Extracellular vesicles (EVs) released from pathogenic protozoans play crucial roles in host–parasite communication and disease pathogenesis. Naegleria fowleri is a free-living protozoan causing primary amoebic meningoencephalitis, a fatal disease in the central nervous system. This study aims to explore the roles of N. fowleri-derived EVs (Nf-EVs) in host–pathogen interactions using the THP-1 cell line as a model. The Nf-EVs were isolated from the N. fowleri trophozoite culture supernatant using sequential centrifugation and characterized by nanoparticle tracking analysis and transmission electron microscopy. The functional roles of Nf-EVs in the apoptosis and immune response induction of THP-1 monocytes and macrophages were examined by flow cytometry, quantitative PCR, and ELISA. Results showed that Nf-EVs displayed vesicles with bilayer membrane structure approximately 130–170 nm in diameter. The Nf-EVs can be internalized by macrophages and induce macrophage responses by induction of the expression of costimulatory molecules CD80, CD86, HLA-DR, and CD169 and the production of cytokine IL-8. However, Nf-EVs did not affect the apoptosis of macrophages. These findings illustrate the potential role of Nf-EVs in mediating the host immune cell activation and disease pathogenesis.

Monocytes, one of the main target cells for dengue virus (DENV) infection, contribute to the resolution of viremia and to pathogenesis. We performed a longitudinal study by a detailed phenotypic comparison of classical (CD14++CD16−, non-classical (CD14+CD16++) and intermediate (CD14++CD16+) monocyte subsets in blood samples from dengue fever (DF) to the severe dengue hemorrhagic fever (DHF) and healthy individuals. Various costimulatory molecules of CD40, CD80, CD86 and inducible costimulatory ligand (ICOSL) expressed on these three monocyte subsets were also analyzed. DENV-infected patients showed an increase in the frequency of intermediate monocytes and a decrease in the classical monocytes when compared to healthy individuals. Although these differences did not correlate with disease severity, changes during the early phase of infection gradually returned to normal in the defervescence phase. Moreover, decreased frequency of classical monocytes was associated with a significant up-regulation of co-stimulatory molecules CD40, CD86 and ICOSL. Kinetics of these co-stimulatory molecule-expressing classical monocytes showed different patterns throughout the sampling times of acute DENV infection. Different distribution of monocyte subsets and their co-stimulatory molecules in the peripheral blood during acute infection might exacerbate immune responses like cytokine storms and ADE, and future studies on intracellular molecular pathways utilized by these monocyte linages are warranted.