NIH K01 MH117343

Assessment of Mechanisms Underlying B Cell Impacts on Resilience and Susceptibility to Stress

Human Health Relevance

The consequences of mental health disorders stemming from chronic stress are devastating and there is a great clinical need to further elucidate the mechanisms underlying their pathology. For example, major depressive disorder (MDD), a brain disease associated with persistent sadness, guilt, anhedonia, despair, and in some cases, suicide, is a significant issue (Krishnan and Nestler, 2008). Indeed, in 2010, the nearly 300 million global cases accounted for 8.2% of all disease-induced years lived with disability and cost ~$210 billion in the US alone (Ferrari et al., 2010 a & b; Greenberg et al., 2015). Problematically, available pharmacologic treatments targeting presumed dysregulated neural substrates are associated with delayed and inadequate symptom alleviation in most patients (Krishnan and Nestler, 2008; Trivedi et al., 2006). This suggests that the pathology of MDD is more complex than previously appreciated, that the neurotransmitters thought to underlie MDD-associated brain pathology may not be the sole contributors to its presentation, and that therapeutic interventions targeting these systems will remain insufficient at imparting symptomatic relief. Identification and targeting of other contributing systems will likely lead to novel, more effective, faster-acting therapeutic interventions that benefit a larger number of patients. Importantly, it is now accepted that MDD is both a brain disease and an inflammatory disorder (Leonard, 2010; Maes, 2011; Wholeb et al., 2016), meaning that targeting the immune system to alleviate MDD may be a viable novel therapeutic approach that represents a potentially paradigm-shifting departure from currently available brain-directed interventions. The exploration of the role of the B cell in modulating the response to stress will support this clinically relevant endeavor.

Scientific Premise

Although once considered “immune privileged”, a growing body of literature indicates that the central nervous system and the peripheral immune systems engage in bidirectional communication and profoundly influence one another during homeostasis and in pathological/diseased states (Pavlov et al., 2018). Indeed, chronic inflammation mediated by the innate immune response is implicated in a variety of neurodegenerative diseases and new evidence suggests that anti-inflammatory agents may represent a promising therapeutic avenue for their treatment (Lucas et al., 2006). Importantly, that T cells are present in healthy brain parenchyma and can also infiltrate CNS tissue in response to injury or autoimmune disease has fostered major interest in the role of antigen-specific adaptive immunity in normal and abnormal brain function, including within the context of chronic stress (i.e., Dantzer et al., 2018; Filiano et al., 2017; Fletcher et al., 2010; Herkenham and Kigar, 2017; Rayasam et al., 2018). Yet, due to often null or conflicting findings regarding their impact, the role of B cells in brain function and disease was not extensively studied. As a result, rigorous research regarding the role of the immune system in the response to stress progressed with a focus on addressing the proinflammatory and dysregulated T cell profiles consistently reported.

Importantly, new emerging data support a role for B cells in modulating brain function and the response to stress. Indeed, key protective roles for B cells have been identified in the context of multiple sclerosis, stroke, stress and depression (i.e., Ahmetspahic et al., 2018; Funaro et al., 2016; Gurfein et al., 2017; Hernandez et al., 2010; McGregor et al., 2016; Ortega et al., 2020; Selvaraj et al., 2016). Our team’s preliminary data further support this observation and lay the foundations for the work proposed below.

Aim 1: Whether

To supplement the growing evidence for B cells in modulating the stress response and in the context of MDD, my objective for Aim 1 is to characterize the effects that pan-B cell deficiency has on the susceptibility to acute or chronic stress while I develop key expertise to successfully complete this and other aims. Based on my preliminary data noting a maladaptive passive behavioral response to severe acute stress in genetically B cell deficient mice, my working hypothesis is that B cell deficiency promotes stress susceptibility. My approach to test this hypothesis will use two models of B cell deficiency; the genetically B cell deplete model and an antibody-based FDA-approved drug directed against the cell surface marker, CD20, to selectively deplete B cells in vivo. I will evaluate the effect of B cell depletion in a wide variety of stress contexts ranging from acute to chronic in duration and mild to severe in intensity. I will then transfer B cells to some subjects to attempt to reverse B cell depletion-induced behavioral and neurobiological changes. Endpoints measured will include both historic and translationally relevant measures of stress response behaviors, classic as well as contemporary markers of stress/MDD neuropathology, and central and peripheral assessments of immune function.

