Effects of long-term and brain-wide colonization of peripheral bone marrow-derived myeloid cells in the CNS

BACKGROUND

Microglia, the primary resident myeloid cells of the brain, play critical roles in immune defense by maintaining tissue homeostasis and responding to injury or disease. However, microglial activation and dysfunction has been implicated in a number of central nervous system (CNS) disorders, thus developing tools to manipulate and replace these myeloid cells in the CNS is of therapeutic interest.

METHODS

Using whole body irradiation, bone marrow transplant, and colony-stimulating factor 1 receptor inhibition, we achieve long-term and brain-wide (~ 80%) engraftment and colonization of peripheral bone marrow-derived myeloid cells (i.e., monocytes) in the brain parenchyma and evaluated the long-term effects of their colonization in the CNS.

RESULTS

Here, we identify a monocyte signature that includes an upregulation in Ccr1, Ms4a6b, Ms4a6c, Ms4a7, Apobec1, Lyz2, Mrc1, Tmem221, Tlr8, Lilrb4a, Msr1, Nnt, and Wdfy1 and a downregulation of Siglech, Slc2a5, and Ccl21a/b. We demonstrate that irradiation and long-term (~ 6 months) engraftment of the CNS by monocytes induces brain region-dependent alterations in transcription profiles, astrocytes, neuronal structures, including synaptic components, and cognition. Although our results show that microglial replacement with peripherally derived myeloid cells is feasible and that irradiation-induced changes can be reversed by the replacement of microglia with monocytes in the hippocampus, we also observe that brain-wide engraftment of peripheral myeloid cells (relying on irradiation) can result in cognitive and synaptic deficits.

CONCLUSIONS

These findings provide insight into better understanding the role and complexity of myeloid cells in the brain, including their regulation of other CNS cells and functional outcomes.

Low-Level Ionizing Radiation Induces Selective Killing of HIV-1-Infected Cells with Reversal of Cytokine Induction Using mTOR Inhibitors

HIV-1 infects 39.5 million people worldwide, and cART is effective in preventing viral spread by reducing HIV-1 plasma viral loads to undetectable levels. However, viral reservoirs persist by mechanisms, including the inhibition of autophagy by HIV-1 proteins (i.e., Nef and Tat). HIV-1 reservoirs can be targeted by the “shock and kill” strategy, which utilizes latency-reversing agents (LRAs) to activate latent proviruses and immunotarget the virus-producing cells. Yet, limitations include reduced LRA permeability across anatomical barriers and immune hyper-activation. Ionizing radiation (IR) induces effective viral activation across anatomical barriers. Like other LRAs, IR may cause inflammation and modulate the secretion of extracellular vesicles (EVs). We and others have shown that cells may secrete cytokines and viral proteins in EVs and, therefore, LRAs may contribute to inflammatory EVs. In the present study, we mitigated the effects of IR-induced inflammatory EVs (i.e., TNF-α), through the use of mTOR inhibitors (mTORi; Rapamycin and INK128). Further, mTORi were found to enhance the selective killing of HIV-1-infected myeloid and T-cell reservoirs at the exclusion of uninfected cells, potentially via inhibition of viral transcription/translation and induction of autophagy. Collectively, the proposed regimen using cART, IR, and mTORi presents a novel approach allowing for the targeting of viral reservoirs, prevention of immune hyper-activation, and selectively killing latently infected HIV-1 cells.

4-(Nitrophenylsulfonyl)piperazines mitigate radiation damage to multiple tissues

Our ability to use ionizing radiation as an energy source, as a therapeutic agent, and, unfortunately, as a weapon, has evolved tremendously over the past 120 years, yet our tool box to handle the consequences of accidental and unwanted radiation exposure remains very limited. We have identified a novel group of small molecule compounds with a 4-nitrophenylsulfonamide (NPS) backbone in common that dramatically decrease mortality from the hematopoietic acute radiation syndrome (hARS). The group emerged from an in vitro high throughput screen (HTS) for inhibitors of radiation-induced apoptosis. The lead compound also mitigates against death after local abdominal irradiation and after local thoracic irradiation (LTI) in models of subacute radiation pneumonitis and late radiation fibrosis. Mitigation of hARS is through activation of radiation-induced CD11b⁺Ly6G⁺Ly6C⁺ immature myeloid cells. This is consistent with the notion that myeloerythroid-restricted progenitors protect against WBI-induced lethality and extends the possible involvement of the myeloid lineage in radiation effects. The lead compound was active if given to mice before or after WBI and had some anti-tumor action, suggesting that these compounds may find broader applications to cancer radiation therapy.

