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The nervous system exerts potent control over the immune system. A wealth of recent data strongly implicate different branches of the peripheral nervous system in toning immunity, including the sympathetic and parasympathetic as well as the sensory nervous system. Neurotransmitters released by peripheral nerves mediate the interactions with the immune system, which can have pro-or anti-inflammatory effects. In this review, we focus on these recent developments by discussing the effects of norepinephrine, acetylcholine, VIP, NMU, CGRP, substance P as well as dopamine on immune cells.

This study shows loss of sympathetic innervation to the popliteal lymph node to induce IFN-γ expression in CD8 T cells, causing expansion. This is rescued by β2 adrenergic receptor agonists, demonstrating the pro-inflammatory effect of loss of direct adrenergic input., Peripheral nerve injury can cause debilitating disease and immune cell–mediated destruction of the affected nerve. While the focus has been on the nerve-regenerative response, the effect of loss of innervation on lymph node function is unclear. Here, we show that the popliteal lymph node (popLN) receives direct neural input from the sciatic nerve and that sciatic denervation causes lymph node expansion. Loss of sympathetic, adrenergic tone induces the expression of IFN-γ in LN CD8 T cells, which is responsible for LN expansion. Surgery-induced IFN-γ expression and expansion can be rescued by β2 adrenergic receptor agonists but not sensory nerve agonists. These data demonstrate the mechanisms governing the pro-inflammatory effect of loss of direct adrenergic input on lymph node function.

The authors show that autophagy in lymphatic endothelial cells (LECs) dampens collagen-induced arthritis by regulating two distinct pathways: the promotion of T reg cells in LNs and the inhibition of S1P-dependent migration of pathogenic Th17 cells in inflamed organs., Lymphatic endothelial cells (LECs) present peripheral tissue antigens to induce T cell tolerance. In addition, LECs are the main source of sphingosine-1-phosphate (S1P), promoting naive T cell survival and effector T cell exit from lymph nodes (LNs). Autophagy is a physiological process essential for cellular homeostasis. We investigated whether autophagy in LECs modulates T cell activation in experimental arthritis. Whereas genetic abrogation of autophagy in LECs does not alter immune homeostasis, it induces alterations of the regulatory T cell (T reg cell) population in LNs from arthritic mice, which might be linked to MHCII-mediated antigen presentation by LECs. Furthermore, inflammation-induced autophagy in LECs promotes the degradation of Sphingosine kinase 1 (SphK1), resulting in decreased S1P production. Consequently, in arthritic mice lacking autophagy in LECs, pathogenic Th17 cell migration toward LEC-derived S1P gradients and egress from LNs are enhanced, as well as infiltration of inflamed joints, resulting in exacerbated arthritis. Our results highlight the autophagy pathway as an important regulator of LEC immunomodulatory functions in inflammatory conditions.

The bone marrow is the major hematopoietic organ, consisting of distinct microenvironmental niches for the production of hematopoietic cells. Advanced visualizing methods are required to define and better understand the interactions between stromal and hematopoietic cells. In this chapter, we describe an ex vivo whole-mount imaging technique of the bone marrow, which allows for a fast, high-quality, and three-dimensional visualization of different bone marrow components. We provide a guide for conducting adoptive transfer experiments of fluorescently labeled leukocytes and visualizing their location in the bone marrow with respect to the bone marrow vasculature. This method presents a quick, easy, and inexpensive approach to image the bone marrow in three dimensions.

Aims Mental stress substantially contributes to the initiation and progression of human disease, including cardiovascular conditions. We aim to investigate the underlying mechanisms of these contributions since they remain largely unclear. Methods and results Here, we show in humans and mice that leucocytes deplete rapidly from the blood after a single episode of acute mental stress. Using cell-tracking experiments in animal models of acute mental stress, we found that stress exposure leads to prompt uptake of inflammatory leucocytes from the blood to distinct tissues including heart, lung, skin, and, if present, atherosclerotic plaques. Mechanistically, we found that acute stress enhances leucocyte influx into mouse atherosclerotic plaques by modulating endothelial cells. Specifically, acute stress increases adhesion molecule expression and chemokine release through locally derived norepinephrine. Either chemical or surgical disruption of norepinephrine signalling diminished stress-induced leucocyte migration into mouse atherosclerotic plaques. Conclusion Our data show that acute mental stress rapidly amplifies inflammatory leucocyte expansion inside mouse atherosclerotic lesions and promotes plaque vulnerability., Graphical Abstract This study provides novel mechanistic insights into how acute mental stress fuels vascular inflammation and promotes plaque rupture. EC, endothelial cells; HPA, hypothalamic-pituitary-adrenal axis; MACS, macrophages; SAM, sympathetic-adrenal-medullary axis.

The adaptive immune function of lymph nodes is dependent on constant recirculation of lymphocytes. In this article, we identify neutrophils present in the lymph node at steady state, exhibiting the same capacity for recirculation. In germ-free mice, neutrophils still recirculate through lymph nodes, and in mice cohoused with wild microbiome mice, the level of neutrophils in lymph nodes increases significantly. We found that at steady state, neutrophils enter the lymph node entirely via L-selectin and actively exit via efferent lymphatics via an S1P dependent mechanism. The small population of neutrophils in the lymph node can act as reconnaissance cells to recruit additional neutrophils in the event of bacterial dissemination to the lymph node. Without these reconnaissance cells, there is a delay in neutrophil recruitment to the lymph node and a reduction in swarm formation following Staphylococcus aureus infection. This ability to recruit additional neutrophils by lymph node neutrophils is initiated by LTB4. This study establishes the capacity of neutrophils to recirculate, much like lymphocytes via L-selectin and high endothelial venules in lymph nodes and demonstrates how the presence of neutrophils at steady state fortifies the lymph node in case of an infection disseminating through lymphatics.

The immune system is under control of the circadian clock. Many of the circadian rhythms observed in the immune system originate in direct interactions between components of the circadian clock and components of the immune system. The main means of circadian control over the immune system is by direct control of circadian clock proteins acting as transcription factors driving the expression or repression of immune genes. A second circadian control of immunity lies in the acetylation or methylation of histones to regulate gene transcription or inflammatory proteins. Furthermore, circadian clock proteins can engage in direct physical interactions with components of key inflammatory pathways such as members of the NFκB protein family. This regulation is transcription independent and allows the immune system to also reciprocally exert control over circadian clock function. Thus, the molecular interactions between the circadian clock and the immune system are manifold. We highlight and discuss here the recent findings with respect to the molecular mechanisms that control time-of-day-dependent immunity. This review provides a structured overview focusing on the key circadian clock proteins and discusses their reciprocal interactions with the immune system., Graphical abstract Image 1, Highlights • The immune system is under control of the circadian clock. • Circadian clock proteins act as transcription factors controlling genes of the immune system. • Circadian clock proteins engage in direct physical interactions with inflammatory proteins. • Immune factors also reciprocally exert control over circadian clock function.

