Analysis of Butyrophilin-Mediated Activation of γδ T Cells from Human Spleen

There is considerable interest in therapeutically engaging human γδ T cells. However, due to the unique TCRs of human γδ T cells, studies from animal models have provided limited directly applicable insights, and human γδ T cells from key immunological tissues remain poorly characterized. In this study, we investigated γδ T cells from human spleen tissue. Compared to blood, where Vδ2+Vγ9+ T cells are the dominant subset, splenic γδ T cells included a variety of TCR types, with Vδ1+ T cells typically being the most frequent. Intracellular cytokine staining revealed that IFN-γ was produced by a substantial fraction of splenic γδ T cells, IL-17A by a small fraction, and IL-4 was minimal. Primary splenic γδ T cells frequently expressed NKG2D (NK group 2 member D) and CD16, whereas expression of DNAM-1 (DNAX accessory molecule 1), CD28, PD-1, TIGIT, and CD94 varied according to subset, and there was generally little expression of natural cytotoxicity receptors, TIM-3, LAG-3, or killer Ig-like receptors. In vitro expansion was associated with marked changes in expression of these activating and inhibitory receptors. Analysis of functional responses of spleen-derived Vδ2+Vγ9+, Vδ1+Vγ9+, and Vδ1+Vγ9- T cell lines to recombinant butyrophilin BTN2A1 and BTN3A1 demonstrated that both Vδ2+Vγ9+ and Vδ1+Vγ9+ T cells were capable of responding to the extracellular domain of BTN2A1, whereas the addition of BTN3A1 only markedly enhanced the responses of Vδ2+Vγ9+ T cells. Conversely, Vδ1+Vγ9+ T cells appeared more responsive than Vδ2+Vγ9+ T cells to TCR-independent NKG2D stimulation. Thus, despite shared recognition of BTN2A1, differential effects of BTN3A1 and coreceptors may segregate target cell responses of Vδ2+Vγ9+ and Vδ1+Vγ9+ T cells.

Abstract 1815: Rapid serial killing of target cells by Vγ9Vδ2 T Cells in cynomolgus macaques and humanized mice treated with a CD20-directed heterodimeric butyrophilin 2A1/3A1 fusion protein

CD3-directed T cell engagers provide clinical benefit in a variety of hematologic cancers, but it remains unclear whether γδ T cells (GDT) are present at sufficient numbers to be similarly harnessed because they represent only 1-5% of the total T cell pool. The Vγ9Vδ2 T cell receptor is activated by a butyrophilin 2A1 and 3A1 (2A1/3A1) heterodimer as “signal 1”, together with costimulatory signaling through CD28 or NKG2D as “signal 2”. Here we evaluated the ability of a GDT engager (GADLEN) comprising the extracellular domains of 2A1/3A1 adjoined via an Fc to an antibody fragment targeting the CD20 antigen, to mediate target-cell depletion in vivo at physiologically relevant frequencies of GDT. A humanized mouse model was developed wherein human peripheral blood mononuclear cells (PBMC) were engrafted to NSG-IL15 mice and a high degree of human chimerism persisted for over 3 weeks, including a circulating pool of human GDT comprising 0.5 to 3% of total T cells. Across different human donors, the GDT effector to target B cell ratio varied from 1:1 to 1:46. Following treatment with a B cell-targeted GADLEN, rapid and dose-dependent depletion of over 95% of human B cells was achieved in the peripheral blood and spleen, even at E:T ratios of 1:46. Based on these data, a repeat-dose toxicology study was conducted in cynomolgus macaques across a dose range of 0.1 to 25 mg/kg with the CD20-targeted GADLEN. This study also demonstrated dose-dependent B cell depletion within 2 hours of infusion. Durable receptor occupancy was observed on CD20+ cells for over 7 days, whereas transient occupancy was observed on circulating cynomolgus Vγ9Vδ2 T cells. GDT represented approximately 1-3% of total T cells in cynomolgus macaques, and E:T ratios ranged from 1:7 to 1:49. All treatments were well tolerated, and no evidence of cytokine release syndrome nor any other toxicities were observed. Interestingly, in both humanized mice and non-human primates, the proliferation of GDT was not required to achieve B cell depletion. In fact, over several weeks, the circulating frequencies of cynomolgus Vγ9Vδ2 T cells remained stable, and no evidence of either proliferation or migration out of the blood was observed. These observations, together with the speed at which B cell depletion occurred, suggest that the Vγ9Vδ2 T cells eliminated the B cell pool by serial killing following treatment with the GADLEN. Despite a GDT frequency of less than 5% of total T cells in all these studies, Vγ9Vδ2 T cells achieved rapid depletion of target cells following treatment with the GADLEN, without evidence of toxicity, even at high doses. Collectively, these results indicate that low frequencies of GDT can be harnessed to achieve similar target-cell killing potential as the broader CD3+ T cell pool but with potentially less toxicity.

