The 2022 FASEB virtual Catalyst Conference on B Cells in Injury and Regeneration, March 30, 2022

Federation of American Societies for Experimental Biology (FASEB) Catalyst Conferences are a series of half-day, virtual conferences aimed to help build communities in emerging areas of biology. Catalyst Conferences take place on Wednesdays, from October through April, and are free to attend. The second FASEB Catalyst Conference on B Cells in Injury and Regeneration took place on March 30, 2022. The meeting was co-chaired by Dr. Ruxandra F. Sîrbulescu (Principal Investigator, Vaccine and Immunotherapy Center [VIC], and Instructor in Neurology, Massachusetts General Hospital [MGH], and Harvard Medical School [HMS]) and Dr. Mark C. Poznansky (Director, Vaccine and Immunotherapy Center, MGH and Professor in Medicine at HMS, Boston, MA, USA). Together, Sîrbulescu and Poznansky assembled a panel of scientists from multiple institutions across the United States who offered a multifaceted perspective on the diverse functions that B cells can have in the context of tissue injury and regeneration. This edition of the conference focused particularly on central nervous system (CNS) injuries and on aspects of regenerative medicine. The six invited speakers shared recent published and unpublished work from their groups and others' with the stated goal of promoting open collaboration in this emerging field. Dr. Ann M. Stowe (Associate Professor, Departments of Neurology and Neuroscience, University of Kentucky College of Medicine, Lexington, KY, USA) presented recent work exploring the connection between Bcell trafficking in the brain after stroke and brain plasticity leading to improved functional outcome. Previous work from Stowe and collaborators found bilateral diapedesis of B cells into areas remote from the site of acute stroke and Bcell-mediated support for neurogenesis and functional recovery in mice.1 Interestingly, unlike other immune cells, B cells showed a lack of distinct homing to the injury site, with a widespread distribution throughout the ipsilateral and contralateral hemispheres. It is unclear, however, what parenchymal stimuli could activate B cells in remote regions ostensibly lacking canonical “pro-inflammatory” cues. Stowe proposed a mechanism by which B cells could have a trophic and neuroprotective effect after stroke through the secretion of brain-derived neurotrophic factor (BDNF). Preliminary data show that a subset of B cells responds to glutamate levels via N-methyl-D-aspartate (NMDA) receptors. In stroke, GluN2A subunit-containing NMDA receptors on B cells are activated by glutamate, leading to increased BDNF production. However, at very high glutamate concentrations, similar to levels in the epicenter of the lesion, Bcell NMDA receptor activity is reduced, potentially reducing the neuroprotective activity. Overall, Dr. Stowe hypothesizes that a subset of B cells may play a role in functional recovery after stroke by promoting neuronal survival and neuroplasticity in areas of the brain distant from the injury site through BDNF, but potential activation by physiologic levels of glutamate is concentration dependent. While there is great interest in the potential for B cells as promoters of regeneration, there are also instances where their presence at a site of injury can be detrimental. In his talk, Dr. Kristian Doyle (Associate Professor, Department of Neurology, University of Arizona, AZ, USA) described the dual nature of B cells in the context of stroke, highlighting both their neuroprotective effects and their possible role in chronic “smoldering” inflammation. Using a mouse preclinical model of ischemic stroke, Doyle described the immediate and significant increase in angiogenesis within the stroke site, characterized by the formation of new blood vessels that lack tight junctions, causing prolonged blood–brain barrier (BBB) disfunction. Moreover, due to the highly proteolytic microenvironment at the site of stroke, any newly formed tight junctions would likely be disrupted as soon as they were formed. Further complicating the regenerative response, although astrocytic gliosis creates a scar around the infarcted area, the scar is a permeable barrier that allows neurotoxic molecules in the infarct to leak into the surrounding tissue.2 By week 7 post-stroke, the brain shows a drastic loss of symmetry characterized by both edema and atrophy in the ipsilateral hemisphere. Correlated to the structural disruption, animals develop major long-term potentiation (LTP) deficits as well as memory deficits. Doyle pointed out that the neurodegeneration that occurs in the weeks after