Apoptosis-Inducing-Factor-Dependent Mitochondrial Function Is Required for T Cell but Not B Cell Function.

The role of apoptosis inducing factor (AIF) in promoting cell death versus survival remains controversial. We report that the loss of AIF in fibroblasts led to mitochondrial electron transport chain defects and loss of proliferation that could be restored by ectopic expression of the yeast NADH dehydrogenase Ndi1. Aif-deficiency in T cells led to decreased peripheral T cell numbers and defective homeostatic proliferation, but thymic T cell development was unaffected. In contrast, Aif-deficient B cells developed and functioned normally. The difference in the dependency of T cells versus B cells on AIF for function and survival correlated with their metabolic requirements. Ectopic Ndi1 expression rescued homeostatic proliferation of Aif-deficient T cells. Despite its reported roles in cell death, fibroblasts, thymocytes and B cells lacking AIF underwent normal death. These studies suggest that the primary role of AIF relates to complex I function, with differential effects on T and B cells.

Interplay between H1N1 influenza a virus infection, extracellular and intracellular respiratory tract pH, and host responses in a mouse model.

During influenza A virus (IAV) entry, the hemagglutinin (HA) protein is triggered by endosomal low pH to undergo irreversible structural changes that mediate membrane fusion. HA proteins from different isolates vary in the pH at which they become activated in endosomes or become irreversible inactivated if exposed to extracellular acid. Little is known about extracellular pH in the upper respiratory tracts of mammals, how pH may shift during IAV infection, and its impact on replication of viruses that vary in HA activation pH. Here, we inoculated DBA/2J mice intranasally with A/TN/1-560/2009 (H1N1) (activation pH 5.5) or a mutant containing the destabilizing mutation HA1-Y17H (pH 6.0). We measured the kinetics of extracellular pH during infection using an optical pH-sensitive microsensor probe placed in the naris, nasal sinus, soft palate, and trachea. We also measured intracellular pH of single-cell suspensions of live, primary lung epithelial cells with various wavelength pH-sensitive dyes localized to cell membranes, cytosol, endosomes, secretory vesicles, microtubules, and lysosomes. Infection with either virus decreased extracellular pH and increased intracellular pH. Peak host immune responses were observed at 2 days post infection (DPI) and peak pH changes at 5 DPI. Extracellular and intracellular pH returned to baseline by 7 DPI in mice infected with HA1-Y17H and was restored later in wildtype-infected. Overall, IAV infection altered respiratory tract pH, which in turn modulated replication efficiency. This suggests a virus-host pH feedback loop that may select for IAV strains containing HA proteins of optimal pH stability, which may be approximately pH 5.5 in mice but may differ in other species.

Genome editing of HBG1 and HBG2 to induce fetal hemoglobin.

Induction of fetal hemoglobin (HbF) via clustered regularly interspaced short palindromic repeats/Cas9-mediated disruption of DNA regulatory elements that repress γ-globin gene (HBG1 and HBG2) expression is a promising therapeutic strategy for sickle cell disease (SCD) and ß-thalassemia, although the optimal technical approaches and limiting toxicities are not yet fully defined. We disrupted an HBG1/HBG2 gene promoter motif that is bound by the transcriptional repressor BCL11A. Electroporation of Cas9 single guide RNA ribonucleoprotein complex into normal and SCD donor CD34+ hematopoietic stem and progenitor cells resulted in high frequencies of on-target mutations and the induction of HbF to potentially therapeutic levels in erythroid progeny generated in vitro and in vivo after transplantation of hematopoietic stem and progenitor cells into nonobese diabetic/severe combined immunodeficiency/Il2rγ-/-/KitW41/W41 immunodeficient mice. On-target editing did not impair CD34+ cell regeneration or differentiation into erythroid, T, B, or myeloid cell lineages at 16 to 17 weeks after xenotransplantation. No off-target mutations were detected by targeted sequencing of candidate sites identified by circularization for in vitro reporting of cleavage effects by sequencing (CIRCLE-seq), an in vitro genome-scale method for detecting Cas9 activity. Engineered Cas9 containing 3 nuclear localization sequences edited human hematopoietic stem and progenitor cells more efficiently and consistently than conventional Cas9 with 2 nuclear localization sequences. Our studies provide novel and essential preclinical evidence supporting the safety, feasibility, and efficacy of a mechanism-based approach to induce HbF for treating hemoglobinopathies.

Lymphoid progenitors and primary routes to becoming cells of the immune system.

