Mechanistic Basis of ex Vivo Umbilical Cord Blood Stem Progenitor Cell Expansion.

Umbilical cord blood (CB) transplantation has been used successfully in humans for three decades due to its rapid availability for patients lacking a suitable allogeneic donor, less stringent HLA matching requirements, and low rates of relapse and chronic graft-versus-host disease (GVHD). However, CB transplantation is associated with complications, such as delayed hematopoietic engraftment, graft failure, which increases infection and bleeding and causes longer hospital stays, and transplant-related mortality. The majority of these biological limitations are due to the unforeseeable functional potency of multipotent hematopoietic stem cells (HSCs), which reduce the predictability of successful transplantation; however, several strategies have been developed to increase the number of hematopoietic stem progenitor cells (HSPCs) infused during CB transplantation. This review primarily addresses the methods that promote ex vivo CB expansion within the context of symmetrical and asymmetrical HSC division and those that rely on epigenetic mechanisms, along with the reportedly most successful cytokine combinations. We also review recent clinical research on small molecules (StemRegenin-1, UM171, and nicotinamide) in ex vivo expanded CB and discuss yet unvalidated preclinical strategies. Expanding and transplanting CB graft enriched in HSPCs in a single CB unit is a particularly exciting prospect with the potential to improve the use and availability of CB grafts. Greater knowledge of optimal ex vivo expansion strategies, cell longevity, and graft potency will expand the scope of cellular therapies. Also the development of adequate ex vivo HSPC expansion strategies could bring expanded cord blood grafts to the forefront of transplant therapy and regenerative medicine.

Combined immune score of lymphocyte to monocyte ratio and immunoglobulin levels predicts treatment-free survival of multiple myeloma patients after autologous stem cell transplant.

Outcomes after ASCT are highly variable making it difficult to predict risk of disease progression. We analyzed the impact of clinically available immune-related biomarkers on treatment-free survival (TFS) in 130 patients receiving Mel200 and ASCT. Absolute lymphocyte count (ALC), monocyte count (AMC), neutrophil count (ANC), and immunoglobulin (Ig) levels were collected on day -2 and 90 of ASCT. The lymphocyte-monocyte (LMR) and neutrophil-lymphocyte ratios (NLR) were then derived. At Day +90, we found that low ALC (18 versus 23 months, p = 0.04) or AMC (13 versus 25 months, p = 0.02) predicted for worse TFS. A low LMR predicted for worse TFS (16 versus 52 months, p = 0.004). Patients with two or three suppressed Ig levels had worse TFS (17 versus 51 months, p = 0.04). Median TFS for poor (low LMR and 2-3 suppressed Ig), intermediate, and good (high LMR and 0-1 suppressed Ig) risk groups was 7.5 versus 27 versus 79 months, respectively (p = 0.0004). In a multivariate analysis, a low LMR and suppressed Ig levels were strong independent predictors of poor TFS. We propose an immune score combining these available tests to stratify patients at risk for early progression and identify those who may benefit from intensified post-ASCT consolidation or immunotherapy based approaches.

Chromatin-Modifying Agent-Expanded Human Cord Blood Cells Display Reduced Allostimulatory Capacity.

The limited number of hematopoietic stem cells (HSC) within a single unit of human cord blood currently limits its use as an alternate graft source. However, we have developed a strategy using 5-aza-2'-deoxycytidine (5azaD) and trichostatin A (TSA), which expands transplantable HSC 7- to 10-fold. In our current studies, we have assessed the allostimulatory capacity of the 5azaD/TSA-expanded grafts. The coexpression of immunophenotypic dendritic cell (DC) markers, such as HLA-DR/CD86 and HLA-DR/CD11c as determined by flow cytometry, and the allostimulatory capacity of 5azaD/TSA-expanded cells as determined by MLC were both significantly lower than control. It has been previously demonstrated that STAT3 is indispensable for the differentiation of DC from HSC. Real-time quantitative PCR analysis revealed that 5azaD/TSA-expanded cells expressed more STAT3 transcript than control while also expressing increased transcripts for STAT3 inhibitors including SHP1, p21, and GATA1. Western blot analysis indicates that chromatin-modifying agent-expanded grafts displayed a reduced ratio of p-STAT3 to total STAT3 than control cultures, which is likely indicative of STAT3 inactivity in 5azD/TSA-expanded grafts. Culturing 5azaD/TSA-expanded cord blood cells in extended cultures reveals that they are still capable of generating DC. Notably, STAT3 inactivity was transient because the transcript levels of STAT3 and its inhibitors, including SHP1, were comparable between 5azaD/TSA and control cultures following extended culture. Taken together, our studies indicate that the reduced allostimulatory capacity of 5azaD/TSA-expanded cells is likely because of reversible inhibition of STAT3-dependent DC differentiation. These results suggest that a graft composed of 5azaD/TSA-expanded cells possesses relatively less allostimulatory response but is still capable of generating DC in permissive conditions.