Citation Format: Anne Y. Lai, Louis E. Gonzalez, Noah Murr, Karen Lenz, Derek Franklin, Kristen Campbell, Faraha Brewer, Arpita Patel, Kinsley Evans, Mahmud Hussein, Keith Wilson, George Fromm, Taylor H. Schreiber, Suresh de Silva. Rapid serial killing of target cells by Vγ9Vδ2 T Cells in cynomolgus macaques and humanized mice treated with a CD20-directed heterodimeric butyrophilin 2A1/3A1 fusion protein [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 1815.

Cutting Edge: Bispecific γδ T Cell Engager Containing Heterodimeric BTN2A1 and BTN3A1 Promotes Targeted Activation of Vγ9Vδ2+ T Cells in the Presence of Costimulation by CD28 or NKG2D

Vγ9Vδ2⁺ T cell-targeted immunotherapy is of interest to harness its MHC-independent cytotoxic potential against a variety of cancers. Recent studies have identified heterodimeric butyrophilin (BTN) 2A1 and BTN3A1 as the molecular entity providing "signal 1" to the Vγ9Vδ2 TCR, but "signal 2" costimulatory requirements remain unclear. Using a tumor cell-free assay, we demonstrated that a BTN2A1/3A1 heterodimeric fusion protein activated human Vγ9Vδ2⁺ T cells, but only in the presence of costimulatory signal via CD28 or NK group 2 member D. Nonetheless, addition of a bispecific γδ T cell engager BTN2A1/3A1-Fc-CD19scFv alone enhanced granzyme B-mediated killing of human CD19⁺ lymphoma cells when cocultured with Vγ9Vδ2⁺ T cells, suggesting expression of costimulatory ligand(s) on tumor cells is sufficient to satisfy the "signal 2" requirement. These results highlight the parallels of signal 1 and signal 2 requirements in αβ and γδ T cell activation and demonstrate the utility of heterodimeric BTNs to promote targeted activation of γδ T cells.

Trends in Negative Emotions Throughout the COVID-19 Pandemic in the United States

Objectives

Study design

Methods

Results

Conclusions

CDK4/6 inhibition enhances antitumor efficacy of chemotherapy and immune checkpoint inhibitor combinations in preclinical models and enhances T-cell activation in patients with SCLC receiving chemotherapy

Background

Combination treatment with chemotherapy and immune checkpoint inhibitors (ICIs) has demonstrated meaningful clinical benefit to patients. However, chemotherapy-induced damage to the immune system can potentially diminish the efficacy of chemotherapy/ICI combinations. Trilaciclib, a highly potent, selective and reversible cyclin-dependent kinase 4 and 6 (CDK4/6) inhibitor in development to preserve hematopoietic stem and progenitor cells and immune system function during chemotherapy, has demonstrated proof of concept in recent clinical trials. Furthermore, CDK4/6 inhibition has been shown to augment T-cell activation and antitumor immunity in preclinical settings. Therefore, addition of trilaciclib has the potential to further enhance the efficacy of chemotherapy and ICI combinations.