Lymphoid and Myeloid Recovery in Rhesus Macaques Following Total Body X-Irradiation

Recovery from severe immunosuppression requires hematopoietic stem cell reconstitution and effective thymopoiesis to restore a functional immune cell repertoire. Herein, a model of immune cell reconstitution consequent to potentially lethal doses of irradiation is described, which may be valuable in evaluating potential medical countermeasures. Male rhesus macaques were total body irradiated by exposure to 6.00 Gy 250 kVp x-radiation (midline tissue dose, 0.13 Gy min), resulting in an approximate LD10/60 (n = 5/59). Animals received medical management, and hematopoietic and immune cell recovery was assessed (n ≤ 14) through 370 d post exposure. A subset of animals (n ≤ 8) was examined through 700 d. Myeloid recovery was assessed by neutrophil and platelet-related parameters. Lymphoid recovery was assessed by the absolute lymphocyte count and FACS-based phenotyping of B- and T-cell subsets. Recent thymic emigrants were identified by T cell receptor excision circle quantification. Severe neutropenia, lymphopenia, and thrombocytopenia resolved within 30 d. Total CD3+ cells μL required 60 d to reach values 60% of normal, followed by subsequent slow recovery to approximately normal by 180 d post irradiation. Recovery of CD3+4+ and CD3+8+ cell memory and naïve subsets were markedly different. Memory populations were ≥ 100% of normal by day 60, whereas naïve populations were only 57% normal at 180 d and never fully recovered to baseline post irradiation. Total (CD20+) B cells μL were within normal levels by 77 d post exposure. This animal model elucidates the variable T- and B-cell subset recovery kinetics after a potentially lethal dose of total-body irradiation that are dependent on marrow-derived stem and progenitor cell recovery, peripheral homeostatic expansion, and thymopoiesis.

C/EBPδ deficiency sensitizes mice to ionizing radiation-induced hematopoietic and intestinal injury

Knowledge of the mechanisms involved in the radiation response is critical for developing interventions to mitigate radiation-induced injury to normal tissues. Exposure to radiation leads to increased oxidative stress, DNA-damage, genomic instability and inflammation. The transcription factor CCAAT/enhancer binding protein delta (Cebpd; C/EBPδ is implicated in regulation of these same processes, but its role in radiation response is not known. We investigated the role of C/EBPδ in radiation-induced hematopoietic and intestinal injury using a Cebpd knockout mouse model. Cebpd−/− mice showed increased lethality at 7.4 and 8.5 Gy total-body irradiation (TBI), compared to Cebpd+/+ mice. Two weeks after a 6 Gy dose of TBI, Cebpd−/− mice showed decreased recovery of white blood cells, neutrophils, platelets, myeloid cells and bone marrow mononuclear cells, decreased colony-forming ability of bone marrow progenitor cells, and increased apoptosis of hematopoietic progenitor and stem cells compared to Cebpd+/+ controls. Cebpd−/− mice exhibited a significant dose-dependent decrease in intestinal crypt survival and in plasma citrulline levels compared to Cebpd+/+ mice after exposure to radiation. This was accompanied by significantly decreased expression of γ-H2AX in Cebpd−/− intestinal crypts and villi at 1 h post-TBI, increased mitotic index at 24 h post-TBI, and increase in apoptosis in intestinal crypts and stromal cells of Cebpd−/− compared to Cebpd+/+ mice at 4 h post-irradiation. This study uncovers a novel biological function for C/EBPδ in promoting the response to radiation-induced DNA-damage and in protecting hematopoietic and intestinal tissues from radiation-induced injury.