Synergistic Cytotoxic Effect of Busulfan and the PARP Inhibitor Veliparib in Myeloproliferative Neoplasms.

Patients with high-risk myeloproliferative neoplasms (MPNs), and in particular myelofibrosis (MF), can be cured only with allogeneic hematopoietic stem cell transplantation (HSCT). Because MPNs and JAK2V617F-mutated cells show genomic instability, stalled replication forks, and baseline DNA double-strand breaks, DNA repair inhibition with poly(ADP-ribose) polymerase-1 (PARP-1) inhibitors represents a potential novel therapy. Because the alkylating agent busulfan is integral in conditioning regimens for HSCT and leads to stalled replication forks through DNA strand cross-linking, we hypothesized that PARP inhibition with veliparib in combination with busulfan may lead to synergistic cytotoxicity in MPN cells. We first treated 2 MPN cell lines harboring the JAK2V617F mutation (SET2 and HEL) with veliparib at increasing concentrations and measured cell proliferation. SET2 and HEL cells were relatively sensitive to veliparib (IC50 of 11.3 µM and 74.2 µM, respectively). We next treated cells with increasing doses of busulfan in combination with 4 µM veliparib and found that the busulfan IC50 decreased from 27 µM to 4 µM in SET2 cells and from 45.1 µM to 28.1 µM in HEL cells. The mean combination index was .55 for SET2 cells and .40 for HEL cells. Combination treatment of SET2 cells caused G2M arrest in 53% of cells, compared with 30% with veliparib alone and 35% with busulfan alone. G2M arrest was associated with activation of the ATR-Chk1 pathway, as shown by an immunofluorescence assay for phosphorylated Chk1 (p-Chk1). We then tested in vivo the effect of combined low doses of busulfan and veliparib in a JAK2V617F MPN-AML xenotransplant model. Vehicle- and veliparib-treated mice had similar median survival of 39 and 40 days, respectively. Combination treatment increased median survival from 47 days (busulfan alone) to 50 days (P = .02). Finally, we tested the combined effect of busulfan and veliparib on CD34+ cells obtained from the bone marrow or peripheral blood of 5 patients with JAK2V617F-mutated and 2 patients with CALR-mutated MF. MF cells treated with the combination of veliparib and busulfan showed reduced colony formation compared with busulfan alone (87% versus 68%; P = .001). In contrast, treatment of normal CD34+ cells with veliparib did not affect colony growth. Here we show that in vivo confirmation that treatment with the PARP-1 inhibitor veliparib and busulfan results in synergistic cytotoxicity in MPN cells. Our data provide the rationale for testing novel pretransplantation conditioning regimens with combinations of PARP-1 inhibition and reduced doses of alkylators, such as busulfan and melphalan, for high-risk MPNs or MPN-derived acute myelogenous leukemia.

A Novel l-Asparaginase with low l-Glutaminase Coactivity Is Highly Efficacious against Both T- and B-cell Acute Lymphoblastic Leukemias In Vivo.

Acute lymphoblastic leukemia (ALL) is the most common type of pediatric cancer, although about 4 of every 10 cases occur in adults. The enzyme drug l-asparaginase serves as a cornerstone of ALL therapy and exploits the asparagine dependency of ALL cells. In addition to hydrolyzing the amino acid l-asparagine, all FDA-approved l-asparaginases also have significant l-glutaminase coactivity. Since several reports suggest that l-glutamine depletion correlates with many of the side effects of these drugs, enzyme variants with reduced l-glutaminase coactivity might be clinically beneficial if their antileukemic activity would be preserved. Here we show that novel low l-glutaminase variants developed on the backbone of the FDA-approved Erwinia chrysanthemi l-asparaginase were highly efficacious against both T- and B-cell ALL, while displaying reduced acute toxicity features. These results support the development of a new generation of safer l-asparaginases without l-glutaminase activity for the treatment of human ALL.Significance: A new l-asparaginase-based therapy is less toxic compared with FDA-approved high l-glutaminase enzymes Cancer Res; 78(6); 1549-60. ©2018 AACR.

