Phagocytic uptake of Encephalitozoon cuniculi by nonprofessional phagocytes

Encephalitozoon cuniculi is an obligate intracellular, spore-forming parasite belonging to the microsporidia that can cause disseminated infection in immunocompromised persons. E. cuniculi spores infect host cells by germination, i.e., by explosively everting the polar filament, through which the spore contents (sporoplasms) are subsequently injected into the cytoplasm. In addition, we observed intracellular, nongerminated spores in various nonprofessional phagocytes. In MRC5 cells, the number of internalized spores was approximately 10-fold higher than the number of injected sporoplasms. Compared to the rate of uptake by human monocyte-derived macrophages, internalization rates by A549 cells, MRC5 cells, and 293 cells were 0.6, 4.4, and 22.2%, respectively. The mechanism of uptake was studied in MRC5 cells. Killed spores were internalized at the same rate as live spores, indicating that nongerminated parasites do not actively participate in cell entry. Cytochalasin D inhibited uptake of spores by 95%, demonstrating an actin-dependent process. By electron and epifluorescence microscopy, intracellular spores were found in a tightly fitting membrane-bound compartment. The vacuole containing the spores was positive for the lysosomal membrane protein LAMP-1 and colocalized with the late endosomal-lysosomal content marker rhodamine dextran. Our results show that, in addition to the unique way in which microsporidia infect cells, E. cuniculi spores enter nonprofessional phagocytes by phagocytosis and traffic into a late endosomal-lysosomal compartment.

Dynamics of Apis mellifera filamentous virus (AmFV) infections in honey bees and relationships with other parasites

Apis mellifera filamentous virus (AmFV) is a large double stranded DNA virus of honey bees, but its relationship with other parasites and prevalence are poorly known. We analyzed individual honey bees from three colonies at different times post emergence in order to monitor the dynamics of the AmFV gut colonization under natural conditions. Prevalence and loads of microsporidia and trypanosomes were also recorded, as well as five common honey bee RNA viruses. The results show that a high proportion of bees get infected with AmFV during the first week post-emergence (75%) and that AmFV DNA levels remained constant. A similar pattern was observed for microsporidia while trypanosomes seem to require more time to colonize the gut. No significant associations between these three infections were found, but significant positive correlations were observed between AmFV and RNA viruses. In parallel, the prevalence of AmFV in France and Sweden was assessed from pooled honey bee workers. The data indicate that AmFV is almost ubiquitous, and does not seem to follow seasonal patterns, although higher viral loads were significantly detected in spring. A high prevalence of AmFV was also found in winter bees, without obvious impact on overwintering of the colonies.