Immune cell reconstitution after exposure to potentially lethal doses of radiation in the nonhuman primate

Delayed immune reconstitution remains a major cause of morbidity associated with myelosuppression induced by cytotoxic therapy or myeloablative conditioning for stem cell transplant, as well as potentially lethal doses of total- or partial-body irradiation. Restoration of a functional immune cell repertoire requires hematopoietic stem cell reconstitution for all immune cells and effective thymopoiesis for T cell recovery. There are no medical countermeasures available to mitigate damage consequent to high-dose, potentially lethal irradiation, and there are no well characterized large animal models of prolonged immunosuppression to assess efficacy of potential countermeasures. Herein, the authors describe a model of T and B cell reconstitution following lethal doses of partial-body irradiation with 5% bone marrow sparing that includes full exposure of the thymus. Rhesus macaques (n = 31 male, 5.5-11.3 kg body weight) were exposed to midline tissue doses of 9.0-12.0 Gy using 6 MV LINAC-derived photons at a dose rate of 0.80 Gy min, sparing approximately 5% of bone marrow (tibiae, ankles, and feet). All animals received medical management and were monitored for myeloid and lymphoid suppression and recovery through 180 d post-exposure. Myeloid recovery was assessed by neutrophil and platelet-related hematological parameters. Reconstitution of B and T cell subsets was assessed by flow cytometric immunophenotyping, and recent thymic emigrants were identified by RT-PCR of T cell receptor excision circles. Mortality was recorded through 180 d post-exposure. Acute myelo-suppression was characterized by severe neutropenia and thrombocytopenia, followed by recovery 30-60 d post-exposure. Total T (CD3+) and B (CD20+) cells were reduced significantly following exposure and exhibited differential recovery patterns post-exposure. Both CD4+ and CD8+ subsets of naïve T cells and total CD4+ T cell counts remained significantly lower than baseline through 180 d post-exposure. The failure of recent thymic emigrants and naïve T cell subsets to recover to normal baseline values reflects the severe radiation effects on the recovery of marrow-derived stem and early thymic progenitor cells, their mobilization and seeding of receptive thymic niches, and slow endogenous thymic regeneration.

Selective depletion of eosinophils or neutrophils in mice impacts the efficiency of apoptotic cell clearance in the thymus

Developing thymocytes undergo a rigorous selection process to ensure that the mature T cell population expresses a T cell receptor (TCR) repertoire that can functionally interact with major histocompatibility complexes (MHC). Over 90% of thymocytes fail this selection process and die. A small number of macrophages within the thymus are responsible for clearing the large number of dying thymocytes that must be continuously cleared. We studied the capacity of thymic macrophages to clear apoptotic cells under acute circumstances. This was done by synchronously inducing cell death in the thymus and then monitoring the clearance of apoptotic thymocytes. Interestingly, acute cell death was shown to recruit large numbers of CD11b⁺ cells into the thymus. In the absence of a minor CSF-1 dependent population of macrophages, the recruitment of these CD11b⁺ cells into the thymus was greatly reduced and the clearance of apoptotic cells was disrupted. To assess a possible role for the CD11b⁺ cells in the clearance of apoptotic cells, we analyzed mice deficient for eosinophils and mice with defective trafficking of neutrophils. Failure to attract either eosinophils or neutrophils to the thymus resulted in the impaired clearance of apoptotic cells. These results suggested that there is crosstalk between cells of the innate immune system that is necessary for maximizing the efficiency of apoptotic cell removal.

Optimization of a myeloid cell transfusion strategy for infected neutropenic hosts

Although granulocyte transfusion is a logical, therapeutic option for neutropenic patients with refractory infections, significant technical barriers have prevented its widespread use. A novel phagocyte transfusion strategy has been developed based on activation of a human myeloid cell line HL-60. To further define the potential for HL-60 cells to recapitulate white cell transfusions, a shortened duration of activation was evaluated, facile quality control markers were defined, and the impact of low-dose irradiation on cell function was determined. Three days of activation resulted in increased cell viability and in vitro candidacidal capacity but with slightly higher cell replication compared with 7 days of activation. Cell viability and several flow cytometric measurements were accurate, quality control markers for HL-60 activation. In combination with activation, low-dose irradiation abrogated replication while sparing the candidacidal effects of the HL-60 cells. Infusion of irradiated, activated HL-60 cells improved survival of neutropenic, candidemic mice significantly. In summary, activated, irradiated HL-60 cells are microbicidal, have virtually no replicative capacity, and are safe and effective at protecting neutropenic mice against an otherwise 100% fatal candidal infection. With continued development, this strategy to recapitulate neutrophil functions has the potential to serve as an effective alternative to granulocyte transfusions.