Methods

In murine syngeneic tumor models, a schedule of 3 weekly doses of trilaciclib was combined with chemotherapy/ICI regimens to assess the effect of transient CDK4/6 inhibition on antitumor response and intratumor T-cell proliferation and function. Peripheral T-cell status was also analyzed in patients with small cell lung cancer (SCLC) treated with chemotherapy with or without trilaciclib to gain insights into the effect of transient exposure of trilaciclib on T-cell activation.

Results

Preclinically, the addition of trilaciclib to chemotherapy/ICI regimens enhanced antitumor response and overall survival compared with chemotherapy and ICI combinations alone. This effect is associated with the modulation of the proliferation and composition of T-cell subsets in the tumor microenvironment and increased effector function. Transient exposure of trilaciclib in patients with SCLC during chemotherapy treatment both preserved and increased peripheral lymphocyte counts and enhanced T-cell activation, suggesting that trilaciclib not only preserved but also enhanced immune system function.

Conclusions

Transient CDK4/6 inhibition by trilaciclib was sufficient to enhance and prolong the duration of the antitumor response by chemotherapy/ICI combinations, suggesting a role for the transient cell cycle arrest of tumor immune infiltrates in remodeling the tumor microenvironment. These results provide a rationale for combining trilaciclib with chemotherapy/ICI regimens to improve antitumor efficacy in patients with cancer.

Myelopreservation with the CDK4/6 inhibitor trilaciclib in patients with small-cell lung cancer receiving first-line chemotherapy: a phase Ib/randomized phase II trial

BACKGROUND

Chemotherapy-induced damage of hematopoietic stem and progenitor cells (HSPC) causes multi-lineage myelosuppression. Trilaciclib is an intravenous CDK4/6 inhibitor in development to proactively preserve HSPC and immune system function during chemotherapy (myelopreservation). Preclinically, trilaciclib transiently maintains HSPC in G1 arrest and protects them from chemotherapy damage, leading to faster hematopoietic recovery and enhanced antitumor immunity.

PATIENTS AND METHODS

This was a phase Ib (open-label, dose-finding) and phase II (randomized, double-blind placebo-controlled) study of the safety, efficacy and PK of trilaciclib in combination with etoposide/carboplatin (E/P) therapy for treatment-naive extensive-stage small-cell lung cancer patients. Patients received trilaciclib or placebo before E/P on days 1-3 of each cycle. Select end points were prespecified to assess the effect of trilaciclib on myelosuppression and antitumor efficacy.

RESULTS

A total of 122 patients were enrolled, with 19 patients in part 1 and 75 patients in part 2 receiving study drug. Improvements were seen with trilaciclib in neutrophil, RBC (red blood cell) and lymphocyte measures. Safety on trilaciclib+E/P was improved with fewer ≥G3 adverse events (AEs) in trilaciclib (50%) versus placebo (83.8%), primarily due to less hematological toxicity. No trilaciclib-related ≥G3 AEs occurred. Antitumor efficacy assessment for trilaciclib versus placebo, respectively, showed: ORR (66.7% versus 56.8%, P = 0.3831); median PFS [6.2 versus 5.0 m; hazard ratio (HR) 0.71; P = 0.1695]; and OS (10.9 versus 10.6 m; HR 0.87; P = 0.6107).

CONCLUSION

Trilaciclib demonstrated an improvement in the patient's tolerability of chemotherapy as shown by myelopreservation across multiple hematopoietic lineages resulting in fewer supportive care interventions and dose reductions, improved safety profile, and no detriment to antitumor efficacy. These data demonstrate strong proof-of-concept for trilaciclib's myelopreservation benefits.

CLINICAL TRAIL NUMBER

NCT02499770.