T Cell-Mediated Rejection of Human CD34+ Cells Is Prevented by Costimulatory Blockade in a Xenograft Model.

A xenograft model of stem cell rejection was developed by co-transplantating human CD34+ and allogeneic CD3+ T cells into NOD-scid ɣ-chainnull mice. T cells caused graft failure when transplanted at any CD34/CD3 ratio between 1:50 and 1:.1. Kinetics experiments showed that 2 weeks after transplantation CD34+ cells engrafted the marrow and T cells expanded in the spleen. Then, at 4 weeks only memory T cells populated both sites and rejected CD34+ cells. Blockade of T cell costimulation was tested by injecting the mice with abatacept (CTLA4-IgG1) from day -1 to +27 (group A), from day -1 to +13 (group B), or from day +14 to +28 (group C). On day +56 groups B and C had rejected the graft, whereas in group A graft failure was completely prevented, although with lower stem cell engraftment than in controls (P = .03). Retransplantation of group A mice with same CD34+ cells obtained a complete reconstitution of human myeloid and B cell lineages and excluded latent alloreactivity. In this first xenograft model of stem cell rejection we showed that transplantation of HLA mismatched CD34+ cells may be facilitated by treatment with abatacept and late stem cell boost.

LIGHT Elevation Enhances Immune Eradication of Colon Cancer Metastases.

The majority of patients with colon cancer will develop advanced disease, with the liver being the most common site of metastatic disease. Patients with increased numbers of tumor-infiltrating lymphocytes in primary colon tumors and liver metastases have improved outcomes. However, the molecular factors that could empower antitumor immune responses in this setting remain to be elucidated. We reported that the immunostimulatory cytokine LIGHT (TNFSF14) in the microenvironment of colon cancer metastases associates with improved patient survival, and here we demonstrate in an immunocompetent murine model that colon tumors expressing LIGHT stimulate lymphocyte proliferation and tumor cell-specific antitumor immune responses. In this model, increasing LIGHT expression in the microenvironment of either primary tumors or liver metastases triggered regression of established tumors and slowed the growth of liver metastases, driven by cytotoxic T-lymphocyte-mediated antitumor immunity. These responses corresponded with significant increases in tumor-infiltrating lymphocytes and increased expression of lymphocyte-homing signals in the metastatic tumors. Furthermore, we demonstrated evidence of durable tumor-specific antitumor immunity. In conclusion, increasing LIGHT expression increased T-cell proliferation, activation, and infiltration, resulting in enhanced tumor-specific immune-mediated tumor regressions in primary tumors and colorectal liver metastases. Mechanisms to increase LIGHT in the colon cancer microenvironment warrant further investigation and hold promise as an immunotherapeutic strategy. Cancer Res; 77(8); 1880-91. ©2017 AACR.

Clinical grade isolation of regulatory T cells from G-CSF mobilized peripheral blood improves with initial depletion of monocytes.

Clinical isolation of circulating CD4(+)CD25(+) regulatory T cells (Tregs) from peripheral blood mononuclear cells is usually performed by CD4(+) cell negative selection followed by CD25(+) cell positive selection. Although G-CSF mobilized peripheral blood (G-PBSC) contains a high number of Tregs, a high number of monocytes in G-PBSC limits Treg isolation. Using a small scale device (MidiMACS, Miltenyi) we initially demonstrated that an initial depletion of monocytes would be necessary to obtaina separation of CD4(+)CD25(+)FoxP3(+)CD127(-) cells from G-PBSC (G-Tregs) with a consistent purity >70% and inhibitory activity of T cell alloreactivity in-vitro. We then validated the same approach in a clinical scale setting by separating G-Tregs with clinically available antibodies to perform a CD8(+)CD19(+)CD14(+) cell depletion followed by CD25(+) cell selection (2-step process) or by adding an initial CD14(+) cell depletion (3-step process) using a CliniMACS column. The 3-step approach resulted in a better purity (81±12% vs. 35±33%) and yield (66% vs. 39%). Clinically isolated G-Tregs were also FoxP3(+)CD127(dim) and functionally suppressive in-vitro. Our findings suggest that a better and more consistent purity of Tregs can be achieved from G-PBSC by an initial single depletion of monocytes prior to selection of CD4(+)CD25(+) cells.