Dog leukocyte antigen-haploidentical stem cell allografts after anti-CD44 therapy and nonmyeloablative conditioning in a preclinical canine model

BACKGROUND

We previously described a reduced-intensity hematopoietic cell transplantation (HCT) regimen in dog leukocyte antigen (DLA)-haploidentical littermate recipients consisting of 450 cGy total body irradiation (TBI) and anti-CD44 monoclonal antibody (mAb) S5 before and mycophenolate mofetil (MMF)/cyclosporine (CSP) after HCT.

METHODS

We tested a nonmyeloablative regimen of mAb S5 and 200 cGy TBI with postgrafting MMF/CSP in 44 DLA-haploidentical recipients using eight different regimens. Ten dogs also received escalating doses of donor lymphocyte infusion (DLI) alone or with pentostatin to convert to complete donor chimerism.

RESULTS

All dogs achieved initial engraftment between one to two weeks after HCT with peripheral blood mononuclear cell (PBMC) donor chimerism ranging from 2% to 98% (median 37%) on day +35. Twenty-five (57%) dogs rejected their donor grafts at a median of seven (range; 1-19) weeks after HCT. Low levels of PBMC donor chimerism at three weeks predicted graft rejection. DLI neither facilitated conversion to full donor chimerism after HCT nor prevented rejection. Higher total nucleated cells, CD4+, CD8+, and CD14+ cell subset numbers in the PBMC graft were associated with stable full donor engraftment. Dogs given higher cell subset doses of infused PBMC achieved longer duration of mixed chimerism.

CONCLUSIONS

Nonmyeloablative conditioning with 200 cGy TBI and anti-CD44 mAb was sufficient for initial uniform engraftment across DLA haplotype-mismatched barriers. However, sustained donor engraftment was seen in only 43% of recipients. Graft composition and donor-dominant chimerism early after HCT may be the most important factors for sustained donor engraftment.

IL-17 receptor knockout mice have enhanced myelotoxicity and impaired hemopoietic recovery following gamma irradiation

IL-17A is a T cell-derived proinflammatory cytokine required for microbial host defense. In vivo expression profoundly stimulates granulopoiesis. At baseline, the hemopoietic system of IL-17R knockout mice (IL-17Ra(-/-)) is, with the exception of increased splenic progenitor numbers, indistinguishable from normal control mice. However, when challenged with gamma irradiation, hemopoietic toxicity is significantly more pronounced in IL-17Ra(-/-) animals, with the gamma irradiation-associated LD(50) being reduced by 150 rad. In spleen-derived T cells, gamma irradiation induces significant murine IL-17A expression in vivo but not in vitro. After sublethal radiation injury (500 rad), the infusion of purified CD4(+) T cells enhances hemopoietic recovery. This recovery is significantly impaired in IL-17Ra(-/-) animals or after in vivo blockade of IL-17Ra in normal mice, resulting in a reduction of hemopoietic precursors by 50% and of neutrophils by 43%. Following sublethal radiation-induced myelosuppression, in vivo overexpression of murine IL-17A in normal mice substantially enhanced granulopoietic restoration in mice with a 4-fold increase in neutrophils and splenic precursors on day 8 (CFU-granulocyte-macrophage/granulocyte-erythrocyte-megakaryocyte-monocyte, CFU-high proliferative potential), as well as 2- and 3-fold increases of bone marrow precursors, respectively. This establishes IL-17A as a hemopoietic response cytokine to radiation injury in mice and an inducible mechanism that is required for recovery of granulopoiesis after radiation injury.

