Peripheral and central effects of repeated social defeat stress: Monocyte trafficking, microglial activation, and anxiety

“The mechanistic association between inflammation and depression is particularly well-established (Raison et al., 2006; Dantzer et al., 2008; Miller et al., 2009; Norman et al., 2010; Capuron and Miller, 2011), while the case continues to build for the mechanistic association between inflammation…”

“The mechanistic association between inflammation and depression is particularly well-established (Raison et al., 2006; Dantzer et al., 2008; Miller et al., 2009; Norman et al., 2010; Capuron and Miller, 2011), while the case continues to build for the mechanistic association between inflammation…”

“Moreover, data suggest that RSD-induced microglial activation is likely an upstream event of monocyte trafficking to the brain and this is dependent on receptors such as CCR2 and CX3CR1 in that increased microglia cytokine expression was still observed in CCR2KO and CX3CR1 KO mice that lacked both perivascular and par…”

“…are not fully understood, the individual response to prolonged or severe stressors contributes to the development and exacerbation of depression and anxiety (Maes et al., 1998; Kalynchuk et al., 2004; Raison et al., 2006; Norman et al., 2010; Capuron and Miller, 2011; Gilman et al., 2013).”

“This idea that neuronal activity locally influences brain vasculature is reminiscent of the concept of a neurovascular unit (Hawkins and Davis, 2005).”

“, 2011), while the fractalkine receptor is highly expressed by microglia (Jung et al., 2000).”

“Because the brain is highly sensitive to increases in proinflammatory cytokines, microglia tend to be highly regulated by anti-inflammatory ligands and receptors (e.g., CD200/CD200R (Lyons et al., 2007), CX3CR1/CX3CL1 (Cardona et al., 2006), or TGFb/TGFbR (Butovsky et al., 2014)).”

“The idea that the spleen serves as an alternative source of monocyte trafficking is seen in several studies involving inflammatory challenges including myocardial infarction (Swirski et al., 2009) and stroke (Seifert et al., 2012).”

“…examined subordinate non-human primates, who, in response to social reorganization (i.e., a shift in social hierarchy) had reduced body weights, higher levels of circulating GCs (i.e., cortisol), and increased susceptibility to upper respiratory infections (Cohen et al., 1997; Sapolsky, 2005).”

“Under homeostatic conditions within the CNS, fractalkine is highly and primarily expressed by neurons (Bazan et al., 1997; Kim et al., 2011), while the fractalkine receptor is highly expressed by microglia (Jung et al., 2000).”

“Fractalkine receptor stimulation of brain microglia is anti-inflammatory (Cardona et al., 2006; Corona et al., 2010) and promotes microglial quiescence under homeostatic conditions (Wolf et al., 2013).”

“…stress is associated with proinflammatory leukocytic phenotypes that are unresponsive to the anti-inflammatory actions of glucocorticoids (GCs) (Cohen et al., 2012) and a transcriptional profile that is consistent with the expansion and priming of myeloid-derived cells (Miller et al., 2008;…”

“In other contexts, these same microglia-derived cytokines and chemokines are capable of increasing CAM expression on the luminal surface of vascular endothelial cells and increasing the respective receptors on vascular monocytes (Kim et al., 1996; Wang et al., 2007; Shi et al., 2011).”

“…are not fully understood, the individual response to prolonged or severe stressors contributes to the development and exacerbation of depression and anxiety (Maes et al., 1998; Kalynchuk et al., 2004; Raison et al., 2006; Norman et al., 2010; Capuron and Miller, 2011; Gilman et al., 2013).”

“Neuronal fractalkine expression, or lack thereof, is involved in microglial chemotaxis and calcium regulation and plays a fundamental role within neuronal and microglial communication (Harrison et al., 1998).”

“Although it is possible that these two events may act independently of each other, it is well recognized that neuronal activity can regulate local microglial activation through specific chemokine signaling pathways (Kettenmann et al., 2013).”

“If these cells enter the brain parenchyma, cluster of differentiation 45 (CD45) expression is reduced and these cells morphologically and phenotypically become indistinguishable from resident microglia (Mildner et al., 2007).”

“Associated with the experience of social stress, it is this prolonged inflammatory state that has the potential to contribute to the etiology of depression and anxiety (Meduri and Yates, 2004; Barnes and Adcock, 2009).”

“In particular, stress majorly impacts the expression of cytokine and chemokine genes that are instrumental in mobilizing immune cells to aid in the resolution of an insult and restoration of tissue function (Black and Garbutt, 2002).”

