Glial Cell–Elicited Activation of Brain Microvasculature in Response to Brucella abortus Infection Requires ASC Inflammasome–Dependent IL-1β Production

Abstract

Blood–brain barrier activation and/or dysfunction are a common feature of human neurobrucellosis, but the underlying pathogenic mechanisms are largely unknown. In this article, we describe an immune mechanism for inflammatory activation of human brain microvascular endothelial cells (HBMEC) in response to infection with Brucella abortus. Infection of HBMEC with B. abortus induced the secretion of IL-6, IL-8, and MCP-1, and the upregulation of CD54 (ICAM-1), consistent with a state of activation. Culture supernatants (CS) from glial cells (astrocytes and microglia) infected with B. abortus also induced activation of HBMEC, but to a greater extent. Although B. abortus–infected glial cells secreted IL-1β and TNF-α, activation of HBMEC was dependent on IL-1β because CS from B. abortus–infected astrocytes and microglia deficient in caspase-1 and apoptosis-associated speck-like protein containing a CARD failed to induce HBMEC activation. Consistently, treatment of CS with neutralizing anti–IL-1β inhibited HBMEC activation. Both absent in melanoma 2 and Nod-like receptor containing a pyrin domain 3 are partially required for caspase-1 activation and IL-1β secretion, suggesting that multiple apoptosis-associated speck-like protein containing CARD–dependent inflammasomes contribute to IL-1β–induced activation of the brain microvasculature. Inflammasome-mediated IL-1β secretion in glial cells depends on TLR2 and MyD88 adapter-like/TIRAP. Finally, neutrophil and monocyte migration across HBMEC monolayers was increased by CS from Brucella-infected glial cells in an IL-1β–dependent fashion, and the infiltration of neutrophils into the brain parenchyma upon intracranial injection of B. abortus was diminished in the absence of Nod-like receptor containing a pyrin domain 3 and absent in melanoma 2. Our results indicate that innate immunity of the CNS set in motion by B. abortus contributes to the activation of the blood–brain barrier in neurobrucellosis and IL-1β mediates this phenomenon.

The role of NLRP3 and AIM2 in inflammasome activation during Brucella abortus infection

“…Caspase-1 activation and IL-1β secretion in glial cells infected with B. abortus depends on the ASC inflammasomes NLRP3 and AIM2, confirming and extending previous work of murine macrophages [16]. B. abortus -induced glial production of IL-1β that leads to activation of HBMEC is dependent on the adapter molecule Mal/TIRAP and TLR2-mediated [58]. These findings are congruent with our previous observations indicating that TLR2/MyD88 signaling is crucial for inflammatory responses induced by B. abortus [61].…”

“…TNF-α is also involved in the secretion of metalloproteinases by infected astrocytes [57]. Although we have described a major role for TNF-α and IL-6 in the immunopathogenesis of neurobrucellosis [53,57], the role of IL-1β has never been investigated until recently [58]. …”

“…IL-1β secreted by B. abortus -infected microglia and astrocytes was able to activate human brain microvascular endothelial cells (HBMEC). Secretion of IL-1 β by glial cells and concomitant HBMEC activation requires caspase-1 and ASC, indicating that a NLR is involved in the production of IL-1β and the subsequent activation of HBMEC by B. abortus [58]. …”

“…In vitro , the activation of HBMEC by B. abortus -infected glial cells correlates with an increased capacity of the microvascular endothelial cells to promote the transmigration of neutrophils and monocytes. This phenomenon depends on the secretion of IL-1β upon the activation of AIM2 and NLRP3 inflammasomes [58]. This suggests that the interaction of Brucella with innate immunity in vivo may result in an increased transmigration of phagocytes to the brain parenchyma which would explain the pleocytosis observed in neurobrucellosis patients [63,64].…”

Brucella Melitensis 16M Regulates the Effect of AIR Domain on Inflammatory Factors, Autophagy, and Apoptosis in Mouse Macrophage through the ROS Signaling Pathway

“…Macrophages can kill most of the invading Brucella , but a small portion is able to evade the immune system and survive and propagate in macrophages[3]. In addition, Brucella can induce macrophage apoptosis, and thus, evade the immune system[4]. Worse still, Brucella spread apoptotic macrophages throughout the body, causing chronic infection[5].…”