It has long been thought that epilepsy is mainly caused by structural changes in the central nervous system, without much input from outside this system. The blood–brain barrier has, however, been a candidate for helping to protect the brain from seizures1: it maintains ionic and metabolic homeostasis in the central nervous system, excluding serum and circulating cells while facilitating the passage of essential substances such as glucose. Indeed, several studies have shown that compromising this barrier aids the initiation and progression of epilepsy, even in the absence of structural damage to brain tissue. Two reports2, 3, including one by Kim et al.2 on page 191 of this issue, now take us a step further by documenting the involvement of inflammatory cells in the leakiness of the blood–brain barrier in mouse models of seizures and epilepsy.
Infection of the mouse central nervous system with lymphocytic choriomeningitis virus (LCMV) results in the breakdown of the blood–brain barrier and fatal seizures. This condition had been believed to occur when T cells expressing the CD8 marker protein (CD8+ T cells) recognized virus-infected cells, bound to them and killed them. Kim and colleagues2 tested this hypothesis by performing a detailed dynamic study in the LCMV-mouse model, visualizing fluorescently labelled CD8+ T cells through the intact skull using two-photon microscopy.
Of the meninges membranes that enclose the brain, the one adjacent to the brain tissue is separated from the membrane that adheres to the skull's inner surface by the subarachnoid space (SAS). This space contains cerebrospinal fluid and structural connective tissue containing stromal cells. Shortly after LCMV infection, Kim et al. did not detect CD8+ T cells in the brain parenchyma — the functional tissue into which the virus had been inoculated and whence it spreads to the SAS stromal cells. During this time, however, T cells entered the SAS to make short-lived contact with LCMV-infected stromal cells. The transient contact seems to be sufficient for the T cells to recognize the virus and become activated. But the T cells fail to adhere firmly to the infected cells — a step essential for the killing of virus-infected cells — probably because the stromal cells secrete proteins called chemokines, which can impair the special type of adhesiveness involved in immune recognition.
Kim et al.2 also find that, once activated, the CD8+ T cells secrete additional chemokines, which trigger an enormous influx of inflammatory myeloid cells such as neutrophils and monocytes into the SAS (Fig. 1a). Recruitment of the inflammatory cells was associated with the disruption of the blood–brain barrier within the brain parenchyma, presumably because these cells produce inflammatory immune mediators called cytokines; these small proteins diffuse into the brain tissue and compromise the barrier functions of the endothelial cells lining blood vessels, leading to seizures and death. Depleting LCMV-infected mice of the myeloid cells delayed death, pointing at a causal relation between the inflammatory reaction at the meninges, leakiness of the blood–brain barrier and fatal seizures2.
Figure 1: Role of the immune system in epilepsy.
a, Kim et al.2 show that, while surveying the mouse brain, T cells carrying the CD8 marker (CD8+ T cells) recognize and bind to stromal cells infected with lymphocytic choriomeningitis virus (LCMV) in the subarachnoid space (SAS). Consequently, the T cells secrete chemoattractants that trigger the influx of myeloid cells into the SAS and their secretion of inflammatory cytokines. The cytokines then enter the brain, breaking down the blood–brain barrier, causing seizures and death. b, Fabene et al.3 find that systemic injection of the drug pilocarpine triggers adhesion of white blood cells to endothelial cells. This compromises the blood–brain barrier, allowing the entry of pilocarpine, as well as potassium ions (K+) into the brain, which trigger seizures and ultimately chronic epilepsy.
High resolution image and legend (138K)These results vividly demonstrate that CD8+ T cells initially recognize LCMV through immune surveillance in the SAS. Intriguingly, within the SAS, immune recognition of antigenic targets by CD4+ T cells has also been reported4. Together, these papers2, 4 highlight the need to understand the versatile immune apparatus that resides within the SAS5 and supports the notion of surveillance of the central nervous system5 by both CD4+ and CD8+ T cells within the SAS.
Of the previous studies that have convincingly implicated breakdown of the blood–brain barrier in the development of epilepsy, one salient example involved injection of the epilepsy-inducing drug pilocarpine into the systemic circulation of mice6. This treatment causes severe seizures followed by the onset of chronic epilepsy. At the molecular level, pilocarpine injection leads to a mild inflammatory reaction with release of the cytokine interleukin-1
, causing a slightly leaky blood–brain barrier6 and the subsequent entry of low levels of pilocarpine and potassium ions (which lower seizure thresholds) into the central nervous system.
The understanding of how pilocarpine injection leads to breakdown of the blood–brain barrier is extended by Fabene et al. in a study published in Nature Medicine3. The authors propose that pilocarpine injection stimulates white blood cells as well as the endothelial cells of the blood–brain barrier, which then transiently adhere to each other, causing the barrier to become leaky (Fig. 1b). They tested this hypothesis by blocking (using neutralizing antibodies) or genetically eliminating molecules that these two cell types use for adhesion. Remarkably, suppressing interaction between white blood cells and endothelial cells virtually abrogated initiation of seizures. And when adhesion molecules were blocked after a single severe seizure, progression to epilepsy was suppressed.
These findings3 are in line with several previous reports: seizures disrupt the function of the blood–brain barrier and permit entry of serum albumin into the brain7; albumin is taken up into the brain's support cells, the astrocytes, downregulating potassium channels in these cells and impairing their ability to buffer excess potassium8; and chronic seizures induce vessel formation (angiogenesis), increasing vascular surface area and producing a crop of porous, newly formed vascular elements that have poor barrier function9. The blood–brain barrier therefore seems to be involved in the initiation, progression and perpetuation of seizures.
These exciting findings2, 3 document two routes to the development of seizures and epilepsy. Kim et al. show that meningeal inflammation can signal 'inwards' to the brain parenchyma, activating the vasculature there and compromising the blood–brain barrier. Fabene and colleagues confirm that, in the presence of an epilepsy-inducing factor, even mild systemic inflammation leading to minimal disruption of the blood–brain barrier can produce seizures. These authors' results3 are convincing and of potential significance for treatment, as they studied some molecules that could be used to inhibit the adherence of white blood cells to endothelial cells in humans.
But as far as therapeutic application is concerned, these findings are fraught with concerns about how well the animal models reflect epilepsy in humans. What's more, long-term administration of adhesion-molecule blockers would pose some risk10, as they would alter immune function and increase vulnerability to infection. However, short courses of such treatment after brain trauma, for example, which is known to increase liability to seizures, might offer the opportunity both to enhance patients' chances of recovery and to generate a proof-of-principle for the validity in humans of observations derived from animal models.