Gadd45a and Gadd45b protect hematopoietic cells from UV-induced apoptosis via distinct signaling pathways, including p38 activation and JNK inhibition

Gadd45a, Gadd45b, and Gadd45g (Gadd45/MyD118/CR6) are genes that are rapidly induced by genotoxic stress and have been implicated in genotoxic stress-induced responses, notably in apoptosis. Recently, using myeloid-enriched bone marrow (BM) cells obtained from wild-type (WT), Gadd45a-deficient, and Gadd45b-deficient mice, we have shown that in hematopoietic cells Gadd45a and Gadd45b play a survival function to protect hematopoietic cells from DNA-damaging agents, including ultra violet (UV)-induced apoptosis. The present study was undertaken to decipher the molecular paths that mediate the survival functions of Gadd45a and Gadd45b against genotoxic stress induced by UV radiation. It is shown that in hematopoietic cells exposed to UV radiation Gaddd45a and Gadd45b cooperate to promote cell survival via two distinct signaling pathways involving activation of the GADD45a-p38-NF-kappaB-mediated survival pathway and GADD45b-mediated inhibition of the stress response MKK4-JNK pathway.

In vivo persistence of retrovirally transduced murine long-term repopulating cells is not limited by expression of foreign gene products in the fully or minimally myeloablated setting

Many nonmalignant hematologic disorders could potentially be treated by genetic correction of as few as 5-10% of target lineage cells. However, immune system clearance of cells expressing gene products perceived as foreign could be limiting. There is evidence that tolerance to foreign proteins can result when myeloablative conditioning is used, but this limits the overall applicability of such techniques. Therefore, we sought to evaluate the engraftment of hematopoietic stem cells carrying a foreign transgene after low-dose irradiation by comparing in vivo survival of murine long-term repopulating cells (LTRC) transduced with either a retroviral vector expressing the bacterial neomycin phosphotransferase gene (neo) or a vector containing neo gene sequences but modified to prevent protein expression (nonexpression). First, marrow cells from congenic donors were transduced with either vector and transplanted into recipients treated with standard dose irradiation of 800 rads. High-level engraftment and gene marking resulted, without differences in the marking levels or pattern of persistence of the cells between cells transduced with either vector. Low-dose irradiation at 100 rads was tested using higher cell doses. Marking levels as high as 10% overall were obtained, again with no differences between mice receiving cells transduced with the neo versus the nonexpression vectors. To investigate a potentially more immunogenic protein, marrow cells were transduced with a vector containing the green fluorescent protein (GFP) gene, and their persistence was studied in recipient mice receiving 100 rads. Stable GFP expression in 5-10% of circulating cells was observed long term. We conclude that even with very low dose conditioning, engraftment by genetically modified LTRC cells at clinically significant levels can be achieved without evidence for clearance of cells known to be expressing immunogenic proteins.

The chicken Pdcd4 gene is regulated by v-Myb

The retroviral oncogene v-myb encodes a transcription factor (v-Myb) which is responsible for the ability of avian myeloblastosis virus (AMV) to transform myelomonocytic cells. v-Myb is thought to disrupt the differentiation of myelomonocytic cells by affecting the expression of specific target genes. To identify such genes we have analysed the gene expression in a myelomonocytic chicken cell line that carries an estrogen inducible version of v-Myb by differential display. Here we describe the identification of the chicken homolog of the mouse Pdcd4 gene as a novel v-Myb target gene. Pdcd4 is also known as MA-3, TIS and H731 and has recently been shown to suppress the transformation of epidermal cells by tumor promoters. Our results provide the first evidence that v-Myb directly regulates the expression of a potential tumor suppressor gene.

Studies of the action of leucogenenol on the myeloid and lymphoid tissues of the sublethally irradiated mouse

Studies of the action of leucogenenol on the peripheral leukocytes, the myeloid cells found in bone marrow, and the lymphoid cells found in the spleen of sublethally irradiated mice strongly suggest that leucogenenol stimulates the maturation and/or cellular division of cells of both the myeloid and lymphoid series. Accordingly, as indicated by the increase in the number of peripheral leukocytes, as well as the increase in the number of myeloid and lymphoid cells found in the bone marrow and spleen, mice treated with leucogenenol appear to recover more rapidly than untreated controls.