CD32 gene-disrupted mice, the B-cell compartment was significantly enlarged. These experiments provide several lines of evidence that the FcγR expressed on murine B-cell precursors can influence their growth and differentiation.

© 1998 by The American Society of Hematology.

A Regulatory Role for Fcγ Receptors CD16 and CD32 in the Development of Murine B Cells

Early in development, murine B-lineage progenitor cells express two classes of IgG Fc receptors (FcγR) designated as FcγRII (CD32) and FcγRIII (CD16), but mature B lymphocytes only express FcγRII (CD32), which functions as an inhibitor of B-cell activation when it is induced to associate with mIgM. The functions of CD16 and CD32 on B-lineage precursor cells have not previously been investigated. To search for FcγR functions on developing B-lineage cells, normal murine bone marrow cells were cultured in the presence of 2.4G2, a rat monoclonal antibody that binds to CD16 and CD32, or in the presence of control normal rat IgG, and then the B-lineage compartment was analyzed for effects. Cultures that contained 2.4G2 showed enhanced growth and differentiation of B-lineage cells compared with control cultures. The enhancing effect of 2.4G2 also occurred when fluorescence-activated cell-sorted B-cell precursors (B220+, sIgM−, HSAhigh, FcγR+) from normal bone marrow were cocultured with BMS2, a bone marrow stromal cell line, but not when they were cultured in BMS2-conditioned media. The enhancement of B-lineage development induced by 2.4G2 was CD16-dependent and CD32-dependent, because 2.4G2 did not effect B-lineage growth or differentiation in cultures of bone marrow from mice in which either the gene encoding CD16 or CD32 had been disrupted. Analysis of fresh bone marrow from the CD16 gene-disrupted mice showed normal numbers and distribution of cells within the B-cell compartment, but in CD32 gene-disrupted mice, the B-cell compartment was significantly enlarged. These experiments provide several lines of evidence that the FcγR expressed on murine B-cell precursors can influence their growth and differentiation.

© 1998 by The American Society of Hematology.

CD16 Cross-Linking Blocks Rearrangement of the TCRβ Locus and Development of αβ T Cells and Induces Development of NK Cells from Thymic Progenitors

Mouse thymocytes normally develop into T lymphocytes, but the embryonic thymus also contains precursor cells capable of developing into NK cells. Here, we describe conditions that induce pro-T cells to develop into NK cells. CD16 is expressed on thymic pro-T cells. We observed that CD16 cross-linking during culture of embryonic thymic organs suppressed rearrangement of the TCRβ locus (but did not inhibit TCRγ locus rearrangement). Rearrangement of the TCRβ locus is normally required for development to the CD4+CD8+, and this development was also suppressed by CD16 cross-linking. The ability of CD16 cross-linking to block αβT cell development was not attributable to toxic effects, but rather was accompanied by promotion of development into NK cells, identified based on molecular and functional criteria. These results suggest that common lymphoid precursors can respond to environmental signals to commit to the αβT vs NK developmental pathways.

Lineage commitment and differentiation of T and natural killer lymphocytes in the fetal mouse

T cells and natural killer (NK) cells are presumed to share a common intrathymic precursor. The development of conventional alpha beta T lymphocytes begins within the early fetal thymus, after the colonization of multipotent CD117+ precursors. Irrevocable commitment to the T lineage is marked by thymus-induced expression of CD25. However, the contribution of the fetal thymus to NK lineage commitment and differentiation remains largely unappreciated. Recently, we demonstrated that the development of functional mouse NK cells occurs first in the fetal thymus. Moreover, the appearance of mature fetal thymic NK cells (NK1.1+/CD117-) is preceded by a thymus-induced developmental stage (NK1.1+/CD117+) that marks lineage commitment of multipotent hematopoietic precursors to the T and NK-cell fates. Commitment to the T/NK bipotent stage is induced by fetal thymic stroma, but is not thymus dependent. Recent data indicate that CD90+/CD117lo fetal blood prothymocytes exhibit NK lineage potential and are phenotypically and functionally identical to fetal thymic NK1.1+/CD117+ progenitors. This finding also indicates that full commitment of circulating precursors to the T-cell lineage occurs after thymus colonization. In this review, we discuss recent insights into the cellular and molecular events involved in fetal mouse T and NK lineage commitment and differentiation to unipotent progenitors.