Extraordinary progress has been made in charting the maturation of hematopoietic cells. However, these charted processes do not necessarily represent obligate pathways to specialized types of lymphocytes. In fact, there is a degree of plasticity associated with primitive progenitors. Moreover, all lymphocytes of a given kind are not necessarily produced through precisely the same sequence of events. Particularly contentious is the nature of cells that seed the thymus, because different progenitors can generate T cells under experimental circumstances. Non-renewing progenitors with a high density of c-Kit in bone marrow are likely to replenish the thymus under normal circumstances and most closely resemble canonical T cell progenitors.

Early stages of hematopoietic differentiation.

Mouse bone marrow contains hematopoietic stem cells as well as progenitor cells, which are partially differentiated offspring of stem cells. We have utilized several approaches to separate progenitors from stem cells in order to characterize essential differences between these two stages of development. As a first approach, we utilized the supravital fluorescent dye rhodamine-123 (Rh-123) to distinguish quiescent stem cells (Rh-123(low)) from metabolically active progenitor cells (Rh-123(hi)). Analysis of megakaryocyte potential in a tissue culture assay demonstrated that Rh-123(hi) progenitor cells were capable of robust megakaryocyte differentiation, while Rh-123(low) stem cells produced fewer colonies containing megakaryocytes. Transplantation of the two cell populations into irradiated recipients revealed the opposite outcome, suggesting that the tissue culture assay failed to predict behavior in a transplant setting. We also evaluated functional potential of lymphoid progenitors isolated by selecting for differential expression of Thy-1.1 and c-kit. The potential of defined cell populations to differentiate as T or B lymphocytes in vivo was dependent upon the time post transplant at which animals were evaluated. These studies underscore the need for caution in the interpretation of lineage potentials evaluated by both in vitro and in vivo assays.

Id1 has a physiological role in regulating early B lymphopoiesis.

Basic helix-loop-helix E proteins play critical roles in B-cell development by stimulating B cell-specific gene expression and immunoglobulin gene rearrangement. The function of E proteins can be effectively suppressed by their naturally occurring inhibitors, Id1 to 4. Ectopic expression of Id1 has been shown to block B-cell development at the early pro-B cell stage. However, whether Id1 plays a physiological role in controlling B lymphopoiesis was not known. Although Id1-deficient mice do not exhibit significant abnormalities in steady-state B lymphopoiesis, we detected more robust B-cell engraftment in transplant recipients of Id1-deficient bone marrow compared to those of wild-type donor cells. In culture, Id1 ablation dramatically enhances B-lineage cell production without any marked effects on myeloid differentiation. Consistently, Id1 expression was found in pro-B but not pre-B cells as measured by enhanced green fluorescent protein (EGFP) fluorescence and by quantitative reverse transcription-PCR. Although loss of Id1 did not alter the number of B-cell colonies generated from whole bone marrow or the proliferation rate of developing B cells, B-cell colonies were detectable at a much earlier time point and the size of the colonies were larger. Therefore, we infer that Id1-deficient progenitors possess higher potential to differentiate to the pre-B cell stage when a proliferative burst occurs. Taken together, we present evidence to suggest that Id1 plays a physiological role in restraining the developmental progression, which may be important for proper B-cell differentiation in the bone marrow.

Id1 attenuates Notch signaling and impairs T-cell commitment by elevating Deltex1 expression.

Complete inhibition of E protein transcription factors by Id1 blocks the developmental transition of CD4/CD8 double-negative 1 (DN1; CD44(+) CD25(-)) thymocytes to the DN2 (CD44(+) CD25(+)) stage. To understand the underlying mechanisms, we observed that mRNA levels of Deltex1, as well as Deltex4, were dramatically elevated in Id1-expressing thymocytes, which could result in developmental arrest by attenuating Notch function. In support of this hypothesis, we found that Deltex1 ablation enabled Id1-expressing progenitors to differentiate to the DN3 (CD44(-) CD25(+)) stage, which was accompanied by enhanced Notch1 expression in T-cell progenitors. Consistently, constitutive activation of Notch1 drove the differentiation of Id1-expressing progenitors to the DN3 stage. Furthermore, we showed that Gfi1b levels decreased, whereas GATA3 levels increased in Id1 transgenic thymocytes. When overexpressed, GATA3 was able to upregulate Deltex1 transcription. Thus, T-cell commitment may be controlled by the interplay among E proteins, Gfi1b, and GATA3 transcription regulators, which influence Notch function through the expression of Deltex1.

Regulation of lymphocyte development by cell-type-specific interpretation of Notch signals.