Combination of linear accelerator-based intensity-modulated total marrow irradiation and myeloablative fludarabine/busulfan: a phase I study.

Here we examined the addition of intensity-modulated total marrow irradiation (TMI) delivered using a linear accelerator to a myeloablative chemotherapy conditioning regimen before allogeneic hematopoietic stem cell transplantation (HSCT). In this phase I study, we enrolled 14 patients with high-risk hematologic malignancies who received escalating doses of TMI at 3 Gy (n = 3), 6 Gy (n = 3), 9 Gy (n = 6), and 12 Gy (n = 2) in combination with intravenous (i.v.) fludarabine 160 mg/m(2) and targeted busulfan (area under the curve, 4800 µM*minute). Peripheral blood mobilized stem cells were obtained from HLA-matched related (n = 9) or unrelated (n = 4) or 1 antigen-mismatched unrelated (n = 1) donors. All patients rapidly engrafted and recovered their immune cells. Overall, Bearman extrahematologic toxicity were limited to grades 1 or 2, with oral mucositis grade 1 in 64% and grade 2 in 36% of the patients. With a median follow-up of 1126 days (range, 362 to 1469) for living patients, the overall survival was 50% and relapse-free survival was 43%. Of 7 deaths, 3 were due to relapse and 4 to transplantation-related complications. We conclude that 9 Gy TMI can be combined with myeloablative chemotherapy in the design of new preparative regimens for HSCT. This study was registered at clinicaltrials.gov as NCT00988013.

CD11b+GR1+ myeloid cells secrete NGF and promote trigeminal ganglion neurite growth: implications for corneal nerve regeneration.

PURPOSE: We characterized fluorescent bone marrow cells (YFP(+) BMCs) in the thy1-YFP mouse and determine if they promote trigeminal ganglion (TG) cell neurite growth. METHODS: Excimer laser annular keratectomy was performed in thy1-YFP mice, and corneas were imaged. BMCs were harvested from femur and tibia, and the expression of surface markers on YFP(+) BMCs was analyzed by flow cytometry. The immunosuppressive action of BMCs (YFP(+) and YFP(-)) was evaluated in an allogenic mixed lymphocyte reaction (MLR). Neurotrophic action of BMCs (YFP(+) and YFP(-)) was determined in compartmental and transwell cultures of dissociated TG cells. RESULTS: Following annular keratectomy, YFP(+) BMCs infiltrated the cornea. YFP(+) BMCs shared surface markers (CD11b+Gr1+Ly6C+Ly6G-F4/80(low)) with monocytic myeloid-derived suppressor cells (MDSCs), had similar morphology, and suppressed T-cell proliferation in allogenic MLR in a dose-dependent manner. YFP(+) BMCs, but not YFP(-) BMCs, significantly increased growth of TG neurites in vitro. When cultured in a transwell with TG neurites, YFP(+) BMCs expressed neurotrophins and secreted nerve growth factor (NGF) in conditioned medium. YFP(+) BMCs that infiltrated the cornea maintained their phenotype and actions (neuronal and immune). CONCLUSIONS: YFP(+) BMCs in thy1-YFP mice have immunophenotypic features of MDSCs. They secrete NGF and promote neuroregeneration. Their immunosuppressive and neurotrophic actions are preserved after corneal infiltration. These findings increase our understanding of the beneficial roles played by leukocyte trafficking in the cornea and may lead to therapeutic strategies that use NGF-secreting myeloid cells to repair diseased or injured neurons.

Human CD4(+)CD25(+) Cells in Combination with CD34(+) Cells and Thymoglobulin to Prevent Anti-hematopoietic Stem Cell T Cell Alloreactivity.

