Differential response of mesenchymal stromal cells (MSCs) to type 1 ex vivo cytokine priming: implications for MSC therapy.

BACKGROUND AIMS: Mesenchymal stromal cells (MSCs) are polymorphic, adherent cells with the capability to stimulate tissue regeneration and modulate immunity. MSCs have been broadly investigated for potential therapeutic applications, particularly immunomodulatory properties, wound healing and tissue regeneration. The exact physiologic role of MSCs, however, remains poorly understood, and this gap in knowledge significantly impedes the rational development of therapeutic cells. Here, we considered interferon γ (IFN-γ) and tumor necrosis factor alpha (TNF-α), two cytokines likely encountered physiologically and commonly used in cell manufacturing. For comparison, we studied interleukin-10 (IL-10) (anti-inflammatory) and interleukin-4 (IL-4) (type 2 cytokine). METHODS: We directly assessed the effects of these cytokines on bone marrow MSCs by comparing RNA Seq transcriptional profiles. Western blotting and flow cytometry were also used to evaluate effects of cytokine priming. RESULTS: The type 1 cytokines (IFN-γ and TNF-α) induced striking changes in gene expression and remarkably different profiles from one another. Importantly, priming MSCs with either of these cytokines did not increase variability among multiple donors beyond what is intrinsic to non-primed MSCs from different donors. IFN-γ-primed MSCs expressed IDO1 and chemokines that recruit activated T cells. In contrast, TNF-α-primed MSCs expressed genes in alternate pathways, namely PGE2 and matrix metalloproteinases synthesis, and chemokines that recruit neutrophils. IL-10 and IL-4 priming had little to no effect. CONCLUSIONS: Our data suggest that IFN-γ-primed MSCs may be a more efficacious immunosuppressive therapy aimed at diseases that target T cells (ie, graft-versus-host disease) compared with TNF-α-primed or non-primed MSCs, which may be better suited for therapies in other disease settings. These results contribute to our understanding of MSC bioactivity and suggest rational ex vivo cytokine priming approaches for MSC manufacturing and therapeutic applications.

Microbial Changes occurring during oronasal fistula wound healing.

The oral microbiome is a complex community that matures with dental development while oral health is also a recognized risk factor for systemic disease. Despite the oral cavity having a substantial microbial burden, healing of superficial oral wounds occurs quickly and with little scarring. By contrast, creation of an oro-nasal fistula (ONF), often occurring after surgery to correct a cleft palate, is a significant wound healing challenge that is further complicated by a connection of the oral and nasal microbiome. In this study, we characterized the changes in the oral microbiome of mice following a freshly inflicted wound in the oral palate that results in an open and unhealed ONF. Creation of an ONF in mice significantly lowered oral microbiome alpha diversity, with concurrent blooms of Enterococcus faecalis, Staphylococcus lentus, and Staphylococcus xylosus in the oral cavity. Treatment of mice with oral antibiotics one week prior to ONF infliction resulted in a reduction in the alpha diversity, prevented E. faecalis and S. lentus, and S. xylosus blooms, but did not impact ONF healing. Strikingly, delivery of the beneficial microbe Lactococcus lactis subsp. cremoris (LLC) to the wound bed of the freshly inflicted ONF via a PEG-MAL hydrogel vehicle resulted in rapid healing of the ONF. Healing of the ONF was associated with the maintenance of relatively high microbiome alpha diversity, and limited the abundance of E. faecalis and S. lentus, and S. xylosus in the oral cavity. These data demonstrate that a freshly inflicted ONF in the murine palate is associated with a dysbiotic oral microbiome state that may prevent ONF healing, and a bloom of opportunistic pathogens. The data also demonstrate that delivery of a specific beneficial microbe, LLC, to the ONF can boost wound healing, can restore and/or preserve oral microbiome diversity, and inhibit blooms of opportunistic pathogens.

Unrecognized introductions of SARS-CoV-2 into the US state of Georgia shaped the early epidemic.

In early 2020, as diagnostic and surveillance responses for severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) ramped up, attention focused primarily on returning international travelers. Here, we build on existing studies characterizing early patterns of SARS-CoV-2 spread within the USA by analyzing detailed clinical, molecular, and viral genomic data from the state of Georgia through March 2020. We find evidence for multiple early introductions into Georgia, despite relatively sparse sampling. Most sampled sequences likely stemmed from a single or small number of introductions from Asia three weeks prior to the state's first detected infection. Our analysis of sequences from domestic travelers demonstrates widespread circulation of closely related viruses in multiple US states by the end of March 2020. Our findings indicate that the exclusive focus on identifying SARS-CoV-2 in returning international travelers early in the pandemic may have led to a failure to recognize locally circulating infections for several weeks and point toward a critical need for implementing rapid, broadly targeted surveillance efforts for future pandemics.