“Moreover, data suggest that RSD-induced microglial activation is likely an upstream event of monocyte trafficking to the brain and this is dependent on receptors such as CCR2 and CX3CR1 in that increased microglia cytokine expression was still observed in CCR2KO and CX3CR1 KO mice that lacked both perivascular and par…”

“…are not fully understood, the individual response to prolonged or severe stressors contributes to the development and exacerbation of depression and anxiety (Maes et al., 1998; Kalynchuk et al., 2004; Raison et al., 2006; Norman et al., 2010; Capuron and Miller, 2011; Gilman et al., 2013).”

“Functional studies from other labs identified this CD11b+/CD45hi population as brain macrophages (Sedgwick et al., 1991; Ford et al., 1995).”

“Along these same lines, RSD primes and activates peripherally derived monocytes inducing increased cytokine expression and resistance to the immunosuppressive effects of GCs (Engler et al., 2005; Wohleb et al., 2011, 2014a; Powell et al., 2013).”

“Although similar in morphology and phenotype to microglia, peripherally derived brain macrophages tend to be more inflammatory and less subject to immune regulation (Perry and Teeling, 2013).”

“Functional studies from other labs identified this CD11b+/CD45hi population as brain macrophages (Sedgwick et al., 1991; Ford et al., 1995).”

“Along these same lines, RSD primes and activates peripherally derived monocytes inducing increased cytokine expression and resistance to the immunosuppressive effects of GCs (Engler et al., 2005; Wohleb et al., 2011, 2014a; Powell et al., 2013).”

“Stress-induced brain macrophage trafficking promotes anxiety-like behavior One key finding is that the establishment of anxiety-like behavior following RSD is promoted by myeloid-derived cell trafficking to the brain (Wohleb et al., 2013).”

“By the end of six cycles of RSD, the ultimate behavior displayed by the defeated residents is subordination, which involves huddling in a corner and, upon approach, rearing on hind legs to bare their ventral body surface to the aggressor (Blanchard and Blanchard, 1977; Avitsur et al., 2007).”

“…with proinflammatory leukocytic phenotypes that are unresponsive to the anti-inflammatory actions of glucocorticoids (GCs) (Cohen et al., 2012) and a transcriptional profile that is consistent with the expansion and priming of myeloid-derived cells (Miller et al., 2008; Powell et al., 2013).”

“In a myocardial infarct model, monocytes recruited to damaged tissues were replenished by extramedullary monocytopoiesis that occurred in the spleen (Leuschner et al., 2012).”

“These findings also strongly resemble work by Tynan et al. (2010) that revealed region-specific microglial activation following chronic restraint stress in rats.”

“In other contexts, these same microglia-derived cytokines and chemokines are capable of increasing CAM expression on the luminal surface of vascular endothelial cells and increasing the respective receptors on vascular monocytes (Kim et al., 1996; Wang et al., 2007; Shi et al., 2011).”

“Influx and efflux of cells and mediators to and from the bone marrow affects the differentiation and release of immune cells (Dorshkind, 1990).”

“For instance, reports from both Wohleb et al. (2011) and Hinwood et al. (2012) indicate a region-specific coupling between neuronal and microglial activation in the context of two different stress paradigms.”

“Competition for access to resources causes the formation of social hierarchies within the cage consisting of a dominant alpha, co-dominants, and subordinates (Ginsburg and Alee, 1975).”

“These mice express GFP under the LsyM promoter that drives myeloidderived cells to strongly express GFP (e.g., monocytes and granulocytes) (Rieger et al., 2009).”

“In humans, longitudinal studies of social defeat involving bullying or loss of social status in the workplace have demonstrated increased incidence of depression, anxiety, loss of self-esteem, increased incidence of illness, and other aberrant physiological and behavioral symptoms (Marmot and Feeney, 1997; Griffin et…”

“…et al., 2006; Dantzer et al., 2008; Miller et al., 2009; Norman et al., 2010; Capuron and Miller, 2011), while the case continues to build for the mechanistic association between inflammation and anxiety (Maes et al., 1998; Pitsavos et al., 2006; O’Donovan et al., 2010; Pace and Heim, 2012).”

“Mouse model of social stress: RSD paradigm RSD is an ethologically relevant resident–intruder paradigm involving intermale aggression and loss of social status by the disruption of an established social hierarchy (Avitsur et al., 2001; Stark et al., 2001).”

“Fractalkine receptor stimulation of brain microglia is anti-inflammatory (Cardona et al., 2006; Corona et al., 2010) and promotes microglial quiescence under homeostatic conditions (Wolf et al., 2013).”

