Exploring the role of E. faecalis Enterococcal Polysaccharide Antigen
(EPA) and lipoproteins in evasion of phagocytosis
Abstract
Enterococcus faecalis is an opportunistic pathogen frequently
causing nosocomial infections. The virulence of this organism is
underpinned by its capacity to evade phagocytosis, allowing
dissemination in the host. Immune evasion requires a surface
polysaccharide produced by all enterococci, known as the Enterococcal
Polysaccharide Antigen (EPA). EPA consists of a cell wall-anchored
rhamnose backbone substituted by strain-specific polysaccharides called
“decorations”, essential for the biological activity of this polymer.
However, the structural determinants required for innate immune evasion
remain unknown, partly due to a lack of suitable validated assays. Here,
we describe a quantitative, in vitro assay to investigate how EPA
decorations alter phagocytosis. Using the E. faecalis model
strain OG1RF, we demonstrate that a mutant with a deletion of the locus
encoding EPA decorations can be used as a platform strain to express
heterologous decorations, thereby providing an experimental system to
investigate the inhibition of phagocytosis by strain-specific
decorations. We show that the aggregation of cells lacking decorations
is increasing phagocytosis and that this process does not involve the
recognition of lipoproteins by macrophages. Collectively, our work
provides novel insights into innate immune evasion by enterococci and
paves the way for further studies to explore the structure/function
relationship of EPA decorations.