Cortical contusions are traumatic lesions in the brain that hemorrhage. Following injury, cellular and macromolecular components in blood can traverse the disrupted BBB and enter the CNS. This damage triggers an array of signaling cascades that activate the innate immune system (Jakeman et al., 2014). Dying cells release genomic DNA and other damage-associated molecular patterns (DAMPs; Vénéreau et al., 2015) which serve as biochemical cues (Figure 2A). Microglia sense these DAMPs and are polarized toward phagocytic, pro-inflammatory phenotypes which upregulate membrane-bound CD68 and release signaling molecules such as IL-1, IL-6, and TNF-α. These activated microglia also upregulate ionized calcium binding adaptor molecule 1 (Iba1; Ohsawa et al. (2004)), a protein that participates in microglia phagocytosis. As a response to brain injury, astrocytes also undergo major changes and will upregulate glial fibrillary acidic protein (GFAP) as well as release cytokines and chemokines to attract microglia, circulating monocytes, and other immune cells from the periphery to the site of injury. Interestingly, inflammatory peptides such as bradykinin can cause astrocytes to secrete IL-6 and lead to further BBB permeabilization early on, suggesting that temporarily heightened peripheral immune surveillance supports an inflammatory response before astrocyte-mediated repair of the BBB (Schwaninger et al., 1999; Karve et al., 2016). Following cytokine and chemokine release soon after injury, peripheral immune cells such as neutrophils and circulating monocytes enter the brain. These circulating monocytes differentiate into macrophages and can persist for several days to weeks (Jakeman et al., 2014; Jin and Yamashita, 2016) while exhibiting both pro-inflammatory and wound-healing phenotypes.

Dramatic changes to the ECM occur following contusions. Following stimulation by signaling molecules such as IL-6, IFNγ, and TGFβ, astrocytes secrete chondroitin sulfate (CS) and CS-containing proteoglycans (CSPGs). CS levels increase 1–7 days following injury, and can remain elevated for weeks before reduction ~2 months later (Jakeman et al., 2014); they also have a profound effect on neurons (Siebert et al., 2014). CSPGs inhibit axonal growth, and upregulation of CS in vivo by astrocytes in the area of injury results in a reduction of regenerative capacity. This self-defense mechanism may be an endogenous way to wall off injury and prevent its propagation, and controlling it is an area of research for regenerative medicine (Jakeman et al., 2014; Siebert et al., 2014).

Strokes are the second leading cause of death worldwide (WHO Global Health Estimates), and preventing them or limiting their damage has been the focus of extensive research. Strokes are disruptions of blood flow to the brain, triggered either by a blood clot causing ischemia in about 80% of cases (Aguilar, 2015), or by hemorrhagic vessel disruptions. This paragraph will focus on the consequences of ischemic strokes, which share some immune cascades with contusions.

Ischemia leads to the disruption of the BBB through several mechanisms (Baeten and Akassoglou, 2011). At the cellular level, ischemia causes a disruption of tight junctions between endothelial cells, with subsequent opening of the endothelial and astrocytic end feet barriers. Ischemia affects the ECM as well by causing a degradation of the BM due to proteolysis by matrix metalloproteases (MMPs). Fibrin and fibrinogen extravasation from the bloodstream through the disrupted BBB leads to secondary damage in the brain as well.

As with contusions, this damage in the brain from ischemia triggers an orchestrated neuro-inflammatory cascade that begins with an influx of microglia to the site of injury. Soluble factors released as a consequence of the cell injury in the parenchyma or from blood extravasation activate microglia (Qin et al., 2019). These activated microglia can play multiple roles at the BBB in ischemic stroke. Starting in the initial hours after ischemic stroke and peaking a week after, microglia produce high levels of pro-inflammatory cytokines and chemokines, which enhance the infiltration of T and B leukocytes (Selvaraj and Stowe, 2017; Zhang et al., 2021). In latter stages, microglia exhibit wound-healing phenotypes and support synpatic remodeling. They release VEGF, an angiogenic factor, to promote reformation of vasculature. Microglia also triggers expression of synaptic proteins and plays a role in neurogenesis, axonal regeneration, and other neural circuit recovery processes (Yu et al., 2021).

Following the influx of microglia, other cellular players traffic to the site of injury. In addition to circulating macrophages, T and B lymphocytes, dendritic cells (DCs), and neutrophils enter the brain and play a role in the neuro-inflammatory and healing cascades that follow (Selvaraj and Stowe, 2017). The role of the adaptive immune system in recovery following ischemia is still not fully understood. Interestingly, in animal models of stroke, wild-type mice fare worse than immune-deficient mice, and IFN-γ and IL-17 have been identified as key players in T cell-mediated tissue damage (Yilmaz et al., 2006; Shichita et al., 2009; Gelderblom et al., 2012; Selvaraj and Stowe, 2017 The long-term effects of adaptive immune cell influx into the brain following stroke remain an active area of research. Further investigations into their acute and chronic functions, and interactions with neurons, could open up new therapeutic strategies that address this large unmet medical need.