Single-Amplicon Multiplex Real-Time Reverse Transcription-PCR with Tiled Probes To Detect SARS-CoV-2 spike Mutations Associated with Variants of Concern.

To provide an accessible and inexpensive method to surveil for severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) mutations, we developed a multiplex real-time reverse transcription-PCR (rRT-PCR) assay, the Spike single-nucleotide polymorphism (SNP) assay, to detect specific mutations in the spike receptor binding domain. A single primer pair was designed to amplify a 348-bp region of spike, and probes were initially designed to detect K417, E484K, and N501Y. The assay was evaluated using characterized variant sample pools and residual nasopharyngeal samples. Variant calls were confirmed by SARS-CoV-2 genome sequencing in a subset of samples. Subsequently, a fourth probe was designed to detect L452R. The lower limit of 95% detection was 2.46 to 2.48 log10 genome equivalents (GE)/ml for the three initial targets (∼1 to 2 GE/reaction). Among 253 residual nasopharyngeal swabs with detectable SARS-CoV-2 RNA, the Spike SNP assay was positive in 238 (94.1%) samples. All 220 samples with threshold cycle (CT) values of <30 for the SARS-CoV-2 N2 target were detected, whereas 18/33 samples with N2 CT values of ≥30 were detected. Spike SNP results were confirmed by sequencing in 50/50 samples (100%). Addition of the 452R probe did not affect performance for the original targets. The Spike SNP assay accurately identifies SARS-CoV-2 mutations in the receptor binding domain, and it can be quickly modified to detect new mutations that emerge.

Pimozide and Imipramine Blue Exploit Mitochondrial Vulnerabilities and Reactive Oxygen Species to Cooperatively Target High Risk Acute Myeloid Leukemia.

Acute myeloid leukemia (AML) is a heterogeneous disease with a high relapse rate. Cytokine receptor targeted therapies are therapeutically attractive but are subject to resistance-conferring mutations. Likewise, targeting downstream signaling pathways has been difficult. Recent success in the development of synergistic combinations has provided new hope for refractory AML patients. While generally not efficacious as monotherapy, BH3 mimetics are very effective in combination with chemotherapy agents. With this in mind, we further explored novel BH3 mimetic drug combinations and showed that pimozide cooperates with mTOR inhibitors and BH3 mimetics in AML cells. The three-drug combination was able to reach cells that were not as responsive to single or double drug combinations. In Flt3-internal tandem duplication (ITD)-positive cells, we previously showed pimozide to be highly effective when combined with imipramine blue (IB). Here, we show that Flt3-ITD+ cells are sensitive to an IB-induced dynamin 1-like (Drp1)-p38-ROS pathway. Pimozide contributes important calcium channel blocker activity converging with IB on mitochondrial oxidative metabolism. Overall, these data support the concept that antioxidants are a double-edged sword. Rationally designed combination therapies have significant promise for further pre-clinical development and may ultimately lead to improved responses.

Metagenomic Sequencing To Detect Respiratory Viruses in Persons under Investigation for COVID-19.

Broad testing for respiratory viruses among persons under investigation (PUIs) for SARS-CoV-2 has been performed inconsistently, limiting our understanding of alternative viral infections and coinfections in these patients. RNA metagenomic next-generation sequencing (mNGS) offers an agnostic tool for the detection of both SARS-CoV-2 and other RNA respiratory viruses in PUIs. Here, we used RNA mNGS to assess the frequencies of alternative viral infections in SARS-CoV-2 RT-PCR-negative PUIs (n = 30) and viral coinfections in SARS-CoV-2 RT-PCR-positive PUIs (n = 45). mNGS identified all viruses detected by routine clinical testing (influenza A [n = 3], human metapneumovirus [n = 2], and human coronavirus OC43 [n = 2], and human coronavirus HKU1 [n = 1]). mNGS also identified both coinfections (1, 2.2%) and alternative viral infections (4, 13.3%) that were not detected by routine clinical workup (respiratory syncytial virus [n = 3], human metapneumovirus [n = 1], and human coronavirus NL63 [n = 1]). Among SARS-CoV-2 RT-PCR-positive PUIs, lower cycle threshold (CT ) values correlated with greater SARS-CoV-2 read recovery by mNGS (R2, 0.65; P < 0.001). Our results suggest that current broad-spectrum molecular testing algorithms identify most respiratory viral infections among SARS-CoV-2 PUIs, when available and implemented consistently.

