Multiple Immune and Genetic Mechanisms Contribute to Cmv5s-Driven Susceptibility and Tissue Damage during Acute Murine Cytomegalovirus Infection

The MHC class I molecule H-2Dk conveys resistance to acute murine CMV infection in both C57L (H-2Dk transgenic) and MA/My mice. M.H2k/b mice are on an MA/My background aside from a C57L-derived region spanning the MHC (Cmv5s), which diminishes this resistance and causes significant spleen histopathology. To hone in on the effector elements within the Cmv5s interval, we generated several Cmv5-recombinant congenic mouse strains and screened them in vivo, allowing us to narrow the phenotype-associated interval >6-fold and segment the genetic mechanism to at least two independent loci within the MHC region. In addition, we sought to further characterize the Cmv5s-associated phenotypes in their temporal appearance and potential direct relationship to viral load. To this end, we found that Cmv5s histopathology and NK cell activation could not be fully mirrored in the MA/My mice with increased viral dose, and that marginal zone destruction was the first apparent Cmv5s phenotype, being reliably quantified as early as 2 d postinfection in the M.H2k/b mice, prior to divergence in viral load, weight loss, or NK cell phenotype. Finally, we further dissect NK cell involvement, finding no intrinsic differences in NK cell function, despite increased upregulation of activation markers and checkpoint receptors. In conclusion, these data dissect the genetic and immunologic underpinnings of Cmv5 and reveal a model in which polymorphism within the MHC region of the genome leads to the development of tissue damage and corrupts protective NK cell immunity during acute viral infection.

Altered-Self MHC Class I Sensing via Functionally Disparate Paired NK Cell Receptors Counters Murine Cytomegalovirus gp34-Mediated Immune Evasion

The murine CMV (MCMV) immunoevasin m04/gp34 escorts MHC class I (MHC I) molecules to the surface of infected cells where these complexes bind Ly49 inhibitory receptors (IRs) and prevent NK cell attack. Nonetheless, certain self-MHC I-binding Ly49 activating and inhibitory receptors are able to promote robust NK cell expansion and antiviral immunity during MCMV infection. A basis for MHC I-dependent NK cell sensing of MCMV-infected targets and control of MCMV infection however remains unclear. In this study, we discovered that the Ly49R activation receptor is selectively triggered during MCMV infection on antiviral NK cells licensed by the Ly49G2 IR. Ly49R activating receptor recognition of MCMV-infected targets is dependent on MHC I Dk and MCMV gp34 expression. Remarkably, although Ly49R is critical for Ly49G2-dependent antiviral immunity, blockade of the activation receptor in Ly49G2-deficient mice has no impact on virus control, suggesting that paired Ly49G2 MCMV sensing might enable Ly49R+ NK cells to better engage viral targets. Indeed, MCMV gp34 facilitates Ly49G2 binding to infected cells, and the IR is required to counter gp34-mediated immune evasion. A specific requirement for Ly49G2 in antiviral immunity is further explained by its capacity to license cytokine receptor signaling pathways and enhance Ly49R+ NK cell proliferation during infection. These findings advance our understanding of the molecular basis for functionally disparate self-receptor enhancement of antiviral NK cell immunity.

Astrocytic trans-Differentiation Completes a Multicellular Paracrine Feedback Loop Required for Medulloblastoma Tumor Growth

The tumor microenvironment (TME) is critical for tumor progression. However, the establishment and function of the TME remain obscure because of its complex cellular composition. Using a mouse genetic system called mosaic analysis with double markers (MADMs), we delineated TME evolution at single-cell resolution in sonic hedgehog (SHH)-activated medulloblastomas that originate from unipotent granule neuron progenitors in the brain. First, we found that astrocytes within the TME (TuAstrocytes) were trans-differentiated from tumor granule neuron precursors (GNPs), which normally never differentiate into astrocytes. Second, we identified that TME-derived IGF1 promotes tumor progression. Third, we uncovered that insulin-like growth factor 1 (IGF1) is produced by tumor-associated microglia in response to interleukin-4 (IL-4) stimulation. Finally, we found that IL-4 is secreted by TuAstrocytes. Collectively, our studies reveal an evolutionary process that produces a multi-lateral network within the TME of medulloblastoma: a fraction of tumor cells trans-differentiate into TuAstrocytes, which, in turn, produce IL-4 that stimulates microglia to produce IGF1 to promote tumor progression.