Epidermal Growth Factor (EGF) Modulates Fetal Thymocyte Growth and Differentiation: Partial Reversal by Insulin, Mimicking by Specific Inhibitors of EGF Receptor Tyrosine Kinase Activity, and Differential Expression of CD45 Phosphatase Isotypes

We have recently reported that epidermal growth factor (EGF) modulates thymocyte development in fetal thymus organ cultures. Exogenously added EGF arrested thymocyte growth and differentiation, acting at the transition from the CD4−CD8− (double-negative (DN)) to the CD4+CD8+ (double-positive (DP)) phenotype. In this study, we further investigate some molecular aspects of this blockade. This inhibitory effect could be mimicked by tyrphostins, which are selective inhibitors of EGF receptor kinase activity. An attempt to use insulin (INS) as a synergizing effector resulted in partial restoration of lobe cellularity, leading to expansion of the CD44−CD25+ DN subset. However, INS did not overcome the EGF-driven blockade of the thymocyte DN → DP transition. Analysis of CD45 phosphatase showed that this transition was preceded by a rise in CD45RB isotype expression. At the end of a 7-day culture, the remaining DN cells from both EGF- and EGF+INS-treated fetal thymus organ cultures showed a CD45RB− phenotype and were negative for the EGF-immunoreactive molecule described previously on the fetal thymocyte surface. This finding implies that neither molecule is related to the growth capability of cells at this early developmental stage; it is more likely that the molecules are related to subsequent events in the thymocyte pathway to the DP phenotype. Thus, our data support the concept that EGF receptor-related circuitry may be relevant in thymus ontogeny. Additionally, evidence is provided for the duality between growth and differentiation at this particular early stage of thymocyte development.

A p56lck-independent pathway of CD2 signaling involves Jun kinase

The p56 Src family non-receptor tyrosine kinase has been shown to be critical for T lymphocyte differentiation and activation. Hence in the absence of p56, T cell receptor triggered activation does not occur. We now provide evidence for a CD2-based signaling pathway which, in contrast to that of the T cell receptor, is independent of p56. CD2-mediated interleukin-2 production occurs via activation of Jun kinase in cell lines lacking p56. Jun kinase then facilitates the binding of c-Jun/c-Fos heterodimers to the AP-1 consensus site and the subsequent transcriptional activity of the interleukin-2 promoter. These data elucidate differences between TCR and CD2 signaling pathways in the same T cells.

p56lck signals for regulating thymocyte development can be distinguished by their dependency on Rho function

The tyrosine kinase p56lck regulates the differentiation and proliferative expansion of pre-T cells. However, nothing is known about other signaling molecules that operate with p56lck to mediate the pleiotropic changes that occur at this stage of thymocyte development. We used a genetic strategy to examine the requirement for the GTPase Rho in p56lck-mediated signals in the thymus. By generating mice double transgenic for a constitutively activated form of p56lck (p56lckF505) and the Rho inhibitor C3 transferase we were able to compare thymocyte development in mice expressing active p56lck on a wild-type or Rho- background. Thymocytes expressing active p56lck show enhanced proliferation of pre-T cells resulting in increased numbers of late pre-T cells, however, this dramatic effect on pre-T cell proliferation is lost when the p56lck transgene is expressed in thymocytes lacking endogenous Rho GTPase function. Expression of active p56lck also generates double positive (DP) thymocytes with low levels of CD2 antigen expression. Again, p56lck cannot prevent expression of CD2 when expressed on a Rho- background. CD4(+)CD8(+) DP cells expressing active p56lck have been shown to lack functional alpha/beta-T cell receptor (TCR) complexes due to p56lck-mediated inhibition of TCR gene Vbeta-Dbeta rearrangement. This inhibition of TCR expression by active p56lck is unimpaired in the absence of Rho function. The signaling pathways that are mediated by p56lck and control thymocyte proliferation, alpha/beta-TCR and CD2 antigen expression can thus be distinguished by their dependency on Rho function.

Requirement for the thymus in alphabeta T lymphocyte lineage commitment

We recently identified a fetal thymic developmental stage (NK1.1+/CD117(lo)) that characterizes committed T/NK progenitors. We now report the existence of phenotypically and functionally identical T/NK progenitors in mouse fetal blood and spleen but not in fetal liver. These precursors are indistinguishable from previously characterized fetal blood "prothymocytes" (CD90+/CD117(lo)), with the exception that they express NK1.1, lack markers associated with T lineage commitment, maintain a germline TCRbeta locus, and can give rise to both T and NK cells. Moreover, NK1.1+/CD90+/CD117(lo) fetal blood precursors are present in athymic nude mice. These results suggest that the T/NK lineage commitment pathway is thymus-independent. In contrast, full commitment to the alphabeta T lineage does not precede thymus colonization.