Metagenomic sequencing to detect respiratory viruses in persons under investigation for COVID-19.

We used metagenomic next-generation sequencing (mNGS) to assess the frequencies of alternative viral infections in SARS-CoV-2 RT-PCR negative persons under investigations (PUIs) (n=30) and viral co-infections in SARS-CoV-2 RT-PCR positive PUIs (n=45). mNGS identified both co-infections and alternative viral infections that were not detected by routine clinical workup.

Imipramine blue sensitively and selectively targets FLT3-ITD positive acute myeloid leukemia cells.

Aberrant cytokine signaling initiated from mutant receptor tyrosine kinases (RTKs) provides critical growth and survival signals in high risk acute myeloid leukemia (AML). Inhibitors to FLT3 have already been tested in clinical trials, however, drug resistance limits clinical efficacy. Mutant receptor tyrosine kinases are mislocalized in the endoplasmic reticulum (ER) of AML and play an important role in the non-canonical activation of signal transducer and activator of transcription 5 (STAT5). Here, we have tested a potent new drug called imipramine blue (IB), which is a chimeric molecule with a dual mechanism of action. At 200-300 nM concentrations, IB is a potent inhibitor of STAT5 through liberation of endogenous phosphatase activity following NADPH oxidase (NOX) inhibition. However, at 75-150 nM concentrations, IB was highly effective at killing mutant FLT3-driven AML cells through a similar mechanism as thapsigargin (TG), involving increased cytosolic calcium. IB also potently inhibited survival of primary human FLT3/ITD+ AML cells compared to FLT3/ITDneg cells and spared normal umbilical cord blood cells. Therefore, IB functions through a mechanism involving vulnerability to dysregulated calcium metabolism and the combination of fusing a lipophilic amine to a NOX inhibiting dye shows promise for further pre-clinical development for targeting high risk AML.

Synergistic cell death in FLT3-ITD positive acute myeloid leukemia by combined treatment with metformin and 6-benzylthioinosine.

Current therapy for acute myeloid leukemia (AML) primarily includes high-dose cytotoxic chemotherapy with or without allogeneic stem cell transplantation. Targeting unique cellular metabolism of cancer cells is a potentially less toxic approach. Monotherapy with mitochondrial inhibitors like metformin have met with limited success since escape mechanisms such as increased glycolytic ATP production, especially in hyperglycemia, can overcome the metabolic blockade. As an alternative strategy for metformin therapy, we hypothesized that the combination of 6-benzylthioinosine (6-BT), a broad-spectrum metabolic inhibitor, and metformin could block this drug resistance mechanism. Metformin treatment alone resulted in significant suppression of ROS and mitochondrial respiration with increased glycolysis accompanied by modest cytotoxicity (10-25%). In contrast, 6-BT monotherapy resulted in inhibition of glucose uptake, decreased glycolysis, and decreased ATP with minimal changes in ROS and mitochondrial respiration. The combination of 6-BT with metformin resulted in significant cytotoxicity (60-70%) in monocytic AML cell lines and was associated with inhibition of FLT3-ITD activated STAT5 and reduced c-Myc and GLUT-1 expression. Therefore, although the anti-tumor and metabolic effects of metformin have been limited by the metabolic reprogramming within cells, the novel combination of 6-BT and metformin targets this bypass mechanism resulting in reduced glycolysis, STAT5 inhibition, and increased cell death.

Nanoproteomic assays on hematopoietic stem cells.

Dysregulation of cytokine signaling pathways is associated with benign and malignant hematologic disorders. Improvements in therapy rely on understanding the biology of the pathways and the proteins involved. Studying these pathways in patient samples is challenging as samples are difficult to obtain, contain fewer cells, and are heterogeneous in nature. To address some of these difficulties, we have utilized the technique of microcapillary electrophoresis. Using the NanoPro 1000 system (ProteinSimple) which is built on an automated, capillary-based immunoassay platform, we have developed rapid and quantitative assays for specific proteins from relatively small sample sizes. The NanoPro provides precise and quantitative data of the phosphorylation states of a specific protein of interest. We describe our experience with NanoPro assay development and optimization with specific application toward understanding aberrant cytokine signaling in human leukemia cells.

