Group B Streptococcal Membrane Vesicles Induce Proinflammatory Cytokine Production and Are Sensed in an NLRP3 Inflammasome-Dependent Mechanism in a Human Macrophage-like Cell Line

Abstract

Group B Streptococcus (GBS) is a major cause of fetal and neonatal mortality worldwide. Many of the adverse effects of invasive GBS are associated with inflammation; therefore, understanding bacterial factors that promote inflammation is of critical importance. Membrane vesicles (MVs), which are produced by many bacteria, may modulate host inflammatory responses. While it is known that mice injected intra-amniotically with GBS MVs exhibit large-scale leukocyte infiltration, preterm birth, and subsequent fetal death, the immune effectors driving this response remain unclear. Here, we hypothesized that THP-1 macrophage-like cells respond to GBS-derived MVs by producing proinflammatory cytokines and are recognized through one or more pattern recognition receptors. We show that THP-1s produce high levels of neutrophil- and monocyte-specific chemokines in response to MVs derived from different clinical isolates of GBS. Using antibody microarrays and multiplex Luminex assays, we found that GBS MVs elicit significantly (p < 0.05) higher levels of CCL1, CCL2, CCL20, CXCL1, CXCL10, and IL-1β relative to untreated THP-1s. Using chemical inhibitors in combination with caspase-1 activity assays and Luminex assays, we further demonstrate that GBS MVs upregulated IL-1β production in a caspase-1 and NLRP3-dependent manner, ultimately identifying NLRP3 as a sensor of GBS MVs. These data indicate that MVs contain one or more pathogen-associated molecular patterns that can be sensed by the immune system and show that the NLRP3 inflammasome is a novel sensor of GBS MVs. Our data additionally indicate that MVs may serve as immune effectors that can be targeted for immunotherapeutics.