stroke correlates with the appearance of B cells at the stroke site. Conversely, Bcell-deficient mice, both from knockout and depletion, were protected from LTP and memory deficits.3 One hypothesis was that due to the permeability of the astrocytic scar, antibodies and pro-inflammatory cytokines produced locally by B cells within the compartmentalized infarcted site leak out into the surrounding tissue. Doyle also noted, however, that the Bcell response in the brain is heterogeneous and that different subsets of B cells could have different effects. For example, B1 cells, which are a type of B cell that are part of the innate immune system and produce low-affinity antibodies independently of T cell help, are one of the major sub-populations of B cell within chronic stroke infarcts.4 He discussed and presented data that supported the view that disrupted lipid metabolism due to stroke is almost certainly a major driver of the chronic inflammatory response to stroke and that these B1 cells may be playing an important role in clearing myelin lipid debris. Shifting the focus toward the role neuroinflammation in spinal cord injury (SCI), Dr. Phillip Popovich (Professor and Chair, Department of Neuroscience, Ohio State University, OH, USA) highlighted the importance of microglia in coordinating repair in the injured CNS. The talk began with a histological overview of SCI and showed how macrophages, derived from infiltrating peripheral monocytes, are the distinctly dominant immune cell population at the site of injury, a phenomenon that is conserved across many species.5-7 Interestingly, systemic depletion of monocytes resulted in reduced axon loss, improved plasticity, and functional recovery after SCI in rodent models.8 By contrast, when microglia are depleted, lesions increase in size and complexity, while spontaneous recovery is greatly impaired. New RNA sequencing data from Popovich's team showed that microglia have a critical and highly complex role in regulating the post-SCI transcriptome. When microglia were depleted using PLX5622, more than half of the genes usually upregulated after SCI failed to change expression levels. In these studies, microglia even appear to dictate the phenotype of other cell populations after SCI, altering the transcriptional fate of monocyte-derived macrophages at the injury site, as well as that of astrocytes. In the overall context of neuroinflammatory response to SCI, Popovich noted that the absence of B cells improved motor recovery after SCI in mice, supporting a potentially detrimental aspect of infiltrating B cells at the injury site.9 Taking a broader perspective on the functional and neurobehavioral aspects of immune activation and neurological function, Dr. Elizabeth Engler-Chiurazzi (Assistant Professor, Department of Neurosurgery, Tulane University, LA, USA) gave an insightful overview of existing knowledge on Bcell contributions to mental health, psychosocial stress, and major depression. She presented highly original research from her laboratory and others showing that the abundance of peripheral B cells is reduced in major depressive disorder as well as in healthy individuals in the context of stress. Interestingly, chronic stress can alter splenic Bcell counts in a subset-dependent matter. In preclinical mouse models, it has been shown that acute or prolonged stress alters the balance of immune cell populations, reducing the abundance of certain B cell subsets while increasing the ratio of CD4+ to CD8+ T cells. Engler-Chiurazzi's research has shown that Bcell deficiency is associated with increased susceptibility to severe acute stress and that this depressive-like phenotype is age related and sex specific. Moreover, reconstitution of Bcell-deficient mice with purified splenic B cells increased the behavioral resilience of mice to forced swim stress. This work provided a unique view on the functional relevance of B cells served to broaden the definition of CNS “injury” to include psychological stressors and linked this aspect of adaptive immunity to higher cognitive functions across the lifespan. In a talk that focused on the potential for targeted molecular manipulation of B cells, Dr. Christian LeGuern (Associate Professor of Surgery [Immunology], Harvard Medical School, MA, USA) presented work performed in collaboration with the group of Dr. James Markmann (Claude E. Welch Professor of Surgery, HMS, MA, USA) characterizing the therapeutic potential of regulatory B cells (Bregs) in organ transplantation. Their group developed in vitro methods of stimulating B cells to induce a regulatory phenotype.10 LeGuern showed how B cells can be induced