Capillary nano-immunoassay for Akt 1/2/3 and 4EBP1 phosphorylation in acute myeloid leukemia.

BACKGROUND: Overall cure rates in acute myeloid leukemia (AML) continue to range between 60-65% with disease relapse being a major cause of mortality. The PI3K-Akt-mTOR kinase pathway plays a vital role in pro-survival signals within leukemic cells and inhibition of this pathway is being investigated to improve patient outcomes. Tracking activation of multiple signaling proteins simultaneously in patient samples can be challenging especially with limiting cell numbers within rare sub-populations. METHODS: The NanoPro 1000 system (ProteinSimple) is built on an automated, capillary-based immunoassay platform and enables a rapid and quantitative analysis of specific proteins and their phosphorylation states. We have utilized this nano-immunoassay to examine activation of Akt 1/2/3 and downstream mTOR target--eukaryotic initiation factor 4E-Binding Protein 1 (4EBP1). RESULTS: Assays for Akt 1/2/3 and 4EBP1 were standardized using AML cell lines (MV4-11, MOLM-14, OCI-AML3 and HL-60) prior to testing in patient samples. Target inhibition was studied using mTOR 1/2 inhibitor AZD-8055 and results were corroborated by Western blotting. The assay was able to quantify nanogram amounts of 4EBP1 and Akt 1/2/3 in AML cell lines and primary pediatric AML samples and results were quantifiable, consistent and reproducible. CONCLUSION: Our data provides a strong basis for testing this platform on a larger scale and our long term aim is to utilize this nano-immunoassay prospectively in de-novo AML to be able to identify poor responders who might benefit from early introduction of targeted therapy.

Hematopoietic reconstitution with androgenetic and gynogenetic stem cells.

Parthenogenetic embryonic stem (ES) cells with two oocyte-derived genomes (uniparental) have been proposed as a source of autologous tissue for transplantation. The therapeutic applicability of any uniparental cell type is uncertain due to the consequences of genomic imprinting that in mammalian uniparental tissues causes unbalanced expression of imprinted genes. We transplanted uniparental fetal liver cells into lethally irradiated adult mice to test their capacity to replace adult hematopoietic tissue. Both maternal (gynogenetic) and paternal (androgenetic) derived cells conveyed long-term, multilineage reconstitution of hematopoiesis in recipients, with no associated pathologies. We also establish that uniparental ES cells can differentiate into transplantable hematopoietic progenitors in vitro that contribute to long-term hematopoiesis in recipients. Hematopoietic tissue in recipients maintained fidelity of parent-of-origin methylation marks at the Igf2/H19 locus; however, variability occurred in the maintenance of parental-specific methylation marks at other loci. In summary, despite genomic imprinting and its consequences on development that are particularly evident in the androgenetic phenotype, uniparental cells of both parental origins can form adult-transplantable stem cells and can repopulate an adult organ.

A STAT5 modifier locus on murine chromosome 7 modulates engraftment of hematopoietic stem cells during steady-state hematopoiesis.

Homologous disruption of expression of signal transducer and activator of transcription 5a (STAT5a) and STAT5b (STAT5ab(-/-)) in mice results in hematopoietic stem cells (HSCs) that can engraft irradiated hosts alone but are noncompetitive against wild-type HSCs. To explore mechanisms for this phenotype, we crossed the STAT5 mutations onto an HW80 background congenic to the original C57BL/6 that differs in a small chromosome 7 genomic locus. We previously demonstrated that C57BL/6 or HW80 background STAT5ab(-/-) bone marrow (BM) cells showed equal repopulating function either competitively or noncompetitively in irradiated hosts. However, one intraperitoneal injection of wild-type green fluorescent protein (GFP) transgenic BM cells into unconditioned newborn STAT5ab(-/-) recipients of either background was sufficient for high-level donor engraftment. Furthermore, haploinsufficiency of STAT5 (STAT5ab(+/-)) allowed improved engraftment over wild-type recipients, indicating a dose-dependent requirement for STAT5 activation. In reciprocal experiments, STAT5ab(-/-) BM was transplanted into nonirradiated W/W(v) hosts. In these mice, C57BL/6 STAT5ab(-/-) BM cells were 10-fold more defective in long-term engraftment than control wild-type BM cells and HW80 STAT5ab(-/-) BM cells were 5- to 10-fold more defective than C57BL/6 STAT5ab(-/-) BM cells. Therefore, we conclude that STAT5 plays a critical role during steady-state HSC engraftment and a chromosome 7 modifier locus regulates this activity.