Abstract 1752: Transient exposure of trilaciclib, a CDK4/6 inhibitor, modulates gene expression in tumor immune infiltrates to promote a pro-inflammatory tumor microenvironment

While immune checkpoint inhibitors (ICIs) are efficacious and lead to durable responses in patients with various cancers, only a minority of patients respond. An approach to increase the response rate of ICIs is to combine them with chemotherapy to induce immunogenic cell death, leading to an enhanced antitumor response. However, chemotherapy can cause damage to cell types of the immune system, potentially diminishing the activity of the chemotherapy/ICI combination. Therefore, a targeted approach to preserve immune system function during chemotherapy could allow optimal efficacy of chemotherapy alone and chemotherapy/ICI combinations. Trilaciclib, a short-acting intravenous CDK4/6 inhibitor, preserves hematopoietic stem and progenitor cells and enhances immune system function during chemotherapy. While combining chemotherapeutic agents such as oxaliplatin or 5-FU with an ICI against PD-1 or PD-L1 delayed tumor growth in MC38 and CT26 mouse tumor models, the addition of intermittent dosing of trilaciclib to chemotherapy/ICI regimens led to further tumor regression and significantly enhanced the durability of response to chemotherapy/ICI. These preclinical results support the hypothesis that intermittent dosing of trilaciclib with chemotherapy/ICI can preserve and/or enhance an antitumor response. To gain insight into the effect of transient exposure of trilaciclib on the tumor microenvironment, we examined the proliferation status, cellular composition, and gene expression of tumor immune infiltrates from MC38 tumor-bearing mice post-treatment. All major intratumor immune cell types (T, NK, and MDSC subsets) are highly proliferative and sensitive to CDK4/6 inhibition. Cell proliferation was reduced by more than 75% after trilaciclib exposure, but was fully restored 48 hours post-treatment. While this brief proliferation arrest did not affect the frequency of each immune population, the overall expression of 59 genes functionally enriched for lymphocyte activation and upregulation of the pro-inflammatory cytokine interferon-γ was significantly enhanced. In parallel, several genes involved in immunosuppressive reactive oxygen species metabolic processes were downregulated. These findings suggest that transient cell-cycle arrest in tumor immune infiltrates can lead to modulation of gene expression, resulting in a pro-inflammatory tumor microenvironment that is favorable for increasing ICI activity. Additional roles of trilaciclib in preserving and enhancing antitumor response in combination with chemotherapy/ICIs are currently being investigated. A phase 2 study to assess the safety and efficacy of trilaciclib or placebo with carboplatin, etoposide, and atezolizumab in first-line extensive stage SCLC patients is ongoing (NCT03041311).

Citation Format: Anne Y. Lai, Jessica A. Sorrentino, Jay C. Strum, Patrick J. Roberts. Transient exposure of trilaciclib, a CDK4/6 inhibitor, modulates gene expression in tumor immune infiltrates to promote a pro-inflammatory tumor microenvironment [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 1752.

Abstract 5628: Trilaciclib (G1T28), a CDK4/6 inhibitor, enhances the efficacy of combination chemotherapy and immune checkpoint inhibitor treatment in preclinical models

While immune checkpoint inhibitors are efficacious and lead to durable responses in patients with various cancers, only a minority of patients respond. An approach to increase the response rate of immune checkpoint inhibitors is to combine them with chemotherapy in order to enhance immunogenic cell death and “prime” the immune system. However, chemotherapy itself can cause damage to various cell types of the immune system, potentially diminishing the activity of the chemotherapy/checkpoint inhibitor combination. Therefore, a targeted approach to preserve immune system function during chemotherapy would allow optimal activity of the checkpoint inhibitor therapy.

Trilaciclib (G1T28), a potent and selective IV CDK4/6 inhibitor, preserves hematopoietic stem cells and enhances immune system function during chemotherapy. We and others have previously shown T lymphocytes are extremely sensitive to CDK4/6 inhibition causing a transient arrest of proliferation and protection from damage by chemotherapy. Post treatment with trilaciclib and chemotherapy, T lymphocytes are released from the transient arrest in the presence of chemotherapy-induced immunogenic cell death allowing better priming of an anti-tumor immune response. To evaluate the effect of trilaciclib combination treatments, MC38 tumor bearing mice were treated with trilaciclib, oxaliplatin, or anti-PD-L1 alone or in combination, and tumor size was measured during and post treatment for 100 days.