Cotransplantation of human CD34(+) hematopoietic stem cells (HSC) and CD4(+)CD25(+)FoxP3(+) regulatory T cells (Tregs) could prevent anti-HSC alloreactivity and reduce the risk of rejection in HLA mismatched transplants. To pursue this hypothesis we cocultured CD34(+) cells and CD4(+)CD25(+) cells immunomagnetically isolated (Milteny) from human peripheral blood (unmanipulated or granulocyte-colony stimulating factor [G-CSF] mobilized) or cord blood. Enriched Tregs obtained from the same source (autologous) of CD34(+) cells showed greater inhibitory effect on T cell alloreactivity than third-party (allogeneic) Tregs. The immunosuppressive activity of Tregs was maintained after stimulation with allogeneic CD34(+) cells and Tregs did not modify the clonogenic activity of CD34(+) cells in vitro. Cotransplantation of Tregs with CD34(+) cells at 1:1 or 2:1 ratios in nonobese diabetic/severe combined immunodeficiency (NOD/SCID) mice resulted in normal hematopoietic stem cell engraftment. Incubation with physiologic doses of rabbit antithymocyte globulin (rATG, thymoglobulin) did not affect the number of Tregs in 6-day culture. Upon exposure to thymoglobulin Tregs maintained their suppressive activity, increased expression of CCR7, and released multiple cytokines, primarily interleukin (IL)10. Our findings suggest that human autologous or allogeneic Tregs could be cotransplanted with CD34(+) cells after preparative regimens including thymoglobulin.

Cord blood nucleated cells induce delayed T cell alloreactivity.

Cord blood (CB) mononuclear cells (MNCs) can be transplanted in HLA mismatched recipients with limited graft rejection or graft-versus-host disease (GVHD). Previous studies have shown that naive T cells and hyporesponsive dendritic cells are largely represented in CB. Data presented here demonstrate that CB MNCs are unable to stimulate allogeneic T cell proliferative or cytotoxic responses in standard in vitro assays. However, a suppressive effect of CB MNCs was ruled out because purified CD34(+) cells or CD14(+) monocytes stimulated T cell responses that were not inhibited by add-back of CB MNCs. The lack of antigen-presenting cell (APC) activity of CB MNCs in primary mixed lymphocyte culture (MLC) did not induce allogeneic T cell anergy. In fact, rechallenge of T cells with CB CD34(+) cells, or immature monocyte-derived dendritic cells (iMo-DCs) in secondary MLC induced potent T cell proliferative responses. A delayed APC activity of CB MNCs was observed after stimulation with irradiated allogeneic T cells for 6 days, likely because of the upregulation of CD86 and HLA-DR on CB cells. Cytotoxic lymphocytes (CTL) were generated after stimulation of blood T cells with CB MNCs for 4 weeks or CB-derived iMo-DCs for 1 week. Concomitant stimulation of T cells with CB iMo-DC obtained from 2 CB units resulted in the generation of CTLs specific for each CB, independently of the CB:CB cell ratio. These data suggest that the APC activity of CB cells possibly increases posttransplant, and may contribute to delayed graft rejection and/or acute and chronic GVHD.

Allogeneic T cells induce rapid CD34+ cell differentiation into CD11c+CD86+ cells with direct and indirect antigen-presenting function.

Dendritic cells (DCs) derive from CD34+ cells or monocytes and stimulate alloimmune responses in transplantation. We hypothesized that the interaction between CD34+ cells and allogeneic T cells would influence the function of hematopoietic stem cells (HSCs). Cord blood (CB) CD34+ cells proliferated briskly in response to allogeneic, but not autologous, T cells when mixed with irradiated T cells for 6 days in vitro. This proliferation was significantly inhibited by an anti-HLA class II monoclonal antibody (mAb), by an anti-TNFalpha mAb, or by CTLA4-Ig. Allogeneic T cells induced the differentiation of CD34+ progenitors into cells with the morphology of dendritic monocytic precursors and characterized by the expression of HLA-DR, CD86, CD40, CD14, and CD11c, due to an endogenous release of TNFalpha. Cotransplantation of CD34+ cells with allogeneic T cells into nonobese diabetic-severe combined immunodeficiency (NOD/SCID) mice resulted in a greater engraftment of myeloid CD1c+ dendritic cells compared with cotransplantation with autologous T cells. In vitro, CD34+ cell-derived antigen-presenting cells (APCs) were functionally capable of both direct and indirect presentation of alloantigens. Based on these findings, we hypothesize that in HSC transplantation the initial cross talk between allogeneic T cells and CD34+ cells may result in the increased generation of APCs that can present host alloantigens and possibly contribute to the development of graft-versus-host disease.

