llluminating the live-cell dynamics of early interactions between neutrophils and the microsporidian parasite...

Encephalitozoon cuniculi is an obligate intracellular parasite that affects birds and mammals, including humans, and belongs to the phylum Microsporidia. The innate immune system serves as the first line of defense against infection, but it only partially clears the parasite. Microsporidia are known to modulate the effector functions of phagocytes. They can survive and multiply inside macrophages (MOs) to evade destruction after phagocytosis and also modify the functional properties of dendritic cells. However, the role of neutrophils (PMNs) in microsporidiosis remains poorly understood. In a mouse ear skin model, inflammatory cells, including MOs and PMNs, were rapidly recruited to the cutaneous injection site, and intact spores persisted within phagocytes for several days. We hypothesized that Microsporidia also manipulate the antimicrobial mechanisms of PMNs, including phagocytosis and the formation of neutrophil extracellular traps (NETs), to enhance their survival.

Using live and static cell imaging in vitro, along with the mouse ear pinna model, we investigated the dynamics of interactions between PMNs and E. cuniculi parasites. Our observations revealed that the parasites rapidly adhere to and are phagocytosed by PMNs. After activation, infected PMNs exhibited modified structural and functional properties, such as the extension of pseudopodia, parasite lysis inside phagolysosomes, modified motility and shape parameters, and NETs formation. Interestingly, a small proportion of the parasites survived intracellularly, with early developmental stages (meronts, sporonts) observed within PMNs at 24 and 48 h post-infection. Importantly, infected PMNs harboring intact and infectious intracellular spores were later phagocytosed by MOs.

Using innovative microscopy and imaging approaches, we discovered that E. cuniculi Microsporidia manipulate PMNs, key innate immune cells, by using them as intracellular niches for their development, and as potential vehicles for their silent transfer to other host target cells.