Circadian rhythms are a ubiquitous feature of virtually all living organisms, regulating a wide diversity of physiological systems. It has long been established that the circadian clockwork plays a key role in innate immune responses, and recent studies reveal that several aspects of adaptive immunity are also under circadian control. We discuss the latest insights into the genetic and biochemical mechanisms linking immunity to the core circadian clock of the cell and hypothesize as to why the immune system is so tightly controlled by circadian oscillations. Finally, we consider implications for human health, including vaccination strategies and the emerging field of chrono-immunotherapy.

Lymphocytes circulate through lymph nodes (LN) in search for antigen in what is believed to be a continuous process. Here, we show that lymphocyte migration through lymph nodes and lymph occurred in a non-continuous, circadian manner. Lymphocyte homing to lymph nodes peaked at night onset, with cells leaving the tissue during the day. This resulted in strong oscillations in lymphocyte cellularity in lymph nodes and efferent lymphatic fluid. Using lineage-specific genetic ablation of circadian clock function, we demonstrated this to be dependent on rhythmic expression of promigratory factors on lymphocytes. Dendritic cell numbers peaked in phase with lymphocytes, with diurnal oscillations being present in disease severity after immunization to induce experimental autoimmune encephalomyelitis (EAE). These rhythms were abolished by genetic disruption of T cell clocks, demonstrating a circadian regulation of lymphocyte migration through lymph nodes with time-of-day of immunization being critical for adaptive immune responses weeks later.

This article contains errors in Figs. 5 and 6, for which we apologize. In Fig. 5f, the image ‘E12.5 tail’ was inadvertently replaced with a duplicate of the image ‘E12.5 trunk’ from the same panel. In Figure 6d, the image ‘E9.5/OH-TAM E8.5, embryo’ was inadvertently replaced with a duplicate of the image ‘E10.5/ OH-TAM E8.5, embryo’ from Fig. 6b. The corrected versions of these figures appear in the Author Correction associated with this Article.

The number of leukocytes circulating in blood in mammals is under circadian control (i.e., ∼24h). We summarize here latest findings on the mechanisms governing leukocyte migration from the blood into various organs, focusing on the distinct leukocyte subtype- and tissue-specific molecules involved. We highlight the oscillatory expression patterns of adhesion molecules, chemokines, and their receptors that are expressed on endothelial cells and leukocytes, and which are crucial regulators of rhythmic leukocyte recruitment. We also discuss the relevance of clock genes for leukocyte function and migration. Finally, we compare immune cell rhythms under steady-state conditions as well as during inflammation and disease, and we postulate how these findings provide potential new avenues for therapeutic intervention.

The role of integrin-mediated adhesion during T cell progenitor homing to and differentiation within the thymus is ill-defined, mainly due to functional overlap. To circumvent compensation, we disrupted the hematopoietic integrin regulator kindlin-3 in mice and found a progressive thymus atrophy that is primarily caused by an impaired homing capacity of T cell progenitors to the vascularized thymus. Notably, the low shear flow conditions in the vascular system at midgestation allow kindlin-3-deficient fetal liver-derived T cell progenitors to extravasate via pharyngeal vessels and colonize the avascular thymus primordium. Once in the thymus, kindlin-3 promotes intrathymic T cell proliferation by facilitating the integrin-dependent crosstalk with thymic antigen presenting cells, while intrathymic T cell migration, maturation into single positive CD4 and CD8 T cells and release into the circulation proceed without kindlin-3. Thus, kindlin-3 is dispensable for integrin-mediated T cell progenitor adhesion and signalling at low and indispensable at high shear forces.

Onset of cardiovascular complications as a consequence of atherosclerosis exhibits a circadian incidence with a peak in the morning hours. Although development of atherosclerosis extends for long periods of time through arterial leukocyte recruitment, we hypothesized that discrete diurnal invasion of the arterial wall could sustain atherogenic growth. Here, we show that myeloid cell recruitment to atherosclerotic lesions oscillates with a peak during the transition from the activity to the resting phase. This diurnal phenotype is regulated by rhythmic release of myeloid cell-derived CCL2, and blockade of its signaling abolished oscillatory leukocyte adhesion. In contrast, we show that myeloid cell adhesion to microvascular beds peaks during the early activity phase. Consequently, timed pharmacological CCR2 neutralization during the activity phase caused inhibition of atherosclerosis without disturbing microvascular recruitment. These findings demonstrate that chronic inflammation of large vessels feeds on rhythmic myeloid cell recruitment, and lay the foundation for chrono-pharmacology-based therapy.

Summary The number of leukocytes present in circulation varies throughout the day, reflecting bone marrow output and emigration from blood into tissues. Using an organism-wide circadian screening approach, we detected oscillations in pro-migratory factors that were distinct for specific vascular beds and individual leukocyte subsets. This rhythmic molecular signature governed time-of-day-dependent homing behavior of leukocyte subsets to specific organs. Ablation of BMAL1, a transcription factor central to circadian clock function, in endothelial cells or leukocyte subsets demonstrated that rhythmic recruitment is dependent on both microenvironmental and cell-autonomous oscillations. These oscillatory patterns defined leukocyte trafficking in both homeostasis and inflammation and determined detectable tumor burden in blood cancer models. Rhythms in the expression of pro-migratory factors and migration capacities were preserved in human primary leukocytes. The definition of spatial and temporal expression profiles of pro-migratory factors guiding leukocyte migration patterns to organs provides a resource for the further study of the impact of circadian rhythms in immunity., Graphical Abstract, Highlights • Leukocyte subsets show time-of-day-dependent migration patterns to organs • This relies on lineage- and tissue-specific oscillations in pro-migratory factors • Loss of circadian clocks in the endothelium or leukocytes ablates rhythmicity • The efficacy of blocking leukocyte migration is time-of-day dependent, Leukocytes continuously circulate throughout the body. He et al. demonstrate that trafficking patterns of major leukocyte subsets occur in a rhythmic manner dependent on the time-of-day-dependent expression of lineage- and tissue-specific factors. This influences the inflammatory response and leukemic tumor burden and translates to the migration behavior of human primary lymphocytes.