“Along these same lines, RSD primes and activates peripherally derived monocytes inducing increased cytokine expression and resistance to the immunosuppressive effects of GCs (Engler et al., 2005; Wohleb et al., 2011, 2014a; Powell et al., 2013).”

“Under homeostatic conditions within the CNS, fractalkine is highly and primarily expressed by neurons (Bazan et al., 1997; Kim et al., 2011), while the fractalkine receptor is highly expressed by microglia (Jung et al., 2000).”

“…et al., 2006; Dantzer et al., 2008; Miller et al., 2009; Norman et al., 2010; Capuron and Miller, 2011), while the case continues to build for the mechanistic association between inflammation and anxiety (Maes et al., 1998; Pitsavos et al., 2006; O’Donovan et al., 2010; Pace and Heim, 2012).”

“Mouse model of social stress: RSD paradigm RSD is an ethologically relevant resident–intruder paradigm involving intermale aggression and loss of social status by the disruption of an established social hierarchy (Avitsur et al., 2001; Stark et al., 2001).”

“…et al., 2006; Dantzer et al., 2008; Miller et al., 2009; Norman et al., 2010; Capuron and Miller, 2011), while the case continues to build for the mechanistic association between inflammation and anxiety (Maes et al., 1998; Pitsavos et al., 2006; O’Donovan et al., 2010; Pace and Heim, 2012).”

“…with substantial CNS-related tissue pathology and inflammation, e.g., multiple sclerosis, traumatic injury, stroke, Alzheimer’s disease, or infection (Hafler et al., 2005; Gate et al., 2010; Donnelly et al., 2011; Hawthorne and Popovich, 2011; McGavern and Kang, 2011; de Vries et al., 2012).”

“In the bone marrow, RSD increases production, egress, and trafficking of proinflammatory immune cells that are insensitive to GCs (Avitsur et al., 2002; Engler et al., 2004, 2005; Kinsey et al., 2007; Hanke et al., 2012; Wohleb et al., 2012) (Fig.”

“This increased egress was associated with more severe pathology (Ajmo et al., 2008) that was dependent upon peripheral noradrenergic signaling (Ajmo et al., 2009).”

“In the laboratory, murine social stress is modeled using resident–intruder paradigms whereby the response to social stress involves protecting the resources of the cage from intruders (Tamashiro et al., 2005).”

“Because the brain is highly sensitive to increases in proinflammatory cytokines, microglia tend to be highly regulated by anti-inflammatory ligands and receptors (e.g., CD200/CD200R (Lyons et al., 2007), CX3CR1/CX3CL1 (Cardona et al., 2006), or TGFb/TGFbR (Butovsky et al., 2014)).”

“…with substantial CNS-related tissue pathology and inflammation, e.g., multiple sclerosis, traumatic injury, stroke, Alzheimer’s disease, or infection (Hafler et al., 2005; Gate et al., 2010; Donnelly et al., 2011; Hawthorne and Popovich, 2011; McGavern and Kang, 2011; de Vries et al., 2012).”

“…are not fully understood, the individual response to prolonged or severe stressors contributes to the development and exacerbation of depression and anxiety (Maes et al., 1998; Kalynchuk et al., 2004; Raison et al., 2006; Norman et al., 2010; Capuron and Miller, 2011; Gilman et al., 2013).”

“1a) and traffic to the spleen (Engler et al., 2004), lung (Curry et al., 2010), and brain (Wohleb et al., 2013) (Fig.”

“Along these same lines, RSD primes and activates peripherally derived monocytes inducing increased cytokine expression and resistance to the immunosuppressive effects of GCs (Engler et al., 2005; Wohleb et al., 2011, 2014a; Powell et al., 2013).”

“Fractalkine receptor stimulation of brain microglia is anti-inflammatory (Cardona et al., 2006; Corona et al., 2010) and promotes microglial quiescence under homeostatic conditions (Wolf et al., 2013).”

“Anxiety-like behavior is most robustly observed following the six cycles of RSD, but also occurs in a cycle-dependent manner persisting to at least 8 days past the termination of the stressor (Kinsey et al., 2007; Wohleb et al., 2013).”

“…are not fully understood, the individual response to prolonged or severe stressors contributes to the development and exacerbation of depression and anxiety (Maes et al., 1998; Kalynchuk et al., 2004; Raison et al., 2006; Norman et al., 2010; Capuron and Miller, 2011; Gilman et al., 2013).”