Deletion of the SAPS1 subunit of protein phosphatase 6 in mice increases radiosensitivity and impairs the cellular DNA damage response

Cellular responses to DNA damage include activation of DNA-dependent protein kinase (DNA-PK) through, among others, the serine/threonine protein phosphatase 6 (PP6). We previously showed that recognition of DNA-PKcs is mediated by the SAPS1 PP6 regulatory subunit. Here, we report and characterize a SAPS1 null mouse and investigate the effects of deletion on DNA damage signaling and repair. Strikingly, neither SAPS1-null animals nor cells derived from them show gross defects, unless subjected to DNA damage by radiation or chemical agents. The overall survival of SAPS1-null animals following whole body irradiation is significantly shortened as compared to wild-type mice, and the clonogenic survival of null cells subjected to ionizing radiation is reduced. The dephosphorylation of DNA damage/repair markers, such as γH2AX, p53 and Kap1, is diminished in SAPS1-null cells as compared to wild-type controls. Our results demonstrate that loss of SAPS1 confers sensitivity to DNA damage and confirms previously reported cellular phenotypes of SAPS1 knock-down in human glioma cells. The results support a role for PP6 regulatory subunit SAPS1 in DNA damage responses, and offer a novel target for sensitization to enhance current tumor therapies, with a potential for limited deleterious side effects.

c-Myb Coordinates Survival and the Expression of Genes That Are Critical for the Pre-BCR Checkpoint

The c-Myb transcription factor is required for adult hematopoiesis, yet little is known about c-Myb function during lineage-specific differentiation due to the embryonic lethality of Myb-null mutations. We previously used tissue-specific inactivation of the murine Myb locus to demonstrate that c-Myb is required for differentiation to the pro-B cell stage, survival during the pro-B cell stage, and the pro-B to pre-B cell transition during B lymphopoiesis. However, few downstream mediators of c-Myb-regulated function have been identified. We demonstrate that c-Myb regulates the intrinsic survival of CD19⁺ pro-B cells in the absence of IL-7 by repressing expression of the proapoptotic proteins Bmf and Bim and that levels of Bmf and Bim mRNA are further repressed by IL-7 signaling in pro-B cells. c-Myb regulates two crucial components of the IL-7 signaling pathway: the IL-7Rα-chain and the negative regulator SOCS3 in CD19⁺ pro-B cells. Bypassing IL-7R signaling through constitutive activation of Stat5b largely rescues survival of c-Myb-deficient pro-B cells, whereas constitutively active Akt is much less effective. However, rescue of pro-B cell survival is not sufficient to rescue proliferation of pro-B cells or the pro-B to small pre-B cell transition, and we further demonstrate that c-Myb-deficient large pre-B cells are hypoproliferative. Analysis of genes crucial for the pre-BCR checkpoint demonstrates that, in addition to IL-7Rα, the genes encoding λ5, cyclin D3, and CXCR4 are downregulated in the absence of c-Myb, and λ5 is a direct c-Myb target. Thus, c-Myb coordinates survival with the expression of genes that are required during the pre-BCR checkpoint.

c-Myb promotes the survival of CD4+CD8+ double-positive thymocytes through upregulation of Bcl-xL

Mechanisms that regulate the lifespan of CD4(+)CD8(+) double-positive (DP) thymocytes help shape the peripheral T cell repertoire. However, the molecular mechanisms controlling DP thymocyte survival remain poorly understood. The Myb proto-oncogene encodes a transcription factor required during multiple stages of T cell development. We demonstrate that Myb mRNA expression is upregulated as thymocytes differentiate from the double-negative into the metabolically quiescent, small, preselection DP stage during T cell development. Using a conditional deletion mouse model, we demonstrate that Myb-deficient DP thymocytes undergo premature apoptosis, resulting in a limited Tcralpha repertoire biased toward 5' Jalpha segment usage. Premature apoptosis occurs specifically in the small preselection DP compartment in an alphabetaTCR-independent manner and is a consequence of decreased Bcl-xL expression. Forced Bcl-xL expression is able to rescue survival, and reintroduction of c-Myb restores both Bcl-xL expression and the small preselection DP compartment. We further demonstrate that c-Myb promotes transcription at the Bcl2l1 locus via a genetic pathway that is independent of the expression of T cell-specific factor-1 or RORgammat, two transcription factors that induce Bcl-xL expression in T cell development. Thus, Bcl-xL is a novel mediator of c-Myb activity during normal T cell development.