Anticancer chemotherapy drugs challenge hematopoietic tissues to regenerate but commonly produce long-term sequelae. Chemotherapy-induced deficits in hematopoietic stem or stromal cell function have been described, but the mechanisms mediating hematopoietic dysfunction remain unclear. Administration of multiple cycles of cisplatin chemotherapy causes substantial sensory neuropathy. Here we demonstrate that chemotherapy-induced nerve injury in the bone marrow of mice is a crucial lesion impairing hematopoietic regeneration. Using pharmacological and genetic models, we show that the selective loss of adrenergic innervation in the bone marrow alters its regeneration after genotoxic insult. Sympathetic nerves in the marrow promote the survival of constituents of the stem cell niche that initiate recovery. Neuroprotection by deletion of Trp53 in sympathetic neurons or neuroregeneration by administration of 4-methylcatechol or glial-derived neurotrophic factor (GDNF) promotes hematopoietic recovery. These results demonstrate the potential benefit of adrenergic nerve protection for shielding hematopoietic niches from injury.

Mesenchymal stem cells (MSCs) are self-renewing precursor cells that can differentiate into bone, fat, cartilage, and stromal cells of the bone marrow. Recent studies suggest that MSCs themselves are critical for forming a niche that maintains hematopoietic stem cells (HSCs). The ease by which human MSC-like and stromal progenitor cells can be isolated from the bone marrow and other tissues has led to the rapid development of clinical investigations exploring their anti-inflammatory properties, tissue preservation capabilities, and regenerative potential. However, the identity of genuine MSCs and their specific contributions to these various beneficial effects have remained enigmatic. In this article, we examine the definition of MSCs and discuss the importance of rigorously characterizing their stem cell activity. We review their role and that of other putative niche constituents in the regulation of bone marrow HSCs. Additionally, how MSCs and their stromal progeny alter immune function is discussed, as well as potential therapeutic implications.

Circadian rhythms, which have long been known to play crucial roles in physiology, are emerging as important regulators of specific immune functions. Circadian oscillations of immune mediators coincide with the activity of the immune system, possibly allowing the host to anticipate and handle microbial threats more efficiently. These oscillations may also help to promote tissue recovery and the clearance of potentially harmful cellular elements from the circulation. This Review summarizes the current knowledge of circadian rhythms in the immune system and provides an outlook on potential future implications.

Inhibition of leukocyte adhesion to the vascular endothelium represents a novel and important approach for decreasing sickle cell disease (SCD) vaso-occlusion. Using a humanized SCD–mouse-model of tumor necrosis factor-α–induced acute vaso-occlusion, we herein present data demonstrating that short-term administration of either hydroxyurea or the phosphodiesterase 9 (PDE9) inhibitor, BAY73-6691, significantly altered leukocyte recruitment to the microvasculature. Notably, the administration of both agents led to marked improvements in leukocyte rolling and adhesion and decreased heterotypic red blood cell-leukocyte interactions, coupled with prolonged animal survival. Mechanistically, these rheologic benefits were associated with decreased endothelial adhesion molecule expression, as well as diminished leukocyte Mac-1-integrin activation and cyclic guanosine monophosphate (cGMP)–signaling, leading to reduced leukocyte recruitment. Our findings indicate that hydroxyurea has immediate beneficial effects on the microvasculature in acute sickle-cell crises that are independent of the drug's fetal hemoglobin-elevating properties and probably involve the formation of intravascular nitric oxide. In addition, inhibition of PDE9, an enzyme highly expressed in hematopoietic cells, amplified the cGMP-elevating effects of hydroxyurea and may represent a promising and more tissue-specific adjuvant therapy for this disease.

Over the last few years, molecular evidence has clearly shown a direct impact of daily rhythms on the immune system, both in steady state and under inflammatory conditions. Circadian oscillations have been demonstrated in the regulation of cytokine and chemokine levels as well as immune cell numbers in blood and tissues. A possible explanation for these rhythms is that species are more prone to be exposed to a variety of acute threats such as injuries and microbial infections at specific times during their circadian cycle. Also some chronic diseases follow a circadian rhythm, indicating that a greater understanding of the underlying molecular mechanisms will permit the development of new and improved therapeutic strategies for treating inflammatory diseases that incorporate the element of time.

Myocardial infarction (MI) is the leading cause of death in Western countries. Epidemiological studies show acute MI to be more prevalent in the morning and to be associated with a poorer outcome in terms of mortality and recovery. The mechanisms behind this association are not fully understood. Here, we report that circadian oscillations of neutrophil recruitment to the heart determine infarct size, healing, and cardiac function after MI Preferential cardiac neutrophil recruitment during the active phase (Zeitgeber time, ZT13) was paralleled by enhanced myeloid progenitor production, increased circulating numbers of CXCR2(hi) neutrophils as well as upregulated cardiac adhesion molecule and chemokine expression. MI at ZT13 resulted in significantly higher cardiac neutrophil infiltration compared to ZT5, which was inhibited by CXCR2 antagonism or neutrophil-specific CXCR2 knockout. Limiting exaggerated neutrophilic inflammation at this time point significantly reduced the infarct size and improved cardiac function., info:eu-repo/semantics/published

Background: Therapeutic targeting of arterial leukocyte recruitment in the context of atherosclerosis has been disappointing in clinical studies. Reasons for such failures include the lack of knowledge of arterial-specific recruitment patterns. Here we establish the importance of the cathepsin G (CatG) in the context of arterial myeloid cell recruitment. Methods: Intravital microscopy of the carotid artery, the jugular vein, and cremasteric arterioles and venules in Apoe –/– and CatG-deficient mice ( Apoe –/– Ctsg –/– ) was used to study site-specific myeloid cell behavior after high-fat diet feeding or tumor necrosis factor stimulation. Atherosclerosis development was assessed in aortic root sections after 4 weeks of high-fat diet, whereas lung inflammation was assessed after inhalation of lipopolysaccharide. Endothelial deposition of CatG and CCL5 was quantified in whole-mount preparations using 2-photon and confocal microscopy. Results: Our observations elucidated a crucial role for CatG during arterial leukocyte adhesion, an effect not found during venular adhesion. Consequently, CatG deficiency attenuates atherosclerosis but not acute lung inflammation. Mechanistically, CatG is immobilized on arterial endothelium where it activates leukocytes to firmly adhere engaging integrin clustering, a process of crucial importance to achieve effective adherence under high-shear flow. Therapeutic neutralization of CatG specifically abrogated arterial leukocyte adhesion without affecting myeloid cell adhesion in the microcirculation. Repetitive application of CatG-neutralizing antibodies permitted inhibition of atherogenesis in mice. Conclusions: Taken together, these findings present evidence of an arterial-specific recruitment pattern centered on CatG-instructed adhesion strengthening. The inhibition of this process could provide a novel strategy for treatment of arterial inflammation with limited side effects.