Tissue inhibitor of matrix metalloproteinase-1 overexpression in M1 myeloblasts impairs IL-6-induced differentiation.

The balance between matrix metalloproteinase (MMP) and tissue inhibitor of matrix metalloproteinase (TIMP) is important for extracellular matrix interactions of hematopoietic cells. MMP-independent growth modulating activity for TIMP-1 on B lymphocytes and erythroid progenitors has also been described, but a role for TIMP-1 in myelomonocytic differentiation has not been previously reported. In this study, we demonstrate that TIMP-1 overexpression impairs differentiation of the myeloblastic M1 cell line following interleukin (IL)-6 stimulation. We generated retroviral vectors coexpressing human TIMP-1 and the green fluorescent protein (GFP) and stably transduced murine M1 myeloid cells. TIMP-1 expressing cells showed a large reduction in IL-6-induced macrophage differentiation in vitro that was reversible with a specific monoclonal antibody. The differentiation delay in M1/TIMP-1 cells was also specifically reversible by pharmacologic phosphatidylinositol-3 kinase (PI3-K) inhibition. Additionally, overexpression of a TIMP-1/GFP fusion protein also impaired M1 differentiation and this protein was localized to the cell surface, consistent with an autocrine receptor-mediated mechanism. Surprisingly, TIMP-1 transduced cells had a selective advantage for growth in IL-6, indicating that functional effects on growth and differentiation of M1 cells were primarily through an autocrine mechanism. Intrinsic TIMP-1 expression in myeloid leukemia cells might thus impact upon survival or differentiation.

Increased numbers of committed myeloid progenitors but not primitive hematopoietic stem/progenitors in mice lacking STAT6 expression.

Signal transducer and activator of transcription-6 (STAT6) plays important roles in cytokine signaling via interleukin-4 and -13 receptors (IL-4R and IL-13R). Mice in which STAT6 has been disrupted by homologous recombination show defects in T helper cell type 2 (Th2) lymphocyte production, resulting in an accumulation of Th1 cells. In addition to defects in differentiation and proliferation of T lymphocytes, STAT6-deficient mice show increased cell-cycle activation and frequency of myeloid progenitors. Although this has been shown to be mediated through Oncostatin M production by T cells, IL-4Ralpha and STAT6 have also recently been found to be enriched for expression in primitive hematopoietic stem cells (HSCs) in gene expression-profiling studies. Therefore, we have investigated whether defects in hematopoietic function in mice lacking STAT6 expression extended into the primitive hematopoietic compartments of the bone marrow. Here, we report that STAT6 deficiency increased bone marrow-committed myeloid progenitors but did not alter the number of cells enriched for HSC/multipotent progenitors, primitive cobblestone area-forming cells assayed in vitro, or bone marrow short-term or long-term repopulating cells assayed in vivo. Therefore, the requirement for STAT6 activation during hematopoiesis is limited, and primitive hematopoietic cell types are insulated against possible effects of cytokine stimulation by Th1 cells.

Hematopoietic-repopulating defects from STAT5-deficient bone marrow are not fully accounted for by loss of thrombopoietin responsiveness.

Signal transducer and activator of transcription-5 (STAT5) plays an important role in repopulating activity of hematopoietic stem cells (HSCs). However, the relationship of STAT5 activation with early acting cytokine receptors is not well established. We have directly compared bone marrow (BM) from mice mutant for STAT5a and STAT5b (STAT5ab(-/-)) with that from mice lacking c-Mpl (c-Mpl(-/-)), the thrombopoietin receptor. Both STAT5 and c-Mpl deficiency only mildly affected committed myeloid progenitors assayed in vitro, but STAT5ab(-/-) BM showed lower Gr-1+ (4.4-fold), B220+ (23-fold), CD4+ (20-fold), and Ter119+ (17-fold) peripheral blood repopulating activity than c-Mpl(-/-) BM against wild-type competitor in long-term repopulating assays in vivo. Direct head-to-head competitions of STAT5ab(-/-) BM and c-Mpl(-/-) BM showed up to a 25-fold reduction in STAT5ab(-/-) contribution. Differences affecting reconstitution of primitive c-Kit+Lin-Sca-1+ multipotent progenitor (MPP)/HSC (1.8-fold) and c-Kit+Lin-Sca-1- oligopotent progenitor BM fractions (3.3-fold) were more modest. In serial transplantation experiments, STAT5ab(-/-) and c-Mpl(-/-) BM both failed to provide consistent engraftment in tertiary hosts and could not radioprotect lethally irradiated quaternary recipients. These results indicate substantial overlap in c-Mpl-STAT5 signaling defects at the MPP/HSC level but indicate that STAT5 is activated independent of c-Mpl to promote multilineage hematopoietic differentiation.