“…of social status in the workplace have demonstrated increased incidence of depression, anxiety, loss of self-esteem, increased incidence of illness, and other aberrant physiological and behavioral symptoms (Marmot and Feeney, 1997; Griffin et al., 2002; Stansfeld et al., 2003; Bilgel et al., 2006).”

“Although the literature regarding myeloid-derived cell trafficking from the spleen is relatively sparse, in the context of cardiovascular disease and stroke, substantial gains have been made (Leuschner et al., 2012; Seifert et al., 2012).”

“Additionally, the control chimeric mice in these studies displayed little or no GFP+ cells within the CNS parenchyma, which is consistent with previous studies demonstrating that microglia are not bone marrowderived under homeostatic conditions (Kierdorf et al., 2013).”

“This increased egress was associated with more severe pathology (Ajmo et al., 2008) that was dependent upon peripheral noradrenergic signaling (Ajmo et al., 2009).”

“It is possible that cytokines or neuroendocrine mediators induce GC-insensitivity by altering the number or function of GC receptors (Kino et al., 2003).”

“…of social status in the workplace have demonstrated increased incidence of depression, anxiety, loss of self-esteem, increased incidence of illness, and other aberrant physiological and behavioral symptoms (Marmot and Feeney, 1997; Griffin et al., 2002; Stansfeld et al., 2003; Bilgel et al., 2006).”

“stress and severe infection causes a rapid release of neutrophils and monocytes into circulation referred to as neutrophilia and monocytosis, respectively subsequently leading to the increased production of these cells in the bone marrow (Stefanski and Engler, 1998; Stefanski, 2000; Powell et al., 2013).”

“GM-CSF, in particular, is a potent chemoattractant, and, by up-regulating CD11b expression, increases the adhesion of blood neutrophils and monocytes to the vascular endothelium (Socinski et al., 1988; Kirby et al., 2007).”

“…are not only GC-insensitive, but also possess a primed and activated phenotype, i.e., they have elevated levels of Toll-like receptors (TLRs), adhesion molecules, and co-stimulatory receptors on their surface (Avitsur et al., 2003; Engler et al., 2004; Bailey et al., 2007; Powell et al., 2009).”

“…and severe infection causes a rapid release of neutrophils and monocytes into circulation referred to as neutrophilia and monocytosis, respectively subsequently leading to the increased production of these cells in the bone marrow (Stefanski and Engler, 1998; Stefanski, 2000; Powell et al., 2013).”

“…context of stress, bAR activation appears to primarily mediate the proinflammatory actions of the high surge of stress-induced catecholamines and is able to increase the production of cytokines in immune cells (e.g., Izeboud et al., 1999; Tynan et al., 2010; Johnson et al., 2013; Kim et al., 2014).”

“…with substantial CNS-related tissue pathology and inflammation, e.g., multiple sclerosis, traumatic injury, stroke, Alzheimer’s disease, or infection (Hafler et al., 2005; Gate et al., 2010; Donnelly et al., 2011; Hawthorne and Popovich, 2011; McGavern and Kang, 2011; de Vries et al., 2012).”

“In other contexts, these same microglia-derived cytokines and chemokines are capable of increasing CAM expression on the luminal surface of vascular endothelial cells and increasing the respective receptors on vascular monocytes (Kim et al., 1996; Wang et al., 2007; Shi et al., 2011).”

“Quan et al. (2003) examined the molecular mechanisms of GC-insensitivity in RSD splenocytes and found that the GC receptor in RSD splenocytes (CD11b+ cells) did not have the ability to suppress NFjB activity due to a failure of the GC receptor to translocate to the nucleus (Quan et al., 2003).”

“, they have elevated levels of Toll-like receptors (TLRs), adhesion molecules, and co-stimulatory receptors on their surface (Avitsur et al., 2003; Engler et al., 2004; Bailey et al., 2007; Powell et al., 2009).”

“…context of stress, bAR activation appears to primarily mediate the proinflammatory actions of the high surge of stress-induced catecholamines and is able to increase the production of cytokines in immune cells (e.g., Izeboud et al., 1999; Tynan et al., 2010; Johnson et al., 2013; Kim et al., 2014).”

“Along these same lines, RSD primes and activates peripherally derived monocytes inducing increased cytokine expression and resistance to the immunosuppressive effects of GCs (Engler et al., 2005; Wohleb et al., 2011, 2014a; Powell et al., 2013).”

“, multiple sclerosis, traumatic injury, stroke, Alzheimer’s disease, or infection (Hafler et al., 2005; Gate et al., 2010; Donnelly et al., 2011; Hawthorne and Popovich, 2011; McGavern and Kang, 2011; de Vries et al., 2012).”