Notch signaling pathways exert diverse biological effects depending on the cellular context where Notch receptors are activated. How Notch signaling is integrated with environmental cues is a central issue. Here, we show that Notch activation accelerates ubiquitin-mediated and mitogen-activated protein kinase (MAPK)-dependent degradation of E2A transcription factors and Janus kinases, molecules essential for both B- and T-lymphocyte development. However, these events occur in B lymphocytes, but not T lymphocytes, due to their different levels of MAPK, thus providing one mechanism whereby Notch inhibits B-cell development without impairing T-cell differentiation. Lymphoid progenitors expressing a Notch-resistant E2A mutant differentiated into B-lineage cells on stromal cells expressing Notch ligands and in the thymus of transplant recipients. Bone marrow transplant assays and examination of steady-state B lymphopoiesis also revealed that the expression of Notch-resistant E2A and constitutively active STAT5 in mice neutralized the effects of Notch-induced degradation, allowing B-cell development through a bone marrow-like program in the thymus. These findings illustrate that Notch function can be influenced by MAPKs, producing distinct outcomes in different cellular contexts.

Id1, but not Id3, directs long-term repopulating hematopoietic stem-cell maintenance.

E-proteins are widely expressed basic helix-loop-helix (HLH) transcription factors that regulate differentiation in many cell lineages, including lymphoid, muscle, and neuronal cells. E-protein function is controlled by HLH inhibitors such as Id and SCL/TAL1 proteins, which recently have been suggested to play a role in hematopoietic stem cell (HSC) differentiation. However, the precise stages when these proteins are expressed and their specific functions are not entirely clear. Using a knock-in mouse model where the sequence for the enhanced green fluorescent protein (GFP) was inserted downstream of the Id1 promoter, we were able to track Id1 expression on an individual cell basis and detected Id1 expression in long-term repopulating HSCs (LT-HSCs). Functional assays showed that the Id1/GFP(+)Lin(-)Sca1(+)c-kit(Hi) population was highly enriched for LT-HSCs. Consistent with this expression pattern, Id1 deficiency led to a 2-fold reduction in the number of LT-HSCs defined as Lin(-)Sca1(+)c-kit(Hi)CD48(-)CD150(+). Primary bone marrow transplantation studies revealed that Id1 is dispensable for short-term engraftment. In contrast, both Id1(-/-) whole bone marrow and Lin(-)Sca1(+)c-kit(Hi)Thy1.1(Lo)-enriched HSCs, but not Id3(-/-) marrow, displayed impaired engraftment relative to wild-type controls in secondary transplantation assays. These findings suggest a unique role for Id1 in LT-HSC maintenance and hematopoietic development.

Interferon-producing killer dendritic cells (IKDCs) arise via a unique differentiation pathway from primitive c-kitHiCD62L+ lymphoid progenitors.

Interferon-producing killer dendritic cells (IKDCs) have only recently been described and they share some properties with plasmacytoid dendritic cells (pDCs). We now show that they can arise from some of the same progenitors. However, IKDCs expressed little or no RAG-1, Spi-B, or TLR9, but responded to the TLR9 agonist CpG ODN by production of IFNgamma. The RAG-1(-)pDC2 subset was more similar to IKDCs than RAG-1(+) pDC1s with respect to IFNgamma production. The Id-2 transcriptional inhibitor was essential for production of IKDCs and natural killer (NK) cells, but not pDCs. IKDCs developed from lymphoid progenitors in culture but, unlike pDCs, were not affected by Notch receptor ligation. While IKDCs could be made from estrogen-sensitive progenitors, they may have a slow turnover because their numbers did not rapidly decline in hormone-treated mice. Four categories of progenitors were compared for IKDC-producing ability in transplantation assays. Of these, Lin(-)Sca-1(+)c-Kit(Hi)Thy1.1(-)L-selectin(+) lymphoid progenitors (LSPs) were the best source. While NK cells resemble IKDCs in several respects, they develop from different progenitors. These observations suggest that IKDCs may arise from a unique differentiation pathway, and one that diverges early from those responsible for NK cells, pDCs, and T and B cells.

Primitive lymphoid progenitors in bone marrow with T lineage reconstituting potential.

Multiple subsets of the bone marrow contain T cell precursors, but it remains unclear which is most likely to replenish the adult thymus. Therefore, RAG-1+ early lymphoid progenitors (RAG-1+ ELP), and CD62L/L-selectin+ progenitors (LSP), as well as common lymphoid progenitors from C57BL6-Thy1.1-RAG-1/GFP mouse bone marrow were directly compared in transplantation assays. The two c-Kit(high) populations vigorously regenerated the thymus and were superior to common lymphoid progenitors in magnitude and frequency of thymic reconstitution. Regeneration was much faster than the 22 days described for transplanted stem cells, and RAG-1+ ELP produced small numbers of lymphocytes within 13 days. As previously reported, LSP were biased to a T cell fate, but this was not the case for RAG-1+ ELP. Although RAG-1+ ELP and LSP had reduced myeloid potential, they were both effective progenitors for T lymphocytes and NK cells. The LSP subset overlapped with and included most RAG-1+ ELP and many RAG-1- TdT+ ELP. LSP and RAG-1+ ELP were both present in the peripheral circulation, but RAG-1+ ELP had no exact counterpart among immature thymocytes. The most primitive of thymocytes were similar to Lin- c-Kit(high) L-selectin+ TdT+ RAG-1- progenitors present in the marrow, suggesting that this population is normally important for sustaining the adult thymus.