Mobilization of neutrophils from the bone marrow determines neutrophil blood counts and thus is medically important. Balanced neutrophil mobilization from the bone marrow depends on the retention-promoting chemokine CXCL12 and its receptor CXCR4 and the egression-promoting chemokine CXCL2 and its receptor CXCR2. Both pathways activate the small guanosine triphosphatase Rac, leaving the role of this signaling event in neutrophil retention and egression ambiguous. On the assumption that active Rac determines persistent directional cell migration, we generated a mathematical model to link chemokine-mediated Rac modulation to neutrophil egression time. Our computer simulation indicated that, in the bone marrow, where the retention signal predominated, egression time strictly depended on the time it took Rac to return to its basal activity (namely, adaptation). This prediction was validated in mice lacking the Rac inhibitor ArhGAP15. Neutrophils in these mice showed prolonged Rac adaptation and cell-autonomous retention in the bone marrow. Our model thus demonstrates that mobilization in the presence of two spatially defined opposing chemotactic cues strictly depends on inhibitors shaping the time course of signal adaptation. Furthermore, our findings might help to find new modes of intervention to treat conditions characterized by excessively low or high circulating neutrophils.

Hematopoietic stem cells (HSCs) reside in specialized bone marrow (BM) niches regulated by the sympathetic nervous system (SNS). Here, we have examined whether mononuclear phagocytes modulate the HSC niche. We defined three populations of BM mononuclear phagocytes that include Gr-1hi monocytes (MOs), Gr-1lo MOs, and macrophages (MΦ) based on differential expression of Gr-1, CD115, F4/80, and CD169. Using MO and MΦ conditional depletion models, we found that reductions in BM mononuclear phagocytes led to reduced BM CXCL12 levels, the selective down-regulation of HSC retention genes in Nestin+ niche cells, and egress of HSCs/progenitors to the bloodstream. Furthermore, specific depletion of CD169+ MΦ, which spares BM MOs, was sufficient to induce HSC/progenitor egress. MΦ depletion also enhanced mobilization induced by a CXCR4 antagonist or granulocyte colony-stimulating factor. These results highlight two antagonistic, tightly balanced pathways that regulate maintenance of HSCs/progenitors in the niche during homeostasis, in which MΦ cross talk with the Nestin+ niche cell promotes retention, and in contrast, SNS signals enhance egress. Thus, strategies that target BM MΦ hold the potential to augment stem cell yields in patients that mobilize HSCs/progenitors poorly.

Objective—Junctional adhesion molecule-C (JAM-C) is an adhesion molecule that has multiple roles in inflammation and vascular biology, but many aspects of its functions under pathological conditions are unknown. Here we investigated the role of JAM-C in leukocyte migration in response to ischemia reperfusion (I/R) injury.Methods and Results—Pretreatment of mice with soluble JAM-C (sJAM-C), used as a pharmacological blocker of JAM-C-mediated reactions, significantly suppressed leukocyte migration in models of kidney and cremaster muscle I/R injury (39 and 51% inhibition, respectively). Furthermore, in the cremaster muscle model (studied by intravital microscopy), both leukocyte adhesion and transmigration were suppressed in JAM-C-deficient mice (JAM-C−/−) and enhanced in mice overexpressing JAM-C in their endothelial cells (ECs). Analysis of JAM-C subcellular expression by immunoelectron microscopy indicated that in I/R-injured tissues, EC JAM-C was redistributed from cytoplasmic vesicles and EC junctional sites to nonjunctional plasma membranes, a response that may account for the role of JAM-C in both leukocyte adhesion and transmigration under conditions of I/R injury.Conclusions—The findings demonstrate a role for EC JAM-C in mediating leukocyte adhesion and transmigration in response to I/R injury and indicate the existence of a novel regulatory mechanism for redistribution and hence function of EC JAM-C in vivo.

JAM-C is an adhesion molecule that is expressed on cells within the vascular compartment and epithelial cells and, to date, has been largely studied in the context of inflammatory events. Using immunolabeling procedures in conjunction with confocal and electron microscopy, we show here that JAM-C is also expressed in peripheral nerves and that this expression is localized to Schwann cells at junctions between adjoining myelin end loops. Sciatic nerves from JAM-C–deficient [having the JAM-C gene knocked out (KO)] mice exhibited loss of integrity of the myelin sheath and defective nerve conduction as indicated by morphological and electrophysiological studies, respectively. In addition, behavioral tests showed motor abnormalities in the KO animals. JAM-C was also expressed in human sural nerves with an expression profile similar to that seen in mice. These results demonstrate that JAM-C is a component of the autotypic junctional attachments of Schwann cells and plays an important role in maintaining the integrity and function of myelinated peripheral nerves.

Blood polymorphonuclear neutrophils provide immune protection against pathogens, but may also promote tissue injury in inflammatory diseases. Although neutrophils are generally considered to be a relatively homogeneous population, evidence for heterogeneity is emerging. Under steady-state conditions, neutrophil heterogeneity may arise from ageing and replenishment by newly released neutrophils from the bone marrow. Aged neutrophils upregulate CXCR4, a receptor allowing their clearance in the bone marrow, with feedback inhibition of neutrophil production via the IL-17/G-CSF axis, and rhythmic modulation of the haematopoietic stem-cell niche. The aged subset also expresses low levels of L-selectin. Previous studies have suggested that in vitro-aged neutrophils exhibit impaired migration and reduced pro-inflammatory properties. Here, using in vivo ageing analyses in mice, we show that neutrophil pro-inflammatory activity correlates positively with their ageing whilst in circulation. Aged neutrophils represent an overly active subset exhibiting enhanced αMβ2 integrin activation and neutrophil extracellular trap formation under inflammatory conditions. Neutrophil ageing is driven by the microbiota via Toll-like receptor and myeloid differentiation factor 88-mediated signalling pathways. Depletion of the microbiota significantly reduces the number of circulating aged neutrophils and dramatically improves the pathogenesis and inflammation-related organ damage in models of sickle-cell disease or endotoxin-induced septic shock. These results identify a role for the microbiota in regulating a disease-promoting neutrophil subset.

Acute cardiovascular complications occur predominantly at the onset of the behavioral activity phase and are the major cause of death in humans. Recent evidence points to a critical role for rhythm...