“…are not fully understood, the individual response to prolonged or severe stressors contributes to the development and exacerbation of depression and anxiety (Maes et al., 1998; Kalynchuk et al., 2004; Raison et al., 2006; Norman et al., 2010; Capuron and Miller, 2011; Gilman et al., 2013).”

“However, this concept has expanded to explain vascular–neuronal interactions in the context of inflammatory CNS pathology, e.g., Alzheimer’s disease and stroke (Iadecola and Gorelick, 2004).”

“Associated with the experience of social stress, it is this prolonged inflammatory state that has the potential to contribute to the etiology of depression and anxiety (Meduri and Yates, 2004; Barnes and Adcock, 2009).”

“…data using the RSD paradigm had an intense focus on the stress-induced modulation of peripheral immune compartments (Avitsur et al., 2001; Bailey et al., 2004; Engler et al., 2004, 2008; Hanke et al., 2012), recent focus has shifted toward the effects of social defeat on behavior and the brain.”

“, multiple sclerosis, traumatic injury, stroke, Alzheimer’s disease, or infection (Hafler et al., 2005; Gate et al., 2010; Donnelly et al., 2011; Hawthorne and Popovich, 2011; McGavern and Kang, 2011; de Vries et al., 2012).”

“For instance, the RSD-induced establishment and resolution of GC-insensitivity (Avitsur et al., 2002) and increased myelopoiesis (Engler et al., 2004; Wohleb et al., 2013) follow similar kinetics to the development of anxiety-like behavior (Wohleb et al., 2013).”

“Interestingly, the stress associated with simply observing other mice undergoing foot shock was sufficient to recruit monocytes to the brain parenchyma (Ataka et al., 2013).”

“Additionally, both chemokine (C–C motif) receptor 2 (CCR2) and bAR signaling contributed to the trafficking of bone marrow-derived monocytes to the brain (Ataka et al., 2013; Sawada et al., 2014).”

“One response to RSD is the development of anxietylike behavior, which is evident in tests like open field, novel object, and light/dark assays (Kinsey et al., 2007; Bailey et al., 2009; Wohleb et al., 2011; Hanke et al., 2012).”

“…are not only GC-insensitive, but also possess a primed and activated phenotype, i.e., they have elevated levels of Toll-like receptors (TLRs), adhesion molecules, and co-stimulatory receptors on their surface (Avitsur et al., 2003; Engler et al., 2004; Bailey et al., 2007; Powell et al., 2009).”

“For example, both RSD and PTSD involve a sensitizing event that is often both physical and psychological in nature (Bailey et al., 2004; American Psychiatric Association, 2013; Freeman et al., 2013).”

“Defeated subordinate animals receive bite wounds and experience pain that is not life-threatening, but contributes to significant changes in the immune system, cellular activation in the CNS, and expression of prolonged anxiety-like behavior (Bailey et al., 2004).”

“For example, following 5 days of foot shock stress, bone marrow-derived monocytes trafficked to the ventral hippocampus and demonstrated a ramified, microglia-like morphological phenotype (Brevet et al., 2010).”

“…context of stress, bAR activation appears to primarily mediate the proinflammatory actions of the high surge of stress-induced catecholamines and is able to increase the production of cytokines in immune cells (e.g., Izeboud et al., 1999; Tynan et al., 2010; Johnson et al., 2013; Kim et al., 2014).”

“1a) and traffic to the spleen (Engler et al., 2004), lung (Curry et al., 2010), and brain (Wohleb et al., 2013) (Fig.”

“Likewise, in a model of inflammatory liver disease, Kerfoot and Kubes (2005) demonstrated that recruitment of TNF-a-expressing monocytes to the brain promoted depressive-like behavior and fatigue.”

“It is important to note that not all RSD mice develop this state of GC-insensitivity in the spleen; it is most robust in mice that exhibited more subordinate behaviors during the stress treatment (Avitsur et al., 2007).”

“The murine spleen has low myelopoietic activity and most of the granulocytes and monocytes that circulate come from the bone marrow (Ohno et al., 1993).”

“…of monocytes originating from the periphery in the brain (Wohleb et al., 2013), suggesting that blood brain-barrier (BBB) permeability may be affected by stress and could contribute to the establishment of depressive-like behaviors and mood disorders (Wohleb et al., 2014b, Reader et al., 2015).”

“Repeated social defeat (RSD) is an ethologically relevant social stressor that recapitulates many of the stress-induced immune and behavioral outcomes of stress (Reader et al, 2015).”