toward a regulatory phenotype either by a CD40 ligand stimulation or a via a toll-like receptor (TLR)-mediated pathway, specifically involving both TLR-9 stimulation by CpG dinucleotides and TLR-4 stimulation by lipopolysaccharides. Using murine allogeneic islet and skin transplantation models, the group found that both Breg subsets achieved prolongation of graft survival. Interestingly, CD40-induced Breg cells appear to act mostly through the release of interleukin 10 while TLR-induced Bregs (Breg-TLR) produce a large amount of transforming growth factor beta (TGFβ). The group further focused on characterizing subsets of Breg-TLR cells through single-cell RNA sequencing, showing that B cells stimulated through the TLR-9 and TLR-4 pathways have a unique expression profile, characterized by high expression of TGFβ. Interestingly, donor-specific Bregs-TLR are more efficient at promoting graft survival, suggesting that local targeting via the Bcell receptor may play a role in specific homing of the regulatory cells to the graft.11 A recent study from the same group further showed that Bregs-TLR can inhibit cognate Bcell activation, proliferation, and development into plasma cells. It was hypothesized that because Bregs-TLR express very high levels of major histocompatibility complex class II, they may induce CD4+ T cell anergy by presenting relevant antigens in a tolerogenic milieu. This work underlined the potential for ex vivo stimulated B cells to improve the success of graft survival in allogeneic transplantation. In the final segment of the conference, Dr. Marion Buckwalter (Professor of Neurology and Neurosurgery, Stanford University, CA, USA) gave a compelling talk on the role of B cells in post-stroke dementia. Buckwalter began by highlighting the serious implications of stroke and post-stroke pathologies, particularly dementia, which has been shown to double in likelihood following stroke. Among the histopathological changes observed after acute ischemic stroke, neuroinflammation and peripheral immune cell infiltration are common hallmarks. Interestingly, findings in post-mortem brain samples indicate that more B cells are found in the lesion core in individuals that have also developed dementia after stroke.3 Stroke can lead to immunodepression including lymphopenia and monocyte suppression, which amplifies the risk of infection. At the same time, the tissue disruption associated with the stroke lesion can lead to the release of brain antigens, potentially triggering autoimmune responses. It was hypothesized that generalized infection may amplify the overall inflammatory response and thus also autoimmunity. Buckwalter's group is actively investigating peripheral immune signatures after stroke that may be associated with future cognitive deterioration. Buckwalter shared the design of an ongoing clinical study at Stanford Hospital which is aimed at linking immunological responses to stroke with long-term post-stroke cognitive trajectories.12 Samples are collected from patients and participants longitudinally, and immune profiling of peripheral blood mononuclear cells and plasma proteomics are used for analysis. Preliminary data indicate that numerous proteins associated with immunomodulation are significantly altered in the plasma of enrolled patients confirmed to have had an acute ischemic stroke. Moreover, there are significant correlations between these alterations and post-stroke mood. The outcomes from this and other studies with a similar design will represent a significant contribution to our understanding of the temporal dynamics of inflammatory involvement in the development of post-stroke dementia. Overall, the Conference on B Cells in Injury and Regeneration provided fascinating insights into the functions that B cells have in the context of various pathologies of the CNS, as well as evidence of potential therapeutic applications, fostering a highly collaborative discussion between subject area experts from diverse scientific fields. Liam J. Dwyer, Mark C. Poznansky, and Ruxandra F. Sîrbulescu contributed equally to writing the article. All authors contributed equally to editing. We are grateful to Phoebe Ingram at the Vaccine and Immunotherapy Center and to Ying Zhu at FASEB for help with organizing the conference. The work presented by K.P. Doyle was supported by the US National Institutes of Health/National Institute of Neurological Disorders and Stroke (NIH/NINDS; R01NS096091) and the Leducq Foundation. The authors declare that no conflict of interest exists. Data sharing not applicable to this article as no datasets were generated or analyzed during the current study.