In acute and chronic inflammation, neutrophils and platelets, both of which promote monocyte recruitment, are often activated simultaneously. We investigated how secretory products of neutrophils and platelets synergize to enhance the recruitment of monocytes. We found that neutrophil-borne human neutrophil peptide 1 (HNP1, a-defensin) and platelet-derived CCL5 form heteromers. These heteromers stimulate monocyte adhesion through CCR5 ligation. We further determined structural features of HNP1-CCL5 heteromers and designed a stable peptide that could disturb proinflammatory HNP1-CCL5 interactions. This peptide attenuated monocyte and macrophage recruitment in a mouse model of myocardial infarction. These results establish the in vivo relevance of heteromers formed between proteins released from neutrophils and platelets and show the potential of targeting heteromer formation to resolve acute or chronic inflammation.

The functions of blood cells extend well beyond the immune functions of leucocytes or the respiratory and hemostatic functions of erythrocytes and platelets. Seen as a whole, the bloodstream is in charge of nurturing and protecting all organs by carrying a mixture of cell populations in transit from one organ to another. To optimize these functions, evolution has provided blood and the vascular system that carries it with various mechanisms that ensure the appropriate influx and egress of cells into and from the circulation where and when needed. How this homeostatic control of blood is achieved has been the object of study for over a century, and although the major mechanisms that govern it are now fairly well understood, several new concepts and mediators have recently emerged that emphasize the dynamism of this liquid tissue. Here we review old and new concepts that relate to the maintenance and regulation of leucocyte homeostasis in blood and briefly discuss the mechanisms for platelets and red blood cells.

Saltatory conduction in the nervous system is enabled through the intimate association between the leading edge of the myelin sheath and the axonal membrane to demarcate the node of Ranvier. The 186 kDa neuron specific isoform of the adhesion molecule neurofascin (Nfasc186) is required for the clustering of voltage gated Na+ channels at the node, whilst the 155 kDa glial specific isoform (Nfasc155) is required for the assembly of correct paranodal junctions. In order to understand the relationship between these vital structures and how they are affected in multiple sclerosis we have examined the expression of Nfasc155 and Nfasc186 in areas of inflammation, demyelination and remyelination from post-mortem brains. Fourteen cases of neuropathologically confirmed multiple sclerosis (8 female and 6 male; post-mortem delay 7-24 h; age 37-77 years; and disease duration 15-40 years), comprising 20 tissue blocks with 32 demyelinating or remyelinating lesions, were used in this study. A significant early alteration in Nfasc155+ paranodal structures occurs within and adjacent to actively demyelinating white matter lesions that are associated with damaged axons. Shaker-type Kv1.2 channels, normally located distally to the paranode, overlapped with the disrupted Nfasc155+ structures. In the absence of Nfasc155, Kv1.2 channels abutted normally clustered Nfasc186+ nodes, indicating that complete disruption of the paranodal structure and movement of Kv1.2 channels precede alterations at the node itself. Within areas of partial remyelination, a number of atypical triple-Nfasc155+ structures were noted that may represent transient oligodendrocyte-axonal contacts during the process of myelin repair or aberrant interactions. Within shadow plaques discretely clustered Na+v, Nfasc186+ and Nfasc155+ domains indicated the restoration of normal nodal architecture. The alterations in oligodendrocyte Nfasc155 expression that accompany inflammation and demyelination suggest an ongoing disruption to the axonal-oligodendrocyte complex within newly forming as well as established lesions in multiple sclerosis, resulting in destruction of the Nfasc186+/Na+v nodal complex vital to successful fast neurotransmission in the CNS.

The mechanism of leukocyte migration through venular walls in vivo is largely unknown. By using immunofluorescence staining and confocal microscopy, the present study demonstrates the existence of regions within the walls of unstimulated murine cremasteric venules where expression of key vascular basement membrane (BM) constituents, laminin 10, collagen IV, and nidogen-2 (but not perlecan) are considerably lower (

The molecular mechanisms that control leukocyte migration to sites of inflammation have been a major focus of immunologists over the past 25 years. Recently, an additional dimension has been added to the physiological orchestration of leukocyte trafficking: time. On page 1483 in this issue, Nguyen et al. ( 1 ) describe a daily oscillation in monocytes present in different mouse tissues, in which there is an increase in number during the animal's time of rest and a decrease during the mouse's active phase. Moreover, this pattern of behavior is governed by a circadian gene in myeloid cells. This suggests that rhythms within this lineage control the trafficking behavior of monocytes in the organism.

In the bone marrow, hematopoietic stem cells (HSCs) lodge in specialized microenvironments that tightly control the proliferative state of HSCs to adapt to the varying needs for replenishment of blood cells while also preventing HSC exhaustion. All putative niche cells suggested thus far have a nonhematopoietic origin. Thus, it remains unclear how feedback from mature cells is conveyed to HSCs to adjust their proliferation. Here we show that megakaryocytes (MKs) can directly regulate HSC pool size in mice. Three-dimensional whole-mount imaging revealed that endogenous HSCs are frequently located adjacent to MKs in a nonrandom fashion. Selective in vivo depletion of MKs resulted in specific loss of HSC quiescence and led to a marked expansion of functional HSCs. Gene expression analyses revealed that MKs are the source of chemokine C-X-C motif ligand 4 (CXCL4, also named platelet factor 4 or PF4) in the bone marrow, and we found that CXCL4 regulates HSC cell cycle activity. CXCL4 injection into mice resulted in a reduced number of HSCs because of their increased quiescence. By contrast, Cxcl4(-/-) mice exhibited an increased number of HSCs and increased HSC proliferation. Combined use of whole-mount imaging and computational modeling was highly suggestive of a megakaryocytic niche capable of independently influencing HSC maintenance by regulating quiescence. These results indicate that a terminally differentiated cell type derived from HSCs contributes to the HSC niche, directly regulating HSC behavior.

SummaryThe multistep sequence leading to leukocyte migration is thought to be locally regulated at the inflammatory site. Here, we show that broad systemic programs involving long-range signals from the sympathetic nervous system (SNS) delivered by adrenergic nerves regulate rhythmic recruitment of leukocytes in tissues. Constitutive leukocyte adhesion and migration in murine bone marrow (BM) and skeletal-muscle microvasculature fluctuated with circadian peak values at night. Migratory oscillations, altered by experimental jet lag, were implemented by perivascular SNS fibers acting on β-adrenoreceptors expressed on nonhematopoietic cells and leading to tissue-specific, differential circadian oscillations in the expression of endothelial cell adhesion molecules and chemokines. We showed that these rhythms have physiological consequences through alteration of hematopoietic cell recruitment and overall survival in models of septic shock, sickle cell vaso-occlusion, and BM transplantation. These data provide unique insights in the leukocyte adhesion cascade and the potential for time-based therapeutics for transplantation and inflammatory diseases.