c-Myb is required for pro-B cell differentiation

The c-Myb transcription factor is required for normal adult hematopoiesis. However, the embryonic lethality of Myb-null mutations has been an impediment to identifying roles for c-Myb during lymphocyte development. We have used tissue-specific inactivation of the Myb locus in early progenitor cells to demonstrate that c-Myb is absolutely required for the differentiation of CD19(+) B-lineage cells and B cell differentiation is profoundly blocked beyond the pre-pro-B cell stage in Myb(f/f) Mb1-cre mice. We demonstrate that c-Myb is required for the intrinsic survival of CD19(+) pro-B cells as well as the proper expression of the alpha-chain of the IL-7 receptor (CD127) and Ebf1. However, survival of c-Myb-deficient CD19(+) pro-B cells cannot be rescued by transduction with CD127-producing retrovirus, suggesting that c-Myb controls a survival pathway independent of CD127. Furthermore, c-Myb-deficient progenitor cells inefficiently generate CD19(+) B-lineage cells during stromal cell culture but this process can be partially rescued with exogenous Ebf1. Thus, c-Myb does not appear to be required for commitment to B cell differentiation but is crucial for B cell differentiation to the CD19(+) pro-B cell stage as well as survival of CD19(+) pro-B cells. Surprisingly, forced c-Myb expression in lymphoid-primed multipotent progenitors favors differentiation toward the myeloid lineage, suggesting that proper c-Myb expression is crucial for B-lineage development.

c-Myb is a novel regulator of Bcl-xL in the CD4+ and CD8+ double positive stage of T cell development (85.2)

The lifespan of quiescent CD4+ and CD8+ double positive (DP) thymocytes regulates the composition of the peripheral T cell repertoire. However, the molecular mechanisms controlling DP thymocyte survival remain poorly understood. The c-Myb transcription factor is highly expressed in quiescent pre-selection DP thymocytes. Using a conditional deletion mouse model, we demonstrate that pre-selection DP thymocytes lacking c-Myb undergo premature apoptosis due to decreased expression of the key anti-apoptotic factor Bcl-xL. Moreover, c-Myb deficient DP thymocytes display a skewed TCRα repertoire, with a bias towards 3' Jα?usage, as a consequence of premature apoptosis occurring shortly after the initiation of TCRα rearrangements in vivo. Finally, re-introduction of c-Myb to c-Myb deficient DP thymocytes restored control levels of Bcl-xL mRNA and protein. Taken together, our study identifies c-Myb as a novel regulator of Bcl-xL expression in the DP stage of T cell development. We demonstrate that c-Myb acts through a novel pathway, independently of RORγt and TCF-1, two transcription factors previously shown to promote Bcl-xL expression in DP thymocytes. This study sheds new light on the regulation of the survival of pre-selection DP thymocytes and provides a novel mechanism by which c-Myb guards the balance between life, death and differentiation during hematopoiesis.

Cytokine-facilitated transduction leads to low-level engraftment in nonablated hosts

Using a murine bone marrow transplantation model, we evaluated the long-term engraftment of retrovirally transduced bone marrow cells in nonmyeloablated hosts. Male bone marrow was stimulated in a cocktail of interleukin-3 (IL-3), IL-6, IL-11, and stem cell factor (SCF) for 48 hours, then cocultured on the retroviral producer line MDR18.1 for an additional 24 hours. Functional transduction of hematopoietic progenitors was detected in vitro by reverse transcriptase-polymerase chain reaction (RT-PCR) amplification of multiple drug resistance 1 (MDR1) mRNA from high proliferative potential-colony forming cell (HPP-CFC) colonies. After retroviral transduction, male bone marrow cells were injected into nonablated female mice. Transplant recipients received three TAXOL (Bristol-Myers, Princeton, NJ) injections (10 mg/kg) over a 14-month period. Transplant recipient tissues were analyzed by Southern blot and fluorescence in situ hybridization for Y-chromosome-specific sequences and showed donor cell engraftment of approximately 9%. However, polymerase chain reaction amplification of DNAs from bone marrow, spleen, and peripheral blood showed no evidence of the transduced MDR1 gene. RT-PCR analysis of total bone marrow RNA showed that transcripts from the MDR1 gene were present in a fraction of the engrafted donor cells. These data show functional transfer of the MDR1 gene into nonmyeloablated murine hosts. However, the high rates of in vitro transduction into HPP-CFC, coupled with the low in vivo engraftment rate of donor cells containing the MDR1 gene, suggest that the majority of stem cells that incorporated the retroviral construct did not stably engraft in the host. Based on additional studies that indicate that ex vivo culture of bone marrow induces an engraftment defect concomitantly with progression of cells through S phase, we propose that the cell cycle transit required for proviral integration reduces or impairs the ability of transduced cells to stably engraft.

