The outcome of Cryptococcus neoformans intracellular pathogenesis in human monocytes

Abstract

Background: Cryptococcus neoformans is an encapsulated yeast that is a facultative intracellular pathogen. The interaction between macrophages and C. neoformans is critical for extrapulmonary dissemination of this pathogenic yeast. C. neoformans can either lyse macrophages or escape from within them through a process known as phagosomal extrusion. However, most studies of intracellular pathogenesis have been made with mouse cells and their relevance to human infection is uncertain. In this study we extended studies of C. neoformans-macrophage cellular interaction/s to human peripheral blood monocytes.

Results: This study demonstrated that C. neoformans can shed polysaccharide within human monocytes, spread from cell to cell, and be extruded from them. Furthermore, human monocytes responded to ingestion of C. neoformans with cell cycle progression from G1 to S.

Conclusion: Similarities between mouse and human cells support the suitability of mouse cells for the study of intracellular pathogenesis mechanisms. Given that these hosts diverged over 70 million years ago, the similar pathogenic strategies for C. neoformans in murine and human cells supports the hypothesis that the mechanism that underlies the mammalian intracellular pathogenesis of C. neoformans originated from interactions with a third host, possibly soil amoeboid predators, before the mammalian radiation.

Figure 1

Intracellular replication leads to extrusion of C. neoformans phagosome. HPBMs were incubated with C. neoformans strain H99. Following incubation, C. neoformans budding occurred every 2–3 hours as evidenced by the small arrows. This was followed by extrusion of the C. neoformans phagosomes as evidenced by the large arrow. Images were collected at 10×.

Figure 2

Intracellular polysaccharide shedding by C. neoformans cells. Polysaccharide shedding capacity of C. neoformans strain H99 was tested in HPBMs. Top panel: Intracellular shedding of cryptococcal polysaccharide from C. neoformans cells into HPBMs after 18 h incubation. Bottom panel: HPBMs lacking intracellular cryptococcal cells showed no fluorescence. Bar = 10 μM

Figure 3

Cell-to-cell spread of C. neoformans leads to infection of previously uninfected cell. Following phagocytosis, human peripheral blood monocytes closely apposed to each other underwent fusion leading to cell to cell spread of C. neoformans. The small arrow points to the uninfected monocyte approaching the infected monocyte to sequester the yeast cells while the large arrow indicates the C. neoformans cells that have been fully transferred to the previously uninfected human monocyte. Bar = 10 μM

Figure 4

Fc- and complement-receptor activation stimulates cell cycle progression of human peripheral blood monocytes from G1 to S. Phagocytosis of C. neoformans strain 20467 mediated by 18B7 and C. neoformans strain H99 mediated by human serum was followed by an increase in S phase cell distribution of human monocytes. Percentage of G1, S and G2 cells are indicated in the control group (C. neoformans added – and C. neoformans ingested -) and the phagocytosis assay group (C. neoformans added +) which was further separated into the non-phagocytic (C. neoformans added + and C. neoformans ingested -) and the phagocytic (C. neoformans added + and C. neoformans ingested +) groups. Comparison of G1, S and G2 percentages between non-phagocytic and phagocytic groups revealed statistically significant differences (p < 0.001).