Sickle cell disease (SCD) is a genetic blood disorder characterized by repeated episodes of vaso-occlusion, in which neutrophils play a primary function through their recruitment in inflamed venules and interactions with sickle red blood cells (sRBCs) via activated αMβ2 integrin (Mac-1). Intravital microscopy analyses revealed considerable heterogeneity in Mac-1 activation on adherent neutrophils, suggesting that subsets of neutrophils differ markedly in their pro-inflammatory activity (Nat Med 2009; 15:384). Recently, we found that aged neutrophils, marked by CD62Llo CXCR4hi, represented an overly active subset exhibiting enhanced Mac-1 activation (Blood 2013; 122:324). To further characterize aged neutrophils, we compared their transcriptome with control and in vivo- activated neutrophils. Gene set enrichment analyses revealed that aged neutrophils up-regulated several pathways that were also enhanced during activation, including integrin, leukocyte adhesion, toll-like receptor (TLR), and NFκB signalling pathways (P < 0.05), suggesting a contribution by exogenous inflammatory mediators. Based on the expression profile, we hypothesized that microbiota-derived signals may influence neutrophil aging in the circulation. To test this idea, we analyzed aged neutrophil numbers in antibiotics (ABX)-treated and germ-free (GF) mice, in which the gut microbiota was largely depleted or deficient, respectively. We found significant reductions of aged neutrophil numbers in both models, and the numbers could be significantly restored by either lipopolysaccharide (LPS) gavage or fecal transplantation (Ctrl / ABX / ABX+LPS: 98±14 / 36±11 / 107±25 cells per μl blood; SPF / GF / GF-FT: 56±12 / 4±1 / 15±4 cells per μl blood; P < 0.05). We next investigated whether microbiota-driven neutrophil aging was mediated by TLRs and Myd88 signalling using chimeric mice reconstituted with a mixture of WT and Myd88, TLR4 or TLR2-deficient hematopoietic cells. Interestingly, we observed significant reductions in the aged subset of Myd88, TLR4 or TLR2-deficient neutrophils compared to WT neutrophils in the same mice, and in their capacity to capture fluosphere beads that specifically bound to activated Mac-1 (WT / LysM-cre/Myd88-flox: 10.4±0.3 / 7.0±0.6 %, 1.0±0.1 / 0.4±0.1 bead per cell; WT / Tlr4-/-: 10.3±0.2 / 6.0±0.2 %, 1.0±0.1 / 0.4±0.1 bead per cell; WT / Tlr2-/-: 10.0±0.4 / 6.8±0.5 %, 1.3±0.2 / 0.8±0.1 bead per cell; P < 0.01). Analysis of SCD mice (BERK) revealed that the aged neutrophil population was significantly increased (by >10-fold) compared to hemizygous (SA) mice, and the expansion was completely abrogated by microbiota depletion (SA / SCD (SS)-Ctrl / SS-ABX: 71±11 / 981±261 / 124±27 cells per μl blood, P < 0.01). We then challenged SA, untreated and ABX-treated SCD mice with TNF-α to induce acute vaso-occlusion. SCD mice exhibited significant increases in neutrophil adhesion, Mac-1 integrin activation and heterotypic interactions with RBCs compared to SA mice, all of which were markedly reduced by microbiota depletion (SA / SS-Ctrl / SS-ABX: 6.4±0.6 / 18.1±1.0 / 11.2±1.0 adherent neutrophil per vessel; 0.5±0.1 / 1.0±0.2 / 0.3±0.1 bead per cell; 0.1±0.1 / 0.5±0.1 / 0.2±0.1 RBC interaction per vessel per min; P < 0.05), resulting in significantly enhanced blood flow and prolonged survival in ABX-treated SCD mice (P < 0.05). Most interestingly, the splenomegaly of SCD mice was significantly reduced, and liver damage including fibrosis, necrosis and inflammation was dramatically alleviated in ABX-treated SCD mice (P < 0.05). Finally, we evaluated whether the numbers of circulating aged neutrophils were altered in patients with SCD. As Penicillin V antibiotic prophylaxis therapy is recommended for children < 5 years or older patients with immune defects to prevent life-threatening infections, we determined aged neutrophil numbers in this patient population. We found that SCD patients exhibited a marked increase in the numbers of circulating aged neutrophils compared to healthy controls, which were significantly reduced in patients on Penicillin V prophylaxis (Ctrl / SS / SS-PV: 261±40 / 1175±205 / 439±87 cells per μl blood, P < 0.05). Our results raise the possibility that targeting neutrophil aging and the microbiota may have broad implications in our understanding of sickle cell disease pathogenesis and its management that should be further studied in clinical trials. Disclosures No relevant conflicts of interest to declare.

Purpose of review Neutrophil infiltration is an important feature in inflammatory scenarios. Before these cells infiltrate tissues, however, they contribute to crucial intravascular events in which neutrophil microdomains mediate heterotypic interactions with endothelial cells, red blood cells and/or platelets. In vascular diseases, this can result in exacerbated neutrophil activation, subsequent vascular injury and obstruction of microcirculatory blood flow. This review discusses recent advances in elucidating these neutrophil domains and their associated functions in cell adhesion. Recent findings Neutrophil recruitment is mediated by sequential interactions with the endothelium, termed rolling, adhesion and extravasation. Evidence points to novel signaling pathways induced during the rolling phase resulting in the transition to leukocyte adhesion, which appear to contribute to chemokine mediated activation. In addition, specific neutrophil microdomains are important for interactions with other hematopoietic cells inducing reductions in microvascular flow and injury. Summary Neutrophils integrate signals received from the endothelium to act as linkers between the vessel wall and a variety of vascular components (i.e. endothelial cells, platelets, red blood cells) in acute and chronic inflammatory conditions to mediate interactions that can result in vascular injury and vasoocclusion.

Monocyte recruitment from the vasculature involves sequential engagement of multiple receptors, culminating in transendothelial migration and extravasation. Junctional adhesion molecule-C (JAM-C) is localized at endothelial intercellular junctions and plays a role in monocyte transmigration. Here, we show that blockade of JAM-B/-C interaction reduced monocyte numbers in the extravascular compartment through increased reverse transmigration rather than by reduced transmigration. This was confirmed in vivo, showing that an anti–JAM-C antibody reduced the number of monocytes in inflammatory tissue and increased the number of monocytes with a reverse-transmigratory phenotype in the peripheral blood. All together, our results suggest a novel mechanism of reducing accumulation of monocytes at inflammation sites by disruption of JAM-C–mediated monocyte retention.