Our results demonstrate that the addition of trilaciclib to an oxaliplatin/anti-PD-L1 combination (TOP) treatment significantly improves anti-tumor activity. Specifically, twice as many mice treated with TOP for 15 days had a complete response (CRs) when compared to oxaliplatin/anti-PD-L1 (OP); 6/10 CRs vs 3/10 CRs, respectively. In addition, the CRs were durable and without any evidence of recurrence at 100 days. Furthermore, TOP treatment caused a 60% increase in overall survival (OS) compared to mice treated with OP; median OS for TOP was 98 days compared to 61 days (HR, 0.53) for the OP treatment group. Assessment of ex-vivo stimulation of cytokine release from harvested splenocytes to measure T-cell activation, and evaluation of immune infiltrates and T-cell receptor repertoire within the tumors are ongoing. Taken together, we demonstrate that the short-acting IV CDK4/6 inhibitor, trilaciclib, which has been shown to preserve immune function during chemotherapy, enhances the anti-tumor activity of chemotherapy/anti-PD-L1 combination therapy.

Citation Format: Jessica A. Sorrentino, Anne Y. Lai, Jay C. Strum, Patrick J. Roberts. Trilaciclib (G1T28), a CDK4/6 inhibitor, enhances the efficacy of combination chemotherapy and immune checkpoint inhibitor treatment in preclinical models [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 5628. doi:10.1158/1538-7445.AM2017-5628

Diethylstilbestrol (DES)-stimulated hormonal toxicity is mediated by ERα alteration of target gene methylation patterns and epigenetic modifiers (DNMT3A, MBD2, and HDAC2) in the mouse seminal vesicle

BACKGROUND

Diethylstilbestrol (DES) is a synthetic estrogen associated with adverse effects on reproductive organs. DES-induced toxicity of the mouse seminal vesicle (SV) is mediated by estrogen receptor α (ERα), which alters expression of seminal vesicle secretory protein IV (Svs4) and lactoferrin (Ltf) genes.

OBJECTIVES

We examined a role for nuclear receptor activity in association with DNA methylation and altered gene expression.

METHODS

We used the neonatal DES exposure mouse model to examine DNA methylation patterns via bisulfite conversion sequencing in SVs of wild-type (WT) and ERα-knockout (αERKO) mice.

RESULTS

The DNA methylation status at four specific CpGs (-160, -237, -306, and -367) in the Svs4 gene promoter changed during mouse development from methylated to unmethylated, and DES prevented this change at 10 weeks of age in WT SV. At two specific CpGs (-449 and -459) of the Ltf gene promoter, DES altered the methylation status from methylated to unmethylated. Alterations in DNA methylation of Svs4 and Ltf were not observed in αERKO SVs, suggesting that changes of methylation status at these CpGs are ERα dependent. The methylation status was associated with the level of gene expression. In addition, gene expression of three epigenetic modifiers-DNMT3A, MBD2, and HDAC2-increased in the SV of DES-exposed WT mice.

CONCLUSION

DES-induced hormonal toxicity resulted from altered gene expression of Svs4 and Ltf associated with changes in DNA methylation that were mediated by ERα. Alterations in gene expression of DNMT3A, MBD2, and HDAC2 in DES-exposed male mice may be involved in mediating the changes in methylation status in the SV.

CITATION

Li Y, Hamilton KJ, Lai AY, Burns KA, Li L, Wade PA, Korach KS. 2014. Diethylstilbestrol (DES)-stimulated hormonal toxicity is mediated by ERα alteration of target gene methylation patterns and epigenetic modifiers (DNMT3A, MBD2, and HDAC2) in the mouse seminal vesicle. Environ Health Perspect 122:262-268; http://dx.doi.org/10.1289/ehp.1307351.

