Detection of microbial products by host inflammasomes is critical for innate immune surveillance. Inflammasomes activate the CASPASE-1 (CASP1) protease, which processes the cytokines interleukin(IL)-1β and -18, and initiates a lytic host cell death called pyroptosis1. To identify novel CASP1 functions in vivo, we devised a strategy for cytosolic delivery of bacterial flagellin, a specific ligand for the NAIP5 (NLR family, apoptosis inhibitory protein 5)/NLRC4 (NLR family, CARD domain containing 4) inflammasome2–4. Here we show that systemic inflammasome activation by flagellin leads to loss of vascular fluid into the intestine and peritoneal cavity, resulting in rapid (< 30 minutes) death in mice. This unexpected response depends on the inflammasome components NAIP5, NLRC4, and CASP1, but is independent of IL-1β/-18 production. Instead, inflammasome activation results, within minutes, in an ‘eicosanoid storm’ – a pathological release of signaling lipids that rapidly initiate inflammation and vascular fluid loss. Mice deficient in cyclooxygenase-1 (COX-1), a critical enzyme in prostaglandin biosynthesis, are resistant to these rapid pathological effects of systemic inflammasome activation by either flagellin or anthrax lethal toxin. Inflammasome-dependent biosynthesis of eicosanoids is mediated by activation of cPLA2 (cytosolic phospholipase A2) in resident peritoneal macrophages, which are specifically primed for production of eicosanoids by high expression of eicosanoid biosynthetic enzymes. Thus, our results identify eicosanoids as a novel cell type-specific signaling output of the inflammasome with dramatic physiological consequences in vivo.

Polymorphonuclear leukocytes (PMN) are key in innate immunity but their role in viral pathogenesis is incompletely understood. In infection with West Nile virus (WNV), we found that expression of two PMN-attracting chemokines, Cxcl1 and Cxcl2, was rapidly and dramatically elevated in macrophages. PMN are rapidly recruited to the site of WNV infection in mice and support efficient replication of WNV. Mice depleted of PMN after WNV inoculation developed higher viremia and earlier death compared to the control group, suggesting a protective role for PMN. In contrast, when PMN were depleted prior to infection with WNV, and in mice deficient in Cxcr2, a chemokine receptor gene, viremia was reduced and survival was enhanced or delayed. Collectively, these data suggest that PMN have a biphasic response to WNV infection, serving as a reservoir for replication and dissemination in early infection and later contributing to viral clearance.

The role of antibody in protection against human immunodeficiency virus (HIV-1) has been difficult to study in animal models because most primary HIV-1 strains do not infect nonhuman primates. Using a chimeric simian/human immunodeficiency virus (SHIV) based on the envelope of a primary isolate (HIV-89.6), we performed passive-transfer experiments in rhesus macaques to study the role of anti-envelope antibodies in protection. Based on prior in vitro data showing neutralization synergy by antibody combinations, we evaluated HIV immune globulin (HIVIG), and human monoclonal antibodies (MAbs) 2F5 and 2G12 given alone, compared with the double combination 2F5/2G12 and the triple combination HIVIG/2F5/2G12. Antibodies were administered 24 h prior to intravenous challenge with the pathogenic SHIV-89.6PD. Six control monkeys displayed high plasma viremia, rapid CD4+-cell decline, and clinical AIDS within 14 weeks. Of six animals given HIVIG/2F5/2G12, three were completely protected; the remaining three animals became SHIV infected but displayed reduced plasma viremia and near normal CD4+-cell counts. One of three monkeys given 2F5/2G12 exhibited only transient evidence of infection; the other two had marked reductions in viral load. All monkeys that received HIVIG, 2F5, or 2G12 alone became infected and developed high-level plasma viremia. However, compared to controls, monkeys that received HIVIG or MAb 2G12 displayed a less profound drop in CD4+ T cells and a more benign clinical course. These data indicate a general correlation between in vitro neutralization and protection and suggest that a vaccine that elicits neutralizing antibody should have a protective effect against HIV-1 infection or disease.

The development of experimental Lyme arthritis has been correlated with the expression of a number of chemokines and cytokines, however, none of these have been measured directly from the arthritic joint. We examined the temporal expression of IL-1β, IL-4, IL-6, IL-10, IL-12p70, GM-CSF, IFN-γ, TNF-α, macrophage inflammatory protein-2, KC, macrophage inflammatory protein-1α, and monocyte chemoattractant protein-1 directly from the tibiotarsal joint in arthritis-resistant C57BL/6 (B6) and -susceptible C3H/He (C3H) mice. Only the chemokines KC and monocyte chemoattractant protein-1 were differentially expressed in joints of B6 and C3H mice and correlated with the development of Lyme arthritis. Infection of CXCR2−/− mice on either genetic background resulted in a significant decrease in the development of pathology, although infection of CCR2−/− mice had little or no effect. Neutrophils in CXCR2−/− mice were marginalized within blood vessels and could not enter the joint tissue. These results suggest that chemokine-mediated recruitment of neutrophils into the infected joint is a key requirement for the development of experimental Lyme arthritis.