Glioblastomas (GBs) are aggressive grade IV astrocytomas and are the most common primary intracranial tumors (Bondy et al., 2008). GBs are universally lethal and have among the worst clinical outcomes of all cancer types. They develop a tumor microenvironment with altered cellular and extracellular composition that is crucial to their progression and therapeutic resistance, characterized by metabolic, hypoxic, osmotic and pH changes (Qiu et al., 2021). The neuro-ECM in this tumor environment supports cancer stem cells and is involved in tumor progression and chemoresistance (Ros et al., 2018) (Figure 2B). It provides receptors able to anchor stem cells in the niche and secretes growth factors that lead to their proliferation, such as laminin alpha-2 (Lathia et al., 2012), or regulation, such as integrin alpha-6 (Corsini and Martin-Villalba, 2010).

GBs cause BBB disruption, resulting in cerebral edema and tumor microenvironment communication with the blood stream (Wiranowska et al., 1992; Schneider et al., 2004). Disruption is not merely due to astrocytic dysfunction, but also due to GB cells secreting ECM-disrupting soluble factors (Schneider et al., 2004), for instance hypoxia-induced VEGF-A, also responsible for angiogenic stimulation (Zhao et al., 2018). They lead to a lower expression of claudin-1, claudin-5, and occludin, resulting in pathological fenestration in endothelial cells tight junctions, and increase the secretion of MMPs by endothelial cells, thus disrupting the ECM (Ishihara et al., 2008; Zhao et al., 2018).

However, the GB-induced BBB disruption, associated with the concept of a blood-brain tumor barrier, does not enable higher permeability of therapeutic macromolecules to the tumor. On the contrary, it creates a local microenvironment favorable for tumor growth and resistance against a variety of therapeutic modalities (Ros et al., 2018). The neuro-ECM is partly responsible for the increased stiffness of glioblastoma tissue and ECM compared to non-tumoral tissue, which, along the local hyperosmotic character, interferes with vessel integrity and is a significant obstacle to macromolecule transport across the BBB and the recruitment of inflammatory cells.

Brain tumors also have direct immuno-modulatory potential. Tumors are commonly thought of as “continuous wounds” with respect to their immunological characteristics and similarities of tumor-associated immune cells to those cells participating in wound-healing (Dvorak, 2015). GBs recruit immune cells by releasing soluble factors such as chemokines and cytokines, and can polarize them toward pro-tumor phenotypes. For example, microglia and peripheral monocytes/macrophages can be recruited and polarized to tumor-associated microglia (TAMs), and strikingly, these cells make up approximately 40% of a brain tumor's volume (Buonfiglioli and Hambardzumyan, 2021). TAMs are critical for tumor growth. They help reformulate the ECM to become more suitable for cancer cells, suppress effector immune cell surveillance, and enable brain tumor resistance to chemo- or radio-therapy (Buonfiglioli and Hambardzumyan, 2021).

The ability of neurons to contribute to tumor progression by secreting soluble factors and directly forming synapses with tumor cells has recently gained significant attention (Venkatesh et al., 2015, 2017, 2019; Venkataramani et al., 2019; Zeng et al., 2019; Monje et al., 2020; Pan et al., 2021). Of particular importance is a subset of glioma cells that resemble oligodendrocyte precursor cells (OPCs), referred to here as OPC-like cells (Neftel et al., 2019). When exposed to soluble forms of the neuronal synaptic protein neuroligin-3 (NLGN3), OPC-like tumor cells proliferated faster (Venkatesh et al., 2015) (Figure 2B). An experiment in vitro showed that tumor proliferation monotonically increased with increasing NLGN3 concentrations. Strinkingly, patient-derived cancer cells were not able to appreciably grow in vivo in Nlgn3-deficient mice. These data highlighted that soluble factors derived from neurons can modulate the growth of tumors, and preventing the cleavage of this post-synaptic protein is a viable therapeutic approach to reduce the rate of tumor progression (Venkatesh et al., 2017).