Stem cell transplantation in the normal nonmyeloablated host: relationship between cell dose, schedule, and engraftment

In previous studies we have shown high rates of stable engraftment when 40 million male BALB/c cells were infused intravenously daily for 5 days (a total of 200 million cells) to normal nonmyeloablated female hosts. The present studies evaluate engraftment of male BALB/c bone marrow cells in female host marrow, spleen, and thymus 20-25 weeks after transplantation using varying cell dosages within a 5-day schedule. Engraftment in recipient mice was assessed by detection of male specific sequence in recipient DNA from each organ. When 40 million cells were given per daily injection for 1, 2, 3, 4, or 5 days, engraftment percentages in host marrow were 11 +/- 0.83, 20 +/- 2.0, 23 +/- 2.5, 32 +/- 6.3, and 39% +/- 5.7 (+/- standard error of mean), respectively, yielding engraftment percentages per million cells infused of 0.28, 0.25, 0.19, 0.20, and 0.20%, respectively. When levels of 2.5, 5, 10, 20, or 40 million cells were injected 5 times over a 5-day schedule into normal BALB/c female hosts, progressively increasing levels of engraftment from 3 +/- 0.6 to 39% +/- 5.7 were seen in host marrow. Highest levels of engraftment per million cells injected were obtained on days 1 and 2 of a 5-day schedule and with a level of 10 million cells given daily over 5 days. Engraftment profiles varied with spleen and thymus and percent engraftment was generally lower than for marrow. The present work indicates that regardless of cell level infused or number of infusions, rates of engraftment observed in marrow approached or exceeded the highest rates of engraftment estimated by theoretical calculations based on replacing host cells ("replacement model") or adding to host cells ("incremental model"). Engraftment in spleen and thymus was lower, but also at times approached or exceeded theoretical maxima. These data show extraordinary levels of engraftment in normal hosts, suggesting that rates in this competitive model are superior to those seen in irradiated hosts; alternatively, there may be selective repression of host stem cell proliferation and differentiation.

High levels of engraftment with a single infusion of bone marrow cells into normal unprepared mice

Repetitive infusion of 40 million male murine marrow cells (total 200 million cells) into normal unprepared female BALB/c hosts for 5 consecutive days results in high levels of engraftment at 1-25 months postinfusion, as determined by Southern blot analysis using a Y chromosome-specific probe. We investigated the importance of the schedule of injections in this engraftment model. Surprisingly, a single infusion of 200 x 10(6) male BALB/c bone marrow cells analyzed at 7-14 weeks postinfusion resulted in engraftment levels in individual female mice of over 50% with mean values of 25 +/- 2% for 44 individual transplant points. Engraftment levels in spleen and thymus were 14 +/- 1% and 18 +/- 3%, respectively. Including heparin in the infusion increased engraftment in marrow, spleen, and thymus. Administration of the cells over five or 10 separate infusions, rather than in one injection, did not increase engraftment levels. If the infused bone marrow cells seeded equally between host spleen, thymus, and bone marrow, and if all cells engrafted, the bone marrow engraftment seen here approaches the theoretical maximum. This suggests either a large number of available "niches" or the displacement of host marrow cells by infused marrow. The latter possibility is upheld by cell counts per tibia/femur and total seven-factor HPP-CFC/tibia, which were not increased. These data suggest that a single infusion of marrow homes quantitatively to spleen, thymus, and bone marrow, possibly displacing host cells in the process.