Distinct activation profiles in microglia of different ages: a systematic study in isolated embryonic to aged microglial cultures

Microglia have been implicated in disease progression for several age-related brain disorders. However, while microglia's contribution to the progression of these disorders is accepted, the effect of aging on their endogenous cellular characteristics has received limited attention. In fact, a comprehensive study of how the structure and function of microglia changes as a function of developmental age has yet to be performed. Here, we describe the functional response characteristics of primary microglial cultures prepared from embryonic, neonatal (Neo), 2-3month-old, 6-8month-old, 9-11month-old, and 13-15month-old rats. Microglial morphology, glutamate (GLU) uptake, and release of trophic and inflammatory factors were assessed under basal conditions and in microglia activated with adenosine 5'-triphosphate (ATP) or lipopolysaccharide. We found that microglia from different age groups were both morphologically and functionally distinct. Upon activation by ATP, Neo microglia were the most reactive, upregulating nitric oxide, tumor necrosis factor-α, and brain-derived neurotrophic factor release as well as GLU uptake. This upregulation translated into neurotoxicity in microglia-neuron co-cultures that were not observed with microglia of different developmental ages. Interestingly, 13-15month-old microglia exhibited similar activation profiles to Neo microglia, whereas microglia from younger adults and embryos were activated less by ATP. Our data also identify age-dependent differences in purinergic receptor subtype expression that contribute to the regulation of neuronal survival. Combined, our data demonstrate that microglial activation and purinergic receptor profiles vary non-linearly with developmental age, a potentially important finding for studies examining the role of microglia in neurodegenerative disorders.

DNA methylation profiling in human B cells reveals immune regulatory elements and epigenetic plasticity at Alu elements during B-cell activation

Memory is a hallmark of adaptive immunity, wherein lymphocytes mount a superior response to a previously encountered antigen. It has been speculated that epigenetic alterations in memory lymphocytes contribute to their functional distinction from their naive counterparts. However, the nature and extent of epigenetic alterations in memory compartments remain poorly characterized. Here we profile the DNA methylome and the transcriptome of B-lymphocyte subsets representing stages of the humoral immune response before and after antigen exposure in vivo from multiple humans. A significant percentage of activation-induced losses of DNA methylation mapped to transcription factor binding sites. An additional class of demethylated loci mapped to Alu elements across the genome and accompanied repression of DNA methyltransferase 3A. The activation-dependent DNA methylation changes were largely retained in the progeny of activated B cells, generating a similar epigenetic signature in downstream memory B cells and plasma cells with distinct transcriptional programs. These findings provide insights into the methylation dynamics of the genome during cellular differentiation in an immune response.

Cancer biology and NuRD: a multifaceted chromatin remodelling complex

The nucleosome remodelling and histone deacetylase (NuRD; also known as Mi-2) complex regulates gene expression at the level of chromatin. The NuRD complex has been identified - using both genetic and molecular analyses - as a key determinant of differentiation in mouse embryonic stem cells and during development in various model systems. Similar to other chromatin remodellers, such as SWI/SNF and Polycomb complexes, NuRD has also been implicated in the regulation of transcriptional events that are integral to oncogenesis and cancer progression. Emerging molecular details regarding the recruitment of NuRD to specific loci during development, and the modulation of these events in cancer, are used to illustrate how the inappropriate localization of the complex could contribute to tumour biology.

DNA methylation prevents CTCF-mediated silencing of the oncogene BCL6 in B cell lymphomas

Aberrant DNA methylation commonly occurs in cancer cells where it has been implicated in the epigenetic silencing of tumor suppressor genes. Additional roles for DNA methylation, such as transcriptional activation, have been predicted but have yet to be clearly demonstrated. The BCL6 oncogene is implicated in the pathogenesis of germinal center-derived B cell lymphomas. We demonstrate that the intragenic CpG islands within the first intron of the human BCL6 locus were hypermethylated in lymphoma cells that expressed high amounts of BCL6 messenger RNA (mRNA). Inhibition of DNA methyltransferases decreased BCL6 mRNA abundance, suggesting a role for these methylated CpGs in positively regulating BCL6 transcription. The enhancer-blocking transcription factor CTCF bound to this intronic region in a methylation-sensitive manner. Depletion of CTCF by short hairpin RNA in neoplastic plasma cells that do not express BCL6 resulted in up-regulation of BCL6 transcription. These data indicate that BCL6 expression is maintained during lymphomagenesis in part through DNA methylation that prevents CTCF-mediated silencing.