Aim 2: How

B cells have a number of functionally important roles in the immune system. In addition to mounting antibody-mediated responses, B cells have been recently been recognized to down-modulate immune responses through the activity of regulatory B cells (Bregs) (Hoffman et al., 2016; Lund, 2008). Although additional immunoregulatory mechanisms have been identified, Bregs predominantly inhibit the immune system through the expression of the signaling molecule interleukin (IL)-10. While this cytokine plays a protective role in various CNS inflammatory settings (Lobo-Silva et al., 2016) and converging data support its critical role in mediating stress susceptibility and MDD (Gazal et al., 2015; Laumet et al., 2018; Mezquita et al., 2008), the contribution of Breg-secreted IL-10 on stress susceptibility is not yet known, though a recent report noted Bregs (but not total B cell counts) were lower in MDD patients (Ahmetspahic et al., 2018). My objective for Aim 2 is to determine if B cell modulation of stress susceptibility is dependent on Bregs and the IL-10 they secrete. My working hypothesis is that lack of IL-10 from Breg cells promotes stress susceptibility. My approach to test this hypothesis will utilize models that are deficient in Bregs, or B cell-derived IL-10, and assess the extent to which these subjects, or genetically B cell deficient subjects administered their Breg- or IL-10-deficient B cells, adopt a maladaptive behavioral response to various stressors and display MDD brain dysfunction.

Aim 3: Where

While the CNS was once considered immune-privileged, lymphocytes, and their secreted products (ie. antibodies, cytokines) have been localized in brain meninges and parenchyma in low numbers in surveillance roles or higher numbers in response to a neuroimmune insult (i.e., Anthony et al., 2003; Laumet et al., 2018; Ortega et al., 2020). My objective for this aim is to determine if B cells accumulate at brain target sites and subsequent secretion of pro-resilience cytokines (i.e. IL-10). My working hypothesis is that peripheral B cells promote stress resilience by migrating to mood/stress-relevant brain regions and secreting protective anti-inflammatory factors; susceptibility occurs in the absence of such a mechanism. My approach will assess the extent to which B cells traffic to the brain and secrete protective factors among mice that adopt a behavioral response to stress along the spectrum of resilience to susceptibility.

Future Directions and Broader Impacts

Completion of these studies and training proposed in this grant will support the development of my independent scientific research program by providing the evidential basis for continued exploration of this topic and enhance my competitiveness for the successful acquisition of extramural funding. Indeed, data generated here will provide insight into the mechanisms involved. In doing so, I will advance my career goal to positively impact the mental health field as a productive scientist, help improve clinical practice with the discovery of new therapeutic targets and approaches, and ultimately ease the burden of chronic stress disorders.

Findings from studies conducted in this K01 grant will lay the foundations for continued exploration of this intriguing research topic. Given that leukocyte recruitment to central nervous system readily occurs in response to brain injury and cellular stress, an immediate next step of this program of study is to address how psychosocial stress-induced B cell trafficking to the brain is regulated by inhibiting various cell adhesion molecules including beta-1 integrins. Future work will also address the antibody-secreting ability of B cells as well as their function as antigen-presenting cells and their interactions with CD4+ T helper cells, which have also been critically implicated in chronic stress disorders.

From a broader perspective, if sufficient data are collected to allow us to accept the premise that B cells underly the response to stress, we can broaden our scope of research focus to include a wide variety of debilitating disorders in which the function of the immune system is fundamentally altered and depressed mood is a component of the co-morbid symptomology. Blue text indicates areas of inital exploration by the Liz’s Lab team.