Cell intrinsic defects in cytokine responsiveness of STAT5-deficient hematopoietic stem cells.

Secreted growth factors are integral components of the bone marrow (BM) niche and can regulate survival, proliferation, and differentiation of committed hematopoietic stem cells (HSCs). However, downstream genes activated in HSCs by early-acting cytokines are not well characterized. To better define intracellular cytokine signaling in HSC function, we have analyzed mice lacking expression of both signal transducer and activator of transcription 5a (STAT5a) and STAT5b (STAT5ab(-/-)). These studies specifically avoided possible autoimmune and/or splenomegaly disease-mediated indirect effects on HSC function by using 2 independent approaches: (1) by crossing onto the C57Bl/6 RAG2(-/-) background, and (2) by generation of wild-type chimeric mice reconstituted with transplanted STAT5ab(-/-) BM cells. These experiments demonstrated that STAT5-deficient HSCs have cell autonomous defects in competitive long-term repopulating activity. Furthermore, in the chimeric mice, injected wild-type BM cells showed a progressive multilineage competitive repopulating advantage in vivo, demonstrating that steady-state hematopoiesis was also highly STAT5-dependent. Consistent with the in vivo repopulating deficiency, when Sca-1(+)c-kit(+)lin(-) (KLS) cells were isolated and stimulated with growth factors in vitro, up to a 13-fold reduced expansion of total nucleated cells was observed in response to cocktails containing interleukin 3 (IL-3), IL-6, stem cell factor (SCF), Flt3 ligand, and thrombopoietin. Notably, a 10-fold reduction in expansion was observed with IL-3 and SCF. However, STAT5 activation was not required for regeneration of the KLS pool in vivo following transplant or for secondary repopulating ability. These studies support a major role for STAT5 activation as a cellular determinant of cytokine-mediated HSC repopulating potential but not self-renewal capacity.

Reduced lymphomyeloid repopulating activity from adult bone marrow and fetal liver of mice lacking expression of STAT5.

Signal transducers and activators of transcription (STATs) are intracellular mediators of cytokine receptor signals. Because many early-acting growth factors have been implicated in STAT5 activation, this study sought to investigate whether STAT5 may be a transcriptional regulator of hematopoietic stem cell (HSC) long-term repopulating activity. To test this possibility, bone marrow (BM) and fetal liver (FL) cells from mice containing homozygous deletions of both STAT5a and STAT5b genes (STAT5ab(-/-)) were characterized for hematopoietic repopulating activities. BM and FL grafts were capable of repopulating lymphoid and myeloid lineages of lethally irradiated primary and secondary hosts, with defects observed primarily in T-lymphocyte engraftment. Because only a fraction of normal HSC function is required to reconstitute hematopoiesis, competitive repopulation assays of adult BM or FL cells were used against wild type adult BM or FL cells to quantitate stem cell function. In these analyses, average 25-, 28-, 45-, and 68-fold decreases in normal repopulating activity were evident in granulocyte (Gr-1(+)), macrophage (Mac-1(+)), erythroid progenitor (Ter119(+)), and B-lymphocyte (B220(+)) populations, respectively, with T lymphocytes (CD4(+)) always undetectable from the STAT5ab(-/-) graft. Consistent with previous reports of divergence between stem cell phenotype and function in cases of perturbed hematopoiesis, the absolute number of cells within Sca-1(+)c-kit(+)lin(-) or lin(-) Hoechst 33342 side population fractions was not significantly different between wild type and STAT5ab(-/-) BM or FL cells. These results demonstrate that a significant proportion of the growth factor signals required for multilineage reconstitution potential of HSCs is STAT5 dependent.