Characterization of thymic progenitors in adult mouse bone marrow.

Thymic cellularity is maintained throughout life by progenitor cells originating in the bone marrow. In this study, we describe adult mouse bone cells that exhibit several features characteristic of prothymocytes. These include 1) rapid thymic engraftment kinetics following i.v. transplantation, 2) dramatic expansion of thymic progeny, and 3) limited production of hemopoietic progeny other than thymocytes. The adult mouse bone marrow population that is depleted of cells expressing any of a panel of lineage-specific Ags, stem cell Ag-1 positive, and not expressing the Thy1.1 Ag (Thy1.1(-)) (Thy1.1(-) progenitors) can repopulate the thymus 9 days more rapidly than can hemopoietic stem cells, a rate of thymic repopulation approaching that observed with transplanted thymocytes. Additionally, Thy1.1(-) progenitors expand prolifically to generate thymocyte progeny comparable in absolute numbers to those observed from parallel hemopoietic stem cell transplants, and provide a source of progenitors that spans multiple waves of thymic seeding. Nevertheless, the Thy1.1(-) population yields relatively few B cells and rare myeloid progeny posttransplant. These observations describe the phenotype of an adult mouse bone marrow population highly enriched for rapidly engrafting, long-term thymocyte progenitors. Furthermore, they note disparity in B and T cell expansion from this lymphoid progenitor population and suggest that it contains the progenitor primarily responsible for seeding the thymus throughout life.

L-selectin defines a bone marrow analog to the thymic early T-lineage progenitor.

The recent description of an early T-lineage progenitor (ETP) population in adult mouse thymus implies the presence of a bone marrow predecessor that has not yet been identified. Here we describe a Lin(Neg) Sca-1(Pos) c-kit(Hi) Thy-1.1(Neg) L-selectin(Pos) adult mouse bone marrow population that resembles the thymic ETP in both antigen expression phenotype and posttransplantation lineage potential. These cells produce wavelike kinetics of thymic seeding and reconstitute the irradiated thymus with kinetics comparable to a thymocyte graft after intravenous transplantation. Transient B-lineage reconstitution is also observed, but little myeloid potential can be detected in transplant experiments. A second subset of progenitors is L-selectin(Neg) and is highly enriched for rapid and persistent T- and B-lineage potential, as well as some myeloid potential. L-selectin (CD62L) is therefore an effective marker for separating lymphoid progenitors from myeloid progenitors and hematopoietic stem cells in mouse bone marrow.

The spleen is a major site of megakaryopoiesis following transplantation of murine hematopoietic stem cells.

The stem cell pool can be fractionated by using the mitochondrial dye, rhodamine-123, into Rho(low) hematopoietic stem cells and Rho(high) progenitors. Rho(low) stem cells permanently engraft all lineages, whereas Rho(high) progenitors transiently produce erythrocytes, without substantial platelet or granulocyte production. We hypothesized that the inability of the Rho(high) cells to produce platelets in vivo was due to the fact that these cells preferentially engraft in the spleen and lack marrow engraftment. Initially, we demonstrated that Rho(high) progenitors produced more megakaryocytes in vitro than Rho(low) stem cells did. To study the activity of the Rho(low) and Rho(high) subsets in vivo, we used mice allelic at the hemoglobin and glucose phosphate isomerase loci to track donor-derived erythropoiesis and thrombopoiesis. Rho(low) stem cells contributed to robust and long-term erythroid and platelet engraftment, whereas Rho(high) progenitors contributed only to transient erythroid engraftment and produced very low numbers of platelets in vivo. Donor-derived megakaryopoiesis occurred at higher densities in the spleen than in the bone marrow in animals receiving Rho(low) stem cells and peaked around day 28. Blockade of splenic engraftment using pertussis toxin did not affect the peak of splenic megakaryopoiesis, supporting the hypothesis that these megakaryocytes were derived from progenitors that originated in the bone marrow. These data emphasize that in vitro behavior of hematopoietic progenitor cell subsets does not always predict their behavior following transplantation. This study supports a major role for the spleen in thrombopoiesis following engraftment of transplanted stem cells in irradiated mice.