Differentiation stage-specific activation of p38 mitogen-activated protein kinase isoforms in primary human erythroid cells.

p38alpha, p38beta, p38gamma, and p38delta are four isoforms of p38 mitogen-activated protein (MAP) kinase (MAPK) involved in multiple cellular functions such as cell proliferation, differentiation, apoptosis, and inflammation response. In the present study, we examined the mRNA expression pattern of each of the four isoforms during erythroid differentiation of primary erythroid progenitors. We show that p38alpha and p38gamma transcripts are expressed in early hematopoietic progenitors as well as in late differentiating erythroblasts, whereas p38delta mRNA is only expressed and active during the terminal phase of erythroid differentiation. On the other hand, p38beta is minimally expressed in early CD34(+) hematopoietic progenitors but not expressed in lineage-committed erythroid progenitors. We also determined the phosphorylation/activation of p38alpha, MAPK kinase 3/6, and MAPKAP-2 in response to erythropoietin and stem cell factor. We found that phosphorylation of p38alpha, MAPK kinase kinase 3/6 and MAPKAP-2 occurs only upon growth factor withdrawal in primary erythroid progenitors. Moreover, our data indicate that activation of p38alpha does not induce apoptosis or promote proliferation of erythroid progenitors. On the other hand, under steady-state culture conditions, both p38alpha and p38delta isoforms are increasingly phosphorylated activated in the terminal phase of differentiation. This increased phosphorylation/activity was accompanied by up-regulation of heat shock protein 27 phosphorylation. Finally, we demonstrate that tumor necrosis factor alpha, an inflammatory cytokine that is modulated by p38alpha, is expressed by differentiating erythroblasts and inhibition of p38alpha or tumor necrosis factor alpha results in reduction in differentiation. Taken together, our data demonstrate that both p38alpha and delta isoforms function to promote the late-stage differentiation of primary erythroid progenitors and are likely to be involved in functions related to erythrocyte membrane remodeling and enucleation.

Regulation of myeloma cell growth through Akt/Gsk3/forkhead signaling pathway.

The interleukin-6 (IL-6) signaling pathway contributes to myeloma cell growth and viability through activation of the PI3/Akt kinase pathway. To understand the downstream signaling elements in the PI3/Akt kinase pathway that are involved in the regulation of myeloma cell growth, we determined the role played by glycogen synthase kinase 3 (Gsk3) and forkhead transcription factors (FH) in the RPMI-8226 myeloma cell line. We demonstrate that both Gsk3 and FH transcription factors FKHRL1 (FOX3a), FKHR (FOXO1a), and AFX (FOXO4) are phosphorylated (inactivated) by IL-6. Further, we show that inhibitors of Gsk3 induce dephosphorylation of FKHRL1 and FKHR at their threonine sites and upregulate the cyclin-dependent kinase inhibitor p27(kip1). Finally, we show that inhibition of Gsk3 activity is sufficient to suppress cell growth and induce apoptosis thus overriding the effects of IL-6 in myeloma cells.

Phosphorylation of forkhead transcription factors by erythropoietin and stem cell factor prevents acetylation and their interaction with coactivator p300 in erythroid progenitor cells.

The mammalian forkhead transcription factors, FOXO3a (FKHRL1), FOXO1a (FKHR) and FOXO4 (AFX) are negatively regulated by PKB/Akt kinase. In the present study we examined the engagement of forkhead family of transcription factors in erythropoietin (Epo)- and stem cell factor (SCF)-mediated signal transduction. Our data show that all three forkhead family members, FOXO3a, FOXO1a and FOXO4 are phosphorylated in human primary erythroid progenitors. Experiments performed to determine various upstream signaling pathways contributing to phosphorylation of forkhead family members show that only PI-3-kinase pathway is required for inactivation of FOXO3a. Our data also demonstrate that during Epo deprivation FOXO3a interacts with the transcriptional coactivator p300 and such interaction is disrupted by stimulation of cells with Epo. To determine the domains in FOXO3a, mediating its interaction with p300, we performed GST pull-down assays and found that the N-terminus region containing the first 52 amino acids was sufficient for binding p300. Finally, our data demonstrate that FOXO3a and FOXO1a are acetylated during growth factor deprivation and such acetylation is reversed by stimulation with Epo. Thus mammalian forkhead transcription factors are involved in Epo and SCF signaling in primary erythroid progenitors and may play a role in the induction of apoptotic and mitogenic signals.