IL-7 specifies B cell fate at the common lymphoid progenitor to pre-proB transition stage by maintaining early B cell factor expression

IL-7 plays a critical role in B cell fate decision by regulating early B cell factor (EBF) expression. However, it was not clear when IL-7 stimulation is necessary in hemato-/lymphopoiesis in adult mice. Here we show that pre-proB cells derived from IL-7-/- mice have lost B cell potential, despite up-regulation of EBF expression following IL-7 stimulation. Pre-proB cells from wild-type mice can give rise to proB cells in the absence of IL-7. In this case, EBF up-regulation during the transition from the pre-proB to proB stages occurs normally. In contrast, EBF expression by IL-7-/- pre-proB cells after IL-7 stimulation is approximately 20 times lower than wild-type pre-proB cells. In addition, only multipotent progenitors with higher levels of ectopic EBF can give rise to proB cells in the absence of IL-7. Therefore, the primary function of IL-7 before the pre-proB stage in B cell development is to maintain the EBF expression level above a certain threshold, which is necessary for pre-proB cells to further transit to the proB stage.

T and B lymphocyte differentiation from hematopoietic stem cell

Until the past few years, it has been thought that lymphoid and myeloid lineage segregation represents the first step of lineage restriction during hematopoiesis from hematopoietic stem cell. Recent investigation of the cell populations within multipotent progenitors in the bone marrow has led to new understanding of how hematopoietic stem cells diversify into different hematopoietic cell types. This review focuses on the recent advances in understanding the developmental events that occur during hematopoietic stem cell specification into the T and B lymphocyte lineages in adult mice.

Identification of a bone marrow precursor of the earliest thymocytes in adult mouse

The thymus requires continuous replenishment of progenitors from the bone marrow (BM) to sustain T cell development. However, it remains unclear which hematopoietic progenitors downstream from hematopoietic stem cells in the BM home to the thymus in adult mice. In this work, we demonstrate that although multiple BM populations have intrinsic T lineage differentiation potential, a small subset of multipotent progenitors (MPPs) expressing CCR9 preferentially homes to the thymus. These CCR9(+) MPPs are phenotypically similar to the most immature early T lineage progenitors (ETPs) in the thymus and are present in the peripheral blood. Similar to ETPs, CCR9(+) MPPs undergo Notch signaling, as indicated by higher expression of Notch1 and downstream target Hes1 genes compared with other MPP subsets. Furthermore, CCR9(+) MPPs possess differentiation potential similar to that of ETPs, with very limited granulocyte/macrophage differentiation potential, but they can differentiate into T, B, and dendritic cells. These characteristics implicate CCR9(+) MPPs as the BM precursors of the earliest thymic progenitors. In addition, our data suggest that before transition from BM to thymus, MPPs are lymphoid-specified and primed for T lineage differentiation.

Asymmetrical lymphoid and myeloid lineage commitment in multipotent hematopoietic progenitors

The mechanism of lineage commitment from hematopoietic stem cells (HSCs) is not well understood. Although commitment to either the lymphoid or the myeloid lineage is popularly viewed as the first step of lineage restriction from HSCs, this model of hematopoietic differentiation has recently been challenged. The previous identification of multipotent progenitors (MPPs) that can produce lymphocytes and granulocyte/macrophages (GMs) but lacks erythroid differentiation ability suggests the existence of an alternative HSC differentiation program. Contribution to different hematopoietic lineages by these MPPs under physiological conditions, however, has not been carefully examined. In this study, we performed a refined characterization of MPPs by subfractionating three distinct subsets based on Flt3 and vascular cell adhesion molecule 1 expression. These MPP subsets differ in their ability to give rise to erythroid and GM lineage cells but are equally potent in lymphoid lineage differentiation in vivo. The developmental hierarchy of these MPP subsets demonstrates the sequential loss of erythroid and then GM differentiation potential during early hematopoiesis. Our results suggest that the first step of lineage commitment from HSCs is not simply a selection between the lymphoid and the myeloid lineage.