Junctional adhesion molecule-A (JAM-A) is a transmembrane protein expressed at tight junctions of endothelial and epithelial cells and on the surface of platelets and leukocytes. The role of JAM-A in leukocyte transmigration in vivo was directly investigated by intravital microscopy using both a JAM-A–neutralizing monoclonal antibody (mAb) (BV-11) and JAM-A–deficient (knockout [KO]) mice. Leukocyte transmigration (but not adhesion) through mouse cremasteric venules as stimulated by interleukin 1β (IL-1β) or ischemia/reperfusion (I/R) injury was significantly reduced in wild-type mice treated with BV-11 and in JAM-A KO animals. In contrast, JAM-A blockade/genetic deletion had no effect on responses elicited by leukotriene B4 (LTB4) or platelet-activating factor (PAF). Furthermore, using a leukocyte transfer method and mice deficient in endothelial-cell JAM-A, evidence was obtained for the involvement of endothelial-cell JAM-A in leukocyte transmigration mediated by IL-1β. Investigation of the functional relationship between JAM-A and PECAM-1 (CD31) determined that dual blockade/deletion of these proteins does not lead to an inhibitory effect greater than that seen with blockade/deletion of either molecule alone. The latter appeared to be due to the fact that JAM-A and PECAM-1 can act sequentially to mediate leukocyte migration through venular walls in vivo.

ICAM-2 has been implicated in leukocyte transmigration in vitro, but there is little in vivo evidence to support this. To address this, neutrophil migration was investigated in ICAM-2–deficient mice (KO) and in wild-type (WT) mice treated with an anti–ICAM-2 blocking monoclonal antibody (mAb) (3C4). In a peritonitis model, IL-1β–induced accumulation of neutrophils was significantly reduced in mice treated with 3C4 (51% inhibition) and in KO mice (41% inhibition). In contrast, TNF-α– or thioglycolate-induced responses were not suppressed in KO mice. Analysis of IL-1β–induced leukocyte responses in cremasteric venules of KO animals by intravital microscopy indicated a defect in transmigration (44% inhibition) but not rolling or adhesion. As found before, TNF-α–induced leukocyte transmigration was unaltered in the KO mice. WT mice treated with the anti–ICAM-2 mAb also exhibited a selective reduction in leukocyte transmigration in response to IL-1β while an anti–ICAM-1 mAb inhibited both leukocyte adhesion and transmigration. Interestingly, mAb 3C4 significantly suppressed IL-1β–induced neutrophil transmigration in PE-CAM-1 KO animals in the peritonitis model but not in the cremaster muscle. The findings provide direct evidence for the involvement of ICAM-2 in neutrophil transmigration in vivo, though this role appears to be stimulus specific. Furthermore, ICAM-2 appears capable of mediating PECAM-1–independent leukocyte transmigration.

Proteasome inhibitors exhibit antitumor activity against malignancies of different histology. Yet, the mechanisms underlying this effect are poorly understood. Recent evidence indicates that antiapoptotic factors may also accumulate as a consequence of exposure to these drugs, possibly reducing their cytotoxicity. These include the Bcl-2 family member Mcl-1, whose down-regulation has been proposed to initiate apoptosis in response to genotoxic stimuli. In this study, we found that proteasome inhibitors release cyotochrome c and second mitochondria-derived activator of caspase (SMAC)/Diablo and trigger the subsequent apoptotic cascade in spite of concomitant Mcl-1 increase. However, our data indicate that subtraction of Mcl-1 during apoptosis, although not required for early release of proapoptotic factors, is probably relevant in speeding up cell demise, since RNA interference-mediated Mcl-1 silencing is lethal in lymphoma cells. Consistent with this, the cytotoxic effects of proteasome inhibitors are enhanced when Mcl-1 increase is impeded. Thus, this study identifies Mcl-1 accumulation as an unwanted molecular consequence of exposure to proteasome inhibitors, which slows down their proapoptotic effects. Pharmacologic or genetic approaches targeting Mcl-1, including therapeutic RNAi, may increase the effectiveness of these compounds. (Blood. 2005;105:3255-3262)

Hematopoietic stem cells (HSCs) adapt to the varying needs for the replenishment of terminally differentiated blood cells by switching between quiescence, self-renewal and differentiation. A tight regulation of this balance is crucial to ensure hematopoietic homeostasis over a lifetime. In the bone marrow (BM), HSCs reside in specialized microenvironments, or niches, that regulate HSC behavior. Although all niche cells identified thus far have a non-hematopoietic origin (e.g., mesenchymal stem cells, osteoprogenitors, endothelial cells and sympathetic nerves), less is known about HSC regulation by its progeny. During confocal imaging studies of whole-mount BM, we have noted an association between HSCs and megakaryocytes (Mk); indeed up to 50% of Lin-CD41-CD48-CD150+ HSC were in direct contact with Mk. HSC were located at an average of 15.4 µm-distance to Mk. This distribution was not random (p=6x10-8) as demonstrated by two-sample Kolmogorov-Smirnov test. To assess the functional role of Mk in HSC regulation in vivo, we bred PF4-cre mice (in which the Cre recombinase gene is under the control of the Mk-specific platelet factor 4 (PF4) promoter) with iDTR mice (where cre-mediated recombination induces expression of the diphtheria toxin (DT) receptor). After 7 days of DT treatment PF4-cre:iDTR mice showed specific BM Mk depletion (5.3-fold reduction; P Disclosures: Lambert: Amgen: Research Funding; Nestle: Consultancy; GSK: Research Funding.