In follow-up studies, it was identified that OPC-like tumor cells form bona fide synapses with neurons in vivo (Venkataramani et al., 2019; Venkatesh et al., 2019). Serving as post-synaptic cells, OPC-like tumor cells had increased proliferation with higher activity independent of the soluble NLGN3 mechanism. Ex vivo slice patch-clamp experiments identified that synaptic communication occurs through AMPA receptors (AMPARs) and could be inhibited by traditional AMPAR antagonists such as NBQX or sodium channel blockers such as tetrodotoxin (TTX). A distinct, prolonged (>1 s) electrophysiological response was also observed in vivo that was not blocked by either AMPAR inhibitors or TTX. This was identified as a potassium current from a gap junction-coupled network, and this depolarization also promoted tumor growth. An unanswered question in this paradigm is whether TAMs have a role in neuron-tumor interactions. Given the role of microglia in sculpting neuronal synapses in healthy postnatal brains, an interesting question is whether TAMs support the formation of neuron-tumor synapses as well.

Multiple sclerosis (MS) is a leading cause of neurological disability, particularly in young adults (WHO Global Health Estimates), and is caused by auto-immune destruction of the myelin sheath around neurons of the CNS. It impedes all types of cerebral and spinal chord functions, including motricity, sensitivity, vision and cognition. MS is currently incurable, though there are approved treatments that slow down disease progression (Martin et al., 2016).

Demyelination and oligodendrocyte loss are the primary pathological hallmarks in MS (Figure 2C). Peripheral leukocytes contribute to this pathology, where coordinated attacks involving misrecognition of myelin, antigen presentation, antibody production, and phagocytic or lytic attack of CNS tissue accelerate disease progression. The underlying mechanisms underlying leukocyte trafficking to the brain has been an active area of research. To recapitulate these observations in pre-clinical mice models, experimental autoimmune encephalitis (EAE) evoked by an injection of myelin proteins is commonly used. Both T and B cells are strongly implicated in MS (Chastain et al., 2011; van Langelaar et al., 2020; Jain and Yong, 2021). Self-reactive T cells in the choroid plexus support neuroimmune homeostasis in the CNS, but in MS and EAE, cerebrospinal fluid T cells skew toward a pathogenic T helper 17 (Th17) phenotype (Mundt et al., 2019) that are detrimental for brain function. Inflamed T leukocytes are more prone to rolling and adhesion along the luminal surface of blood vessels in the brain (Figure 1; Nourshargh and Alon, 2014). “Stickiness” among circulating leukocytes is partly mediated by upregulation of adhesion molecules on the leukocytes themselves, and by alterations in the cell-surface profile of vascular endothelial cells (Pinheiro et al., 2016). Strikingly, upregulation of integrins and cadherins enable Th17 cells to “crawl” for long distances (hundreds of microns) against the direction of blood flow in EAE (Engelhardt and Ransohoff, 2012). During crawling, Th17 cells utilize actin filaments, scaffolding proteins, and ECM-degrading enzymes to infiltrate the brain at fenestrated capillaries in the choroid plexus and penetrate tight junctions at the cerebrovascular interface (Mundt et al., 2019; Samus et al., 2020).

Once in the brain, Th17 cells support neuroinflammation that is the hallmark of MS. T cell diapedesis is evoked by antigen presentation, and in MS/EAE, myelin and myelin-associated proteins are the predominant antigens recognized by T cell receptors (Pinheiro et al., 2016; Mundt et al., 2019). Microglia are the professional antigen-presenting cells (APCs) in the brain parenchyma, but astrocytes are also capable of antigen presentation under certain conditions (Mundt et al., 2019). In the cerebrovasculature, endothelial cells are also capable of antigen presentation in EAE and other chronic inflammatory conditions (Pinheiro et al., 2016). B cells in the dural meninges are also efficient APCs, and they accumulate and skew toward a pro-inflammatory phenotype in EAE (Jain and Yong, 2021). The diversity of cells that can serve as APCs (Chastain et al., 2011) highlight the complicated signaling cascades that underlie neuroinflammation in MS, but also serve as opportunities for therapeutic intervention.

While T cells have historically been the dominant lymphocyte implicated in MS, the role of B cells has received outsized attention in recent years. In a landmark 2017 clinical trial, depletion of CD20+ B cells with the monoclonal antibody Ocrelizumab was identified to be an effective therapy for MS (Hauser et al., 2017). More recently, an epidemiological study of over 10 million active duty U.S. military personnel found strong evidence that Epstein-Barr virus (EBV) serves as the trigger for MS (Bjornevik et al., 2022). In a separate study, it was found that EBV infection triggers B cells to produce antibodies against the EBV transcription factor EBNA1, and strikingly these clones undergo further affinity maturation in the brain to produce auto-antibodies which are also cross-reactive against the CNS protein GlialCAM (Lanz et al., 2022). These consequential studies highlight the potentially detrimental role that mimcry of viral antigens to self-antigens can have in MS and other neuro-inflammatory diseases, and are exciting new avenues to explore for therapeutic interventions.