Engraftment of bone marrow cells into normal unprepared hosts: effects of 5-fluorouracil and cell cycle status

Bone marrow from animals treated with 5-fluorouracil (5FU) competes equally with normal marrow when assessed in vivo in an irradiated mouse, but shows markedly defective engraftment when transplanted into noncytoablated hosts. Using Southern Blot analysis and a Y-chromosome specific probe, we determined the level of engraftment of male donor cells in the bone marrow, spleen, and thymus of unprepared female hosts. We have confirmed the defective engraftment of marrow harvested 6 days after 5FU (FU-6) and transplanted into unprepared hosts and shown that this defect is transient; by 35 days after 5FU (FU-35), engraftment has returned to levels seen with normal marrow. FU-6 marrow represents an actively cycling population of stem cells, and we hypothesize that the cycle status of the stem cell may relate to its capacity to engraft in the nonirradiated host. Accordingly, we have evaluated the cycle status of engrafting normal and FU-6 marrow into normal hosts using an in vivo hydroxyurea technique. We have shown that those cells engrafting from normal marrow and over 70% of the cells engrafting from FU-6 marrow were quiescent, demonstrating no killing with hydroxyurea. We have also used fluorescent in situ hybridization (FISH) analysis with a Y-chromosome probe and demonstrated that normal and post-5FU engraftment patterns in peripheral blood were similar to those seen in bone marrow, spleen, and thymus. Altogether these data indicate that cells engrafting in normal, unprepared hosts are dormant, and the defect that occurs after 5FU is concomitant with the induction of these cells to transit the cell cycle.

Engraftment of normal murine marrow into nonmyeloablated host mice

When 200 x 10(6) male BALB/c cells are given by tail vein injection to female nonmyeloablated hosts in one injection, a relatively low engraftment percentage is seen, but when the same total number of cells is given over five injections (separated by 24 hours), the observed engraftment is much higher. A further increase in engraftment appears to occur when the same number of cells is given in 10 injections separated by at least 24 hours. These data suggest that somewhere between 5 and 10% of marrow niches are available at intervals of 24 or more hours and that the keys to high levels of engraftment are the cell cycle status of the engrafting stem cell and the schedule of engraftment.

Long-term engraftment of normal and post-5-fluorouracil murine marrow into normal nonmyeloablated mice

We report the successful long-term engraftment of normal male donor bone marrow (BM) transfused into noncytoablated female mice, challenging the assumption that "niches" need to be created for marrow to engraft. We have used chromosomal banding and Southern blot analysis to identify transplanted male marrow cells, and shown the long-term stability of the chimeric marrows. Balb/C, BDF1, or CBA-J female hosts (no irradiation) received for 5 consecutive days 40 x 10(6) male cells (per day) of the same strain, and repopulation patterns were observed. Parallel studies were performed using tibia/femur equivalents of normal marrow or marrow from Balb/C mice pretreated 6 days previously with 150 mg/kg 5-fluorouracil (5-FU). Chromosome banding techniques showed that 5% to 46% of marrow cells were male 3 to 9 months posttransplant with normal donor marrow. Southern blot analysis, using the pY2 probe, showed continued engraftment at 21 to 25 months posttransplant, ranging from 15% to 42% male engrafted cells in marrow. Normal donor male marrow engrafted significantly better than 5-FU-pretreated male marrow as shown 1 to 12 months posttransplant in non-cytoablated female recipients. Percentages of male engrafted cells in BM ranged from 23% to 78% for recipients of normal donor marrow and from 0.1% to 39% for recipients of 5-FU marrow. Mean engraftment for 6 mice receiving normal marrow was 38%, whereas that for 6 mice receiving post-5-FU marrow was 8%, as assayed 1 to 3 months posttransplant. At 10 to 12 months, mean engraftment for the normal donor group was 46%, compared with 16% for the 5-FU group. The patterns of engraftment with normal and 5-FU marrow were similar for spleen and thymus. These results show that long-term chimerism can be established after transplantation of normal donor marrow to normal nonirradiated host mice and indicate that marrow spaces do not have to be created for successful engraftment. They suggest that transplanted marrow competes equally with host marrow for marrow space. Finally, these data show that post-5-FU Balb/C male marrow is markedly inferior in the repopulation of Balb/C female host marrow, spleen, and thymus, and suggest that this population of cells may not be the ideal population for gene transfer studies.