Abstract 638 A prevailing idea suggests that quiescent hematopoietic stem cells (HSCs) reside near osteoblasts in the bone marrow (BM) whereas actively cycling HSCs are found near sinusoids. However, this idea is not been proven experimentally. Our recent studies have identified Nestin+ perivascular cells containing all BM mesenchymal stem cell activity as a candidate HSC niche cell (Nature 2010;466:829). To gain more insight in the 3-dimensional (3D) structure of the HSC niche, we have established a novel whole-mount immunofluorescence imaging technique in which the 3D spatial relationships between vascular structures and endogenous CD150+CD48−CD41−Lineage− HSCs in the sternal or femoral BM can be precisely determined. Our imaging analyses in both bone types revealed a prominent sinusoidal network interrupted by rare small caliber (∼10μm) arterioles whose identity was confirmed by staining with artery-specific Alexa Fluor 633 (Nat Methods 2012;9:273) or Sca-1 (Cell Stem Cell 2009;4:263). 3D analyses using Nes-GFP transgenic mice showed two distinct types of Nestin+ cells associated with distinct vascular structures: Nes-GFPbright cells exhibiting a pericyte-like morphology were tightly associated with arterioles (referred to as periarteriolar Nestin (Nesperi) cells) whereas more abundant Nes-GFPdim cells exhibiting a reticular shape (Nesretic cells) were largely associated with sinusoids. We sorted Nesperi and Nesretic cells based on GFP expression and cell size for further characterization. Nesperi cells were very rare (∼0.002% of BM) and expressed ∼3-fold and 10–50-fold higher levels of genes associated with HSC maintenance and retention (Cxcl12, Kitl, Angpt1, Vcam1) compared to Nesretic cells and Nes-GFPneg CD45−CD31−Ter119− stromal cells, respectively. These factors were down-regulated after G-CSF stimulation in Nesperi cells but not in Nesretic cells, which led us to hypothesize that Nesperi cells played an important role in HSC maintenance. We examined the spatial correlation between endogenous HSCs and Nesperi cells by whole-mount immunostaining and found that 38.6 ± 2.9% and 54.9 ± 6.8% of HSCs were localized within 20μm and 40μm distance from Nesperi cells, respectively. After G-CSF treatment, HSCs entered cell cycle (control/G-CSF; 86.8 ± 2.6%/52.2 ± 3.8% in G0, as defined by Ki67 and Hoechst 33342 staining, p=0.002) and expanded around Nesperi cells (control/G-CSF; 34.3 ± 1.0%/48.2 ± 1.6% in 0–20μm proximity, p 80μm distance, p80μm distance, p Disclosures: No relevant conflicts of interest to declare.

Abstract 1099 Leukocyte recruitment is a crucial component of immune homeostasis. The sequential multistep process leading to leukocyte migration is thought to be regulated on demand at the inflammatory site. Here, we show that broad systemic programs involving long-range signals from the sympathetic nervous system (SNS) delivered by adrenergic nerves regulate rhythmic recruitment of leukocytes in tissues. While oscillations in the numbers of circulating leukocytes ranged from 4.1 to 7.8 × 106 cells/ml blood (P Disclosures: No relevant conflicts of interest to declare.

Abstract 139 Chemotherapy regimens commonly produce long-term hematopoietic injury, possibly through a lesion to the hematopoietic stem cell (HSC) and/or its niche but the mechanisms have yet to be elucidated. Nestin-expressing mesenchymal stem cells, under regulation of the sympathetic nervous system (SNS), are a critical component of the HSC niche (Nature. 2010; 466:829). Since peripheral neuropathy is a common complication of chemotherapy, we hypothesized that chemotherapy-induced sympathetic bone marrow (BM) neuropathy could impair BM regeneration. To test this issue, we sympathectomized mice using 6-hydroxydopamine (6OHDA) or diphtheria toxin (DT) injection into TH-Cre-iDTR mice, which express the DT receptor exclusively on catecholaminergic neurons. In both models, SNS ablation led to reduced BM regeneration following 5-fluorouracil (5FU) challenge, as determined by the delayed recovery in BM cellularity (1.7-fold↓*), clonogenic progenitors (CFU-C; 2.5-fold↓*) and Lin−Sca-1+c-Kit+Flt3− cells (LSKF cells 2.5-fold↓*). Further, inhibition of β3 adrenergic signaling with the antagonist SR5920A impaired BM recovery to the same extent as 6OHDA sympathectomy: BM cells (1.6-fold↓*), CFU-C (1.6-fold↓*), LSKF (1.6-fold↓*). We then treated mice with the chemotherapy drug cisplatin, which induces neuropathies in mice (2.8-fold↓* in SNS fibers in BM). Cisplatin-treated mice exhibited reduced survival after BM transplantation (66% vs 100%* for cisplatin-treated and control, respectively) due to delayed regeneration: BM cells (4.6-fold↓*), CFU-C (8.8-fold↓*), LSKF (6.4-fold↓*). To evaluate the potential benefit of neuroprotection, we treated mice with 4-methylcatechol (4-MC), a powerful inducer of Nerve Growth Factor (NGF), together with cisplatin or 6OHDA, which led to the protection of SNS fibers (1.8-fold↑* and 11-fold↑* respectively). This prevented transplantation-associated death and increased BM recovery following transplantation or 5FU administration: BM cells (1.9- and 1.6-fold↑, respectively *), CFU-C (3.6- and 1.4-fold↑*), LSKF (4.5- and 2.4-fold↑*). To ascertain whether 4-MC/NGF acted specifically on the nervous system, rather than on hematopoietic or stromal cells, we used TrkAneo/neo mice, which are genetically deficient in the NGF receptor in non-neuronal tissues (Development. 2004;131:5185). Administration of 4-MC to sympathectomized TrkAneo/neo mice enhanced BM regeneration after 5FU: BM cells (2.0-fold↑*), CFU-C (2.0-fold↑*), LSKF (2.2-fold↑*), suggesting that 4-MC acts directly on neurons to rescue hematopoiesis. Since deletion of p53 promotes neuron survival after genotoxic damage (Neurodegeneration. 1996;5:233), we intercrossed mice expressing TH-Cre mice with p53flox/flox animals to delete p53 specifically in catecholaminergic neurons. SNS fibers of TH-Cre;p53flox/flox mice were protected after cisplatin treatment compared to TH-Cre;p53+/flox controls, and this was associated with improved BM recovery after transplantation: BM cells (2.4-fold↑*), CFU-C (2.9-fold↑*), LSKF (6.5-fold↑*) when compared to TH-Cre;p53+/flox animals. Thus, these various pharmacologic and genetic models strongly suggest that chemotherapy-induced sympathetic neuropathy prevents BM regeneration. To evaluate further the mechanisms, we investigated whether BM neuropathy might lead to dysfunctional HSC niche. SNS injury using 6OHDA or cisplatin treatment led to Nestin+ cell expansion and accumulation (1.7- and 3.8-fold↑*, respectively). Treatment with 4-MC reverted this phenotype (1.8-fold↓* for 6OHDA and 1.5-fold↓* for cisplatin), indicating that Nestin+ cell expansion is due to reduced SNS signaling. SNS injury also led to increased Nestin+ cell sensitivity to genotoxic insult as determined by increased Nestin+ cell death and apoptosis (1.4-fold↑*) 24h after 5FU injection, which reduced Nestin+ cell numbers during regeneration (2.1-fold↓* 12 days after 5FU). By contrast, the number of osteoblasts and endothelial cells were unaffected. These results indicate that adrenergic signals promote the quiescence of HSC niche cells and that sympathetic neuropathy renders the niche vulnerable to genotoxic insult, impairing hematopoietic regeneration. Neural protection may represent a novel strategy to maintain hematopoietic reserve and enhance transplantation efficiency in chemotherapy-treated patients. *P Disclosures: No relevant conflicts of interest to declare.