Antagonistic effect of CCAAT enhancer-binding protein-alpha and Pax5 in myeloid or lymphoid lineage choice in common lymphoid progenitors

Lymphoid lineage-committed progenitors, such as common lymphoid progenitors (CLPs), maintain a latent myeloid differentiation potential, which can be initiated by stimulation through exogenously expressed cytokine receptors, including IL-2 receptors. Here we show that the transcription factor CCAAT enhancer-binding protein-alpha (C/EBPalpha) is promptly up-regulated in CLPs upon ectopic IL-2 stimulation. Enforced C/EBPalpha expression is sufficient to initiate myeloid differentiation from CLPs, as well as from proT and proB cells, even though proB cells do not give rise to myeloid cells after ectopic IL-2 stimulation. Expression of Pax5, a B lymphoid-affiliated transcription factor, is completely suppressed by enforced C/EBPalpha but not by ectopic IL-2 stimulation in proB cells. Introduction of Pax5 blocks ectopic IL-2 receptor-mediated myeloid lineage conversion in CLPs. These data suggest that C/EBPalpha is a proximal target of cytokine-induced lineage conversion in lymphoid progenitors. Furthermore, complete loss of Pax5 expression triggered by up-regulation of C/EBPalpha is a critical event for lineage conversion from lymphoid to myeloid lineage in CLPs and proB cells.

Heterogeneity of Flt3-expressing multipotent progenitors in mouse bone marrow

Mechanisms of lymphoid and myeloid lineage choice by hemopoietic stem cells remain unclear. In this study we show that the multipotent progenitor (MPP) population, which is immediately downstream of hemopoietic stem cells, is heterogeneous and can be subdivided in terms of VCAM-1 expression. VCAM-1(+) MPPs were fully capable of differentiating into both lymphoid and myeloid lineages. In contrast, VCAM-1(-) MPPs gave rise to lymphocytes predominately in vivo. T and B cell development from VCAM-1(-) MPPs was 1 wk faster than that from VCAM-1(+) MPPs. Furthermore, VCAM-1(+) MPPs gave rise to common myeloid progenitors and VCAM-1(-) MPPs in vivo, indicating that VCAM-1(-) MPPs are progenies of VCAM-1(+) MPPs. VCAM-1(-) MPPs, in turn, developed into lymphoid lineage-restricted common lymphoid progenitors. These results establish a hierarchy of developmental relationship between MPP subsets and lymphoid and myeloid progenitors. In addition, VCAM-1(+) MPPs may represent the branching point between the lymphoid and myeloid lineages.

IL-7 receptor signaling is necessary for stage transition in adult B cell development through up-regulation of EBF

Cytokine receptor signals have been suggested to stimulate cell differentiation during hemato/lymphopoiesis. Such action, however, has not been clearly demonstrated. Here, we show that adult B cell development in IL-7^−/− and IL-7Rα^2/− mice is arrested at the pre–pro-B cell stage due to insufficient expression of the B cell–specific transcription factor EBF and its target genes, which form a transcription factor network in determining B lineage specification. EBF expression is restored in IL-7^−/− pre–pro-B cells upon IL-7 stimulation or in IL-7Rα^−/− pre–pro-B cells by activation of STAT5, a major signaling molecule downstream of the IL-7R signaling pathway. Furthermore, enforced EBF expression partially rescues B cell development in IL-7Rα^−/− mice. Thus, IL-7 receptor signaling is a participant in the formation of the transcription factor network during B lymphopoiesis by up-regulating EBF, allowing stage transition from the pre–pro-B to further maturational stages.