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Glycerol monolaurate (GML) is a naturally occurring antimicrobial agent used commercially in numerous products and food items. GML is also used as a homeopathic agent and is being clinically tested to treat several human diseases. In addition to its anti-microbial function, GML suppresses immune cell proliferation and inhibits primary human T cell activation. GML suppresses T cell activation by altering membrane dynamics and disrupting the formation of protein clusters necessary for intracellular signaling. The ability of GML to disrupt cellular membranes suggests it may alter other cell types. To explore this possibility, we tested how GML affects human B cells. We found that GML inhibits BCR-induced cytokine production, phosphorylation of signaling proteins, and protein clustering, while also changing cellular membrane dynamics and dysregulating cytoskeleton rearrangement. Although similar, there are also differences between how B cells and T cells respond to GML. These differences suggest that unique intrinsic features of a cell may result in differential responses to GML treatment. Overall, this study expands our understanding of how GML impacts the adaptive immune response and contributes to a broader knowledge of immune modulating monoglycerides.

 

ABSTRACT Streptococcus sanguinis is a common cause of infective endocarditis (IE). Efforts by research groups are aimed at identifying and characterizing virulence factors that contribute to the ability of this organism to cause IE. This Gram-positive pathogen causes heart infection by gaining access to the bloodstream, adhering to host extracellular matrix protein and/or platelets, colonizing the aortic endothelium, and incorporating itself into the aortic vegetation. While many virulence factors have been reported to contribute to the ability of S. sanguinis to cause IE, it is noteworthy that type IV pili (T4P) have not been described to be a virulence factor in this organism, although S. sanguinis strains typically encode these pili. Type IV pili are molecular machines that are capable of mediating diverse virulence functions and surface motility. T4P have been shown to mediate twitching motility in some strains of S. sanguinis , although in most strains it has been difficult to detect twitching motility. While we found that T4P are dispensable for direct in vitro platelet binding and aggregation phenotypes, we show that they are critical to the development of platelet-dependent biofilms representative of the cardiac vegetation. We also observed that T4P are required for in vitro invasion of S. sanguinis into human aortic endothelial cells, which indicates that S. sanguinis may use T4P to take advantage of an intracellular niche during infection. Importantly, we show that T4P of S. sanguinis are critical to disease progression (vegetation development) in a native valve IE rabbit model. The results presented here expand our understanding of IE caused by S. sanguinis and identify T4P as an important virulence factor for this pathogen. IMPORTANCE This work provides evidence that type IV pili produced by Streptococcus sanguinis SK36 are critical to the ability of these bacteria to attach to and colonize the aortic heart valve (endocarditis). We found that an S. sanguinis type IV pili mutant strain was defective in causing platelet-dependent aggregation in a 24-h infection assay but not in a 1-h platelet aggregation assay, suggesting that the type IV pili act at later stages of vegetation development. In a rabbit model of disease, a T4P mutant strain does not develop mature vegetations that form on the heart, indicating that this virulence factor is critical to disease and could be a target for IE therapy.