Cytokine expression from bone marrow derived macrophages

Monocytes and macrophages show marked phenotypic variation dependent on their tissue of origin. Peripheral blood monocytes have been found to be sources of a variety of cytokines, but isolated marrow macrophages have not been characterized in this regard. Marrow macrophages form a predominant component of murine adherent Dexter stromal cells and can be isolated by sequential explant culture in colony-stimulating factor-1 (CSF-1). We have studied murine (Balb/c) bone marrow macrophage (BMM) cytokine production in the presence or absence of CSF-1, the lectin pokeweed mitogen (PWM) or interleukin-3 (IL-3). Biologic activity in conditioned media (cm) from control and induced BMM was assessed using the factor-dependent cell lines 32D, NFS-60, T1165, MC-6 and FDC-P1. Cell line stimulation and antibody blocking indicated the presence of c-kit ligand, interleukin-6 (IL-6) and granulocyte colony-stimulating factor (G-CSF). This stimulatory activity was increased by exposure to PWM or the combination of CSF-1 and PWM or CSF-1 and IL-3. CSF-1, as determined by radioimmunoassay (RIA), was essentially undetectable in baseline cm and induction was not seen with PWM or CSF-1. Baseline or "constitutive" expression of BMM and mRNA for CSF-1 and c-kit ligand was seen. Uninduced BMM did not express mRNA for G-CSF, granulocyte-macrophage CSF (GM-CSF), IL-6 or IL-3. CSF-1 induced increased expression of IL-6 mRNA, PWM induced increased expression of G-CSF and IL-6 mRNA and the combination of PWM and CSF-1 induced expression of CSF-1, G-CSF and IL-6 mRNA. Varying levels of CSF-1 had differential effects on cytokine production. Increasing levels of CSF-1 increased IL-6 mRNA and downmodulated CSF-1 mRNA expression. There was a biphasic response of c-kit ligand mRNA expression to CSF-1 exposure; low levels of CSF-1 (50 U/mL) induced, while higher levels (2000 U/mL) inhibited, expression. These data indicate that BMM (and by analogy the macrophage component of Dexter culture stroma), are important sources of CSF-1 and c-kit ligand but not GM-CSF or IL-3. BMM can also be induced to express IL-6 and/or G-CSF. Lastly, CSF-1, by differentially modulating BMM cytokine production in a holocrine or autocrine manner, may function as a central regulator of stromal based hematopoiesis.

Biologic significance of constitutive and subliminal growth factor production by bone marrow stroma

The "stromal" or adherent cells of long-term murine Dexter explant bone marrow cultures provide the best in vitro model of the bone marrow microenvironment. Colony-stimulating factor-1 (CSF-1) is produced constitutively by these cells and is easily detected, but most investigators have not found constitutive production of the other hemolymphopoietic cytokines. We have previously reported the detection of granulocyte-macrophage-CSF (GM-CSF) in murine stromal cultures and its induction by the lectin Pokeweed mitogen. The present studies analyzing stromal cytokine messenger RNA (mRNA) production by standard Northern blot analysis show constitutive production of mRNAs for CSF-1, GM-CSF, granulocyte-CSF (G-CSF), c-kit ligand (KL), and interleukin-6 (IL-6), but not IL-3, IL-4, or IL-5 by 3-week irradiated or nonirradiated murine Dexter stromal cells. Exposure of stromal cells to Pokeweed mitogen or IL-1 16 hours before RNA harvest induces the messages for GM-CSF, G-CSF, KL, and IL-6, but not IL-3, IL-4, IL-5, or CSF-1. Polymerase chain reaction amplification of cDNA made with reverse transcriptase from stromal RNA using two separate sets of IL-3-specific primers shows the presence of IL-3 message in irradiated stromal cells, which is only detectable with this more sensitive technique. The factor-dependent cell lines FDC-P1 and 32D are supported by the stromal cells without the addition of exogenous growth factors, demonstrating a cytokine activity in these cultures that is inhibited by the addition of anti-IL-3 or anti-GM-CSF antibodies. These data indicate that murine Dexter stromal cells constitutively produce CSF-1, GM-CSF, G-CSF, IL-6, KL, and IL-3. This growth factor production could explain the support of granulocyte, macrophage, and megakaryocyte production and stem cell maintenance in Dexter-type long-term murine bone marrow cultures.