Alzheimer´s Disease (AD) is the world’s most common dementing illness. Deposition of amyloid beta peptide (Aβ) drives cerebral neuroinflammation by activating microglia1,2. Indeed, Aβ activation of the NLRP3 inflammasome in microglia is fundamental for IL-1β maturation and subsequent inflammatory events3. However, it remains unknown whether NLRP3 activation contributes to AD in vivo. Here, we demonstrate strongly enhanced active caspase-1 expression in human MCI and AD brains suggesting a role for the inflammasome in this neurodegenerative disease. NLRP3−/− or caspase-1−/− mice carrying mutations associated with familiar AD were largely protected from loss of spatial memory and other AD-associated sequelae and demonstrated reduced brain caspase-1 and IL-1β activation as well as enhanced Aβ clearance. Furthermore, NLRP3 inflammasome deficiency skewed microglial cells to an M2 phenotype and resulted in the decreased deposition of Aβ in the APP/PS1 model of Alzheimer’s disease. These results reveal an important role for the NLRP3 / caspase-1 axis in AD pathogenesis, and suggest that NLRP3 inflammasome inhibition represents a novel therapeutic intervention for AD.

The lymph node vasculature is essential to immune function, but mechanisms regulating lymph node vascular maintenance and growth are not well understood. Vascular endothelial growth factor (VEGF) is an important mediator of lymph node endothelial cell proliferation in stimulated lymph nodes. It is expressed basally in lymph nodes and up-regulated upon lymph node stimulation, but the identity of VEGF-expressing cells in lymph nodes is not known. We show that, at homeostasis, fibroblast-type reticular stromal cells (FRC) in the T zone and medullary cords are the principal VEGF-expressing cells in lymph nodes and that VEGF plays a role in maintaining endothelial cell proliferation, although peripheral node addressin (PNAd)+ endothelial cells are less sensitive than PNAd− endothelial cells to VEGF blockade. Lymphotoxin β receptor (LTβR) blockade reduces homeostatic VEGF levels and endothelial cell proliferation, and LTβR stimulation of murine fibroblast-type cells up-regulates VEGF expression, suggesting that LTβR signals on FRC regulate lymph node VEGF levels and, thereby, lymph node endothelial cell proliferation. At the initiation of immune responses, FRC remain the principal VEGF mRNA-expressing cells in lymph nodes, suggesting that FRC may play an important role in regulating vascular growth in stimulated nodes. In stimulated nodes, VEGF regulates the proliferation and expansion of both PNAd+ and PNAd− endothelial cells. Taken together, these data suggest a role for FRC as paracrine regulators of lymph node endothelial cells and suggest that modulation of FRC VEGF expression may be a means to regulate lymph node vascularity and, potentially, immune function.

Lymph node expansion during immune responses is accompanied by rapid vascular expansion. The reestablishment of quiescence and stabilization of the newly expanded vasculature and the regulatory mechanisms involved have not been well studied. We show that while initiation of vascular expansion in immune-stimulated nodes is associated with upregulated endothelial cell proliferation, increased high endothelial venule trafficking efficiency and VCAM-1 expression, and disrupted perivascular fibroblastic reticular cell organization, the reestablishment of vascular quiescence and stabilization after expansion is characterized by reversal of these phenomena. While CD11cmed cells are associated with the initiation of vascular expansion, CD11chiMHCIImed dendritic cells accumulate later and their short-term depletion in mice abrogates the reestablishment of vascular quiescence and stabilization. CD11chiMHCIImed cells promote endothelial cell quiescence in vitro and, in vivo, mediate quiescence at least in part by mediating reduced lymph node VEGF. Disrupted vascular quiescence and stabilization in expanded nodes is associated with attenuated T cell-dependent B cell responses. These results describe a novel mechanism whereby CD11chiMHCIImed dendritic cells regulate the reestablishment of vascular quiescence and stabilization after lymph node vascular expansion and suggest that these dendritic cells function in part to orchestrate the microenvironmental alterations required for successful immunity.